Phosphorylation of serine 51 in initiation factor 2α (eIF2α) promotes complex formation between eIF2α(P) and eIF2B and causes inhibition in the guanine nucleotide exchange activity of eIF2B

Phosphorylation of serine 51 residue on the a-subunit of eukaryotic initiation factor 2 (eIF2 alpha) inhibits the guanine nucleotide exchange (GNE) activity of eIF2B, presumably, by forming a tight complex with eIF2B. Inhibition of the GNE activity of eIF2B leads to impairment in eIF2 recycling and protein synthesis. We have partially purified the wild-type (wt) and mutants of eIF2 alpha in which the serine 51 residue was replaced with alanine (51A mutant) or aspartic acid (51D mutant) in the baculovirus system. Analysis of these mutants has provided novel insight into the role of 51 serine in the interaction between eIF2 and eIF2B. Neither mutant was phosphorylated in vitro. Both mutants decreased eIF2 alpha phosphorylation occurring in hemin and poly(IC)-treated reticulocyte lysates due to the activation of double-stranded RNA-dependent protein kinase (PKR). However, addition of 51D, but not 51A mutant eIF2 alpha protein promoted inhibition of the GNE activity of eIF2B in hemin-supplemented rabbit reticulocyte lysates in which relatively little or no endogenous eIF2 alpha phosphorylation occurred. The 51D mutant enhanced the inhibition in GNE activity of eIF2B that occurred in hemin and poly(IC)-treated reticulocyte lysates where PKR is active. Our results show that the increased interaction between eIF2 and elF2B protein, occurring in reticulocyte lysates due to increased eIF2 alpha phosphorylation, is decreased significantly by the addition of mutant 51A protein but not 51D. Consistent with the idea that mutant 51D protein behaves like a phosphorylated eIF2 alpha, addition of this partially purified recombinant subunit, but not 51A or wt eIF2 alpha, increases the interaction between eIF2 and 2B proteins in actively translating hemin-supplemented lysates. These findings support the idea that phosphorylation of the serine 51 residue in eIF2 alpha promotes complex formation between eIF2 alpha>(*) over bar * (P) and eIF2B and thereby inhibits the GNE activity of eIF2B.