A NOVEL YEAST SCREEN FOR MITOTIC ARREST MUTANTS IDENTIFIES DOC1, A NEW GENE INVOLVED IN CYCLIN PROTEOLYSIS

B-type cyclins are rapidly degraded at the transition between metaphas e and anaphase and their ubiquitin-mediated proteolysis is required fo r cells to exit mitosis. We used a novel enrichment to isolate new bud ding mutants that arrest the cell cycle in mitosis. Most of these muta nts lie in the CDC16, CDC23, and CDC27 genes, which have already been shown to play a role in cyclin proteolysis and encode components of a 20S complex (called the cyclosome or anaphase promoting complex) that ubiquitinates mitotic cyclins. We show that mutations in CDC26 and a n ovel gene, DOC1, also prevent mitotic cyclin proteolysis. Mutants in e ither gene arrest as large budded cells with high levels of the major mitotic cyclin (Clb2) protein at 37 degrees C and cannot degrade Clb2 in G(1)-arrested cells. Cdc26 associates in vivo with Doc1, Cdc16, Cdc 23, and Cdc27. In addition, the majority of Doc1 cosediments at 20S wi th Cdc27 in a sucrose gradient, indicating that Cdc26 and Doc1 are com ponents of the anaphase promoting complex.