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 that targets proteins containing destruction-box sequences for degradation and that uses a ubiquitin ligase known as the anaphase-promoting complex/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 cyclins at the end of mitosis. Recently, two highly conserved WD40-repeat proteins, Cdc20 and Cdh1/Hct1, have been identified as substrate-specific regulators of APe-dependent proteolysis in the budding yeast Saccharomyces cerevisiae. Here, we have investigated the cell cycle regulation of Cdc20 and Cdh1/Hct1. Results: Whereas the levels of CDH1/HCT1 RNA and Cdh1/Hct1 protein are constant throughout the cell cycle, CDC20 RNA and Cdc20 protein are present only during late S phase and mitosis and Cdc20 protein is unstable throughout the entire cell cycle. The instability of Cdc20 depends on CDC23 and CDC27, which encode components of the APC. During the G1 phase, a destruction box within Cdc20 mediates its instability, but during S phase and mitosis, although Cdc20 destruction is still dependent on CDC23 and CDC27 it does not depend on the Cdc20 destruction box. Conclusions: There are remarkable differences in the regulation of Cdc20 and Cdh1/Hct1. Furthermore, the APC activator Cdc20 is itself a substrate of the APe-dependent proteolysis machinery, and the APC subunits Cdc23 and Cdc27 have a role in the degradation of Cdc20 during S phase and early mitosis that is not mediated by its destruction box.