The Importance of Structural Transitions of the Switch II Region for the Functions of Elongation Factor Tu on the Ribosome

Abstract

Elongation factor Tu (EF-Tu) undergoes a large conformational transition when switching from the GTP to GDP forms. Structural changes in the switch I and II regions in the G domain are particularly important for this rearrangement. In the switch II region, helix α2 is flanked by two glycine residues: Gly83 in the consensus element DXXG at the N terminus and Gly94 at the C terminus. The role of helix α2 was studied by pre-steady-state kinetic experiments using Escherichia coli EF-Tu mutants where either Gly83, Gly94, or both were replaced with alanine. The G83A mutation slows down the association of the ternary complex EF-Tu·GTP·aminoacyl-tRNA with the ribosome and abolishes the ribosome-induced GTPase activity of EF-Tu. The G94A mutation strongly impairs the conformational change of EF-Tu from the GTP- to the GDP-bound form and decelerates the dissociation of EF-Tu·GDP from the ribosome. The behavior of the double mutant is dominated by the G83A mutation. The results directly relate structural transitions in the switch II region to specific functions of EF-Tu on the ribosome.