Role of a ubiquitin-conjugating enzyme in degradation of S- and M-phase cyclins

CELL cycle progression in eukaryotes is controlled by the p34^cdc21CDC28 protein kinase and its Short-lived, phase-specific regulatory subunits called cyclins^{1, 2}. In Xenopus oocytes, degradation of M-phase (B-type) cyclins is required for exit from mitosis and is mediated by the ubiquitin-dependent proteolytic system³. Here we show that B-type-cyclin degradation in yeast involves an essential nuclear ubiquitin-conjugating enzyme, UBC9. Repression of UBC9 synthesis prevents cell cycle progression at the G2 or early M phase, causing the accumulation of large budded cells with a single nucleus, a short spindle and replicated DNA. In ubc9 mutants both CLB5, an S-phase cyclin^{4, 5}, and CLB2, an M-phase cyclin^{6, 7}, are stabilized. In wild-type cells the CLB5 protein is unstable throughout the cell cycle, whereas CLB2 turnover occurs only at a specific cell-cycle stage⁸. Thus distinct degradation signals or regulated interaction with the ubiquitin-protein ligase system may determine the cell-cycle specificity of cyclin proteolysi.