Effect of trypsin modification of the Escherichia coli elongation factor Tu on the ternary complex with aminoacyl-tRNA

Ternary complex between elongation factor Tu.GTP and Phe-tRNA(Phe)

The effect of aminoacylation and ternary complex formation with elongation factor Tu.GTP on the tertiary structure of yeast tRNA(Phe) was examined by 1H-NMR spectroscopy. Esterification of phenylalanine to tRNA(Phe) does not lead to changes with respect to the secondary and tertiary base pair interactions of tRNA. Complex formation of Phe-tRNA(Phe) with elongation factor Tu.GTP results in a broadening of all imino proton resonances of the tRNA. The chemical shifts of several NH proton resonances are slightly changed as compared to free tRNA, indicating a minor conformational rearrangement of Phe-tRNA(Phe) upon binding to elongation factor Tu.GTP. All NH proton resonances corresponding to the secondary and tertiary base pairs of tRNA, except those arising from the first three base pairs in the aminoacyl stem, are detectable in the Phe-tRNA(Phe)-elongation factor Tu-GTP ternary complex. Thus, although the interactions between elongation factor Tu and tRNA accelerate the rate of NH proton exchange in the aminoacyl stem-region, the Phe-tRNA(Phe) preserves its typical L-shaped tertiary structure in the complex. At high (> 10(-4) M) ligand concentrations a complex between tRNA(Phe) and elongation factor Tu-GDP can be detected on the NMR time-scale. Formation of this complex is inhibited by the presence of any RNA not related to the tRNA structure. Using the known tertiary structures of yeast tRNA(Phe) and Thermus thermophilus elongation factor Tu in its active, GTP form, a model of the ternary complex was constructed.

Isolation and functional analysis of histidine-tagged elongation factor Tu

The study of the structure/function relationships of the Escherichia coli elongation factor Tu (EF-Tu) via mutagenesis has been hampered by difficulties encountered in separating the mutated factor from other proteins, in particular native EF-Tu. Here we describe a novel system for the purification of EF-Tu mutant species, based on metal-ion affinity chromatography. To facilitate rapid and efficient purification we designed a recombinant EF-Tu with an additional C-terminal sequence of one serine and six histidine residues. A cell extract containing the His-tagged EF-Tu (EF-TuHis) is applied to a Ni(2+)-nitrilotriacetic acid column. EF-TuHis can be selectively eluted with an imidazole containing buffer, yielding a preparation of more than 95% purity, free of wild-type EF-Tu. In-vitro and in-vivo functional analyses show that EF-TuHis resembles the wild-type EF-Tu, which makes this one-step isolation procedure a promising tool for the study of the interactions of mutant EF-Tu with the various components of the elongation cycle. The new isolation procedure was successfully applied for the purification of a mutant EF-TuHis with a Glu substitution for Lys237, a residue possibly involved in the binding of aminoacyl-tRNA.

Identification of the sites in the eukaryotic elongation factor 1 alpha involved in the binding of elongation factor 1 beta and aminoacyl-tRNA

In this article we report the identification of the sites which are involved in the binding of the GDP-exchange factor EF-1 beta and aminoacyl tRNA to the alpha-subunit of the eukaryotic elongation factor 1 (EF-1) from Artemia. For this purpose the polypeptide chain of EF-1 alpha, having 461 amino acid residues, was proteolytically cleaved into large fragments by distinct proteases. Under well defined conditions, a mixture of two large fragments, free from intact EF-1 alpha and with molecular masses of 37 kDa and 43 kDa, was obtained. The 37-kDa and 43-kDa fragments comprise the residues 129-461 and 69-461, respectively. However, in aqueous solution and under non-denaturing conditions, the mixture still contained a short amino-terminal peptide, encompassing the residues 1-36, that remained tightly bound. The ability of the mixture of the 37+43-kDa fragments, including this amino-terminal peptide 1-36, to bind GDP or to facilitate aminoacyl tRNA binding to salt-washed ribosomes was severely reduced, compared to intact EF-1 alpha. However, both of these complexes were able to bind to the GDP-exchange-stimulating subunit EF-1 beta. A 30-kDa fragment, comprising the residues 1-287, was generated after treatment of the protein with endoproteinase Glu-C. This fragment contained the complete guanine nucleotide binding pocket. Although it was able to bind GDP and to transport aminoacyl tRNA to the ribosome, no affinity towards EF-1 beta was observed. We propose that the guanine-nucleotide-exchange stimulation by EF-1 beta is induced through binding of this factor to the carboxy-terminal part of EF-1 alpha. As a result, a decreased susceptibility towards trypsin of the guanine-nucleotide-binding pocket of EF-1 alpha, especially in the region of its presumed effector loop is induced.

Stabilization of the Escherichia coli elongation factor Tu-GTP-aminoacyl-tRNA complex

The effect of ammonium sulfate on the Escherichia coli elongation factor Tu-GTP-aminoacyl-tRNA complex has been studied. The half-lives of 12 E. coli aminoacyl-tRNA species were determined at 37 degrees C in the presence and absence of an equimolar amount of EF-Tu-GTP and in the presence and absence of 1.5 M ammonium sulfate. The results indicate that the addition of 1.5 M ammonium sulfate to the ternary complex increased the stability of all 12 complexes studied. In addition, the effects of various salts and crystallization agents on the stability of the E. coli EF-Tu-GTP-phenylalanyl-tRNA complex was studied in detail. Binding parameters were also measured under various conditions at 37 degrees C. The results indicate that the stability and the Kassoc of the ternary complex, using phenylalanyl-tRNA, can be increased by the presence of polyethylene glycol or ammonium sulfate.

A conserved amino acid sequence around Arg-68 of Artemia elongation factor 1 alpha is involved in the binding of guanine nucleotides and aminoacyl transfer RNAs

The rate of trypsin cleavage of elongation factor 1 alpha having bound GDP is low and increases on exchange of GDP for GTP. The cleavage occurs at a unique position of the protein chain, namely at arginine-68 of Artemia EF-1 alpha. This increase in trypsin sensitivity is enhanced further in the presence of charged or uncharged transfer RNA. The local unfolding of EF-alpha at residue 68 is discussed in terms of a model in which GTP hydrolysis controls the positioning of a short 3'-terminal section of transfer RNA near the centre of peptide bond synthesis.