Dispensable nature of phosphatidylglycerol in Escherichia coli: dual roles of anionic phospholipids

The intricate link between membrane lipid structure and composition and membrane structural properties in bacterial membranes

It is now evident that the cell manipulates lipid composition to regulate different processes such as membrane protein insertion, assembly and function. Moreover, changes in membrane structure and properties, lipid homeostasis during growth and differentiation with associated changes in cell size and shape, and responses to external stress have been related to drug resistance across mammalian species and a range of microorganisms. While it is well known that the biomembrane is a fluid self-assembled nanostructure, the link between the lipid components and the structural properties of the lipid bilayer are not well understood. This perspective aims to address this topic with a view to a more detailed understanding of the factors that regulate bilayer structure and flexibility. We describe a selection of recent studies that address the dynamic nature of bacterial lipid diversity and membrane properties in response to stress conditions. This emerging area has important implications for a broad range of cellular processes and may open new avenues of drug design for selective cell targeting.

Interaction between glycolipid MPIase and proteinaceous factors during protein integration into the cytoplasmic membrane of E. coli

Protein integration into biomembranes is an essential biological phenomenon common to all organisms. While various factors involved in protein integration, such as SRP, SecYEG and YidC, are proteinaceous, we identified a glycolipid named MPIase (Membrane Protein Integrase), which is present in the cytoplasmic membrane of E. coli. In vitro experiments using inverted membrane vesicles prepared from MPIase-depleted strains, and liposomes containing MPIase showed that MPIase is required for insertion of a subset of membrane proteins, which has been thought to be SecYEG-independent and YidC-dependent. Also, SecYEG-dependent substrate membrane proteins require MPIase in addition. Furthermore, MPIase is also essential for insertion of proteins with multiple negative charges, which requires both YidC and the proton motive force (PMF). MPIase directly interacts with SecYEG and YidC on the membrane. MPIase not only cooperates with these factors but also has a molecular chaperone-like function specific to the substrate membrane proteins through direct interaction with the glycan chain. Thus, MPIase catalyzes membrane insertion by accepting nascent membrane proteins on the membrane through its chaperone-like function, i.e., direct interaction with the substrate proteins, and then MPIase functionally interacts with SecYEG and YidC for substrate delivery, and acts with PMF to facilitate and complete membrane insertion when necessary. In this review, we will outline the mechanisms underlying membrane insertion catalyzed by MPIase, which cooperates with proteinaceous factors and PMF.

Role of the lipid bilayer in outer membrane protein folding in Gram-negative bacteria

β-Barrel outer membrane proteins (OMPs) represent the major proteinaceous component of the outer membrane (OM) of Gram-negative bacteria. These proteins perform key roles in cell structure and morphology, nutrient acquisition, colonization and invasion, and protection against external toxic threats such as antibiotics. To become functional, OMPs must fold and insert into a crowded and asymmetric OM that lacks much freely accessible lipid. This feat is accomplished in the absence of an external energy source and is thought to be driven by the high thermodynamic stability of folded OMPs in the OM. With such a stable fold, the challenge that bacteria face in assembling OMPs into the OM is how to overcome the initial energy barrier of membrane insertion. In this review, we highlight the roles of the lipid environment and the OM in modulating the OMP-folding landscape and discuss the factors that guide folding in vitro and in vivo We particularly focus on the composition, architecture, and physical properties of the OM and how an understanding of the folding properties of OMPs in vitro can help explain the challenges they encounter during folding in vivo Current models of OMP biogenesis in the cellular environment are still in flux, but the stakes for improving the accuracy of these models are high. OMP folding is an essential process in all Gram-negative bacteria, and considering the looming crisis of widespread microbial drug resistance it is an attractive target. To bring down this vital OMP-supported barrier to antibiotics, we must first understand how bacterial cells build it.

Coevolution of Resistance Against Antimicrobial Peptides

Antimicrobial peptides (AMPs) are produced by all forms of life, ranging from eukaryotes to prokaryotes, and they are a crucial component of innate immunity, involved in clearing infection by inhibiting pathogen colonization. In the recent past, AMPs received high attention due to the increase of extensive antibiotic resistance by these pathogens. AMPs exhibit a diverse spectrum of activity against bacteria, fungi, parasites, and various types of cancer. AMPs are active against various bacterial pathogens that cause disease in animals and plants. However, because of the coevolution of host and pathogen interaction, bacteria have developed the mechanisms to sense and exhibit an adaptive response against AMPs. These resistance mechanisms are playing an important role in bacterial virulence within the host. Here, we have discussed the different resistance mechanisms used by gram-positive and gram-negative bacteria to sense and combat AMP actions. Understanding the mechanism of AMP resistance may provide directions toward the development of novel therapeutic strategies to control multidrug-resistant pathogens.

Folding and Misfolding of Human Membrane Proteins in Health and Disease: From Single Molecules to Cellular Proteostasis

Advances over the past 25 years have revealed much about how the structural properties of membranes and associated proteins are linked to the thermodynamics and kinetics of membrane protein (MP) folding. At the same time biochemical progress has outlined how cellular proteostasis networks mediate MP folding and manage misfolding in the cell. When combined with results from genomic sequencing, these studies have established paradigms for how MP folding and misfolding are linked to the molecular etiologies of a variety of diseases. This emerging framework has paved the way for the development of a new class of small molecule "pharmacological chaperones" that bind to and stabilize misfolded MP variants, some of which are now in clinical use. In this review, we comprehensively outline current perspectives on the folding and misfolding of integral MPs as well as the mechanisms of cellular MP quality control. Based on these perspectives, we highlight new opportunities for innovations that bridge our molecular understanding of the energetics of MP folding with the nuanced complexity of biological systems. Given the many linkages between MP misfolding and human disease, we also examine some of the exciting opportunities to leverage these advances to address emerging challenges in the development of therapeutics and precision medicine.

Syntheses and Activities of the Functional Structures of a Glycolipid Essential for Membrane Protein Integration

MPIase is the first known glycolipid that is essential for membrane protein integration in the inner membrane of E. coli. Since the amount of natural MPIase available for analysis is limited and it contains structural heterogeneity, precisely designed synthetic derivatives are promising tools for further elucidation of its membrane protein integration mechanism. Thus, we synthesized the minimal unit of MPIase, a trisaccharyl pyrophospholipid termed mini-MPIase-3, and its derivatives. Integration assays revealed that the chemically synthesized trisaccharyl pyrophospholipid possesses significant activity, indicating that it includes the essential structure for membrane integration. Structure-activity relationship studies demonstrated that the number of trisaccharide units and the 6- O-acetyl group on N-acetylglucosamine contribute to efficient integration. Furthermore, anchoring in the membrane by a lipid moiety was essential for the integration. However, the addition of phosphorylated glycans devoid of the lipid moiety in the assay solution modulated the integration activity of MPIase embedded in liposomes, suggesting an interaction between phosphorylated glycans and substrate proteins in aqueous solutions. The prevention of protein aggregation required the 6- O-acetyl group on N-acetylglucosamine, a phosphate group at the reducing end of the glycan, and a long glycan chain. Taken together, we verified the mechanism of the initial step of the translocon-independent pathway in which a membrane protein is captured by a glycan of MPIase, which maintains its structure to be competent for integration, and then MPIase integrates it into the membrane by hydrophobic interactions with membrane lipids.

Dynamic remodeling of lipids coincides with dengue virus replication in the midgut of Aedes aegypti mosquitoes

We describe the first comprehensive analysis of the midgut metabolome of Aedes aegypti, the primary mosquito vector for arboviruses such as dengue, Zika, chikungunya and yellow fever viruses. Transmission of these viruses depends on their ability to infect, replicate and disseminate from several tissues in the mosquito vector. The metabolic environments within these tissues play crucial roles in these processes. Since these viruses are enveloped, viral replication, assembly and release occur on cellular membranes primed through the manipulation of host metabolism. Interference with this virus infection-induced metabolic environment is detrimental to viral replication in human and mosquito cell culture models. Here we present the first insight into the metabolic environment induced during arbovirus replication in Aedes aegypti. Using high-resolution mass spectrometry, we have analyzed the temporal metabolic perturbations that occur following dengue virus infection of the midgut tissue. This is the primary site of infection and replication, preceding systemic viral dissemination and transmission. We identified metabolites that exhibited a dynamic-profile across early-, mid- and late-infection time points. We observed a marked increase in the lipid content. An increase in glycerophospholipids, sphingolipids and fatty acyls was coincident with the kinetics of viral replication. Elevation of glycerolipid levels suggested a diversion of resources during infection from energy storage to synthetic pathways. Elevated levels of acyl-carnitines were observed, signaling disruptions in mitochondrial function and possible diversion of energy production. A central hub in the sphingolipid pathway that influenced dihydroceramide to ceramide ratios was identified as critical for the virus life cycle. This study also resulted in the first reconstruction of the sphingolipid pathway in Aedes aegypti. Given conservation in the replication mechanisms of several flaviviruses transmitted by this vector, our results highlight biochemical choke points that could be targeted to disrupt transmission of multiple pathogens by these mosquitoes.

Unraveling the expression of genes involved in the biosynthesis pathway of cardiolipin and phosphatidylethanolamine in Salmonella Hadar grown under static magnetic field 200 mT

We aimed in this work to evaluate the effect of static magnetic field 200 mT (SMF) on the expression of genes involved in the biosynthetic pathway of cardiolipin: g3pd, plsB, cdsA, pgsA, pgpA, cls and phosphatidylethanolamine: pssA and psd in Salmonella enterica subsp enterica serovar Hadar. Bacteria were exposed to a SMF during 3, 6 and 9 h. RNA extraction was followed by Reverse Transcriptase Polymerase Chain Reaction RT-PCR. The relative quantification of mRNA expression levels using 16S rRNA doesn't change during the time exposure. RT-PCR was done for two exposure experiments. The gene expression using RT-PCR present no significant difference in case of plsB, cdsA, pgpA, pgsA and psd genes during the different exposure times. However, a significant increase was observed in the expression of g3pd and pssA genes after 6 h and for cls gene after 3 h of exposure, but any variation was notified after 9 h of exposure. So we can conclude from this study that cls, g3pd and pssA genes are required in the adaptation of Salmonella Hadar to SMF.

In Vivo and in Vitro Synthesis of Phosphatidylglycerol by an Escherichia coli Cardiolipin Synthase

Phosphatidylglycerol (PG) makes up 5-20% of the phospholipids of Escherichia coli and is essential for growth in wild-type cells. PG is synthesized from the dephosphorylation of its immediate precursor, phosphatidylglycerol phosphate (PGP) whose synthase in E. coli is PgsA. Using genetic, biochemical, and highly sensitive mass spectrometric approaches, we identified an alternative mechanism for PG synthesis in E. coli that is PgsA independent. The reaction of synthesis involves the conversion of phosphatidylethanolamine and glycerol into PG and is catalyzed by ClsB, a phospholipase D-type cardiolipin synthase. This enzymatic reaction is demonstrated herein both in vivo and in vitro as well as by using the purified ClsB protein. When the growth medium was supplemented with glycerol, the expression of E. coli ClsB significantly increased PG and cardiolipin levels, with the growth deficiency of pgsA null strain also being complemented under such conditions. Identification of this alternative mechanism for PG synthesis not only expands our knowledge of bacterial anionic phospholipid biosynthesis, but also sheds light on the biochemical functions of the cls gene redundancy in E. coli and other bacteria. Finally, the PGP-independent PG synthesis in E. coli may also have important implications for the understanding of PG biosynthesis in eukaryotes that remains incomplete.

Phospholipids and glycolipids mediate proton containment and circulation along the surface of energy-transducing membranes

Proton bioenergetics provides the energy for growth and survival of most organisms in the biosphere ranging from unicellular marine phytoplankton to humans. Chloroplasts harvest light and generate a proton electrochemical gradient (proton motive force) that drives the production of ATP needed for carbon dioxide fixation and plant growth. Mitochondria, bacteria and archaea generate proton motive force to energize growth and other physiologies. Energy transducing membranes are at the heart of proton bioenergetics and are responsible for catalyzing the conversion of energy held in high-energy electrons→electron transport chain→proton motive force→ATP. Whereas the electron transport chain is understood in great detail there are major gaps in understanding mechanisms of proton transfer or circulation during proton bioenergetics. This paper is built on the proposition that phospho- and glyco-glycerolipids form proton transport circuitry at the membrane's surface. By this proposition, an emergent membrane property, termed the hyducton, confines active/unbound protons or hydronium ions to a region of low volume close to the membrane surface. In turn, a von Grotthuß mechanism rapidly moves proton substrate in accordance with nano-electrochemical poles on the membrane surface created by powerful proton pumps such as ATP synthase.

The Power of Asymmetry: Architecture and Assembly of the Gram-Negative Outer Membrane Lipid Bilayer

Determining the chemical composition of biological materials is paramount to the study of natural phenomena. Here, we describe the composition of model gram-negative outer membranes, focusing on the predominant assembly, an asymmetrical bilayer of lipid molecules. We also give an overview of lipid biosynthetic pathways and molecular mechanisms that organize this material into the outer membrane bilayer. An emphasis is placed on the potential of these pathways as targets for antibiotic development. We discuss deviations in composition, through bacterial cell surface remodeling, and alternative modalities to the asymmetric lipid bilayer. Outer membrane lipid alterations of current microbiological interest, such as lipid structures found in commensal bacteria, are emphasized. Additionally, outer membrane components could potentially be engineered to develop vaccine platforms. Observations related to composition and assembly of gram-negative outer membranes will continue to generate novel discoveries, broaden biotechnologies, and reveal profound mysteries to compel future research.

Rapidly Probing Antibacterial Activity of Graphene Oxide by Mass Spectrometry-based Metabolite Fingerprinting

Application of nanomaterials as anti-bacteria agents has aroused great attention. To investigate the antibacterial activity and antibacterial mechanism of nanomaterials from a molecular perspective is important for efficient developing of nanomaterial antibiotics. In the current work, a new mass spectrometry-based method was established to investigate the bacterial cytotoxicity of graphene oxide (GO) by the metabolite fingerprinting of microbes. The mass spectra of extracted metabolites from two strains DH5α and ATCC25922 were obtained before and after the incubation with nanomaterials respectively. Then principal component analysis (PCA) of these spectra was performed to reveal the relationship between the metabolism disorder of microbes and bactericidal activity of GO. A parameter “D” obtained from PCA scores was proposed that is capable to quantitatively evaluate the antibacterial activity of GO in concentration and time-dependent experiments. Further annotation of the fingerprinting spectra shows the variabilities of important metabolites such as phosphatidylethanolamine, phosphatidylglycerol and glutathione. This metabolic perturbation of E. coli indicates cell membrane destruction and oxidative stress mechanisms for anti-bacteria activity of graphene oxide. It is anticipated that this mass spectrometry-based metabolite fingerprinting method will be applicable to other antibacterial nanomaterials and provide more clues as to their antibacterial mechanism at molecular level.

Organization and function of anionic phospholipids in bacteria

In addition to playing a central role as a permeability barrier for controlling the diffusion of molecules and ions in and out of bacterial cells, phospholipid (PL) membranes regulate the spatial and temporal position and function of membrane proteins that play an essential role in a variety of cellular functions. Based on the very large number of membrane-associated proteins encoded in genomes, an understanding of the role of PLs may be central to understanding bacterial cell biology. This area of microbiology has received considerable attention over the past two decades, and the local enrichment of anionic PLs has emerged as a candidate mechanism for biomolecular organization in bacterial cells. In this review, we summarize the current understanding of anionic PLs in bacteria, including their biosynthesis, subcellular localization, and physiological relevance, discuss evidence and mechanisms for enriching anionic PLs in membranes, and conclude with an assessment of future directions for this area of bacterial biochemistry, biophysics, and cell biology.

Septal localization by membrane targeting sequences and a conserved sequence essential for activity at the COOH-terminus of Bacillus subtilis cardiolipin synthase

The acidic phospholipid cardiolipin (CL) is localized on polar and septal membranes and plays an important physiological role in Bacillus subtilis cells. ClsA, the enzyme responsible for CL synthesis, is also localized on septal membranes. We found that GFP fusion proteins of the enzyme with NH2-terminal and internal deletions retained septal localization. However, derivatives with deletions starting from the COOH-terminus (Leu482) ceased to localize to the septum once the deletion passed the Ile residue at 448, indicating that the sequence responsible for septal localization is confined within a short distance from the COOH-terminus. Two sequences, Ile436-Leu450 and Leu466-Leu478, are predicted to individually form an amphipathic α-helix. This configuration is known as a membrane targeting sequence (MTS) and we therefore refer to them as MTS2 and MTS1, respectively. Either one has the ability to affect septal localization, and each of these sequences by itself localizes to the septum. Membrane association of the constructs of this enzyme containing the MTSs was verified by subcellular fractionation of the cells. CL synthesis, in contrast, was abolished after deleting just the last residue, Leu482, in the COOH-terminal four amino acid residue sequence, Ser-Pro-Ile-Leu, which is highly conserved among bacterial CL synthases.

E. coli MG1655 modulates its phospholipid composition through the cell cycle

This paper describes a study of the phospholipid profile of Escherichia coli MG1655 cultures at the B and D periods of the cell cycle. The results indicate that the phosphatidyl glycerol fraction grows relatively rapidly and that the size of the cardiolipin (CL) fraction does not grow at all during cell elongation. This is consistent with observations that CL is located preferentially at the poles of E. coli. It also suggests that lipid production is controlled as a function of the cell cycle.

The membrane: transertion as an organizing principle in membrane heterogeneity

The bacterial membrane exhibits a significantly heterogeneous distribution of lipids and proteins. This heterogeneity results mainly from lipid-lipid, protein-protein, and lipid-protein associations which are orchestrated by the coupled transcription, translation and insertion of nascent proteins into and through membrane (transertion). Transertion is central not only to the individual assembly and disassembly of large physically linked groups of macromolecules (alias hyperstructures) but also to the interactions between these hyperstructures. We review here these interactions in the context of the processes in Bacillus subtilis and Escherichia coli of nutrient sensing, membrane synthesis, cytoskeletal dynamics, DNA replication, chromosome segregation, and cell division.

Lantibiotic resistance

The dramatic rise in the incidence of antibiotic resistance demands that new therapeutic options will have to be developed. One potentially interesting class of antimicrobials are the modified bacteriocins termed lantibiotics, which are bacterially produced, posttranslationally modified, lanthionine/methyllanthionine-containing peptides. It is interesting that low levels of resistance have been reported for lantibiotics compared with commercial antibiotics. Given that there are very few examples of naturally occurring lantibiotic resistance, attempts have been made to deliberately induce resistance phenotypes in order to investigate this phenomenon. Mechanisms that hinder the action of lantibiotics are often innate systems that react to the presence of any cationic peptides/proteins or ones which result from cell well damage, rather than being lantibiotic specific. Such resistance mechanisms often arise due to altered gene regulation following detection of antimicrobials/cell wall damage by sensory proteins at the membrane. This facilitates alterations to the cell wall or changes in the composition of the membrane. Other general forms of resistance include the formation of spores or biofilms, which are a common mechanistic response to many classes of antimicrobials. In rare cases, bacteria have been shown to possess specific antilantibiotic mechanisms. These are often species specific and include the nisin lytic protein nisinase and the phenomenon of immune mimicry.

Polar localization of a tripartite complex of the two-component system DcuS/DcuR and the transporter DctA in Escherichia coli depends on the sensor kinase DcuS

The C4-dicarboxylate responsive sensor kinase DcuS of the DcuS/DcuR two-component system of E. coli is membrane-bound and reveals a polar localization. DcuS uses the C4-dicarboxylate transporter DctA as a co-regulator forming DctA/DcuS sensor units. Here it is shown by fluorescence microscopy with fusion proteins that DcuS has a dynamic and preferential polar localization, even at very low expression levels. Single assemblies of DcuS had high mobility in fast time lapse acquisitions, and fast recovery in FRAP experiments, excluding polar accumulation due to aggregation. DctA and DcuR fused to derivatives of the YFP protein are dispersed in the membrane or in the cytosol, respectively, when expressed without DcuS, but co-localize with DcuS when co-expressed at appropriate levels. Thus, DcuS is required for location of DctA and DcuR at the poles and formation of tripartite DctA/DcuS/DcuR sensor/regulator complexes. Vice versa, DctA, DcuR and the alternative succinate transporter DauA were not essential for polar localization of DcuS, suggesting that the polar trapping occurs by DcuS. Cardiolipin, the high curvature at the cell poles, and the cytoskeletal protein MreB were not required for polar localization. In contrast, polar localization of DcuS required the presence of the cytoplasmic PAS(C) and the kinase domains of DcuS.

Coordinated response of phospholipids and acyl components of membrane lipids in Pseudomonas putida A (ATCC 12633) under stress caused by cationic surfactants

The present study assessed the role of membrane components of Pseudomonas putida A (ATCC 12633) under chemical stress conditions originated by treatment with tetradecyltrimethylammonium bromide (TTAB), a cationic surfactant. We examined changes in fatty acid composition and in the fluidity of the membranes of cells exposed to TTAB at a specific point of growth as well as of cells growing with TTAB. The addition of 10-50 mg TTAB l(-1) promoted an increase in the saturated/unsaturated fatty acid ratio. By using fluorescence polarization techniques, we found that TTAB exerted a fluidizing effect on P. putida A (ATCC 12633) membranes. However, a complete reversal of induced membrane fluidification was detected after 15 min of incubation with TTAB. Consistently, the proportion of unsaturated fatty acids was lower in TTAB-treated cells as compared with non-treated cells. In the presence of TTAB, the content of phosphatidylglycerol increased (120 %), whilst that of cardiolipin decreased (60 %). Analysis of the fatty acid composition of P. putida A (ATCC 12633) showed that phosphatidylglycerol carried the major proportion of saturated fatty acids (89 %), whilst cardiolipin carried an elevated proportion of unsaturated fatty acids (18 %). The increase in phosphatidylglycerol and consequently in saturated fatty acids, together with a decrease in cardiolipin content, enabled greater membrane resistance, reversing the fluidizing effect of TTAB. Therefore, results obtained in the present study point to changes in the fatty acid profile as an adaptive response of P. putida A (ATCC 12633) cells to stress caused by a cationic surfactant.

Polar localization of Escherichia coli chemoreceptors requires an intact Tol-Pal complex

Subcellular biomolecular localization is critical for the metabolic and structural properties of the cell. The functional implications of the spatiotemporal distribution of protein complexes during the bacterial cell cycle have long been acknowledged; however, the molecular mechanisms for generating and maintaining their dynamic localization in bacteria are not completely understood. Here we demonstrate that the trans-envelope Tol-Pal complex, a widely conserved component of the cell envelope of Gram-negative bacteria, is required to maintain the polar positioning of chemoreceptor clusters in Escherichia coli. Localization of the chemoreceptors was independent of phospholipid composition of the membrane and the curvature of the cell wall. Instead, our data indicate that chemoreceptors interact with components of the Tol-Pal complex and that this interaction is required to polarly localize chemoreceptor clusters. We found that disruption of the Tol-Pal complex perturbs the polar localization of chemoreceptors, alters cell motility, and affects chemotaxis. We propose that the E. coli Tol-Pal complex restricts mobility of the chemoreceptor clusters at the cell poles and may be involved in regulatory mechanisms that co-ordinate cell division and segregation of the chemosensory machinery.

Importance of the proline-rich region for the regulatory function of RcsF, an outer membrane lipoprotein component of the Escherichia coli Rcs signal transduction system

The outer membrane lipoprotein RcsF is an essential component of the Rcs phosphorelay signal transduction system in Escherichia coli. It senses stresses imposed on the cell envelope and conveys the information to histidine kinase RcsC in the cytoplasmic membrane. Mislocalization of RcsF to the periplasm, effected by fusing it to the periplasmic maltose-binding protein, or to the cytoplasmic membrane, brought about by changing the lipoprotein sorting signal, leads to high activation of the Rcs system, suggesting that RcsF functions as a ligand for RcsC in activating the system. Here, we focus on the proline-rich region (PRR) in the N-terminal half of RcsF, a region which also contains many basic amino acid residues. Deletion of the PRR in the mislocalized RcsF resulted in even higher activation of the Rcs system. The same deletion in wild-type RcsF lipoprotein that is correctly localized to the outer membrane, however, blocked activation of the system under stresses that normally should activate it. It is highly likely that the PRR plays an important role in the regulation of the function of RcsF in activating the Rcs system.

Depletion of acidic phospholipids influences chromosomal replication in Escherichia coli

In Escherichia coli, coordinated activation and deactivation of DnaA allows for proper timing of the initiation of chromosomal synthesis at the origin of replication (oriC) and assures initiation occurs once per cell cycle. In vitro, acidic phospholipids reactivate DnaA, and in vivo depletion of acidic phospholipids, results in growth arrest. Growth can be restored by the expression of a mutant form of DnaA, DnaA(L366K), or by oriC-independent DNA synthesis, suggesting acidic phospholipids are required for DnaA- and oriC-dependent replication. We observe here that when acidic phospholipids were depleted, replication was inhibited with a concomitant reduction of chromosomal content and cell mass prior to growth arrest. This global shutdown of biosynthetic activity was independent of the stringent response. Restoration of acidic phospholipid synthesis resulted in a resumption of DNA replication prior to restored growth, indicating a possible cell-cycle-specific growth arrest had occurred with the earlier loss of acidic phospholipids. Flow cytometry, thymidine uptake, and quantitative polymerase chain reaction data suggest that a deficiency in acidic phospholipids prolonged the time required to replicate the chromosome. We also observed that regardless of the cellular content of acidic phospholipids, expression of mutant DnaA(L366K) altered the DNA content-to-cell mass ratio.

Cardiolipin binding in bacterial respiratory complexes: structural and functional implications

The structural and functional integrity of biological membranes is vital to life. The interplay of lipids and membrane proteins is crucial for numerous fundamental processes ranging from respiration, photosynthesis, signal transduction, solute transport to motility. Evidence is accumulating that specific lipids play important roles in membrane proteins, but how specific lipids interact with and enable membrane proteins to achieve their full functionality remains unclear. X-ray structures of membrane proteins have revealed tight and specific binding of lipids. For instance, cardiolipin, an anionic phospholipid, has been found to be associated to a number of eukaryotic and prokaryotic respiratory complexes. Moreover, polar and septal accumulation of cardiolipin in a number of prokaryotes may ensure proper spatial segregation and/or activity of proteins. In this review, we describe current knowledge of the functions associated with cardiolipin binding to respiratory complexes in prokaryotes as a frame to discuss how specific lipid binding may tune their reactivity towards quinone and participate to supercomplex formation of both aerobic and anaerobic respiratory chains. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

Activation of the Cpx phosphorelay signal transduction system in acidic phospholipid-deficient pgsA mutant cells of Escherichia coli

The pgsA gene encodes the enzyme for the committed step in the synthesis of acidic phospholipids in Escherichia coli, and the pssA gene does the same for zwitterionic phospholipid. It has been reported that the Rcs and Cpx phosphorelay signal transduction systems are activated in pgsA- and pssA-defective mutants, respectively. In this study, we show that the Cpx system is activated also in a pgsA mutant, whereas the Rcs system was not activated in a pssA mutant. Lack of phosphatidylglycerol in pgsA mutants causes inadequate modification of lipoproteins, resulting in poor localization to the outer membrane. The outer membrane lipoprotein RcsF is necessary for the response of the Rcs system to various stimuli, and Rcs activation in pgsA mutants involves inner membrane mislocalization of this lipoprotein. The outer membrane lipoprotein NlpE, however, while necessary for the surface adhesion-induced Cpx response, was not involved in Cpx activation in the pgsA mutant.

Exploring the relationship between lipoprotein mislocalization and activation of the Rcs signal transduction system in Escherichia coli

The Rcs phosphorelay signal transduction system controls genes for capsule production and many other envelope-related functions and is implicated in biofilm formation. We investigated the activation of the Rcs system in a pgsA null mutant of Escherichia coli, which completely lacks the major acidic phospholipids phosphatidylglycerol and cardiolipin. We found that the Rcs activation, and consequent thermosensitivity, were suppressed by overexpression of the lgt gene, encoding diacylglyceryltransferase, which catalyses the modification of prolipoproteins that is the first step in the maturation and localization process of lipoproteins, and is a prerequisite for the later steps. The outer-membrane lipoprotein RcsF is an essential component of Rcs signalling. This lipoprotein was poorly localized to the outer membrane in the pgsA null mutant, probably because of the absence of phosphatidylglycerol, the major donor of diacylglycerol in the Lgt reaction. Even in a pgsA(+) background, the Rcs system was activated when RcsF was mislocalized to the inner membrane by alteration of the residues at positions 2 and 3 of its mature form to inner-membrane retention signals, or when it was mislocalized to the periplasm by fusing the mature form to maltose-binding protein. These results suggest that RcsF functions as a ligand for RcsC in activating Rcs signalling. Mislocalized versions of RcsF still responded to mutations pgsA, mdoH and tolB, further activating the Rcs system, although the rfaP mutation barely caused activation. It seems that RcsF must be localized in the outer membrane to respond effectively to stimuli from outside the cell.

Supramolecular organization in prokaryotic respiratory systems

Prokaryotes are characterized by an extreme flexibility of their respiratory systems allowing them to cope with various extreme environments. To date, supramolecular organization of respiratory systems appears as a conserved evolutionary feature as supercomplexes have been isolated in bacteria, archaea, and eukaryotes. Most of the yet identified supercomplexes in prokaryotes are involved in aerobic respiration and share similarities with those reported in mitochondria. Supercomplexes likely reflect a snapshot of the cellular respiration in a given cell population. While the exact nature of the determinants for supramolecular organization in prokaryotes is not understood, lipids, proteins, and subcellular localization can be seen as key players. Owing to the well-reported supramolecular organization of the mitochondrial respiratory chain in eukaryotes, several hypotheses have been formulated to explain the consequences of such arrangement and can be tested in the context of prokaryotes. Considering the inherent metabolic flexibility of a number of prokaryotes, cellular distribution and composition of the supramolecular assemblies should be studied in regards to environmental signals. This would pave the way to new concepts in cellular respiration.

Phase transitions and spatially ordered counterion association in ionic-lipid membranes: theory versus experiment

Aqueous dispersions of phosphatidylglycerol (PG) lipids may present an anomalous chain-melting transition at low ionic strengths, as seen by different experimental techniques such as calorimetry or light scattering. The anomaly disappears at high ionic strengths or for longer acyl-chain lengths. In this article, we use a statistical model for the bilayer that distinguishes both lipid chain and headgroup states in order to compare model and experimental thermotropic and electrical properties. The effective van der Waals interactions among hydrophobic chains compete with the electrostatic repulsions between polar headgroups, which may be ionized (counterion dissociated) or electrically neutral (associated with counterions). Electric degrees of freedom introduce new thermotropic charge-ordered phases in which headgroup charges may be spatially ordered, depending on the electrolyte ionic strength, introducing a new rationale for experimental data on PGs. The thermal phases presented by the model for different chain lengths, at fixed ionic strength, compare well with an experimental phase diagram constructed on the basis of differential scanning calorimetry profiles. In the case of dispersions of DMPG (dimyristoyl phosphatidylglycerol) with added monovalent salt, the model properties reproduce the main features displayed by data from differential scanning calorimetry as well as the characteristic profile for the degree of ionization of the bilayer surface across the anomalous transition region, obtained from the theoretical interpretation of electrokinetic (conductivity and electrophoretic mobility) measurements.

Biophysical studies of the membrane-embedded and cytoplasmic forms of the glucose-specific Enzyme II of the E. coli phosphotransferase system (PTS)

The glucose Enzyme II transporter complex of the Escherichia coli phosphotransferase system (PTS) exists in at least two physically distinct forms: a membrane-integrated dimeric form, and a cytoplasmic monomeric form, but little is known about the physical states of these enzyme forms. Six approaches were used to evaluate protein-protein and protein-lipid interactions in this system. Fluorescence energy transfer (FRET) using MBP-II(Glc)-YFP and MBP-II(Glc)-CFP revealed that the homodimeric Enzyme II complex in cell membranes is stable (FRET(-)) but can be dissociated and reassociated to the heterodimer only in the presence of Triton X100 (FRET(+)). The monomeric species could form a heterodimeric species (FRET(+)) by incubation and purification without detergent exposure. Formaldehyde cross linking studies, conducted both in vivo and in vitro, revealed that the dimeric MBP-II(Glc) activity decreased dramatically with increasing formaldehyde concentrations due to both aggregation and activity loss, but that the monomeric MBP-II(Glc) retained activity more effectively in response to the same formaldehyde treatments, and little or no aggregation was observed. Electron microscopy of MBP-II(Glc) indicated that the dimeric form is larger than the monomeric form. Dynamic light scattering confirmed this conclusion and provided quantitation. NMR analyses provided strong evidence that the dimeric form is present primarily in a lipid bilayer while the monomeric form is present as micelles. Finally, lipid analyses of the different fractions revealed that the three lipid species (PE, PG and CL) are present in all fractions, but the monomeric micellar structure contains a higher percentage of anionic lipids (PG & CL) while the dimeric bilayer form has a higher percentage of zwitterion lipids (PE). Additionally, evidence for a minor dimeric micellar species, possibly an intermediate between the monomeric micellar and the dimeric bilayer forms, is presented. These results provide convincing evidence for interconvertible physical forms of Enzyme-II(Glc).

Tolerance to changes in membrane lipid composition as a selected trait of membrane proteins

Membrane lipid composition can vary dramatically across the three domains of life and even within single organisms. Here we review evidence that the lipid-exposed surfaces of membrane proteins have generally evolved to maintain correct structure and function in the face of major changes in lipid composition. Such tolerance has allowed evolution to extensively remodel membrane lipid compositions during the emergence of new species without having to extensively remodel the associated membrane proteins. The tolerance of membrane proteins also permits single-cell organisms to vary their membrane lipid composition in response to their changing environments and allows dynamic and organelle-specific variations in the lipid compositions of eukaryotic cells. Membrane protein structural biology has greatly benefited from this seemingly intrinsic property of membrane proteins: the majority of structures determined to date have been characterized under model membrane conditions that little resemble those of native membranes. Nevertheless, with a few notable exceptions, most experimentally determined membrane protein structures appear, to a good approximation, to faithfully report on native structure.

Three phosphatidylglycerol-phosphate phosphatases in the inner membrane of Escherichia coli

The phospholipids of Escherichia coli consist mainly of phosphatidylethanolamine, phosphatidylglycerol (PG), and cardiolipin. PG makes up ∼25% of the cellular phospholipid and is essential for growth in wild-type cells. PG is synthesized on the inner surface of the inner membrane from cytidine diphosphate-diacylglycerol and glycerol 3-phosphate, generating the precursor phosphatidylglycerol-phosphate (PGP). This compound is present at low levels (∼0.1% of the total lipid). Dephosphorylation of PGP to PG is catalyzed by several PGP-phosphatases. The pgpA and pgpB genes, which encode structurally distinct PGP-phosphatases, were identified previously. Double deletion mutants lacking pgpA and pgpB are viable and still make PG, suggesting the presence of additional phosphatase(s). We have identified a third PGP-phosphatase gene (previously annotated as yfhB but renamed pgpC) using an expression cloning strategy. A mutant with deletions in all three phosphatase genes is not viable unless covered by a plasmid expressing either pgpA, pgpB, or pgpC. When the triple mutant is covered with the temperature-sensitive plasmid pMAK705 expressing any one of the three pgp genes, the cells grow at 30 but not 42 °C. As growth slows at 42 °C, PGP accumulates to high levels, and the PG content declines. PgpC orthologs are present in many other bacteria.

Packing behaviour of two predominant anionic phospholipids of bacterial cytoplasmic membranes

Phosphatidylglycerol and cardiolipin represent the most abundant anionic phospholipid components of cytoplasmic bacterial membranes and thus are used as constituents for membrane mimetic systems. In this study, we have characterized the temperature dependent phase behaviour of the binary system dipalmitoyl-phosphatidylglycerol (DPPG) and tetramyristoyl-cardiolipin (TMCL) using microcalorimetry and X-ray scattering techniques. Both lipids exhibited a very similar main transition temperature (∼ 41 °C), showing a minimum (39.4 °C) for the binary mixtures at X_DPPG = 0.8, and exhibited low-temperature phase transitions, which were abolished by incorporation of small amounts (≤ 10 mol%) of the other lipid component. Therefore, over a wide temperature and composition range a lamellar L_β gel phase is the predominant structure below the chain melting transition, characterized by a relatively broad wide-angle peak for X_DPPG ≤ 0.8. This observation suggests the existence of packing inconsistencies of the TMCL/DPPG hydrocarbon lattices in the gel phase, supported by the small average size of lipid clusters (∼ 50 lipids) within this composition range. The bilayer thickness for the lamellar-gel phase showed a monotonic increase (56 Å for TMCL to about 58 Å for X_DPPG = 0.8 at 30 °C), which may be explained by different degrees of partial interdigitation of the acyl chains to compensate for the differences in the hydrocarbon lengths of DPPG and TMCL in the L_β phase.

Involvement of sigmaS accumulation in repression of the flhDC operon in acidic phospholipid-deficient mutants of Escherichia coli

Escherichia coli pgsA mutations, which cause acidic phospholipid deficiency, repress transcription of the flagellar master operon flhDC, and thus impair flagellar formation and motility. The molecular mechanism of the strong repression of flhDC transcription in the mutant cells, however, has not yet been clarified. In order to shed light on this mechanism we isolated genes which, when supplied in multicopy, suppress the repression of flhD, and found that three genes, gadW, metE and yeaB, were capable of suppression. Taking into account a previous report that gadW represses sigma(S) production, the level of sigma(S) in the pgsA3 mutant was examined. We found that pgsA3 cells had a high level of sigma(S) and that introduction of a gadW plasmid into pgsA3 cells did reduce the sigma(S) level. The pgsA3 cells exhibited a sharp increase in sigma(S) levels that can only be partially attributed to the slight increase in rpoS transcription; the largest part of the effect is due to a post-transcriptional accumulation of sigma(S). GadW in multicopy exerts its effect by post-transcriptionally downregulating sigma(S). YeaB and MetE in multicopy also exert their effect via sigma(S). Disruption of rpoS caused an increase of the flhD mRNA level, and induction from P(trc)-rpoS repressed the flhD mRNA level. The strong repression of flhD transcription in pgsA3 mutant cells is thus suggested to be caused by the accumulated sigma(S).

Involvement of PlsX and the acyl-phosphate dependent sn-glycerol-3-phosphate acyltransferase PlsY in the initial stage of glycerolipid synthesis in Bacillus subtilis

The gene responsible for the first acylation of sn-glycerol-3-phosphate (G3P) in Bacillus subtilis has not yet been determined with certainty. The product of this first acylation, lysophosphatidic acid (LPA), is subsequently acylated again to form phosphatidic acid (PA), the primary precursor to membrane glycerolipids. A novel G3P acyltransferase (GPAT), the gene product of plsY, which uses acyl-phosphate formed by the plsX gene product, has recently been found to synthesize LPA in Streptococcus pneumoniae. We found that in B. subtilis growth arrests after repression of either a plsY homologue or a plsX homologue were overcome by expression of E. coli plsB, which encodes an acyl-acylcarrier protein (acyl-ACP)-dependent GPAT, although in the case of plsX repression a high level of plsB expression was required. B. subtilis has, therefore, a capability to use the acyl-ACP dependent GPAT of PlsB. Simultaneous expression of plsY and plsX suppressed the glycerol requirement of a strict glycerol auxotrophic derivative of the E. coli plsB26 mutant, although either one alone did not. Membrane fractions from B. subtilis cells catalyzed palmitoylphosphate-dependent acylation of [14C]-labeled G3P to synthesize [14C]-labeled LPA, whereas those from DeltaplsY cells did not. The results indicate unequivocally that PlsY is an acyl-phosphate dependent GPAT. Expression of plsX corrected the glycerol auxotrophy of a DeltaygiH (the deleted allele of an E. coli homologue of plsY) derivative of BB26-36 (plsB26 plsX50), suggesting an essential role of plsX other than substrate supply for acyl-phosphate dependent LPA synthesis. Two-hybrid examinations suggested that PlsY is associated with PlsX and that each may exist in multimeric form.

Characterization and subcellular localization of a bacterial flotillin homologue

The process of endospore formation in Bacillus subtilis is complex, requiring the generation of two distinct cell types, a forespore and larger mother cell. The development of these cell types is controlled and regulated by cell type-specific gene expression, activated by a sigma-factor cascade. Activation of these cell type-specific sigma factors is coupled with the completion of polar septation. Here, we describe a novel protein, YuaG, a eukaryotic reggie/flotillin homologue that is involved in the early stages of sporulation of the Gram-positive model organism B. subtilis. YuaG localizes in discrete foci in the membrane and is highly dynamic. Purification of detergent-resistant membranes revealed that YuaG is associated with negatively charged phospholipids, e.g. phosphatidylglycerol (PG) or cardiolipin (CL). However, localization of YuaG is not always dependent on PG/CL in vivo. A yuaG disruption strain shows a delay in the onset of sporulation along with reduced sporulation efficiency, where the spores develop to a certain stage and then appear to be trapped at this stage. Our results indicate that YuaG is involved in the early stage of spore development, probably playing a role in the signalling cascade at the onset of sporulation.

Phosphatidic acid and N-acylphosphatidylethanolamine form membrane domains in Escherichia coli mutant lacking cardiolipin and phosphatidylglycerol

The pgsA null Escherichia coli strain, UE54, lacks the major anionic phospholipids phosphatidylglycerol and cardiolipin. Despite these alterations the strain exhibits relatively normal cell division. Analysis of the UE54 phospholipids using negativeion electrospray ionization mass spectrometry resulted in identification of a new anionic phospholipid, N-acylphosphatidylethanolamine. Staining with the fluorescent dye 10-N-nonyl acridine orange revealed anionic phospholipid membrane domains at the septal and polar regions. Making UE54 null in minCDE resulted in budding off of minicells from polar domains. Analysis of lipid composition by mass spectrometry revealed that minicells relative to parent cells were significantly enriched in phosphatidic acid and N-acylphosphatidylethanolamine. Thus despite the absence of cardiolipin, which forms membrane domains at the cell pole and division sites in wild-type cells, the mutant cells still maintain polar/septal localization of anionic phospholipids. These three anionic phospholipids share common physical properties that favor polar/septal domain formation. The findings support the proposed role for anionic phospholipids in organizing amphitropic cell division proteins at specific sites on the membrane surface.

Molecular genetic approaches to defining lipid function

Lipids fulfill multiple and diverse functions in cells. Establishing the molecular basis for these functions has been challenging due to the lack of catalytic activity of lipids and the pleiotropic effects of mutations that affect lipid composition. By combining molecular genetic manipulation of membrane lipid composition with biochemical characterization of the resulting phenotypes, the molecular details of novel lipid functions have been established. This review summarizes the results of such a combined approach to defining lipid function in bacteria.

Hyperexpression of the osmB gene in an acidic phospholipid-deficient Escherichia coli mutant

An Escherichia coli pgsA null mutant deficient in acidic phospholipids shows a thermosensitive cell lysis phenotype because of activation of the Rcs phosphorelay signal transduction system. We conducted a DNA microarray analysis with special attention to the genes affected by growth temperature in the mutant deficient in acidic phospholipids. Among the genes identified as highly expressed at high temperature in the pgsA null mutant, the osmB gene was shown to be dependent on the Rcs system for the high expression by dot blot hybridization. Induction of the cloned osmB in the pgsA null mutant caused the thermosensitive defect even in the absence of the Rcs system. Although the deletion of osmB did not suppress the thermosensitivity in the presence of the Rcs system, indicating a multifactorial nature of the deleterious effect of the Rcs activation, we suggest that the osmB hyperexpression is one of the causes of the Rcs-dependent lysis phenotype of the pgsA null mutant.

The essential role of phosphatidylglycerol in photosynthesis

Since the first identification of phosphatidylglycerol in Scenedesmus by Benson and Maruo in 1958, researchers have studied many biological functions of this phospholipid. Genetic, biochemical, and structural studies of photosynthetic organisms have revealed that phosphatidylglycerol is crucial to the photosynthetic transport of electrons, the development of chloroplasts, and tolerance to chilling. In this review, we summarize our present understanding of the biochemical and physiological functions of phosphatidylglycerol in cyanobacteria and higher plants.