Structural similarity between the pocket region of retinoblastoma tumour suppressor and the cyclin-box.

The pocket region of retinoblastoma tumour suppressor (Rb) is essential for tumour suppressing activity. The Rb pocket is primarily composed of two domains, A and B. We have determined the X-ray crystal structure of domain A (residues 378-562) at 2.3 A resolution. Domain A consists of nine alpha-helices. The overall arrangement of helices in domain A is remarkably similar to the cyclin-box folds found in the crystal structures of cyclin A and TFIIB. This structure, along with domain B which is predicted to be homologous to the cyclin-box, suggests that the Rb pocket is composed of two cyclin-box fold domains. We present the structural/functional features of the Rb pocket, and the potential binding region for cellular or viral proteins within domain A.