Phosphoglycerides and phospholipase C in membrane fractions of Escherichia coli B

Engineering diverse fatty acid compositions of phospholipids in Escherichia coli

Bacterial fatty acids (FAs) are an essential component of the cellular membrane and are an important source of renewable chemicals as they can be converted to fatty alcohols, esters, ketones, and alkanes, and used as biofuels, detergents, lubricants, and commodity chemicals. Most prior FA bioconversions have been performed on the carboxylic acid group. Modification of the FA hydrocarbon chain could substantially expand the structural and functional diversity of FA-derived products. Additionally, the effects of such modified FAs on the growth and metabolic state of their producing cells are not well understood. Here we engineer novel Escherichia coli phospholipid biosynthetic pathways, creating strains with distinct FA profiles enriched in ω7-unsaturated FAs (ω7-UFAs, 75%), Δ5-unsaturated FAs (Δ5-UFAs, 60%), cyclopropane FAs (CFAs, 55%), internally-branched FAs (IBFAs, 40%), and Δ5,ω7-double unsaturated FAs (DUFAs, 46%). Although bearing drastically different FA profiles in phospholipids, UFA, CFA, and IBFA enriched strains display wild-type-like phenotypic profiling and growth. Transcriptomic analysis reveals DUFA production drives increased differential expression and the induction of the fur iron starvation transcriptional cascade, but higher TCA cycle activation compared to the UFA producing strain. This likely reflects a slight cost imparted for DUFA production, which resulted in lower maximum growth in some, but not all, environmental conditions. The IBFA-enriched strain was further engineered to produce free IBFAs, releasing 96 mg/L free IBFAs from 154 mg/L of the total cellular IBFA pool. This work has resulted in significantly altered FA profiles of membrane lipids in E. coli, greatly increasing our understanding of the effects of FA structure diversity on the transcriptome, growth, and ability to react to stress.

Mechanisms of Enhanced Antibacterial Activity by Reduced Chitosan-Intercalated Nontronite

Previous studies have documented the antibacterial activity of certain iron-containing clays. However, the repulsion between negatively charged bacteria and the clay surface makes this process inefficient. The objective of this study is to improve the bactericidal efficiency of clays by reversing their surface charge from negative to positive. To achieve this objective, positively charged chitosan, a nontoxic and biodegradable polymer, was intercalated into nontronite NAu-2. Chitosan-intercalated NAu-2 (C-NAu-2) was chemically reduced to obtain reduced C-NAu-2 (rC-NAu-2). Relative to reduced nontronite (rNAu-2), the antibacterial activity of rC-NAu-2 is higher and more persistent over a pH range of 6-8. The close spatial association between positively charged rC-NAu-2 and negatively charged bacteria increases the chances of cell membrane attack by extracellular ROS, the influx of soluble Fe²⁺ into the bacterial cell, and the yield of intracellular ROS. All these factors contribute to the enhanced antibacterial activity of rC-NAu-2. In contrast to rNAu-2 treated E. coli cells, where membrane damage and intracellular ROS/Fe accumulation are restricted to the polar regions, the close bacteria-clay association in rC-NAu-2 results in nonselective membrane damage and more uniform intracellular ROS/Fe distribution across whole bacterial cells. These results advance the antibacterial model by highlighting the importance of bacteria-clay interactions to the antibacterial activity of Fe-bearing clays.

Thermodynamic studies and binding mechanisms of cell-penetrating peptides with lipids and glycosaminoglycans

Cell-penetrating peptides (CPPs) traverse the membrane of biological cells at low micromolar concentrations and are able to take various cargo molecules along with. Despite large differences in their chemical structure, CPPs share the structural similarity of a high cationic charge density. This property confers to them the ability to bind electrostatically membrane constituents such as anionic lipids and glycosaminoglycans (GAGs). Controversies exist, however, about the biological response after the interaction of CPPs with such membrane constituents. Present review compares thermodynamic binding studies with conditions of the biological CPP uptake. It becomes evident that CPPs enter biological cells by different and probably competing mechanisms. For example, some amphipathic CPPs traverse pure lipid model membranes at low micromolar concentrations--at least in the absence of cargos. In contrast, no direct translocation at these conditions is observed for non-amphipathic CPPs. Finally, CPPs bind GAGs at low micromolar concentrations with potential consequences for endocytotic pathways.

Genetic characterization of a phospholipase C gene from Candida albicans: presence of homologous sequences in Candida species other than Candida albicans

Phospholipase C (PLC) enzymes are essential in regulating several important cellular functions in eukaryotes, including yeasts. In this study, PCR was used to identify a gene encoding PLC activity in Candida albicans, using oligonucleotide primers complementary to sequences encoding highly conserved amino acid regions within the X domains of previously characterized eukaryotic phospholipase C genes. The nucleotide sequence of the C. albicans gene, CAPLC1 (2997 bp), was determined from a recombinant clone containing C. albicans 132A genomic DNA; it encoded a polypeptide of 1099 amino acids with a predicted molecular mass of 124.6 kDa. The deduced amino acid sequence of this polypeptide (CAPLC1) exhibited many of the features common to previously characterized PLCs, including specific X and Y catalytic domains. The CAPLC1 protein also exhibited several unique features, including a novel stretch of 18-19 amino acid residues within the X domain and an unusually long N-terminus which did not contain a recognizable EF-hand Ca(2+)-binding domain. An overall amino acid homology of more than 27% with PLCs previously characterized from Saccharomyces cerevisiae and Schizosaccharomyces pombe suggested that the CAPLC1 protein is a delta-form of phosphoinositide-specific PLC (PI-PLC). PLC activity was detected in cell-free extracts of both yeast and hyphal forms of C. albicans 132A following 7 h and 24 h growth using the PLC-specific substrate p-nitrophenylphosphorylcholine (p-NPPC). In addition, CAPLC1 mRNA was detected by reverse transcriptase PCR in both yeast and hyphal forms of C. albicans 132A at the same time intervals. Expression of CAPLC1 activity was also detected in extracts of Escherichia coli DH5 alpha harbouring plasmids which contained portions of the CAPLC1 gene lacking sequences encoding part of the N-terminus. Southern hybridization and PCR analyses revealed that all C. albicans and Candida dubliniensis isolates examined possessed sequences homologous to CAPLC1. Sequences related to CAPLC1 were detected in some but not all isolates of Candida tropicalis, Candida glabrata and Candida parapsilosis tested, but not in the isolates of Candida krusei, Candida kefyr, Candida guillermondii and Candida lusitaniae examined. This paper reports the first description of the cloning and sequencing of a PLC gene from a pathogenic yeast species.

Mode of action of the lanthionine-containing peptide antibiotics duramycin, duramycin B and C, and cinnamycin as indirect inhibitors of phospholipase A2

Effects of the lanthionine-containing peptide antibiotics duramycin, duramycin B, duramycin C and cinnamycin on the activity of phospholipase A2 from six different sources were studied, and their mode of action was investigated. The four antibiotics inhibited potently all tested phospholipases A2, with IC50 values of around 1 microM, using phosphatidylethanolamine or [1-14C]oleate-labelled Escherichia coli, whose phospholipids are rich in phosphatidylethanolamine, as substrates. No inhibition was observed when the substrate was phosphatidylcholine. Binding of the antibiotics to the lipid fraction of E. coli could be demonstrated by co-sedimentation with whole, but not with lipid-depleted E. coli. In addition, preincubation of duramycin B with vesicles of phosphatidylethanolamine, but not those of phosphatidylcholine, prevented the inhibition of phospholipase A2 activity. The interaction of duramycin B and C, but not that of the biologically inactive compounds actagardine and the duramycin B trisulphoxide, with phosphatidylethanolamine was demonstrated using circular dichroism studies. On the other hand, no interaction of duramycin B with phosphatidylcholine could be demonstrated. A strict correlation between the physico-chemical interaction of the studied lantibiotics, demonstrated by circular dichroism spectroscopy, and their inhibition of phospholipase A2 was observed. These results suggest that lanthionine-containing peptide antibiotics inhibit phospholipase A2 indirectly by specifically sequestering the substrate phosphatidylethanolamine. This mode of action is analogous to the one described for the protein lipocortin.

Localization and orientation of the VirD4 protein of Agrobacterium tumefaciens in the cell membrane

The virD4 gene of Agrobacterium tumefaciens is essential for the formation of crown galls. Analysis of the nucleotide sequence of virD4 has suggested that the N-terminal region of the encoded protein acts as a signal peptide for the transport of the VirD4 protein to the cell membrane of Agrobacterium. We have examined the localization and orientation of this protein in the cell membrane. When the nucleotides encoding the first 30 to 41 amino acids from the N-terminus of the VirD4 protein were fused to the gene for alkaline phosphatase from which the signal sequence had been removed, alkaline phosphatase activity was detectable under appropriate conditions. Immunoblotting with VirD4-specific antiserum indicated that the VirD4 protein could be recovered exclusively from the membrane fraction of Agrobacterium cells. Moreover, when the membrane fraction was separated into inner and outer membrane fractions by sucrose density-gradient centrifugation, VirD4 protein was detected in the inner-membrane fraction and in fractions that sedimented between the inner and outer membrane fractions. By contrast, the VirD4'/alkaline phosphatase fusion protein with the N-terminal sequence from VirD4 was detected only in the inner membrane fraction. Treatment of spheroplasts of Agrobacterium cells with proteinase K resulted in digestion of the VirD4 protein. These results indicate that the VirD4 protein is transported to the bacterial membrane and anchored on the inner membrane by its N-terminal region. In addition, the C-terminal portion of the VirD4 protein probably protrudes into the periplasmic space, perhaps in association with some unidentified cellular factor(s).

The 'Bayer bridges' confronted with results from improved electron microscopy methods

In electron micrographs of conventionally prepared thin sections of Escherichia coli one observes (i) a wavy appearance of the two membranes showing frequent appositions (named adhesion sites) and (ii) intermembrane bridges after plasmolysis which, it is claimed, occur at the adhesion sites and are related to intermembrane protein transport (transmigration). When chemical fixation is replaced by cryofixation, the observations are very different. (a) The two membranes are equally spaced and no contacts, adhesions or other sorts of connections are visible. (b) After plasmolysis the protoplast is shrunken, but the typical bridges are no longer produced. (c) In addition, when peptidoglycan is stained on conventionally prepared sections, it is revealed as a 7-nm-thick sacculus which is not interrupted at the sites of apposition. In view of the new observations, the structural concepts derived from conventionally prepared material must be revised. It is proposed that the intermembrane space is entirely filled by a gel, the outer part of which is the 7 nm thick, very stable, chemically resistant peptidoglycan (or murein). The inner part is much less stable and is proposed to undergo rapid autolytic changes upon cell death. The large 'Bayer bridges' might then tentatively be explained as an artificial post-mortem enhancement of either a stream of proteins transmigrating across the periplasm or of a pre-existing, but not yet resolved, structure. This enhancement probably occurs during the 7-10 min between plasmolysis and fixation that are prescribed for the procedure necessary for revealing 'Bayer bridges'.

Diacylglycerol-phosphatidylethanolamine exchange in Escherichia coli

The conversion of [3H]- or [14C]diacylglycerol to labelled phosphatidylethanolamine by dialysed, particulate fraction of Escherichia coli was studied. The reaction occurred in the presence of hydroxylamine, under which conditions, the synthesis of [14C]phosphatidylethanolamine from CDP-diacylglycerol and [14C]serine did not occur. The conversion was enhanced by addition of dilauroyl- or dioleoylphosphatidylethanolamine. A conversion of [3H]- or [14C]phosphatidylethanolamine to labelled diacylglycerol could also be readily demonstrated provided unlabelled diacylglycerol was added. Double-labelled [acyl-3H, 32P]phosphatidylethanolamine was converted to labelled diacylglycerol without formation of labelled water-soluble products. The formation of double-labelled phosphatidylethanolamine from [3H]diacyl[14C]glycerol or of double-labelled diacylglycerol from [acyl-3H,glycerol-14C]phosphatidylethanolamine occurred without significant change in isotope ratio. When [acyl-3H]phosphatidylethanolamine was incubated with increasing concentrations of [acyl-14C]diacylglycerol, correspondingly increasing concentrations of [14C]phosphatidylethanolamine were formed which matched the concentrations of [3H]diacylglycerol produced concurrently. It was concluded that E. coli extracts can catalyze an exchange between the diacylglycerol moiety of phosphatidylethanolamine and free diacylglycerol with complete sparing of the phosphoethanolamine moiety.

Localization and assembly into the Escherichia coli envelope of a protein required for entry of colicin A

Mutations in tolQ, previously designated fii, render cells tolerant to high concentrations of colicin A. In addition, a short deletion in the amino-terminal region of colicin A (amino acid residues 16 to 29) prevents its lethal action, although this protein can still bind the receptor and forms channels in planar lipid bilayers in vitro. These defects in translocation across the outer membrane in the tolQ cells or the colicin A mutant cannot be bypassed by osmotic shock. The TolQ protein, which is constitutively expressed at a low level, was studied in recombinant plasmid constructs allowing the expression of various TolQ fusion proteins under the control of the inducible caa promoter. The TolQ protein was thus "tagged" with an epitope from the colicin A protein for which a monoclonal antibody is available. A fusion protein containing the entire TolQ protein plus the 30 N-terminal residues of colicin A was shown to complement the tolQ mutation. Pulse-chase labeling followed by gradient fractionation indicated that the bulk of the overproduced fusion protein was rapidly incorporated into the inner membrane, with small amounts localized to regions corresponding to the attachment sites between inner and outer membranes and to the outer membrane itself. However, most of the protein was rapidly degraded, leaving only that localized to the attachment sites and the outer membrane remaining at very late times of chase.

Association of thioredoxin with the inner membrane and adhesion sites in Escherichia coli

The intracellular localization of thioredoxin in Escherichia coli was determined by immunoelectron microscopy and correlated to previous biochemical data which had suggested that thioredoxin resides at inner-outer membrane adhesion sites. Since a considerable amount of thioredoxin was lost during preparation of cells for electron microscopy, we immobilized the protein with the heterobifunctional photoactivatable cross-linker p-azidophenacylbromide before the cells were fixed with aldehyde and embedded in Lowicryl K4M. Thin sections were labeled with affinity-purified antithioredoxin antiserum and protein A-gold complexes. Densities of immunolabel in a designated membrane-associated area and in the rest of the cytoplasm were compared and the data were statistically evaluated. Wild-type strain W3110 and strain SK3981, an overproducer of thioredoxin, exhibited increased labeling at the inner membrane and its adjacent cytoplasmic area. In contrast, the more centrally located cytoplasm of both strains showed much lower label density. This label distribution did not change with cell growth or in the stationary phase. Immunolabel was often found at bridges between the inner and outer membranes; this result is consistent with a model which places at least a portion of the thioredoxin at membrane adhesion sites, corresponding to an osmotically sensitive cytoplasmic compartment bounded by a hybrid inner-outer membrane (C.A. Lunn and V. Pigiet, J. Biol. Chem. 257:11424-11430, 1982; C.A. Lunn and V. Pigiet, J. Biol. Chem. 261:832-838, 1986). Specific label was absent in the periplasmic space.

Specific location of penicillin-binding proteins within the cell envelope of Escherichia coli

This communication deals with the location of penicillin-binding proteins in the cell envelope of Escherichia coli. For this purpose, bacterial cells have been broken by various procedures and their envelopes have been fractioned. To do so, inner (cytoplasmic) and outer membranes were separated by isopycnic centrifugation in sucrose gradients. Some separation methods (Osborn et al., J. Biol. Chem. 247:3962-3972, 1972; J. Smit, Y. Kamio, and H. Nikaido, J. Bacteriol. 124:942-958, 1975) revealed that penicillin-binding proteins are not exclusively located in the inner membrane. They are also found in the outer membrane (A. Rodríguez-Tébar, J. A. Barbas, and D. Vásquez, J. Bacteriol. 161:243-248, 1985). Under the milder conditions for cell rupture used in this work, an intermembrane fraction, sedimenting between the inner and outer membrane, can be recovered from the gradients. This fraction has a high content of both penicillin-binding proteins and phospholipase B activity and may correspond to the intermembrane adhesion sites (M. H. Bayer, G. P. Costello, and M. E. Bayer, J. Bacteriol. 149:758-769, 1982). We postulate that this intermembrane fraction is a labile structure that contains a high amount of all penicillin-binding proteins which are usually found in both the inner and outer membranes when the adhesion sites are destroyed by the cell breakage and fractionation procedures.

Effects of bacteriophage fd infection on Escherichia coli HB11 envelope: a morphological and biochemical study

Phage fd-infected host bacteria revealed three characteristic changes in their envelope. (i) The preferred cleavage plane during freeze-fracturing shifted from the inner to the outer membrane (OM). (ii) The total lipids of the OM of the infected cells increased by 25% without major alterations in the relative concentration of phospholipids. We propose that such an increase would to some extent contribute to the change in the freeze-fracture behavior of the OM; however, additional factors will have to play a role in the apparent fracture resistance of the inner membrane. (iii) Ultrathin sectioning and immunolabeling methods revealed that extrusion of fd phages takes place at membrane adhesion sites of the infected cells.