The F-Box Protein Met30 Is Required for Multiple Steps in the Budding Yeast Cell Cycle

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Abstract
The Saccharomyces cerevisiae ubiquitin ligase SCFMet30 is essential for cell cycle progression. To identify and characterize SCFMet30-dependent cell cycle steps, we used temperature-sensitive met30 mutants in cell cycle synchrony experiments. These experiments revealed a requirement for Met30 during both G1/S transition and M phase, while progression through S phase was unaffected by loss of Met30 function. Expression of the G1-specific transcripts CLN1, CLN2, and CLB5 was very low in met30 mutants, whereas expression of CLN3 was unaffected. However, overexpression of Cln2 could not overcome the G1 arrest. Interestingly, overexpression of Clb5 could induce DNA replication in met30 mutants, albeit very inefficiently. Increased levels of Clb5 could not, however, suppress the cell proliferation defect of met30 mutants. Consistent with the DNA replication defects, chromatin immunoprecipitation experiments revealed significantly lower levels of the replication factors Mcm4, Mcm7, and Cdc45 at replication origins in met30 mutants than in wild-type cells. These data suggest that Met30 regulates several aspects of the cell cycle, including G1-specific transcription, initiation of DNA replication, and progression through M phase.