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ApUpG, the oligoribonucleotide homologous to the initiation codon, as well as the tetranucleotides ApUpGpA and ApUpGpG block initiation of protein synthesis in the rabbit reticulocyte lysate. These oligonucleotides are recognized as translational initiation sites by the ribosomes, leading to a very large accumulation of complete, but inactive, 80 S initiation complexes, containing methionylated initiator tRNA and ApUpG in a 1:1 stoichiometry. ApUpG appears to inhibit by competing with endogenous globin mRNA for 80 S ribosomal couples, since the inhibition of protein synthesis by ApUpG can be largely relieved by increasing the globin mRNA. The 80 S · Met-tRNAiMet · ApUpG complexes are not formed in the absence of hemin, demonstrating that their formation requires the active recycling of eukaryotic initiation factor 2. In addition the trinucleotide correctly directs the Met-tRNAiMet into the ribosomal donor site, since the methionyl residue is puromycin-reactive. 相似文献
86.
Both NaCl and NaF promoted PGE2 binding to epididymal adipocyte membranes by apparent increase in the binding affinity. In order to distinguish between the effect of fluoride and the ‘salt effect’ of sodium on PGE2 binding, the effects of Mg2+ and guanyl nucleotides on PGE2 binding in the presence of NaCl or NaF were compared. Mg2+ decreased PGE2 binding; high NaF concentration abolished this inhibition, while increased NaCl concentratipns did not affect the Mg2+ inhibition. In the presence of Mg2+ the effects of NaCl and NaF were additive. The enhancement of PGE2 binding by fluoride, unlike sodium, was dependent on the presence of Mg2+. Induction of the membranes with GDPβS, Gpp(NH)p, GTP or GTPγS increased PGE, binding. Gradual increase in NaF concentrations in the presence of guanyl nucleotides resulted in stimulation of PGE2 binding at low NaF concentrations and inhibition of PGE2 binding at higjh NaF concentrations. No changes in the stimulatory action of NaCl on PGE2 binding were observed in the simulatenous presence of NaCl and guanyl nucleotides. A biphasic effect on PGE2 binding was observed with a wide concentration range of guanyl nucleotides. Treatment of the isolated membranes with cholera or pertussis toxins stimulated the adenylyl cyclase activity of the membranes, but failed to influence PGE2 binding. The implications of these findings are discussed. 相似文献
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Plant Molecular Biology - 相似文献
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G protein-coupled signaling is utilized by a wide variety of eukaryotes for communicating information from the extracellular environment. Signal termination is achieved by the action of the arrestins, which bind to activated, phosphorylated G protein-coupled receptors. We describe here crystallographic studies of visual arrestin in its basal conformation. The salient features of the structure are a bipartite molecule with an unusual polar core. This core is stabilized in part by an extended carboxy-terminal tail that locks the molecule into an inactive state. In addition, arrestin is found to be a dimer of two asymmetric molecules, suggesting an intrinsic conformational plasticity. In conjunction with biochemical and mutagenesis data, we propose a molecular mechanism by which arrestin is activated for receptor binding. 相似文献
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