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Cytoskeleton

The cytoskeleton is a highly dynamic and intricate network of protein filaments present throughout the cytoplasm of eukaryotic cells, functioning as an internal scaffold that maintains cellular shape, organizes organelles and other cytoplasmic components, and enables critical processes such as intracellular transport, cell motility, and division.^[1] Prokaryotic cells also possess a cytoskeleton composed of homologous proteins, such as FtsZ and MreB, which support analogous functions including cell division and shape maintenance.^[2] Composed primarily of three distinct types of filaments—actin filaments (also known as microfilaments), intermediate filaments, and microtubules—the cytoskeleton provides both structural integrity and functional versatility, with each filament type exhibiting unique assembly mechanisms, diameters, and roles.^[3]^[1] Actin filaments, the thinnest components at approximately 7 nm in diameter, consist of polymerized globular actin (G-actin) monomers forming flexible, double-helical structures that drive cellular movements, support the plasma membrane, and facilitate cytokinesis during cell division.^[4]^[1] Intermediate filaments, with diameters ranging from 8 to 12 nm, are rope-like assemblies of diverse fibrous proteins (such as keratins, vimentin, and neurofilaments) that impart mechanical resilience, resist tensile stress, and anchor the nucleus and other organelles to the cell periphery.^[5]^[6] Microtubules, the thickest at about 25 nm in diameter, form rigid, hollow tubes from dimers of α-tubulin and β-tubulin proteins, serving as tracks for motor proteins like kinesins and dyneins to transport vesicles and organelles, while also forming the mitotic spindle essential for chromosome segregation in mitosis.^[1]^[6] Beyond structural support, the cytoskeleton dynamically reorganizes through polymerization and depolymerization events regulated by accessory proteins, integrating with signaling pathways to coordinate responses to environmental cues, including adhesion to the extracellular matrix and intercellular communication.^[7]^[1] This interconnected meshwork not only connects intracellular elements to the cell's exterior but also senses and transduces mechanical forces, influencing processes like wound healing, immune response, and tissue development.^[8]^[3]

Introduction

Definition and Composition

The cytoskeleton is an intricate intracellular scaffold composed of protein polymers that provides mechanical support, maintains cell shape, and facilitates the spatial organization of cellular components in both prokaryotic and eukaryotic cells.^[3] This dynamic network interconnects the plasma membrane, nucleus, and various organelles, forming a versatile framework capable of rapid remodeling in response to cellular needs.^[7] Its filamentous architecture spans the cytoplasm, enabling structural integrity while allowing adaptability through polymerization and depolymerization processes.^[3] In eukaryotic cells, the cytoskeleton primarily consists of three classes of filaments: actin filaments (microfilaments), microtubules, and intermediate filaments, each with distinct molecular compositions and diameters.^[8] Actin filaments, approximately 7 nm in diameter, are assembled from globular actin (G-actin) monomers that polymerize into double-helical structures known as F-actin.^[3] Microtubules, with a diameter of about 25 nm, form hollow tubular arrays from α- and β-tubulin heterodimers arranged into 13 protofilaments.^[3] Intermediate filaments, ranging from 8 to 12 nm in diameter, exhibit a ropelike structure and are built from a diverse array of proteins, including keratins in epithelial cells, vimentin in mesenchymal cells, and lamins in the nuclear envelope.^[8] These components interact to create a cohesive meshwork that integrates with accessory proteins for stability and function.^[7] Prokaryotic cells possess homologs of these eukaryotic filaments, such as MreB (actin-like for cell shape maintenance), FtsZ (tubulin-like for division), and crescentin (intermediate filament-like for curvature), forming simpler cytoskeletal systems.^[9] This evolutionary conservation underscores the presence of filamentous proteins across all domains of life, with the greater complexity in eukaryotes emerging through gene duplication events that expanded protein diversity and regulatory mechanisms.^[9]

General Functions

The cytoskeleton provides essential mechanical support to cells, enabling them to resist deformation and maintain structural integrity under physical stress. This framework helps preserve cell polarity and asymmetry, which are critical for specialized cellular functions in tissues. For instance, in epithelial cells, the cytoskeleton counters tensile forces from the extracellular matrix, preventing collapse or irregular shaping during tissue stretching.^[8] Beyond structural roles, the cytoskeleton drives cell motility and division, facilitating processes such as crawling migration and cytokinesis. During cell division, it orchestrates chromosome segregation and ensures equitable partitioning of cellular contents to daughter cells, a process indispensable for organismal development and tissue renewal. In motile cells like fibroblasts, cytoskeletal dynamics enable directed movement across substrates, supporting immune surveillance and tissue remodeling.^[1] The cytoskeleton also positions organelles and coordinates intracellular trafficking, anchoring structures like mitochondria and the endoplasmic reticulum to maintain metabolic efficiency and spatial organization. Vesicle transport along cytoskeletal tracks delivers proteins and lipids to specific destinations, sustaining cellular homeostasis and responsiveness. This positioning is vital for energy production and secretory pathways in diverse cell types.^[10] Furthermore, the cytoskeleton integrates signal transduction by sensing mechanical cues from the environment and relaying them to intracellular pathways, often linking extracellular matrix interactions to nuclear gene expression. Such mechanosensing influences cell fate decisions, including proliferation and differentiation, through effectors like YAP/TAZ that respond to cytoskeletal tension. At the tissue level, collective cytoskeletal behaviors contribute to wound healing by promoting cell migration and contraction at injury sites, as well as embryogenesis through coordinated morphogenetic movements that shape developing organs. In immune responses, cytoskeletal rearrangements enable leukocyte extravasation and phagocytosis, bolstering host defense. The cytoskeleton's multifaceted roles underscore its indispensability for eukaryotic life, as disruptions lead to severe pathologies like developmental defects and cancer.^[11]^[12]

History

Early Observations

The earliest microscopic observations of cytoskeletal elements date back to the 19th century, when researchers began describing dynamic, thread-like structures within the cytoplasm using rudimentary light microscopy. In 1835, Félix Dujardin identified a viscous, granular substance termed sarcode in protozoan cells, noting its ability to extend into thread-like forms during movement and exudation. These observations laid the groundwork for recognizing the cytoplasm as a living, contractile material, later extended to plant cells where similar threads were seen in streaming protoplasm. In the 1860s, Max Schultze further characterized this substance as protoplasm or bioplasm, emphasizing its thread-like organization in various cell types, including plant cells, based on detailed examinations of Foraminifera and other organisms.^[13] Despite these insights, light microscopy's resolution limit of approximately 200 nm constrained observations to coarse fibrils in structures like amoeboid pseudopods and muscle fibers, without revealing the underlying fine architecture. Researchers could identify bundled threads in amoebae during locomotion and striated patterns in muscle cells, but the technology could not resolve individual filaments below the diffraction limit of visible light. This era's findings, while descriptive, highlighted the cytoplasm's fibrillar nature without molecular detail. Advances in the early 20th century improved visualization through vital staining techniques. In 1900, Leonor Michaelis introduced Janus green B as a supravital dye that selectively stained mitochondria in living cells, uncovering associated filamentous networks that suggested interconnected cytoplasmic components.^[14] Complementing this, Hugo de Vries's studies in the 1880s on protoplasmic streaming in plant cells documented rotational movements and thread formations, providing early evidence of the cytoplasm's dynamic organization.^[15] The shift toward higher resolution came with the advent of electron microscopy in the mid-20th century. In 1945, Keith Porter and colleagues examined chick embryo fibroblasts, revealing intricate networks of the endoplasmic reticulum intertwined with filamentous elements, which surpassed light microscopy's capabilities and opened the door to ultrastructural analysis.

Key Discoveries and Researchers

In the 1950s and 1960s, advances in electron microscopy enabled the visualization of intricate networks of filaments within the cytoplasm of diverse cell types, laying the groundwork for the modern concept of the cytoskeleton as a dynamic structural framework. The term "cytoskeleton" was first proposed by Russian cytologist Nikolai Koltsov in 1903 to describe a network of tubules that determine cell shape. Keith R. Porter, a pioneer in biological electron microscopy, played a central role in these observations, capturing high-resolution images of filamentous elements in animal and plant cells that suggested an interconnected cytoplasmic architecture. These findings, including the identification of tubular structures in plant cell cortices, highlighted the ubiquity of such filaments across eukaryotes and spurred further biochemical investigations. The discovery of actin as a key cytoskeletal component began with its biochemical isolation from rabbit muscle by Ferenc Bruno Straub in 1942, marking a breakthrough in understanding contractile proteins. This work was refined in 1954 by Hugh E. Huxley and Jean Hanson, who proposed the sliding filament model of muscle contraction, demonstrating how actin filaments interact with myosin to generate force through overlapping arrays. By the 1970s, Thomas D. Pollard and Edward D. Korn extended these insights to non-muscle cells, purifying actin from sources like the amoeba Acanthamoeba castellanii and showing its polymerization into filaments essential for motility and shape maintenance in diverse cell types. Microtubules emerged as another major cytoskeletal element through electron microscopy studies by Myron C. Ledbetter and Keith R. Porter in 1963, who first described these hollow, tubular structures in the cortices of dividing plant cells, linking them to cell wall formation. Subsequent biochemical advances came in 1972 when Richard C. Weisenberg developed a method to purify tubulin, the protein subunit of microtubules, by exploiting its high-affinity binding to colchicine, an alkaloid that disrupts microtubule assembly; this enabled in vitro polymerization studies confirming tubulin's role in dynamic cytoskeletal networks.^[16] Intermediate filaments were recognized as a distinct cytoskeletal class in the late 1970s, with Gale S. Bennett and colleagues demonstrating in 1978 that their 10-nm-diameter subunits vary by cell type, forming tissue-specific networks that provide mechanical resilience. For instance, William W. Schlaepfer's 1977 work isolated and characterized neurofilaments from rat peripheral nerves, revealing their composition of high-molecular-weight proteins and their importance in neuronal structural integrity. The identification of prokaryotic cytoskeletal homologs expanded the field's scope in the 1990s and early 2000s. In 1991, Erfei F. Bi and Joe Lutkenhaus visualized FtsZ forming a contractile ring at the division site in Escherichia coli, establishing it as a tubulin ancestor essential for bacterial cytokinesis.^[17] Similarly, Laurence J. Jones and colleagues reported in 2001 that MreB, an actin homolog in Bacillus subtilis, assembles into helical filaments beneath the membrane, directing rod-shaped cell morphogenesis.^[18] Recent advances up to 2025 have leveraged cryo-electron microscopy (cryo-EM) to resolve high-resolution structures of dynamic cytoskeletal assemblies, building on the 2017 Nobel Prize in Chemistry awarded to Jacques Dubochet, Joachim Frank, and Richard Henderson for developing cryo-EM techniques that revolutionized imaging of proteins like tubulin dimers and microtubule ends. Additionally, optogenetic tools have enabled precise light-mediated control of filament dynamics; for example, studies since 2020 have used photoactivatable systems to manipulate actin polymerization and microtubule stability in living cells, revealing spatiotemporal regulation of cytoskeletal remodeling during migration and division.^[19]

Eukaryotic Cytoskeleton

Microfilaments

Microfilaments, primarily composed of actin, are the thinnest elements of the eukaryotic cytoskeleton, formed by the polymerization of globular actin monomers (G-actin) into double-stranded, right-handed helical filaments known as filamentous actin (F-actin).^[20] Each G-actin subunit is a 42 kDa protein that binds ATP or ADP, and polymerization occurs head-to-tail, resulting in a filament with inherent structural polarity: the barbed (plus) end, where subunits add more rapidly, and the pointed (minus) end, where addition is slower.^[21] These filaments have a uniform diameter of approximately 7 nm and are relatively flexible compared to other cytoskeletal elements, allowing them to bend and form networks essential for cellular mechanics.^[20] The dynamic assembly and disassembly of microfilaments are governed by treadmilling, a steady-state process driven by ATP hydrolysis. G-actin-ATP subunits preferentially associate with the barbed end, promoting filament elongation, while hydrolysis to ADP-Pi and subsequent phosphate release at the pointed end facilitates subunit dissociation, creating a net flux of subunits from the plus to the minus end without overall length change under critical monomer concentrations.^[22] This polarity-dependent dynamics, first described in vitro, maintains a pool of unpolymerized G-actin and enables rapid remodeling in response to cellular signals.^[23] Numerous actin-binding proteins regulate microfilament polymerization and architecture. The Arp2/3 complex, activated by nucleation-promoting factors, binds to existing filament sides to nucleate branched actin networks with a characteristic 70-degree angle, facilitating dendritic array formation.^[24] In contrast, formins processively elongate unbranched filaments by stabilizing the barbed end and preventing subunit loss, often in linear bundles.^[25] Capping proteins like CapZ bind tightly to the barbed end in a calcium-independent manner, inhibiting further elongation and stabilizing filament length, while severing proteins such as cofilin bind ADP-actin segments to increase filament flexibility and create new ends for depolymerization or renucleation.^[26]^[27] In eukaryotic cells, microfilaments are particularly abundant in lamellipodia, where Arp2/3-mediated branching drives actin polymerization against the plasma membrane to generate protrusive forces for cell motility.^[28] During cytokinesis, they assemble into a contractile ring at the cell equator, where myosin II cross-links and slides antiparallel filaments to constrict the ring and divide the cytoplasm.^[29] In humans, cytoplasmic beta-actin is encoded by the ACTB gene on chromosome 7, while skeletal muscle alpha-actin is encoded by ACTA1 on chromosome 1; mutations in ACTA1, such as missense variants disrupting polymerization, are a leading cause of nemaline myopathy, characterized by muscle weakness and rod-like inclusions in myofibers.^[30]^[31]

Intermediate Filaments

Intermediate filaments (IFs) form a diverse superfamily of cytoskeletal proteins in eukaryotic cells, characterized by their rope-like structure with a diameter of approximately 10 nm, intermediate between actin microfilaments and microtubules. These proteins are encoded by over 70 genes in the human genome, divided into six major types (I–VI) based on sequence homology, structure, and tissue-specific expression. Type I and II IFs consist of acidic and basic keratins, respectively, primarily expressed in epithelial cells where they provide mechanical resilience to tissues exposed to external stress. Type III includes vimentin, desmin, and glial fibrillary acidic protein (GFAP), found in mesenchymal cells, muscle, and glia, respectively, supporting cellular integrity during development and repair. Type IV comprises neurofilaments in neurons, essential for axonal diameter and transport. Type V lamins form the nuclear lamina, underlying the inner nuclear membrane to maintain nuclear shape and chromatin organization. Type VI encompasses specialized proteins like nestin in neural progenitors and synemin in muscle, often co-assembling with other types.^[32]^[33]^[34] The assembly of IFs begins with the formation of α-helical coiled-coil dimers from two polypeptide chains, which associate laterally into antiparallel tetramers, the basic soluble unit. These tetramers further anneal end-to-end and side-to-side to create protofilaments, ultimately bundling into apolar 10-nm filaments without defined plus or minus ends, unlike the polar actin or microtubule polymers. This hierarchical, non-polar assembly allows for dynamic reorganization through end-to-end fusion and severing, enabling the formation of extensive, interconnected networks that span from the nucleus to the plasma membrane. The process is driven by the central rod domain's conserved sequence motifs, with head and tail domains contributing to lateral interactions and filament stability.^[35]^[36] IFs exhibit exceptional mechanical properties, including high tensile strength and extensibility, allowing individual filaments to stretch up to 240% of their initial length before fracturing, far exceeding the elasticity of actin or microtubules. Networks of IFs display strain-stiffening behavior, where stiffness increases nonlinearly under deformation, providing a viscoelastic response that absorbs and dissipates mechanical stress without permanent damage. For instance, desmin IFs in muscle cells yield at stresses around 10 MPa and harden progressively, enabling cells to withstand cyclic loading. This resilience contrasts with the more brittle failure of other cytoskeletal elements, positioning IFs as key shock absorbers in mechanically challenged tissues.^[37]^[38]^[39] In eukaryotic cells, IFs fulfill specialized structural roles tailored to cellular and tissue demands. Lamins (type V) ensure nuclear lamina integrity, anchoring chromatin and supporting nuclear envelope stability during cell division and migration. Keratins (types I/II) anchor to desmosomes in epithelial tissues like skin, resisting shear forces and preventing blistering under tension. Neurofilaments (type IV) provide radial support in axons, regulating caliber and facilitating long-distance transport in neurons. Vimentin (type III) in mesenchymal cells maintains cytoskeletal organization during epithelial-to-mesenchymal transitions, while also integrating briefly with actin and microtubules for overall network cohesion. These functions underscore IFs' role in cellular architecture, with mutations often leading to tissue fragility diseases.^[33]^[40]^[41] The genetic diversity of IFs, with approximately 70–73 genes in humans, reflects their evolutionary adaptation from ancient, IF-like proteins in early metazoans, where they likely emerged to support multicellularity and tissue differentiation. Phylogenetic analyses trace type III proteins as the most ancestral, with subsequent diversification into tissue-specific classes across vertebrates. This expansion enabled specialized expressions, such as keratins in amniotes for epidermal barriers, highlighting IFs' conserved yet versatile contribution to eukaryotic structural complexity.^[32]^[42]^[43]

Microtubules

Microtubules are rigid, hollow cylindrical filaments essential to the eukaryotic cytoskeleton, serving as tracks for intracellular transport and structural supports during processes like cell division. They are polymerized from heterodimers of α-tubulin and β-tubulin, each approximately 8 nm in length, which align head-to-tail to form linear protofilaments. These protofilaments associate laterally to create a tubular structure typically consisting of 13 protofilaments arranged around a central hollow core, resulting in an outer diameter of about 25 nm and an inner diameter of roughly 15 nm. The assembly process is polarized: the plus end, exposing β-tubulin, grows preferentially by the addition of GTP-bound tubulin dimers, while the minus end, capped by α-tubulin, is more stable and often anchored. This polarity imparts directional properties to microtubules, influencing their interactions with cellular components. Microtubule dynamics are characterized by a phenomenon known as dynamic instability, where individual filaments undergo stochastic switches between phases of slow growth and rapid shrinkage. During growth, GTP-tubulin dimers add to the plus end, forming a stabilizing "GTP cap" that prevents hydrolysis of the bound GTP to GDP; loss of this cap triggers catastrophe, a sudden depolymerization from the plus end at rates up to 20 times faster than growth. Conversely, rescue events allow regrowth by recapping the end with GTP-tubulin, enabling microtubules to explore cellular space efficiently. The minus ends are generally less dynamic and are anchored at microtubule-organizing centers (MTOCs), such as the centrosome in animal cells, which nucleate and stabilize the microtubule network. Several microtubule-associated proteins (MAPs) regulate microtubule stability and dynamics. For instance, tau, a prominent neuronal MAP, binds along the microtubule lattice to promote assembly and inhibit depolymerization, thereby enhancing structural integrity in axons. In contrast, plus-end tracking proteins (+TIPs), such as EB1, autonomously track growing plus ends, recognizing the GTP-bound conformation and recruiting other factors to modulate polymerization rates and end structure. These proteins collectively fine-tune microtubule behavior in response to cellular needs. In eukaryotic cells, microtubules organize into distinct arrays depending on the cell type and cycle stage. Astral microtubules radiate from centrosomes during mitosis to position the spindle apparatus, while spindle microtubules form the bipolar mitotic spindle that segregates chromosomes. Cortical arrays, prominent in interphase cells especially in plants, align parallel to the plasma membrane to guide cell wall deposition and shape. Nucleation primarily occurs at the centrosome in animal cells, where γ-tubulin ring complexes (γ-TuRCs) template the minus ends. In plants, which lack centrosomes, γ-TuRCs associate with dispersed sites like the nuclear envelope or cortical regions to initiate microtubule formation. Post-translational modifications (PTMs) of tubulin further diversify microtubule properties and longevity. Acetylation of lysine residues on α-tubulin, typically on stable microtubules, increases lattice flexibility and resistance to mechanical stress, promoting long-term stability without altering polymerization rates. Detyrosination, the removal of the C-terminal tyrosine from α-tubulin, marks older microtubules and enhances interactions with certain MAPs and motors, indirectly stabilizing the lattice while selectively influencing motor protein binding affinity. These PTMs create a "tubulin code" that spatially and temporally regulates microtubule functions across cellular compartments.

Accessory Structures

Accessory structures of the eukaryotic cytoskeleton encompass specialized proteins and complexes that support, link, or organize the primary filament systems of microfilaments, intermediate filaments, and microtubules, enhancing their functional integration without forming the core polymeric networks themselves. These elements include GTP-binding septins, which assemble into filament-like scaffolds, and the spectrin-based membrane skeleton, which provides mechanical reinforcement beneath the plasma membrane. Other accessories, such as microtubule-organizing centers (MTOCs) and actin cross-linkers like fascin, facilitate nucleation, anchoring, and bundling to maintain cellular architecture and dynamics.^[44] Septins are a family of conserved GTP-binding proteins that polymerize into hetero-oligomeric complexes, typically consisting of four to eight subunits, which further assemble into non-polar filaments and higher-order structures such as rings and sheets. These filaments act as scaffolds to recruit other proteins at sites of cell division, including the contractile ring during cytokinesis, and at the bases of cilia and flagella where they stabilize microtubule arrays. In budding yeast (Saccharomyces cerevisiae), septins form hourglass-shaped rings at the bud neck early in the cell cycle, transitioning to paired rings that delimit the division site and coordinate membrane remodeling during cytokinesis. Originally identified through genetic screens for cell division mutants in yeast, septins have been recognized as the "fourth" cytoskeletal component due to their filament-forming capacity and roles in compartmentalization and polarity establishment across eukaryotes.^[45]^[46] The spectrin cytoskeleton consists of α- and β-spectrin heterodimers that self-associate head-to-head to form flexible tetramers, which cross-link short actin filaments into a two-dimensional polygonal network underlying the plasma membrane, particularly prominent in erythrocytes. In red blood cells, spectrin tetramers bind ankyrin to anchor the network to the lipid bilayer via band 3 and 4.2 proteins, generating cortical tension that confers biconcave shape and resistance to shear stress during circulation. Beyond erythrocytes, spectrin isoforms in non-erythroid cells, such as neurons and endothelial cells, contribute to mechanosensing by transducing extracellular forces through the network to ion channels and adhesion complexes, thereby regulating processes like axonal integrity and vascular permeability.^[44]^[47] In yeast, the cytoskeleton integrates accessory elements with core filaments for specialized functions: actin assembles into polarized cables that guide vesicular transport for secretion and polarized growth, while cortical patches mediate endocytosis, and astral microtubules emanating from the spindle pole body position the mitotic spindle for proper segregation. Septins at the bud neck integrate with these systems by recruiting actin cables via the scaffold protein Hof1, ensuring targeted exocytosis and membrane ingression during budding. Microtubule-organizing centers (MTOCs), such as the centrosome in animal cells or the spindle pole body in yeast, nucleate and anchor microtubules through γ-tubulin ring complexes, directing spindle assembly and intracellular trafficking. Actin cross-linkers like fascin bundle filaments into parallel arrays, promoting the formation of protrusive structures such as filopodia in motile cells by increasing filament rigidity and stability. These accessories collectively enable the cytoskeleton to adapt to mechanical cues and coordinate multicomponent assemblies for cellular homeostasis.^[46]^[48]^[49]

Prokaryotic Cytoskeleton

FtsZ

FtsZ is a highly conserved GTPase protein that serves as the primary structural homolog of eukaryotic tubulin in prokaryotes, playing a central role in cell division by forming the Z-ring, a contractile structure at the midcell that coordinates septum formation.^[50] Found in nearly all bacteria and most archaea, FtsZ polymerizes into single-stranded protofilaments that exhibit treadmilling dynamics, driven by GTP hydrolysis, enabling the filaments to assemble and disassemble rapidly for precise positioning.^[51] These protofilaments associate laterally to create the Z-ring, which constricts during cytokinesis to invaginate the cell membrane and facilitate division, often in coordination with the divisome complex that includes peptidoglycan synthases for cell wall remodeling in bacteria.^[52] The assembly of FtsZ filaments is tightly regulated to ensure accurate midcell localization and prevent aberrant septation. FtsZ monomers bind GTP and polymerize head-to-tail into dynamic filaments, where GTP hydrolysis induces curvature and depolymerization, promoting turnover; this process is modulated by accessory proteins such as FtsA and ZipA, which tether the Z-ring to the membrane.^[53] The Min system, comprising MinC, MinD, and MinE proteins, further regulates positioning by oscillating between cell poles, inhibiting FtsZ polymerization at undesirable sites like the poles and thus restricting Z-ring formation to the midcell, thereby avoiding division over nucleoids or at cell ends.^[54] In the bacterial divisome, the Z-ring recruits downstream proteins like FtsI and FtsW, which synthesize peptidoglycan to complete septum formation.^[55] Evolutionarily, FtsZ shares low sequence identity (10-18%) with tubulin but exhibits high structural homology, reflecting a common ancestral GTPase fold that supports polymerization into cytoskeletal filaments, with cryo-EM structures from the 2010s onward revealing curved protofilaments in FtsZ that mirror tubulin's conformational changes during microtubule dynamics.^[51]^[56] High-resolution cryo-EM analyses, such as those of Klebsiella pneumoniae FtsZ, have shown single protofilaments in a polymerization-preferred straight conformation transitioning to curved forms upon GTP hydrolysis, underscoring the mechanistic parallels to eukaryotic microtubules.^[57] In archaea, FtsZ variations include the formation of broader, wheel-like bands rather than tight rings, often anchored by SepF instead of bacterial FtsA, adapting the system to diverse membrane topologies while maintaining binary fission.^[58] Due to its essential role and conservation, FtsZ is a promising antibacterial target; for instance, the small-molecule inhibitor PC190723 stabilizes FtsZ filaments, preventing dynamic turnover and Z-ring constriction, leading to bactericidal effects against pathogens like Staphylococcus aureus without impacting eukaryotic cells.^[59] This compound binds an allosteric site on FtsZ, promoting excessive polymerization and disrupting divisome assembly, highlighting FtsZ's therapeutic potential in combating antibiotic-resistant bacteria.^[60]

MreB

MreB is the primary actin homolog in prokaryotes, essential for maintaining the rod-shaped morphology of many bacterial cells by organizing cell wall synthesis during elongation. As a structural and functional analog to eukaryotic actin, MreB assembles into dynamic filaments that guide the insertion of new peptidoglycan units, ensuring uniform sidewall growth and preventing aberrant shapes. Unlike eukaryotic actin, MreB polymerization does not require ATP or GTP hydrolysis for assembly, relying instead on conformational changes and interactions with the cytoplasmic membrane to form stable structures.^[61] The structure of MreB consists of monomeric subunits that polymerize into antiparallel double protofilaments, forming a double-ring configuration highly similar to that of actin, as determined by X-ray crystallography and cryo-electron microscopy studies (e.g., in Caulobacter crescentus, PDB ID: 4CZF). These protofilaments assemble laterally into sheet-like or helical filaments that localize beneath the inner cytoplasmic membrane, adopting a circumferential orientation perpendicular to the cell's long axis. This membrane association is crucial for MreB's function, as it positions the filaments to scaffold cell wall biosynthetic enzymes. Polymerization occurs independently of nucleotide hydrolysis, with assembly driven by membrane crowding and lipid binding, allowing for rapid treadmilling and turnover.^[61]^[62] In terms of function, MreB filaments act as a cytoskeletal scaffold that directs the elongasome complex—comprising peptidoglycan synthases and hydrolases—to sites of sidewall insertion, promoting lateral cell expansion while maintaining cylindrical geometry. MreB interacts with accessory proteins MreC and MreD, which bridge the cytoskeleton to extracytoplasmic cell wall synthesis machinery, such as penicillin-binding proteins (PBPs), ensuring coordinated peptidoglycan deposition. These interactions enable MreB patches to rotate circumferentially around the cell, distributing growth evenly and counteracting turgor pressure-induced widening. Disruptions in MreB function, such as through depletion or inhibition (e.g., via A22 drug), result in loss of rod shape, leading to spherical or irregular cells prone to lysis due to unbalanced wall synthesis.^[63]^[64]30580-X) MreB is widely distributed in rod-shaped (bacillar) bacteria, such as Escherichia coli and Bacillus subtilis, where it is indispensable for maintaining elongated morphology during vegetative growth. It is notably absent in spherical cocci, like Staphylococcus aureus, which lack the need for lateral elongation machinery and instead rely on septal division for shape maintenance. In organisms possessing MreB, paralogs (e.g., MreB, Mbl, MreBH in B. subtilis) often form overlapping helical networks to redundantly support shape determination. MreC and MreD, co-encoded in the mreBCD operon, are conserved in MreB-containing species and facilitate linkage to wall synthesis, with their absence mimicking MreB loss by producing round cells.00292-7)^[65]^[66] The dynamics of MreB are characterized by rapid, oscillatory movements visualized through fluorescent tagging (e.g., GFP-MreB fusions), revealing filaments that treadmill at speeds of approximately 5–20 nm/s and rotate around the cell's long axis at rates coupled to peptidoglycan assembly. These motions form transient patches or helices that sweep the sidewall, ensuring isotropic growth; rotation halts upon inhibition of wall synthesis, indicating that MreB dynamics are powered by the mechanical forces of polymer insertion rather than independent motor activity. In E. coli, MreB filaments exhibit a pitch angle inversely correlated with cell diameter, adapting to maintain shape homeostasis. Mutations in mreB, such as point substitutions or deletions, disrupt these dynamics, causing cells to adopt spherical forms with increased width and reduced viability, underscoring MreB's role in force generation and curvature control.^[67]30580-X)^[61] Evolutionarily, MreB represents an ancient prokaryotic innovation predating eukaryotic actin, with structural homology evident in its double-ring fold and filament architecture, suggesting a common ancestry for cytoskeletal polymers across domains of life. Recent studies from the 2020s have highlighted MreB's ability to sense and induce membrane curvature through nucleotide-dependent twisting (e.g., ~10° left-handed twist in ATP-bound states), enabling it to respond to local geometry and stabilize elongating membranes. This curvature-sensing mechanism likely evolved to integrate cytoskeletal mechanics with cell envelope biogenesis, as seen in conserved roles from bacteria to plant thylakoids via MreB homologs.^[61]

Other Prokaryotic Elements

In addition to the core components like FtsZ and MreB, prokaryotes possess a variety of specialized cytoskeletal proteins that facilitate intracellular organization, particularly for DNA segregation and cell curvature. One prominent example is ParM, an actin-like ATPase encoded by certain low-copy-number plasmids in bacteria such as Escherichia coli. ParM polymerizes into dynamic, bipolar filaments that assemble around plasmid-bound ParR-parC complexes, elongating bidirectionally to generate force and separate replicated DNA molecules to opposite poles of the cell via ATP-dependent treadmilling. This mechanism ensures stable plasmid inheritance during cell division, mimicking aspects of eukaryotic mitotic spindles but in a simplified, plasmid-specific context.00451-9) Another key element is Crescentin, a coiled-coil protein in bacteria like Caulobacter crescentus that exhibits intermediate filament-like assembly properties. Crescentin forms stable, 10-nm-wide filaments that localize along the inner, concave side of the cell envelope, providing longitudinal rigidity and resisting bending forces to maintain the characteristic vibrioid or crescent shape. Mutants lacking Crescentin adopt a straight-rod morphology, highlighting its role in cell curvature independent of peptidoglycan synthesis. This protein represents an ancient bacterial adaptation for morphological diversity, with homologs identified in other curved bacteria.00935-8) Further examples include TubZ, a tubulin-like GTPase involved in partitioning large plasmids and viral elements in bacteria such as Bacillus thuringiensis and bacteriophages. TubZ assembles into protofilaments that exhibit dynamic instability and treadmilling, positioning DNA cargoes at mid-cell for equitable distribution during replication. In archaea, actin-like proteins, such as those encoded in Asgard archaea genomes, form branched and bundled filaments that contribute to complex cell architectures, including vesicle formation and shape maintenance, bridging prokaryotic and eukaryotic cytoskeletal features. Bactofilins, another class of bacterial cytoskeletal polymers like BacA in C. crescentus, organize envelope biogenesis by forming rigid filaments that guide protein localization during stalk development.^[68]^[69] Genomic analyses from the 2010s onward have uncovered this expanding diversity of prokaryotic cytoskeletal elements, revealing over a dozen families of filament-forming proteins across bacteria and archaea that enable targeted mechanical functions without compartmentalized organelles. These systems support essential processes like viral capsid transport and asymmetric division, often through homology to eukaryotic counterparts but adapted for prokaryotic simplicity. However, prokaryotic cytoskeletons typically lack the extensive cross-linking and regulatory networks seen in eukaryotes, focusing instead on discrete tasks such as force generation for segregation or localized stiffening for morphology.00350-X)^[70]

Cytoskeletal Dynamics

Assembly and Disassembly

The assembly of actin microfilaments proceeds via a nucleation-elongation mechanism, in which the initial formation of a trimeric or tetrameric nucleus is thermodynamically unfavorable and rate-limiting, followed by rapid addition of globular actin (G-actin) monomers primarily to the barbed (plus) end of the filament.^[71] Polymerization occurs above a critical concentration of approximately 0.1–0.5 μM for ATP-bound G-actin at the barbed end, below which net disassembly dominates, ensuring steady-state treadmilling where subunits add to the barbed end and dissociate from the pointed (minus) end.^[72] ATP hydrolysis within the filament, coupled to polymerization, converts ATP-actin to ADP-Pi-actin and eventually ADP-actin, which destabilizes the filament lattice and promotes disassembly from the pointed end, recycling monomers for further growth at the barbed end.^[72] Microtubule assembly and disassembly exhibit dynamic instability, characterized by stochastic switches between phases of slow growth and rapid shrinkage at the plus ends.^[73] The net growth rate during the growth phase is governed by the equation

\text{growth rate} = k_{\text{on}} [\text{tubulin}] - k_{\text{off}},

where k_{\text{on}} and k_{\text{off}} are the association and dissociation rate constants, respectively, and [tubulin] is the free tubulin dimer concentration; above a critical concentration of about 5–10 μM, net assembly occurs, but stochastic loss of a stabilizing GTP cap at the plus end triggers catastrophe and rapid depolymerization.^[73] The GTP cap, formed by GTP-bound tubulin dimers at the growing end, maintains lattice stability by preventing curling of protofilaments, while GTP hydrolysis to GDP within the body of the microtubule weakens interdimer bonds, facilitating disassembly upon cap loss.^[74] In contrast to actin and microtubules, intermediate filaments display slower subunit exchange and turnover, with half-lives typically ranging from hours to days or longer, depending on the filament type and cellular context, reflecting their role in mechanical resilience rather than rapid remodeling.^[75]^[76] Disassembly is primarily regulated by site-specific phosphorylation, such as by cyclin-dependent kinases (CDKs) at consensus motifs in the head and tail domains of filament proteins like vimentin or keratins, which disrupts lateral interactions and promotes filament fragmentation into soluble tetramers or octamers.^[77] Across filament types, assembly is facilitated by nucleating factors that lower the energy barrier for initial oligomer formation. For actin, the Arp2/3 complex binds to existing filaments and nucleates branched daughter filaments by mimicking a actin dimer, enabling dendritic network formation.^[78] Microtubule nucleation is templated by the γ-tubulin ring complex (γ-TuRC), which provides a helical scaffold matching the microtubule lattice and promotes addition of 13–14 protofilaments from α/β-tubulin dimers. Depolymerization is enhanced by severing proteins: katanin, an AAA ATPase, uses ATP hydrolysis to cut microtubules internally, increasing the number of ends available for disassembly, while ADF/cofilin binds ADP-actin filaments, inducing twists that promote severing and pointed-end depolymerization to recycle monomers.^[79] Prokaryotic cytoskeletal elements exhibit analogous but simplified dynamics. FtsZ, a tubulin homolog essential for division, forms treadmilling protofilaments in a GTP-dependent manner without a stabilizing cap, where GTP hydrolysis drives subunit flux along the filament, facilitating Z-ring constriction.^[80] MreB, an actin homolog involved in cell wall elongation, assembles into dynamic, short filaments with rapid turnover (half-life ~20–40 seconds), enabling circumferential motion and peptidoglycan synthesis guidance.^[81]

Motor Proteins and Regulation

Motor proteins are ATP-dependent enzymes that generate force and movement by "walking" along cytoskeletal filaments, converting chemical energy from ATP hydrolysis into mechanical work.^[82] These proteins interact with actin microfilaments or microtubules as tracks, enabling processes such as contraction and directed transport within eukaryotic cells.^[83] The core mechanochemical cycle involves binding to the track, hydrolysis of ATP to ADP and inorganic phosphate, a conformational change that propels the motor forward, and release of products to reset the cycle.^[84] Myosins constitute a superfamily of motor proteins that translocate along actin filaments toward the plus (barbed) end.01016-6) Class II myosins, such as non-muscle myosin II, form bipolar filaments that drive actin-myosin sliding for cellular contraction in processes like cytokinesis and wound healing.01016-6) In contrast, myosin V, a processive dimer, facilitates vesicle and organelle transport by taking ~36 nm steps per ATP hydrolyzed, matching the actin helical repeat.^[85] The power stroke in myosins arises from a swinging lever arm mechanism, where ATP binding detaches the head from actin, hydrolysis cocks the lever, and phosphate release triggers the forward swing.^[84] Kinesins and dyneins are microtubule-based motors that move cargos along microtubules, with most kinesins directing toward the plus end and dyneins toward the minus end.^[86] Conventional kinesin-1 (KIF5) advances in hand-over-hand steps of approximately 8 nm per ATP molecule, with its two heads alternating to ensure processivity.^[83] Dynein, a large AAA+ ATPase complex, exhibits a similar stepping mechanism but often requires dynactin and adaptor proteins for efficient cargo binding and motility.^[83] Both families hydrolyze ATP to drive conformational changes that bias movement along the microtubule lattice.^[87] Regulation of motor proteins and cytoskeletal dynamics is mediated by signaling pathways that modulate activity, localization, and filament interactions. Rho GTPases, small G proteins that cycle between GTP-bound (active) and GDP-bound (inactive) states, orchestrate actin organization: Cdc42 activates the Arp2/3 complex for branched actin networks in lamellipodia, while RhoA promotes linear stress fibers through formin-mediated polymerization and myosin II activation.^[88] Calcium ions, bound to calmodulin, regulate myosins by activating myosin light chain kinase (MLCK), which phosphorylates the regulatory light chain of myosin II to relieve inhibition and enable actin binding.^[89] Cross-talk between motors and the cytoskeleton involves microtubule-associated proteins (MAPs) that link motors to cargo and modulate track accessibility. For instance, MAPs like tau and MAP2 can compete with kinesins for microtubule binding sites, thereby regulating motor processivity and cargo distribution.^[90] Phosphorylation by kinases such as Rho-associated coiled-coil kinase (ROCK), a RhoA effector, enhances myosin II activity by inhibiting myosin light chain phosphatase, promoting contractility, and also influences microtubule stability via LIM kinase activation.^[91] In prokaryotes, true ATP-powered walking motors like myosins, kinesins, or dyneins are absent, with cytoskeletal functions relying instead on polymerization-depolymerization dynamics for force generation.^[92] For example, ParM, an actin homolog in plasmid segregation systems, forms dynamic filaments that push DNA apart through bistable polymerization rather than motor walking.^[93]

Cellular Functions

Cell Shape and Mechanics

The cytoskeleton plays a pivotal role in determining and maintaining cell shape through the coordinated action of its major components: actin filaments, microtubules (MTs), and intermediate filaments (IFs). The actin cortex, a thin meshwork of crosslinked actin filaments beneath the plasma membrane, generates protrusive and retractile forces that drive lamellipodia and filopodia formation, enabling dynamic changes in cell morphology during migration and spreading.^[94] Microtubules contribute to cellular asymmetry and polarity by organizing the spatial distribution of organelles and signaling molecules, with their stiffness balancing cortical tension to establish elongated or polarized shapes in adherent cells.^[95] Intermediate filaments provide tensile strength and mechanical resilience, distributing compressive and shear forces across the cell to prevent rupture under stress, particularly in tissues subjected to mechanical strain.^[96] In mechanobiology, the cytoskeleton integrates with extracellular matrix (ECM) cues via focal adhesions, where actin stress fibers connect to integrin receptors through adaptor proteins like talin, facilitating force sensing and transmission. Talin unfolding under mechanical load exposes cryptic binding sites, reinforcing adhesion complexes and propagating signals that modulate cytoskeletal remodeling in response to substrate rigidity.^[97]^[98] This linkage allows cells to adapt their shape to environmental forces, such as shear stress or ECM stiffness, thereby influencing processes like tissue morphogenesis. The cytoskeleton exhibits viscoelastic properties, behaving as a composite material where actin and MTs provide elasticity and IFs contribute viscosity, enabling cells to deform reversibly while resisting permanent damage.^[8] The tensegrity model describes this at the whole-cell level, positing that prestressed cytoskeletal networks—supported by compressive MTs and tensile actomyosin—maintain structural integrity and distribute forces globally, consistent with observations of cell stiffening under tension.^[99] In prokaryotes, analogous elements enforce shape maintenance without a true nucleus. MreB filaments, actin homologs, form dynamic helices along the cell membrane that guide peptidoglycan insertion, enforcing rod-like morphology in bacteria like Escherichia coli by counteracting turgor pressure.^[100] FtsZ, a tubulin homolog, assembles into contractile rings at division sites, generating tension that constricts the membrane during septation while coordinating with MreB to couple elongation and division.^[101] Experimental techniques have elucidated these mechanics. Micropipette aspiration quantifies cortical tension by applying negative pressure to deform cell membranes, revealing actin-dependent elasticity on the order of 0.1–1 nN/μm in eukaryotic cells.^[102] Recent advances in traction force microscopy (TFM) during the 2020s, including uncertainty-aware algorithms, map subcellular force patterns with high resolution, showing how cytoskeletal disruptions alter traction stresses up to 1000 Pa in migrating fibroblasts.^[103]^[104]

Intracellular Transport

The cytoskeleton enables directed intracellular transport of vesicles, organelles, and macromolecules by providing structural tracks for motor proteins. In eukaryotic cells, microtubules act as primary highways for long-distance vesicle trafficking, where kinesin-1 motors facilitate anterograde movement of cargo from the cell center toward the periphery.^[105] Dynein motors, in contrast, power retrograde transport along microtubules, returning vesicles to the perinuclear region.^[106] For short-range delivery near the plasma membrane, actin filaments paired with myosin motors support localized vesicle movements, such as in synaptic regions.^[107] Organelle positioning also depends on cytoskeletal networks. Microtubules maintain the central localization of the Golgi apparatus and endoplasmic reticulum through motor-driven anchoring and dynamic interactions with post-translationally modified tubulin.^[108] In budding yeast, actin cables and myosin V mediate vacuole inheritance, ensuring proper partitioning to daughter cells during cell division.^[109] Chromosome segregation during mitosis involves microtubule-based transport mechanisms. Kinesin-5 generates sliding forces between antiparallel microtubules to elongate the spindle and separate chromosomes.^[110] Cortical dynein, anchored at the cell periphery, pulls on astral microtubules to orient and position the spindle apparatus.^[111] In prokaryotes, intracellular transport is constrained by small cell size and relies primarily on diffusion, but specialized systems like ParM filaments use ATP-driven polymerization to actively push plasmids toward opposite cell poles for equitable segregation.^[112] Disruptions in these transport processes, such as dynein mutations, impair axonal trafficking and contribute to neurodegeneration in diseases like amyotrophic lateral sclerosis.^[113]

Cytoplasmic Streaming

Cytoplasmic streaming, also known as cyclosis, refers to the directed flow of cytoplasm within cells, primarily driven by interactions between the actin cytoskeleton and motor proteins. In plant cells, this process is powered by class XI myosins (myosin XI) that associate with organelles and move processively along bundles of actin filaments, entraining surrounding cytoplasm to generate bulk flow. These actin cables are often organized into thick bundles aligned parallel to the cell's long axis, particularly in elongated cells such as those in characean algae and vascular plants. The streaming velocity varies by species and cell type, reaching up to 70 μm/s in characean algae, while in Arabidopsis thaliana cells it typically ranges from 4 to 8 μm/s, directly correlating with the stepping rate of myosin XI motors under physiological ATP concentrations.^[114]^[115]^[116]^[117] In animal cells, cytoplasmic streaming occurs in specific contexts, such as the rapid ooplasmic flows in Drosophila oocytes driven by actin-myosin interactions, which can reach velocities up to 20 μm/s and facilitate the localization of mRNAs and organelles during early development. Similar streaming aids in organelle distribution in large cells like amoebae or neuronal processes.^[118] The structural organization of cytoplasmic streaming relies on transvacuolar strands—cytoplasmic bridges spanning the central vacuole—that contain bundled actin filaments serving as tracks for myosin-driven movement. These strands maintain cytoplasmic continuity and facilitate the circulation of organelles like chloroplasts and mitochondria. Regulation occurs through cytosolic calcium ions (Ca²⁺), where elevated concentrations inhibit myosin XI activity and halt streaming, acting as a brake on motility during stress responses. Plant hormones, such as auxin, indirectly modulate streaming by influencing actin organization and nutrient diffusion, thereby linking it to developmental signaling. In prokaryotes, true cytoplasmic streaming is absent, but analogous surface motility occurs via type IV pili in bacteria like Myxococcus xanthus, where pilus extension and retraction enable gliding over substrates without internal bulk flow.^[119]^[120]^[121]^[122] Functionally, cytoplasmic streaming enhances molecular diffusion in large plant cells by creating convective currents that distribute nutrients, metabolites, and signaling molecules more efficiently than passive diffusion alone, preventing nutrient gradients in expansive volumes. In reproductive structures, it supports pollen tube growth by directing cytoplasmic flow toward the tip, aiding vesicle delivery and polarized elongation essential for fertilization. Additionally, streaming contributes to gravitropism by facilitating the redistribution of hormones like auxin, enabling asymmetric growth responses to gravity. Disruptions, such as those induced by actin-disrupting agents like cytochalasin B, impair metabolite exchange between chloroplasts, reducing photosynthetic efficiency and non-photochemical quenching under varying light conditions.^[123]^[124]^[121]^[125] Recent studies in the 2020s, including analyses of myosin XI dynamics and autoregulation, have reinforced the causal role of actin-myosin interactions in streaming through in vitro motility assays and genetic mutants, showing that alterations in myosin stepping modes directly impact flow rates. For instance, dual-stepping mechanisms in Arabidopsis myosin XI isoforms enable high-speed processivity, confirming the molecular basis for observed velocities. These findings, combined with observations of streaming defects in hormone-treated cells, underscore its integration with broader cytoskeletal regulation.^[116]^[126]

Evolutionary and Comparative Aspects

Similarities Across Domains

The cytoskeleton demonstrates profound conservation across Bacteria, Archaea, and Eukarya, reflecting shared evolutionary origins that predate the last universal common ancestor (LUCA). Core homologies include FtsZ proteins in prokaryotes, which are structural and functional relatives of eukaryotic tubulins; both form protofilamentous structures and utilize GTP hydrolysis to enable treadmilling and dynamic instability essential for processes like cell division.^[9] Similarly, bacterial MreB proteins exhibit homology to eukaryotic actin, polymerizing into filaments powered by ATP hydrolysis to generate forces for cell wall synthesis and shape maintenance.^[127] Crescentin, found in certain bacteria, shares biochemical properties and domain organization with eukaryotic intermediate filaments (IFs), forming stable, apolar structures that provide mechanical resilience without nucleotide dependence.^[128] These homologies underscore a universal reliance on nucleotide hydrolysis—GTP for tubulin/FtsZ and ATP for actin/MreB—to regulate filament assembly, disassembly, and force production across domains.^[129] Filament polymerization serves conserved functions in force generation, cell division, and morphology throughout life. In all domains, these polymers create contractile rings or scaffolds that constrict during cytokinesis, as seen with FtsZ rings in bacteria and archaea mirroring tubulin-based spindles in eukaryotes.^[130] MreB and actin homologs similarly drive elongation and polarity, ensuring rod-like shapes in prokaryotes and motility in eukaryotes through polarized growth and depolymerization.^[100] This shared architecture allows cytoskeletal elements to transduce chemical energy into mechanical work, facilitating division and structural integrity from simple prokaryotic cells to complex eukaryotes.^[9] Archaeal cytoskeletal proteins provide critical insights into the prokaryotic-eukaryotic transition, bridging gaps with actin-like and IF-like elements. Crenactin, an archaeal actin homolog in hyperthermophilic Crenarchaeota, polymerizes into double-helical filaments strikingly similar to F-actin, regulated by ATP hydrolysis and implicated in cell shape determination.^[131] Complementing this, coiled-coil-rich proteins (CCRPs) in archaea, particularly in Asgard archaea, form IF-like filaments that organize the cell envelope and support membrane dynamics, resembling eukaryotic IFs in stability and multifunctionality.^[130] These archaeal systems highlight intermediate forms that likely contributed to eukaryotic complexity. Evolutionary models posit that the last eukaryotic common ancestor (LECA) acquired cytoskeletal sophistication through endosymbiosis, integrating prokaryotic precursors with novel regulatory elements to enable phagocytosis and organelle positioning.^[132] Recent genomic and phylogenomic studies from the 2020s, analyzing Asgard archaea and deep-branching lineages, confirm these homologies originated before LUCA, with diversified cytoskeletal gene families evident in early cellular organization around 4.2 billion years ago.^[133] Such findings reveal a pre-LUCA cytoskeletal toolkit that evolved through gene duplication and functional specialization across domains.^[134]

Differences Between Prokaryotes and Eukaryotes

The cytoskeleton in prokaryotes and eukaryotes differs fundamentally in complexity, with prokaryotic systems lacking the motor proteins and cross-linkers that characterize eukaryotic networks. Prokaryotic filaments, such as FtsZ (a tubulin homolog) and MreB (an actin homolog), operate without associated molecular motors like kinesin or myosin, limiting their ability to generate directed force or enable active motility.^[135] In contrast, eukaryotic cytoskeletons integrate motors and cross-linkers—such as fascin, alpha-actinin, and dynein—that facilitate filament bundling, sliding, and force transmission, creating a highly interactive framework essential for cellular processes.^[2] These absences in prokaryotes result in filaments that are primarily task-specific and peripheral, rather than forming the extensive, interconnected cytoplasmic networks seen in eukaryotes.^[136] In terms of size and organization, prokaryotic cytoskeletal elements are typically smaller and membrane-associated, relying on interactions with the cell envelope for positioning and stability, without dedicated organizing centers. For instance, FtsZ rings form at midcell during division via membrane tethering, but do not span the cytoplasm like eukaryotic microtubules.^[137] Eukaryotes, however, feature larger-scale organization through microtubule-organizing centers (MTOCs), such as centrosomes, and connections to the nuclear envelope, allowing for polarized structures that extend across the cell and coordinate with organelles.^[138] This organizational disparity reflects the smaller cell volume in prokaryotes (often 1-10 μm³) versus the larger, compartmentalized eukaryotic cells (up to 10,000 μm³), where cytoskeletal networks provide mechanical support over greater distances.^[135] Dynamics also vary markedly, with eukaryotic cytoskeletons capable of rapid remodeling driven by signaling cascades, such as Rho GTPases regulating actin polymerization, enabling responses to environmental cues within seconds to minutes.^[139] Prokaryotic dynamics, by comparison, are slower and predominantly polymerization-driven, as seen in MreB treadmilling for peptidoglycan synthesis, lacking the layered regulatory controls that allow eukaryotic filaments to assemble, disassemble, or reorganize swiftly.^[2] These differences underscore how eukaryotic systems support dynamic processes like cell migration, while prokaryotic ones prioritize stable, growth-linked functions.^[136] Protein diversity further highlights these contrasts, with eukaryotes employing over 100 distinct cytoskeletal and accessory proteins— including numerous actin- and tubulin-binding partners—to achieve functional versatility across cell types.^[140] Prokaryotes, however, utilize a limited repertoire of approximately 5-10 homologs per species, such as FtsZ, MreB, and ParM in bacteria, or crescentin in Caulobacter, with composition varying by organism but generally far less elaborate.^[141] This reduced diversity in prokaryotes constrains their cytoskeletal roles to basic maintenance, without the specialization seen in eukaryotic isoforms.^[135] Functionally, prokaryotic cytoskeletons exhibit gaps in capabilities like true intracellular transport, lacking the motor-driven vesicle trafficking that eukaryotes use for secretion and organelle positioning.^[2] While prokaryotes achieve some cargo movement via filament polymerization (e.g., plasmid segregation by ParM), it does not equate to the bidirectional, long-range transport in eukaryotes.^[139] Moreover, eukaryotic cytoskeletons underpin multicellularity through adhesion complexes and tissue mechanics, features absent in predominantly unicellular prokaryotes.^[137] These divergences, despite shared homologies like tubulin and actin ancestors, emphasize the evolutionary elaboration of eukaryotic systems for complex life.^[138]

Long-Range Organization

The long-range organization of the cytoskeleton emerges from collective interactions among filaments and associated proteins, leading to ordered patterns that span entire cells or tissues. In eukaryotic cells, microtubule (MT) asters exhibit liquid-crystal-like alignment, where MTs radiate from centrosomes in a nematic fashion, driven by motor-induced sliding and bundling that establishes global polarity during mitosis. Similarly, in migrating cells, actin flow fields create coherent retrograde flows from the leading edge, coordinating protrusion and retraction across the cell body to maintain directional persistence. These patterns arise beyond local filament dynamics, reflecting emergent properties of active cytoskeletal networks. Self-organization mechanisms underpin these large-scale structures, particularly through motor protein cross-linking. For instance, kinesin-5 motors bind antiparallel MT overlaps and generate sliding forces that separate poles, promoting bipolar spindle formation essential for chromosome segregation. This process exemplifies how ATP-dependent motility couples filament interactions to produce ordered architectures without external templates. In actin networks, myosin-II minifilaments similarly cross-link and contract bundles, fostering alignment in contractile arrays. At multi-scale levels, cytoskeletal organization manifests in highly periodic structures like sarcomeres in striated muscle, where actin and myosin filaments interdigitate in repeating units to enable force transmission over millimeters. Analogous sarcomere-like motifs appear in non-muscle stress fibers, where periodic myosin-II assemblies drive contractility in response to mechanical cues. Theoretical models of active matter describe these phenomena as non-equilibrium systems where filament polarity and motor activity generate spontaneous flows and instabilities, analogous to flocking or turbulence in physical systems. In prokaryotes, long-range organization is exemplified by oscillatory MreB helices, which form dynamic spirals along the cell length in rod-shaped bacteria like Escherichia coli, guiding peptidoglycan synthesis to maintain global cylindrical shape and chirality. These actin homologs rotate circumferentially, ensuring uniform cell wall deposition over the entire envelope. Recent advances in the 2020s, including large-scale simulations and super-resolution imaging, have illuminated how phase separation contributes to organization. Molecular dynamics simulations reveal how MT-associated proteins phase separate into condensates that nucleate and stabilize bundles, enhancing long-range alignment. Super-resolution techniques, such as STORM, have visualized liquid-like condensates of actin-binding proteins like tropomyosin, which sequester filaments to form ordered domains under stress, integrating phase behavior with active mechanics.

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Jul 10, 2023 · Here, we determine the cryoEM structure of a single protofilament of FtsZ from Klebsiella pneumoniae (KpFtsZ) in a polymerization-preferred conformation.Results · Comparison Of Ftsz In The... · Cryoem Image Processing And...
[54]
SepF is the FtsZ anchor in archaea, with features of an ancestral cell ...
Jun 4, 2021 · Most archaea divide by binary fission using an FtsZ-based system similar to that of bacteria, but they lack many of the divisome components ...Missing: variations | Show results with:variations
[55]
The antibacterial cell division inhibitor PC190723 is an FtsZ polymer ...
May 7, 2010 · PC190723 is an effective bactericidal cell division inhibitor that targets FtsZ in the pathogen Staphylococcus aureus and Bacillus subtilis and does not affect ...Missing: antibiotic | Show results with:antibiotic
[56]
The Antibacterial Cell Division Inhibitor PC190723 Is an FtsZ ... - NIH
PC190723 is an effective bactericidal cell division inhibitor that targets FtsZ in the pathogen Staphylococcus aureus and Bacillus subtilis and does not affect ...
[57]
Chiral twisting in a bacterial cytoskeletal polymer affects filament ...
Mar 16, 2020 · Our analyses indicate that MreB double protofilaments can exhibit left-handed twisting that is dependent on the bound nucleotide and membrane binding.
[58]
Membrane molecular crowding enhances MreB polymerization to ...
Jan 13, 2020 · Our study demonstrates that the self-assembly of MreB into a mechanically dynamic cytoskeleton is driven by molecular crowding in two dimensions ...
[59]
The cell-shape protein MreC interacts with extracytoplasmic ... - PNAS
... MreB cytoskeleton spatially coordinates the activity of the peptidoglycan synthesizing enzymes (PBPs). Because MreC resided in the periplasm where ...
[60]
The actin homologue MreB organizes the bacterial cell membrane
Mar 7, 2014 · Upon binding, MreB forms a complex with the conserved membrane proteins MreC and MreD, and with proteins involved in peptidoglycan synthesis ...Mreb Generates Membrane... · Absence Of Mreb Changes... · Mreb Stimulates Membrane...
[61]
Supramolecular structure in the membrane of Staphylococcus aureus
However, MreB is missing in S. aureus and other coccal cells, whereas MreC and MreD are present (9). S. aureus deprived of MreC grew identically to the parent, ...
[62]
https://www.pnas.org/doi/10.1073/pnas.1914656117
[63]
The bacterial actin MreB rotates, and rotation depends on cell-wall ...
MreB Structures Persistently Rotate Around the Long Axis of the Cell. To gain insight into both the dynamics and function of MreB, we carefully imaged the ...
[64]
Actin cytoskeleton and complex cell architecture in an Asgard ...
Dec 21, 2022 · Asgard archaea are considered to be the closest known relatives of eukaryotes. Their genomes contain hundreds of eukaryotic signature ...
[65]
Bactofilins, a ubiquitous class of cytoskeletal proteins mediating ...
Dec 3, 2009 · Here, we report the identification of a new class of polymer-forming proteins, termed bactofilins, that are widely conserved among bacteria.
[66]
The discovery of the prokaryotic cytoskeleton: 25th anniversary - NIH
Feb 1, 2017 · Later work has discovered multiple prokaryotic actins with a variety of cytoskeletal functions, although the functions of most are unknown.
[67]
Actin polymerization kinetics, cap structure, and fluctuations - PNAS
We show that growth rates and the critical concentration at the barbed end are intimately related to cap structure and dynamics.
[68]
Actin Polymerization and ATP Hydrolysis - Science
During actin polymerization, ATP bound to G-actin is hydrolyzed to ADP bound to F-actin, occurring in two steps: ATP cleavage and phosphate release.
[69]
Dynamic instability of individual microtubules analyzed by video ...
Abstract. We have developed video microscopy methods to visualize the assembly and disassembly of individual microtubules at 33-ms intervals.Missing: seminal paper
[70]
Microtubule dynamic instability: A new model with coupled GTP ...
Mar 27, 2013 · A key question in understanding microtubule dynamics is how GTP hydrolysis leads to catastrophe, the switch from slow growth to rapid shrinkage.
[71]
Role of Phosphorylation on the Structural Dynamics and Function of ...
Phosphorylation regulates intermediate filament assembly/disassembly, affects their structure, and is spatiotemporally regulated, impacting their disposition.
[72]
The Human Arp2/3 Complex Is Composed of Evolutionarily ...
A similar Arp2/3 complex was first discovered by profilin affinity chromatography of cell extracts from Acanthamoeba castellanii (Machesky et al., 1994). In ...
[73]
Actin Depolymerizing Factor (ADF/Cofilin) Enhances the Rate ... - NIH
Abstract. Actin-binding proteins of the actin depolymerizing factor (ADF)/cofilin family are thought to control actin-based motile processes.
[74]
FtsZ dynamics in bacterial division: what, how, and why? - PMC
Recent studies have documented treadmilling of FtsZ polymer clusters both in cells and in vitro. Emerging evidence suggests that FtsZ dynamics are regulated ...
[75]
Motion of variable-length MreB filaments at the bacterial cell ... - NIH
Our study reveals a novel mode of filament dynamics of MreB, namely the fusion of filaments. Figure 2 shows an example of two filaments meeting from ...Missing: paper | Show results with:paper
[76]
How Molecular Motors Move | Science
Myosin and kinesin motor proteins use the energy obtained from adenosine triphosphate (ATP) hydrolysis to transport organelles and vesicles by moving along the ...
[77]
Kinesin and Dynein Mechanics: Measurement Methods and ... - NIH
Kinesin and dynein have many attributes in common, both being motor proteins dependent on microtubules and ATP to catalyze a cyclic stepping motion to transport ...
[78]
Swinging lever mechanism of myosin directly shown by time ...
Apr 9, 2025 · Myosins produce force and movement in cells through interactions with F-actin1. Generation of movement is thought to arise through ...
[79]
Myosin V: regulation by calcium, calmodulin, and the tail domain - NIH
Calcium activates the ATPase activity of tissue-purified myosin V, but not that of shorter expressed constructs. Here, we resolve this discrepancy by comparing ...
[80]
Editorial: Microtubule-associated molecular motors - Frontiers
Most kinesins transport cargo towards the plus-end of microtubule (called as anterograde transport), whereas dyneins walk towards the minus-end of microtubule ( ...
[81]
Dynein and kinesin share an overlapping microtubule‐binding site
Dyneins and kinesins are microtubule‐based molecular motors that play important roles in various cellular processes, including axonal transport, chromosome ...
[82]
Rho GTPases and their role in organizing the actin cytoskeleton
Mar 1, 2011 · Rho GTPases bind to a variety of effectors, including protein kinases (Zhao and Manser, 2005) and some actin-binding proteins.Introduction · Actin polymerization I... · Actin depolymerization and...
[83]
Regulation of non-muscle myosin assembly by calmodulin ... - Nature
Sep 18, 1980 · Calcium activates a specific calmodulin-dependent kinase8–11 which phosphorylates the light chain, initiating actin–myosin interaction. Although ...
[84]
Competition between microtubule-associated proteins directs motor ...
Here the authors show how MAP competition controls microtubule access to determine the distribution and balance of motor activity.
[85]
Effect of the Rho-Kinase/ROCK Signaling Pathway on Cytoskeleton ...
ROCK regulates microtubules by phosphorylation of several microtubule-associated proteins such as MAP2/Tau, collapsin response mediator protein 2 (CRMP2) and ...
[86]
Overview of the Cytoskeleton from an Evolutionary Perspective - PMC
The cytoskeleton is made of protein polymers that help establish shapes, maintain mechanical integrity, divide, and move cells. Eukaryotes have actin, ...Missing: general | Show results with:general
[87]
Prokaryotic DNA segregation by an actin-like filament - PubMed
We show here that the ParM ATPase encoded by par forms dynamic actin-like filaments with properties expected for a force-generating protein.Missing: motor | Show results with:motor
[88]
The actin cortex at a glance | Journal of Cell Science
Jul 19, 2018 · Summary: A summary of the composition, architecture, mechanics and function of the cellular actin cortex, which determines the shape of animal
[89]
Balance of microtubule stiffness and cortical tension determines the ...
The shape of animal cells is determined by the cytoskeleton, including microtubules (MTs), contractile networks of actin filaments, intermediate filaments, and ...
[90]
Intermediate Filaments in Cellular Mechanoresponsiveness - Frontiers
In this Mini Review we discuss how cytoskeletal crosstalk confers mechanosensitivity to cells and tissues, with a particular focus on the role of intermediate ...
[91]
Talin as a mechanosensitive signaling hub | Journal of Cell Biology
Talin plays a central role in cell adhesion, first by converting integrins to high-affinity states (“activation”) and by coupling integrins to the cytoskeleton.
[92]
The mechanical response of talin | Nature Communications
Jul 7, 2016 · Our results reveal that force-dependent stochastic unfolding and refolding of talin rod domains make talin a very effective force buffer.
[93]
Mechanical behavior in living cells consistent with the tensegrity model
Alternative models of cell mechanics depict the living cell as a simple mechanical continuum, porous filament gel, tensed cortical membrane, or tensegrity ...
[94]
MreB: unraveling the molecular mechanisms of bacterial shape ...
Aug 22, 2025 · MreB generates circumferential forces along bacterial membranes to direct cell wall synthesis and morphogenesis, whereas actin's stably ...
[95]
A Mechanical Explanation for Cytoskeletal Rings and Helices in ...
Most of the polymer structures that are possible on rod-shaped membranes have been observed in live cells. FtsZ forms a ring (8), MreB forms helices (11), and ...
[96]
Advances in Micropipette Aspiration: Applications in Cell ...
With five decades of sustained application, micropipette aspiration has enabled a wide range of biomechanical studies in the field of cell mechanics.Missing: 2020s | Show results with:2020s
[97]
Uncertainty-Aware Traction Force Microscopy - bioRxiv
Jul 9, 2024 · This manuscript presents an uncertainty-aware TFM technique that estimates the variability in the magnitude and direction of the traction stress vector ...
[98]
A new approach for high-content traction force microscopy to ... - Sciety
Jul 8, 2025 · Traction Force Microscopy (TFM) is a technology that can quantify the contractile forces exerted by cells via measuring the displacement of ...
[99]
Mitochondria-adaptor TRAK1 promotes kinesin-1 driven transport in ...
Jun 19, 2020 · Intracellular trafficking of organelles, driven by kinesin-1 stepping along microtubules, underpins essential cellular processes.
[100]
Retrograde axonal transport and motor neuron disease - PMC
1). Actin filaments are used by myosin motor proteins for short-range, dispersive distribution of vesicles, and/or organelles to the cell periphery. Transport ...
[101]
Molecular Motors in Neurons: Transport Mechanisms and Roles in ...
Nov 18, 2010 · Myosin V is suggested to function in the short-distance transport of vesicles including in AMPAR trafficking. Myo5a is responsible for the ...
[102]
ER proteins decipher the tubulin code to regulate organelle ... - Nature
Dec 15, 2021 · Organelles move along differentially modified microtubules to establish and maintain their proper distributions and functions.
[103]
Actin and myosin function in directed vacuole movement during cell ...
The vacuole inheritance defect in this strain appears to result from the loss of a specific actin function; the actin cytoskeleton is intact and protein ...
[104]
Mitotic Functions of Kinesin-5 - PMC
Mechanistically, cortical dynein pulling on astral microtubules is thought to generate outward forces that drive centrosome separation (Sharp et al., 1999); ...
[105]
Dynein‐mediated pulling forces drive rapid mitotic spindle ...
We describe that early spindle elongation is supported by kinesin‐5 and that at the transition to rapid spindle elongation, dynein appears at plus‐ends of ...
[106]
Article Bacterial Mitosis: ParM of Plasmid R1 Moves Plasmid DNA by ...
The ParM protein interacts with the ParR/parC complex whereby its ATPase activity is stimulated (Jensen and Gerdes, 1997). Recently, we found that ParM forms ...
[107]
Mutations in cytoplasmic dynein and its regulators cause ... - PubMed
Abnormalities in intracellular transport are thought to be a critical factor in the degeneration and death of neurons in both the central and peripheral nervous ...
[108]
Plant-Specific Myosin XI, a Molecular Perspective - Frontiers
Velocity and Directionality The velocity of myosin XI directly reflects the velocity of cytoplasmic streaming, because this process is generated by sliding of ...
[109]
Cytoplasmic Streaming Velocity as a Plant Size Determinant
Nov 11, 2013 · Cytoplasmic streaming velocity varies among species and cells, ranging from 1 to 70 μm × s−1. It directly reflects the velocity of myosin XI ...
[110]
Autoregulation and dual stepping mode of MYA2, an Arabidopsis ...
Feb 24, 2022 · Cytoplasmic streaming is generated by organelle-associated plant-specific class XI myosin sliding along actin filaments10.
[111]
Roles of actin filaments in cytoplasmic streaming and organization of ...
The present results suggest that actin filaments work as a track of cytoplasmic streaming and as a cytoskeleton to maintain the transvacuolar strand. The ...
[112]
The molecular mechanism and physiological role of cytoplasmic ...
Jul 17, 2015 · Regulations of myosin XI Cytoplasmic streaming in plant cells stops when cytosolic calcium ion (Ca2+) concentration is elevated [27, 28].
[113]
https://rupress.org/jcb/article/169/4/561/51655/A-mutation-in-dynein-rescues-axonal-transport
[114]
Type IV pili and cell motility - Wiley Online Library
Mar 1, 2002 · Type IV pili (Tfp) mediate the movement of bacteria over surfaces without the use of flagella. These movements are known as social gliding ...Missing: analogs streaming
[115]
Analytical methods for cytoplasmic streaming in elongated cells
Intracellular flows are driven by a slip velocity U ( z , t ) tangential to the cell boundary (schematized as purple arrows) which has an axial component U ( z ...
[116]
Mechanics of Pollen Tube Elongation: A Perspective - Frontiers
Pollen tube (PT) serves as a vehicle that delivers male gametes (sperm cells) to a female gametophyte during double fertilization, which eventually leads to ...
[117]
Effect of cytoplasmic streaming on photosynthetic activity of ...
Dec 23, 2011 · Cytoplasmic streaming plays an important role in cell processes since it promotes solute exchange between the cytoplasm and organelles and ...Missing: disruptions | Show results with:disruptions
[118]
Dynamics of full-length Arabidopsis myosin XI and its involvement in ...
Aug 21, 2025 · Plant myosin XI plays a crucial role in intracellular transport, known as cytoplasmic streaming. Previous studies have identified associations ...
[119]
Evolution of cytomotive filaments: The cytoskeleton from prokaryotes ...
We have identified three classes of cytomotive filaments that originated in prokaryotes and acquired new functions in eukaryotes.
[120]
Bacterial intermediate filaments: in vivo assembly, organization, and ...
Crescentin, which is the founding member of a rapidly growing family of bacterial cytoskeletal proteins, was previously proposed to resemble eukaryotic ...
[121]
Evolution of the cytoskeleton - PMC - NIH
The eukaryotic cytoskeleton appears to have evolved from ancestral precursors related to prokaryotic FtsZ and MreB.
[122]
Primer Bacterial and archaeal cytoskeletons - ScienceDirect.com
May 24, 2021 · Tubulin and actin homologues in bacteria and archaea form nucleotide-dependent filaments, most of which display cytomotive properties, such as ...
[123]
Crenactin forms actin-like double helical filaments regulated ... - eLife
Nov 17, 2016 · Here we report that crenactin forms bona fide double helical filaments that show exceptional similarity to eukaryotic F-actin.
[124]
The last eukaryotic common ancestor (LECA): Acquisition of ... - PNAS
Aug 29, 2006 · Three possibilities for the origin of the cytoskeleton include (i) ancient origin from our genetically-not-yet-annealed ancestors (3), (ii) ...
[125]
The nature of the last universal common ancestor and its impact on ...
Jul 12, 2024 · Here we infer that LUCA lived ~4.2 Ga (4.09–4.33 Ga) through divergence time analysis of pre-LUCA gene duplicates, calibrated using microbial ...
[126]
Evolution of Cellular Organization Along the First Branches of the ...
Jul 17, 2024 · Current evidence suggests that some form of cellular organization arose well before the time of the last universal common ancestor (LUCA).
[127]
Origin and Evolution of the Self-Organizing Cytoskeleton in the ...
The eukaryotic cytoskeleton evolved from prokaryotic cytomotive filaments. But it has additional features (eg, motor proteins) not found in prokaryotes.
[128]
The Bacterial Cytoskeleton - PMC - PubMed Central
In eukaryotic cells, actin polymerizes into polarized structures, with addition of actin-ATP being favored at one end and ATP hydrolysis and subunit release ...
[129]
The prokaryotic cytoskeleton | Nature Reviews Molecular Cell Biology
Dec 1, 2008 · Actin and tubulin are the major components of the eukaryotic cytoskeleton. It was previously thought that prokaryotes lacked a cytoskeleton.
[130]
The evolution of the cytoskeleton - PMC - PubMed Central - NIH
Aug 22, 2011 · The cytoskeleton is a system of intracellular filaments crucial for cell shape, division, and function in all three domains of life.Missing: general | Show results with:general<|control11|><|separator|>
[131]
Evolution of the cytoskeleton - PubMed - NIH
The eukaryotic cytoskeleton appears to have evolved from ancestral precursors related to prokaryotic FtsZ and MreB. FtsZ and MreB show 40-50% sequence identity ...
[132]
https://www.pnas.org/doi/10.1073/pnas.0604985103
[133]
An expanded view of the eukaryotic cytoskeleton - PMC - NIH
Jun 1, 2013 · The three well-established components of the eukaryotic cytoskeleton are actin filaments, microtubules, and intermediate filaments (Pollard and ...Missing: differences | Show results with:differences
[134]
[PDF] Prokaryotic cytoskeletons: protein filaments organizing small cells
Jan 22, 2017 · FtsZ is localized near the membrane at future division sites in almost all bacteria and most archaeal phyla6,22 and forms a ring-like structure ...