ACTIVITY OF PROTEIN PHOSPHATASES AGAINST INITIATION FACTOR-II AND ELONGATION FACTOR-II

The protein phosphatases active against phosphorylase a, elongation factor-2 (EF-2) and the α-subunit of initiation factor-2 (eIF-2) [eIF-2(αP)] were studied in extracts of rabbit reticulocytes, Swiss-mouse 3T3 fibroblasts and rat hepatocytes, by use of the specific phosphatase inhibitors okadaic acid and inhibitor proteins-1 and -2. In all three extracts tested, both phosphatase-1 and phosphatase-2A contributed to overall phosphatase activity against phosphorylase and eIF-2(αP), but phosphatases-2B and -2C did not. In contrast, only protein phosphatase-2A was active against EF-2. Furthermore, in hepatocytes there was substantial type-2C phosphatase activity against EF-2, but not against phosphorylase or eIF-2α. These findings in cell extracts were borne out by data obtained by studying the activities of purified protein phosphatase-1 and -2A against eIF-2(αP) and EF-2. eIF-2(αP) was a moderately good substrate for both enzymes (relative to phosphorylase a). In contrast, EF-2 was a very poor substrate for protein phosphatase-1, but was dephosphorylated faster than phosphorylase a by protein phosphatase-2A. The implications of these findings for the control of translation and their relationships to previous work are discussed.