CACUL1 (CDK2-associated and cullin domain-containing protein 1; also known as CAC1 / C10orf46) is a poorly characterized 369-amino-acid human protein containing a single cullin-repeat-like domain with disordered N- and C-terminal regions. Despite its name and cullin-fold homology, it is much shorter than canonical cullins (~750-900 aa) and there is no experimental evidence that it nucleates a functional cullin-RING ubiquitin ligase or carries a neddylation/RBX module. The best-supported activity, from a single study, is physical association with the cyclin-dependent kinase CDK2 and promotion of CDK2 kinase activity, consistent with a role in G1/S cell-cycle progression and proliferation; the protein is highly expressed in cancer tissues and cell lines, and its abundance varies across the cell cycle. CACUL1 also acts as a nuclear-receptor transcriptional corepressor: it is a SIRT1-interacting protein that binds PPARgamma (and estrogen receptor alpha) and represses their transcription by coordinating SIRT1 and the histone demethylase LSD1/KDM1A at target promoters (lowering H3K9 acetylation and raising H3K9 methylation), thereby suppressing adipocyte differentiation. Separately, although CACUL1 does not itself nucleate a cullin-RING ligase, it binds the Cul3-Keap1-Rbx1 (CRL3-Keap1) ubiquitin-ligase complex and attenuates its ubiquitination of the transcription factor Nrf2 (NFE2L2), stabilizing Nrf2 and promoting the antioxidant response. CACUL1 has also been recovered as a yeast two-hybrid interactor of ARMC5. Overall, CACUL1 is a multifunctional protein linking CDK2-associated cell-cycle progression, nuclear-receptor corepression, and negative regulation of Nrf2 ubiquitination.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000082 G1/S transition of mitotic cell cycle | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic (IBA) transfer of a G1/S cell-cycle role. This is consistent with the direct experimental evidence in the same gene (PMID:19829063), where CACUL1/CAC1 knockdown induces G1/S arrest. Retained but as a non-core/secondary process annotation since the defining molecular activity is the CDK2 interaction rather than the broader cell-cycle process itself. Reason: Supported indirectly by direct knockdown evidence (PMID:19829063) that CACUL1 loss causes G1/S arrest, but this is a downstream process rather than a precise molecular function; treat as a non-core involvement. Supporting Evidence: PMID:19829063 Knockdown of CAC1 by RNAi inhibits cell proliferation and induces G(1)/S arrest. |
| GO:0019901 protein kinase binding | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) transfer of protein kinase binding. This is the same molecular function directly demonstrated for CACUL1 via its physical interaction with CDK2 (PMID:19829063). Accept as it reflects the best-supported molecular activity of the protein. Reason: Directly corroborated by experimental CDK2 binding (PMID:19829063); the IBA call coincides with the strongest molecular evidence for this protein. Supporting Evidence: PMID:19829063 this protein is physically associated with CDK2 |
| GO:0006511 ubiquitin-dependent protein catabolic process | IEA GO_REF:0000002 | MODIFY | Summary: The IEA InterPro2GO transfer (cullin-repeat domain IPR001373) implies direct participation in ubiquitin-dependent catabolism, which remains unsupported: CACUL1 is too short to be a CRL scaffold and carries no RBX/neddylation module. However, Kigoshi et al. 2015 (PMID:26238671) show CACUL1 binds the Cul3-Keap1-Rbx1 ubiquitin-ligase complex and ATTENUATES Nrf2 ubiquitination, stabilizing Nrf2. The experimentally supported role is therefore NEGATIVE regulation of (Nrf2) ubiquitination, not direct participation in catabolism; modify accordingly. Reason: Direct participation in ubiquitin-dependent catabolism is a domain-only over-annotation, but CACUL1 has experimentally demonstrated activity as a negative regulator of Nrf2 ubiquitination via the Cul3-Keap1-Rbx1 complex (PMID:26238671); replace with negative regulation of protein ubiquitination. Proposed replacements: negative regulation of protein ubiquitination Supporting Evidence: PMID:26238671 CACUL1 attenuates Nrf2 ubiquitination PMID:26238671 CACUL1 stabilizes Nrf2 by regulating Nrf2 ubiquitination |
| GO:0031625 ubiquitin protein ligase binding | IEA GO_REF:0000002 | ACCEPT | Summary: Although the IEA call was originally a pure cullin-domain (IPR001373) transfer, it is now experimentally supported: Kigoshi et al. 2015 (PMID:26238671) demonstrate that CACUL1 binds Keap1, Cul3 and Rbx1 - the components of the ubiquitin ligase (CRL3) responsible for Nrf2 ubiquitination - and thereby modulates that ligase's activity toward Nrf2. The ubiquitin-protein-ligase-binding molecular function is therefore retained with direct experimental support. Reason: Experimentally demonstrated binding to the Cul3-Keap1-Rbx1 ubiquitin ligase complex (PMID:26238671) directly supports this molecular function; it is no longer a domain-only over-annotation. Supporting Evidence: PMID:26238671 we further assessed CACUL1 binding with Keap1, Cul3 and Rbx1, the components of the Ubiquitin ligase responsible for Nrf2 ubiquitination. |
| GO:0005515 protein binding | IPI PMID:23178685 Negative regulation of ERΞ± by a novel protein CAC1 through a... | MODIFY | Summary: This IPI annotation (WITH ESR1/P03372) records the physical interaction between CACUL1/CAC1 and estrogen receptor alpha (ERalpha), corroborated by IntAct (NbExp=5). The bare "protein binding" term is uninformative; a more specific molecular function is warranted given the established, directional ERalpha interaction. Reason: The interaction partner is a specific nuclear receptor (ERalpha); replace the uninformative protein binding term with nuclear estrogen receptor binding to capture the actual molecular function. Proposed replacements: nuclear estrogen receptor binding Supporting Evidence: PMID:23178685 The CoRNR box of CAC1 was required for the binding to and inactivation of ERΞ±. |
| GO:0005515 protein binding | IPI PMID:28169274 Armc5 deletion causes developmental defects and compromises ... | MARK AS OVER ANNOTATED | Summary: This IPI annotation (WITH ARMC5/Q96C12) derives from a yeast two-hybrid screen in which CACUL1 was one of 16 ARMC5-binding partners. The paper characterizes ARMC5, not CACUL1, and provides no functional context for CACUL1. The bare "protein binding" term is uninformative and rests on a single high-throughput Y2H datum. Reason: Uninformative bare protein binding from a single Y2H hit in an ARMC5-focused study; no specific function or biology for CACUL1 is established. Supporting Evidence: PMID:28169274 Yeast 2-hybrid assays identify 16 ARMC5-binding partners. |
| GO:0000082 G1/S transition of mitotic cell cycle | IMP PMID:19829063 Identification and characterization of CAC1 as a novel CDK2-... | KEEP AS NON CORE | Summary: Direct experimental evidence: RNAi knockdown of CACUL1/CAC1 induces G1/S arrest, indicating involvement in the G1/S transition. This is a genuine process annotation but downstream of the protein's molecular role in activating CDK2; retained as a non-core involvement. Reason: Well-supported by knockdown phenotype, but the broad cell-cycle process is secondary to the precise CDK2-activating molecular function. Supporting Evidence: PMID:19829063 Knockdown of CAC1 by RNAi inhibits cell proliferation and induces G(1)/S arrest. |
| GO:0008284 positive regulation of cell population proliferation | IMP PMID:19829063 Identification and characterization of CAC1 as a novel CDK2-... | KEEP AS NON CORE | Summary: Direct experimental evidence: CACUL1/CAC1 knockdown inhibits cell proliferation, supporting a positive role in proliferation. This is a real but downstream phenotypic process annotation; kept as non-core relative to the underlying CDK2-activating molecular function. Reason: Supported by knockdown phenotype but is a broad downstream process rather than the precise molecular activity. Supporting Evidence: PMID:19829063 Knockdown of CAC1 by RNAi inhibits cell proliferation and induces G(1)/S arrest. |
| GO:0019901 protein kinase binding | IPI PMID:19829063 Identification and characterization of CAC1 as a novel CDK2-... | ACCEPT | Summary: Direct experimental evidence (IPI, WITH CDK2/P24941) for physical association of CACUL1/CAC1 with the cyclin-dependent kinase CDK2. This is the best-supported molecular function of the protein and represents a core activity. Reason: Directly demonstrated CDK2 binding; an informative molecular-function term reflecting the protein's defining characterized interaction. Supporting Evidence: PMID:19829063 this protein is physically associated with CDK2 |
| GO:0045860 positive regulation of protein kinase activity | IMP PMID:19829063 Identification and characterization of CAC1 as a novel CDK2-... | ACCEPT | Summary: Direct experimental evidence that CACUL1/CAC1 promotes CDK2 kinase activity. This captures the functional consequence of the CDK2 interaction and is a core aspect of the protein's characterized activity, though the molecular mechanism of activation remains undefined. Reason: Supported by experiments showing CAC1 promotes CDK2 kinase activity; a well-evidenced functional annotation. Supporting Evidence: PMID:19829063 CAC1 interacts with CDK2 and promotes the kinase activity of CDK2 protein |
| GO:0003714 transcription corepressor activity | IDA PMID:29233982 CACUL1 reciprocally regulates SIRT1 and LSD1 to repress PPAR... | NEW | Summary: Proposed NEW molecular-function annotation (not in current GOA). CACUL1 represses PPARgamma- (PMID:29233982) and ERalpha- (PMID:23178685) dependent transcription by coordinating SIRT1 and LSD1 at target promoters, i.e. it acts as a nuclear-receptor transcription corepressor. This is the informative MF underlying the existing bare protein-binding / nuclear estrogen receptor binding annotations. Reason: Two independent studies show CACUL1 physically binds nuclear receptors (PPARgamma, ERalpha) and represses their transcriptional activity via SIRT1/LSD1; transcription corepressor activity captures this MF. Supporting Evidence: PMID:29233982 CACUL1, identified as a novel SIRT1 interacting protein, physically interacts with PPARΞ³ and represses its transcriptional activity, suppressing adipocyte differentiation PMID:23178685 CAC1, associated with LSD1, functions as an ERΞ± corepressor |
| GO:0042975 peroxisome proliferator activated receptor binding | IPI PMID:29233982 CACUL1 reciprocally regulates SIRT1 and LSD1 to repress PPAR... | NEW | Summary: Proposed NEW molecular-function annotation (not in current GOA). CACUL1 physically interacts with PPARgamma, providing the receptor-binding basis for its corepressor activity at PPARgamma-responsive promoters. Reason: Direct physical interaction of CACUL1 with PPARgamma demonstrated (PMID:29233982); a specific, informative nuclear-receptor-binding MF. Supporting Evidence: PMID:29233982 CACUL1, identified as a novel SIRT1 interacting protein, physically interacts with PPARΞ³ and represses its transcriptional activity, suppressing adipocyte differentiation |
| GO:0031397 negative regulation of protein ubiquitination | IMP PMID:26238671 CACUL1/CAC1 Regulates the Antioxidant Response by Stabilizin... | NEW | Summary: Proposed NEW process annotation (not in current GOA), also offered as the MODIFY replacement for the domain-only GO:0006511 IEA. CACUL1 binds the Cul3-Keap1-Rbx1 complex and attenuates Nrf2 ubiquitination, stabilizing Nrf2 - a negative regulatory role in protein ubiquitination. Reason: Experimentally demonstrated attenuation of Nrf2 ubiquitination by CACUL1 (PMID:26238671) supports negative regulation of protein ubiquitination. Supporting Evidence: PMID:26238671 CACUL1 attenuates Nrf2 ubiquitination PMID:26238671 CACUL1 stabilizes Nrf2 by regulating Nrf2 ubiquitination file:human/CACUL1/CACUL1-deep-research-falcon.md CACUL1 associates with the Cul3-Keap1-Rbx1 ubiquitin ligase complex |
| GO:0050821 protein stabilization | IMP PMID:26238671 CACUL1/CAC1 Regulates the Antioxidant Response by Stabilizin... | NEW | Summary: Proposed NEW process annotation (not in current GOA). By attenuating its ubiquitination, CACUL1 stabilizes the transcription factor Nrf2 (half-life ~10 -> ~30 min), the downstream consequence of its CRL3-Keap1 modulation. Reason: CACUL1 lengthens Nrf2 half-life by limiting its ubiquitination (PMID:26238671), consistent with a protein-stabilization role. Supporting Evidence: PMID:26238671 CACUL1 stabilizes Nrf2 by regulating Nrf2 ubiquitination |
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Download this section (compressed HTML)Q: Does CACUL1 act as a bona fide CDK2 regulator through a direct structural/allosteric mechanism, or indirectly via cyclin or CDK-inhibitor levels?
Q: Is the nuclear-receptor co-regulatory activity of CACUL1 (ERalpha repression via LSD1, and the separately reported androgen-receptor co-regulation) physiologically significant, and how is its directionality (corepressor vs coactivator) determined?
Q: CACUL1 binds the Cul3-Keap1-Rbx1 (CRL3) complex and negatively regulates Nrf2 ubiquitination (PMID:26238671) without nucleating its own ligase - does its cullin-repeat-like domain mediate this CRL3 association, and is the regulation Nrf2-specific or a more general modulation of CRL3-Keap1 substrate ubiquitination?
Experiment: In vitro reconstituted CDK2 kinase assays with purified recombinant CACUL1 to test whether it directly stimulates CDK2 activity, and structural/biophysical mapping (e.g., cryo-EM or co-crystallization, HDX-MS) of the CACUL1-CDK2 interface.
Hypothesis: CACUL1 directly binds and allosterically activates CDK2 independent of cyclin levels.
Type: biochemical/structural
Experiment: Affinity purification-mass spectrometry of endogenous CACUL1 across cell-cycle stages to define its stable interactome and test for any RBX/neddylation or cullin-RING ligase components.
Hypothesis: CACUL1 does not assemble a canonical cullin-RING ligase and instead functions through CDK2 and nuclear-receptor/chromatin partners.
Type: proteomics
Experiment: ChIP-seq and reporter assays for CACUL1 at ERalpha- and androgen-receptor- responsive promoters with and without LSD1, to define its co-regulatory direction and chromatin effects.
Hypothesis: CACUL1 modulates nuclear-receptor target genes through LSD1-dependent changes in histone methylation.
Type: genomics/transcription
What is not known β curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The direct biochemical mechanism by which CACUL1 promotes CDK2 kinase activity remains unresolved: it is unclear whether CACUL1 allosterically activates CDK2, alters cyclin/CDK-inhibitor availability, changes substrate engagement, or acts through another cell-cycle regulatory complex.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: CACUL1 physically associates with CDK2, promotes CDK2 kinase activity, is expressed in a cell-cycle-dependent manner, and RNAi knockdown causes reduced proliferation with G1/S arrest. The unresolved part is the immediate molecular mechanism connecting CACUL1 binding to increased CDK2 activity.
Significance: CDK2 regulation is the best-supported CACUL1 activity, but without a mechanism it is hard to distinguish a direct CDK2 co-regulator from an indirect cell-cycle phenotype.
What would resolve it: Reconstitute purified CACUL1-CDK2-cyclin complexes for kinase kinetics, map the binding interface, and test whether CACUL1 changes cyclin binding, CDK-inhibitor binding, or substrate phosphorylation in cells.
Provenance (the field's own admissions):
Gap: The role of CACUL1's cullin-repeat-like domain in CRL3-Keap1/Nrf2 regulation is unresolved, and CACUL1 should not be curated as a bona fide cullin-RING scaffold unless future experiments show that it assembles or positions an E3 ubiquitin ligase complex.
OPEN BIOLOGYCURATION RESIDUAL_SUBGAP
What is known: CACUL1 binds Keap1, Cul3, and Rbx1 and attenuates Nrf2 ubiquitination, supporting ubiquitin-ligase binding and negative regulation of Nrf2 ubiquitination. The gap is whether the short cullin-like region is the binding/regulatory interface and whether CACUL1 modulates only CRL3-Keap1/Nrf2 or broader CRL3 substrate ubiquitination.
Significance: This boundary prevents domain-only over-propagation of cullin scaffold activity while preserving the experimentally supported CRL3-Keap1/Nrf2 regulatory role.
What would resolve it: Map CACUL1 deletion mutants against Keap1, Cul3, Rbx1, and Nrf2; assay CRL3 ubiquitination activity with purified components; and test whether CACUL1 affects other CRL3-Keap1 or Cul3 substrates.
Provenance (the field's own admissions):
Gap: The physiological context and directionality of CACUL1 nuclear-receptor coregulation remain only partly defined, especially whether ERalpha, androgen receptor, and PPARgamma effects represent one general chromatin mechanism or distinct receptor- and cell-type-specific activities.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: CACUL1 can bind nuclear receptors and repress ERalpha- and PPARgamma-dependent transcription with LSD1/SIRT1-linked chromatin changes. The unresolved part is which tissues and metabolic or hormonal states require this activity in vivo, and when CACUL1 acts as a corepressor versus a co-regulator with different directionality.
Significance: Nuclear-receptor corepression is experimentally supported but may be highly context-dependent; resolving context will determine whether CACUL1 should be treated as a broadly relevant transcriptional regulator or a narrower adipocyte and hormone-response modulator.
What would resolve it: Combine receptor-specific ChIP-seq/CUT&RUN, transcriptomics, and CACUL1 loss- and gain-of-function in relevant adipocyte and hormone-responsive models, followed by in vivo perturbation where feasible.
Provenance (the field's own admissions):
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