Calcineurin regulation of the mammalian G(0)/G(1) checkpoint element, cyclin dependent kinase 4

Cyclin dependent kinase 4 (cdk8) activity is controlled by the binding of r egulatory subunits and inhibitory factors, as well as tyrosine and serine/t hreonine phosphorylation, More recently the influence of calcium levels hav e been demonstrated. Using transient transfections in Jurkat cells, we obse rved specific binding between cdk4 and the calcium and calmodulin activated serine/threonine phosphatase, calcineurin. Furthermore, we demonstrated th at the inhibition of the phosphatase activity of calcineurin with FK506 and cyclosporin A resulted in an overall increase in cdk4 kinase activity, sug gesting that the phosphatase activity of calcineurin was inhibitory to the kinase activity of cdkA In contrast, we were not able to observe a similar effect on the kinase activity of either cdk6 or cdk2, indicating that the p hosphatase activity of calcineurin was specific for cdk4, In addition, usin g an in vitro phosphatase assay for calcineurin, we observed that the exoge nous addition of calcineurin resulted in the dephosphorylation of cdk4, an event that downregulated the kinase activity of cdk4 Calcineurin could, the refore, play an opposing role to the action of the cyclin activating kinase complex, an enzyme that upregulates the kinase activity of cdk4, an import ant G(0)/G(1) checkpoint element in mammalian cells.