Effect of hemin on site-specific phosphorylation of eukaryotic initiation factor 2.

Initiation factor 2 (eIF-2) is phosphorylated in vitro by two different cyclic nucleotide-independent protein kinases. As previously shown, a protein kinase activity that comigrates with the major casein kinase activity from rabbit reticulocytes phosphorylates eIF-2beta. In addition, a second protein kinase that specifically phosphorylates eIF-2alpha has been identified. Both protein kinase activities demonstrate cyclic nucleotide-independent activity and are not inhibited by the inhibitor protein diagnostic for cyclic AMP-regulated protein kinase activities. Phosphorylation of eIF-2alpha is almost completely inhibited by 20--35 muM hemin, whereas phosphorylation of eIF-2beta is only partially inhibited. Hemin acts by decreasing the rate of incorporation of phosphate into eIF-2alpha. The protein kinase activity that modifies eIF-2alpha has been shown to have inhibitory activity in the cell-free protein-synthesizing system, whereas the protein kinase for eIF-2beta has no effect. The identity of the former enzyme with the hemin-controlled repressor and role of hemin in the control of initiation are discussed.