Second phosphorylation site on alpha subunit of eIF-2 in rabbit reticulocyte lysate.
Eukaryotic protein synthesis initiation factor 2, eIF-2, was purified from either hemin-supplemented (translationally active) or hemin-deficient (translationally inactive) rabbit reticulocyte lysate under conditions chosen and demonstrated to preserve the in situ phosphorylation state. Direct analysis of the phosphate content of the alpha-subunit of eIF-2 was determined by chemical analysis of the isolated alpha-subunit or by a combination of vertical slab gel isoelectric focusing and immunoblotting. These results were compared with those obtained from an indirect analysis utilising the incorporation of [gamma-32P]ATP into eIF-2 by the heme-sensitive eIF-2 alpha-kinase. All three analyses demonstrate that the phosphorylation site specific for the heme-sensitive kinase is unoccupied in translationally active lysate and 25-30% occupied in translationally inhibited lysate. In addition, both direct analyses support the existence of a second phosphorylation site on the alpha-subunit, not regulated by hemin and distinct from that phosphorylated by the heme-sensitive kinase. Different reticulocyte lysate batches vary with respect to the activity of the kinase responsible for phosphorylation of the second site. Further investigations demonstrated that this kinase is a membrane-associated protein.