Okadaic acid‐sensitive protein phosphatases dephosphorylate MARCKS, a major protein kinase C substrate

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