clb2

clb2 is a gene in the budding yeast Saccharomyces cerevisiae that encodes the B-type cyclin Clb2p. As a regulatory subunit of the Cdc28-dependent protein kinase, Clb2p partners with Cdc28 to drive progression from late G2 into mitosis and to coordinate early mitotic events such as spindle formation and chromosome segregation.

Expression and regulation of Clb2p are tightly timed. The CLB2 transcript reaches its peak in late G2 and early M phase, and the protein is subsequently degraded as cells exit mitosis. Transcriptional control involves the Forkhead-like transcription factors that coordinate late cell-cycle expression, while post-translational regulation relies on the anaphase-promoting complex (APC/C) targeting Clb2p for destruction, ensuring timely mitotic exit. Clb2p contains destruction motifs that direct its degradation by APC/C in collaboration with coactivators Cdc20 and Cdh1.

Genetically, CLB2 is one of several B-type cyclin genes (including CLB1, CLB3, CLB4, CLB5, and CLB6). CLB2 plays a major role in mitotic entry and progression, but is not strictly essential for viability; cells lacking CLB2 exhibit delays in mitosis and abnormalities in spindle function or the timing of mitotic events. The phenotype is often more pronounced in combination with mutations in other CLB genes, reflecting partial redundancy among B-type cyclins. Clb2p activity is coordinated with G2/M regulators and the morphogenesis of the spindle apparatus, contributing to the proper ordering of mitotic processes.

Overall, CLB2 is a well-studied example of a cyclin–CDK module that governs mitotic entry and progression in budding yeast, illustrating how cyclin fluctuations regulate cell-cycle transitions.