Budding yeasts have a single CDK catalytic subunit, encoded by the gene CDC28, that collaborates with no fewer than 9 different cyclins to regulate events in the cell cycle. The cyclins can be categorized into 2 major classes, G 1 and B type, and these classes can then be divided into subclasses on the basis of both primary sequence and pattern of expression. Each class appears to be optimally suited for a specific function in the cell cycle. This specificity is due, in part, to the pattern of accumulation of the cyclins. We have focused our attention on the regulation of initiation of the cell cycle during G 1 in the budding yeast Saccharomyces cerevisiae.
Initiation involves a complex choreography of events driven by the sequential activation of CDKs.
Efforts are under way to elucidate the mechanism of activation of G 1 -specific transcription. Accumulation of the proteins Cln1 and Cln2 activates the Cdc28 CDK, driving the formation of a bud, the presumptive daughter cell, and duplication of spindle poles. These key proteins include the G 1 cyclins themselves and Sic1, a specific inhibitor of CDK activity associated with B-type cyclins. Understanding the basis for specific recognition of phosphorylated substrates by SCF is one goal of our efforts in this area. Cyclin-CDK activity is also regulated in response to environmental signals and during cell differentiation.