samsn1b (SAM domain, SH3 domain and nuclear localization signals 1b) is one of two zebrafish co-orthologs of vertebrate SAMSN1 (HACS1, SLy2, NASH1); its paralog is samsn1a. It encodes an intracellular adaptor of the SLy/SASH1 family with an N-terminal disordered region, a central SH3 domain and a C-terminal SAM domain. In mammals SAMSN1 is expressed mainly in hematopoietic cells, is found in cytoplasm and nucleus, is induced by B-cell activation signals, binds tyrosine-phosphorylated proteins and, through its SH3 domain, the inhibitory receptor PIRB, and acts as an inhibitory adaptor that dampens B-cell activation (mouse knockout B cells are hyper-responsive). The zebrafish protein keeps a conserved SH3 domain (77% identical to human SAMSN1) but is much longer than human SAMSN1 and only 38% identical to samsn1a overall. samsn1b has a brief gastrula expression peak and is otherwise mainly larval and adult; in situ hybridization detects it in the pineal organ (epiphysis) and the retinal photoreceptor layer, and bulk RNA-seq places it highest in retina and granulocytes, with lower calls in spleen and head kidney. No functional study of the zebrafish gene has been published.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003674 molecular_function | ND GO_REF:0000015 | ACCEPT | Summary: Root-term ND placeholder recording that no molecular function has been curated for samsn1b. Reason: Accurate. No molecular activity has been measured for samsn1b, and the mammalian ortholog is characterized only by its binding partners (tyrosine-phosphorylated proteins, PIRB). The paralog samsn1a lacks this ND row only because of how the two records were processed; the asymmetry is a bookkeeping artefact. Supporting Evidence: PMID:15381729 HACS1 associates with tyrosine-phosphorylated proteins after B cell activation and binds in vitro to the inhibitory molecule paired Ig-like receptor B. |
| GO:0005575 cellular_component | ND GO_REF:0000015 | ACCEPT | Summary: Root-term ND placeholder for cellular component. Reason: Accurate. No localization has been determined for the zebrafish protein. Mammalian SAMSN1 is mainly cytoplasmic with some nuclear signal, but no location term has been propagated to either zebrafish copy, so the placeholder reflects the state of annotation. Supporting Evidence: PMID:11536050 Immunostaining and cellular fractionation studies localized the HACS1 protein predominantly to the cytoplasm. |
| GO:0008150 biological_process | ND GO_REF:0000015 | ACCEPT | Summary: Root-term ND placeholder for biological process. Reason: Accurate record that no process has been curated from zebrafish data. The process rows that samsn1b does carry are phylogenetic (IBA) inferences, so the ND placeholder still describes the absence of direct evidence. The gene's own literature-search record finds no study of this gene product in this species that identifies a biological process. Supporting Evidence: file:DANRE/samsn1a/samsn1a-bioinformatics/RESULTS.md samsn1a is the zygotic, larval and hematopoietic copy; samsn1b has a small gastrula peak and curated pineal and photoreceptor in situ expression. file:DANRE/samsn1b/samsn1b-notes.md its text beyond gene lists; no functional study exists. |
| GO:0050869 negative regulation of B cell activation | IBA GO_REF:0000033 | ACCEPT | Summary: Negative regulation of B cell activation, propagated from the SAMSN1 node PTN002642349, whose experimental seed is the mouse Hacs1 (Samsn1) knockout. Reason: The IBA places the function at a node that contains both zebrafish SAMSN1 co-orthologs. The seed evidence is sound (Hacs1-/- B cells hyper-proliferate on BCR stimulation and show increased tyrosine phosphorylation), and the SH3 domain that mediates the PIRB interaction is 77% identical between samsn1b and human SAMSN1. Although the curated in situ data for samsn1b are pineal and photoreceptor, bulk RNA-seq detects samsn1b in granulocytes, spleen and head kidney, so there is no evidence that this copy has left immune cells. The function has not been tested in zebrafish; the annotation is accepted as a phylogenetic inference, and the same call is made for samsn1a. If paralog-resolved single-cell data later show samsn1b absent from lymphocytes, this row should be revisited. Supporting Evidence: PMID:19923443 Purified splenic B cells from Hacs1(-/-) mice showed increased cell proliferation on BCR (B-cell receptor) stimulation. PMID:19923443 This study suggests that Hacs1 is an immunoinhibitory adaptor that might be a useful target for immune suppression therapy. file:DANRE/samsn1a/samsn1a-bioinformatics/RESULTS.md The SH3 domain is the best-conserved part of both copies |
| GO:1902531 regulation of intracellular signal transduction | IBA GO_REF:0000033 | ACCEPT | Summary: Regulation of intracellular signal transduction, propagated from the SLy/SASH1 family node PTN000266438 (seeded by human SASH1). Reason: A broad but appropriate process for an SH3-SAM adaptor of the SLy/SASH1 family. The mammalian family members act as intracellular signalling adaptors, and SAMSN1 itself associates with tyrosine-phosphorylated proteins after B-cell receptor stimulation. samsn1b keeps the SH3 and SAM domains that define the family. The term is general, but no more specific process is established across the family. Supporting Evidence: PMID:33710696 Intracellular adaptor proteins are indispensable for the transduction of receptor-derived signals, as they recruit and connect essential downstream effectors. PMID:15381729 HACS1 associates with tyrosine-phosphorylated proteins after B cell activation and binds in vitro to the inhibitory molecule paired Ig-like receptor B. |
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Download this section (compressed HTML)Q: Is samsn1b expressed in any zebrafish lymphocyte or myeloid population at the single-cell level, or only in retina, pineal and granulocyte-rich bulk samples?
Q: Does gar SAMSN1 show pineal or photoreceptor expression, which would make the samsn1b domain ancestral rather than new?
Experiment: Paralog-specific HCR in situ for samsn1a and samsn1b in larval retina, pineal, kidney marrow and circulating blood, together with gar SAMSN1 in situ on gar larvae.
Hypothesis: samsn1b is a photoreceptor and pineal copy, while samsn1a is the blood-cell copy.
Type: expression analysis
Experiment: Compare single and double mutants (RNA-less alleles where possible) for adult B-cell responses and larval myeloid inflammatory responses; check transcriptional adaptation of the paralog in each single mutant.
Hypothesis: samsn1b contributes to immune regulation redundantly with samsn1a.
Type: genetic interaction
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