Hsl1p, a Swe1p inhibitor, is degraded via the anaphase-promoting complex

Ubiquitination and subsequent degradation of critical cell cycle regulators is a key mechanism exploited by the cell to ensure an irreversible progres sion of cell cycle events. The anaphase-promoting complex (APC) is a ubiqui tin ligase that targets proteins for degradation by the 26S proteasome. Her e we identify the Hsl1p protein kinase as an APC substrate that interacts w ith Cdc20p and Cdh1p, proteins that mediate APC ubiquitination of protein s ubstrates. Hsl1p is absent in G(1), accumulates as cells begin to bud, and disappears in late mitosis. Hsl1p is stabilized by mutations in CDH1 and CD C23, both of which result in compromised APC activity. Unlike Hsl1p, Gin4p and Kcc4p, protein kinases that have sequence homology to Hsl1p, were stabl e in G(1)-arrested cells containing active APC. Mutation of a destruction b ox motif within Hsl1p (Hsl1p(db-mut)) stabilized Hsl1p. Interestingly, this mutation also disrupted the Hsl1p-Cdc20p interaction and reduced the assoc iation between Hsl1p and Cdh1p in coimmunoprecipitation studies. These find ings suggest that the destruction box motif is required for Cdc20p and, to a lesser extent, for Cdh1p to target Hsl1p to the APC for ubiquitination. H sl1p has been previously shown to inhibit Swe1p, a protein kinase that nega tively regulates the cyclin-dependent kinase Cdc28p, by promoting Swe1p deg radation via SCFMet30 in a bud morphogenesis checkpoint. Results of the pre sent work indicate that Hsl1p is degraded in an APC-dependent manner and su ggest a link between the SCF (Skp1-cullin-F box) and APC-proteolytic system s that may help to coordinate the proper progression of cell cycle events.