Roles of a 67-kDa polypeptide in reversal of protein synthesis inhibition in heme-deficient reticulocyte lysate

Abstract

During heme deficiency in reticulocyte lysates, the heme-regulated protein synthesis inhibitor, HRI, phosphorylates the alpha subunit of eukaryotic initiation factor 2 (eIF-2) and thus inhibits protein synthesis. Two factors, eIF-2 and a reticulocyte-lysate supernatant factor that we term RF, reverse this inhibition. We now report the following. (i) An active eIF-2 preparation contained, in addition to the three subunits (alpha, beta, and gamma), a 67-kDa polypeptide. Pretreatment of eIF-2 with polyclonal antibodies against either isolated alpha subunit or 67-kDa polypeptide almost completely inhibited the reversal activity. Upon further fractionation, three-subunit eIF-2 and the 67-kDa polypeptide were resolved. Neither the three-subunit eIF-2 nor the 67-kDa polypeptide alone was active in protein synthesis inhibition reversal. The activity was, however, restored by combining both the three-subunit eIF-2 and the 67-kDa polypeptide. (ii) Active RF preparations contained eIF-2 alpha (unphosphorylated) and beta subunits and the 67-kDa polypeptide. As with eIF-2, prior treatment of the RF preparation with antibodies to either the alpha subunit or the 67-kDa polypeptide almost completely inhibited the reversal activity. The RF preparation devoid of eIF-2 gamma subunit did not form ternary complex (Met-tRNA(fMet).eIF-2.GTP). The eIF-2 gamma subunit in the free form was isolated, and addition of this isolated gamma subunit to RF promoted significant ternary-complex formation. (iii) Purified HRI efficiently phosphorylated the alpha subunit in the three subunit eIF-2. However, the extent of such phosphorylation was significantly reduced when eIF-2 containing the 67-kDa polypeptide was used. The 67-kDa polypeptide apparently protected eIF-2 alpha subunit from HRI-catalyzed phosphorylation but did not inhibit HRI activity. Based on these results, we suggest that the protein synthesis inhibition reversal activity in both eIF-2 and RF is due to the same components--namely, eIF-2 alpha subunit and the 67-kDa polypeptide. The 67-kDa polypeptide protects eIF-2 alpha subunit from HRI-catalyzed phosphorylation and may also be a necessary component of the functioning eIF-2 molecule.