Quantitative differences between cyclin-dependent kinases underlie the unique functions of CDK1 in human cells.

Summary One of the most intriguing features of cell-cycle control is that, although there are multiple cyclin-dependent kinases (CDKs) in higher eukaryotes, a single CDK is responsible for both G1-S and G2-M in yeasts. By leveraging a rapid conditional silencing system in human cell lines, we confirm that CDK1 assumes the role of G1-S CDK in the absence of CDK2. Unexpectedly, CDK1 deficiency does not prevent mitotic entry. Nonetheless, inadequate phosphorylation of mitotic substrates by noncanonical cyclin B-CDK2 complexes does not allow progression beyond metaphase and underscores deleterious late mitotic events, including the uncoupling of anaphase A and B and cytokinesis. Elevation of CDK2 to a level similar to CDK1 overcomes the mitotic defects caused by CDK1 deficiency, indicating that the relatively low concentration of CDK2 accounts for the defective anaphase. Collectively, these results reveal that the difference between G2-M and G1-S CDKs in human cells is essentially quantitative.