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VOL. 72 (4), 611-627, 2006 HEME RESPONSIVENESS IN VITRO IS A COMMON...

INTRODUCTION

In eukaryotic cells, protein synthesis is mainly regulated at
the level of initiation of mRNA translation. The reversible
phosphorylation of the a-subunit of eukaryotic translation initiation
factor 2 (eIF2a) is a well-characterized mechanism of translational
control in response to a wide variety of cellular stresses, including
nutrient starvation, iron deficiency, heat shock, UV irradiation
and viral infection (1, 2). Four different eIF2a kinases have been
identified that specifically phosphorylate eIF2a on Ser-51. All known
eIF2a kinases share a conserved kinase domain linked to unique
regulatory regions. Thus, HRI is activated both by heme deficiency
and under conditions of heat shock and oxidative stress (3). PKR
is induced by interferon (IFN) and activated by double-stranded
RNA (dsRNA) during viral infection (4). GCN2 (general control
non-derepressible-2) is an eIF2a kinase that is activated by amino
acid or serum deprivation and UV irradiation (5-7). The fourth eIF2a
kinase, PERK, is activated by unfolded proteins in the endoplasmic
reticulum (ER) (8, 9). It has recently cloned and characterized two
novel eIF2a kinases from the fission yeast S. pombe (named SEK1/
Hri1p and SEK2/Hri2p) both of which show higher homology with
mammalian HRI than with other eIF2a kinases (10).

HRI, found most abundantly in erythroid cells, serves to limit
globin protein synthesis when the levels of available heme are
low. However, the presence of HRI mRNA and HRI activity in
non-erythroid tissues and in NIH 3T3 cells raises the possibility
of additional regulatory roles for HRI (11). The eIF2a kinase
GCN2 was originally characterized in Saccharomyces cerevisiae
and GCN2 homologs have been identified in Drosophila melanogaster
(12) and mammals (5), suggesting that GCN2 might be the founding
and also the best conserved member of the eIF2a kinase family (2).
PERK was originally identified in rat pancreatic islet cells (8) and,
recently, PERK homologs have been found in mouse, human,
and Drosophila melanogaster (9, 13). Interestingly, when activated by
their cognate upstream stress signals, the mammalian PERK and
GCN2 repress translation of most mRNAs but selectively increase
translation of ATF4 mRNA, a member of the activating transcription
family (14).

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