The importance of structural transitions of the switch II region for the functions of elongation factor Tu on the ribosome

Elongation factor Tu (EF-Tu) undergoes a large con formational transition w hen 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 rearran gement. In the snitch II region, helix alpha2 is flanked by two glycine res idues: Gly(83) in the consensus element DXXG at the N terminus and Gly(94) the C terminus. The role of helix alpha2 was studied by pre-steady state ki netic experiments using Escherichia coli EF-Tu mutants where either Gly(83) , Gly(94), Or both were replaced with alanine. The G83A mutation slows down the association of the ternary complex EF-Tu.GTP.aminoacyl-tRNA with the r ibosome and abolishes the ribosome-induced GTPase activity of EF-Tu. The G9 4A mutation strongly impairs the conformational change of EF-Tu from the GT P- 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 m utation. The results directly relate structural transitions in the switch I I region to specific functions of EF-Tu on the ribosome.