THE REGULATION OF CDC20 PROTEOLYSIS REVEALS A ROLE FOR THE APC COMPONENTS CDC23 AND CDC27 DURING S-PHASE AND EARLY MITOSIS

Background: in eukaryotic cells, a specialized proteolysis machinery t hat targets proteins containing destruction-box sequences for degradat ion and that uses a ubiquitin ligase known as the anaphase-promoting c omplex/cyclosome (APC) plays a key role in the regulation of mitosis, APC-dependent proteolysis triggers the separation of sister chromatids at the metaphase-anaphase transition and the destruction of mitotic c yclins at the end of mitosis, Recently, two highly conserved WD40-repe at proteins, Cdc20 and Cdh1/Hct1 have been identified as substrate-spe cific regulators of APC-dependent proteolysis in the budding yeast Sac charomyces cerevisiae. Here, we have investigated the cell cycle regul ation of Cdc20 and Cdh1/Hct1. Results: Whereas the levels of CDH1/HCT1 RNA and Cdh1/Hct1 protein are constant throughout the cell cycle, CDC 20 RNA and Cdc20 protein are present only during late S phase and mito sis and Cdc20 protein is unstable throughout the entire cell cycle. Th e instability of Cdc20 depends on CDC23 and CDC27, which encode compon ents of the APC, During the G1 phase, a destruction box within Cdc20 m ediates its instability, but during S phase and mitosis, although Cdc2 0 destruction is still dependent on CDC23 and CDC27, it does not depen d on the Cdc20 destruction box, Conclusions: There are remarkable diff erences in the regulation of Cdc20 and Cdh1/Hct1. Furthermore, the APC activator Cdc20 is itself a substrate of the APC-dependent proteolysi s machinery, and the APC subunits Cdc23 and Cdc27 have a role in the d egradation of Cdc20 during S phase and early mitosis that is not media ted by its destruction box. (C) Current Biology Ltd.