OKADAIC ACID-SENSITIVE PROTEIN PHOSPHATASES DEPHOSPHORYLATE MARCKS, AMAJOR PROTEIN-KINASE-C SUBSTRATE

The myristoylated alanine-rich C kinase substrate (MARCKS) undergoes a rapid and, in certain circumstances, transient increase in phosphoryl ation in response to stimuli that activate protein kinase C. We have i nvestigated the protein-serine/threonine phosphatase activity responsi ble for reversing the phosphorylation of MARCKS. In cell-free assays, protein phosphatases 1, 2A and 2C (PP1, PP2A and PP2C) all dephosphory late recombinant MARCKS or a synthetic peptide based on its phosphoryl ation site domain. In intact Swiss 3T3 cells, okadaic acid, a specific inhibitor of PP1 and PP2A, had little effect on MARCKS phosphorylatio n on its own, but largely prevented the dephosphorylation of MARCKS th at occured following activation of protein kinase C by bombesin with s ubsequent receptor blockade. These results indicate that although the dephosphorylation of MARCKS can be mediated by PP2C in vitro, this pro tein is dephosphorylated by okadaic acid-sensitive phosphatases in the intact cell.