CLB1CLB4

Clb1Clb4 refers to the four B-type cyclins encoded by the CLB1, CLB2, CLB3 and CLB4 genes in the budding yeast Saccharomyces cerevisiae. These proteins bind to the cyclin-dependent kinase Cdc28 to form active CDK complexes that drive cells from late G2 into mitosis. As a group, Clb1–Clb4 are central regulators of mitotic entry and progression in normal cell division.

During mitosis, Clb1–Clb4 coordinate events such as spindle assembly, chromosome condensation, sister chromatid separation, and mitotic exit. They activate Cdc28 substrates that promote spindle dynamics, chromosome segregation, and other mitotic processes, with timing that depends on the individual cyclin family member.

Genetic and functional relationships among Clb1–Clb4 show partial redundancy and specialization. Clb2 is the dominant mitotic cyclin, while Clb1 can partly compensate for Clb2 loss in some conditions. Clb3 and Clb4 contribute to early mitotic events and spindle dynamics, and together with Clb1/Clb2 support timely progression through mitosis. Loss of multiple Clb cyclins yields strong arrest phenotypes.

Regulation and degradation: Clb1–Clb4 levels rise during late S to G2 and are degraded by the anaphase-promoting complex (APC/C) through Cdc20 during anaphase, followed by Cdh1-mediated degradation to exit mitosis. This proteolysis lowers CDK activity and helps reset the cell cycle after mitosis.

Clb1–Clb4 are widely used in studies of cyclin-dependent kinase regulation and mitotic control, illustrating redundancy, specificity, and the interplay between cyclins and the CDK in a eukaryotic model. Insights from yeast Clb1–Clb4 have informed understanding of cyclin B function in higher organisms.