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

The proteasome has been shown to be involved in exit from mitosis by bringi ng 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 stru cturally related peptide-aldehydes prevent nuclear envelope breakdown in th eir order of inhibitory efficacies against the proteasome. Their efficacies in blocking exit from S phase and exit from mitosis correlate well, indica ting that the proteasome is involved at both these steps. Mitotic histone H I kinase activation and tyrosine dephosphorylation of p34(cdc2) kinase are blocked by inhibition of the proteasome, indicating that the proteasome pla ys an important role in the pathway that leads to embryonic p34(cdc2) kinas e activation. Arrested embryos continued to incorporate [H-3]thymidine and characteristically developed large nuclei. Pre-mitotic arrest can be overco me by treatment with caffeine, a manoeuvre that is known to override the DN A replication checkpoint, These data demonstrate that the proteasome is inv olved in the control of termination of S phase and consequently in the init iation of M phase of the first embryonic cell cycle.