Partial purification and characterization of reticulocyte phosphatase with activity for phosphorylated peptide...

An enzyme fraction containing phosphatase activity for phosphorylated eukaryotic peptide initiation factor 2 (eIF-2) has been isolated from rabbit reticulocytes and partially characterized. The enzyme efficiently catalyzes release of phosphate from the small subunit of eIF-2 (eIF-2 alpha) that has been phosphorylated by the hemin-controlled repressor. It is shown to restore activity of this phosphorylated eIF-2 for binding of methionyl-tRNAf to 40 S ribosomal subunits in a partial reaction of peptide initiation. The enzyme fraction also has phosphatase activity for eIF-2 phosphorylated in its largest subunit and for the 100,000-dalton peptide associated with the eIF-2 alpha kinase activity of the hemin-controlled repressor. The phosphoprotein phosphatase has been isolated by a procedure involving precipitation with ethanol at room temperature and has an apparent molecular weight in the order of 76,000. Its phosphatase activity for eIF-2 alpha is stimulated about 3-fold by optimal concentrations of Mn2+, but is not stimulated by Ca2+ or Mg2+. The enzyme is strongly inhibited by Fe2+ and by purine nucleoside diphosphates.