THE UBC3 (CDC34) UBIQUITIN-CONJUGATING ENZYME IS UBIQUITINATED AND PHOSPHORYLATED IN-VIVO

The transition from G(1) to S phase of the cell cycle in Saccharomyces cerevisiae requires the activity of the Ubc3 (Cdc34) ubiquitin-conjug ating enzyme. S. cerevisiae cells lacking a functional UBC3 (CDC34) ge ne are able to execute the Start function that initiates the cell cycl e but fail to form a mitotic spindle or enter S phase. The Ubc3 (Cdc34 ) enzyme has previously been shown to catalyze the attachment of multi ple ubiquitin molecules to model substrates, suggesting that the role of this enzyme in cell cycle progression depends on its targeting an e ndogenous protein(s) for degradation. In this report, we demonstrate t hat the Ubc3 (Cdc34) protein is itself a substrate for both ubiquitina tion and phosphorylation. Immunochemical localization of the gene prod uct to the nucleus renders it likely that the relevant substrates simi larly reside within the nucleus.