Studies on the mechanism of action of a eukaryotic codon-dependent factor specific for initiator Met-tRNAf and ribosomal 40 S subunits

Thiazolidinediones expand body fluid volume through PPARgamma stimulation of ENaC-mediated renal salt absorption

Thiazolidinediones (TZDs) are widely used to treat type 2 diabetes mellitus; however, their use is complicated by systemic fluid retention. Along the nephron, the pharmacological target of TZDs, peroxisome proliferator-activated receptor-gamma (PPARgamma, encoded by Pparg), is most abundant in the collecting duct. Here we show that mice treated with TZDs experience early weight gain from increased total body water. Weight gain was blocked by the collecting duct-specific diuretic amiloride and was also prevented by deletion of Pparg from the collecting duct, using Pparg (flox/flox) mice. Deletion of collecting duct Pparg decreased renal Na(+) avidity and increased plasma aldosterone. Treating cultured collecting ducts with TZDs increased amiloride-sensitive Na(+) absorption and Scnn1g mRNA (encoding the epithelial Na(+) channel ENaCgamma) expression through a PPARgamma-dependent pathway. These studies identify Scnn1g as a PPARgamma target gene in the collecting duct. Activation of this pathway mediates fluid retention associated with TZDs, and suggests amiloride might provide a specific therapy.

The effect of non-permissive temperature on met-tRNA synthetase in Saccharomyces cerevisiae temperature-sensitive mutant ts 7-45

The effects of incubation of yeast spheroplasts at elevated temperature (40 degrees C) on a number of activities involved in protein biosynthesis have been examined in preparations obtained from wild-type cells (wt A364A ) and a temperature-sensitive mutant (ts 7-45) derived from it. With wild-type cells, preincubation of spheroplasts at the elevated temperature had little or no effect on the following: the ribosomal subunit-polysome pattern; the translation of exogenous natural mRNA in postpolysomal extracts devoid of endogenous mRNA; the translation of poly(U) in postpolysomal extracts; the incorporation of methionine into 40 S preinitiation and 80 S initiation complexes; the synthesis of Met-tRNA in postribosomal (cytosol) extracts; and the formation of eIF-2 X GTP X Met-tRNAf ternary complex in the cytosol. With temperature-sensitive spheroplasts that had not been preincubated at the elevated temperature, the concentration of free, native 40 S subunits appeared to be lower and that of 60 S subunits higher than in wild-type cells; translation of exogenous natural mRNA in postpolysomal extracts was somewhat lower than in wild-type preparations, but all of the other reactions and components measured were comparable to those in wild-type preparations. Preincubation of temperature-sensitive spheroplasts at 40 degrees C resulted in: a further decrease in the level of 40 S subunits; disaggregation of polysomes; loss of ability to translate natural mRNA but not poly(U); decreased ability to form 40 S preinitiation intermediates; and production of an activity, found in the cytosol, that inhibited Met-tRNA synthetase reversibly. The inhibitor had the characteristics of a protein and did not appear to be a proteinase, nuclease, or nucleotidase.

Effects of preparative procedures on the sedimentation behaviour of 40-S ribosomal subunits

The sedimentation characteristics on sucrose gradients of derived 40-S subunits from rat liver depend on the ionic conditions used in their preparation. Following the procedure of Wettenhall, R.E.H., Wool, I.G. and Sherton, C.C. (Biochemistry 12 (1973) 2403-2411) in which 12.5 mM Mg2+ is present throughout the preparation of the subunits, sucrose gradients of the 40-S fraction show two peaks consistent with the presence of a mixed and non-interchanging population of monomers and dimers. By contrast, when ribosomes and subunits are prepared in 2 mM Mg2+ it is shown here that 40-S subunits sediment as a single peak at a rate intermediate between that of monomers and dimers as established by prior fixation with glutaraldehyde or formaldehyde - a phenomenon observed by Spirin, A.S., Belitsina, N.V. and Lishnevskaya, E.B. (FEBS Lett. 24 (1972) 219-224) for the Escherichia coli 30-S--50-S couple but not previously shown for subunits of eukaryotic origin. Under conditions of low [Mg2+] it is suggested that the subunits are of a uniform population capable of dimerization and that equilibrium can continue during sedimentation. In the present experiments it was not possible to find tRNA in 40-S subunits as shown by Wettenhall et al. By fixation before sedimentation the relative proportions of monomers and dimers of 40 S incubated under a variety of conditions was determined. Warming from 0 to 37 degrees C increased the proportion of monomers markedly, but variation of [Mg2+] had little effect. Incubation with albumin or tRNA or spermine also had little effect, but addition of ribosomal salt-wash increased the proportion of monomers. It seems most likely that many of the 40-S subunits prepared in 12.5 mM Mg2+ have initiation factors or other proteins bound to them which prevent dimerization, but that subunits prepared at any stage with buffer containing only 2 mM Mg2+ have lost these proteins and therefore are able to dimerize to a degree depending on concentration. Different relative proportions of monomers and dimers were observed depending on whether glutaraldehyde or formaldehyde was used as fixative. Reasons are suggested for believing that formaldehyde is to be preferred to glutaraldehyde for fixation in experiments of this nature.

Activity of Euglena gracilis chloroplast ribosomes with procaryotic and eucaryotic initiation factors

A method that permits the preparation of Euglena gracilis chloroplast 30 S ribosomal subunits that are largely free of endogenous initiation factors and that are active in the binding of fMet-tRNA in response to poly(A, U, G), has been developed. These 30 S subunits have been tested for activity in initiation complex formation with initiation factors from both procaryotes and eucaryotes. We have observed that Escherichia coli IF-2 binds fMet-tRNA nearly as well to Euglena chloroplast ribosomal subunits as it does to its homologous subunits. Neither wheat germ eIF-2 nor Euglena eIF-2A can bind fMet-tRNA efficiently to Euglena chloroplast or E. coli 30 S subunits although both are active with wheat germ 40 S ribosomal subunits. Euglena chloroplast 68 S ribosomes will also bind the initiator tRNA. Both E. coli IF-2 and E. coli IF-3 stimulate this reaction on chloroplast ribosomes with approximately the same efficiency as they do on their homologous ribosomes. E. coli IF-1 enhances the binding of fMet-tRNA to the chloroplast 68 S ribosomes when either IF-2 or IF-3 is limiting. The chloroplast ribosomes unlike E. coli ribosomes show considerable activity over a broad range of Mg2+ ion concentrations.