Clathrin assembly protein AP180 from the longfin inshore squid (Doryteuthis pealeii). AP180 is a monomeric clathrin adaptor protein that promotes clathrin lattice assembly via its C-terminal clathrin assembly domain containing multiple DLL motifs. The N-terminal ANTH (AP180 N-terminal homology) domain binds phosphatidylinositol-4,5-bisphosphate, targeting the protein to membranes. AP180 plays an essential role in synaptic vesicle endocytosis at the squid giant synapse, where it is required for clathrin-coated pit formation and maintenance of the synaptic vesicle pool. Squid AP180 shares 40% sequence identity with mouse AP180 and has conserved clathrin assembly activity despite divergent C-terminal primary sequence (PMID:10575017). Eps15 potently stimulates AP180-mediated clathrin assembly via interactions between Eps15 EH domains and NPF motifs on AP180 (PMID:12807910).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000149 SNARE binding | IEA GO_REF:0000118 | UNDECIDED | Summary: TreeGrafter prediction of SNARE binding for AP180. While some AP180/CALM family members have been reported to interact with VAMP/synaptobrevin SNAREs in other organisms, there is no direct evidence for SNARE binding by squid AP180 in the available literature (PMID:10575017, PMID:12807910). The known molecular interactions of squid AP180 are with clathrin, Eps15, and phosphoinositides. This annotation may reflect functions of mammalian orthologs (CALM/PICALM) that may not apply directly to the squid protein. Reason: No direct evidence for SNARE binding by squid AP180. However, the ANTH domain of AP180/CALM family members binds directly to the SNARE motif of VAMP2/synaptobrevin, and this interaction is conserved from yeast to mammals. AP180-knockout mice show greatly reduced VAMP2 in synaptic vesicles, and yeast lacking AP180 homologs mislocalize the VAMP analog Snc1. While no squid-specific SNARE binding data exists, the high conservation of the ANTH domain (the SNARE-binding region) across species makes this annotation plausible. Retained as UNDECIDED pending direct experimental confirmation in squid. |
| GO:0005543 phospholipid binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based annotation of phospholipid binding via the ANTH domain (IPR011417). AP180 contains an N-terminal ANTH domain that binds phosphoinositides, consistent with this annotation. However, phospholipid binding is overly broad; the more specific term phosphatidylinositol-4,5-bisphosphate binding (GO:0005546) is already annotated and better reflects the known specificity of the ANTH domain. Reason: Phospholipid binding is a valid parent term for the ANTH domain activity. While GO:0005546 (PtdIns(4,5)P2 binding) is more specific and also annotated, this broader InterPro-derived annotation is not incorrect and captures general lipid binding capacity of the ANTH domain. The UniProt record notes the protein "Binds clathrin and phosphatidylinositol 4,5-bisphosphate." |
| GO:0005545 1-phosphatidylinositol binding | IEA GO_REF:0000120 | ACCEPT | Summary: Combined IEA annotation for 1-phosphatidylinositol binding based on InterPro ANTH domain superfamily (IPR014712) and PANTHER. The ANTH domain of AP180 family proteins binds phosphoinositides; the N-terminal domain of squid AP180 is described as the "inositide-binding domain" (PMID:10575017). Reason: Consistent with the known function of the ANTH domain. The abstract of PMID:10575017 explicitly refers to the "N-terminal inositide-binding domain" of squid AP180. This term is appropriate alongside the more specific PtdIns(4,5)P2 binding annotation. Supporting Evidence: PMID:10575017 Alignment of squid AP180 with other AP180 homologs shows that amino acid identity was highest in the N-terminal inositide-binding domain of the protein |
| GO:0005546 phosphatidylinositol-4,5-bisphosphate binding | IEA GO_REF:0000118 | ACCEPT | Summary: TreeGrafter prediction of PtdIns(4,5)P2 binding. The ANTH domain of AP180 binds PtdIns(4,5)P2 to target the protein to membranes. This is well-established for the AP180/CALM family and the UniProt record explicitly states the protein "Binds clathrin and phosphatidylinositol 4,5-bisphosphate." The N-terminal inositide-binding domain is highly conserved (PMID:10575017). Reason: Well-supported core molecular function. PtdIns(4,5)P2 binding via the ANTH domain is a conserved and essential function of AP180 family proteins, enabling membrane targeting during clathrin-coated pit formation. Supporting Evidence: PMID:10575017 Alignment of squid AP180 with other AP180 homologs shows that amino acid identity was highest in the N-terminal inositide-binding domain of the protein |
| GO:0005794 Golgi apparatus | IEA GO_REF:0000044 | MARK AS OVER ANNOTATED | Summary: UniProt subcellular location mapping annotation placing AP180 at the Golgi apparatus. This is based on ARBA prediction. While AP180/CALM family members may associate with Golgi membranes in some cell types, the primary characterized localization of squid AP180 is at the presynaptic terminal, specifically at clathrin-coated pits and the periactive zone membrane (PMID:10575017). There is no experimental evidence for Golgi localization in squid. Reason: No experimental evidence for Golgi localization of squid AP180. The primary known site of action is the presynaptic terminal plasma membrane and clathrin-coated pits. This ARBA-derived annotation likely reflects general family properties that may not apply to the synaptic context of squid AP180. |
| GO:0005905 clathrin-coated pit | IEA GO_REF:0000120 | ACCEPT | Summary: Combined IEA annotation for clathrin-coated pit localization. Squid AP180 functions at clathrin-coated pits during synaptic vesicle endocytosis. When AP180 function is disrupted by peptide injection, coated pit formation is inhibited (PMID:10575017, PMID:12807910). The UniProt subcellular location also lists clathrin-coated pit. Reason: Strongly supported by experimental evidence. PMID:10575017 shows that blocking AP180 function depletes coated vesicles, and PMID:12807910 demonstrates that disrupting Eps15-AP180 interactions "inhibit the formation of clathrin-coated pits and coated vesicles during synaptic vesicle endocytosis." Supporting Evidence: PMID:12807910 when injected into squid giant presynaptic nerve terminals, these peptides inhibit the formation of clathrin-coated pits and coated vesicles during synaptic vesicle endocytosis PMID:10575017 peptide-injected terminals had far fewer synaptic vesicles, were depleted of coated vesicles |
| GO:0006898 receptor-mediated endocytosis | IEA GO_REF:0000117 | MARK AS OVER ANNOTATED | Summary: ARBA prediction of involvement in receptor-mediated endocytosis. While AP180 family proteins function in clathrin-mediated endocytosis, the specific role of squid AP180 is in synaptic vesicle endocytosis at the giant synapse, not receptor-mediated endocytosis per se. Synaptic vesicle endocytosis (GO:0048488) is already annotated and is more appropriate. Reason: The characterized function of squid AP180 is in synaptic vesicle endocytosis, not receptor-mediated endocytosis. While the clathrin machinery is shared, receptor-mediated endocytosis implies cargo selection via receptor internalization signals, which is not what squid AP180 has been shown to do. The more accurate term GO:0048488 (synaptic vesicle endocytosis) is already annotated with IMP evidence. |
| GO:0006900 vesicle budding from membrane | IEA GO_REF:0000118 | KEEP AS NON CORE | Summary: TreeGrafter prediction of vesicle budding from membrane. AP180 promotes clathrin coat assembly, which is essential for membrane budding during synaptic vesicle endocytosis. This is a reasonable general annotation, though more specific terms (clathrin coat assembly, synaptic vesicle endocytosis) are already annotated. Reason: Vesicle budding from membrane is a valid but very general description of AP180 function. The protein's role is specifically in clathrin-mediated vesicle budding at the synapse. More specific terms (GO:0048268 clathrin coat assembly, GO:0048488 synaptic vesicle endocytosis) capture the core function better. This annotation is retained as it is not incorrect but is non-core. |
| GO:0008021 synaptic vesicle | IEA GO_REF:0000118 | MODIFY | Summary: TreeGrafter prediction of synaptic vesicle localization. Squid AP180 functions at the presynaptic terminal in synaptic vesicle recycling. The protein is found in the presynaptic terminal where it associates with clathrin coats during vesicle reformation. However, AP180 is not a synaptic vesicle resident protein; it is a cytosolic/peripheral membrane protein that transiently associates with membranes during clathrin coat assembly. Reason: AP180 is not a component of mature synaptic vesicles. It is a cytosolic adaptor protein that transiently associates with the presynaptic plasma membrane and nascent clathrin-coated pits during endocytosis. A more appropriate localization would be the presynaptic endocytic zone or cytosol. Proposed replacements: extrinsic component of presynaptic endocytic zone membrane |
| GO:0012505 endomembrane system | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA prediction of endomembrane system localization. This is a very broad cellular component term. AP180 transiently associates with membranes via its ANTH domain binding to PtdIns(4,5)P2, so it is not entirely wrong, but it provides minimal information about the actual localization. Reason: While very general, this is not incorrect as AP180 does associate with endomembrane compartments (clathrin-coated pits and vesicles). More specific CC terms are already annotated (GO:0005905 clathrin-coated pit, GO:0030136 clathrin-coated vesicle, GO:0098894 presynaptic endocytic zone membrane). |
| GO:0016020 membrane | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA prediction of membrane localization. This is an extremely broad term. AP180 associates with membranes transiently via PtdIns(4,5)P2 binding during clathrin coat assembly, but it is primarily a cytosolic protein. Reason: While extremely general, membrane association is not incorrect. AP180 binds PtdIns(4,5)P2 at membranes as part of its function. More specific CC terms are already present. This broad IEA annotation does not misrepresent the protein. |
| GO:0016185 synaptic vesicle budding from presynaptic endocytic zone membrane | IEA GO_REF:0000118 | ACCEPT | Summary: TreeGrafter prediction of synaptic vesicle budding from presynaptic endocytic zone membrane. This is a highly specific and appropriate term for squid AP180 function. The protein promotes clathrin coat assembly at the presynaptic terminal, and blocking its function prevents coated vesicle formation and depletes the synaptic vesicle pool (PMID:10575017, PMID:12807910). Reason: This is an excellent and specific annotation for AP180 function at the squid giant synapse. The experimental evidence from PMID:10575017 directly demonstrates that AP180 is required for synaptic vesicle budding from the presynaptic membrane: blocking AP180 function caused depletion of synaptic vesicles, loss of coated vesicles, and increased plasma membrane perimeter, all consistent with a block in vesicle budding from the presynaptic endocytic zone. Supporting Evidence: PMID:10575017 peptide-injected terminals had far fewer synaptic vesicles, were depleted of coated vesicles, and had a larger plasma membrane perimeter than terminals injected with control solutions |
| GO:0030136 clathrin-coated vesicle | IEA GO_REF:0000120 | ACCEPT | Summary: Combined IEA annotation for clathrin-coated vesicle localization. AP180 promotes the assembly of clathrin coats that form clathrin-coated vesicles. The protein transiently associates with the clathrin coat during vesicle formation. The UniProt subcellular location lists clathrin-coated vesicle. Reason: Consistent with AP180 function. The protein assembles clathrin coats and is associated with the clathrin-coated vesicle compartment during synaptic vesicle recycling. |
| GO:0030276 clathrin binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based annotation of clathrin binding. This is a core molecular function of AP180. Squid AP180 directly binds and assembles clathrin via its C-terminal domain containing multiple DLL motifs (PMID:10575017, PMID:11102472). Recombinant squid AP180 was shown to assemble clathrin in vitro. Reason: Clathrin binding is a central and well-demonstrated function of squid AP180. The protein directly binds clathrin triskelia via DLL motifs in its C-terminal domain and promotes their assembly into clathrin lattices (PMID:10575017, PMID:11102472). Supporting Evidence: PMID:10575017 Recombinant squid AP180 was able to assemble clathrin in vitro, suggesting a conserved three-dimensional structure that mediates clathrin assembly despite the divergent primary sequence of the C-terminal domain PMID:11102472 AP180, like AP-2 and AP-3, binds to the N-terminal domain of clathrin |
| GO:0032050 clathrin heavy chain binding | IEA GO_REF:0000118 | ACCEPT | Summary: TreeGrafter prediction of clathrin heavy chain binding. This is a more specific child of clathrin binding (GO:0030276). Morgan et al. (2000) demonstrated that AP180 binds to the N-terminal domain of the clathrin heavy chain via DLL motifs (PMID:11102472). Reason: Well-supported by experimental evidence from the squid system. AP180 binds the N-terminal domain of clathrin heavy chain, and progressive deletion of DLL motifs caused gradual reduction in clathrin assembly ability (PMID:11102472). This is a more informative and specific term than the broader clathrin binding. Supporting Evidence: PMID:11102472 AP180, like AP-2 and AP-3, binds to the N-terminal domain of clathrin |
| GO:0040011 locomotion | IEA GO_REF:0000117 | REMOVE | Summary: ARBA prediction of involvement in locomotion. There is no evidence that squid AP180 is directly involved in locomotion. AP180 is a clathrin assembly protein that functions in synaptic vesicle endocytosis. While synaptic transmission is necessary for locomotion in the squid, this is an extremely indirect and non-informative annotation for a clathrin adaptor protein. Reason: This annotation is inappropriate for a clathrin assembly protein. AP180 functions in clathrin coat assembly and synaptic vesicle endocytosis. Any connection to locomotion would be extremely indirect (via synaptic transmission). This appears to be a spurious ARBA prediction and does not represent a meaningful function of the protein. |
| GO:0048268 clathrin coat assembly | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based annotation of clathrin coat assembly. This is the core biological process function of AP180. Squid AP180 directly promotes clathrin lattice assembly via its C-terminal domain (PMID:10575017, PMID:12807910). This IEA annotation is consistent with the experimental IDA annotations for the same term. Reason: Clathrin coat assembly is the defining function of AP180. This IEA annotation is fully supported by IDA evidence from PMID:10575017 and PMID:12807910 for the same gene. |
| GO:0048488 synaptic vesicle endocytosis | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA prediction of synaptic vesicle endocytosis. This is a core function of squid AP180, directly demonstrated by microinjection experiments at the squid giant synapse (PMID:10575017). This IEA annotation is consistent with the IMP evidence for the same term. Reason: Synaptic vesicle endocytosis is a well-established core function of squid AP180, supported by IMP evidence from PMID:10575017. The ARBA prediction is correct. |
| GO:0072583 clathrin-dependent endocytosis | IEA GO_REF:0000120 | ACCEPT | Summary: Combined IEA annotation for clathrin-dependent endocytosis based on InterPro AP180-like family (IPR045192) and PANTHER. AP180 is a clathrin adaptor protein essential for clathrin-dependent endocytosis at the squid giant synapse. This is well-supported by the experimental literature (PMID:10575017, PMID:12807910). Reason: Clathrin-dependent endocytosis is the pathway in which AP180 functions. The experimental evidence from the squid system directly demonstrates that AP180 clathrin assembly activity is required for clathrin-mediated synaptic vesicle endocytosis. Augustine et al. (2006) concluded that "a clathrin-dependent pathway is the primary means of SV recycling at this synapse under physiological conditions" (PMID:16417485). |
| GO:0098894 extrinsic component of presynaptic endocytic zone membrane | IEA GO_REF:0000118 | ACCEPT | Summary: TreeGrafter prediction of localization as an extrinsic component of the presynaptic endocytic zone membrane. AP180 is a peripheral/extrinsic membrane protein that binds PtdIns(4,5)P2 via its ANTH domain to associate with the presynaptic membrane. The protein functions at the presynaptic terminal of the squid giant synapse (PMID:10575017, PMID:12807910). Reason: Highly appropriate and specific CC annotation. AP180 is not a transmembrane protein but rather an extrinsic membrane-associated adaptor that is recruited to the presynaptic membrane via PtdIns(4,5)P2 binding. The experimental work was performed at the squid giant presynaptic terminal, placing AP180 precisely at the presynaptic endocytic zone. Supporting Evidence: PMID:10575017 Microinjection of the C-terminal domains of either mouse or squid AP180 into the giant presynaptic terminal of squid enhanced synaptic transmission |
| GO:0048268 clathrin coat assembly | IDA PMID:12807910 Eps15 homology domain-NPF motif interactions regulate clathr... | ACCEPT | Summary: Direct experimental evidence (IDA) for clathrin coat assembly from Morgan et al. (2003). This study demonstrated that Eps15 potently stimulates AP180-mediated clathrin assembly at physiological pH. Peptides that block Eps15-AP180 NPF motif interactions inhibited clathrin-coated pit and vesicle formation when injected into squid giant presynaptic terminals (PMID:12807910). Reason: Strong IDA evidence from direct biochemical and in vivo experiments at the squid giant synapse. The study showed that AP180 mediates clathrin coat assembly and that this activity is regulated by Eps15 via EH domain-NPF motif interactions. Supporting Evidence: PMID:12807910 it potently stimulates the ability of the clathrin adaptor protein, AP180, to assemble clathrin at physiological pH PMID:12807910 when injected into squid giant presynaptic nerve terminals, these peptides inhibit the formation of clathrin-coated pits and coated vesicles during synaptic vesicle endocytosis |
| GO:0007269 neurotransmitter secretion | IMP PMID:10575017 A role for the clathrin assembly domain of AP180 in synaptic... | KEEP AS NON CORE | Summary: IMP evidence for neurotransmitter secretion from Morgan et al. (1999). Microinjection of an AP180 C-terminal peptide that inhibits clathrin assembly completely blocked synaptic transmission at the squid giant synapse. Conversely, injection of the full C-terminal domain enhanced synaptic transmission. While AP180 clearly affects neurotransmitter secretion, this is an indirect effect: AP180 maintains the synaptic vesicle pool via its role in endocytosis, and vesicle depletion leads to transmission failure (PMID:10575017). Reason: The effect on neurotransmitter secretion is real and experimentally demonstrated, but it is an indirect consequence of blocking synaptic vesicle endocytosis rather than a direct role in exocytosis/secretion. AP180 does not participate in the secretory machinery itself. The core function is clathrin coat assembly for synaptic vesicle endocytosis, and the impact on secretion is secondary to vesicle pool depletion. The IMP evidence code is appropriate as the phenotype (blocked transmission) results from disrupting AP180 function. Supporting Evidence: PMID:10575017 a peptide from the C-terminal domain of squid AP180 that inhibited clathrin assembly in vitro completely blocked synaptic transmission when it was injected into the giant presynaptic terminal PMID:10575017 We conclude that the clathrin assembly domain of AP180 is important for synaptic vesicle recycling at physiological rates of activity and that assembly of clathrin by AP180 is necessary for maintaining a pool of releasable synaptic vesicles |
| GO:0048268 clathrin coat assembly | IDA PMID:10575017 A role for the clathrin assembly domain of AP180 in synaptic... | ACCEPT | Summary: Direct experimental evidence (IDA) for clathrin coat assembly from Morgan et al. (1999). Recombinant squid AP180 was shown to assemble clathrin in vitro, and a peptide from the C-terminal clathrin assembly domain that inhibited assembly in vitro also blocked synaptic vesicle endocytosis in vivo (PMID:10575017). Reason: Strong IDA evidence from the foundational study on squid AP180. The in vitro clathrin assembly assay directly demonstrates the clathrin coat assembly function of this protein. This is a core function of AP180. Supporting Evidence: PMID:10575017 Recombinant squid AP180 was able to assemble clathrin in vitro, suggesting a conserved three-dimensional structure that mediates clathrin assembly despite the divergent primary sequence of the C-terminal domain |
| GO:0048488 synaptic vesicle endocytosis | IMP PMID:10575017 A role for the clathrin assembly domain of AP180 in synaptic... | ACCEPT | Summary: IMP evidence for synaptic vesicle endocytosis from Morgan et al. (1999). Microinjection of an AP180 inhibitory peptide into the squid giant presynaptic terminal caused dramatic ultrastructural changes consistent with blocked endocytosis: depletion of synaptic vesicles, loss of coated vesicles, increased plasma membrane perimeter, and larger remaining vesicle diameter (PMID:10575017). Reason: Excellent IMP evidence directly demonstrating AP180 is required for synaptic vesicle endocytosis. The electron microscopic analysis provides compelling morphological evidence for endocytosis blockade when AP180 function is disrupted. This is a core function of squid AP180. Supporting Evidence: PMID:10575017 Electron microscopic analysis revealed several structural changes consistent with the inhibition of synaptic vesicle endocytosis; peptide-injected terminals had far fewer synaptic vesicles, were depleted of coated vesicles, and had a larger plasma membrane perimeter than terminals injected with control solutions PMID:10575017 In addition, the remaining synaptic vesicles were significantly larger in diameter |
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