Identification of the phosphorylation sites in elongation factor-2 from rabbit reticulocytes.

The sites in eukaryotic elongation factor eEF-2 phosphorylated by the Ca2+/calmodulin-dependent eEF-2 kinase in vitro have been identified. The kinase catalysed the rapid incorporation of one mol of phosphate per mol eEF-2 and the slower incorporation of a second mol. All the phosphorylation sites in eEF-2 are contained in the CNBr fragment corresponding to residues 22-155. Tryptic digestion of phosphorylated eEF-2 yielded 3 phosphopeptides, one being unique to monophosphorylated eEF-2. The phosphorylation sites were identified as threonine residues 56 and 58, the former being more rapidly phosphorylated. Ala-Gly-Glu-Thr-Phe-Thr56-Asp-Thr58-Arg. The same sites are labelled in eEF-2 isolated from reticulocyte lysates.