TPR PROTEINS REQUIRED FOR ANAPHASE PROGRESSION MEDIATE UBIQUITINATIONOF MITOTIC B-TYPE CYCLINS IN YEAST

The abundance of B-type cyclin-CDK complexes is determined by regulate d synthesis and degradation of cyclin subunits. Cyclin proteolysis is required for the final exit from mitosis and for the initiation of a n ew cell cycle. In extracts from frog or clam eggs, degradation is acco mpanied by ubiquitination of cyclin. Three genes, CDC16, CDC23, and CS E1 have recently been shown to be required specifically for cyclin B p roteolysis in yeast. To test whether these genes are required for cycl in ubiquitination, we prepared extracts from G1-arrested yeast cells c apable of conjugating ubiquitin to the B-type cyclin Clb2. The ubiquit ination activity was cell cycle regulated, required Clb2's destruction box, and was low if not absent in cdc16, cdc23, cdc27, and cse1 mutan ts. Furthermore all these mutants were also defective in ubiquitinatio n of another mitotic B-type cyclin, Clb3. The Cdc16, Cdc23, and Cdc27 proteins all contain several copies of the tetratricopeptide repeat an d are subunits of a complex that is required for the onset of anaphase . The finding that gene products that are required for ubiquitination of Clb2 and Clb3 are also required for cyclin proteolysis in vivo prov ides the best evidence so far that cyclin B is degraded via the ubiqui tin pathway in living cells. Xenopus homologues of Cdc16 and Cdc27 hav e meanwhile been shown to be associated with a 20S particle that appea rs to function as a cell cycle-regulated ubiquitin-protein ligase.