Limited proteolysis and amino acid replacements in the effector region of Thermus thermophilus elongation factor Tu

Abstract

The effector region of the elongation factor Tu (EF-Tu) from Thermms thermophilus was modified by limited proteolysis or via site-directed mutagenesis. The biochemical properties of the obtained EF-Tu variants were investigated with respect to partial reactions of the functional cycle of EF-Tu. EF-Tu that was cleaved at the Arg59-Gly60 peptide bond [EF-TU-(1-59)/EF-Tu-(60-405)] bound GDP, EF-Ts and aminoacyl-tRNA, had normal intrinsic GTPase activity and was active in poly(U)-dependent poly(Phe) synthesis. However, the GTPase activity of EF-Tu-(1-59)/EF-Tu-(60-405) was not stimulated by T. thermophilus 70S ribosomes, and its GTP-dissociation rate was increased compared with that of intact EF-Tu. EF-Tu cleaved at the Lys52-Ala53 peptide bond has properties similar to EF-Tu-(1-59)/EF-Tu-(60-405). By means of site-directed mutagenesis, Glu55 was replaced by Leu, Glu56 by Ala and Arg59 by Thr in T. thermophilus EF-Tu. These amino acid substitutions did not substantially affect either the affinity of EF-Tu GTP for aminoacyl-tRNA or the interactions with GDP, GTP or EF-Ts. Similarly the intrinsic GTPase activity is not influenced. Replacement of Glu56 by Ala led to strong reduction in the ribosome-induced GTPase activity. This effect is specific since replacement of the neighbouring Glu55 by Leu did not affect the ribosome-induced GTPase activity. The results demonstrate that the structure of the effector region of EF-Tu in the vicinity of Arg59 is important for the control of the GTPase activity by ribosomes.