Clb3

Clb3 is a B-type cyclin of the budding yeast Saccharomyces cerevisiae. It binds the cyclin-dependent kinase Cdc28 to form a mitotic CDK complex that regulates progression from late G2 into mitosis and early mitotic events. The Clb3–Cdc28 kinase activity supports initiation of mitosis, proper spindle formation, and chromosome alignment and segregation. Clb3 expression is cell-cycle regulated, rising during late S phase and peaking in G2/M, followed by degradation as cells exit mitosis.

Degradation of Clb3 is controlled by the ubiquitin–proteasome system through the anaphase-promoting complex (APC/C), with activators such as Cdc20 during early mitosis and Cdh1 later in mitotic exit. This regulated destruction helps coordinate the timing of mitotic events and mitotic exit.

Genetic studies show that CLB3 is not absolutely essential but contributes to efficient mitosis. clb3Δ mutants exhibit delays in mitotic progression and spindle abnormalities. The phenotype is often milder on its own due to redundancy with CLB4; double mutants lacking CLB3 and CLB4 display more severe defects and reduced viability, highlighting functional overlap between these B-type cyclins.

Clb3 operates within a network of cell cycle regulators, including other Clb cyclins and the CDK inhibitor Swe1. Its activity is coordinated with Clb4 and Clb5/6 to ensure proper timing of DNA replication, spindle assembly, and chromosome segregation, illustrating the cooperative control of mitosis in budding yeast.