Clb4Cdc28

Clb4Cdc28 refers to the mitotic cyclin-dependent kinase complex in the budding yeast Saccharomyces cerevisiae, composed of the catalytic subunit Cdc28 and the B-type cyclin Clb4. As a member of the CDK–cyclin regulatory system, Clb4Cdc28 participates in driving the cell cycle through mitosis by phosphorylating a variety of substrates involved in spindle dynamics, chromosome alignment, and late mitotic events.

During the cell cycle, Clb4-Cdc28 activity rises to peak around late G2 and early M phase, coordinating processes required for mitotic entry and progression. It functions in concert with other Clb cyclins, particularly Clb2, to ensure proper spindle assembly, chromosome condensation and segregation, and timely progression through mitosis. The activities of Clb4-Cdc28 are defined by phosphorylation of substrate proteins that regulate microtubule dynamics, kinetochore function, and mitotic exit.

Regulation of Clb4-Cdc28 involves transcriptional control of the CLB4 gene, post-translational modulation of Clb4 stability, and degradation by the anaphase-promoting complex/cyclosome (APC/C) with its activators, such as Cdh1. This ensures orderly mitotic progression and exit. Clb4 belongs to a subfamily of B-type cyclins (along with Clb1, Clb2, and Clb3) that show overlapping functions but may have specialized roles in late mitosis and spindle behavior. Genetic analyses indicate that CLB4 works redundantly with other Clb cyclins, and loss of CLB4 yields mitotic defects that can be partially compensated by other cyclins.

Overall, Clb4Cdc28 is a key component of the yeast cell cycle machinery, enabling proper mitotic progression and spindle-related events through regulated CDK activity.