Glucosamine-induced phosphorylation of the α-subunit of eukaryotic initiation factor 2 is mediated by the protein kinase R-like endoplasmic-reticulum associated kinase

ABSTRACT Protein synthesis is regulated by the phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α) in response to different environmental stresses. One member of the eIF2α kinase family, heme-regulated inhibitor kinase (HRI), is activated under heme-deficient conditions and blocks protein synthesis, principally globin, in mammalian erythroid cells. We identified two HRI-related kinases from Schizosaccharomyces pombe which have full-length homology with mammalian HRI. The two HRI-related kinases, named Hri1p and Hri2p, exhibit autokinase and kinase activity specific for Ser-51 of eIF2α, and both activities were inhibited in vitro by hemin, as previously described for mammalian HRI. Overexpression of Hri1p, Hri2p, or the human eIF2α kinase, double-stranded-RNA-dependent protein kinase (PKR), impeded growth of S. pombe due to elevated phosphorylation of eIF2α. Cells from strains with deletions of the hri1+ and hri2+ genes, individually or in combination, exhibited a reduced growth rate when exposed to heat shock or to arsenic compounds. Measurements of in vivo phosphorylation of eIF2α suggest that Hri1p and Hri2p differentially phosphorylate eIF2α in response to these stress conditions. These results demonstrate that HRI-related enzymes are not unique to vertebrates and suggest that these eIF2α kinases are important participants in diverse stress response pathways in some lower eukaryotes.

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Inhibition of protein synthesis initiation by oxidized glutathione: Activation of a protein kinase that phosphorylates the subunit of eukaryotic initiation factor 2

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.75.9.4110 ◽

1978 ◽

Vol 75 (9) ◽

pp. 4110-4114 ◽

Cited By ~ 63

Author(s):

V. Ernst ◽

D. H. Levin ◽

I. M. London

Keyword(s):

Protein Synthesis ◽

Protein Kinase ◽

Initiation Factor ◽

Oxidized Glutathione ◽

Eukaryotic Initiation Factor ◽

Eukaryotic Initiation Factor 2 ◽

Inhibition Of Protein Synthesis ◽

Initiation Factor 2 ◽

Protein Synthesis Initiation

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Modulation of the Eukaryotic Initiation Factor 2 α-Subunit Kinase PERK by Tyrosine Phosphorylation

Journal of Biological Chemistry ◽

10.1074/jbc.m704612200 ◽

2007 ◽

Vol 283 (1) ◽

pp. 469-475 ◽

Cited By ~ 53

Author(s):

Qiaozhu Su ◽

Shuo Wang ◽

Hong Qing Gao ◽

Shirin Kazemi ◽

Heather P. Harding ◽

...

Keyword(s):

Tyrosine Phosphorylation ◽

Initiation Factor ◽

Eukaryotic Initiation Factor ◽

Α Subunit ◽

Eukaryotic Initiation Factor 2 ◽

Initiation Factor 2

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Parasite-specific eIF2 (eukaryotic initiation factor-2) kinase required for stress-induced translation control

Biochemical Journal ◽

10.1042/bj20040262 ◽

2004 ◽

Vol 380 (2) ◽

pp. 523-531 ◽

Cited By ~ 61

Author(s):

William J. SULLIVAN ◽

Jana NARASIMHAN ◽

Micah M. BHATTI ◽

Ronald C. WEK

Keyword(s):

Initiation Factor ◽

Induced Response ◽

Alkaline Stress ◽

Eukaryotic Initiation Factor ◽

Translation Control ◽

Serine Residue ◽

Α Subunit ◽

Eukaryotic Initiation Factor 2 ◽

Initiation Factor 2

The ubiquitous intracellular parasite Toxoplasma gondii (phylum Apicomplexa) differentiates into an encysted form (bradyzoite) that can repeatedly re-emerge as a life-threatening acute infection (tachyzoite) upon impairment of immunity. Since the switch from tachyzoite to bradyzoite is a stress-induced response, we sought to identify components related to the phosphorylation of the α subunit of eIF2 (eukaryotic initiation factor-2), a well-characterized event associated with stress remediation in other eukaryotic systems. In addition to characterizing Toxoplasma eIF2α (TgIF2α), we have discovered a novel eIF2 protein kinase, designated TgIF2K-A (Toxoplasma gondiiinitiation factor-2kinase). Although the catalytic domain of TgIF2K-A contains sequence and structural features that are conserved among members of the eIF2 kinase family, TgIF2K-A has an extended N-terminal region that is highly divergent from other eIF2 kinases. TgIF2K-A specifically phosphorylates the regulatory serine residue of yeast eIF2α in vitro and in vivo, and can modulate translation when expressed in the yeast model system. We also demonstrate that TgIF2K-A phosphorylates the analogous regulatory serine residue of recombinant TgIF2α in vitro. Finally, we demonstrate that TgIF2α phosphorylation in tachyzoites is enhanced in response to heat shock or alkaline stress, conditions known to induce parasite differentiation in vitro. Collectively, this study suggests that eIF2 kinase-mediated stress responses are conserved in Apicomplexa, and a novel family member exists that may control parasite-specific events, including the clinically relevant conversion into bradyzoite cysts.

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