Inhibiting proteasome activity causes overreplication of DNA and blocks entry into mitosis in sea urchin embryos

Abstract

The proteasome has been shown to be involved in exit from mitosis by bringing about destruction of mitotic cyclins. Here, we present evidence that the proteasome is also required for proper completion of S phase and for entry into mitosis in the sea urchin embryonic cleavage cycle. A series of structurally related peptide-aldehydes prevent nuclear envelope breakdown in their order of inhibitory efficacies against the proteasome. Their efficacies in blocking exit from S phase and exit from mitosis correlate well, indicating that the proteasome is involved at both these steps. Mitotic histone HI kinase activation and tyrosine dephosphorylation of p34(cdc2) kinase are blocked by inhibition of the proteasome, indicating that the proteasome plays an important role in the pathway that leads to embryonic p34(cdc2)kinase activation. Arrested embryos continued to incorporate [(3)H]thymidine and characteristically developed large nuclei. Pre-mitotic arrest can be overcome by treatment with caffeine, a manoeuvre that is known to override the DNA replication checkpoint. These data demonstrate that the proteasome is involved in the control of termination of S phase and consequently in the initiation of M phase of the first embryonic cell cycle.

EIF4E/4E-BP dissociation and 4E-BP degradation in the first mitotic division of the sea urchin embryo

“…Proteolysis has been shown to be important for the initiation of the first mitotic division in sea urchins. Indeed, inhibition of the proteasome activity specifically delays the onset of nuclear envelope breakdown in embryonic cell cycle (Kawahara et al, 2000). Nevertheless, the suppression of entry into mitosis in proteasome-inhibited embryos is caused by a defect in the regulatory mechanism of CDK1 activation rather than by any effect on cyclin synthesis (Kawahara et al, 2000).…”

“…Indeed, inhibition of the proteasome activity specifically delays the onset of nuclear envelope breakdown in embryonic cell cycle (Kawahara et al, 2000). Nevertheless, the suppression of entry into mitosis in proteasome-inhibited embryos is caused by a defect in the regulatory mechanism of CDK1 activation rather than by any effect on cyclin synthesis (Kawahara et al, 2000). Therefore, 4E-BP degradation is unlikely to be dependent of the proteasome pathway.…”

Embryon d'oursin et séquençage du génome de l'espèce S. purpuratus : quels apports à l'étude du cycle cellulaire ?

“…Les protéolyses sont avec les phosphorylations les mécanismes post-traductionnels les plus utilisés dans la régulation du cycle cellulaire. Chez l'oursin le turnover de protéines dû aux dégradations par le protéasome s'active 15 à 20 minutes après la fécondation et son inhibition à ce stade provoque une sur-réplication (Kawahara et al, 2000). Bien qu'on ignore encore le mécanisme sous-jacent, cette observation étend le rôle de la dégradation protéique comme régulateur du cycle au-delà des étapes de sortie de mitose où cette fonction est bien caractérisée.…”