Partial purification and characterization of a 90,000-dalton peptide involved in activation of the eIF-2 alpha protein kinase of the hemin-controlled translational repressor

In the absence of heme, a negative translational control system is activated in reticulocytes or their lysates that causes the phosphorylation of the smallest subunit of peptide initiation factor 2 and the inhibition of peptide initiation. Two partially purified enzyme fractions are shown to give a concerted effect for phosphorylation of this subunit of initiation factor 2 and binding of methionyl-tRNAf to 40S ribosomal subunits. One enzyme fraction contains a 90,000-dalton peptide that functions in activation of an enzyme containing a 100,000-dalton peptide of the other fraction. Phosphorylation of the 100,000-dalton peptide is correlated with activation of the kinase for the smallest subunit of initiation factor 2. Antibodies against the 90,000-dalton peptide decrease phosphorylation of both the 100,000-dalton peptide and the subunit of initiation factor 2. The results indicate that at least two components function in a sequence of reactions that inhibits protein synthesis by phosphorylation of the smallest subunit of eucaryotic initiation factor 2. The same sequence may be activated in the presence of heme by a cascade type of reactions initiated by a heat-stable protein, HS [Henderson, A.B., Miller, A.H., & Hardesty, B. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 2605-2609].