CACYBP (Calcyclin-Binding Protein; also known as SIP, Siah-Interacting Protein, and S100A6-binding protein) is a small nucleocytoplasmic adaptor protein that acts as a molecular bridge in a Siah1/Siah2-based, SKP1-containing E3 ubiquitin ligase complex. Through an N-terminal dimerization domain that binds Siah1 and a C-terminal domain that binds SKP1, it scaffolds substrate and the E2 enzyme into apposition, enabling ubiquitination and proteasomal degradation of target proteins, most notably beta-catenin (CTNNB1) in a p53-responsive pathway. CACYBP binds proteins of the S100 family (calcyclin/S100A6, S100A1, S100B, S100P, S100A12) in a calcium-dependent manner, linking calcium signaling to this ubiquitination machinery, and it forms homodimers. It is found in the cytoplasm at low calcium and redistributes between cytoplasm and nucleus upon calcium increase and certain stimuli.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005634 nucleus | IBA GO_REF:0000033 | ACCEPT | Summary: Nuclear localization is supported; CACYBP redistributes to the nucleus upon calcium increase and stimulation, consistent with its role in degradation of nuclear/cytoplasmic substrates. Reason: CACYBP is documented in the nucleus and cytoplasm, with calcium-dependent nuclear redistribution. Supporting Evidence: file:human/CACYBP/CACYBP-uniprot.txt it localizes in both the nucleus and cytoplasm |
| GO:0019005 SCF ubiquitin ligase complex | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: The Siah1-SIP-Skp1-Ebi complex CACYBP participates in is a SIAH1 RING-finger E3 ligase, not a canonical SCF (SKP1-Cullin1-F-box) complex - it shares SKP1 and an F-box subunit but lacks the defining cullin scaffold. The specific complex term GO:0030877 (beta-catenin destruction complex) is already accepted for this gene with IDA evidence, so the SCF complex assignment is a phylogenetic over-transfer from SKP1/F-box component sharing. Reason: The Siah1-based E3 complex is non-cullin and therefore not an SCF complex as defined by GO:0019005, even though it shares SKP1 and an F-box subunit. The more specific GO:0030877 (beta-catenin destruction complex) annotation already captures the actual complex membership accurately. Supporting Evidence: PMID:16085652 The E3 complex comprises, in addition to Siah1, Siah-interacting protein (SIP), the adaptor protein Skp1, and the F-box protein Ebi |
| GO:0031625 ubiquitin protein ligase binding | IBA GO_REF:0000033 | ACCEPT | Summary: CACYBP binds the RING E3 ligases SIAH1/SIAH2 directly, a core molecular function underlying its bridging role. Reason: Direct interaction of CACYBP/SIP with Siah1 is structurally characterized and is central to assembly of the beta-catenin-destruction E3 complex. Supporting Evidence: PMID:16085652 SIP engages Siah1 by means of two elements |
| GO:0060090 molecular adaptor activity | IBA GO_REF:0000033 | ACCEPT | Summary: CACYBP serves as a molecular bridge/adaptor in ubiquitin E3 complexes, bringing substrate and E2 into apposition; this is its core molecular function. Reason: CACYBP scaffolds Siah1 and Skp1 and provides the surface that brings substrate and the E2 enzyme together. Supporting Evidence: PMID:16085652 this surface provides the scaffold for bringing substrate and the E2 enzyme into apposition in the functional complex |
| GO:0007507 heart development | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Heart development is a broad developmental process not supported by direct mechanistic evidence for human CACYBP; the protein's documented role is as an E3 ligase adaptor. Reason: No direct evidence links CACYBP to heart development as a core function; this appears to be a phylogenetic/orthology-based broad transfer. |
| GO:0005634 nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: Nuclear localization is supported by direct subcellular localization data. Reason: CACYBP localizes to the nucleus, especially after calcium increase. Supporting Evidence: file:human/CACYBP/CACYBP-uniprot.txt it localizes in both the nucleus and cytoplasm |
| GO:0005737 cytoplasm | IEA GO_REF:0000120 | ACCEPT | Summary: Cytoplasmic localization is well supported; CACYBP is cytoplasmic at low calcium concentrations. Reason: Direct localization data place CACYBP in the cytoplasm. Supporting Evidence: file:human/CACYBP/CACYBP-uniprot.txt Cytoplasmic at low calcium |
| GO:0015631 tubulin binding | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: Tubulin binding is an InterPro-based electronic inference without direct human experimental support; cytoskeletal roles are reported but not as a clear core molecular function. Reason: No direct experimental evidence in the cached literature supports tubulin binding as a core CACYBP function; it is an automated domain-based transfer. |
| GO:0031625 ubiquitin protein ligase binding | IEA GO_REF:0000120 | ACCEPT | Summary: CACYBP binds the E3 ligase Siah1 directly; core molecular function (consistent with the IBA annotation). Reason: Direct Siah1 interaction is structurally established. Supporting Evidence: PMID:16085652 SIP engages Siah1 by means of two elements |
| GO:0044548 S100 protein binding | IEA GO_REF:0000002 | ACCEPT | Summary: Calcium-dependent binding to S100 family proteins (calcyclin/S100A6 and others) is the basis of the protein's name and a core molecular function linking calcium signaling to its ubiquitination machinery. Reason: CACYBP interacts with multiple S100 proteins in a calcium-dependent manner. Supporting Evidence: file:human/CACYBP/CACYBP-uniprot.txt Interacts with proteins of the S100 family S100A1, S100A6, S100B, S100P and S100A12 in a calcium-dependent manner |
| GO:0005515 protein binding | IPI PMID:25036637 A quantitative chaperone interaction network reveals the arc... | MARK AS OVER ANNOTATED | Summary: Generic protein binding from a high-throughput chaperone interaction (LUMIER) network; uninformative. Reason: Bare protein binding does not convey a specific CACYBP molecular function. |
| GO:0005515 protein binding | IPI PMID:31837246 High-throughput competitive fluorescence polarization assay ... | MARK AS OVER ANNOTATED | Summary: The underlying data are S100 family interaction profiling, but the GO term used is generic protein binding; S100 protein binding is the informative term. Reason: Bare protein binding is uninformative; the specific S100 binding is captured by GO:0044548. |
| GO:0005515 protein binding | IPI PMID:31980649 Extensive rewiring of the EGFR network in colorectal cancer ... | MARK AS OVER ANNOTATED | Summary: Generic protein binding from an EGFR-network interactome study; uninformative. Reason: Bare protein binding from a high-throughput screen does not identify a core CACYBP function. |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | MARK AS OVER ANNOTATED | Summary: Generic protein binding from the BioPlex proteome-scale interactome; uninformative. Reason: Bare protein binding is too general to represent CACYBP function. |
| GO:0005515 protein binding | IPI PMID:36115835 Quantitative fragmentomics allow affinity mapping of interac... | MARK AS OVER ANNOTATED | Summary: Generic protein binding from a PDZ-affinity fragmentomics interactome; uninformative. Reason: Bare protein binding does not convey a specific CACYBP molecular function. |
| GO:0005641 nuclear envelope lumen | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: Nuclear envelope lumen is an orthology-transferred localization with no direct support; inconsistent with the documented nucleoplasmic/cytoplasmic distribution. Reason: An automated Ensembl Compara transfer not supported by direct human localization evidence. |
| GO:0007507 heart development | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: Heart development is a broad orthology-transferred process without direct mechanistic support for human CACYBP. Reason: Automated transfer; CACYBP's core role is as an E3 ligase adaptor, not a defined cardiac developmental factor. |
| GO:0019005 SCF ubiquitin ligase complex | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: As for the IBA SCF complex annotation above, this is a non-cullin Siah1-based RING E3 ligase that shares SKP1/F-box subunits with SCF complexes but is not itself an SCF complex (no cullin scaffold). The specific GO:0030877 (beta-catenin destruction complex) is the accurate term. Reason: Same rationale as the IBA SCF annotation - the Siah1 E3 complex is non-cullin and therefore not an SCF complex as defined by GO:0019005. Supporting Evidence: PMID:16085652 the adaptor protein Skp1, and the F-box protein Ebi |
| GO:0019904 protein domain specific binding | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: Protein domain specific binding is a broad orthology-transferred molecular function without specific support; more informative terms (Siah1 binding, S100 binding) are captured elsewhere. Reason: Too general and automatically transferred; superseded by the specific binding terms. |
| GO:0044297 cell body | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: Cell body is an orthology-transferred neuronal localization without direct human support. Reason: Automated Ensembl Compara transfer not supported by direct evidence for human CACYBP. |
| GO:0045740 positive regulation of DNA replication | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: Positive regulation of DNA replication is a broad orthology-transferred process not supported by direct mechanistic evidence for human CACYBP. Reason: Automated transfer; not a documented core function. |
| GO:0055007 cardiac muscle cell differentiation | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: Cardiac muscle cell differentiation is an orthology-transferred developmental process without direct support for human CACYBP. Reason: Automated transfer; not a documented core function. |
| GO:0060416 response to growth hormone | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: Response to growth hormone is an orthology-transferred process without direct mechanistic support for human CACYBP. Reason: Automated transfer; not a documented core function. |
| GO:0071277 cellular response to calcium ion | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Cellular response to calcium ion is consistent with CACYBP's calcium-dependent S100 binding and calcium-regulated localization, but the term is broad and the annotation is an orthology transfer. Reason: The calcium-dependence of CACYBP function is real, but this broad process term is best retained as non-core relative to its E3-adaptor molecular role. Supporting Evidence: file:human/CACYBP/CACYBP-uniprot.txt in a calcium-dependent manner |
| GO:0005654 nucleoplasm | IDA GO_REF:0000052 | ACCEPT | Summary: Nucleoplasmic localization is supported by direct immunofluorescence and the documented nuclear redistribution. Reason: CACYBP localizes to the nucleus, consistent with nucleoplasmic detection. Supporting Evidence: file:human/CACYBP/CACYBP-uniprot.txt it localizes in both the nucleus and cytoplasm |
| GO:0005829 cytosol | IDA GO_REF:0000052 | ACCEPT | Summary: Cytosolic localization is well supported; CACYBP is cytoplasmic at low calcium. Reason: Direct localization data place CACYBP in the cytosol. Supporting Evidence: file:human/CACYBP/CACYBP-uniprot.txt Cytoplasmic at low calcium |
| GO:0070062 extracellular exosome | HDA PMID:20458337 MHC class II-associated proteins in B-cell exosomes and pote... | MARK AS OVER ANNOTATED | Summary: High-throughput exosome proteomics localization; not a functionally meaningful localization for a nucleocytoplasmic E3-adaptor protein. Reason: Mass-spectrometry co-purification hit without evidence for a functional extracellular/exosomal role. |
| GO:0005515 protein binding | IPI PMID:16085652 Structural analysis of Siah1-Siah-interacting protein intera... | MARK AS OVER ANNOTATED | Summary: The underlying evidence is the specific CACYBP/SIP-Siah1 and CACYBP-Skp1 interactions; the generic protein binding term is uninformative relative to those specific terms. Reason: Bare protein binding is too general; the informative interactions (Siah1/ubiquitin protein ligase binding, adaptor activity) are captured by other terms. |
| GO:0030877 beta-catenin destruction complex | IDA PMID:16085652 Structural analysis of Siah1-Siah-interacting protein intera... | ACCEPT | Summary: CACYBP/SIP is a component of the Siah1-based complex that targets beta-catenin for degradation; core function. Reason: CACYBP is directly shown to be part of the multiprotein E3 complex that destroys beta-catenin in response to p53. Supporting Evidence: PMID:16085652 a multiprotein E3 ubiquitin ligase complex that targets beta-catenin for destruction in response to p53 activation |
| GO:0042803 protein homodimerization activity | IPI PMID:16085652 Structural analysis of Siah1-Siah-interacting protein intera... | ACCEPT | Summary: CACYBP/SIP forms a homodimer via its N-terminal dimerization domain, which is required for Siah1 binding and beta-catenin destruction; core molecular property. Reason: The N-terminal dimerization domain of SIP is structurally characterized and functionally required. Supporting Evidence: PMID:16085652 An N-terminal dimerization domain of SIP sits across the saddle-shaped upper surface of Siah1 |
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Download this section (compressed HTML)Q: Which substrates beyond beta-catenin are degraded through the Siah1-CACYBP/SIP-SKP1 E3 complex, and how does S100/calcium binding modulate substrate selection or complex assembly?
Q: Are the orthology-transferred developmental annotations (heart development, cardiac muscle cell differentiation) supported by any direct mechanistic role, or are they purely consequences of the E3-adaptor function in particular tissues?
Q: Does CACYBP/SIP have a bona fide protein phosphatase activity toward ERK1/2 (and possibly p38/tau), as suggested by review-level reports, and is this activity regulated by PKC (Ser22/Thr23), CKII (Thr184), and Ca2+/S100A6 binding? This would represent a molecular function distinct from its E3-ligase adaptor role and needs primary biochemical confirmation.
Q: What is the functional consequence of SUMOylation at Lys16 (via Ubc9) for CACYBP/SIP, given the atypical cytoplasmic enrichment of the SUMO-conjugated form, and does it modulate E3-complex assembly, localization, or the proposed phosphatase activity?
Experiment: Reconstitute the Siah1-CACYBP-Skp1-Ebi E3 complex in vitro and test whether calcium-loaded S100A6 binding to CACYBP enhances or inhibits beta-catenin ubiquitination, to define the calcium-to-ubiquitination coupling.
Experiment: Perform quantitative degradomics in CACYBP-knockout vs wild-type cells under calcium stimulation to identify the substrate repertoire dependent on CACYBP.
Experiment: Test for direct, purified-protein phosphatase activity of CACYBP/SIP toward phospho-ERK1/2 in vitro, and determine whether PKC/CKII phosphorylation or Ca2+/S100A6 binding modulate this activity, to confirm or refute the proposed ERK1/2 phosphatase function independent of the E3-adaptor role.
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