Otx (SpOtx) is the single orthodenticle-class (paired/bicoid-type, K50) homeodomain transcription factor of the purple sea urchin. One gene produces an alpha form and several beta forms (otx-beta1/2) by alternative promoter use and splicing; the forms share the homeodomain and C-terminus but differ in their N-termini. Otx is supplied maternally, is transcribed in all cells of the cleaving embryo, and its transcripts later become restricted to endoderm, oral ectoderm and the ciliated band. The protein is initially held in the cytoplasm, entering micromere nuclei precociously at the 16-cell stage and the nuclei of all other cells around the 60-120-cell stage. Otx binds TAATCC/T motifs and acts as a transcriptional activator with N- and C-terminal transactivation domains. In the micromeres it cooperates with nuclear beta-catenin/Tcf to switch on pmar1. In the vegetal-plate endoderm the otx-beta1/2 cis-regulatory module integrates Gatae, Blimp1 (Krox) and Otx inputs as an AND gate, and Otx in turn directly activates blimp1b, gatae, foxa, brachyury and endo16, forming the positive feedback loops that lock down the endoderm regulatory state. In the ectoderm Otx activates the aboral-ectoderm differentiation gene spec2a through multiple Otx sites, and dominant-negative Engrailed-Otx blocks both endoderm and aboral ectoderm formation while Otx overexpression converts the embryo into aboral ectoderm.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000978 RNA polymerase II cis-regulatory region sequence-specific DNA binding | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT inference from the Otx/orthodenticle family (human OTX1/OTX2, CRX, zebrafish otx2). Directly supported for the sea urchin protein: SpOtx binds the TAATCC/T Otx consensus sites in the aboral-ectoderm spec2a RSR enhancer in bandshift assays, and mutagenesis of those sites abolishes enhancer activity, so the protein binds cis-regulatory DNA in a sequence-specific manner. Reason: The phylogenetic inference is corroborated by direct bandshift and site-mutagenesis evidence on a Pol II cis-regulatory element in this species. Supporting Evidence: PMID:8088441 Bandshift analysis revealed that the Otx sites were able to bind SpOtx, suggesting that this transcription factor mediates positive control at these sites. PMID:7875376 Orthodenticle-related proteins bind with high affinity to DNA containing the sequence motif TAATCC/T. |
| GO:0000981 DNA-binding transcription factor activity, RNA polymerase II-specific | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT inference of Pol II transcription factor activity from the orthodenticle family. This is the core molecular function of SpOtx: it has N- and C-terminal transactivation domains, binds Otx sites in the spec2a enhancer and the otx beta1/2 module, and its overexpression effects require the homeodomain and the transactivation domains. The sign (activator) is established, so a more specific NEW row for GO:0001228 is added. Reason: Core function, directly supported by transactivation assays and cis-regulatory reporter analyses in this species. Supporting Evidence: PMID:8625836 These same N-terminal and C-terminal regions were shown to be transactivation domains in a yeast transactivation assay, indicating that the biological effects of overexpressing SpOtx were due to its action as a transcription factor. |
| GO:0000981 DNA-binding transcription factor activity, RNA polymerase II-specific | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the homeobox conserved-site signature (IPR017970). Redundant with the IBA row for the same term but correct for SpOtx, which is an experimentally characterised Pol II transcription factor. Reason: Correct term; corroborated by the experimental literature cited on the IBA row. Supporting Evidence: PMID:8625836 These same N-terminal and C-terminal regions were shown to be transactivation domains in a yeast transactivation assay, indicating that the biological effects of overexpressing SpOtx were due to its action as a transcription factor. |
| GO:0003677 DNA binding | IEA GO_REF:0000002 | ACCEPT | Summary: Domain-level inference from the homeodomain (IPR001356, residues 131-190). Correct but generic: SpOtx binds DNA as a monomer at TAATCC/T motifs, which is already captured more informatively by the GO:0000978 row. Reason: Correct but uninformative parent of the sequence-specific cis-regulatory binding term; kept as a domain-level statement. Supporting Evidence: PMID:8088441 Bandshift analysis revealed that the Otx sites were able to bind SpOtx, suggesting that this transcription factor mediates positive control at these sites. |
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT inference of nuclear activity. Immunostaining shows SpOtx in nuclei of all cells at blastula and pluteus stages, and in micromere nuclei transiently at the 16-cell stage; the protein is cytoplasmic in other early-cleavage blastomeres before its 60-120-cell-stage nuclear translocation, which is recorded as a separate NEW cytoplasm row. Reason: Nuclear localisation of the active transcription factor is directly observed in this species. Supporting Evidence: PMID:7875376 In contrast, SpOtx protein was found in nuclei of all cells at both blastula and pluteus stages. PMID:8952065 SpOtx appears transiently in nuclei of micromeres but not in nuclei of mesomeres and macromeres. |
| GO:0005634 nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: Swiss-Prot subcellular-location mapping. Consistent with the immunolocalisation of SpOtx to nuclei from the 60-120-cell stage onward and to micromere nuclei at the 16-cell stage. Reason: Correct location, directly observed by immunostaining. Supporting Evidence: PMID:8625836 SpOtx was found in the cytoplasm of early cleavage stage embryos and was translocated into nuclei between the 60- and 120-cell stage, coincident with Spec gene activation. |
| GO:0006355 regulation of DNA-templated transcription | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the homeobox signature. Correct but very general; the Pol II-specific IBA row (GO:0006357) and the NEW positive-regulation row (GO:0045944) are more informative statements of the same process. Reason: Correct generic parent term; the specific process is carried by the more informative rows. Supporting Evidence: PMID:8088441 Using site-directed mutagenesis, we showed that three Otx consensus binding sites were responsible for the activity of the enhancer, acting in a non-cooperative manner to yield full activity. |
| GO:0006357 regulation of transcription by RNA polymerase II | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT inference from the family. SpOtx regulates Pol II transcription of spec2a, endo16, blimp1b, gatae, pmar1 and its own beta1/2 transcription unit, as shown by site mutagenesis in reporter constructs and morpholino perturbation. The sign is positive in every characterised case, so a NEW GO:0045944 row is added. Reason: Core process, directly supported by cis-regulatory analyses in this species. Supporting Evidence: PMID:15110718 It requires gatae, otx, and krox inputs, as predicted, and it operates as an "AND" logic processor in that removal of any one of these inputs essentially destroys activity. PMID:11401403 Strongylocentrotus purpuratus Otx (SpOtx) is required simultaneously in sea urchin development for the activation of endo16 in the vegetal plate and for the activation of spec2a in the aboral ectoderm. |
| GO:0001228 DNA-binding transcription activator activity, RNA polymerase II-specific | IDA PMID:8625836 Altering cell fates in sea urchin embryos by overexpressing ... | NEW | Summary: SpOtx is a sequence-specific activator. Its N- and C-terminal regions are transactivation domains in yeast, and the homeodomain plus both regions are required for the overexpression phenotype. Consensus Otx sites fused to a basal promoter are sufficient to activate reporter expression in all cell types; the Otx sites in the spec2a enhancer mediate positive control and their mutation abolishes enhancer activity; the Otx site in the otx beta1/2 module is one of three required activating inputs. Applies to both the alpha and beta forms, which share the homeodomain and C-terminal activation region. Reason: Sign of the activity is established by transactivation assays and by loss-of-activity on mutating Otx sites in Pol II cis-regulatory modules; more informative than the unsigned GO:0000981 rows. Supporting Evidence: PMID:8625836 These same N-terminal and C-terminal regions were shown to be transactivation domains in a yeast transactivation assay, indicating that the biological effects of overexpressing SpOtx were due to its action as a transcription factor. PMID:11401403 We show here that consensus Otx binding sites fused to basal promoters are sufficient to activate CAT reporter gene expression in all cell types, although expression in endomesoderm progenitors is enhanced. PMID:8088441 Bandshift analysis revealed that the Otx sites were able to bind SpOtx, suggesting that this transcription factor mediates positive control at these sites. PMID:15882584 SpOtx, a broadly distributed transcriptional activator |
| GO:0045944 positive regulation of transcription by RNA polymerase II | IDA PMID:15110718 An otx cis-regulatory module: a key node in the sea urchin e... | NEW | Summary: Otx positively regulates transcription of a set of zygotic Pol II genes through identified Otx target sites: its own beta1/2 cis-regulatory module (an AND gate with Gatae and Krox/Blimp1 inputs, where mutation of the Otx site or otx morpholino destroys activity), the aboral ectoderm gene spec2a (three Otx sites in the RSR enhancer), endo16 in the vegetal plate, blimp1b (direct regulation), and the endoderm genes gatae, foxa and brachyury. In micromeres Otx together with beta-catenin/Tcf activates pmar1. Reason: Otx is the gene product performing the activation step (participation test passed); direct cis-regulatory evidence exists for spec2a, endo16 and the otx beta1/2 module. Supporting Evidence: PMID:15110718 For spatial expression in the endoderm, one particular pair of Gata sites is essential and these function synergistically with an adjacent Otx site. PMID:8088441 Using site-directed mutagenesis, we showed that three Otx consensus binding sites were responsible for the activity of the enhancer, acting in a non-cooperative manner to yield full activity. PMID:19895806 An earlier report showed that Otx directly regulates the expression of blimp1b (Smith et al. PMID:18413610 The pmar1 genes are activated by the Ξ²-catenin/Tcf transcription complex plus Otx, i.e., by two of the micromere-specific inputs just enumerated ( 25 , 30 ). |
| GO:0001714 endodermal cell fate specification | IMP PMID:10433832 Requirement of SpOtx in cell fate decisions in the sea urchi... | NEW | Summary: A dominant-negative Engrailed-SpOtx fusion reduces endoderm-specific gene expression and blocks endoderm formation, and activated beta-catenin cannot bypass the block, placing Otx downstream of or parallel to nuclear beta-catenin in endoderm specification. In the GRN, the otx beta1/2 transcription unit is a key endoderm node: it is driven by Gatae, Blimp1 (Krox) and Otx, and Otx directly activates blimp1b, gatae, foxa, brachyury and endo16, forming the positive feedback loops that lock down the endoderm regulatory state. The beta1/2 forms are the isoforms expressed in the vegetal-plate endoderm; UniProt lists no isoform accessions so the isoform field is not populated. Reason: Otx itself performs regulatory steps of endoderm specification (direct activation of endoderm regulatory genes) rather than being merely required upstream; loss-of-function blocks endoderm formation. Comparable direct endoderm regulators (GATA4/6, Sox17 in vertebrates) carry this term. Supporting Evidence: PMID:10433832 We show here that the homeodomain transcription factor SpOtx is required for endoderm and aboral ectoderm formation during sea urchin embryogenesis. PMID:10433832 The Engrailed-SpOtx fusion protein reduced the expression of endoderm- and aboral ectoderm-specific genes and inhibited the formation of endoderm and aboral ectoderm cell types. PMID:17570356 These two genes cross-regulate, generating a positive feedback loop which serves to lock down the state of endoderm specification PMID:19895806 Another key function in the early endoderm regulatory network is provided by the homeodomain transcription factor Otx. |
| GO:0001715 ectodermal cell fate specification | IMP PMID:10433832 Requirement of SpOtx in cell fate decisions in the sea urchi... | NEW | Summary: SpOtx is necessary and sufficient for the aboral ectoderm fate: the Engrailed-SpOtx repressor fusion inhibits aboral ectoderm formation and aboral-ectoderm gene expression; SpOtx mRNA injection converts the embryo into epithelial balls of aboral ectoderm; and SpOtx restores aboral ectoderm gene expression and morphology in C-cadherin embryos lacking nuclear beta-catenin. Mechanistically Otx activates the aboral-ectoderm differentiation gene spec2a through Otx sites, with spatial restriction supplied by repressors (Gsc, GataE) acting at neighbouring elements. Later GRN work also places otx-beta1/2 among the early nodal-independent oral ectoderm genes and the ciliated band, so the generic ectodermal specification term is used rather than an aboral-specific one. Reason: Necessity (dominant negative), sufficiency (overexpression, rescue) and a direct target (spec2a) are all shown for this gene product in the ectoderm. Supporting Evidence: PMID:10433832 Coexpressing SpOtx with C-cadherin restored aboral ectoderm-specific gene expression and aboral ectoderm morphology, but with C-cadherin present, SpOtx was not sufficient for endoderm formation. PMID:8625836 Eggs injected with SpOtx mRNA developed into epithelial balls of aboral ectoderm suggesting that SpOtx redirected nonaboral ectoderm cells to an aboral ectoderm fate. PMID:19268450 These are the early oral ectoderm and later ciliated band genes hnf6 and otxΞ²1/2 |
| GO:0005737 cytoplasm | IDA PMID:8625836 Altering cell fates in sea urchin embryos by overexpressing ... | NEW | Summary: Maternal SpOtx protein is cytoplasmic in early cleavage-stage blastomeres (except the 16-cell-stage micromeres) and is translocated into nuclei between the 60- and 120-cell stage. Cytoplasmic retention is proposed to be mediated by an interaction between a proline-rich SH3-binding-like region of SpOtx and alpha-actinin. This is a regulated, stage-specific location rather than the site of transcriptional activity. Reason: Directly observed by immunostaining in this species; records the developmentally regulated nuclear import that gates Otx activity. Non-core location. Supporting Evidence: PMID:8625836 SpOtx was found in the cytoplasm of early cleavage stage embryos and was translocated into nuclei between the 60- and 120-cell stage, coincident with Spec gene activation. PMID:8952065 A yeast two-hybrid analysis showed a specific interaction between the proline-rich region of SpOtx and a putative SH3 domain of the sea urchin alpha-actinin. |
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Download this section (compressed HTML)Q: Which otx isoform (alpha, beta1 or beta2) does Swiss-Prot Q26417 correspond to, and should UniProt add isoform records so that isoform-specific annotations can be made?
Q: Does Otx bind the pmar1 cis-regulatory region directly, and does knockdown of the alpha form alone impair skeletogenic micromere specification?
Q: Is the aboral-ectoderm role of Otx (Klein lab, 1994-1999) carried by the alpha form while the beta1/2 forms act in oral ectoderm/ciliated band and endoderm, as the expression data suggest?
Experiment: Isoform-specific perturbation with translation-blocking morpholinos against the unique alpha and beta N-terminal exons, followed by nanostring/QPCR of the endoderm (blimp1b, gatae, foxa, brachyury, endo16) and ectoderm (spec2a, hnf6, gsc) gene batteries.
Hypothesis: The beta1/2 forms carry the endoderm GRN feedback function whereas the alpha form carries the early micromere and aboral ectoderm functions.
Experiment: ChIP or DNA-affinity assays with tagged Otx on the pmar1, blimp1b, gatae, foxa and brachyury cis-regulatory modules, with mutation of candidate TAATCC/T sites in reporter constructs.
Hypothesis: Otx occupies TAATCC/T sites in these modules and its input is direct.
What is not known β curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: UniProt Q26417 carries no isoform accessions although the otx gene produces an alpha form and independently regulated beta1 and beta2 transcripts; it is not established which form Q26417 represents, and isoform-specific perturbation (the beta1/2 module drives the endoderm GRN role, the alpha form the early ubiquitous/micromere role) cannot be recorded in the isoform field.
OPEN BIOLOGY
Provenance (the field's own admissions):
Gap: Whether Otx binds the pmar1 cis-regulatory module directly, and whether Otx loss of function affects micromere specification, has not been shown; the Otx input to pmar1 rests on the GRN model and precocious micromere nuclearization.
OPEN BIOLOGY
Provenance (the field's own admissions):
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