Countin-1 (CtnA) is a secreted, glycosylated protein of the social amoeba Dictyostelium discoideum and the principal bioactive component of counting factor (CF), a ~450 kDa multi-subunit extracellular complex (also containing CF50, CF45-1 and CF60) that regulates the size of the multicellular structures formed during development. When cells starve they aggregate into streams that break up into groups of roughly 2x10^4 cells, each of which becomes a fruiting body; secreted CF reports local cell number, and high CF levels promote stream breakup so that fewer cells accumulate per group. CtnA carries a saposin B-type domain, is exported through a signal peptide after N-glycosylation, and binds a small number of high-affinity cell-surface sites to trigger rapid signal transduction. Through this signaling it represses cell-cell adhesion, increases cell motility and F-actin polymerization, lowers intracellular glucose by inhibiting glucose-6-phosphatase, and modulates the cAMP and cGMP relay pulses. Loss of CtnA abolishes detectable CF activity, so aggregation streams fail to break up and cells form abnormally few, very large aggregates and fruiting bodies.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: CtnA is a secreted protein exported via an N-terminal signal peptide, so extracellular localization is correct. This is a core aspect of its function as the diffusible component of counting factor. Reason: The secreted/extracellular location is directly established experimentally and is essential to CtnA's role as an extracellular size-regulating signal. Supporting Evidence: PMID:10444594 The predicted protein, which we have named countin, is hydrophilic, has a potential signal sequence upstream of where we observe the amino terminus of the secreted protein |
| GO:0006971 hypotonic response | IDA PMID:38986731 Trafficking of adhesion and aggregation-modulating proteins ... | UNDECIDED | Summary: This IDA annotation derives from a trafficking study whose full text is not available. The cached abstract describes CtnA localization, glycosylation and secretion pathways but does not mention any hypotonic or osmotic response for CtnA, so the supporting evidence cannot be verified. Reason: Full text is unavailable and the abstract does not cover a hypotonic response. Per policy, an experimental annotation that cannot be verified from the accessible text is left UNDECIDED rather than removed. |
| GO:0031410 cytoplasmic vesicle | IDA PMID:38986731 Trafficking of adhesion and aggregation-modulating proteins ... | ACCEPT | Summary: Before secretion, intracellular CtnA localizes to cytoplasmic vesicles and punctae, as directly shown in the trafficking study. This is a valid subcellular localization reflecting the secretory route of the protein. Reason: The abstract explicitly states CtnA localizes to cytoplasmic vesicles during growth and starvation, supporting this located_in annotation. Supporting Evidence: PMID:38986731 During growth and starvation, CtnA localizes to cytoplasmic vesicles and punctae |
| GO:0140582 adenylate cyclase-activating G protein-coupled cAMP receptor signaling pathway | IDA PMID:37921687 Collective signalling drives rapid jumping between cell stat... | MARK AS OVER ANNOTATED | Summary: This annotation comes from a single-cell transcriptomics study of collective cAMP signalling in which ctnA is one of many genes whose expression responds to cAMP oscillations. The paper does not present CtnA as a component that acts within the adenylate cyclase-activating cAMP receptor pathway; rather ctnA expression is downstream of that signalling. Treating a cAMP-responsive marker gene as involved_in the pathway over-interprets the evidence. Reason: In PMID:37921687 ctnA is a cAMP-regulated expression marker, not a demonstrated participant in the adenylate cyclase-activating cAMP receptor signaling pathway. CtnA's genuine modulation of cAMP relay is captured separately by the annotations from PMID:11371560. |
| GO:0051156 glucose 6-phosphate metabolic process | IMP PMID:16606621 A protein in crude cytosol regulates glucose-6-phosphatase a... | KEEP AS NON CORE | Summary: Counting factor (whose bioactivity resides in countin) represses internal glucose levels by increasing the Km of glucose-6-phosphatase, which hydrolyzes glucose-6-phosphate. This is a genuine downstream effect of CtnA signal transduction rather than a core molecular function of the protein. Reason: The link to glucose-6-phosphate metabolism is a downstream consequence of CtnA signaling on glucose-6-phosphatase, established by IMP, but it is not the core evolved function of the secreted signal. Supporting Evidence: PMID:16606621 The CF signal transduction pathway involves CF-repressing internal glucose levels by increasing the K(m) of glucose-6-phosphatase |
| GO:0001678 intracellular glucose homeostasis | IMP PMID:15643062 Exposure of cells to a cell number-counting factor decreases... | KEEP AS NON CORE | Summary: A brief exposure of cells to recombinant countin decreases intracellular glucose, part of the CF signal transduction pathway that lowers internal glucose. This is a downstream signaling effect, not the core function. Reason: CtnA signaling lowers intracellular glucose (via glucose-6-phosphatase inhibition), a supported downstream effect that is peripheral to the core size-counting function. Supporting Evidence: PMID:15643062 an 8-min exposure of cells to recombinant countin decreases intracellular glucose levels |
| GO:0031157 regulation of aggregate size involved in sorocarp development | IMP PMID:10444594 A cell-counting factor regulating structure size in Dictyost... | ACCEPT | Summary: This is the central, well-established function of CtnA. Disrupting the countin gene abolishes CF secretion so aggregation streams do not break up and cells form huge aggregates and fruiting bodies. This term precisely captures CtnA's core role. Reason: Loss-of-function directly demonstrates that CtnA regulates the size of aggregates during fruiting-body (sorocarp) development; this is the core function of the gene. Supporting Evidence: PMID:10444594 there is no detectable secretion of counting factor, and the aggregation streams do not break up PMID:10444594 it behaves as a complex of polypeptides with an effective molecular mass of 450 kD |
| GO:0042593 glucose homeostasis | IMP PMID:12912898 CF45-1, a secreted protein which participates in Dictyosteli... | KEEP AS NON CORE | Summary: Cells lacking countin have elevated cytosolic glucose, showing CtnA contributes to glucose homeostasis. This is a downstream physiological effect of CtnA signaling rather than its core function. Reason: The glucose homeostasis phenotype of countin-null cells is a supported downstream effect of CF signaling, secondary to the core aggregate-size counting role. Supporting Evidence: PMID:12912898 cells lacking countin or CF50 have higher glucose levels, higher cell-cell adhesion, and lower motilities |
| GO:0040015 negative regulation of multicellular organism growth | IMP PMID:10444594 A cell-counting factor regulating structure size in Dictyost... | KEEP AS NON CORE | Summary: Countin disruption produces considerably larger fruiting bodies, consistent with CtnA limiting the size of the multicellular structure. This overlaps with the more precise aggregate-size term and is a broader restatement of the same size-limiting role. Reason: The larger-structure phenotype supports a size-limiting role, but GO:0031157 (regulation of aggregate size) more precisely captures the core function; this broader growth term is retained as non-core. Supporting Evidence: PMID:10444594 these were considerably larger than those of the parental cells |
| GO:0098727 maintenance of cell number | IDA PMID:11371560 A cell number-counting factor regulates group size in Dictyo... | KEEP AS NON CORE | Summary: Counting factor regulates the number of cells per group/fruiting body. This IDA reflects CtnA's role in setting group cell number, which is closely related to but broader than the specific aggregate-size term, so it is kept as non-core. Reason: CtnA sets the number of cells per group, supporting this annotation, but the size-counting role is more precisely captured by GO:0031157; retained as a non-core aspect. Supporting Evidence: PMID:11371560 A secreted counting factor (CF), regulates the size of Dictyostelium discoideum fruiting bodies in part by regulating cell-cell adhesion |
| GO:0106071 positive regulation of adenylate cyclase-activating G protein-coupled receptor signaling pathway | IMP PMID:11371560 A cell number-counting factor regulates group size in Dictyo... | KEEP AS NON CORE | Summary: A brief exposure of cells to purified CF increases the cAMP-induced cAMP pulse, i.e. it positively modulates the adenylate cyclase-activating cAMP relay. This is a supported downstream signaling effect of CtnA/CF. Reason: CF increases the cAMP-induced cAMP pulse, supporting positive regulation of the adenylate cyclase-activating pathway; this is one of several signaling outputs of CtnA and is non-core relative to size counting. Supporting Evidence: PMID:11371560 A 1-min exposure of cells to purified CF increases the cAMP-induced cAMP pulse |
| GO:0010225 response to UV-C | IDA PMID:25858552 Response of Dictyostelium discoideum to UV-C and involvement... | MARK AS OVER ANNOTATED | Summary: In this study ctnA is one of several developmental marker genes whose mRNA level was measured by RT-PCR after UV-C irradiation, and ctnA expression was markedly reduced. This shows ctnA expression responds to UV-C, not that CtnA mediates the UV-C response; annotating the protein as involved in the UV-C response over-interprets an expression readout. Reason: ctnA is used as a downstream expression marker of UV-C-induced developmental defects; its reduced transcription does not establish a functional role for CtnA in the UV-C response. Supporting Evidence: PMID:25858552 expression of csA and ctnA was markedly reduced |
| GO:0048870 cell motility | IDA PMID:12070154 Cells respond to and bind countin, a component of a multisub... | KEEP AS NON CORE | Summary: Recombinant countin increases cell motility, one of the rapid cellular responses it evokes as a secreted signal. This is a downstream effect of CtnA signaling rather than the protein's own core molecular function. Reason: CtnA increases cell motility as part of its signaling output, supported by direct assay of recombinant countin; this is a non-core downstream effect. Supporting Evidence: PMID:12070154 Recombinant countin increases cell motility, decreases cell-cell adhesion |
| GO:0010754 negative regulation of receptor guanylyl cyclase signaling pathway | IMP PMID:11371560 A cell number-counting factor regulates group size in Dictyo... | KEEP AS NON CORE | Summary: CF slowly down-regulates the cAMP-induced cGMP pulse by inhibiting guanylyl cyclase activity, a supported signaling output of CtnA/CF. This is a downstream regulatory effect and is non-core relative to size counting. Reason: The inhibition of the cAMP-induced cGMP pulse via guanylyl cyclase is a documented CF signaling effect, one of several parallel outputs and not the core function. Supporting Evidence: PMID:11371560 down-regulates the cAMP-induced cGMP pulse by inhibiting guanylyl cyclase |
| GO:0005576 extracellular region | IDA PMID:10444594 A cell-counting factor regulating structure size in Dictyost... | ACCEPT | Summary: Direct evidence establishes that countin is secreted; it is purified from conditioned medium and its secretion is lost on gene disruption. The extracellular location is core to its function as a diffusible signal. Reason: CtnA is directly shown to be secreted into the extracellular medium, a core feature of its role as the diffusible counting-factor signal. Supporting Evidence: PMID:10444594 there is no detectable secretion of counting factor, and the aggregation streams do not break up |
| GO:0007162 negative regulation of cell adhesion | IMP PMID:11090633 A precise group size in Dictyostelium is generated by a cell... | ACCEPT | Summary: Counting factor regulates group size by repressing cell-cell adhesion; high CF (and thus low adhesion) causes streams to break into small groups. This adhesion-repressing activity is a principal mechanism by which CtnA controls group size. Reason: Repression of cell-cell adhesion is a central, well-supported mechanism of CtnA/CF action on group size. Supporting Evidence: PMID:11090633 CF regulates group size by repressing cell-cell adhesion |
| GO:0007162 negative regulation of cell adhesion | IDA PMID:12070154 Cells respond to and bind countin, a component of a multisub... | ACCEPT | Summary: Recombinant countin directly decreases cell-cell adhesion, confirming by direct assay the adhesion-repressing activity inferred from CF. This is a key mechanistic output of CtnA signaling. Reason: Direct assay of recombinant countin shows it decreases cell-cell adhesion, supporting this annotation and the mechanism of group-size control. Supporting Evidence: PMID:12070154 Recombinant countin increases cell motility, decreases cell-cell adhesion |
| GO:0030041 actin filament polymerization | IDA PMID:12070154 Cells respond to and bind countin, a component of a multisub... | KEEP AS NON CORE | Summary: A brief exposure to countin causes an increase in F-actin polymerization, part of the rapid cytoskeletal signal-transduction response it triggers. This is a downstream cellular effect rather than the core function of the secreted protein. Reason: The increase in F-actin polymerization is a rapid downstream response to CtnA signaling, supported by direct assay but peripheral to the core size-counting role. Supporting Evidence: PMID:12070154 causes an increase in F-actin polymerization |
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