Three phosphorylation sites in elongation factor 2

Abstract
Elongation factor 2 (EF‐2) of rabbit reticulocytes was phosphorylated in vitro by incubation with partially purified EF‐2 kinase and (γ32P)ATP. After exhaustive tryptic hydrolysis 4 phosphopeptides were revealed by two‐dimensional peptide mapping. The phosphopeptides were isolated by high performance liquid chromatography and sequenced. A comparison of the primary structure of the phosphopeptides with that of EF‐2 showed that all 4 phosphopeptides originated from one region of EF‐2 located near the N‐terminus that contains 3 threonine residues: Thr‐53, Thr‐56, Thr‐58. A direct estimation of localization of radioactive phosphate in the phosphopeptides demonstrated that all the enumerated threonine residues in EF‐2 can be phosphorylated in vitro.