GTPase mechanisms and functions of translation factors on the ribosome

The elongation factors (EF) Tu and G and initiation factor 2 (IF2) from bacteria are multidomain GTPases with essential functions in the elongation and initiation phases of translation. They bind to the same site on the ribosome where their low intrinsic GTPase activities are strongly stimulated. The factors differ fundamentally from each other, and from the majority of GTPases, in the mechanisms of GTPase control, the timing of P-i release, and the functional role of GTP hydrolysis. EF-Tu GTP forms a ternary complex with aminoacyl-tRNA, which binds to the ribosome, Only when a matching codon is recognized, the GTPase of EF-Tu is stimulated, rapid GTP hydrolysis and P-i release take place, EF-Tu rearranges to the GDP form, and aminoacyl-tRNA is released into the peptidyltransferase center. In contrast, EF-G hydrolyzes GTP immediately upon binding to the ribosome, stimulated by ribosomal protein L7/12. Subsequent translocation is driven by the slow dissociation of P-i, suggesting a mechano-chemical function of EF-G, Accordingly, different conformations of EF-G on the ribosome are revealed by cryo-electron microscopy. GTP hydrolysis by IF2 is triggered upon formation of the 70S initiation complex, and the dissociation of P-i and/or IF2 follows a rearrangement of the ribosome into the elongation-competent state.