Functional interaction of mammalian valyl-tRNA synthetase with elongation factor EF-1 alpha in the complex with EF-1H

In mammalian cells valyl-tRNA synthetase (ValRS) forms a high M-r complex w ith the four subunits of elongation factor EF-1H. The beta, gamma, and delt a subunits, that contribute the guanine nucleotide exchange activity of EF- 1H, are tightly associated with the NH2-terminal polypeptide extension of v alyl-tRNA synthetase, In this study, we have examined the possibility that the functioning of the companion enzyme EF-1 alpha could regulate valyl-tRN A synthetase activity. We show here that the addition of EF-1 alpha and GTP in excess in the aminoacylation mixture is accompanied by a 2-fold stimula tion of valyl-tRNA(Val) synthesis catalyzed by the vall-tRNA synthetase com ponent of the ValRS EF-1H complex. This effect is not observed in the prese nce of EF-1 alpha and GDP or EF-Tu GTP and requires association of valyl-tR NA synthetase within the ValRS EF-1H complex. Since valyl-tRNA synthetase a nd elongation factor EF-1 alpha catalyze two consecutive steps of the in vi vo tRNA cycle, aminoacylation and formation of the ternary complex EF-1 alp ha.GTP.Val-tRNA(Val) that serves as a vector of tRNA from the synthetase to the ribosome, the data suggest a coordinate regulation of these two succes sive reactions. The EF-1 alpha.GTP-dependent stimulation of valyl-tRNA synt hetase activity provides further evidence for tRNA channeling during protei n synthesis in mammalian cells.