Two different bindings of p21 Cdk inhibitor to cyclin/Cdk complex

The cyclin-dependent kinase inhibitor p21(p21 CKI) has been found to inhibit the activity of several Cdks. Of interest wre reports that more than one molecule of p21 CKI appear to be necessary for Cdk kinase inhibition. In this report, we first determined the two different regions of p21CKI that were important for binding to cyclin D1/Cdk4 complex. Mutant analysis revealed that the either binding site is enough for their binding but for Cdk4 kinase inhibition both sites are necessary. Since the mutants (delta 17-22 or W49G) which lacks either binding site could complement each other for kinase inhibition, two molecules of p21 CKI with different binding mode might be a mechanism of cyclin D1/Cdk4 kinase inhibition.