Partial purification and characterization of a 90000-dalton peptide involved in activation of the eIF-2.alpha. protein kinase of the hemin-controlled translational repressor

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Abstract
In the absence of heme, a negative translational control system is activated in [rabbit] 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 gave 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 the 100,000 dalton peptide and the subunit of initiation factor 2. At least 2 components function in a sequence of reactions that apparently inhibit 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.

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