Differential effect of Mn2+ on the hemin-controlled translational repressor and the double-stranded RNA-activated inhibitor

The inhibition of protein synthesis that occurs when rabbit reticulocyte lysate is incubated in the absence of hemin is due to the activation of a protein kinase termed the hemin-controlled translational repressor, and that occurring when reticulocyte lysate is incubated with a low level of double-stranded RNA is mediated by the activation of a separate protein kinase termed the double-stranded RNA-activated inhibitor. Both the hemin-controlled translational repressor and the double-stranded RNA-activated inhibitor act by phosphorylating the M_r = 35 000 (α) subunit of eIF-2. MnCl₂ (0.5 mM) partly reverses the inhibition of protein synthesis produced by hemin deficiency but not that induced by double-stranded RNA. In addition, Mn²⁺ reverses the inhibition of binding of [³⁵S]Met-tRNA_f to reticulocyte ribosomal components, isolated on Sepharose 6B, produced by the hemin-controlled translational repressor but not by the double-stranded RNA-activated inhibitor. The effect of Mn²⁺ is mediated at the level of activation and eIF-2α kinase activity of these two regulatory protein kinases. Specifically, Mn²⁺ inhibits activation of the hemin-controlled translational repressor in the absence of hemin and the phosphorylation of eIF-2α by pre-activated translational repressor. In contrast, the phosphorylation of eIF-2α by the double-stranded RNA-activated inhibitor is not suppressed by Mn²⁺, and the activation and autophosphorylation of this inhibitor is enhanced by Mn²⁺. Finally, while the activation and inactivation of the hemin-controlled translational repressor does not appear to be mediated by autophosphorylation and dephosphorylation, the activation of the double-stranded RNA-activated inhibitor does appear to require autophosphorylation.