Functional domains in cyclin D1: pRb-kinase activity is not essential for transformation

Although cyclin D1 plays a major role during cell cycle progression and is involved in human tumourigenesis, its domain structure is still poorly understood. In the present study, we have generated a series of cyclin D1 N- and C-terminal deletion constructs. These mutants were used to define the domains required for transformation of rat embryonal fibroblasts (REF) in cooperation with activated Ha-ras and and to establish correlations with defined biochemical properties of cyclin D1. Protein binding and REF assays showed that the region of the cyclin box required for the interaction with CDK4 as well as C-terminal sequences determining protein stability were crucial for transformation. Surprisingly, however, the N-terminal deletion of 20 amino acids which impaired pRb kinase activity did not affect the transforming ability of cyclin D1. Likewise, no effect on transformation was observed with mutants defective in p21CIP interaction. These observations argue against a crucial role of pRb inactivation or p21CIP squelching in cyclin D1-mediated transformation.