ClbCdc28

ClbCdc28 is the cyclin-dependent protein kinase complex in budding yeast Saccharomyces cerevisiae formed by the catalytic subunit Cdc28 and one of six B-type cyclins (Clb1–Clb6). It acts as the central driver of cell cycle progression, phosphorylating numerous substrates to advance from G1 through S phase and into mitosis. Activity and substrate specificity depend on the particular Clb partner; Cln-Cdc28 handles G1/S, while Clb-Cdc28 handles S and M.

The complex is tightly regulated by synthesis and degradation of Clb cyclins, CDK-activating phosphorylation, and inhibitors such as Sic1; Clb-CDK complexes are subject to ubiquitin-mediated degradation by the APC/C at appropriate cell-cycle transitions, which prevents premature progression.

Functional specifics include that Clb5 and Clb6 promote DNA replication and origin firing in S phase; Clb1–Clb4 contribute to spindle assembly and chromosome segregation during mitosis; Clb2 plays a key role in mitotic progression and spindle dynamics. There is redundancy among B-type cyclins, so single deletions are often viable but combinations can cause defects.

Genetic and experimental importance: The Clb-Cdc28 axis was central to early yeast cell-cycle studies and remains a paradigm for CDK-cyclin regulation, including temporal regulation of activity, checkpoint integration, and controlled proteolysis.

Additional notes: The complex’s activity is essential for viability in budding yeast; misregulation can lead to cell-cycle arrest or abnormal division.