Interaction of spermidine with viral RNA and its influence on protein synthesis

3D-Membrane Stacks on Supported Membranes Composed of Diatom Lipids Induced by Long-Chain Polyamines

Long-chain polyamines (LCPAs) are intimately involved in the biomineralization process of diatoms taking place in silica deposition vesicles being acidic compartments surrounded by a lipid bilayer. Here, we addressed the question whether and how LCPAs interact with lipid membranes composed of glycerophospholipids and glyceroglycolipids mimicking the membranes of diatoms and higher plants. Solid supported lipid bilayers and monolayers containing the three major components that are unique in diatoms and higher plants, i.e., monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), and sulfoquinovosyldiacylglycerol (SQDG), were prepared by spreading small unilamellar vesicles. The integrity of the membranes was investigated by fluorescence microscopy and atomic force microscopy showing continuous flat bilayers and monolayers with small protrusions on top of the membrane. The addition of a synthetic polyamine composed of 13 amine groups separated by a propyl spacer (C3N13) results in flat but three-dimensional membrane stacks within minutes. The membrane stacks are connected with the underlying membrane as verified by fluorescence recovery after photobleaching experiments. Membrane stack formation was found to be independent of the lipid composition; i.e., neither glyceroglycolipids nor negatively charged lipids were required. However, the formation process was strongly dependent on the chain length of the polyamine. Whereas short polyamines such as the naturally occurring spermidine, spermine, and the synthetic polyamines C3N4 and C3N5 do not induce stack formation, those containing seven and more amine groups (C3N7, C3N13, and C3N18) do form membrane stacks. The observed stack formation might have implications for the stability and expansion of the silica deposition vesicle during valve and girdle band formation in diatoms.

Incomplete polypeptides are formed in vitro by premature chain termination

The mechanism of incomplete polypeptides formation during protein synthesis was studied in the wheat germ cell-free system programmed with brome mosaic virus RNA 4. The synthesis of coat protein, the complete product of RNA 4 translation, was accompanied by the appearance of polypeptides of lower molecular mass. It was shown that incomplete products are formed by translation of different lengths of RNA 4, always from the first 5' AUG codon, and were due neither to proteolysis of coat protein nor to the translation of nucleolytic fragments of mRNA. The molecular masses of incomplete products were determined and the nucleotide sequence of RNA 4 was examined in the regions where wheat germ ribosomes stop translating. It was found that they contained, on average, a slightly higher guanosine content than the total coding part of RNA 4. Translation of RNA 4 in the reticulocyte lysate resulted in a marked diminution of incomplete polypeptides. Addition of high-speed supernatant from reticulocyte lysate prevented the formation of incomplete products during translation of RNA 4 in the wheat germ system. This suggests that reticulocyte lysate contains some factor(s) which facilitate the movement of ribosomes beyond the regions where the elongation is retarded.

Effect of dinucleotides on wheat germ translation system

The effect of ribodinucleoside monophosphates on total protein synthesis was studied in a wheat germ cell-free system, using brome mosaic virus (BMV) RNA as a messenger. Dinucleotides inhibit total protein synthesis to different extents. Of those tested the most inhibitory is CpA. The inhibitory effect of dinucleotides is due to their adverse effect on initiation and not on elongation of polypeptide synthesis. It seems that the dinucleotides complementary to the initiation codon are able to compete with the initiator tRNA during initiation of protein synthesis. The comparison of the effect exerted by different dinucleotides suggests that under conditions of the in vitro protein synthesis RNA 4 is an mRNA molecule with the initiation codon and its immediate neighbourhood being exposed.