REGULATION OF SYNTHESIS AND ACTIVITY OF THE PLSTIRE PROTEIN (CYCLIN-DEPENDENT KINASE-6 (CDK6)), A MAJOR CYCLIN-D-ASSOCIATED CDK4 HOMOLOG INNORMAL HUMAN T-LYMPHOCYTES

The PLSTIRE protein (cyclin-dependent kinase 6 (cdk6)), which shares e xtensive sequence homology (similar to 70%) with cdk4, was identified as the earliest inducible member of the cdk family of proteins in huma n T lymphocytes induced to proliferate in vitro by stimulation either with phorbol 12,13-dibutyrate and ionomycin (PDB/I) or PHA. The p40(cd k6) protein was present in resting cells and increased amounts were de tected 6 h after stimulation. It increased in amount throughout the fi rst cell cycle but was present in reduced amounts at later times. Acti vity of the kinase, determined by in vitro phosphorylation of recombin ant truncated retinoblastoma tumor suppressor gene (Rb) protein (p60(R b)), paralleled p40(cdk6) protein amounts. Cyclins D2 and D3 were the major cyclins associated with p40(cdk6), With D2 predominating in earl y G1 phase. Both PDB and ionomycin were required for maximal accumulat ion of p40(cdk6), but either agent alone stimulated some increase in a mount and activity of the protein. p40(cdk6) also increased in amount in cells activated in the presence of cyclosporin A or FK506, drugs th at inhibit production of IL-2 and cell proliferation, suggesting that initial induction occurred independently of IL-2-mediated cell cycle p rogression. Furthermore, increased accumulation of p40(cdk6) protein a nd activity occurred in cells rendered ''competent'' (responsive to IL -2) by a brief treatment with PDB/I. Thus, increased accumulation of t he protein and its activity begin before IL-2/IL-2 receptor interactio n, suggesting that the cdk6-cyclin D2 complex might be involved in acq uisition of the competent state in human T lymphocytes.