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 HI and their cyclin subunits in an indistinguishable manner, irrespective of the catalytic subunit, p33^cdk2 or p34^cdc2. 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-p33^cdk2 complex phosphorylated p107 in vitro at most of its sites that are also phosphorylated in human cells, suggesting that the cyclin A - p33^cdk2 complex is a major kinase for p107 in vivo.