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Eukaryotic protein synthesis is a highly regulated process with many of the key proteins being controlled by phosphorylation (for reviews, see Rhoads 1993; Redpath and Proud 1994; Jefferies and Thomas 1996; Merrick and Hershey 1996; Sonenberg and Gingras 1998). Several initiation factors, as well as ribosomal proteins and aminoacyl-tRNA synthetases, are phosphoproteins. While in some cases the physiological role of phosphorylation of these factors is well characterized, in others it remains less clear. In the latter category is elongation factor-2 (eEF2), which catalyzes the translocation of peptidyl-tRNA from the A-site to the P-site on the ribosome. eEF2 is phosphorylated by a highly conserved and specific Ca²⁺/calmodulin-dependent kinase, termed EF2 kinase (also known as CaM kinase III) at Thr56 and Thr58 (Palfrey and Nairn 1995; Nairn and Palfrey 1996). In vitro, Thr56 is more rapidly …