Eukaryotic initiation factor 2-associated glycoprotein, p67, shows differential effects on the activity of certain kinases during serum-starved conditions

Abstract

Phosphorylation of the α-subunit of eukaryotic initiation factor 2 is the major regulatory step in the initiation of protein synthesis in mammals. P67, a cellular glycoprotein, protects phosphorylation of eIF2α from kinases. P67 has five conserved amino acid residues at the D251, D262, H331, E364, and E459 positions. To determine the roles of these conserved amino acid residues in eIF2α phosphorylation during serum-starved conditions, we constitutively expressed D251A, D262A, H331A, E364A, and E459A mutants in rat tumor hepatoma cells. We find that the point mutants D251A, H331A, and E364A lower the levels of eIF2α phosphorylation. These low levels of phosphorylation decrease when serum-starved cells are grown in medium containing serum. To understand the mechanism of action of the p67 mutants in eIF2α phosphorylation during serum-starvation, we performed detailed biochemical analyses with the D251A mutant. We find that neither the O-GlcNAc modification on the D251A mutant nor the binding of D251A mutant with eIF2γ has significant effects on eIF2α phosphorylation during serum-starved conditions. However, the D251A mutant inhibits p67’s activity to suppress the activity of ERK1/2. Our data suggest that both p67 and the D251A mutant bind to ERK1, thus strengthening the idea that p67 regulates the activity of ERK1. During serum-starvation conditions, both PKR and PERK are phosphorylated and the D251A mutant shows increased stability of PERK as well as a slight decrease in its activity. Altogether, our data provide evidence to suggest that p67 modulates the expression and activity of certain eIF2α-specific kinases.