Regulation of eIF-2 α-Subunit Phosphorylation in Reticulocyte Lysate

An eIF-2 associated 67-kDa protein (p⁶⁷) protects eIF-2 α-subunit from eIF-2 kinase(s) catalyzed phosphorylation and promotes protein synthesis in the presence of active eIF-2 kinase(s). p⁶⁷ is a glycoprotein and contains multiple O-linked GlcNAc moieties. We have now studied the roles of hemin, p⁶⁷, and the glycosyl residues on p⁶⁷ in the regulation of eIF-2 α-subunit phosphorylation in reticulocyte lysates. The results are as follows: (i) Both hemin and p⁶⁷ inhibited HRI (heme-regulated protein synthesis inhibitor) and dsI (double-stranded RNA activated protein synthesis inhibitor) catalyzed phosphorylation of eIF-2 α-subunit in vitro. However, only hemin, and not p⁶⁷, inhibited casein kinase catalyzed phosphorylation of eIF-2 β-subunit, (ii) Only p⁶⁷, and not hemin, inhibited eIF-2 α-subunit phosphorylation by eIF-2 kinase(s) in reticulocyte lysate. Significant eIF-2 α-subunit phosphorylation was observed even in the presence of hemin when p⁶⁷ in the reticulocyte lysate was removed by treatment with p⁶⁷ antibodies, (iii) Reticulocyte lysate contains a p⁶⁷-deglycosylase in latent form, and hemin prevents activation of this deglycosylase. In the absence of hemin, this p⁶⁷-deglycosylase is activated. Once activated in the absence of hemin, the activated deglycosylase deglycosylates p⁶⁷, even in the presence of hemin. This inactivates p⁶⁷ and allows eIF-2 kinase to phosphorylate eIF-2 α-subunit and inhibit protein synthesis. Protein synthesis in reticulocyte lysate is thus regulated by two novel cascades of covalent modifications: protein deglycosylation leading to protein phosphorylation.