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Protein filament

Protein filaments are linear, polymeric assemblies of protein monomers that form the primary structural elements of the cytoskeleton in eukaryotic cells, typically ranging from 6 to 25 nm in diameter and providing mechanical integrity, shape maintenance, and dynamic functions such as motility and transport.^[1] These filaments self-assemble through non-covalent interactions between subunits, allowing for rapid polymerization and depolymerization in response to cellular needs.^[2] The three major classes of protein filaments differ in composition, diameter, and function. Microfilaments, also known as actin filaments, are the thinnest at approximately 7 nm and consist of globular actin (G-actin) monomers that polymerize into double-helical filaments (F-actin), playing key roles in cell crawling, cytokinesis, and muscle contraction.^[3] Intermediate filaments, with diameters around 10 nm, are rope-like structures formed by diverse proteins such as keratins in epithelial cells, vimentin in mesenchymal cells, and neurofilaments in neurons; they provide tensile strength and resilience against mechanical stress.^[4] Microtubules, the largest at 25 nm, are hollow tubes composed of α- and β-tubulin dimers arranged in protofilaments, essential for mitosis, intracellular trafficking via motor proteins like kinesin and dynein, and maintenance of cell polarity.^[1] Beyond structural support, protein filaments interact dynamically with motor proteins and regulatory factors to enable cellular processes like division, adhesion, and signaling.^[5] Dysfunctions in these filaments are implicated in diseases, including cancers from microtubule defects and neurodegenerative disorders from intermediate filament mutations.^[6]^[7]

Overview

Definition and Characteristics

Protein filaments are linear polymers formed by the self-assembly of protein monomers into elongated, rod-like structures that provide essential mechanical support and organization to cellular architecture. These supramolecular assemblies typically range in diameter from 5 to 25 nanometers, with actin filaments measuring approximately 6-7 nm, intermediate filaments around 10 nm, and microtubules about 25 nm. Unlike soluble proteins that function primarily as individual monomers or small oligomers in metabolic processes, protein filaments achieve their structural integrity through noncovalent interactions that enable the formation of extended, stable polymers capable of spanning significant distances within the cell.^[2]^[1]^[2] A hallmark of many protein filaments is their architectural organization, often featuring helical or tubular arrangements of subunits; for instance, actin filaments consist of two twisted protofilaments forming a double helix, while microtubules are hollow tubes composed of 13 parallel protofilaments. Structural polarity is another key characteristic, particularly in actin filaments and microtubules, which possess distinct plus and minus ends that dictate directional growth and motor protein interactions—though intermediate filaments generally lack this polarity due to their rope-like, apolar assembly. Filament lengths vary widely, typically extending from tens to hundreds of micrometers in cellular contexts, and can reach centimeters in specialized structures such as neuronal axons.^[2]^[1]^[2] Protein filaments are evolutionarily conserved across prokaryotes and eukaryotes, with homologs such as bacterial MreB (actin-like) and FtsZ (tubulin-like) performing analogous roles in cell shape maintenance and division, reflecting an ancient origin dating back over 3 billion years to the last universal common ancestor. This conservation underscores their fundamental importance in cellular organization, from bacterial cytoskeletal networks to the eukaryotic cytoskeleton.^[8]^[8]

Biological Importance

Protein filaments are indispensable components of the cytoskeleton, providing critical mechanical support that maintains cell shape and structural integrity while enabling cells to withstand and respond to mechanical stresses. In eukaryotic cells, these filaments form a dynamic network that anchors organelles, facilitates intracellular transport, and protects against external forces such as shear stress or compression, ensuring cellular resilience in diverse physiological contexts.^[1]^[9]^[10] Beyond single cells, protein filaments play pivotal roles in multicellular organization, contributing to tissue formation, cohesion, and repair processes like wound healing. For instance, intermediate filaments integrate with cell-cell junctions to distribute mechanical loads across tissues, promoting stable architectures in epithelia and connective tissues, while actin filaments drive collective cell migration essential for embryonic development and regeneration.^[11]^[12] From an evolutionary standpoint, protein filaments represent ancient structures that predate eukaryotic cells, with homologs identified in bacteria and archaea that underpin basic cellular division and shape maintenance, ultimately enabling the complexity of multicellular life forms. The conservation of these filament systems across domains of life highlights their foundational role in the transition from prokaryotic simplicity to eukaryotic sophistication and metazoan organization.^[13]^[14]^[8] Mutations in genes encoding protein filament components are linked to a broad spectrum of human diseases, underscoring their physiological indispensability; for example, over 70 intermediate filament-associated disorders have been identified, ranging from dermatological conditions to cardiomyopathies and neurodegenerative diseases.^[15]^[16]^[17]

Molecular Structure

Building Block Proteins

Protein filaments are primarily composed of monomeric protein subunits that polymerize to form the filament structures. These building block proteins include actin for microfilaments, tubulin for microtubules, and a diverse family of intermediate filament proteins (IFPs) for intermediate filaments, each with specific structural features that enable assembly and function. Actin, specifically the globular form known as G-actin, serves as the monomer for microfilaments and consists of 375 amino acids, forming a compact globular structure with a nucleotide-binding cleft that accommodates ATP. This ATP-binding site is crucial for actin's conformational dynamics during polymerization. G-actin's structure features four subdomains arranged around the nucleotide site, with the protein's molecular weight approximately 42 kDa, allowing it to adopt different conformations upon ATP hydrolysis. Tubulin, the building block for microtubules, exists primarily as α-β tubulin heterodimers, where each subunit binds GTP at distinct sites: the exchangeable site on β-tubulin and the non-exchangeable site on α-tubulin. GTP hydrolysis at the β-tubulin site drives conformational changes essential for microtubule dynamics. In humans, there are nine α-tubulin isotypes and nine β-tubulin isotypes, encoded by genes on different chromosomes, which provide tissue-specific variations in stability and function.^[18] Each tubulin monomer comprises about 450 amino acids, with a core structure of two domains connected by an intermediate helix, forming a heterodimer of roughly 100 kDa. Intermediate filament proteins (IFPs) form a large, diverse family classified into six types (I-VI) based on sequence and expression patterns, with monomers featuring a central α-helical rod domain flanked by non-helical head and tail domains. Type I and II IFPs include acidic and basic keratins, respectively, which assemble as obligate heterodimers (e.g., keratin 18 with keratin 8 in epithelial cells), while Type III IFPs like vimentin form homodimers and are expressed in mesenchymal cells. These proteins vary in size from 40 to 220 kDa, with the rod domain typically 310-320 residues long, enabling coiled-coil dimerization as the initial assembly step. Post-translational modifications significantly influence the stability and dynamics of these building block proteins. Phosphorylation occurs on actin and IFPs, altering polymerization rates—for instance, on the head domains of IFPs like vimentin to regulate filament disassembly. Acetylation and detyrosination are prominent on tubulin: acetylation of α-tubulin at lysine 40 stabilizes microtubules in stable structures like cilia, while detyrosination of α-tubulin's C-terminal tyrosine exposes epitopes for motor proteins, enhancing microtubule longevity in neurons. These modifications, often mediated by specific enzymes like HDAC6 for deacetylation, fine-tune filament properties without altering primary sequences.

Architectural Features

Protein filaments exhibit a supramolecular organization characterized by the helical assembly of protein subunits into elongated structures, where individual protofilaments—linear chains of monomers—twist together to form either cylindrical tubes or rope-like bundles, providing mechanical rigidity and directional properties essential for cellular function.^[2] This helical model is a common architectural feature across cytoskeletal filaments, with the twisting pattern determined by the geometry of subunit interactions, resulting in periodic crossovers that define the filament's pitch and overall symmetry.^[2] A key aspect of this architecture is the inherent polarity of many protein filaments, arising from the oriented addition of asymmetric monomers, which creates distinct ends that influence directional growth and interactions with motor proteins. For instance, in actin filaments formed from globular actin monomers, the plus or barbed end is structurally distinct from the minus or pointed end, with the helical twist featuring a crossover distance of approximately 36-38 nm along the filament axis. Similarly, microtubules display polarity with a plus end (exposed β-tubulin) and a minus end (exposed α-tubulin), their cylindrical structure composed of typically 13 parallel protofilaments arranged longitudinally around a hollow core, approximately 25 nm in diameter. In contrast, intermediate filaments lack overall polarity due to the antiparallel arrangement of their dimeric building blocks, forming a rope-like cross-section of eight intertwined protofilaments.^[2] The stability of these filament architectures relies on a combination of non-covalent interactions between subunits, including ionic bonds that provide electrostatic reinforcement and hydrophobic interactions that drive close packing within protofilaments and between adjacent strands. Longitudinal contacts along protofilaments are particularly strong, mediated by hydrophobic cores and salt bridges, while lateral interactions between protofilaments contribute to the overall cylindrical or bundled form but are generally weaker, allowing flexibility without compromising structural integrity. These interaction networks ensure the filaments maintain their dimensions—typically 7-10 nm in diameter for actin and intermediate filaments, and 25 nm for microtubules—under physiological conditions.^[2]

Assembly and Dynamics

Polymerization Mechanisms

Protein filaments assemble through a series of biophysical processes involving the ordered addition of monomeric protein subunits, a phenomenon first theoretically described for helical polymers like actin by Oosawa and Kasai in 1962.^[19] This polymerization is characterized by distinct phases, beginning with nucleation followed by elongation, and is influenced by the intrinsic polarity of the filament structure, where subunits add preferentially to one end over the other.^[20] The nucleation phase represents the rate-limiting step in filament formation, where soluble monomers initially collide to form unstable dimers or trimers that serve as seeds for further assembly. For actin, this process is highly unfavorable due to the energetic cost of aligning globular monomers into a protofilament, resulting in a critical monomer concentration below which polymerization does not occur—approximately 0.1 μM for ATP-bound actin at the barbed (plus) end.^[21] Similar nucleation barriers exist in microtubule assembly, where tubulin dimers form an oligomeric nucleus before rapid elongation.^[22] Intermediate filaments assemble differently through a hierarchical process. Monomers first form parallel coiled-coil dimers, which laterally associate into antiparallel tetramers. These tetramers then assemble longitudinally into unit-length filaments (ULFs) of about 60 nm, followed by end-to-end annealing of ULFs and radial compaction to produce mature 10 nm filaments. Unlike actin and microtubules, this assembly does not involve a distinct nucleation phase and proceeds obligatorily without a critical concentration.^[23] Once nucleated, filaments undergo elongation, with monomers adding more readily to the plus end than the minus end, leading to treadmilling under steady-state conditions where the filament maintains constant length despite net subunit flux. In actin filaments, this involves faster association at the plus end (rate constant ~10 μM⁻¹ s⁻¹) and slower dissociation at the minus end, resulting in subunits effectively "treadmilling" from plus to minus.^[24] Microtubules exhibit analogous dynamics, with GTP-tubulin adding preferentially to the plus end.^[25] Polymerization is energetically coupled to nucleotide hydrolysis, which introduces nonequilibrium dynamics by altering subunit affinity post-incorporation. In actin, ATP hydrolysis to ADP occurs shortly after monomer addition, rendering ADP-actin less stable and promoting dissociation primarily from the minus end, thus driving treadmilling.^[26] For microtubules, GTP hydrolysis destabilizes the lattice, facilitating catastrophe events, though the initial addition requires GTP-bound tubulin.^[27] Theoretically, Oosawa's model describes the equilibrium length distribution of polymers as a function of monomer concentration, governed by association and dissociation rates. The steady-state monomer concentration at the critical point satisfies k_{\text{on}} [M] = k_{\text{off}}, where k_{\text{on}} is the association rate constant and [M] is the free monomer concentration, predicting an exponential distribution of filament lengths above the critical concentration.^[19] This framework has been validated experimentally for both actin and tubulin systems.

Regulation and Turnover

The regulation of protein filament assembly and disassembly is primarily mediated by accessory proteins that modulate polymerization, branching, and motor-driven transport, ensuring dynamic responses to cellular needs. For actin filaments, the ARP2/3 complex nucleates branched networks by binding to existing filaments and promoting the addition of actin monomers, a process essential for lamellipodia formation during cell migration.^[28] In microtubules, motor proteins such as kinesins and dyneins facilitate transport along filaments and influence stability by crosslinking or depolymerizing tubulin subunits, contributing to general principles of force generation and spatial organization across actin filaments and microtubules. Intermediate filament dynamics are regulated mainly by post-translational modifications like phosphorylation, which promote disassembly into subunits during mitosis or stress responses, with slower turnover compared to actin and microtubules.^[29]^[30] Signaling pathways, particularly involving Rho GTPases, tightly control filament dynamics through effectors like formins, which elongate unbranched actin filaments. RhoA, a key Rho GTPase, activates formins such as mDia1 to drive linear actin assembly in stress fibers, linking extracellular cues to cytoskeletal remodeling via GTPase cycling between active and inactive states.^[31] These pathways integrate upstream signals from integrins and growth factors to balance assembly and disassembly, preventing uncontrolled polymerization. Filament turnover is characterized by rapid cycling, with actin filaments exhibiting a half-life of approximately 30 seconds in motile structures due to severing and depolymerization at pointed ends.^[32] Microtubules display dynamic instability, alternating between growth and shrinkage, with typical catastrophe frequencies around 10^{-3} s^{-1} in vitro, governed by GTP hydrolysis rates that destabilize the polymer lattice.^[33] Intermediate filaments exhibit slower turnover, primarily through subunit exchange at filament ends and severing/reannealing processes, occurring on timescales of minutes to hours in cells. This turnover, occurring on timescales of minutes in cells for actin and microtubules, allows filaments to adapt to spatial demands, such as during mitosis. Recent advances have illuminated these processes through optogenetic tools that enable light-inducible control of Rho GTPases, allowing precise spatiotemporal manipulation of filament contractility and assembly in living cells.^[34] Cryo-electron microscopy has revealed transient states, such as nucleotide-dependent conformational changes in actin filaments during aging and tubulin switching in microtubules, providing structural insights into instability mechanisms post-2020.^[35]^[36]

Cellular Functions

Structural Roles

Protein filaments play crucial structural roles in providing mechanical stability and resilience to cells, acting as a framework that maintains cellular integrity under various physical stresses. In the tensegrity model of the cytoskeleton, these filaments function as tensile elements (such as actin microfilaments and intermediate filaments) that balance compressive forces from microtubules, creating a prestressed architecture analogous to tensegrity structures in engineering. This model posits that the continuous tension in filaments distributes mechanical loads across the cell, enabling shape maintenance and resistance to deformation without relying solely on rigid supports.^[37] A key aspect of their structural function is the ability to absorb and dissipate shear stress, particularly through intermediate filaments, which can endure strains exceeding 100% without fracturing or losing elasticity. For instance, vimentin intermediate filament networks demonstrate remarkable extensibility, allowing cells to withstand large deformations such as those encountered during tissue stretching or compression. This resilience arises from the filaments' coiled-coil structure, which permits reversible unfolding under stress, thereby preventing catastrophic failure and contributing to overall cellular toughness.^[38] Protein filaments also anchor cells to the extracellular matrix (ECM) via integrins, transmembrane receptors that link cytoskeletal elements to ECM components like fibronectin and collagen. These connections transmit mechanical forces bidirectionally, stabilizing cell adhesion and distributing external loads to the internal filament network for enhanced structural support. In terms of material properties, cytoskeletal filament networks exhibit a Young's modulus of approximately 0.1-10 kPa, orders of magnitude lower than metals (which typically range from 10^4 MPa upward), allowing flexible yet robust responses to physiological forces.^[39]

Dynamic Processes

Protein filaments play crucial roles in dynamic cellular processes, enabling remodeling and movement essential for cell division, transport, and signaling. In cytokinesis, the final stage of cell division, actin filaments assemble with myosin II motors to form a contractile ring at the cell equator, which constricts to separate daughter cells. This ring's formation is regulated by RhoA GTPase, which activates formins to polymerize actin and recruits myosin II, generating the contractile force needed for furrow ingression.^[40] The process is highly conserved across eukaryotic cells, ensuring precise cytokinesis without disrupting other cellular structures. Intracellular trafficking relies on microtubule-based filaments as tracks for motor proteins, facilitating the directed movement of vesicles, organelles, and other cargoes within the cell. Kinesin motors typically move cargoes toward the plus ends of microtubules at velocities of 0.5-1 μm/s, while dynein drives minus-end-directed transport at similar speeds, enabling bidirectional flow in processes like axonal transport.^[41] These motors hydrolyze ATP to generate stepwise motion along the filament, with run lengths often spanning several micrometers, which is critical for efficient delivery over long distances in neurons or during mitosis.^[42] Filament dynamics, including polymerization and depolymerization, further adapt tracks to changing cellular needs, such as reorganizing during cell migration. Filament tension also contributes to mechanosensing in signal transduction pathways, where physical forces on the cytoskeleton trigger biochemical responses. In the YAP/TAZ pathway, increased actin filament stress from substrate stiffness or cell spreading promotes nuclear translocation of YAP/TAZ transcription factors, activating genes involved in proliferation and differentiation.^[43] This mechanotransduction integrates cytoskeletal tension with Hippo pathway kinases, where low tension leads to YAP/TAZ phosphorylation and cytoplasmic retention, while high tension enables dephosphorylation and nuclear entry.^[44] Such sensing allows cells to respond to extracellular matrix properties, influencing behaviors like tissue homeostasis. Recent advances in artificial intelligence have enabled modeling of filament networks to predict cellular outcomes, including migration speeds. Machine learning approaches, such as deep neural networks trained on fluorescence microscopy images, reconstruct cytoskeletal architectures and forecast migration dynamics by integrating actin and microtubule interactions.^[45] For instance, diffusion-based models predict actin stress fiber geometry from cell morphology, correlating network organization with migration velocities up to 1-2 μm/min in motile cells.^[46] These AI-driven simulations, developed between 2023 and 2025, enhance understanding of how filament remodeling drives collective cell movement in development and disease.^[47]

Types

Microfilaments

Microfilaments, composed of filamentous actin (F-actin) polymers formed from globular actin (G-actin) monomers, are the thinnest components of the cytoskeleton with a diameter of 7-9 nm.^[48] Each G-actin monomer, approximately 42 kDa in size, assembles into a polar double-helical structure where subunits rotate by about 166° relative to the previous one, resulting in a helical twist that repeats every 13 subunits over roughly 36-38 nm.^[48]^[49] This architecture provides flexibility and polarity, with distinct barbed (plus) and pointed (minus) ends that dictate directional growth and shrinkage.^[48] The dynamics of microfilaments are characterized by rapid turnover, with an average half-life of about 30-60 seconds in motile structures such as lamellipodia, enabling quick remodeling in response to cellular signals.^[50] This high turnover is facilitated by treadmilling, where actin subunits add preferentially to the barbed end and dissociate from the pointed end, a process that aligns with general polymerization mechanisms but is amplified by accessory proteins.^[50] Branching of microfilaments occurs primarily through the Arp2/3 complex, which nucleates new filaments at a 70° angle from existing ones, creating dendritic networks essential for protrusion and motility.^[51] In cellular functions, microfilaments drive lamellipodia formation at the leading edge of migrating cells by generating branched actin networks that push the plasma membrane forward via Arp2/3-mediated nucleation.^[51] In striated muscle, they interact with myosin II motors to enable contraction through the cross-bridge cycle: ATP binding to myosin releases it from actin, hydrolysis cocks the myosin head into a high-energy state, calcium-triggered binding forms a cross-bridge, the power stroke slides actin filaments past myosin (releasing ADP and inorganic phosphate), and ATP rebinding detaches the cycle for repetition.^[52] This sliding filament mechanism generates force and shortening in sarcomeres.^[52] Pathological disruptions highlight microfilament importance; mutations in the ACTA1 gene, encoding skeletal muscle α-actin, cause nemaline myopathy, a congenital disorder featuring muscle weakness, hypotonia, and rod-like aggregates in myofibers due to impaired filament assembly.^[53] Pharmacologically, latrunculin disrupts microfilament polymerization by sequestering G-actin monomers in a 1:1 complex, preventing filament elongation and leading to network disassembly.^[54]

Microtubules

Microtubules are rigid, hollow cylindrical polymers assembled from α- and β-tubulin heterodimers, forming the largest components of the cytoskeleton with an outer diameter of approximately 25 nm and an inner lumen of about 15 nm.^[55] These structures consist of 13 linearly arranged protofilaments, which are longitudinal chains of tubulin dimers that associate laterally to create the tubular architecture.^[56] Microtubules exhibit inherent polarity, with a fast-growing plus end and a slower-growing or anchored minus end, enabling directional assembly and disassembly; the plus end typically elongates at rates ranging from 0.2 to 5 μm/min under physiological conditions.^[57] Assembly involves the binding and subsequent hydrolysis of GTP by β-tubulin, which drives the polymerization process but is not strictly required for initial dimer incorporation.^[58] In cellular organization, microtubules radiate outward from microtubule-organizing centers (MTOCs), such as centrosomes in animal cells, forming astral arrays that extend toward the cell periphery to maintain shape and facilitate intracellular positioning.^[59] During mitosis, these dynamic polymers employ a "search-and-capture" mechanism, where growing plus ends explore the cytoplasm and stabilize upon binding kinetochores on chromosomes, thereby contributing to bipolar spindle formation.^[60] This organization ensures efficient chromosome alignment and segregation, with astral microtubules anchoring the spindle to the cortex for proper orientation. Key functions of microtubules include their role in the mitotic spindle, where dynamic instability—characterized by stochastic switches between phases of plus-end growth and rapid shrinkage (catastrophe)—allows rapid adaptation to cellular needs and error correction during division.^[61] In neurons, microtubules serve as polarized tracks for axonal transport, powering the movement of vesicles, organelles, and signaling molecules via motor proteins like kinesin and dynein over long distances.^[62] Dysregulation of microtubule dynamics underlies pathologies such as Alzheimer's disease, where hyperphosphorylated tau protein aggregates, detaches from microtubules, and leads to destabilization and neuronal dysfunction.^[63] Conversely, anticancer agents like paclitaxel exploit microtubule stabilization by binding to the polymer lattice, suppressing dynamic instability and inducing mitotic arrest in rapidly dividing cancer cells.^[64]

Intermediate Filaments

Intermediate filaments (IFs) form a diverse class of cytoskeletal polymers that assemble into rope-like structures approximately 10 nm in diameter, intermediate between the 6 nm microfilaments and 25 nm microtubules.^[65] These filaments are non-polar, arising from the staggered, antiparallel arrangement of their basic building blocks—dimer pairs that form soluble tetramers, which further associate laterally and longitudinally into unit-length filaments (ULFs) and then compact into mature fibers of 8–10 nm width.^[66] This apolar architecture contrasts with the polarity of actin and tubulin polymers, enabling IFs to provide stable, tensile strength rather than directional transport or rapid remodeling. IFs are encoded by over 70 genes in humans and exhibit tissue-specific expression, contributing to mechanical resilience across cell types.^[67] IFs are classified into six types based on amino acid sequence homology, gene structure, and expression patterns. Type I comprises acidic keratins (e.g., K9–K20), and Type II includes neutral-basic keratins (e.g., K1–K8), which form obligate heterodimers in epithelial tissues. Type III proteins, such as vimentin, desmin, glial fibrillary acidic protein (GFAP), and peripherin, are expressed in mesenchymal cells, muscle, glia, and peripheral neurons, respectively. Type IV encompasses neuronal proteins like the neurofilament triplet (NF-L, NF-M, NF-H) and α-internexin, supporting axonal integrity. Type V consists of nuclear lamins (A, B1, B2, C), which line the inner nuclear envelope. Type VI is represented by nestin, prominent in neural progenitor cells during development.^[67] In epithelial cells, Type I/II keratins assemble into networks that anchor to desmosomes and hemidesmosomes, forming a protective scaffold that maintains tissue barrier integrity against mechanical stress and environmental insults.^[68] These filaments distribute forces across the epithelium, preventing cell rupture and ensuring wound healing and tissue regeneration. In contrast, Type V lamins form a meshwork beneath the nuclear envelope, providing structural support to the nucleus and facilitating mechanotransduction by linking cytoskeletal forces to nuclear responses, such as gene expression changes under tension.^[69] Lamin networks buffer nuclear deformation, integrating extracellular signals to regulate cellular stiffness and motility.^[70] Mutations in IF genes underlie various diseases, highlighting their structural roles. Dominant mutations in keratin genes KRT5 or KRT14 disrupt filament assembly in basal keratinocytes, causing epidermolysis bullosa simplex (EBS), a blistering skin disorder triggered by minor trauma due to fragile epidermal barriers.^[71] Similarly, a recurrent point mutation in LMNA (c.1824C>T, p.G608G) activates a cryptic splice site, producing progerin—a truncated lamin A that accumulates in the nuclear envelope, leading to Hutchinson-Gilford progeria syndrome (HGPS), characterized by premature aging, cardiovascular defects, and nuclear blebbing; this mutation was identified in 2003 as the primary cause of HGPS. Recent advances include CRISPR-based therapies targeting the LMNA mutation to reduce progerin expression and restore nuclear architecture, such as a 2019 study demonstrating lifespan extension in progeria mouse models via single-dose editing, showing promise for future clinical applications.^[72] As of 2025, related preclinical efforts, including RNA-based progerin targeting, continue to explore therapeutic potential.^[73]

Beyond Cytoskeleton

Extracellular Filaments

Extracellular protein filaments are essential components of the extracellular matrix (ECM) in multicellular organisms, providing structural support, mechanical strength, and signaling cues to tissues. Unlike intracellular cytoskeletal filaments, these structures are assembled outside the cell, often through secretion of precursor proteins followed by polymerization and stabilization in the extracellular space. Collagen represents the predominant extracellular filament, comprising up to 30% of total protein in animals and forming hierarchical fibrils that impart tensile resilience to connective tissues such as skin, bone, and tendons.^[74] The basic unit of collagen fibrils is tropocollagen, a rod-like molecule approximately 300 nm in length and 1.5 nm in diameter, composed of three polypeptide α-chains that fold into a right-handed triple helix stabilized by hydrogen bonds and hydroxyproline residues. There are 28 known collagen types (I–XXVIII), classified by their supramolecular assembly; fibril-forming types such as I, II, and III are the most abundant and assemble into staggered arrays that produce a characteristic 67 nm D-periodic banding pattern visible under electron microscopy, arising from the quarter-staggered overlap of tropocollagen molecules. These fibrils exhibit remarkable tensile strength, estimated at around 100 MPa in tendon collagen, comparable to that of steel on a per-weight basis, due to their covalent cross-links and hierarchical organization.^[75]^[74]^[76] Beyond collagen, other extracellular filaments include fibrin, which forms during blood clotting as a transient scaffold for wound healing. Fibrinogen, a soluble plasma protein, is cleaved by thrombin to generate fibrin monomers that spontaneously polymerize into protofibrils and branch into a three-dimensional network, further stabilized by cross-linking via factor XIII, a calcium-dependent transglutaminase that forms γ-glutamyl-ε-lysine bonds between α- and γ-chains. This cross-linking enhances clot mechanical stability and resistance to fibrinolysis.^[77] Amyloid fibrils, another class, are pathological extracellular aggregates associated with diseases like Alzheimer's and systemic amyloidosis; they feature a cross-β sheet architecture where β-strands align perpendicular to the filament axis, forming rigid, insoluble fibers up to several micrometers long that disrupt tissue function.^[78] Assembly of extracellular filaments typically begins with cellular secretion: procollagen is synthesized in the endoplasmic reticulum, processed by proprotein convertases in the Golgi, and secreted into the ECM where N- and C-terminal propeptides are cleaved to allow fibril nucleation. Fibrillogenesis proceeds via self-assembly driven by electrostatic interactions and pH changes, with fibrils maturing through enzymatic cross-linking mediated by lysyl oxidase (LOX), a copper-dependent enzyme that oxidizes lysine and hydroxylysine residues to form aldimine cross-links, enhancing fibril diameter and tissue stiffness. In 2025, bioengineered collagen scaffolds have advanced tissue engineering, with recombinant type I collagen hydrogels modified for tunable stiffness and bioactivity, enabling applications in dermal regeneration and cartilage repair by mimicking native ECM architecture and promoting cell adhesion via integrin binding.^[79]^[80]^[81]

Filaments in Pathogens and Applications

Protein filaments play critical roles in the structure and function of pathogens, enabling motility and protection of genetic material. In bacteria, flagella are composed of polymerized flagellin proteins forming long, helical filaments that propel the cell through liquid environments, facilitating infection and colonization of hosts by pathogens such as Salmonella enterica.^[82] These flagellar filaments, approximately 20 nm in diameter and up to several micrometers long, rotate via a motor at the base to achieve speeds of up to 100 body lengths per second, enhancing pathogen virulence by promoting dissemination to invasion sites.^[83] Similarly, certain viruses assemble helical protein filaments into capsids that encapsulate their nucleic acid. For instance, the tobacco mosaic virus (TMV) forms rigid, rod-shaped virions consisting of 2,130 coat protein subunits arranged in a helix around a single-stranded RNA filament, measuring 300 nm in length and 18 nm in diameter, which protects the genome during transmission between plant hosts.^[84] This helical architecture exemplifies how protein filaments in pathogens ensure structural integrity and efficient replication. Beyond pathogenesis, protein filaments inspire biomedical applications, particularly in targeted therapies. Synthetic nanofibers designed to mimic the filamentous structure of actin have been developed using self-assembling peptide amphiphiles, forming high-aspect-ratio structures that encapsulate and release drugs in a controlled manner for localized delivery.^[85] These biomimetic filaments, with diameters around 10 nm and lengths exceeding 1 μm, enhance cellular uptake and bioavailability compared to spherical nanoparticles, enabling sustained release over days in tissue engineering contexts. In cancer therapy, nanotube structures inspired by microtubules—assembled from tubulin or its analogs—serve as carriers for microtubule-targeting agents (MTAs) like paclitaxel, disrupting tumor cell division while minimizing off-target effects.^[86] Recent preclinical studies, including those evaluating tubulin-derived nanotubes in 2023-2024 models, demonstrate improved tumor penetration and efficacy in solid tumors, with ongoing efforts toward clinical translation through analog optimization.^[87] Filament-based biomarkers also hold promise for diagnostics, particularly in neurodegenerative diseases. Neurofilament light chain (NfL), a component of neuronal intermediate filaments, is released into biofluids upon axonal damage and serves as a sensitive indicator of neurodegeneration. In amyotrophic lateral sclerosis (ALS), plasma NfL levels are elevated approximately fourfold compared to healthy controls, correlating with disease progression and severity.^[88] This elevation, often exceeding twofold in early stages, enables non-invasive monitoring via blood tests, outperforming other markers for tracking therapeutic responses in clinical trials.^[89] Emerging advances leverage genetic engineering to enhance filament utility in gene therapy. Post-2023 developments include CRISPR/Cas9 editing of viral capsid proteins in filamentous vectors, such as modified filamentous bacteriophages, to improve tropism and payload capacity for delivering CRISPR components to target cells. These edited filaments reduce immunogenicity and enhance specificity, as demonstrated in preclinical models for monogenic disorders, paving the way for safer in vivo applications.^[90]

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During the polymerization process, adenosine 5′-triphosphate (ATP) that is bound to G-actin is hydrolyzed to adenosine 5′-diphosphate (ADP) that is bound to F- ...
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Microtubules and Microtubule-Associated Proteins - PMC
Dynamic instability allows microtubules to explore intracellular space and remodel in response to intracellular and extracellular cues.
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Formin proteins were recognized as effectors of Rho GTPases some 15 years ago. They contribute to different cellular actin cytoskeleton structures.
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Mechanical behavior in living cells consistent with the tensegrity model
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Keratin filaments are essential for the maintenance of cellular integrity in the face of stress—A role that is defective in epithelial fragility disorders.
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Recent Advances in Microtubule Targeting Agents for Cancer Therapy
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