Cell cycle start from quiescence controlled by tyrosine phosphorylation ofCdk4

In mammals Cdk4 (or Cdk6 in some cell types) is required for starting the c ell cycle. Recently we showed that Cdk4 is regulated by tyrosine phosphoryl ation and dephosphorylation, and that this regulation is required for a DNA damage-induced G(1) arrest. We report here that a generic anti-phosphotyro sine antibody can detect tyrosine-phosphorylated Cdk4 and that as revealed by immunoblot detection and kinase assay, this regulation is employed for D IVA damage-responsive checkpoint control during cell cycle start from quies cence. In rat fibroblasts traversing G(1) or arrested in G(1) by deprivatio n of anchorage, Cdk4 does not undergo tyrosine phosphorylation, Tyrosine ph osphorylation occurs only during cell's arrest in quiescence and dephosphor ylation during their cell cycle start. Ultraviolet irradiation blocks depho sphorylation and concomitant activation of Cdk4, thereby preventing the sta rt of cell cycling, Thus, unlike tyrosine phosphorylation of Cdc2, which co ntrols phase transition in the regular cell cycle, tyrosine phosphorylation of Cdk4 is employed for controlling cell cycle start from quiescence in a rat fibroblast.