Résumé
The elongation factor Tu (EF-Tu) is historically the first GTP-binding protein that was studied with respect to the relation between structure and function (for refs, see Weijland et al., 1992). It is therefore not surprising that it has widely been used as a model to investigate the common properties of the diverse families of these proteins that act as molecular switches in fundamental cellular pathways (Bourne et al., 1990; 1991). EF-Tu is a key component for protein biosynthesis and cell growth. As all GTP-binding proteins, it cycles between a GTPand a GDP-bound state; in the GTP-induced state, it forms a complex with aa-tRNA that binds to the A-site of the mRNA-programmed ribosome. The interactions with aa-tRNA and ribosome are disrupted by the hydrolysis of the bound GTP that causes the release of EF-Tu•GDP, because its affinity for aa-tRNA is lower than that of EF-Tu•GTP by five orders of magnitude. The EF-Tu•GDP complex is several hundreds of times tighter than the EF-Tu•GTP complex and is characterized by a slow dissociation rate. Therefore the intervention of elongation factor Ts (EF-Ts), a GDP dissociation stimulator of EF-Tu, is crucial for a rapid regeneration of the active EF-Tu•GTP complex, as is required for the physiological rate of protein biosynthesis (10–20 amino acid residues incorporated•sec-1). This canonical scheme of the EF-Tu cycle presents some controversial aspects: recently it has been proposed that two molecules of EF-Tu•GTP, instead of one, interact with one molecule of aa-tRNA, thus forming a pentameric complex; two EF-Tu-bound GTP molecules should be hydrolyzed per each amino acid incorporated (Ehrenberg et al., 1990; Bilgin et al., 1992).