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Jean-Claude Labbé

Publications and source records attributed to Jean-Claude Labbé.

8 recordsLinked to original sources
PubMed2003

[Mitosis under control].

The mitotic checkpoint is essential to ensure accurate chromosome segregation by allowing a mitotic delay in response to a spindle defect. This checkpoint postpones the onset of anaphase until all the chromosomes are attached and correctly aligned onto the mitotic spindle. The checkpoint functions by preventing an ubiquitin ligase called the anaphase-promoting complex (APC) from ubiquitinylating proteins whose degradation is required for anaphase onset. Loss of this checkpoint results in chromosome missegregation in higher eukaryotes and may contribute to the genomic instability observed in most of the tumour cells.

Anaphase-Promoting Complex-Cyclosome↗
PubMed2003

Xkid is degraded in a D-box, KEN-box, and A-box-independent pathway.

During mitosis, the Xenopus chromokinesin Kid (Xkid) provides the polar ejection forces needed at metaphase for chromosome congression, and its degradation is required at anaphase to induce chromosome segregation. Despite the fact that the degradation of Xkid at anaphase seems to be a key regulatory factor to induce chromosome movement to the poles, little is known about the mechanisms controlling this proteolysis. We investigated here the degradation pathway of Xkid. We demonstrate that Xkid is degraded both in vitro and in vivo by APC/Cdc20 and APC/Cdh1. We show that, despite the presence of five putative D-box motifs in its sequence, Xkid is proteolyzed in a D-box-independent manner. We identify a domain within the C terminus of this chromokinesin, with sequence GxEN, whose mutation completely stabilizes this protein by both APC/Cdc20 and APC/Cdh1. Moreover, we show that this degradation sequence acts as a transposable motif and induces the proteolysis of a GST-GXEN fusion protein. Finally, we demonstrate that both a D-box and a GXEN-containing peptides completely block APC-dependent degradation of cyclin B and Xkid, indicating that the GXEN domain might mediate the recognition and association of Xkid with the APC.

Amino Acid Motifs↗
PubMed2002

The D-Box-activating domain (DAD) is a new proteolysis signal that stimulates the silent D-Box sequence of Aurora-A.

We have demonstrated previously that Xenopus Aurora-A is degraded at late mitosis by the APC/Fizzy-Related in a D-Box-dependent manner. Here we demonstrate that, although Aurora-B possesses the same D-Box as Aurora-A, Aurora-B is not degraded by this ubiquitin ligase. We have constructed a chimera Aurora-A/B with the N-terminus of Aurora-A and the C-terminus of Aurora-B and we have examined its degradation by APC/Fizzy-Related. We demonstrate that the N-terminus of Aurora-A confers degradation capacity on the C-terminus of Aurora-B and that this feature is blocked by mutation of the conserved D-Box sequence. We characterize the minimal degradation signal at the N-terminus of Aurora-A and demonstrate that its deletion blocks the degradation of this protein by APC/Fizzy-Related. Thus, we conclude that two different degradation signals are required for proteolysis of Aurora-A. The first one, which we designated D-Box-activating domain, within the N-terminal domain of Aurora-A confers the functionality to the second, a silent D-Box, present within the C-terminus of the kinase.

Animals↗
PubMed2002

Initial activation of cyclin-B1-cdc2 kinase requires phosphorylation of cyclin B1.

At the G(2)/M transition of the cell cycle, the cdc25c phosphatase dephosphorylates inhibitory residues of cdc2, and cyclin-B-cdc2 kinase (MPF) is activated. Phosphorylation of cyclin B1 induces its nuclear accumulation, and, since cdc25c is also believed to accumulate and activate shortly before G(2)/M in the nucleus, it has been proposed that this induces cyclin-B1-cdc2 kinase activation. We demonstrate that cyclin B1 phosphorylation has another essential function in vivo: it is required for cdc25c and MPF activation, which does not require nuclear accumulation of cyclin B1, and occurs in the cytoplasm.

Animals↗
PubMed2002

APC/Fizzy-Related targets Aurora-A kinase for proteolysis.

Aurora-A kinase is a mitotic spindle-pole-associated protein that has been implicated in duplication and separation of centrosomes and in spindle assembly. The proper timing and amplitude of Aurora-A expression seems to be important, as elevated levels of this protein have been associated with centrosome abnormalities and aneuploidy in mammalian cells. We show that Aurora-A increases at the G2-M transistion and disappears completely at G1 in XL2 cells. Using Xenopus oocyte extracts, we demonstrate that degradation of Aurora-A is mediated by the anaphase-promoting complex (APC) and is regulated by Fizzy-Related but not by Fizzy. Degradation of Aurora-A depends on a D-Box, but not on its KEN-Box motif, as mutation of its C-terminal D-Box sequence induces stabilization of the protein. Accordingly, addition into the extracts of a cyclin B-type D-Box-motif-containing peptide completely suppresses its degradation. Furthermore, APC/Fizzy-Related ubiquitylates the wild type but not a D-Box mutant form of Aurora-A in vitro. Consistent with these data, ectopic expression of Fizzy-Related in Xenopus oocytes induces complete degradation of endogenous Aurora-A. Aurora-A is thus the first protein, at least in our assay system, that undergoes a D-Box-dependent degradation mediated by APC/Fizzy-Related but not by APC/Fizzy.

Amino Acid Motifs↗
PubMed2002

Using RNA interference to identify genes required for RNA interference.

RNA interference (RNAi) is a phenomenon in which double-stranded RNA (dsRNA) silences endogenous gene expression. By injecting pools of dsRNAs into Caenorhabditis elegans, we identified a dsRNA that acts as a potent suppressor of the RNAi mechanism. We have used coinjection of dsRNAs to identify four additional candidates for genes involved in the RNAi mechanism in C. elegans. Three of the genes are C. elegans mes genes, some of which encode homologs of the Drosophila chromatin-binding Polycomb-group proteins. We have used loss-of-function mutants to confirm a role for mes-3, -4, and -6 in RNAi. Interestingly, introducing very low levels of dsRNA can bypass a requirement for these genes in RNAi. The finding that genes predicted to encode proteins that associate with chromatin are involved in RNAi in C. elegans raises the possibility that chromatin may play a role in RNAi in animals, as it does in plants.

Animals↗
PubMed2002

A new role for Mos in Xenopus oocyte maturation: targeting Myt1 independently of MAPK.

The resumption of meiosis in Xenopus arrested oocytes is triggered by progesterone, which leads to polyadenylation and translation of Mos mRNA, then activation of MAPK pathway. While Mos protein kinase has been reported to be essential for re-entry into meiosis in Xenopus, arrested oocytes can undergo germinal vesicle breakdown (GVBD) independently of MAPK activation, leading us to question what the Mos target might be if Mos is still required. We now demonstrate that Mos is indeed necessary, although is independent of the MAPK cascade, for conversion of inactive pre-MPF into active MPF. We have found that Myt1 is likely to be the Mos target in this process, as Mos interacts with Myt1 in oocyte extracts and Mos triggers Myt1 phosphorylation on some sites in vivo, even in the absence of MAPK activation. We propose that Mos is involved, not only in the MAPK cascade pathway, but also in a mechanism that directly activates MPF in Xenopus oocytes.

Animals↗