Cdc28cyclin

Cdc28cyclin refers to the complexes formed by the essential budding yeast cyclin-dependent kinase Cdc28 and its various regulatory cyclins. In Saccharomyces cerevisiae, Cdc28 functions as the core kinase of the cell cycle, and its activity is determined by which cyclin partner is bound. Together, Cdc28 and specific cyclins drive distinct cell cycle transitions by phosphorylating a range of substrates.

The yeast Cdc28–cyclin family includes several partners. G1 cyclins Cln1, Cln2 and Cln3 pair with Cdc28 to promote START and commitment to another cell cycle, with Cln3 often acting as a rate-limiting trigger. S-phase cyclins Clb5 and Clb6 partner with Cdc28 to facilitate DNA replication, while mitotic cyclins Clb1, Clb2, Clb3 and Clb4 drive progression through mitosis, spindle dynamics, and chromosome segregation. The combination of cyclin type and timing determines substrate specificity and the pace of cell-cycle events.

Regulation of Cdc28 activity involves multiple layers. A CAK-like kinase, Cak1, activates Cdc28 by phosphorylating the T-loop. Inhibitory phosphorylation by Swe1 can restrain Cdc28 activity, which Mih1 counteracts by removing the phosphate to reactivate the kinase. Cyclin synthesis and destruction modulate the availability of Cdc28–cyclin complexes, and Sic1, a CDK inhibitor, is degraded to permit S-phase entry. Overall, Cdc28–cyclin activity coordinates critical transitions from G1 to S phase, through DNA replication, to mitosis and exit, ensuring orderly cell-cycle progression.

Cdc28cyclin complexes are a central model for studying cyclin-dependent kinase regulation, substrate specificity, and the control of eukaryotic cell division.