Phosphorylation of protein synthesis initiation factor-2. Identification of the site in the α-subunit phosphorylated in reticulocyte lysates

The data presented here show that serine-51 of the α-subunit of eukaryotic initiation factor eIF-2 is the only residue phosphorylated by the eIF-2α-specific kinases HCR (haem-controlled repressor) and dsI (double-stranded RNA-activated inhibitor) in vitro. This confirms our earlier finding that serine-48 is not labelled by either kinase. Methodology appropriate for the examination of phosphorylation sites in eIF-2α in whole cells and their extracts has been developed, and used to study the site(s) in eIF-2α labelled in reticulocyte lysates. Only serine-51 became phosphorylated under conditions of haem-deficiency or in the presence of double-stranded RNA. No evidence for a second phosphorylation site on the α-subunit was obtained with the lysates and conditions used here.