Differential regulation of the cyclin-dependent kinase inhibitors p21Cip1 and p27Kip1 by phosphorylation directed by the cyclin encoded by Murine Herpesvirus 68

Members of the γ2-herpesvirus family encode cyclin-like proteins that have the ability to deregulate mammalian cell cycle control. Here we report the key features of the viral cyclin encoded by Murine Herpesvirus 68, M cyclin. M cyclin preferentially associated with and activated cdk2; the M cyclin/cdk2 holoenzyme displayed a strong reliance on phosphorylation of the cdk T loop for activity. cdk2 associated with M cyclin exhibited substantial resistance to the cdk inhibitor proteins p21Cip and p27Kip. Furthermore, M cyclin directed cdk2 to phosphorylate p27Kip1 on threonine 187 (T187) and cellular expression of M cyclin led to down-regulation of p27Kip1 and the partial subversion of the associated G1 arrest. Mutation of T187 to a non-phosphorylatable alanine rendered the p27Kip1-imposed G1 arrest resistant to M cyclin expression. Unlike the related K cyclin, M cyclin was unable to circumvent the G1 arrest associated with p21Cip1 and was unable to direct its associated catalytic subunit to phosphorylate this cdk inhibitor. These results imply that M cyclin has properties that are distinct from other viral cyclins and that M cyclin expression alone is insufficient for S phase entry.