THE ROLE OF PROTEOLYSIS IN CELL-CYCLE PROGRESSION IN SCHIZOSACCHAROMYCES-POMBE

A cell-free system derived from Xenopus eggs nas used to identify the 'destruction box' of the Schizosaccharomyces pombe B-type cyclin, Cdc1 3, as residues 59-67: RHALDDVSN. Expression of indestructible Cdc13 fr om a regulated promoter in S.pombe blocked cells in anaphase and inhib ited septation, showing that destruction of Cdc13 is necessary for exi t from mitosis, but not for sister chromatid separation, In contrast, strong expression of a polypeptide comprising the N-terminal 70 residu es of Cdc13, which acts as a competitive inhibitor of destruction bos- mediated proteolysis, inhibited both sister chromatid separation and t he destruction of Cdc13, whereas an equivalent construct with a mutate d destruction box did not, Appropriately timed expression of this N-te rminal fragment of Cdc13 overcame the G(1) arrest seen in cdc10 mutant strains, suggesting that proteins required for the initiation of S ph ase are subject to destruction by the same proteolytic machinery as cy clin.