structure of CDK4/cyclin D3 has implications for models of CDK activation

Cyclin-dependent kinase 4 (CDK4)/cyclin D complexes are expressed early in the G₁ phase of the cell cycle and stimulate the expression of genes required for G₁ progression by phosphorylation of the product of the retinoblastoma gene, pRb. To elaborate the molecular pathway of CDK4 activation and substrate selection we have determined the structure of nonphosphorylated CDK4/cyclin D3. This structure of an authentic CDK/cyclin complex shows that cyclin binding may not be sufficient to drive the CDK active site toward an active conformation. Phosphorylated CDK4/cyclin D3 is active as a pRb kinase and is susceptible to inhibition by p27Kip¹. Unlike CDK2/cyclin A, CDK4/cyclin D3 can be inactivated by treatment with λ-phosphatase, implying that phosphorylated T172 is accessible to a generic phosphatase while bound to a cyclin. Taken together, these results suggest that the structural mechanism of CDK4/cyclin D3 activation differs markedly from that of previously studied CDK/cyclin complexes.