Cell cycle regulation by the ubiquitin pathway

In the past 2 years, two ubiquitin-dependent proteolytic pathways have been established as important players in the regulation of the cell division cycle. In S, cerevisiae, the entry into S phase requires ubiquitin-mediated degradation of a cdk inhibitor, p40(Sic1), in a pathway that involves the E2 enzyme Cdc34, Recent studies reviewed herein show that the Cdc34 pathway targets phosphorylated substrates. A second pathway that regulates chromosome segregation and mitotic exit by degrading anaphase inhibitors and mitotic cyclins involves a different E2 and a large molecular weight E3 complex, called the anaphase-promoting complex or cyclosome. This pathway targets substrates containing one or more destruction box motif.