Regulation of eIF-2 alpha-subunit phosphorylation in reticulocyte lysate.
An eIF-2 associated 67-kDa protein (p67) protects eIF-2 alpha-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 alpha-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 alpha-subunit in vitro. However, only hemin, and not p67, inhibited casein kinase catalyzed phosphorylation of eIF-2 beta-subunit. (ii) Only p67, and not hemin, inhibited eIF-2 alpha-subunit phosphorylation by eIF-2 kinase(s) in reticulocyte lysate. Significant eIF-2 alpha-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 alpha-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.
Chakraborty
A
A
Department of Chemistry, University of Nebraska, Lincoln 68588-0304.
Saha
D
D
Bose
A
A
Chatterjee
M
M
Gupta
N K
NK
D000118:Acetylglucosaminidase / Q000378:metabolism; D000626:Aminopeptidases; D004789:Enzyme Activation; D015852:Eukaryotic Initiation Factor-2 / Q000737:chemistry / Q000378:metabolism*; D006023:Glycoproteins / Q000378:metabolism; D006031:Glycosylation; D006427:Hemin / Q000494:pharmacology; D007700:Kinetics; D063208:Methionyl Aminopeptidases; D010446:Peptide Fragments / Q000378:metabolism; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D012156:Reticulocytes / Q000187:drug effects / Q000378:metabolism*; D019892:eIF-2 Kinase
D015852:Eukaryotic Initiation Factor-2; D006023:Glycoproteins; D010446:Peptide Fragments; D006427:Hemin; D017346:Protein Serine-Threonine Kinases; D019892:eIF-2 Kinase; C106539:p67 deglycosylase; D000118:Acetylglucosaminidase; D000626:Aminopeptidases; C081311:METAP2 protein, human; D063208:Methionyl Aminopeptidases