Distribution of lipid-binding regions in human apolipoprotein B-100

The distribution of lipid-binding regions of human apolipoprotein B-100 has been investigated by recombining proteolytic fragments of B-100 with lipids and characterizing the lipid-bound fragments by peptide mapping, amino acid sequencing, and immunoblotting. Fragments of B-100 were generated by digestion of low-density lipoproteins (LDL) in the presence of sodium decyl sulfate with either Staphylococcus aureus V8 protease, pancreatic elastase, or chymotrypsin. Particles with electron microscopic appearance of native lipoproteins formed spontaneously when detergent was removed by dialysis from enzyme digests containing fragments of B-100 and endogenous lipids, or from incubation mixtures of delipidated B-100 fragments mixed with microemulsions of exogenous lipids (cholesteryl oleate and egg phosphatidylcholine). Fractionation of the recombinant particles by isopycnic or density gradient ultracentrifugation yielded complexes similar to native LDL with respect to shape, diameter, electrophoretic mobility, and surface and core compositions. Circular dichroic spectra of these particles showed helicity similar to LDL but a somewhat decreased content of beta-structure. Most of the fragments of B-100 were capable of binding to lipids; 12 were identified by direct sequence analysis and 14 by reaction with antisera against specific sequences within B-100. Our results indicate that lipid-binding regions of B-100 are widely distributed within the protein molecule and that proteolytic fragments derived from B-100 can reassociate in vitro with lipids to form LDL-like particles.     

Entrapment of lipid vesicles and membrane protein-lipid vesicles in gel bead pores

Phospholipid vesicles were entrapped in gel beads of Sepharose 6B and Sephacryl S-1000 during vesicle preparation by dialysis. Egg-yolk phospholipids solubilized with cholate or octyl glucoside were dialysed together with gel beads for 2.5 days in a flat dialysis bag. Some vesicles were formed in gel bead pores and vesicles of sufficient size became trapped. Red cell membrane protein-phospholipid vesicles could be immobilized in the same way. Non-trapped vesicles were carefully removed by chromatographic procedures and by centrifugation. The amount of entrapped vesicles increased with the initial lipid concentration and was dependent on the relative sizes of vesicles and gel pores. The largest amount of trapped vesicles, corresponding to 9.5 mumol of phospholipids per ml gel, was achieved when Sepharose 6B gel beads were dialysed with cholate-solubilized lipids at a concentration of 50 mM. In this case the vesicles had an average diameter of 60 nm and an internal volume of 15 microliters/ml gel. The amount of vesicles trapped in Sephacryl S-1000 gel beads upon dialysis under the same conditions was smaller: 2.2 mumol of phospholipids per ml gel. Probably most of the gel pores were too large to trap such vesicles. Larger vesicles, with an average diameter of 230 nm, were entrapped in the Sephacryl S-1000 matrix in an amount corresponding to 3.0 mumol phospholipids per ml gel upon dialysis of the gel beads and octyl glucoside-solubilized lipids at a concentration of 20 mM. The internal volume of these vesicles was 22 microliters/ml gel. The yield of immobilized phospholipids was up to 19%. The entrapped vesicles were somewhat unstable: 9% of the phospholipids were released during 9 days of storage at 4 degrees C. By the dialysis entrapment method vesicles can be immobilized in the gel beads without using hydrophobic ligands or covalent coupling.     

Novel hybrid maturases in unstable pseudorevertants of maturaseless mutants of yeast mitochondrial DNA.

Unstable pseudorevertants of mitochondrial mutants of Saccharomyces cerevisiae lacking the maturase function encoded by the fourth intron of the cytochrome b gene (bI4) were isolated. They were found to be heteroplasmic cells owing their regained ability to respire (and grow on glycerol medium) to the presence of a rearranged (rho-) mtDNA that contains an in-frame fusion of the reading frames of the group I introns bI4 and intron 4 alpha of the coxl gene encoding subunit I of cytochrome c oxidase (aI4 alpha). The products of those gene fusions suppress the bI4 maturase deficiency still present in those heteroplasmic cells. Similar heteroplasmic pseudorevertants of a group II maturaseless mutant of the first intron of the coxI gene were characterized; they result from partial deletion of the coxI gene that fuses the reading frames of introns 1 and 2. These heteroplasms provide independent support for the existence of RNA maturases encoded by group I and group II introns. Also, since the petite/mit- heteroplasms arise spontaneously at very high frequencies they provide a system that can be used to obtain mutants unable to form or maintain heteroplasmic cells.     

Charge dependence of Fe(II)-catalyzed DNA cleavage.

The effect of charge of the Fe(II) reagent used to induce DNA strand cleavage reactions in the presence of a source of reducing equivalents is investigated using two oligonucleotide models. The first consists of the two strands dA20 and dT20, and an equimolar complex between them. The second is a short four-arm branched DNA complex composed of four 16-mer strands. In the former case, cleavage of the 1:1 complex by three reagents with different formal charge, Fe(II).EDTA2-, Fe(II).EDDA and Fe2+, is comparable in rate to that of the individual dT20 and the dA20 strands. While the three reagents show similar cleavage rates for the duplex and single stranded molecules, they give distinctive cutting patterns in the DNA tetramer, consistent with the presence of a site of excess negative charge at the branch point. Scission induced by Fe(II).EDTA2- shows lower reactivity at the branch site relative to duplex controls, whereas Fe(II)2+ shows enhanced reactivity. Formally neutral Fe(II).EDDA shows weak loss of cutting reactivity at the branch. The position of attack by Fe(II)2+ in the branched tetramer is shifted with respect to those of Fe(II).EDTA2- or Fe(II).EDDA; a slower migrating species is also detected in the scission of dA20.dT20 duplex by Fe(II) reaction. These results suggest that the Fe(II)2+ reaction proceeds by a different mechanism from the other agents. The difference in cutting profiles induced by the neutral and negatively charged chelated complexes is consistent with a local electrostatic repulsion of a negatively charged source of radicals, not a positively charged one.     

A 40 kilodalton rat liver nuclear protein binds specifically to apolipoprotein B mRNA around the RNA editing site.

Apolipoprotein (apo) B-48 mRNA is the product of RNA editing which consists of a C----U conversion changing a CAA codon encoding Gln-2153 in apoB-100 mRNA to a UAA stop codon in apoB-48 mRNA. In the adult rat, RNA editing occurs both in the small intestine and the liver. We have studied the ability of rat liver nuclear extracts to bind to synthetic apoB mRNA segments spanning the editing site. Using an RNA gel mobility shift assay, we found the sequence-specific binding of a protein(s) to a 65-nucleotide apoB-100 mRNA. UV crosslinking followed by T1 ribonuclease digestion and SDS-polyacrylamide gel electrophoresis demonstrated the formation of a 40 kDa protein-RNA complex when 32P-labeled apoB-100 mRNA was incubated with a rat liver nuclear extract but not with HeLa nuclear extract. Binding was specific for the sense strand of apoB mRNA, and was not demonstrated with single-stranded apoB DNA, or antisense apoB RNA. The complex also failed to form if SDS was present during the UV light exposure. Binding experiments using synthetic apoB mRNAs indicate that the 40 kDa protein would also bind to apoB-48 mRNA but not apoA-I, apoA-IV, apoC-II or apoE mRNA. Experiments using deletion mutants of apoB-100 mRNA indicate efficient binding of wildtype 65-nucleotide (W65), 40-nucleotide (W40) and 26-nucleotide (W26) apoB-100 mRNA segments, but not 10-nucleotide (or smaller) segments of apoB-100 mRNA to the 40 kDa protein. In contrast, two other regions of apoB-100 mRNA, B-5' (bases 1128-3003) and B-3' (bases 11310-11390), failed to bind to the protein. The 40 kDa sequence-specific binding protein in rat liver nuclear extract may play a role in apoB-100 mRNA editing.     

Studies on theStellaria longipes complex (Caryophyllaceae): Isozyme variability and the relationship betweenStellaria longipes andS. longifolia

Genetic variation within and the relationship betweenStellaria longipes Goldie andS. longifolia Muhl. were studied. Ten enzyme systems were assessed in eight natural populations ofS. longipes (25 loci) and three ofS. longifolia (20 loci) using starch and polyacrylamide gel electrophoresis. Patterns of population differentiation corresponded to geographic distance. There was no evidence that polyploidS. longipes had greater electrophoretic variability than diploidS. longipes. The isozyme data confirmed extensive population differentiation in these species and, within that context, a relatively close relationship betweenS. longipes andS. longifolia. It was postulated that diploids of these two species might be the progenitors of tetraploidS. longipes.     

血管平滑肌细胞的钙动力学

血管平滑肌细胞外的Ca~(2+)通过多种通道进入细胞内。Ca~(2+)通道的本质是镶嵌在膜脂质双分子层中的糖蛋白,神经介质和药物可影响Ca~(2+)通道的功能。靠近胞膜的肌质网和胞膜内侧面的高亲和性Ca~(2+)结合位点是血管平滑肌细胞内储存和释放Ca~(2+)的主要部位。胞浆[Ca~(2+)]增高后在钙调蛋白的介导下引起血管收缩。高血压等血管性疾病的发生与其平滑肌细胞的钙动力学异常有关。