Phosphorylation of the elongation factor 2: the fifth Ca2+/calmodulin-dependent system of protein phosphorylation.
Elongation factor 2 (EF-2) has been recently shown to be extensively phosphorylated in a Ca2+/calmodulin-dependent manner in extracts of mammalian cells (A. G. Ryazanov (1987) FEBS Lett. 214, 331-334). In the present study, we partially purified the protein kinase which phosphorylates EF-2 from rabbit reticulocytes. The molecular weight of the enzyme determined by gel filtration was about 140,000. Unlike the substrate, the EF-2 kinase had no affinity for RNA and therefore could be separated from EF-2 by chromatography on RNA-Sepharose. After chromatography on hydroxyapatite, the kinase activity became calmodulin-dependent. Two-dimensional separation of the phosphorylated EF-2 according to O'Farrell's technique revealed that there were two phosphorylation sites within the EF-2 molecule; in both cases, the phosphorylated amino acid was threonine. The EF-2 kinase differed from the four known types of Ca2+/calmodulin-dependent protein kinases. Thus, the system of EF-2 phosphorylation represents the novel (fifth) Ca2+/calmodulin-dependent system of protein phosphorylation. This system is supposed to be responsible for the regulation of the elongation rate of protein biosynthesis in eukaryotic cells.