Clb1Clb2

Clb1Clb2 refers to the mitotic cyclin–CDK complex formed in budding yeast by the association of the core cyclin-dependent kinase Cdc28 with the B-type cyclins Clb1 and Clb2. These cyclins activate Cdc28 to drive the G2–M transition and are essential for progression through mitosis. Clb1 and Clb2 are highly similar and largely redundant in function; strains lacking only CLB1 or CLB2 show mild or conditional defects, whereas simultaneous loss of both is lethal, underscoring their overlapping roles in promoting mitotic entry and ensuring proper mitotic progression.

During the cell cycle, Clb1/Clb2–Cdc28 activity promotes key mitotic events, including entry into mitosis, spindle formation and function, chromosome congression and alignment, and timely progression to anaphase. Their substrates include regulators of microtubule dynamics and chromosome segregation, with phosphorylation coordinating the orderly sequence of mitotic events. Activity of the Clb1/Clb2–Cdc28 complex is tightly regulated at multiple levels, including transcriptional control of CLB1 and CLB2 expression, and proteolysis of the cyclins by the anaphase-promoting complex/cyclosome (APC/C) to promote mitotic exit and reset of the cell cycle.

In yeast genetics and cell biology, Clb1Clb2 serves as a model for redundant B-type cyclins and their role in robust mitotic control. The concept also reflects broader themes in eukaryotic cell cycle regulation, where cyclin–CDK activity must be precisely timed and confined to successful cell division. Homologous B-type cyclins exist in other yeasts and higher eukaryotes, illustrating conserved principles of cyclin-dependent kinase regulation.