Regulation of protein synthesis in rabbit reticulocyte lysates: Inhibition of eIF-2 phosphoprotein phosphatase by NaF, pyrophosphate and cations

Abstract

The nature and the role of eIF-2 phosphoprotein phosphatase in rabbit reticulocyte lysates have been examined. The eIF-2 phosphoprotein phosphatase is inhibited by a variety of divalent metal ions (Cd ++>Ag ++> Cu ++>Pb ++>Zn ++>Co ++>Sr ++>Mo ++) in lysates in situ . In addition, PPi, EDTA and NaF inhibit this enzyme. The eIF-2 phosphoprotein phosphatase is also inhibited by NaHSO 3 and Na 2S 2O 5. Na 2S 2O 5 is, however, more effective. Na 2S 2O 5 has been found to be a potent inhibitor of protein synthesis in lysates. This inhibition is associated with the phosphorylation of the 38,000-dalton subunit of initiation factor eIF-2. eIF-2 overcomes this inhibition. These findings suggest that under optimum conditions of protein synthesis the phosphorylation and dephosphorylation of eIF-2 are in a dynamic state of equilibrium in which dephosphorylation is favored. The inhibition of eIF-2 phosphoprotein phosphatase by Na 2S 2O 5 shifts this equilibrium in favor of eIF-2 phosphorylation, consequently, protein synthesis is inhibited. The sulfhydryl nature of eIF-2 phosphoprotein phosphatase has been established.