Direct interaction of p21 cyclin-dependent kinase inhibitor with the retinoblastoma tumor suppressor protein.

The p21CKI forms a physical complex with the retinoblastoma protein (pRb) both in vitro and in vivo. The A/B pocket region of pRb and the N-terminal region of p21 were indispensable for this interaction. Among p21 family members, p57, but not p27, associated with pRb. Overexpression of cyclin D1, Cdk4, and E2F1 in the cells expressing pRb and p21 did not perturb the interaction between p21 and pRb. Coexpression of p21 in cells expressing pRb, cyclin D1, and Cdk4 prevented pRb hyperphosphorylation by cyclin D1/Cdk4. On the other hand, hyperphosphorylation of pRb by an excess amount of cyclin/Cdk disrupted pRb/p21 complex formation in vitro. These findings suggest that pRb may be dynamically regulated by the relative binding and activities of p21 and Cdks.