PHOSPHORYLATION AND INACTIVATION OF PROTEIN PHOSPHATASE-1 BY CYCLIN-DEPENDENT KINASES

Protein phosphatase 1 and protein phosphatase 2A contain potential pho sphorylation sites for cyclin-dependent kinases. In the present study we found that rabbit skeletal muscle protein phosphatase 1, as well as recombinant protein phosphatase 1 alpha and protein phosphatase 1 gam ma 1, but not protein phosphatase 2A, was phosphorylated and inhibited by cdc2/cyclin A and cdc2/cyclin B. Phosphopeptide mapping and phosph o amino acid analysis suggested that the phosphorylation site was loca ted at a C-terminal threonine. Neither cdc2/cyclin A nor cdc2/cyclin B phosphorylated an active form of protein phosphatase 1 alpha in which Thr-320 had been mutated to alanine, indicating that the phosphorylat ion occurred at this threonine residue. Furthermore, protein phosphata se 1, but not protein phosphatase 2A, activity was found to change dur ing the cell cycle of human MG-63 osteosarcoma cells. The observed osc illations in protein phosphatase 1 activity during the cell cycle may be due, at least in part, to phosphorylation of protein phosphatase 1 by cyclin-dependent kinases. Together, the results suggest a mechanism for direct regulation of protein phosphatase 1 activity.