Academic literature on the topic 'APC/C-Cdh1'

The antagonism between cyclin-dependent kinases (Cdks) and the ubiquitin ligase APC/C-Cdh1 is central to eukaryotic cell cycle control. APC/C-Cdh1 targets cyclin B and other regulatory proteins for degradation, whereas Cdks disable APC/C-Cdh1 through phosphorylation of the Cdh1 activator protein at multiple sites. Budding yeast Cdh1 carries nine Cdk phosphorylation sites in its N-terminal regulatory domain, most or all of which contribute to inhibition. However, the precise role of individual sites has remained unclear. Here, we report that the Cdk phosphorylation sites of yeast Cdh1 are organized into autonomous subgroups and act through separate mechanisms. Cdk sites 1–3 had no direct effect on the APC/C binding of Cdh1 but inactivated a bipartite nuclear localization sequence (NLS) and thereby controlled the partitioning of Cdh1 between cytoplasm and nucleus. In contrast, Cdk sites 4–9 did not influence the cell cycle–regulated localization of Cdh1 but prevented its binding to the APC/C. Cdk sites 4–9 reside near two recently identified APC/C interaction motifs in a pattern conserved with the human Cdh1 orthologue. Thus a Cdk-inhibited NLS goes along with Cdk-inhibited APC/C binding sites in yeast Cdh1 to relay the negative control by Cdk1 phosphorylation of the ubiquitin ligase APC/C-Cdh1.

The switch from activation of the anaphase-promoting complex/cyclosome (APC/C) by CDC20 to CDH1 during anaphase is crucial for accurate mitosis. APC/CCDC20 ubiquitinates a limited set of substrates for subsequent degradation, including Cyclin B1 and Securin, whereas APC/CCDH1 has a broader specificity. This switch depends on dephosphorylation of CDH1 and the APC/C, and on the degradation of CDC20. Here we show, in human cells, that the APC/C inhibitor MAD2L2 also contributes to ensuring the sequential activation of the APC/C by CDC20 and CDH1. In prometaphase, MAD2L2 sequestered free CDH1 away from the APC/C. At the onset of anaphase, MAD2L2 was rapidly degraded by APC/CCDC20, releasing CDH1 to activate the dephosphorylated APC/C. Loss of MAD2L2 led to premature association of CDH1 with the APC/C, early destruction of APC/CCDH1 substrates, and accelerated mitosis with frequent mitotic aberrations. Thus, MAD2L2 helps to ensure a robustly bistable switch between APC/CCDC20 and APC/CCDH1 during the metaphase-to-anaphase transition, thereby contributing to mitotic fidelity.

In vivo analysis in budding yeast identifies APC/C-Cdh1–specific minimal degrons carrying either a D or a KEN box and a nuclear localization sequence. APC/C-Cdh1 activity is restricted to the nucleus, maximal in the nucleoplasm, and absent from the cytoplasm, allowing for spatiotemporal control of Cdh1 substrate proteolysis.

ABSTRACT Cdh1 is a coactivator of the anaphase-promoting complex/cyclosome (APC/C) and contributes to mitotic exit and G1 maintenance by facilitating the polyubiquitination and subsequent proteolysis of specific substrates. Here, we report that budding yeast Cdh1 is a component of a cell cycle-regulated complex that includes the 14-3-3 homologs Bmh1 and Bmh2 and a previously uncharacterized protein, which we name Acm1 (APC/C Cdh1 modulator 1). Association of Cdh1 with Bmh1 and Bmh2 requires Acm1, and the Acm1 protein is cell cycle regulated, appearing late in G1 and disappearing in late M. In acm1Δ strains, Cdh1 localization to the bud neck and association with two substrates, Clb2 and Hsl1, were strongly enhanced. Several lines of evidence suggest that Acm1 can suppress APC/CCdh1-mediated proteolysis of mitotic cyclins. First, overexpression of Acm1 fully restored viability to cells expressing toxic levels of Cdh1 or a constitutively active Cdh1 mutant lacking inhibitory phosphorylation sites. Second, overexpression of Acm1 was toxic in sic1Δ cells. Third, ACM1 deletion exacerbated a low-penetrance elongated-bud phenotype caused by modest overexpression of Cdh1. This bud elongation was independent of the morphogenesis checkpoint, and the combination of acm1Δ and hsl1Δ resulted in a dramatic enhancement of bud elongation and G2/M delay. Effects on bud elongation were attenuated when Cdh1 was replaced with a mutant lacking the C-terminal IR dipeptide, suggesting that APC/C-dependent proteolysis is required for this phenotype. We propose that Acm1 and Bmh1/Bmh2 constitute a specialized inhibitor of APC/CCdh1.

ABSTRACT The anaphase-promoting complex/cyclosome (APC/C) is an E3 ubiquitin ligase mediating targeted proteolysis through ubiquitination of protein substrates to control the progression of mitosis. The APC/C recognizes its substrates through two adapter proteins, Cdc20 and Cdh1, which contain similar C-terminal domains composed of seven WD-40 repeats believed to be involved in interacting with their substrates. During the transition from metaphase to anaphase, APC/C-Cdc20 mediates the ubiquitination of securin and cyclin B1, allowing the activation of separase and the onset of anaphase and mitotic exit. APC/C-Cdc20 and APC/C-Cdh1 have overlapping substrates. It is unclear whether they are redundant for mitosis. Using a gene-trapping approach, we have obtained mice which lack Cdc20 function. These mice show failed embryogenesis. The embryos were arrested in metaphase at the two-cell stage with high levels of cyclin B1, indicating an essential role of Cdc20 in mitosis that is not redundant with that of Cdh1. Interestingly, Cdc20 and securin double mutant embryos could not maintain the metaphase arrest, suggesting a role of securin in preventing mitotic exit.

Abstract Introduction We have previously proposed that Cdh1 is a tumor suppressor by maintaining genomic stability. We also found Cdh1 downregulated in several tumor cell lines including AML (Oncogene 2008; 27:907-17). Heterozygous Cdh1 knockout mice develop epithelial tumors, myelodysplasia and plasma cell dyscrasias (Nat. Cell Biol. 2008;10:802-11). By analyzing primary AML samples from bone marrow (BM) or peripheral blood (PB) we detected downregulation of Cdh1 in the vast majority of samples when compared to normal CD34+ HSCs. Progression through the cell cycle is tightly regulated by different cyclin-dependent kinases (Cdks) and their activating cyclin subunits. Stage-specific proteolysis of cyclins and other cell cycle regulators is important for transition to the next cell cycle phase. The anaphase-promoting complex/cyclosome (APC/C) is an E3-ubiquitin ligase that controls mitosis and G1 through degradation of these proteins. Through its activating subunits Cdh1 and Cdc20 the APC/C ensures substrate-specifity. While Cdc20 regulates progression through mitosis, Cdh1 is activated in late mitosis to coordinate accurate entry into S-phase. Thereby, the APC/C is crucial for maintaining genomic stability during the cell cycle. Suppression of APC/C-Cdh1 can lead to unscheduled cyclin expression and Cdk activity, which can cause cell cycle defects leading to the accumulation of DNA alterations and further to malignant transformations. However, the exact nature of the origin of genomic instability upon downregulation of Cdh1 is unclear. Methods To investigate stability of cyclins in Cdh1-knockdown (kd) cells, origin loading and start of replication, cells were released from a mitotic block and samples were taken every 2 h until S-phase entry for FACS and immunoblotting. For live-cell imaging cells were seeded 24 h before imaging in chambered coverslips, after which progression through the cell cycle was analyzed by automated microscopy. Results Characterization of a Cdh1-kd revealed strong stabilization of the substrates cyclin A/B leading to diminished loading of mini-chromosome maintenance (MCM) proteins on replication origins in G1. Stabilization of cyclin A/B and unscheduled Cdk1/2 activity may cause the observed premature entry into S-phase, while the reduced loading of MCMs in G1 could be responsible for the prolonged replication in S-phase seen in Cdh1-kd cells. Accordingly, treatment with the Cdk1 inhibitor RO-3306 restored reduced MCM loading. Polo-like kinase 1 (Plk1) was stabilized in Cdh1-kd cells, which may cause bypass of the Cdc14B-Cdh1-Plk1 dependent DNA damage checkpoint. Indeed, potential replication stress in Cdh1-kd cells did not lead to G2/M arrest, but was enforced by inhibition of the Cdh1 substrate Plk1. Underreplicated DNA and replication intermediates in mitosis may be the reason for increased genomic instability, namely lagging chromosomes, anaphase bridges and micronuclei in Cdh1-kd cells detected by live-cell imaging. In addition, aberrant cytokinesis and the development of polyploid cells generated by misseparation of chromosomes during mitosis were enhanced in Cdh1-kd cells. Finally, monitoring of 53BP1, a DNA-repair marker, in living cells showed amplified DNA-damage through increased double-strand breaks in Cdh1-kd cells. Conclusions Downregulation of the tumor suppressor APC/C-Cdh1 leads to deregulation of DNA-replication by stabilizing cyclin A and B in G1 and reduced loading of replication origins with MCM proteins resulting in the accumulation of enhanced genomic instability and DNA damage. Disclosures: No relevant conflicts of interest to declare.

ABSTRACT The ubiquitin E3 ligase anaphase-promoting complex/cyclosome (APC/C) drives degradation of cell cycle regulators in cycling cells by associating with the coactivators Cdc20 and Cdh1. Although a plethora of APC/C substrates have been identified, only a few transcriptional regulators are described as direct targets of APC/C-dependent ubiquitination. Here we show that APC/C, through substrate recognition by both Cdc20 and Cdh1, mediates ubiquitination and degradation of heat shock factor 2 (HSF2), a transcription factor that binds to the Hsp70 promoter. The interaction between HSF2 and the APC/C subunit Cdc27 and coactivator Cdc20 is enhanced by moderate heat stress, and the degradation of HSF2 is induced during the acute phase of the heat shock response, leading to clearance of HSF2 from the Hsp70 promoter. Remarkably, Cdc20 and the proteasome 20S core α2 subunit are recruited to the Hsp70 promoter in a heat shock-inducible manner. Moreover, the heat shock-induced expression of Hsp70 is increased when Cdc20 is silenced by a specific small interfering RNA (siRNA). Our results provide the first evidence for participation of APC/C in the acute response to protein-damaging stress.

2010/2011
The calpains are a family of intracellular cysteine proteases, among which the best studied isoforms, micro- (CAPN1) and milli-calpain (CAPN2), are heterodimers consisting of a catalytic subunit and a common regulatory subunit, CAPNS1, required for function. Calpain is involved in many processes important for cancer biology, such as autophagy, indeed in calpain-depleted cells autophagy is impaired, with a subsequent increase in apoptosis sensitivity. Calpain is also important in all the stages of the stress response. A proteomic approach was employed for the identification of novel CAPNS1 interacting proteins. Proteins immunoprecipitating with endogenous CAPNS1 in HT1080 cell lysates were analyzed by Mass Spectrometry. We identified novel partners among which the deubiquitinating enzyme USP1, a key regulator of the DNA damage response and genome integrity maintenance via its specific action on FANCD2, involved in DNA repair and protection from chromosome instability, and PCNA, involved in the regulation of translesion DNA synthesis (TLS), that bypasses DNA lesions with low stringency basepairing requirements. We performed co-IP assays in lysates of 293T cells and confirmed that the interaction was specific. Furhermore, we observed that calpain is able to bind a USP1 C-terminal deleted mutant, suggesting that USP1 first 523 aminoacids were sufficient for the binding. To understand what is the effect exerted by calpain upon USP1, we depleted calpain activity in a series of cell lines, and followed the fate of endogenous USP1. We transfected CAPNS1 specific siRNAs, or treated cells with a specific inhibitor of calpain, and we observed a strong decrease in USP1 protein levels. This effect should be at a post-transcriptional level, since any significant change in USP1 mRNA levels is detected. We also obtained the same result by transfecting a siRNA specific for CAPN1, the gene encoding for the catalytic subunit micro-calpain. Moreover, we studied the role of calpain in the PCNA-mediated switch between high fidelity replication and TLS upon UV irradiation. In mouse embryonic fibroblasts knockout for CAPNS1, USP1 downregulation is coupled to an increase in PCNA monoubiquitination. Moreover, CAPNS1-depleted U2OS cells showed an increase in the percentage of nuclei containing PCNA-induced foci upon UV irradiation. Since we demonstrated that calpain can modulate an important regulator of DNA damage response such as USP1, we investigated if calpain could have a role in genome integrity maintenance. CAPNS1 depleted cells showed a reduced rescue in DNA repair compared to control cells, suggesting that increased levels in PCNA monoubiquitination could lead to an increased amount of errore-prone TLS. Calpain plays an important role in autophagy, so we asked if USP1 degradation in absence of calpain activity could involve autophagic pathways. We first blocked macroautophagy by silencing ATG5, and we observed that USP1 was downregulated, suggesting that the depletion of ATG5 could lead to an increased activity of other degradation pathways. To impaire chaperone-mediated autophagy (CMA), we silenced a protein important for autophagosome formation, LAMP-2A. Also in this case we observed a decrease in USP1 protein levels, thus suggesting that USP1 is alternatively degraded by different pathways. However, we observed that USP1 is stabilized upon inhibition of lysosomal enzymes, suggesting that USP1 may be degraded in the lysosome. To better understand the mechanism by which calpain affect USP1 stability we search for an effect of calpain upon USP1 co-factor and activator UAF1/WDR48. CAPNS1-depleted cells showed WDR48 downregulation, but WDR48 overexpression only partially rescue USP1 protein levels in this cells. Furthermore, we provided evidences that calpain regulation of p35/p25 activator of Cdk5 can affect Cdh1 phosphorylation and thus APC/Cdh1 activity, leading to a regulation of USP1 stabilization. In conclusion, we identified USP1 as a novel interactor of calpain, and we found that calpain is important for USP1 stability, since in its absence USP1 is downregulated. The importance of this novel regulation is strengthened by the recent findings that unveiled a role of USP1 in maintenance of a mesenchymal stem cell program in osteosarcoma, and thus placing calpain in a crucial regulatory position for cancer development.
XXIV Ciclo
1983