SIC1 encodes Sic1 (p40, also called Sdb25), a 284-residue, largely intrinsically disordered, non-enzymatic stoichiometric inhibitor of the budding-yeast cyclin-dependent kinase Cdc28 (Cdk1) when Cdc28 is complexed with B-type (Clb) cyclins; it does not inhibit Cln-Cdc28 and is unrelated in sequence to the metazoan Cip/Kip inhibitors (p27, p21) that occupy the same regulatory position. The C-terminal region (roughly residues 215-284, the SIC1_C domain) carries the kinase-inhibitory determinants that bind the cyclin-CDK holoenzyme with subnanomolar affinity, while the disordered N-terminal half is a phosphoregulatory platform bearing nine CDK consensus sites. SIC1 is transcribed in late anaphase by Swi5 (with a contribution from Ace2) once Cdc14 has been released, and the protein accumulates through mitotic exit and G1, where it helps extinguish residual Clb2-Cdc28 activity alongside APC/C-Cdh1-driven cyclin destruction and keeps the S-phase kinases Clb5/6-Cdc28 silent so that pre-replicative complexes can be licensed at replication origins. At Start, Cln1/2-Cdc28 (with Pho85-Pcl1 priming and processive phosphorylation by the Clb5-Cdc28-Cks1 complex to which Sic1 is bound) multisite-phosphorylates the N-terminus; once enough suboptimal Cdc4 phosphodegrons are occupied, Sic1 engages the WD40 substrate receptor Cdc4 of the SCF(Cdc4) ubiquitin ligase in a dynamic, polyvalent complex, is ubiquitinated with the E2 Cdc34, and is destroyed by the proteasome, abruptly releasing Clb5/6-Cdc28 to fire origins. This multisite threshold makes S-phase entry ultrasensitive to G1 CDK activity. Sic1 is predominantly nuclear (it carries a bipartite nuclear localization signal and is degraded by nuclear Cdc4) but also has a cytoplasmic pool whose size depends on carbon source, and it promotes nuclear accumulation of its cognate cyclin Clb5. Its stability is a signalling hub: the stress kinase Hog1 phosphorylates Thr173 to stabilize Sic1 and arrest cells in G1 under osmostress, TORC1 inhibition stabilizes it through Mpk1 and PP2A-Cdc55 control, CK2 phosphorylation of Ser201 strengthens kinase inhibition, Cdc14 dephosphorylates it at mitotic exit, and Pho85 complexed with Clg1, Pcl1 or Pho80 destabilizes it, which links Sic1 to negative control of autophagy via the Rim15 kinase. sic1 null cells are viable but fire fewer origins, prolong S phase and suffer genome instability, whereas non-degradable Sic1 blocks S-phase entry.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000082 G1/S transition of mitotic cell cycle | IGI PMID:7954792 The B-type cyclin kinase inhibitor p40SIC1 controls the G1 t... | ACCEPT | Summary: G1/S transition of mitotic cell cycle, inferred from the genetic interaction between SIC1 and CDC4 (SGD:S000001885) in the founding paper: cdc4 and cdc34 mutants arrest in G1 because they cannot destroy the Clb-Cdc28 inhibitor p40Sic1, and Sic1 proteolysis is an essential aspect of the G1 to S transition. Reason: Sic1 sits at the heart of this transition. It is the stoichiometric inhibitor of Clb5/6-Cdc28 that holds S-phase CDK activity off in G1, and it is the Cln-Cdc28-phosphorylated substrate whose SCF(Cdc4)-Cdc34-dependent destruction releases that activity; Cdc4 is the F-box substrate receptor that recognises phosphorylated Sic1. Sic1 does the inhibiting itself (an activity, not a mere requirement), and the timing of the transition is set by the multisite phosphorylation threshold on Sic1, so the participation test is met. The GOA term is the neutral parent; the direction of Sic1's contribution is captured more precisely by negative regulation of G1/S transition of mitotic cell cycle (GO:2000134), which is used in core_functions, but the parent term is correct as asserted and matches how the analogous metazoan CKI p27 is annotated. Supporting Evidence: PMID:7954792 cdc34 mutants cannot enter S phase because they fail to destroy p40SIC1, which is a potent inhibitor of Clb but not Cln forms of the Cdc28 kinase. PMID:7954792 Proteolysis of a cyclin-specific inhibitor of Cdc28 is therefore an essential aspect of the G1 to S phase transition. PMID:18787112 Such phosphorylation enables Sic1-Cdc4 interaction required for ubiquitination of Sic1. |
| GO:0000082 G1/S transition of mitotic cell cycle | IGI PMID:7954792 The B-type cyclin kinase inhibitor p40SIC1 controls the G1 t... | ACCEPT | Summary: G1/S transition of mitotic cell cycle, inferred from the genetic interaction between SIC1 and CDC53 (SGD:S000002290), the cullin scaffold of the SCF(Cdc4) ubiquitin ligase that destroys phosphorylated Sic1 at Start. Reason: Same biology as the CDC4 row: loss of the SCF cullin stabilises Sic1 and blocks S-phase entry, and deleting SIC1 bypasses the block. Sic1 is the Clb-Cdc28 inhibitor that gates the transition, so the annotation is correct; the negative-regulation child GO:2000134 is the more directional statement and is used in core_functions. Supporting Evidence: PMID:7954792 Proteolysis of a cyclin-specific inhibitor of Cdc28 is therefore an essential aspect of the G1 to S phase transition. PMID:18787112 Degradation of Sic1 is strictly required for S-phase entry ( 3 ), whereas that of Swi5 ensures efficient entry into S phase. |
| GO:0000082 G1/S transition of mitotic cell cycle | IGI PMID:7954792 The B-type cyclin kinase inhibitor p40SIC1 controls the G1 t... | ACCEPT | Summary: G1/S transition of mitotic cell cycle, inferred from the genetic interaction between SIC1 and CDC34 (SGD:S000002461): cdc34 mutants arrest as multibudded G1 cells because they cannot destroy p40Sic1, a phenotype suppressed by SIC1 deletion. Reason: This is the classic and best-supported genetic interaction for Sic1: the E2 Cdc34 is required to ubiquitinate phosphorylated Sic1, and the cdc34 G1 arrest is caused by persistent Sic1-mediated inhibition of Clb-Cdc28. Sic1 is the active inhibitor whose removal is the switch, so involvement in the transition is correct. As with the other IGI rows, GO:2000134 states the direction and is used in core_functions. Supporting Evidence: PMID:7954792 A sextuple clb1-6 mutant arrests as multibudded G1 cells that resemble cells lacking the Cdc34 ubiquitin-conjugating enzyme. PMID:7954792 cdc34 mutants cannot enter S phase because they fail to destroy p40SIC1, which is a potent inhibitor of Clb but not Cln forms of the Cdc28 kinase. |
| GO:0000082 G1/S transition of mitotic cell cycle | IGI PMID:7954792 The B-type cyclin kinase inhibitor p40SIC1 controls the G1 t... | ACCEPT | Summary: G1/S transition of mitotic cell cycle, inferred from the genetic interaction between SIC1 and CLB5 (SGD:S000006324): Clb5-Cdc28 is the S-phase kinase that Sic1 inhibits, and the paper shows that DNA replication requires Clb-Cdc28 activation and that Sic1 is a potent inhibitor of Clb but not Cln forms of the kinase. Reason: This row records the target side of the relationship: Clb5-Cdc28 is the kinase Sic1 restrains in G1 and releases upon its destruction. The inhibition is direct (in vitro kinase inhibition, later measured as subnanomolar) and is the mechanism by which Sic1 participates in the transition, so the annotation is sound. GO:2000134 (negative regulation of the transition) is the more directional statement used in core_functions. Supporting Evidence: PMID:7954792 We show that DNA replication also requires activation of Cdc28 by B-type (Clb) cyclins. PMID:7954792 In wild-type cells, p40SIC1 protein appears at the end of mitosis and disappears shortly before S phase. |
| GO:0004861 cyclin-dependent protein serine/threonine kinase inhibitor activity | IDA PMID:7954792 The B-type cyclin kinase inhibitor p40SIC1 controls the G1 t... | ACCEPT | Summary: Cyclin-dependent protein serine/threonine kinase inhibitor activity, shown directly in the founding cell-cycle paper, which characterises p40Sic1 as a potent inhibitor of Clb-Cdc28 but not Cln-Cdc28 kinase complexes. Reason: This is the defining molecular function of Sic1 and the activity through which all of its cell-cycle roles are executed: it binds Clb5/6-Cdc28 (and Clb2-Cdc28) stoichiometrically and blocks phosphorylation of their substrates. The cyclin selectivity (Clb but not Cln) is an important feature that distinguishes Sic1 from a generic kinase inhibitor and justifies the CDK-specific term. Supporting Evidence: PMID:7954792 cdc34 mutants cannot enter S phase because they fail to destroy p40SIC1, which is a potent inhibitor of Clb but not Cln forms of the Cdc28 kinase. file:yeast/SIC1/SIC1-deep-research-falcon.md Sic1 is a nonenzymatic, stoichiometric inhibitor that binds cyclinβCdk1 complexes and blocks phosphorylation of other substrates, principally restraining Clb5/6βCdk1 before S phase and contributing to inhibition of mitotic ClbβCdk1. |
| GO:0004861 cyclin-dependent protein serine/threonine kinase inhibitor activity | IDA PMID:8421781 An inhibitor of p34CDC28 protein kinase activity from Saccha... | ACCEPT | Summary: Cyclin-dependent protein serine/threonine kinase inhibitor activity from the original purification of p40: a 40 kDa protein first identified as a Cdc28 substrate was purified, bound tightly to p34CDC28 and inhibited its kinase activity. Reason: Direct biochemical evidence from purified protein, and the paper that discovered the activity. It correctly captures the dual nature of Sic1 as both substrate and inhibitor of Cdc28; the cyclin specificity was resolved in the following year (Clb-Cdc28 only). Core molecular function. Supporting Evidence: PMID:8421781 The p40 protein bound tightly to p34CDC28 and inhibited the activity PMID:8421781 originally identified as a substrate of the p34CDC28 protein kinase was |
| GO:0005515 protein binding | IPI PMID:11274204 Characterization of the Net1 cell cycle-dependent regulator ... | REMOVE | Summary: Experimental physical interaction (IPI) between Sic1 and the Cdc14 phosphatase (Q00684), recorded under the bare term 'protein binding'. In the cited paper Sic1 is one of five substrates used to characterise Net1 as a competitive inhibitor of Cdc14; Sic1 is a physiologic Cdc14 target that is dephosphorylated at mitotic exit. Reason: Removal reflects the term, not the data. The interaction is an enzyme-substrate relationship (Cdc14 dephosphorylates Sic1 at anaphase, helping Sic1 accumulate), and 'protein binding' says nothing about what Sic1 does. No more informative molecular-function term is supported for Sic1 from this evidence: being dephosphorylated is not a Sic1 activity, and the GO mechanism for a substrate relationship lives on the enzyme (Cdc14 phosphatase activity with Sic1 as input), not on the substrate. The interaction itself is not disputed. Supporting Evidence: PMID:11274204 substrates including the physiologic targets Swi5 and Sic1 PMID:11274204 competitive inhibitor of its activity (K(i) = 3 nm) with five different |
| GO:0005515 protein binding | IPI PMID:15448699 Hog1 mediates cell-cycle arrest in G1 phase by the dual targ... | MODIFY | Summary: Experimental physical interaction (IPI) between Sic1 and the F-box protein Cdc4 (P07834), recorded under the bare term 'protein binding', from the Hog1 osmostress paper in which Hog1 phosphorylation of Sic1 Thr173 stabilises Sic1 and arrests cells in G1. Reason: The interaction is the well-characterised recognition of phosphorylated Sic1 by Cdc4, the WD40 substrate receptor of the SCF(Cdc4) ubiquitin ligase, and the informative term for it already exists on this gene: SCF ubiquitin ligase complex binding (GO:1905761, annotated from the NMR study of the pSic1-Cdc4 complex). The cached record is abstract-only, so the specific Cdc4 experiment in this paper cannot be checked here, but the curator read the full text and the interaction is independently established by direct binding measurements, so the bare term is resolved to the specific one rather than removed. Note that this is substrate-side binding (how Sic1 is regulated), kept as non-core in the GO:1905761 rows. Proposed replacements: SCF ubiquitin ligase complex binding Supporting Evidence: PMID:15448699 phosphorylates a single residue at the carboxyl terminus of Sic1 PMID:19008353 The disordered cyclin-dependent kinase (CDK) inhibitor Sic1 interacts with a single site on its receptor Cdc4 only upon phosphorylation of its multiple dispersed CDK sites. |
| GO:0005515 protein binding | IPI PMID:15448699 Hog1 mediates cell-cycle arrest in G1 phase by the dual targ... | REMOVE | Summary: Experimental physical interaction (IPI) between Sic1 and the stress-activated MAP kinase Hog1 (P32485), recorded under the bare term 'protein binding': Hog1 binds Sic1 in vivo and in vitro and phosphorylates Thr173, which stabilises Sic1 and is required for the osmostress G1 arrest. Reason: Removal reflects the term, not the data. The Hog1-Sic1 contact is a kinase-substrate encounter in which Sic1 is the substrate; 'protein binding' asserts no Sic1 activity, and re-labelling it as protein kinase binding would only restate that an enzyme touched its substrate. The important biology (Hog1 targets Sic1 to impose a stress-induced G1 arrest, via the same CDK-inhibitory activity already annotated) is recorded in the notes and belongs on Hog1's kinase activity with Sic1 as input. The interaction is not disputed and the T173A mutagenesis is retained in the UniProt record. Supporting Evidence: PMID:15448699 Hog1 interacts physically with Sic1 in vivo and in PMID:15448699 the Sic1 CDK-inhibitor is the molecular target |
| GO:0005515 protein binding | IPI PMID:18787112 A refined two-hybrid system reveals that SCF(Cdc4)-dependent... | MODIFY | Summary: Experimental physical interaction (IPI) between Sic1 and Cdc4 (P07834), recorded under the bare term 'protein binding'. In a refined two-hybrid system using an F-box-deleted Cdc4 in cdc4-1 cells, Sic1 interacts with Cdc4 and the interaction is abolished by sic1-0p mutations that prevent Cdc28/Pho85 phosphorylation. Reason: The evidence is a phosphorylation-dependent Sic1-Cdc4 interaction, i.e. recognition of the Sic1 phosphodegrons by the SCF(Cdc4) substrate receptor, for which the specific term SCF ubiquitin ligase complex binding (GO:1905761) is already on this gene. Bare 'protein binding' conveys none of that, so it is resolved to the informative term. As a substrate-side interaction it describes how Sic1 is regulated rather than what Sic1 does, and is treated as non-core. Proposed replacements: SCF ubiquitin ligase complex binding Supporting Evidence: PMID:18787112 it was abolished by sic1β0p mutations that inhibit phosphorylation by Cdc28 and Pho85 ( 6 ). Such phosphorylation enables Sic1-Cdc4 interaction required for ubiquitination of Sic1. PMID:18787112 Although Sic1 is a substrate of the SCF Cdc4 complex, their interaction could not be detected by the conventional two-hybrid system |
| GO:0005515 protein binding | IPI PMID:19008353 Dynamic equilibrium engagement of a polyvalent ligand with a... | MODIFY | Summary: Experimental physical interaction (IPI) between Sic1 and Cdc4 (P07834), recorded under the bare term 'protein binding', from the NMR/fluorescence study of the pSic1-Skp1-Cdc4 complex: six-fold phosphorylated Sic1(1-90) binds Skp1-Cdc4 with an apparent Kd of about 0.6 uM, while unphosphorylated Sic1 shows no binding, and multiple suboptimal phosphodegrons exchange dynamically on a single Cdc4 site. Reason: This is the most direct and quantitative characterisation of the Sic1-Cdc4 interaction, and the same paper is the source of the specific SCF ubiquitin ligase complex binding (GO:1905761) annotations on this gene. The bare term duplicates them without their information content, so it is resolved to GO:1905761. Substrate-side binding; non-core. Proposed replacements: SCF ubiquitin ligase complex binding Supporting Evidence: PMID:19008353 we measured an apparent overall K d value of β0.6 ΞΌM between the pSic1 preparation and Skp1βCdc4, whereas no binding was detected for Sic1 PMID:19008353 multiple phosphorylated sites on Sic1 interact with Cdc4 in dynamic equilibrium with only local ordering around each site |
| GO:0005515 protein binding | IPI PMID:20489023 A global protein kinase and phosphatase interaction network ... | REMOVE | Summary: Experimental physical interaction (IPI) between Sic1 and the Cdc14 phosphatase (Q00684), recorded under the bare term 'protein binding', from a proteome-scale affinity-purification mass spectrometry survey of kinase and phosphatase complexes. Reason: Removal reflects the term, not the data. This is a high-throughput AP-MS interaction that is consistent with the known enzyme-substrate relationship (Cdc14 dephosphorylates Sic1 at mitotic exit), but 'protein binding' asserts no Sic1 activity and no more informative molecular function for Sic1 follows from being a phosphatase substrate. The cached record is abstract-only and does not mention Sic1; the interaction is not disputed. Supporting Evidence: PMID:20489023 We identified a kinase and phosphatase interaction (KPI) network of 1844 interactions in budding yeast by mass spectrometric analysis of protein complexes. PMID:11274204 substrates including the physiologic targets Swi5 and Sic1 |
| GO:0005634 nucleus | IDA PMID:11080155 Nuclear-specific degradation of Far1 is controlled by the lo... | ACCEPT | Summary: Nuclear localisation of a functional Sic1-GFP fusion, shown in a study of the nuclear-restricted degradation of SCF(Cdc4) substrates: cdc4-1 cells expressing cytoplasm-forced Cdc4 accumulate Sic1 and arrest in G1, indicating that nuclear Cdc4 degrades Sic1 at the G1/S transition. Reason: Direct microscopy of a functional GFP fusion, and biologically coherent: Sic1 inhibits nuclear Clb5/6-Cdc28, is degraded by the nuclear F-box protein Cdc4, and carries a bipartite NLS. Nucleus is the compartment in which Sic1 executes its core function. Supporting Evidence: PMID:11080155 Consistent with this notion, a functional Sic1βGFP fusion protein was localized in the nucleus (Figure 8 E). PMID:11080155 increased levels of Sic1 (Figure 8 D), suggesting that nuclear Cdc4 is required to degrade Sic1 at the G 1 βS transition. |
| GO:0005634 nucleus | IDA PMID:11792824 Relationship between the function and the location of G1 cyc... | ACCEPT | Summary: Nuclear localisation of Sic1, observed in a study of the locations of the G1 cyclins Cln2 and Cln3; the Cln2 substrate Sic1 was found in both nucleus and cytoplasm. Reason: Consistent with the other nuclear localisation evidence and with the site of Sic1's CDK-inhibitory function. The paper is abstract-only in the cache, but the abstract itself states the Sic1 result. Supporting Evidence: PMID:11792824 A substrate of Cln2, Sic1, was also in both compartments. |
| GO:0005634 nucleus | IDA PMID:16294029 Subcellular localization of the cyclin dependent kinase inhi... | ACCEPT | Summary: Nuclear localisation of Sic1 with identification of the bipartite nuclear localisation sequence responsible for it; nuclear Sic1 is required for correct, carbon-source-dependent cell-cycle progression, and Sic1 also promotes nuclear accumulation of its cognate cyclin Clb5. Reason: Direct localisation data plus a mapped NLS, from the group that studied carbon-source modulation of Sic1. Nucleus is where Sic1 inhibits Clb5/6-Cdc28 and is degraded, so this is a core location. The additional observation that Clb5 builds up in the cytoplasm when SIC1 is switched off suggests a Cip/Kip-like nuclear-escort role, noted in suggested_questions rather than annotated. Supporting Evidence: PMID:16294029 nuclear localization sequence responsible for nuclear localization of Sic1 and PMID:16294029 Clb5 is observed upon switching off expression of the SIC1 gene |
| GO:0005634 nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: Nucleus, mapped electronically from the UniProtKB subcellular location annotation (SL-0191). Reason: Redundant with three IDA rows for the same term and consistent with the biology; the electronic mapping is correct. Supporting Evidence: PMID:11080155 Consistent with this notion, a functional Sic1βGFP fusion protein was localized in the nucleus (Figure 8 E). |
| GO:0005737 cytoplasm | IDA PMID:11792824 Relationship between the function and the location of G1 cyc... | ACCEPT | Summary: Cytoplasmic localisation of Sic1, observed alongside its nuclear pool in the study of G1 cyclin locations; Sic1 was found in both compartments. Reason: Direct observation and reproducible: a cytoplasmic Sic1 pool is also seen by the Alberghina group, where its size varies with carbon source, and Sic1 is thought to escort Clb5 into the nucleus. The CDK-inhibitory function is executed mainly in the nucleus, but the cytoplasmic pool is real and the annotation is not an over-statement. Supporting Evidence: PMID:11792824 A substrate of Cln2, Sic1, was also in both compartments. PMID:16294029 cytoplasmic building-up of |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: Cytoplasm, mapped electronically from the UniProtKB subcellular location annotation (SL-0086). Reason: Redundant with the IDA row for the same term and consistent with the observed nucleocytoplasmic distribution of Sic1. Supporting Evidence: PMID:11792824 A substrate of Cln2, Sic1, was also in both compartments. |
| GO:0016242 negative regulation of macroautophagy | IMP PMID:20417603 Positive or negative roles of different cyclin-dependent kin... | KEEP AS NON CORE | Summary: Negative regulation of macroautophagy from mutant phenotypes: overexpression of Sic1 or of the degradation-resistant Sic1-delta3P inhibits rapamycin- and starvation-induced autophagy, sic1 deletion up-regulates it, autophagy is blocked in cdc28-4 and cdc34-2 mutants in which Sic1 accumulates, and RIM15 deletion fully suppresses the sic1 phenotype, placing the Rim15 kinase downstream of Sic1. Reason: The genetics are thorough and internally consistent (over-expression, deletion, stabilised allele, and epistasis with Rim15), so the annotation is retained. It is non-core: it is a secondary consequence of Sic1's CDK-inhibitory activity (or of an undefined Sic1 output), the mechanism linking Sic1 to Rim15 is unresolved (only weak co-immunoprecipitation was detected), and the effect is exerted through the nutrient-signalling Pho85 cyclins Clg1/Pcl1/Pho80 that destabilise Sic1 rather than through the canonical G1/S switch. The mammalian counterpart p27 has the opposite sign, so this is a lineage-specific wiring rather than a conserved function. Supporting Evidence: PMID:20417603 Sic1 is a negative regulator of autophagy, based on the observations that overexpression of Sic1 or Sic1-Ξ3P, a degradation-resistant mutant, significantly inhibited autophagy, and that loss of Sic1 dramatically upregulated autophagy PMID:20417603 deletion of RIM15 completely suppressed the upregulation of autophagy in sic1Ξ cells PMID:20417603 However, only a weak interaction was detected in co-immunoprecipitation experiments (data not shown). |
| GO:1905761 SCF ubiquitin ligase complex binding | EXP PMID:19008353 Dynamic equilibrium engagement of a polyvalent ligand with a... | KEEP AS NON CORE | Summary: SCF ubiquitin ligase complex binding, from the NMR and fluorescence study of the interaction between multiply phosphorylated Sic1(1-90) and the Skp1-Cdc4 substrate-receptor module of SCF(Cdc4): binding requires phosphorylation of the dispersed CDK sites, occurs at a single Cdc4 site, and involves dynamic exchange of several suboptimal phosphodegrons. Reason: Direct, quantitative and correctly stated as binding to the SCF complex (via its F-box substrate receptor Cdc4). It belongs, however, to how Sic1 is regulated rather than to what Sic1 does: Sic1 is the substrate of SCF(Cdc4)-Cdc34 ubiquitination and performs none of the ubiquitination chemistry, so process terms for its own degradation would be inappropriate, and the binding is kept as a non-core interaction. This mirrors the treatment of the analogous ubiquitin protein ligase binding annotation on the metazoan CKI p27 (from the SKP2-CKS1-p27 phosphopeptide structure). The interaction is nonetheless the essential mechanistic context for the core function, since the multisite-phosphorylation threshold it embodies is what makes S-phase entry switch-like. Supporting Evidence: PMID:19008353 The disordered cyclin-dependent kinase (CDK) inhibitor Sic1 interacts with a single site on its receptor Cdc4 only upon phosphorylation of its multiple dispersed CDK sites. PMID:19008353 A requirement for multiple phosphorylation events in Sic1 in principle sets a high threshold for the level of active G 1 CDK required to initiate transition to S phase |
| GO:1905761 SCF ubiquitin ligase complex binding | IDA PMID:19008353 Dynamic equilibrium engagement of a polyvalent ligand with a... | KEEP AS NON CORE | Summary: SCF ubiquitin ligase complex binding (IDA duplicate of the EXP row from the same paper), based on the phosphorylation-dependent binding of Sic1(1-90) to Skp1-Cdc4 measured by intrinsic tryptophan fluorescence and mapped by NMR. Reason: Same evidence and same assessment as the EXP row: a real, direct interaction with the SCF(Cdc4) substrate receptor that describes the regulation of Sic1 (substrate recognition for ubiquitin-dependent destruction at G1/S) rather than an activity Sic1 performs. Retained as non-core context for the CDK-inhibitory function; duplicates with different evidence codes are acceptable. Supporting Evidence: PMID:19008353 we measured an apparent overall K d value of β0.6 ΞΌM between the pSic1 preparation and Skp1βCdc4, whereas no binding was detected for Sic1 PMID:19008353 Phosphorylation by G 1 CDK activity (Cln1/2βCdc28) targets Sic1 to the SCF Cdc4 ubiquitin ligase, resulting in Sic1 ubiquitination and degradation by the proteasome |
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Download this section (compressed HTML)Q: Sic1 carries 'SCF ubiquitin ligase complex binding' although it is the substrate rather than an actor in its own ubiquitination, and the analogous metazoan CKI p27 carries 'ubiquitin protein ligase binding'. Should GO record substrate recognition on the substrate as a molecular-function binding term, or only on the F-box receptor (Cdc4 has-input Sic1), and should the two CKIs be harmonised?
Suggested experts: Tyers M, Sicheri F
Q: By what mechanism does Sic1 negatively regulate autophagy upstream of Rim15? Is it a consequence of Clb-Cdc28 inhibition (a CDK-dependent effect on Rim15 or its regulators) or a CDK-independent output, given that only weak Sic1-Rim15 co-immunoprecipitation was detected?
Suggested experts: Klionsky DJ, Yang Z
Q: Is the proposed Cip/Kip-like 'activator' role of Sic1 (promoting nuclear accumulation of Clb5) reproducible and separable from kinase inhibition, and if so should it be captured with a cyclin binding or adaptor-type annotation in addition to the inhibitor term?
Suggested experts: Alberghina L, Vanoni M
Experiment: Express sic1 alleles lacking the C-terminal inhibitory domain, lacking the bipartite NLS, or carrying both lesions in sic1 null cells, and quantify Clb5-GFP nuclear/cytoplasmic ratio, Clb5-Cdc28 kinase activity, origin-firing efficiency and S-phase timing in synchronised cultures on glucose versus ethanol.
Hypothesis: The CDK-inhibitory and Clb5-nuclear-escort functions of Sic1 are separable and map to different regions of the protein.
Type: separation-of-function mutagenesis with live-cell imaging
Experiment: Compare autophagy induction (GFP-Atg8 processing and Pho8delta60 assays) in sic1 null cells rescued with wild-type Sic1 versus a kinase-inhibition-dead C-terminal mutant that retains stability and localisation, and test whether analogue-sensitive cdc28-as1 inhibition phenocopies Sic1 overexpression in a rim15-dependent manner.
Hypothesis: Sic1 inhibits autophagy through Clb-Cdc28 inhibition rather than through a direct Sic1-Rim15 interaction.
Type: epistasis with chemical genetics
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