Swe1 regulation and transcriptional control restrict the activity of mitotic cyclins toward replication proteins in Saccharomyces cerevisiae

Cyclin-dependent kinases (CDKs) drive the cell cycle through the phosphorylation of substrates that function in genome duplication and cell division. The existence of multiple cyclin subunits and their distinct cell cycle-regulated expression suggests that cyclins impart unique specificities to CDK-substrate interactions that are critical for normal cellular function. This study shows that the combination of early cell cycle expression and deletion of the CDK inhibitor Saccharomyces Wee1 (Swe1) enables the mitotic B-type (Clb) cyclins CIb2, CIb3, and CIb4 of Saccharomyces cerevisiae to initiate S phase with similar effectiveness as the S-phase cyclin CIb5. Although in vivo analysis indicates preferential phosphorylation of a replication substrate by Clb5-Cdk1, this difference is relatively minor compared with the impact of transcriptional control and Swell regulation. Indeed, early expressed Cib2-Cdk1 can activate all essential Clb-Cdk substrates in a strain lacking all other Clbs and Swe1. Thus, Swell regulation and expression timing are key mechanisms that sequester the broad activity of Cib2-Cdk1 from critical substrates. Furthermore, the ability of Swell to inhibit the activity of different B-type cyclins in replication initiation correlates with the normal expression timing of those cyclins, with no apparent in vivo inhibition of CIb5 and CIb6, moderate inhibition of CIb3 and CIb4, and strong inhibition of CIb2. Hence, Swell appears to reinforce the temporal activity of cyclins established through transcriptional control. The conserved nature of CDK function suggests that similar mechanisms regulate CDK specificity in multicellular organisms.