Cyclin-dependent kinase and Cks/Suc1 interact with the proteasome in yeastto control proteolysis of M-phase targets

Cell cycle-specific proteolysis is critical for proper execution of mitosis in all eukaryotes. Ubiquitination and subsequent proteolysis of the mitoti c regulators Clb2 and Pds1 depend on the cyclosome/APC and the 26S proteaso me. We report here that components of the cell cycle machinery in yeast, sp ecifically the cell cycle regulatory cyclin-dependent kinase Cdc28 and a co nserved associated protein Cks1/Suc1, interact genetically, physically, and functionally with components of the 26S proteasome. A mutation in Cdc28 (c dc28-1N) that interferes with Cks1 binding, or inactivation of Cks1 itself, confers stabilization of Clb2, the principal mitotic B-type cyclin in budd ing yeast. Surprisingly, Clb2-ubiquitination in vivo and in vitro is not af fected by mutations in cks1, indicating that Cks1 is not essential for cycl osome/APC activity. However, mutant Cks1 proteins no longer physically inte ract with the proteasome, suggesting that Cks1 is required for some aspect of proteasome function during M-phase-specific proteolysis. We further prov ide evidence that Cks1 function is required for degradation of the anaphase inhibitor Pds1. Stabilization of Pds1 is partially responsible for the met aphase arrest phenotype of cks1 mutants because deletion of PDS1 partially releaves the metaphase block in these mutants.