IDENTIFICATION OF HUMAN AND MOUSE P19, A NOVEL CDK4 AND CDK6 INHIBITOR WITH HOMOLOGY TO P16(INK4)

The cell cycle in mammalian cells is regulated by a series of cyclins and cyclin-dependent kinases (CDKs). The G(1)/S checkpoint is mainly d ictated by the kinase activities of the cyclin D-CDK4 and/or cyclin D- CDK6 complex and the cyclin E-CDK2 complex. These G(1) kinases can in turn be regulated by cell cycle inhibitors, which may cause the cells to arrest at the G(1) phase. In T-cell hybridomas, addition of anti-T- cell receptor antibody results not only in G(1) arrest but also in apo ptosis. In searching for a protein(s) which might interact with Nur77, an orphan steroid receptor required for activation-induced apoptosis of T-cell hybridomas, we have cloned a novel human and mouse CDK inhib itor, p19. The deduced p19 amino acid sequence consists of four ankyri n repeats with 48% identity to p16. The human p19 gene is located on c hromosome 19p13, distinct from the positions of p18, p16, and p15. Its mRNA is expressed in all cell types examined. The p19 fusion protein can associate in vitro with CDK4 but not with CDK2, CDC2, or cyclin A, B, E, or D1 to D3. Addition of p19 protein can lead to inhibition of the in vitro kinase activity of cyclin D-CDK 1 but not that of cyclin E-CDK2. In T-cell hybridoma DO11.10, p19 was found in association with CDK4 and CDK6 in vivo, although its association with Nur77 is not cle ar at this point, Thus, p19 is a novel CDK inhibitor which may play a role in the cell cycle regulation of T cells.