Translocation requires hydrolysis of a molecule of GTP and is mediated by EF-G in bacteria and by eEF2 in eukaryotes. The eukaryotic elongation factor eEF2 is a GTPase involved in the translocation of the peptidyl-tRNA from the A site to the P site on the ribosome. The 95-kDa protein is highly conserved, with 60% amino acid sequence identity between the human and yeast proteins. Two major mechanisms are known to regulate protein elongation and both involve eEF2. First, eEF2 can be modulated by reversible phosphorylation. Increased levels of phosphorylated eEF2 reduce elongation rates presumably because phosphorylated eEF2 fails to bind the ribosomes. Treatment of mammalian cells with agents that raise the cytoplasmic Ca2+ and cAMP levels reduce elongation rates by activating the kinase responsible for phosphorylating eEF2. In contrast, treatment of cells with insulin increases elongation rates by promoting eEF2 dephosphorylation. Second, the protein can be post-translationally modified by ADP-ribosylation. Various bacterial toxins perform this reaction after modification of a specific histidine residue to diphthamide, but there is evidence for endogenous ADP ribosylase activity. Similar to the bacterial toxins, it is presumed that modification by the endogenous enzyme also inhibits eEF2 activity.
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