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A‐ and B‐type cyclins differentially modulate substrate specificity of cyclin‐cdk complexes.

Both cyclins A and B associate with and thereby activate cyclin‐dependent protein kinases (cdks). We have investigated which component in the cyclin‐cdk complex determines its substrate specificity. The A‐ and B‐type cyclin‐cdk complexes phosphorylated histone H1 and their cyclin subunits in an indistinguishable manner, irrespective of the catalytic subunit, p33cdk2 or p34cdc2. In contrast, only the cyclin A‐cdk complexes phosphorylated the Rb‐related p107 protein in vitro. Likewise, binding studies revealed that cyclin A‐cdk complexes bound stably to p107 in vitro, whereas cyclin B‐cdk complexes did not detectably associate with p107, under identical assay conditions. Binding to p107 required both cyclin A and a cdk as neither subunit alone bound to p107. These results demonstrate that although the kinase subunit provides a necessary component for binding, it is the cyclin subunit that plays the critical role in targeting the complex to p107. Finally, we show that the cyclin A‐p33cdk2 complex phosphorylated p107 in vitro at most of its sites that are also phosphorylated in human cells, suggesting that the cyclin A‐p33cdk2 complex is a major kinase for p107 in vivo.