CLB4

Clb4, or Cyclin B4, is a mitotic cyclin in the budding yeast Saccharomyces cerevisiae, encoded by the CLB4 gene. It belongs to the B-type cyclins that activate the cyclin-dependent kinase Cdc28 (CDK1) to drive cell-cycle progression. Expression of CLB4 rises in late S phase and peaks during G2/M, and the Clb4–Cdc28 complex localizes to the nucleus and the mitotic spindle, where it helps regulate spindle formation and chromosome segregation.

Functionally, Clb4 contributes to mitotic entry and progression, with a particular emphasis on spindle organization, chromosome alignment, and timely separation. It operates in parallel and partially redundantly with other G2/M cyclins such as Clb2; clb4Δ mutants show mild defects, whereas clb2 clb4 double mutants exhibit stronger mitotic defects and spindle abnormalities. Clb4 is also implicated in spindle pole body duplication and kinetochore function, influencing proper spindle dynamics and nuclear division.

Regulation of Clb4 follows the standard CDK–cyclin control paradigm. Clb4 is degraded during the exit from mitosis via the APC/C ubiquitin ligase, helping to reset CDK activity for the next cell cycle. Transcriptional control coordinates CLB4 expression with mitotic progression, and kinase activity is modulated by phosphorylation and subcellular localization.

In other yeasts, CLB4 homologs share a general role in promoting G2/M progression, reflecting the conserved function of B-type cyclins in fungal cell division.