MnmE is a GTP-binding protein conserved between bacteria and eukarya. It is a dimeric three-domain protein where the two G domains have to approach each other for activation of the potassium-stimulated GTPase reaction. Together with GidA, in a heterotetrameric alpha(2)beta(2) complex, it is involved in the modification of the wobble uridine base U34 of the first anticodon position of particular tRNAs. Here we show, using various spin-labeled MnmE mutants and EPR spectroscopy, that GidA binding induces large conformational and dynamic changes in MnmE. It stimulates the GTPase reaction by stabilizing the GTP-bound conformation in a potassium-independent manner. Surprisingly, GidA binding influences not only the GTP- but also the GDP-bound conformation. Thus GidA is a new type of regulator for a G protein activated by dimerization.