Interactions of periodate-oxidized guanine nucleotides with Escherichia coli elongation factor G and the ribosome

Structural and functional studies of the interaction of the eukaryotic elongation factor EF-2 with GTP and ribosomes

The structure of the guanosine nucleotide binding site of EF-2 was studied by affinity labelling with the GTP analogue, oxidized GTP (oGTP), and by amino acid sequencing of polypeptides generated after partial degradation with trypsin and N-chlorosuccinimide. Native EF-2 contains two exposed trypsin-sensitive cleavage sites. One site is at Arg66 with a second site at Lys571/Lys572. oGTP was covalently bound to the factor between Arg66 and Lys571. After further cleavage of this fragment with the tryptophan-specific cleavage reagent N-chlorosuccinimide, oGTP was found associated with a polypeptide fragment originating from a cleavage at Trp261 and Trp343. The covalent oGTP . EF-2 complex was capable of forming a high-affinity complex with ribosomes, indicating that oGTP, in this respect, induced a conformation in EF-2 indistinguishable from that produced by GTP. Although GTP could be substituted by non-covalently linked oGTP in the factor and ribosome-dependent GTPase reaction, the factor was unable to utilize the covalently bound oGTP as a substrate. This indicates that the conformational flexibility in EF-2 required for the ribosomal activation of the GTPase was inhibited by the covalent attachment of the nucleotide to the factor. EF-2 cleaved at Arg66 were unable to form the high-affinity complex with ribosomes while retaining the ability to form the low-affinity complex and to hydrolyse GTP. The second cleavage at Lys571/Lys572 was accompanied by a total loss of both the low-affinity binding and the GTPase activity.

Inhibition of human purine nucleoside phosphorylase by acyclic nucleosides and nucleotides

In an effort to develop more potent inhibitors of human purine nucleoside phosphorylase (PNP) as immunosuppressive and cancer chemotherapeutic agents, the affinity of the erythrocytic enzyme for 30 acyclic nucleosides, nucleotides and related compounds was determined. Among the acyclonucleosides, 2'-nordeoxyguanosine [2'NDG, 9-(1,3-dihydroxy-2-propoxymethyl)guanine] had a 3-fold greater affinity than acyclovir, and 8-amino-2'NDG was the best inhibitor with Ki = 2.6 X 10(-7) M. The ether moiety of the acyclovir and 2'NDG side-chains was not important for binding. Phosphorylated 2'NDG analogs appeared to act as multisubstrate analogs with optimal binding at low (1 mM) phosphate concentration. The 2'NDG mono- and triphosphates had higher affinities than those reported for the phosphorylated acyclovir derivatives but the diphosphate had a similar Ki value of 9 X 10(-9) M. Poor affinity, independent of phosphate concentration, was found for 9-(2-phosphonoethyl)guanine. The 3'-phosphate derivative of 8-(3-hydroxypropyl)-9-methylguanine inhibited with a Ki = 2 X 10(-5) M in 1 mM phosphate. The chemical syntheses of new analogs are described.

Interactions of elongation factor 2 (EF-2) with guanine nucleotides and ribosomes. Binding of periodate-oxidized guanine nucleotides to EF-2

Interactions of rat liver elongation factor 2 (EF-2) with guanine nucleotides and ribosomes were studied by equilibrium dialysis and sedimentation methods. GDP (Kd = 0.5 microM) or GDP-Mg2+ (Kd = 1.57 microM) displayed a higher affinity in the formation of a binary complex with EF-2 than GTP (Kd = 2.68 microM), GTP-Mg2+ (Kd = 2.77 microM), or guanosine 5'-[beta, gamma-methylene]triphosphate (GuoPP[CH2]P) (Kd = 24.0 microM). NaIO4-oxidized guanine nucleotides (oGDP) (Kd = 38 microM) and oxidized/reduced guanine nucleotides (orGDP) (Kd = 27 microM) had lower affinites to the binding site on EF-2 than those of GDP or GTP. However, the binding of oGDP, oGTP or oGuoPP[CH2]P to EF-2 resulted in the formation of a stable product which could be recovered by the nitrocellulose filter technique or by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. In the presence of ribosomes and EF-2 the formation of a new binding site (or a different conformation of the binding site) with a higher affinity for GuoPP[CH2]P-Mg2+ (Kd = 0.26 microM) than fof GDP-Mg2+ (kd = 9.3 microM) became apparent. The presence of ribosomes thus appeared to favor the formation of a complex involving guanosine triphosphates. Adenosine diphosphate ribosylated EF-2 (ADP-Rib-EF-2) in its turn could bind to the ribosome with high affinity even without guanosine nucleotides (Kd = 0.18 microM). GuoPP[CH2]P increased to some extent the affinity of ADP-Rib-EF-2 for its ribosomal binding site (Kd = 0.05 microM).

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.