Ser 51 in the NH2-terminal sequence of the α-subunit of eukaryotic peptide initiation factor 2 (eIF-2) has been identified as a second phosphorylation site for the heme-controlled eIF-2α kinase from rabbit reticulocytes. Increased phosphorylation of this serine relative to the previously described phosphorylation site (Ser 48) is observed when the kinase reaction is carried out in the presence of the α-subunit of spectrin. A synthetic peptide corresponding to eIF-2α(41–54) is phosphorylated only in Ser 51 by the eIF-2α kinase.
