Balanced biosynthesis of major membrane components through regulated degradation of the committed enzyme of lipid A biosynthesis by the AAA protease FtsH (HflB) in Escherichia coli

The suppressor mutation, named sfhC21, that allows Escherichia coli ftsH null mutant cells to survive was found to be an allele of fabZ encoding R-3-hydroxyacyl-ACP dehydrase, involved in a key step of fatty acid biosynthesis, and appears to upregulate the dehydrase. The ftsH1(Ts) mutation increased the amount of lipopolysaccharide at 42 degrees C. This was accompanied by a dramatic increase in the amount of UDP-3-O-(R-3-hydroxymyristoyl)-N-acetylglucosamine deacetylase [the IpxC (envA) gene product] involved in the committed step of lipid A biosynthesis. Pulse-chase experiments and in vitro assays with purified components showed that FtsH, the AAA-type membrane-bound metalloprotease, degrades the deacetylase. Genetic evidence also indicated that the FtsH protease activity for the deacetylase might be affected when acyl-ACP pools were altered. The biosynthesis of phospholipids and the lipid A moiety of lipopolysaccharide, both of which derive their fatty acyl chains from the same R-3-hydroxyacyl-ACP pool, is regulated by FtsH.     

The lipid lysyl-phosphatidylglycerol is present in membranes of Rhizobium tropici CIAT899 and confers increased resistance to polymyxin B under acidic growth conditions

Lysyl-phosphatidylglycerol (LPG) is a well-known membrane lipid in several gram-positive bacteria but is almost unheard of in gram-negative bacteria. In Staphylococcus aureus, the gene product of mprF is responsible for LPG formation. Low pH-inducible genes, termed IpiA, have been identified in the gram-negative alpha-proteobacteria Rhizobium tropici and Sinorhizobium medicae in screens for acid-sensitive mutants and they encode homologs of MprF. An analysis of the sequenced bacterial genomes reveals that genes coding for homologs of MprF from S. aureus are present in several classes of organisms throughout the bacterial kingdom. In this study, we show that the expression of lpiA from R. tropici in the heterologous hosts Escherichia coli and Sinorhizobium meliloti causes formation of LPG. A wild-type strain of R. tropici forms LPG (about 1% of the total lipids) when the cells are grown in minimal medium at pH 4.5 but not when grown in minimal medium at neutral pH or in complex tryptone yeast (TY) medium at either pH. LPG biosynthesis does not occur when lpiA is deleted and is restored upon complementation of lpiA-deficient mutants with a functional copy of the lpiA gene. When grown in the low-pH medium, lpiA-deficient rhizobial mutants are over four times more susceptible to the cationic peptide polymyxin B than the wild type.     

Dolichyl-phosphate-glucose is used to make O-glycans on glycoproteins of Trichomonas vaginalis

Trichomonas vaginalis, the protist that causes vaginal itching, has a huge genome with numerous gene duplications. Recently we found that Trichomonas has numerous genes encoding putative dolichyl-phosphate-glucose (Dol-P-Glc) synthases (encoded by ALG5 genes) despite the fact that Trichomonas lacks the glycosyltransferases (encoded by ALG6, ALG8, and ALG10 genes) that use Dol-P-Glc to glucosylate dolichyl-PP-linked glycans. In addition, Trichomonas does not have a canonical DPM1 gene, encoding a dolichyl-P-mannose (Dol-P-Man) synthase. Here we show Trichomonas membranes have roughly 300 times the Dol-P-Glc synthase activity of Saccharomyces cerevisiae membranes and about one-fifth the Dol-P-Man synthase activity of Saccharomyces membranes. Endogenous Dol-P-hexoses of Trichomonas are relatively abundant and contain 16 isoprene units. Five paralogous Trichomonas ALG5 gene products have Dol-P-Glc synthase activity when expressed as recombinant proteins, and these Trichomonas Alg5s correct a carboxypeptidase N glycosylation defect in a Saccharomyces alg5 mutant in vivo. A recombinant Trichomonas Dpm1, which is deeply divergent in its sequence, has Dol-P-Man synthase activity. When radiolabeled Dol-P-Glc is incubated with Trichomonas membranes, Glc is incorporated into reducing and nonreducing sugars of O-glycans of endogenous glycoproteins. To our knowledge, this is the first demonstration of Dol-P-Glc as a sugar donor for O-glycans on glycoproteins.     

Subcellular organelle lipidomics in TLR-4-activated macrophages

Lipids orchestrate biological processes by acting remotely as signaling molecules or locally as membrane components that modulate protein function. Detailed insight into lipid function requires knowledge of the subcellular localization of individual lipids. We report an analysis of the subcellular lipidome of the mammalian macrophage, a cell type that plays key roles in inflammation, immune responses, and phagocytosis. Nuclei, mitochondria, endoplasmic reticulum (ER), plasmalemma, and cytoplasm were isolated from RAW 264.7 macrophages in basal and activated states. Subsequent lipidomic analyses of major membrane lipid categories identified 229 individual/isobaric species, including 163 glycerophospholipids, 48 sphingolipids, 13 sterols, and 5 prenols. Major subcellular compartments exhibited substantially divergent glycerophospholipid profiles. Activation of macrophages by the Toll-like receptor 4-specific lipopolysaccharide Kdo₂-lipid A caused significant remodeling of the subcellular lipidome. Some changes in lipid composition occurred in all compartments (e.g., increases in the levels of ceramides and the cholesterol precursors desmosterol and lanosterol). Other changes were manifest in specific organelles. For example, oxidized sterols increased and unsaturated cardiolipins decreased in mitochondria, whereas unsaturated ether-linked phosphatidylethanolamines decreased in the ER. We speculate that these changes may reflect mitochondrial oxidative stress and the release of arachidonic acid from the ER in response to cell activation.     

Induction of kappa light chain synthesis in 70Z/3 B lymphoma cells by chemically defined lipid A precursors

The murine B cell lymphoma 70Z/3 is a tumor line that resembles an early B cell. It responds to lipopolysaccharide by synthesizing kappa light chains, resulting in the appearance of surface IgM. We now demonstrate that this effect is triggered by the lipid A domain of lipopolysaccharide since a defined tetraacyl disaccharide 1,4'-bisphosphate precursor of mature lipid A, designated lipid IVA (Raetz, C. R. H., Purcell, S., Meyer, M. V., Qureshi, N., and Takayama, K. (1985) J. Biol. Chem. 260, 16080-16088), is fully active in stimulating kappa chain mRNA synthesis. In contrast, the monosaccharide lipid A precursor, lipid X, shows only slight activity; but a large excess of lipid X appears to complete with lipid IVA, partially blocking its effects. Mutants of the 70Z/3 line that are unresponsive to lipopolysaccharide also fail to respond to lipid IVA. The somatic cell system described here, coupled with the use of chemically defined agonists and antagonists, offers a new approach to understanding the early events in lipopolysaccharide/animal cell interactions.     

Chloroform-soluble nucleotides in Escherichia coli. Role of CDP-diglyceride in the enzymatic cytidylylation of phosphomonoester acceptors

CDP-diglyceride, the precursor of all the phospholipids in Escherichia coli, is cleaved in vitro to phosphatidic acid and CMP by a membrane-bound hydrolase. Since the physiological function of CDP-diglyceride hydrolase is unknown, we have explored the possibility that this enzyme acts in vivo as either a phosphatidyl- or cytidylyltransferase. To distinguish between these two alternatives, partially purified hydrolase was incubated with CDP-diglyceride in the presence of 50% H218O. Analysis of the reaction products by 31P NMR showed that 18O is incorporated exclusively into CMP, suggesting that the enzyme is a cytidylyltransferase. This conclusion is further supported by the following experimental results: (i) the hydrolase catalyzes the transfer of CMP from CDP-diglyceride to Pi; (ii) numerous phosphomonoesters, such as glycerol 3-phosphate, phosphoserine, and glucose 1-phosphate also function as CMP acceptors, but the corresponding compounds lacking the phosphate residues are not substrates for the enzyme; and (iii) CDP-diglyceride hydrolase exchanges [32P]phosphatidic acid for the phosphatidyl moiety of CDP-diglyceride and 32Pi for the beta-phosphate residue of CDP, indicating the involvement of a novel CMP-enzyme complex. These data suggest a biosynthetic role for CDP-diglyceride hydrolase, and extend the possible functions of CDP-diglyceride in the E. coli envelope.     

Syntheses, structures and antibiotic activities of LpxC inhibitors based on the diacetylene scaffold

Compounds inhibiting LpxC in the lipid A biosynthetic pathway are promising leads for novel antibiotics against multidrug-resistant Gram-negative pathogens. We report the syntheses and structural and biochemical characterizations of LpxC inhibitors based on a diphenyl-diacetylene (1,4-diphenyl-1,3-butadiyne) threonyl-hydroxamate scaffold. These studies provide a molecular interpretation for the differential antibiotic activities of compounds with a substituted distal phenyl ring as well as the absolute stereochemical requirement at the C2, but not C3, position of the threonyl group.     

Nano-biolistics: a method of biolistic transfection of cells and tissues using a gene gun with novel nanometer-sized projectiles

Background

Biolistic transfection is proving an increasingly popular method of incorporating DNA or RNA into cells that are difficult to transfect using traditional methods. The technique routinely uses 'microparticles', which are ~1 μm diameter projectiles, fired into tissues using pressurised gas. These microparticles are efficient at delivering DNA into cells, but cannot efficiently transfect small cells and may cause significant tissue damage, thus limiting their potential usefulness. Here we describe the use of 40 nm diameter projectiles - nanoparticles - in biolistic transfections to determine if they are a suitable alternative to microparticles.

Results

Examination of transfection efficiencies in HEK293 cells, using a range of conditions including different DNA concentrations and different preparation procedures, reveals similar behaviour of microparticles and nanoparticles. The use of nanoparticles, however, resulted in ~30% fewer damaged HEK293 cells following transfection. Biolistic transfection of mouse ear tissue revealed similar depth penetration for the two types of particles, and also showed that < 10% of nuclei were damaged in nanoparticle-transfected samples, compared to > 20% in microparticle-transfected samples. Visualising details of small cellular structures was also considerably enhanced when using nanoparticles.

Conclusions

We conclude that nanoparticles are as efficient for biolistic transfection as microparticles, and are more appropriate for use in small cells, when examining cellular structures and/or where tissue damage is a problem.