Two heme-binding domains of heme-regulated eukaryotic initiation factor-2alpha kinase. N terminus and kinase insertion.
In heme deficiency, protein synthesis in reticulocytes is inhibited by activation of heme-regulated alpha-subunit of eukaryotic initiation factor-2alpha (eIF-2alpha) kinase (HRI). Previous studies indicate that HRI contains two distinct heme-binding sites per HRI monomer. To study the role of the N terminus in the heme regulation of HRI, two N-terminally truncated mutants, Met2 and Met3 (deletion of the first 103 and 130 amino acids, respectively), were prepared. Met2 and Met3 underwent autophosphorylation and phosphorylated eIF-2alpha with a specific activity of approximately 50% of that of the wild type HRI. These mutants were significantly less sensitive to heme regulation both in vivo and in vitro. In addition, the heme contents of purified Met2 and Met3 HRI were less than 5% of that of the wild type HRI. These results indicated that the N terminus was important but was not the only domain involved in the heme-binding and heme regulation of HRI. Heme binding of the individual HRI domains showed that both N terminus and kinase insertion were able to bind hemin, whereas the C terminus and the catalytic domains were not. Thus, both the N terminus and the kinase insertion, which are unique to HRI, are involved in the heme binding and the heme regulation of HRI.