Inactivation of 50S ribosomal subunits by 2-methoxy-5-nitrotropone

Structure and function of rat-liver ribosomes. Modification by 2-methoxy-5-nitrotropone treatment

Rat liver ribosomes and 60-S ribosome subunits were treated with the primary-amino-group-specific reagent 2-methoxy-5-nitrotropone. Important differences in the sensitivity of several ribosomal activities to inactivation by the reagent were observed. While elongation-factor-dependent activities are totally abolished in the treated particles, peptidyl transferase activity is either unaffected in 60-S ribosomal subunits or even strongly stimulated in 80-S ribosomes. Analysis of the ribosomal proteins modified by nitrotropone made it possible to draw some conclusions on their accessibility in the ribosomal structure and to relate some proteins with their involvement in the ribosome active centers. Thus, proteins L3, L13, L15 and L23 seem to be in a rather well protected position while proteins L10, L35, L37, X1 and X2 are totally exposed to the reagent. The protein accessibility also depends on the ribosome conformation, proteins L14 and L17, for instance, being sensitive in 80-S ribosomes and protected in 60-S subunits. In relation to the implication of proteins in functional centers, the data presented here together with other data obtained from protein-deficient core particles seem to indicate a possible role of proteins L21 and/or L26 in the peptidyl transferase center.

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.

Study of mammalian ribosomal protein reactivity in situ. I. - Effect of 2-methoxy-5-nitrotropone on 40S and 60S subunits

Liver ribosomes and subunits were reacted with increasing concentrations of 2-methoxy-5-nitrotropone. At low reagent concentrations (0.3 mM), the molar uptake by 60S subunits was more efficient than the uptake by 40S subunits, and the amount of reagent bound to 80S ribosomes was less than that bound to both free subunits considered together. At higher reagent concentrations, the molar uptake of both subunits was equivalent. Subunits and ribosomes remained fully active when reacted with up to 0.3 mM and 1 mM of the reagent, respectively. With 2 mM of the reagent, both subunits were half inactivated, although their sedimentation characteristics were unaltered. The reactivity of each ribosomal protein was assessed by two-dimensional gel electrophoresis and quantitative measurement of the unmodified proteins. From these results, considered together with the uptake characteristics and the inactivation curves, a number of tentative conclusions about ribosome topography can be drawn. The over-all sensitivity of the 60S subunits to the reagent is higher than that of the 40S subunits. Both subunits undergo a conformational change when they combine to form 80S ribosomes. Proteins S18, S20, S28 and L5, L9, L11, L15, L16, L25, L29, L30, L31, L34, L37 have NH2 groups exposed in native subunits. These groups are not essential for subunit function.