CLB1CLB6

CLB1CLB6 refers to the six B-type cyclins encoded by the CLB1, CLB2, CLB3, CLB4, CLB5, and CLB6 genes in the budding yeast Saccharomyces cerevisiae. These cyclins partner with the essential cyclin-dependent kinase Cdc28 and together regulate key transitions in the cell cycle. The CLB genes are expressed and degraded in a cell-cycle–dependent manner, enabling stage-specific activation of CDK activity.

Functionally, the CLB family can be divided into groups with distinct timing and roles. CLB5 and CLB6 act predominantly during S phase to promote DNA replication origin firing and progression through early replication steps. By contrast, CLB1, CLB2, CLB3, and CLB4 contribute mainly to mitosis, with roles in mitotic entry, spindle dynamics, chromosome alignment, and segregation. CLB2 often serves as the principal mitotic cyclin, while CLB1–CLB4 provide redundancy and fine-tuning of mitotic events. The diverse expression patterns of these cyclins enable coordinated control of CDK activity across the cell cycle.

Regulation and significance include transcriptional control aligned with cell-cycle cues and targeted degradation by the anaphase-promoting complex (APC/C) to ensure orderly cell-cycle progression and exit from mitosis. Genetic studies show that loss or alteration of multiple CLB genes leads to cell-cycle defects or lethality, underscoring the essential, partially overlapping functions of the CLB1CLB6 family. As a model system, CLB1CLB6 helps illuminate how multiple cyclins coordinate CDK activity to drive replication and division.