Spore coat protein SP70 (also called Beejin) is a developmentally regulated, secreted glycoprotein of the social amoeba Dictyostelium discoideum. It is synthesized specifically in prespore cells during multicellular development, where it is packaged into prespore vesicles as part of a preassembled multiprotein glycoprotein complex (the PsB complex). Late in culmination this complex is secreted and deposited into the spore coat, the cellulose-and-glycoprotein extracellular matrix that surrounds the mature dormant spore. SP70 carries an N-terminal signal peptide, a Dictyostelium spore-coat (DSCP-N) domain and multiple follistatin-like cysteine-rich repeats, and is post-translationally modified by N-linked glycosylation, fucosylation and phosphorylation. Together with the other major coat proteins SP96 (CotC) and SP60 (CotA) it contributes to the structural integrity and permeability barrier of the spore coat; loss of these proteins leaves spores viable but more porous. Because its expression is tightly restricted to prespore cells, cotB is widely used as a molecular marker of the prespore cell type.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0030435 sporulation resulting in formation of a cellular spore | IEA GO_REF:0000117 | ACCEPT | Summary: Electronic (ARBA) annotation placing SP70 in sporulation. SP70 is a prespore-specific spore coat glycoprotein deposited into the spore coat during sporulation, so involvement in spore formation is correct. This is redundant with the experimental IMP/IGI annotations to the same term. Reason: SP70/cotB is a prespore-restricted structural component of the spore coat, and its synthesis and deposition are part of the sporulation program. The electronic inference is consistent with the direct experimental evidence for a role in spore formation. Supporting Evidence: PMID:7813801 lacking the major spore coat proteins, SP96, SP70, or SP60 |
| GO:0031160 spore wall | IEA GO_REF:0000117 | ACCEPT | Summary: Electronic annotation locating SP70 in the spore wall (spore coat). SP70 is secreted from prespore vesicles and incorporated into the spore coat extracellular matrix, so this localization is correct and is supported by direct experimental evidence. Reason: SP70 is a component of the assembled spore coat. Both the electronic inference and experimental data place the protein in this extracellular structure. Supporting Evidence: PMID:8662961 the complex is synchronously secreted from the prespore vesicles and incorporated into the spore coat |
| GO:0031160 spore wall | IDA PMID:499661 Plasma membrane proteins of Dictyostelium: the spore coat pr... | ACCEPT | Summary: Direct assay localizing SP70 to the spore coat. The cited paper (Orlowski & Loomis 1979) is an early biochemical characterization of the Dictyostelium spore coat proteins; the cached record is abstract-only, but the localization of SP70 to the spore coat is independently and robustly supported by later work on the secreted PsB glycoprotein complex. Reason: SP70 is an established structural constituent of the spore coat, the cellulose-and-glycoprotein extracellular matrix surrounding the spore. This is core to the protein's function and is corroborated by direct evidence that the coat glycoprotein complex is secreted and incorporated into the coat. Supporting Evidence: PMID:8662961 the complex is synchronously secreted from the prespore vesicles and incorporated into the spore coat |
| GO:0003677 DNA binding | IDA PMID:7713325 Protein kinase A is a positive regulator of spore coat gene ... | REMOVE | Summary: This annotation assigns DNA binding to the SP70 gene product, but the cited paper describes DNA binding by a distinct trans-acting factor that recognizes CA-rich elements upstream of the cotB gene, not by the SP70 protein itself. SP70 is a secreted, signal-peptide-bearing spore coat glycoprotein with follistatin-like repeats and has no nuclear localization or DNA-binding architecture; DNA binding is biologically implausible for it. Reason: The DNA-binding activity in Hopper et al. 1995 belongs to a separate regulatory (CA-box-binding) protein that controls transcription of the cot genes, whereas cotB/SP70 is the structural spore coat glycoprotein being regulated. Attributing DNA binding to the SP70 product is a misassignment; the protein's secreted, glycosylated, extracellular nature is incompatible with a DNA-binding molecular function. Supporting Evidence: PMID:7713325 the binding activity that recognizes the CA-rich sequences upstream of the cotB gene PMID:7713325 modulating the activity of a DNA binding protein |
| GO:0005515 protein binding | IPI PMID:8662961 PsB multiprotein complex of Dictyostelium discoideum. Demons... | KEEP AS NON CORE | Summary: Interaction-based annotation reflecting SP70's participation in the PsB multiprotein glycoprotein complex. Bare 'protein binding' is uninformative; the biologically meaningful content (membership in the preassembled spore coat glycoprotein complex) is better captured by the glycoprotein complex annotation. Reason: SP70 does engage in protein-protein interactions as a subunit of the PsB complex, but GO:0005515 is too general to convey function. The informative aspect is retained via the glycoprotein complex (GO:0090665) annotation, so this term is kept only as non-core. Supporting Evidence: PMID:8662961 some of these spore coat glycoproteins exist as a preassembled multiprotein complex |
| GO:0030435 sporulation resulting in formation of a cellular spore | IMP PMID:7813801 Structural roles of the spore coat proteins in Dictyostelium... | ACCEPT | Summary: Mutational (knockout) evidence for SP70's role in spore formation. Fosnaugh et al. compared spores of strains lacking SP96, SP70 and/or SP60 and showed altered spore coat properties, supporting a role in sporulation. Reason: Direct knockout analysis of the major spore coat proteins including SP70 establishes their contribution to the spore-formation program. This is a core biological process for the protein. Supporting Evidence: PMID:7813801 lacking the major spore coat proteins, SP96, SP70, or SP60 |
| GO:0030435 sporulation resulting in formation of a cellular spore | IGI PMID:7813801 Structural roles of the spore coat proteins in Dictyostelium... | ACCEPT | Summary: Genetic-interaction evidence from single, double and triple knockouts of the major spore coat proteins, supporting an overlapping, partially redundant role of SP70 with SP96 and SP60 in spore formation. Reason: The combinatorial knockout analysis demonstrates that SP70 acts together with the other major coat proteins in building a functional spore, supporting involvement in sporulation. Supporting Evidence: PMID:7813801 The intensity of staining with fluorescently labeled ricinA increased as the spore coat proteins were incrementally lost |
| GO:0042244 spore wall assembly | IMP PMID:7813801 Structural roles of the spore coat proteins in Dictyostelium... | ACCEPT | Summary: Knockout analysis shows that loss of the major coat proteins including SP70 makes spores more porous, indicating a contribution to the structure and permeability barrier of the assembled spore coat, even though the coat still forms in their absence. Reason: SP70 contributes to the structural completeness of the spore coat; its loss increases porosity and exposes underlying polysaccharide. This places it in spore wall assembly as a structural constituent, which is a core function. Supporting Evidence: PMID:7813801 the spores are more porous which might make them at risk to predators before germination |
| GO:0090665 glycoprotein complex | IDA PMID:8662961 PsB multiprotein complex of Dictyostelium discoideum. Demons... | ACCEPT | Summary: SP70 is part of the preassembled PsB spore coat glycoprotein complex that is stored in prespore vesicles and later secreted and incorporated into the spore coat. This complex membership is a core structural feature of the protein. Reason: The spore coat glycoproteins, including SP70, exist as a preassembled multiprotein glycoprotein complex, directly supporting the glycoprotein complex annotation. Supporting Evidence: PMID:8662961 some of these spore coat glycoproteins exist as a preassembled multiprotein complex PMID:8662961 This unique extracellular matrix is composed of cellulose and glycoproteins |
| GO:0030248 cellulose binding | IMP PMID:8662961 PsB multiprotein complex of Dictyostelium discoideum. Demons... | ACCEPT | Summary: The assembled PsB glycoprotein complex has specific in vitro cellulose binding activity that requires all protein subunits to be assembled. As a subunit, SP70 contributes to this complex-level activity rather than binding cellulose independently, consistent with the contributes_to qualifier. Reason: Analysis of subunit-deletion mutants showed that the PsB complex must be fully assembled for cellulose binding, so individual subunits including SP70 contribute to the activity. The contributes_to qualifier correctly captures this complex-dependent molecular function. Supporting Evidence: PMID:8662961 the PsB complex has a specific in vitro cellulose binding activity PMID:8662961 the protein subunits must be assembled for cellulose binding activity |
| GO:0031153 slug development involved in sorocarp development | IEP PMID:25887420 Leaps and lulls in the developmental transcriptome of Dictyo... | MARK AS OVER ANNOTATED | Summary: This annotation derives from expression profiling in which cotB is used as a marker of prespore-cell differentiation during the slug stage. Expression timing as a cell-type marker indicates when and where the gene is transcribed, not that SP70 functionally drives slug development. SP70's demonstrated role is structural, in the spore coat. Reason: cotB is a prespore marker whose developmental expression profile was measured, but there is no evidence that the SP70 protein participates causally in slug development. An IEP inference from a marker gene's expression pattern overstates a functional role; the gene's core function lies in spore coat structure, not in the morphogenesis of the slug. Supporting Evidence: PMID:25887420 The genes cotB and ecmB are often used to mark the differentiation of prespore and prestalk cell-types, respectively |
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