Complexin (synaphin) is a cytosolic protein that acts as a dual regulator of synaptic vesicle exocytosis at the squid giant synapse: it both clamps SNARE complexes to prevent spontaneous fusion and facilitates rapid Ca2+-triggered synchronous release. It binds preferentially to syntaxin within the assembled SNARE core complex (containing syntaxin-1, synaptobrevin, and SNAP-25), and promotes SNARE complex oligomerization into higher-order structures that form a scaffold for efficient, regulated vesicle fusion. The crystal structure of squid complexin bound to the SNARE complex (2.95 A resolution; PMID:12004067) reveals an alpha-helical segment (residues 25-98) that binds in antiparallel fashion to the SNARE four-helix bundle, contacting syntaxin and synaptobrevin around the ionic zero layer. The N-terminal tip (~residues 1-26) is critical for activating fast Ca2+-triggered fusion but dispensable for clamping. The central and accessory helices mediate the inhibitory clamp function, stabilizing a partially zippered SNARE state. The C-terminal tail is amphipathic, enabling membrane association via curvature sensing (non-CAAX variant, unlike retinal complexin-3/4 isoforms). Complexin cooperates with synaptotagmin-1 to synergistically clamp SNARE assembly, and Ca2+-bound synaptotagmin releases the clamp to trigger synchronous neurotransmitter release. In mammals, complexin knockout causes 3-4 fold increase in spontaneous miniature release and abolishes fast synchronous release. Human CPLX1 loss-of-function mutations cause infantile epileptic encephalopathies (SNAREopathies).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000149 SNARE binding | IEA GO_REF:0000118 | ACCEPT | Summary: TreeGrafter-predicted SNARE binding is strongly supported by direct experimental evidence from the squid giant synapse. Tokumaru et al. (2001) demonstrated that complexin/synaphin preferentially binds to syntaxin within the SNARE complex [PMID:11239399]. Bracher et al. (2002) resolved the crystal structure of squid complexin bound to the SNARE core complex at 2.95 angstrom resolution, showing detailed molecular contacts [PMID:12004067]. SNARE binding is the core molecular function of complexin. The term is appropriate, though syntaxin binding (GO:0019905) would also be valid as a more specific child term given the preferential syntaxin interaction. Reason: SNARE binding is the defining molecular function of complexin, demonstrated directly in this species by both biochemical and structural studies. The term is at the right level of specificity since complexin binds the assembled SNARE complex as a whole, not just syntaxin alone. Supporting Evidence: PMID:11239399 Synaphin/complexin is a cytosolic protein that preferentially binds to syntaxin within the SNARE complex. PMID:12004067 A helical segment of complexin binds in anti-parallel fashion to the four-helix bundle of the core SNARE complex and interacts at its C terminus with syntaxin and synaptobrevin around the ionic zero layer of the SNARE complex. |
| GO:0005829 cytosol | IEA GO_REF:0000044 | ACCEPT | Summary: Cytosol localization is supported by UniProt subcellular location annotation and by direct experimental evidence. Tokumaru et al. (2001) identified complexin as a cytosolic protein in squid optic lobe [PMID:11239399]. The UniProt record confirms cytoplasm/cytosol localization with experimental evidence (ECO:0000269|PubMed:11239399). Reason: Cytosol localization is experimentally confirmed in this species. Complexin is primarily a soluble cytosolic protein that associates with the SNARE complex at the membrane. Supporting Evidence: PMID:11239399 Synaphin/complexin is a cytosolic protein that preferentially binds to syntaxin within the SNARE complex. |
| GO:0006836 neurotransmitter transport | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO mapping from the Synaphin domain (IPR008849) to neurotransmitter transport. This is an overly broad and somewhat imprecise annotation. Complexin does not function as a neurotransmitter transporter; rather, it regulates the exocytotic release of neurotransmitter-containing synaptic vesicles. The more accurate biological process terms are synaptic vesicle exocytosis (GO:0016079) and positive regulation of synaptic vesicle exocytosis (GO:2000302), both of which are already annotated or proposed. Reason: Neurotransmitter transport implies a direct role in moving neurotransmitter molecules, which is misleading. Complexin regulates exocytosis of neurotransmitter-containing synaptic vesicles at a late prefusion step. The correct process is regulation of neurotransmitter secretion or synaptic vesicle exocytosis, not transport per se. Proposed replacements: positive regulation of synaptic vesicle exocytosis Supporting Evidence: PMID:11239399 Injection of this peptide into squid giant presynaptic terminals inhibited neurotransmitter release at a late prefusion step of synaptic vesicle exocytosis. |
| GO:0016020 membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Membrane association is supported by UniProt annotation indicating lipid-anchor (farnesylation) at Cys149. The UniProt record lists membrane localization via lipid-anchor. While complexin is primarily cytosolic, the C-terminal farnesyl modification enables membrane association. This is a very broad CC term but is not incorrect. Reason: Complexin is farnesylated at Cys149 (S-farnesyl cysteine), providing a lipid anchor for membrane association. The membrane annotation is consistent with this post-translational modification. Although a more specific membrane compartment (e.g. presynaptic membrane) would be preferable, the broad membrane term is acceptable given only the farnesylation evidence without specific membrane subcompartment localization data. |
| GO:0016079 synaptic vesicle exocytosis | IEA GO_REF:0000118 | MODIFY | Summary: TreeGrafter-predicted involvement in synaptic vesicle exocytosis is strongly supported by experimental evidence from the squid giant synapse. Tokumaru et al. (2001) showed that injection of an inhibitory complexin peptide into squid giant presynaptic terminals blocked neurotransmitter release at a late prefusion step of synaptic vesicle exocytosis [PMID:11239399]. Bracher et al. (2002) proposed that complexin is part of a multiprotein fusion machinery that regulates vesicle fusion at a late pre-fusion stage [PMID:12004067]. However, complexin is a regulator of this process rather than a direct participant; a more precise annotation would be positive regulation of synaptic vesicle exocytosis (GO:2000302). Reason: While complexin is clearly involved in the synaptic vesicle exocytosis pathway, it functions as a positive regulator rather than a core component of the fusion machinery itself. The term GO:2000302 (positive regulation of synaptic vesicle exocytosis) more accurately captures the regulatory role demonstrated by the antibody and peptide inhibition experiments. Proposed replacements: positive regulation of synaptic vesicle exocytosis Supporting Evidence: PMID:11239399 Injection of this peptide into squid giant presynaptic terminals inhibited neurotransmitter release at a late prefusion step of synaptic vesicle exocytosis. PMID:12004067 We propose that this structure is part of a multiprotein fusion machinery that regulates vesicle fusion at a late pre-fusion stage. |
| GO:0031201 SNARE complex | IEA GO_REF:0000118 | MARK AS OVER ANNOTATED | Summary: TreeGrafter-predicted localization to the SNARE complex. Complexin binds the assembled SNARE complex but is not a core subunit of it. The SNARE complex consists of syntaxin-1, synaptobrevin, and SNAP-25; complexin is an accessory regulatory protein that binds to the outside of the assembled four-helix bundle. Bracher et al. (2002) solved the crystal structure showing complexin bound to the SNARE complex surface [PMID:12004067]. Being a binding partner rather than a subunit, the part_of relationship implied by this CC annotation is questionable. Reason: Complexin is not a core subunit of the SNARE complex; it binds to the exterior surface of the assembled SNARE four-helix bundle as an accessory regulatory protein. The part_of qualifier used in the GOA annotation implies complexin is a structural component of the SNARE complex, which is inaccurate. The SNARE binding MF annotation (GO:0000149) already captures this interaction appropriately. Supporting Evidence: PMID:12004067 A helical segment of complexin binds in anti-parallel fashion to the four-helix bundle of the core SNARE complex and interacts at its C terminus with syntaxin and synaptobrevin around the ionic zero layer of the SNARE complex. PMID:11239399 Synaphin/complexin is a cytosolic protein that preferentially binds to syntaxin within the SNARE complex. |
| GO:0043195 terminal bouton | IEA GO_REF:0000118 | ACCEPT | Summary: TreeGrafter-predicted localization to terminal bouton. Tokumaru et al. (2001) performed experiments by injecting reagents into squid giant presynaptic terminals, confirming that complexin functions at synaptic terminals [PMID:11239399]. While the squid giant synapse is anatomically distinct from a typical bouton, the general concept of presynaptic terminal localization is supported. This annotation is reasonable for a phylogenetically inferred term. Reason: Terminal bouton localization is consistent with the known function of complexin at presynaptic terminals. The squid giant synapse experiments directly demonstrate complexin activity at the presynaptic terminal. While the squid giant synapse is not a classical bouton, the TreeGrafter inference from mammalian complexin orthologs is phylogenetically sound. Supporting Evidence: PMID:11239399 Injection of this peptide into squid giant presynaptic terminals inhibited neurotransmitter release at a late prefusion step of synaptic vesicle exocytosis. |
| GO:0046928 regulation of neurotransmitter secretion | IEA GO_REF:0000118 | ACCEPT | Summary: TreeGrafter-predicted involvement in regulation of neurotransmitter secretion. This is well supported by experimental evidence. Tokumaru et al. (2001) demonstrated that complexin is essential for neurotransmitter release at the squid giant synapse, and that blocking complexin-syntaxin interaction inhibits neurotransmitter release [PMID:11239399]. This term is acceptable but slightly less specific than positive regulation of synaptic vesicle exocytosis (GO:2000302), which better captures the mechanism. Reason: Regulation of neurotransmitter secretion is an accurate annotation for complexin. While GO:2000302 (positive regulation of synaptic vesicle exocytosis) is more mechanistically precise, this broader regulatory term is also correct and captures the physiological outcome. Keeping both provides complementary granularity. Supporting Evidence: PMID:11239399 Injection of this peptide into squid giant presynaptic terminals inhibited neurotransmitter release at a late prefusion step of synaptic vesicle exocytosis. |
| GO:2000302 positive regulation of synaptic vesicle exocytosis | IDA PMID:11239399 SNARE complex oligomerization by synaphin/complexin is essen... | NEW | Summary: Proposed new annotation based on direct experimental evidence from the squid giant synapse. Tokumaru et al. (2001) demonstrated that complexin positively regulates synaptic vesicle exocytosis: blocking complexin function with antibodies or inhibitory peptides inhibited neurotransmitter release at a late prefusion step [PMID:11239399]. UniProt also annotates this protein as positively regulating a late step in synaptic vesicle exocytosis. This is the most precise biological process term for complexin function. Reason: This term precisely captures the experimentally demonstrated positive regulatory role of complexin in synaptic vesicle exocytosis, which is the core biological function of this protein. It is more specific than both GO:0016079 (synaptic vesicle exocytosis) and GO:0046928 (regulation of neurotransmitter secretion). Supporting Evidence: PMID:11239399 SNARE complex oligomerization by synaphin/complexin is essential for synaptic vesicle exocytosis. PMID:11239399 Injection of this peptide into squid giant presynaptic terminals inhibited neurotransmitter release at a late prefusion step of synaptic vesicle exocytosis. |
| GO:0019905 syntaxin binding | IPI PMID:11239399 SNARE complex oligomerization by synaphin/complexin is essen... | NEW | Summary: Proposed new annotation for syntaxin binding, which is noted in the UniProt DR lines (from InterPro) but absent from the GOA TSV. Both key publications demonstrate direct complexin-syntaxin interaction. Tokumaru et al. (2001) showed that complexin preferentially binds to syntaxin within the SNARE complex and that a peptide from the syntaxin-binding domain competitively inhibits this interaction [PMID:11239399]. Bracher et al. (2002) resolved the structural basis showing complexin contacts syntaxin around the ionic zero layer [PMID:12004067]. Reason: Syntaxin binding is a more specific child term of SNARE binding that is directly demonstrated in this species. It captures the preferential interaction of complexin with syntaxin within the SNARE complex. This complements the broader SNARE binding annotation. Supporting Evidence: PMID:11239399 Synaphin/complexin is a cytosolic protein that preferentially binds to syntaxin within the SNARE complex. We find that synaphin promotes SNAREs to form precomplexes that oligomerize into higher order structures. A peptide from the central, syntaxin binding domain of synaphin competitively inhibits these two proteins from interacting. PMID:12004067 A helical segment of complexin binds in anti-parallel fashion to the four-helix bundle of the core SNARE complex and interacts at its C terminus with syntaxin and synaptobrevin around the ionic zero layer of the SNARE complex. |
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