PHOSPHORYLATION AND ACTIVATION OF THE ATP-MG-DEPENDENT PROTEIN PHOSPHATASE BY THE MITOGEN-ACTIVATED PROTEIN-KINASE

Inhibitor-2 (I-2) is the regulatory subunit of the cytosolic ATP-Mg-dependent form of type 1 serine/threonine protein phosphatase and its phosphorylation at Thr-72 by glycogen synthase kinase-3 results in phosphatase activation. Activation of cytosolic type 1 phosphatase has been observed in cells treated with growth factors. Reported here is the phosphorylation and activation of the ATP-Mg-dependent phosphatase by mitogen-activated protein kinase (MAPR). Recombinant I-2 was phosphorylated by activated MAPK to an extent (similar to 0.3 mol of phosphate/mol of polypeptide) similar to that reported for phosphorylation by the alpha isoform of glycogen synthase kinase-3. The phosphorylation of I-2 by MAPK was exclusively at Thr-72, the site involved in the activation of phosphatase. Incubation of MAPK with purified ATP Mg-dependent phosphatase resulted in phosphorylation of the I-2 component and activation of the phosphatase. Ribosomal S6 protein kinase II (p9O(rsh)) was also able to phosphorylate the recombinant I-2; however, this phosphorylation occurred on serines and had no effect on phosphatase activation. Our data may explain growth factor-induced activation of the ATP-Mg-dependent phosphatase and suggest that MAPK may be the physiological kinase responsible for the activation of cytosolic type 1 phosphatase in response to insulin and/or other growth factors.