RESUMO
The recently solved structure of the ternary complex formed between GTP-bound elongation factor Tu and aminoacylated tRNA reveals that the elements of aminoacyl-tRNA that interact with elongation factor Tu can be divided into three groups: the T stem; the 3'-end CCA-Phe; and the 5' end. The conserved residues Arg288, Lys89 and Asn90 are involved in the binding of the 5' end. In the active, GTP-bound form of the elongation factor, Arg288 and Asn90 are involved in the formation of a network of hydrogen bonds connecting the switch regions I and II of domain 1 with the rest of the molecule. This network is disrupted upon formation of the ternary complex. Arg288 was replaced by alanine, isoleucine, lysine or glutamic acid, and the resulting mutants have been subjected to an in vitro characterisation with the aim of clarifying the function of Arg288. Unexpectedly, the mutants behaved like the wild-type factor with regard to the association and dissociation of guanine nucleotides, and the intrinsic GTPase activities are unchanged. Furthermore, the mutants were as efficient as the wild-type factor in carrying out protein synthesis in vitro in the presence of an excess of aminoacyl-tRNA. However, the mutants' abilities to bind aminoacyl-tRNA and protect the labile aminoacyl bond were impaired, especially where the charge had been reversed.
