Properties of the crystalline amino acid polymerization factors from Escherichia coli: binding of G to ribosomes

Interaction of the N-terminal and C-terminal domains of elongation factor G on formation of complexes with guanyl nucleotides

Polarized fluorescence studies of interaction between guanyl nucleotides (GTP and GDP) and elongation factor G and its N-terminal tryptic fragment T*/s, carrying a fluorescent group (aminorhodamine B) at the exposed cysteine residue, has shown that binding on nucleotides by an intact EF-G molecule at neutral pH essentially affects the mobility of the fluorescent group. GTP binding changes its relaxation properties to a greater extent than GDP binding. At the same time it was demonstrated that the spectrum of relaxation time of the fluorescent group practically does not change on binding of nucleotides by the N-terminal fragment T*/2 (in the absence of the C-terminal domain) or in the case when the three-dimensional structure of the intact EF-G molecule is destabilized (pH 10). Comparison of the relaxation properties of EF-G and its N-terminal fragment T*/2, carrying a fluorescent group at the exposed cysteine residue, at pH 7.5 and 10, indicates that the C-terminal domain is involved in the formation of the close environment of the exposed cysteine residue located in the N-terminal part of EF-G. A conclusion is drawn on the nucleotide-induced influence of the C-terminal domain on a change of the exposed cysteine residue environment on guanyl nucleotide binding with EF-G at neutral pH and a hypothetical model of the EF-G molecule is proposed.

Properties and regulation of the GTPase activities of elongation factors Tu and G, and of initiation factor 2

During protein synthesis the interaction with ribosomes of elongation factors Tu (EF-Tu), G (EF-G) and initiation factor 2 (IF-2) is associated with the hydrolysis of GTP which is directly related to the functions of the factors. In this article we review systematically the properties of these GTPase activities in the presence and absence of protein synthesis, and by examining the characteristics of the different minimal systems for the expression of these activities we point to the role of the various effectors and to the enzymological aspects of the systems. For EF-Tu, it has been possible to eliminate any requirement for macromolecular effectors, showing that the factor itself is a GTPase. For EF-G, the presence of at least the 50S ribosomal subunit has remained a requirement, whereas IF-2 needs both the 50S and 30S subunits to exhibit GTPase activity. Between the GTPase activities of the three factors there are some striking similarities, but important differences prevail as a consequence of the specificity of the different functions. This can also be seen by examining the respective ribosomal regions implicated in these reactions. When coupled with protein synthesis, the three GTPase activities reveal characteristics differing from those observed in partial systems.

Cleavage of elongation factor G into compact domains

A limited trypsinolysis of native elongation factor G results in the formation of two large fragments resistant to further proteolysis. The fragments were isolated in homogeneous state in conditions when their native structure is retained. According to circular dichroism and scanning calorimetry data the formed fragments retain the stability and compact structure that they had in the whole protein.

Interactions of guanosine triphosphate analogues with elongation factor G of Escherichia coli

Previous studies from this laboratory have shown that the GTP analogue guanosine 3'-diphosphate 5'-triphosphate (pppGpp) was almost without activity in the translocation reaction catalyzed by elongation factor G (EF-G), while it was fully active with elongation factor T (EF-T) and initiation factor 2 (IF-2). To assess the importance of the 3'-ribose hydroxyl itself in the translocation reaction, we examined polyphenylalamine synthesis and EF-G-dependent formation of N-acetylphenylalanylphenylalanylpuromycin (Ac-Phe2-puromycin) supported by 3'-deoxyguanosine 5'-triphosphate (3'dGTP) and 3'-deoxy-3'-aminoguanosine 5'-triphosphate (3'dNH2GTP). Like pppGpp, these nucleotides were similar to GTP in EF-T and IF-2dependent reactions. We also examined the ability of the dialcohol derived from GTP by periodate oxidation and borohydride reduction (rroGTP) and of ITP to support translocation. These compounds had shown significant, although reduced, activity with EF-T and IF-2. A spectrum of activity was found with these compounds in both poly(Phe) synthesis and Ac-Phe2-puromycin formation. All had significantly reduced activity relative to GTP, but all were significantly more active than pppGpp. A surprising finding was that the activities of all analogues relative to GTP were dependent on the reaction temperature in both poly(Phe) synthesis and Ac-Phe2-puromycin formation; these relative activities were significantly lower at 8 degrees C after than 37 degrees C. Furthermore, the extent of poly(Phe) synthesis with the analogues relative to GTP could be significantly affected by the amounts of EF-G and EF-T in the reaction mixture. Differences were minimized in the presence of rate-limiting EF-T and saturating EF-G and maximized in the presence of rate-limiting EF-G and saturating EF-T. This effect of reducing the level of EF-G in the reaction mixture thus mimicked the effect of lowering the incubation temperature, and a similar observation was made for Ac-Phe2-puromycin formation. GTP-supported formation of Ac-Phe2-puromycin at 8 degress C could be inhibited by 3'dNH2GTP, 3'dGTP, pppGpp, and ITP: these compounds were better inhibitors than they were substrates. Inhibition by other nucleotides was also noted. The GTP analogues were compared to GTP as substrates in EF-G-dependent reactions uncoupled from protein synthesis. Fusidic-acid-dependent binding of nucleotides to ribosomes and ribosome-dependent catalytic nucleotide hydrolysis were both examined, and the activity of the nucleotides as substrates in these ractions showed little correlation with their ability to support translocation.

Elongation factors EF Tu and EF G interact at related sites on ribosomes

The binding of elongation factor EF G to ribosomes inhibits the subsequent reaction of the ribosomes with the ternary complex aminoacyl-tRNA.EF Tu.GTP. Both the hydrolysis of GTP and the binding of aminoacyl-tRNA to ribosomes are nearly abolished by the previous binding of factor EF G to ribosomes in the presence of either fusidic acid plus either GTP or a nonhydrolyzable analog of GTP. The results suggest that each elongation factor binds to the same region on the ribosome. The GTPase activities of both factors EF G and EF Tu may be activated by interaction at the same ribosomal site, as has been previously suggested by others.