Synaptoporin is a brain-enriched, four-pass MARVEL-domain integral membrane glycoprotein of small synaptic vesicles. It is concentrated in selected neuronal populations rather than uniformly across nerve terminals. Family-level reconstitution and genetic studies support overlapping roles with synaptophysin and synaptogyrins in organizing and clustering small synaptic vesicles and in establishing their characteristic small size. Rodent knockdown/rescue studies indicate that, at hippocampal mossy-fiber–CA3 synapses, presynaptic synaptoporin supports activity-dependent structural and functional homeostatic plasticity. An intrinsic ion-channel activity has not been demonstrated.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0030672 synaptic vesicle membrane | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic inference places SYNPR on the synaptic vesicle membrane, its defining and experimentally corroborated localization. Reason: Rat synaptoporin was localized to synaptic vesicles by immunoelectron microscopy, and the conserved human protein is a four-pass synaptic-vesicle membrane protein. The IBA is consistent with direct family evidence and represents the core site of action. Supporting Evidence: PMID:8229211 Immunoelectron microscopy confirmed that syp II is localized to synaptic vesicles |
| GO:0008021 synaptic vesicle | IEA GO_REF:0000002 | ACCEPT | Summary: The InterPro-derived annotation correctly places SYNPR in synaptic vesicles, although the membrane-specific child term is more informative. Reason: Biochemical copurification and immunoelectron microscopy directly support synaptic-vesicle localization. Located_in the vesicle is valid for an integral vesicle-membrane protein, and GOA also contains the more precise synaptic vesicle membrane annotation. Supporting Evidence: PMID:2206533 copurifies with small synaptic vesicles |
| GO:0016020 membrane | IEA GO_REF:0000002 | MODIFY | Summary: SYNPR is a four-pass integral membrane protein, but the generic membrane term discards its defining synaptic-vesicle context. Reason: The annotation is biologically correct but unnecessarily broad. Direct localization and the protein topology justify replacing it with synaptic vesicle membrane. Proposed replacements: synaptic vesicle membrane Supporting Evidence: PMID:8229211 part of a heteromultimeric complex in synaptic vesicle membranes |
| GO:0030672 synaptic vesicle membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping assigns the experimentally supported synaptic-vesicle membrane localization. Reason: This is the most specific established cellular location for SYNPR and is independently supported by biochemical fractionation and immunoelectron microscopy of synaptoporin. Supporting Evidence: PMID:8229211 Immunoelectron microscopy confirmed that syp II is localized to synaptic vesicles |
| GO:0045202 synapse | IEA GO_REF:0000044 | MODIFY | Summary: SYNPR is present at synapses through its residence in presynaptic synaptic vesicles, but the undifferentiated synapse term is too broad. Reason: The evidence places SYNPR specifically in presynaptic vesicles and at mossy-fiber boutons. Presynapse better represents this cellular context without implying a postsynaptic localization. Proposed replacements: presynapse Supporting Evidence: PMID:23312519 Mossy fiber boutons are also selectively marked by the presynaptic vesicle-associated protein synaptoporin |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | MARK AS OVER ANNOTATED | Summary: A proteome-scale binary-interactome screen reports SYNPR interaction context, including GOA WITH/FROM accession Q969F0. Reason: The high-throughput interaction may be useful at partner level, but generic protein binding is not an informative molecular function and does not establish a core biochemical activity of SYNPR. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | Summary: The reference human binary-interactome map contributes multiple SYNPR interaction candidates; GOA currently aggregates nine WITH/FROM accessions under this term-reference group. Reason: A collection of high-throughput binary interactions does not define a specific SYNPR molecular activity. Partner-specific statements can be curated separately if independently validated, but generic protein binding should not be treated as a core function. |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | MARK AS OVER ANNOTATED | Summary: A dual proteome-scale network study reports an additional SYNPR interaction candidate, represented in GOA by WITH/FROM accession Q9C0E8. Reason: This network-scale observation is interaction context rather than evidence for a defined molecular activity. The generic protein-binding term is too broad to convey SYNPR biology. |
| GO:0048499 synaptic vesicle membrane organization | ISS PMID:38985768 Overlapping role of synaptophysin and synaptogyrin family pr... | NEW | Summary: Proposed human annotation by orthology. Mammalian synaptoporin and synapsin coexpression generates clusters of small vesicles, and family knockout alters normal synaptic-vesicle size. Reason: Individual-protein reconstitution and family-level mouse genetics support a redundant role for synaptoporin in arranging the membrane and size of synaptic vesicles. Assignment to human Q8TBG9 is inferred from the conserved mammalian ortholog; this term avoids asserting an unproven channel or membrane-bending molecular activity. Supporting Evidence: PMID:38985768 These results point to an overlapping role of these proteins in the biogenesis and clustering of SV file:human/SYNPR/SYNPR-deep-research-manual.md Its supported roles are overlapping organization/clustering of small synaptic vesicles and a specialized contribution to mossy-fiber–CA3 homeostatic structural plasticity. |
| GO:0097091 synaptic vesicle clustering | ISS PMID:38985768 Overlapping role of synaptophysin and synaptogyrin family pr... | NEW | Summary: Proposed human annotation by orthology. Mammalian synaptoporin coexpression with synapsin is sufficient to produce clusters of small vesicles in COS7 cells. Reason: This heterologous assay supports a role in vesicle clustering, and the human assignment is inferred from the conserved mammalian ortholog. The conclusion is appropriately treated as an overlapping family function because individual family members are redundant in vivo. Supporting Evidence: PMID:38985768 expression in COS7 cells of each of these three other proteins together with synapsin resulted in the formation of clusters of small vesicles similar in size to those generated by synaptophysin and synapsin coexpression |
| GO:0048168 regulation of neuronal synaptic plasticity | ISS PMID:23312519 Mossy fiber-CA3 synapses mediate homeostatic plasticity in m... | NEW | Summary: Proposed human annotation by orthology. Rat synaptoporin knockdown and rescue show that the presynaptic protein is required for homeostatic adaptation at mature hippocampal mossy-fiber–CA3 synapses. Reason: Rat loss-of-function and rescue experiments support a causal, circuit-specific role in neuronal homeostatic synaptic plasticity; assignment to human SYNPR is by orthology. The annotation should not be interpreted as a global role at every neuronal synapse. Supporting Evidence: PMID:23312519 TE formation required presynaptic synaptoporin |
| GO:0051823 regulation of synapse structural plasticity | ISS PMID:23312519 Mossy fiber-CA3 synapses mediate homeostatic plasticity in m... | NEW | Summary: Proposed human annotation by orthology. In rat hippocampal cultures, presynaptic synaptoporin is required for activity-dependent mossy-fiber bouton upregulation and instructs postsynaptic thorny-excrescence formation or growth. Reason: Rat knockdown/rescue evidence connects synaptoporin to structural remodeling of a defined synapse class, making this more informative than a generic synaptic-plasticity annotation alone; the human assignment is inferred by orthology. Supporting Evidence: PMID:23312519 Presynaptic synaptoporin was required for functional MF upregulation, and also provides instructive signals necessary for postsynaptic TE formation and/or growth |
| GO:0098686 hippocampal mossy fiber to CA3 synapse | ISS PMID:23312519 Mossy fiber-CA3 synapses mediate homeostatic plasticity in m... | NEW | Summary: Proposed human annotation by orthology. Endogenous rat synaptoporin marks presynaptic mossy-fiber boutons contacting CA3 neurons and functions at these synapses. Reason: Rat immunolocalization plus functional knockdown/rescue evidence place active synaptoporin at this precisely defined synapse type; assignment to human SYNPR is inferred by orthology. Supporting Evidence: PMID:23312519 Mossy fiber boutons are also selectively marked by the presynaptic vesicle-associated protein synaptoporin |
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Download this section (compressed HTML)Q: Does SYNPR directly bind synapsins at endogenous synaptic vesicles, and which residues or post-translational modifications control that interaction?
Suggested experts: Pietro De Camilli
Q: Does SYNPR itself bend membranes or set vesicle diameter, or does it recruit a separate membrane-shaping factor?
Suggested experts: Pietro De Camilli
Q: Do the two human SYNPR splice isoforms differ in neuronal distribution, trafficking, or support of homeostatic plasticity?
Suggested experts: Yumin Mao
Experiment: Generate SYNPR-null human iPSC-derived dentate granule neurons and perform isoform-resolved rescue. Quantify synaptic-vesicle diameter and number by electron microscopy, vesicle cycling by pH-sensitive reporters, and synaptic transmission during chronic activity suppression.
Hypothesis: SYNPR loss enlarges synaptic vesicles and impairs activity-dependent mossy-fiber-like presynaptic upregulation, with rescue by the principal neuronal isoform.
Type: CRISPR genetics, electrophysiology, live imaging, and electron microscopy
Experiment: Reconstitute purified SYNPR into composition-controlled proteoliposomes and measure membrane curvature, vesicle diameter, and ion or solute conductance with and without synapsin.
Hypothesis: SYNPR promotes membrane curvature and small-vesicle formation with synapsin but does not display an autonomous ion-channel activity.
Type: biochemical reconstitution and electrophysiology
Experiment: Endogenously tag each SYNPR isoform in human neurons and compare localization, turnover, binding partners, and activity-dependent redistribution.
Hypothesis: Alternative splicing changes SYNPR trafficking or partner engagement rather than creating a distinct catalytic activity.
Type: isoform-specific genome editing, microscopy, and quantitative proteomics
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The direct molecular mechanism by which SYNPR and related tetraspanins determine synaptic-vesicle shape and size is unknown.
OPEN BIOLOGY MF_DARK
What is known: SYNPR plus synapsin is sufficient to generate clusters of small vesicles in COS7 cells, and quadruple-family knockout enlarges mouse synaptic vesicles, but these results do not distinguish intrinsic membrane bending from recruitment of another shaping factor.
Significance: Resolving this mechanism would identify SYNPR's biochemical activity and enable a precise molecular-function annotation.
What would resolve it: Purified-protein membrane reconstitution, curvature measurements, systematic partner depletion, and structure-guided mutagenesis should distinguish intrinsic membrane shaping from cofactor recruitment.
Provenance (the field's own admissions):
Gap: Whether SYNPR has any autonomous ion-channel or solute-channel activity remains unestablished.
OPEN BIOLOGYCURATION MF_DARK
What is known: The protein is a four-pass MARVEL-domain synaptic-vesicle membrane protein, but the reviewed literature uses channel language as a putative homology-based model and supplies no direct conductance or selectivity assay.
Significance: Direct evidence is needed before any channel molecular-function term can be assigned to SYNPR.
What would resolve it: Reconstitute purified SYNPR into defined liposomes or planar bilayers and test ion and solute flux over physiologically relevant gradients, with negative and positive channel controls.
Provenance (the field's own admissions):
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