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C-terminus

The C-terminus, also known as the carboxyl terminus or COOH-terminus, is the end of a polypeptide or protein chain where the carboxyl group (-COOH) of the terminal amino acid residue remains free and is not involved in a peptide bond.^[1] This terminal carboxyl group typically exists as a carboxylic acid or carboxylate under physiological conditions, distinguishing it from the N-terminus at the opposite end of the chain.^[1] In protein primary structure, the C-terminus marks the conclusion of the linear sequence of amino acids linked by peptide bonds.^[2] During protein biosynthesis on ribosomes, the C-terminus is the final portion synthesized, as translation proceeds from the N- to C-terminal direction, with release factors terminating elongation at this end.^[3] Structurally, the C-terminus is often intrinsically disordered and solvent-exposed, lacking stable secondary structure in its terminal 5–10 residues, which confers flexibility and accessibility for interactions.^[3] Approximately 87% of C-terminal residues are solvent-accessible, enabling them to adopt transient conformations such as α-helices when interacting with binding partners.^[3] The C-terminus plays pivotal roles in regulating protein function, including localization, stability, trafficking, and signaling through short linear motifs called minimotifs (typically 2–15 residues long).^[3] These motifs facilitate protein-protein interactions (e.g., binding to PDZ domains via sequences like x-[S/T]-x-[L/V]), post-translational modifications (e.g., phosphorylation, prenylation, or GPI anchoring), and subcellular targeting (e.g., the KDEL motif for endoplasmic reticulum retention).^[3]^[2] For instance, the C-terminal domain (CTD) of RNA polymerase II, consisting of up to 52 heptapeptide repeats (YSPTSPS), is essential for coordinating transcription initiation, RNA processing, and mRNA export.^[2] In other cases, C-terminal sequences regulate enzymatic activity and aggregation, as seen in α-synuclein where acidic residues in the C-terminus prevent pathological fibril formation.^[4] Mutations or cleavages at the C-terminus can lead to diseases, such as arrhythmias from connexin-43 truncation or disrupted signaling in procaspase-8 processing.^[3] Overall, the C-terminus's functional versatility underscores its importance in cellular processes, with databases like the C-terminome cataloging over 3,500 verified minimotifs across proteomes.^[3]

Chemistry

Basic Structure

The C-terminus, also known as the carboxy-terminus, is the end of a polypeptide chain that features a free carboxyl group (-COOH) from the last amino acid residue.^[5] Polypeptide chains consist of amino acids linked by peptide bonds, which form through a dehydration reaction between the carboxyl group of one amino acid and the amino group of the subsequent amino acid, resulting in the C-terminal carboxyl group remaining unlinked and exposed.^[5] At physiological pH (approximately 7.4), the C-terminal carboxyl group is predominantly deprotonated, existing as a negatively charged carboxylate ion (-COO⁻), which contributes to the overall charge of the protein and can influence its solubility and molecular interactions.^[6] In contrast, the N-terminus features a free amino group (-NH₂) that is protonated at physiological pH, carrying a positive charge (-NH₃⁺), thereby distinguishing the two termini in terms of electrostatic properties and potential roles in protein behavior.^[7]^[5] The terminology "C-terminus" or "carboxy-terminus" emerged in the mid-20th century amid advances in protein sequencing, particularly through Frederick Sanger's determination of the insulin amino acid sequence in the early 1950s, which identified terminal residues and established the directional convention for polypeptide chains from N- to C-terminus.

Biosynthesis

Protein biosynthesis occurs through ribosomal translation, a process in which messenger RNA (mRNA) is decoded to assemble a polypeptide chain on the ribosome. This synthesis proceeds directionally from the amino (N)-terminus to the carboxyl (C)-terminus, with each new amino acid added to the carboxyl end of the growing chain. The ribosome reads the mRNA sequence in the 5' to 3' direction, ensuring that the N-terminal end is synthesized first and the C-terminus last.^[8] Translation initiates at the start codon AUG, which encodes the amino acid methionine and signals the ribosome to begin assembly. As elongation continues, transfer RNAs (tRNAs) carrying specific amino acids are matched to successive mRNA codons in the ribosomal A site, forming peptide bonds that extend the chain toward the C-terminus. The process terminates when the ribosome encounters one of three stop codons—UAA, UAG, or UGA—in the A site; these codons do not code for amino acids but instead trigger the release of the completed polypeptide. Release factors bind to the stop codon, promoting the hydrolysis of the ester bond linking the C-terminal amino acid to its tRNA, thereby liberating the full protein with a free carboxyl group at the C-terminus. No additional amino acids are incorporated beyond this point, as the stop codon prevents further tRNA binding.^[8] In bacteria, translation termination is specifically mediated by two class I release factors: RF1, which recognizes UAA and UAG stop codons, and RF2, which recognizes UAA and UGA. These factors bind to the ribosomal A site, mimicking the structure of tRNA anticodons to interact with the stop codon via conserved motifs (PxT for RF1 and SPF for RF2). Their GGQ motif then positions a glutamine residue in the peptidyl transferase center, catalyzing the addition of a water molecule to cleave the peptidyl-tRNA bond without incorporating a new amino acid, thus finalizing the C-terminal end.^[9] Each cycle of elongation, including peptide bond formation, is energetically driven by GTP hydrolysis. The elongation factor EF-Tu forms a ternary complex with GTP and aminoacyl-tRNA, delivering it to the A site; upon codon-anticodon matching, EF-Tu hydrolyzes GTP to GDP, releasing the tRNA for accommodation and subsequent peptide bond formation with the prior amino acid. Following peptide bond formation, elongation factor EF-G binds with GTP and promotes translocation of the tRNAs and mRNA, hydrolyzing a second GTP molecule. Two GTP molecules are typically hydrolyzed per elongation cycle: one by EF-Tu during aminoacyl-tRNA delivery to the A site upon codon-anticodon matching, and one by EF-G during translocation following peptide bond formation, ensuring efficient and accurate chain extension up to the C-terminus.^[10]^[8]

Function

Localization Signals

C-terminal amino acid sequences in proteins serve as critical sorting signals that direct trafficking to specific cellular destinations, including plasma membranes, organelles, and the extracellular space, by interacting with dedicated recognition machinery such as receptors and chaperones. These signals ensure precise localization post-translationally, complementing N-terminal mechanisms that often drive co-translational translocation.^[11] The general mechanism involves short motifs, typically 3-10 amino acids long and positioned at the extreme C-terminus, which bind targeting factors through hydrophobic interactions or charged residues. For instance, in tail-anchored proteins, the C-terminal transmembrane domain (approximately 20 residues) exhibits moderate hydrophobicity, enabling chaperone-mediated delivery via factors like TRC40/Get3 to the endoplasmic reticulum or other membranes. Similarly, GPI-anchoring signals feature a conserved ω-site followed by a 5-12 residue spacer and an 11-15 residue hydrophobic tail, recognized during transit through the secretory pathway.^[11]^[12] Broad classes of these signals encompass membrane anchoring motifs (e.g., for GPI attachment or tail-anchoring to the ER and plasma membrane), organelle import sequences (such as those directing to peroxisomes or mitochondria), and motifs facilitating extracellular secretion, distinguishing them from N-terminal signals that primarily initiate ER entry. These classes rely on the C-terminal position to remain accessible after protein folding, allowing recognition by cytosolic or vesicular factors.^[11]^[12] C-terminal localization signals demonstrate evolutionary conservation across eukaryotic lineages, appearing in diverse protein families and enabling refined subcellular distribution, with increased motif diversity in complex organisms like mammals compared to yeast or plants. This conservation underscores their role in adapting protein targeting to varying cellular demands across species.

Retention Signals

Retention signals at the C-terminus of proteins play a crucial role in maintaining the localization of endoplasmic reticulum (ER) resident proteins by preventing their export through the secretory pathway. The canonical ER retention signal is the tetrapeptide sequence -KDEL (Lys-Asp-Glu-Leu), which is appended to the C-terminus of soluble ER luminal proteins and recognized by the KDEL receptor (KDELR) located in the cis-Golgi and intermediate compartments.^[13] This signal ensures that proteins that inadvertently escape the ER are retrieved back, thereby sustaining the protein-folding environment within the ER. Variant retention signals include -HDEL (His-Asp-Glu-Leu) in yeast, which functions analogously to -KDEL by binding to the yeast ortholog of the KDEL receptor, known as Erd2p, to mediate retrieval of ER proteins.^[14] In some mammalian species, -RDEL (Arg-Asp-Glu-Leu) serves a similar role, exhibiting comparable receptor affinity and contributing to ER retention through the same retrieval pathway.^[15] These sequence variations reflect adaptations across species while preserving the core mechanism of receptor-mediated recapture. The retention mechanism relies on the pH-dependent binding affinity of the KDEL receptor for its ligands, exploiting the pH gradient between the neutral ER (pH ~7.2) and the more acidic Golgi (pH ~6.5), which promotes ligand binding in the Golgi and release in the ER.^[16] Upon binding, the receptor-ligand complex is incorporated into COPI-coated vesicles for retrograde transport from the Golgi back to the ER, ensuring efficient recycling and preventing secretion of ER residents.^[17] Representative examples of proteins utilizing the -KDEL signal include BiP (binding immunoglobulin protein, also known as GRP78), a molecular chaperone essential for protein folding in the ER, and protein disulfide isomerase (PDI), which catalyzes disulfide bond formation and isomerization.^[13] Disruption of the -KDEL sequence in these proteins, such as through mutation or deletion, results in their secretion into the extracellular space, leading to defects in ER protein homeostasis and impaired cellular folding capacity.

Peroxisomal Targeting Signal

The peroxisomal targeting signal 1 (PTS1) serves as a key C-terminal motif that directs soluble proteins to the matrix of peroxisomes, single-membrane-bound organelles involved in lipid metabolism and reactive oxygen species detoxification. This signal is typically a tripeptide located at the extreme C-terminus, with the prototypical sequence being Ser-Lys-Leu (-SKL) and common variants including -Ala-Lys-Leu (-AKL) and -Ser-Arg-Leu (-SRL).^[18] The consensus sequence for PTS1 is generally defined as (S/A/C)-(K/R/H)-L, though broader variations extend to (S/A/H/C)-(K/R/H/Q)-(L/M/F), accommodating numerous weak signals that still confer peroxisomal localization. Over 50 such variants have been experimentally validated, primarily in plants but applicable across eukaryotes, with targeting efficiency modulated by the basic residues and the identity of flanking amino acids proximal to the tripeptide, which can enhance or inhibit receptor binding affinity.^[18]^[19]^[20] PTS1 recognition begins in the cytosol, where the signal binds to the tetratricopeptide repeat (TPR) domain of the soluble receptor peroxin 5 (PEX5), forming a stable complex with the cargo protein. This receptor-cargo assembly then docks at the peroxisomal membrane through PEX5's interaction with the integral membrane protein PEX14, enabling translocation across the membrane and release of folded proteins into the matrix—a unique feature of peroxisomal import that accommodates oligomeric and cofactor-bound structures.^[21]^[22] Representative proteins employing PTS1 include catalase, which degrades hydrogen peroxide in the peroxisome and possesses a C-terminal -SKL motif essential for its import, and urate oxidase, involved in purine catabolism with a similar tripeptide signal. Disruptions in PTS1 function, such as mutations altering the signal sequence or impairing PEX5 recognition, contribute to peroxisomal biogenesis disorders; for instance, defects in the PEX5 receptor underlie complementation group 7 of Zellweger spectrum disorders, leading to absent or dysfunctional peroxisomes and severe multi-organ dysfunction.^[23]^[24]^[25]

Degradation Signals

C-terminal degrons are short amino acid motifs located at the extreme C-terminus of proteins that serve as recognition signals for ubiquitin-mediated proteasomal degradation. These degrons typically consist of 2–10 residues and are recognized by specific E3 ubiquitin ligases, which facilitate the attachment of ubiquitin chains to the protein, marking it for breakdown by the 26S proteasome. Unlike N-terminal degrons, C-degrons often function independently of the protein's overall structure when exposed, allowing for precise control of protein half-life.^[26] The mechanism of C-degron-mediated degradation generally involves direct binding of the C-terminal motif to substrate-binding domains in cullin-RING E3 ligases, such as CRL2 or CRL4 complexes, which recruit E2 ubiquitin-conjugating enzymes to polyubiquitinate internal lysine residues on the target protein. For instance, the di-glycine (-GG) C-degron is specifically bound by the Kelch-like domain of KLHDC2 in CRL2, initiating ubiquitination without requiring prior modification of the degron itself. Non-canonical C-degrons, such as exposed hydrophobic patches at the C-terminus upon protein misfolding, can also recruit quality control E3 ligases like CHIP, bypassing traditional ubiquitin dependency in some cases to ensure rapid clearance of aberrant proteins. In variants of the N-end rule adapted for C-ends, terminal residues like glycine or arginine dictate ligase specificity, leading to proteasomal targeting.^[27]^[28] Representative examples illustrate the diversity of C-terminal degrons across organisms. In yeast (Saccharomyces cerevisiae), the C-terminal region of the monocarboxylate transporter Jen1 contains a degron motif responsive to glucose signaling, which recruits the α-arrestin Rod1 to facilitate ubiquitination by the Rsp5 ligase upon nutrient shifts, ensuring transporter turnover. In mammals, C-terminal PEST sequences—rich in proline, glutamic acid, serine, and threonine— in transcription factors like c-Fos promote rapid degradation; the C-terminal tripeptide PTL within this PEST region accelerates ubiquitination via ERK kinase-dependent phosphorylation, limiting the duration of immediate-early gene responses.^[29]^[30] Biologically, C-terminal degrons play critical roles in regulating protein turnover to maintain cellular homeostasis, particularly in dynamic processes like the cell cycle, where they control the timely degradation of cyclins and checkpoints, and the stress response, by destabilizing activated transcription factors once stimuli subside. Defects in these signals, such as mutations eliminating PEST degrons in c-Fos, lead to oncoprotein stabilization and are implicated in cancers, including sarcomas and leukemias, where unchecked transcriptional activity drives proliferation.^[26]^[30]

C-terminal Modifications

Prenylation

Prenylation is a post-translational lipid modification that attaches isoprenoid groups to the cysteine residue in the C-terminal CAAX motif of target proteins, where the motif consists of cysteine (C), followed by two typically aliphatic amino acids (a), and a variable residue (X) such as serine, methionine, alanine, or others that influence prenyl group specificity.^[31] This process enables the anchoring of otherwise soluble proteins to cellular membranes, particularly the inner leaflet of the plasma membrane.^[31] The enzymatic attachment is catalyzed by protein prenyltransferases: farnesyltransferase (FTase) adds a 15-carbon farnesyl group derived from farnesyl pyrophosphate, while geranylgeranyltransferase type I (GGTase-I) attaches a 20-carbon geranylgeranyl group from geranylgeranyl pyrophosphate.^[31] The specificity is largely determined by the X residue in the CAAX motif; for instance, motifs ending in serine, methionine, alanine, cysteine, or glutamine are preferentially farnesylated by FTase, whereas those ending in leucine or isoleucine favor geranylgeranylation by GGTase-I.^[31] Following prenylation, the -AAX tripeptide is cleaved by the endoprotease RCE1, exposing the prenylated cysteine at the new C-terminus.^[31] This is followed by carboxyl methylation of the cysteine thiol by isoprenylcysteine carboxyl methyltransferase (ICMT), which further enhances membrane affinity through increased hydrophobicity.^[31] The primary functional outcome of prenylation is to facilitate the reversible association of proteins with lipid bilayers, allowing for proper localization and activation in signal transduction pathways.^[32] Notable examples include the Ras family of small GTPases (such as H-Ras and K-Ras), which require farnesylation for oncogenic signaling, and Rho GTPases (like RhoA and RhoC), which are typically geranylgeranylated to regulate cytoskeletal dynamics and cell motility.^[31] In cancer therapy, prenylation inhibition has been targeted using farnesyl pyrophosphate analogs like tipifarnib, a selective FTase inhibitor that blocks Ras membrane localization and downstream pathways such as RAF-MEK-ERK.^[32] This approach showed preclinical promise in Ras-driven malignancies, including pancreatic cancer where KRAS mutations occur in over 90% of cases, but clinical trials revealed limited efficacy due to compensatory geranylgeranylation of Ras isoforms.^[32]

GPI Anchors

Glycosylphosphatidylinositol (GPI) anchors are complex glycolipid structures that covalently attach to the C-terminus of certain proteins, tethering them to the outer leaflet of the plasma membrane. The attachment occurs post-translationally in the endoplasmic reticulum (ER), where a preformed GPI glycan is transferred to the ω-site residue—typically a serine, glycine, or occasionally alanine—at the extreme C-terminus, following proteolytic cleavage of a C-terminal signal peptide. This linkage is mediated by a phosphoethanolamine bridge, forming an amide bond between the carboxyl group of the C-terminal residue and the amino group of the ethanolamine phosphate on the GPI glycan.^[33]^[34] The biosynthesis of GPI anchors begins in the ER, where the GPI precursor is assembled on the cytoplasmic face of the membrane before flipping to the luminal side for completion. The C-terminal signal peptide of the precursor protein, which directs it to the ER via the signal recognition particle pathway, features a hydrophilic N-terminal region, a spacer of small uncharged residues, and a C-terminal hydrophobic tail of 15–30 residues that facilitates membrane insertion. The GPI transamidase complex, comprising subunits such as PIG-K, GPAA1, PIG-T, PIG-U, PIG-W, and PIG-S, recognizes this signal peptide and catalyzes the cleavage at the ω-site while simultaneously attaching the GPI anchor, replacing the signal peptide with the glycolipid.^[33]^[34] Functionally, GPI anchors enable the cell-surface expression of proteins that lack transmembrane domains, anchoring them via the lipid moiety embedded in the outer leaflet of the plasma membrane and allowing lateral mobility within lipid rafts for signaling and adhesion roles. These anchors can be dynamically released from the membrane by phospholipases, such as phosphatidylinositol-specific phospholipase C (PI-PLC) or phospholipase D (PLD), which cleave the phosphodiester bond, generating soluble forms of the protein for extracellular functions.^[33]^[34] Representative examples of GPI-anchored proteins include the prion protein (PrP), which is involved in neuroprotection and implicated in prion diseases, and alkaline phosphatase (ALP), an enzyme critical for hydrolysis of phosphate esters in various tissues. Defects in GPI anchor biosynthesis, particularly somatic mutations in the PIGA gene encoding the first enzyme in the pathway, lead to paroxysmal nocturnal hemoglobinuria (PNH), a hemolytic anemia characterized by the absence of GPI-anchored proteins on blood cells, resulting in complement-mediated lysis.^[33]^[35]

Methylation

C-terminal methylation involves the enzymatic addition of a methyl group to the alpha-carboxyl group of specific C-terminal amino acid residues, such as leucine or cysteine.^[36] This post-translational modification is catalyzed by S-adenosylmethionine (SAM)-dependent methyltransferases, including leucine carboxyl methyltransferase 1 (LCMT1), which specifically targets the C-terminal leucine residue (Leu309) of the protein phosphatase 2A catalytic subunit (PP2A-C), promoting holoenzyme assembly and regulatory subunit binding.^[37] Another key enzyme, isoprenylcysteine carboxyl methyltransferase (ICMT), methylates the C-terminal cysteine in prenylated proteins following farnesylation or geranylgeranylation.^[38] The process is reversible, with demethylation mediated by methylesterases such as PME-1, which hydrolyzes the methyl ester bond to restore the negatively charged carboxyl group.^[39] Methylation neutralizes the negative charge at the C-terminus, altering electrostatic properties that influence protein stability, subcellular localization, and molecular interactions; for instance, in PP2A, it facilitates the association with scaffolding and regulatory subunits without affecting catalytic activity.^[40] This charge modulation can enhance hydrophobic interactions, as seen in ICMT-mediated methylation, which completes the maturation of CAAX motifs in small GTPases like Ras and RhoA.^[41] In isoprenylated proteins, post-prenylation C-terminal methylation by ICMT on the terminal cysteine increases membrane affinity by promoting tighter association with lipid bilayers, a step that follows farnesyltransferase or geranylgeranyltransferase activity (as detailed in the Prenylation subsection).^[42] Biologically, C-terminal methylation modulates protein half-life by stabilizing against degradation; for example, inhibiting ICMT reduces the half-life of RhoA from 31 hours to 12 hours in macrophages, leading to decreased signaling efficiency.^[41] It is also implicated in aging processes, such as in Hutchinson-Gilford progeria syndrome, where disrupted ICMT activity on the farnesylated C-terminus of mutant lamin A (progerin) contributes to nuclear envelope abnormalities and premature cellular senescence.^[43] Additionally, carboxymethylation influences the function of DNA repair enzymes like those involved in base excision repair, where age-related declines in methylation efficiency exacerbate protein damage accumulation.^[44]

C-terminal Domains

Definition and Structure

The C-terminal domain represents a discrete structural module located at the carboxyl end of a polypeptide chain, typically comprising 50-200 amino acid residues.^[45] These domains can fold independently from the protein's core domain or be intrinsically disordered, often serving as a functional appendage that interacts with other molecular components without disrupting the overall protein architecture. Structurally, C-terminal domains commonly incorporate alpha-helices, beta-sheets, or intrinsically disordered regions, with alpha-helices showing a slight enrichment compared to N-terminal regions.^[45] These motifs contribute to the domain's stability where structured, which can be further enhanced by intramolecular disulfide bonds or coordination with metal ions in certain contexts, promoting compact folding or conformational adaptability. Disordered segments, particularly in the terminal 5-10 residues, are prevalent, lacking defined electron density in crystallographic data and enabling dynamic interactions. In terms of length and variability, C-terminal domains differ markedly from N-terminal signal peptides, which are shorter (typically 16-30 residues) and subject to proteolytic cleavage during protein maturation. Instead, C-terminal domains persist as integral, non-cleavable units, with examples like PDZ-binding motifs spanning just 3-4 residues to mediate specific modular associations. This persistence underscores their role as enduring structural features rather than transient targeting elements. Biophysically, C-terminal domains are frequently solvent-exposed, with approximately 87% of residues accessible to the aqueous environment, and characterized by high net charge, low sequence complexity, and inherent flexibility.^[45] These properties are illuminated through techniques such as nuclear magnetic resonance (NMR) spectroscopy, which detects inter-residue nuclear Overhauser effects (NOEs) indicative of transient structures, and X-ray crystallography, which highlights flexible linkers connecting the domain to the main chain. Such linkers, often unstructured, allow rotational freedom and facilitate the domain's exposure for regulatory purposes.^[45]

Functional Examples

The C-terminal domain (CTD) of RNA polymerase II exemplifies a structured regulatory element composed of multiple heptapeptide repeats with the consensus sequence YSPTSPS, numbering up to 52 in humans.^[46] Phosphorylation of specific residues within these heptads, particularly serines at positions 2 and 5, dynamically modulates the polymerase's progression through transcription initiation, productive elongation, and termination, while also recruiting factors for mRNA capping, splicing, and polyadenylation.^[47] This heptad-based architecture enables the CTD to serve as a scaffold for transient interactions with enzymatic complexes, ensuring coordinated gene expression.^[48] In ion channels, C-terminal domains often influence gating properties; for instance, the C-terminal region of transient receptor potential (TRP) channels, such as TRPC3, contains loops that alter allosteric coupling between cytoplasmic and transmembrane domains, thereby modulating ion permeation and sensitivity to stimuli like temperature or ligands.^[49] Similarly, in cytoskeletal proteins, the C-terminal domain of dystrophin forms helical spectrin-like repeats that bind syntrophins, stabilizing the dystrophin-glycoprotein complex at the muscle cell membrane and facilitating signal transduction from the extracellular matrix.^[50] These interactions underscore the role of C-terminal domains in maintaining structural integrity and mechanotransduction in muscle fibers.^[51] C-terminal domains frequently mediate regulatory mechanisms through allosteric effects, ligand binding, or scaffolding assemblies. In Src family kinases, the C-terminal tail harbors a tyrosine residue (Tyr530 in humans) whose phosphorylation promotes intramolecular binding to the SH2 domain, enforcing an autoinhibited conformation that prevents aberrant kinase activation.^[52] This autoregulatory switch exemplifies how C-terminal modifications can fine-tune enzymatic activity in response to cellular signals.^[53] Mutations in C-terminal domains contribute to various disorders, highlighting their physiological importance. For example, heterozygous variants in POLR2A, encoding the RNA polymerase II subunit bearing the CTD, underlie a neurodevelopmental syndrome characterized by infantile hypotonia, developmental delay, and cerebellar abnormalities.^[54] In muscular dystrophies, deletions or missense mutations in the dystrophin C-terminal disrupt syntrophin binding, leading to membrane instability and progressive muscle degeneration.^[51] Activating mutations in the Src C-terminal tyrosine, observed in subsets of colon cancers, abolish autoinhibition and drive uncontrolled proliferation.^[55] Therapeutic strategies targeting C-terminal domains hold promise for disease intervention. Inhibitors of cyclin-dependent kinase 9 (CDK9), which phosphorylates the RNA polymerase II CTD, impair transcription of oncogenes and induce apoptosis in cancer cells, with compounds like flavopiridol advancing in clinical trials for hematologic malignancies.^[56] Such approaches leverage the domain's centrality in regulatory networks to selectively disrupt pathological processes.^[57]

References

[1]
Illustrated Glossary of Organic Chemistry - C-terminus
C-terminus: The end of a peptide or protein primary structure in which the amino acid residue's carbonyl group is not part of a peptide bond.Missing: biochemistry | Show results with:biochemistry
[2]
Introduction to N-terminus and C-terminus - Creative Proteomics
The C-terminus, also known as the carboxyl-terminus, COOH-terminus, or C-terminal end, concludes the polypeptide chain. It is distinguished by a free carboxyl ...
[3]
The carboxy-terminus, a key regulator of protein function - PMC - NIH
The C-termini of proteins contain minimotifs, short peptides with an encoded function generally characterized as binding, posttranslational modifications, and ...
[4]
C-Terminus - an overview | ScienceDirect Topics
C terminal refers to the unique protein-sequence signatures at the end of a protein that interact with other proteins, playing a crucial role in various ...<|control11|><|separator|>
[5]
The Shape and Structure of Proteins - Molecular Biology of the Cell
The structural components of a protein. A protein consists of a polypeptide backbone with attached side chains. Each type of protein differs in its sequence and ...
[6]
Recent Advances in the Synthesis of C-Terminally Modified Peptides
Most C-terminal modifications result in neutralization of the negative charge that the carboxylic acid derivative displays at physiological pH. Without this ...
[7]
N-terminal modifications of cellular proteins: The enzymes involved ...
The N-terminal α-amino group is (usually) positively charged at neutral pH. The chemical character of protein N-termini can be modified by, for example ...
[8]
From RNA to Protein - Molecular Biology of the Cell - NCBI Bookshelf
Consequently, a protein is synthesized stepwise from its N-terminal end to its C-terminal end. Throughout the entire process the growing carboxyl end of the ...Missing: directionality | Show results with:directionality
[9]
Structural aspects of translation termination on the ribosome - PMC
In eubacteria, translation termination is mediated by release factor RF1 in response to a UAG or UAA stop codon, and by RF2 in response to a UGA or UAA codon.
[10]
GTP consumption of elongation factor Tu during translation ... - PNAS
Generally one GTP is hydrolyzed by EF-Tu per aminoacyl-tRNA bound and peptide bond formed, and more than one GTP is hydrolyzed only when a particular mRNA ...
[11]
Targeting pathways of C-tail-anchored proteins - ScienceDirect.com
The C-terminal membrane anchor of these tail-anchored (TA) proteins generally represents the sole targeting determinant, and becomes available to targeting ...
[12]
C-Terminal Signals Regulate Targeting of ... - PubMed Central - NIH
These signals consist of a conserved ω domain (the ω or GPI attachment site and the ω + 1 and ω + 2 sites), a spacer domain consisting of 5 to 12 amino acids, ...
[13]
A C-terminal signal prevents secretion of luminal ER proteins
Mar 13, 1987 · We propose that the KDEL sequence marks proteins that are to be retained in the ER and discuss possible retention mechanisms. ... Munro and Pelham ...
[14]
ERD2, a yeast gene required for the receptor-mediated retrieval of ...
Jun 29, 1990 · Resident proteins of the ER lumen carry a specific tetrapeptide signal (KDEL or HDEL) that prevents their secretion.
[15]
The KDEL receptor: New functions for an old protein - FEBS Press
Oct 23, 2009 · By 1990, Lewis and Pelham had identified the first human HDEL receptor homologue, referred to as hERD2 or the KDEL receptor, since the KDEL ...The Kdel Receptor: New... · 6 Regulation Of Kdel... · 7 The Kdel Receptor...
[16]
Structural basis for pH-dependent retrieval of ER proteins ... - Science
Mar 8, 2019 · Here, we present crystal structures of the chicken KDEL receptor in the apo ER state, KDEL-bound Golgi state, and in complex with an antagonistic synthetic ...Missing: primary | Show results with:primary
[17]
Molecular basis for KDEL-mediated retrieval of escaped ER-resident ...
Oct 9, 2020 · Binding to KDEL-tagged cargo triggers the formation of a stable receptor–cargo complex and incorporation into COPI vesicles (2). Following ...
[18]
Identification and analysis of the plant peroxisomal targeting ... - PNAS
PTS2 is located at the N terminus of peroxisomal proteins and is defined by the loose consensus sequence -R-[L/I/Q]-X5-H-L- (reviewed in ref. 4).
[19]
Comprehensive identification of plant peroxisome targeting signal ...
Jul 8, 2024 · For example, the PTS1 sequence may be shielded from the receptor PEX5 by other parts of the protein, or a strong N-terminal targeting signal is ...
[20]
Peroxisomal targeting signal-1 recognition by the TPR ... - PubMed
PEX5, the receptor for PTS1, interacts with the signal via a series of tetratricopeptide repeats (TPRs) within its C-terminal half. Here we report the crystal ...
[21]
Structural basis for competitive interactions of Pex14 with the import ...
The Pex14(N)–Pex5 complex reveals the structural basis for an interaction that is critical for docking of the Pex5 import receptor to the peroxisomal membrane.
[22]
Identification of peroxisomal targeting signal of pumpkin catalase ...
Catalase, a peroxisomal protein, has PTS1-like motifs around or at the C-terminus. However, it remains unclear whether catalase is imported into peroxisome via ...<|separator|>
[23]
Functional Expression and Peroxisomal Targeting of Rat Urate ... - NIH
This study was undertaken to determine whether recombinantly expressed rat urate oxidase in a primate cell system can be targeted to the peroxisome.Missing: examples | Show results with:examples
[24]
Mutations in the PTS1 receptor gene, PXR1, define ... - Nature
Feb 1, 1995 · PXR1, like PAS8, encodes a receptor for proteins with the type–1 peroxisomal targeting signal (PTS1). Mutations in PXR1 define complementation ...
[25]
N-degron and C-degron pathways of protein degradation - PNAS
Jan 7, 2019 · The intracellular protein degradation is mediated largely by the ubiquitin (Ub)-proteasome system (UPS) and by autophagy-lysosome pathways ...
[26]
the N-degron and C-degron pathways of protein degradation
Jul 6, 2020 · The ubiquitin-proteasome system (UPS) represents the major route through which the cell achieves selective protein degradation [1]. The ...
[27]
Orphan quality control by an SCF ubiquitin ligase directed ... - Nature
Dec 15, 2023 · Selective protein degradation typically involves substrate recognition via short linear motifs known as degrons. Various degrons can be ...
[28]
The C-terminal region of the yeast monocarboxylate transporter ...
The C-terminal region of the yeast monocarboxylate transporter Jen1 acts as a glucose signal–responding degron recognized by the α-arrestin Rod1Glucose- ...The Jen1 C-Terminal Region... · Experimental Procedures · Strains, Plasmid...Missing: example | Show results with:example
[29]
Identification of a C-terminal tripeptide motif involved in the control of ...
Further dissection of c-Fos C-terminal region showed that the degradation-accelerating effect is not contributed by the whole PEST sequence but by a short PTL ...
[30]
CAAX-box protein, prenylation process and carcinogenesis - NIH
The prenylation process of the CAAX proteins includes 3 steps: polyisoprenylation, proteolysis, and carboxyl methylation.
[31]
Targeting protein prenylation for cancer therapy - PubMed Central
Farnesyltransferase inhibitors (FTIs) were originally developed to block the function of RAS. However, as numerous studies in vitro and in vivo have shown, ...
[32]
Biosynthesis of GPI-anchored proteins: special emphasis on ... - NIH
GPIs act as membrane anchors of many cell surface proteins after they are covalently attached to the C termini of the proteins as a posttranslational ...
[33]
Recent Research Progress in Glycosylphosphatidylinositol ... - NIH
Jan 4, 2024 · This article reviews the recent advancements in the study of GPI-AP biosynthesis, GPI-AP synthesis, and GPI interaction with the cell membrane utilizing ...
[34]
Deficiency of the GPI anchor caused by a somatic ... - PubMed
Deficiency of the GPI anchor caused by a somatic mutation of the PIG-A gene in paroxysmal nocturnal hemoglobinuria. Cell. 1993 May 21;73(4):703-11. doi ...
[35]
Modified residue | UniProt help
Oct 16, 2025 · Carboxyl methylation can occur either on a C-terminal cysteine, leucine or lysine residue, or on the side chain of a glutamate residue (or ...
[36]
Leucine Carboxyl Methyltransferase 1 (LCMT-1) Methylates Protein ...
LCMT-1 coordinately regulates the carboxyl methylation of PP2A-related phosphatases and, consequently, their holoenzyme assembly and function.
[37]
o60725 · icmt_human - UniProt
Aug 1, 1998 · Catalyzes the post-translational methylation of isoprenylated C-terminal cysteine residues.
[38]
Regulation of PP2A, PP4, and PP6 holoenzyme assembly ... - Nature
Nov 29, 2021 · The role of C-terminal methylation is best understood for PP2A. Methylation removes the negative charge of the carboxyl-terminus, thereby ...
[39]
Loss of LCMT1 and biased protein phosphatase 2A ... - Nature
Aug 29, 2023 · Methyl-PP2A-C loss is associated with prostate cancer progression. LCMT1 catalyzed PP2A-C α-carboxymethylation at L309 is required for the ...
[40]
Fighting cancer by disrupting C-terminal methylation of signaling ...
In mammalian cells, inhibition of methylation in a macrophage cell line results in the decrease of the half-life of the RhoA small G-protein from 31 to 12 hours ...
[41]
Fighting cancer by disrupting C-terminal methylation of signaling ...
Protein methylation at the C-terminus of mammalian isoprenylated proteins has been implicated in membrane attachment, protein-protein interactions, and ...
[42]
Carboxyl Methylation of Deamidated Calmodulin Increases Its ...
Significantly, ions representing the intact N-terminal and C- terminal peptides of CaM can be identified in all samples, confirming that proteolytic ...
[43]
Molecular insights into the premature aging disease progeria
Feb 4, 2016 · The processing of B-type lamins stops at this step, resulting in mature lamin B with a C-terminal farnesyl- and carboxymethyl group. The ...
[44]
Mapping protein carboxymethylation sites provides insights into their ...
Nov 18, 2021 · We identify over 1000 sites of protein carboxymethylation in mouse and primary human cells treated with the glycating agent glyoxal.
[45]
https://doi.org/10.1080/10409238.2019.1586828
[46]
Function and Control of RNA Polymerase II C-Terminal Domain ...
The C-terminal domain of the RNA polymerase II largest subunit (the Rpb1 CTD) is composed of tandem heptad repeats of the consensus sequence Y1S2P3T4S5P6S7.
[47]
The RNA polymerase II CTD coordinates transcription and RNA ...
The CTD functions to help couple transcription and processing of the nascent RNA and also plays roles in transcription elongation and termination.
[48]
The RNA Polymerase II C-terminal Domain - PubMed Central - NIH
INTRODUCTION. The RNA polymerase II (Pol II) C-terminal domain (CTD) is a repetitive disordered domain that extends from the catalytic core of the enzyme.
[49]
Structure–function analyses of the ion channel TRPC3 reveal that its ...
Our findings indicate that the C-terminal loop affects channel gating by altering the allosteric coupling between the cytoplasmic and transmembrane domains.The C-Terminal Loop... · Human Trpc3 Expression And... · Trpc3 In Gdn Sample
[50]
Syntrophin binds directly to multiple spectrin-like repeats in ... - NIH
Following the initial identification of syntrophin as a dystrophin-binding protein, three independent labs identified a region in the C-terminal domain of ...
[51]
The Dystrophin Complex: structure, function and implications for ...
The C-terminus (CT, shown in grey) contains binding sites for syntrophins (SBD) and dystrobrevin (DbBD). (B) Domain structure of the internally-truncated ...
[52]
Structural basis for the recognition of c-Src by its inactivator Csk - PMC
The catalytic activity of the Src family of tyrosine kinases is suppressed by phosphorylation on a tyrosine residue located near the C-terminus (Tyr 527 in ...
[53]
Autophosphorylation activates c-Src kinase through global structural ...
Phosphorylation at a specific tyrosine by C-terminal Src kinase inactivates c-Src, whereas autophosphorylation is essential for the c-Src activation process.
[54]
De Novo Heterozygous POLR2A Variants Cause a ...
Aug 1, 2019 · Experiments in Arabidopsis mutants that encode truncated RPB1 with different sizes of shortened C-terminal domain (CTD) show disturbed cell ...
[55]
Activating SRC mutation in a subset of advanced human colon ...
These results provide, for the first time, genetic evidence that activating SRC mutations may have a role in the malignant progression of human colon cancer.
[56]
Therapeutic Targeting of the General RNA Polymerase II ...
May 9, 2020 · Inhibitors targeting the general RNA polymerase II (RNAPII) transcription machinery are candidate therapeutics in cancer and other complex diseases.
[57]
Cyclin Dependent Kinase 9 Inhibitors for Cancer Therapy
Inhibition of CDK9 mediated phosphorylation of the CTD RNAPII results in reduced levels of antiapoptotic proteins such as Mcl-1 and XIAP, which reinstates the ...