Isolation of differentiated membrane domains from Escherichia coli and Salmonella typhimurium, including a fraction containing attachment sites between the inner and outer membranes and the murein skeleton of the cell envelope

Lipopolysaccharide as an antibiotic target

Gram-negative bacteria, including Escherichia coli, Pseudomonas aeruginosa and Acinetobacter baumannii are amongst the highest priority drug-resistant pathogens, for which new antibiotics are urgently needed. Whilst antibiotic drug development is inherently challenging, this is particularly true for Gram-negative bacteria due to the presence of the outer membrane, a highly selective permeability barrier that prevents the ingress of several classes of antibiotic. This selectivity is largely due to an outer leaflet composed of the glycolipid lipopolysaccharide (LPS), which is essential for the viability of almost all Gram-negative bacteria. This essentiality, coupled with the conservation of the synthetic pathway across species and recent breakthroughs in our understanding of transport and membrane homeostasis has made LPS an attractive target for novel antibiotic drug development. Several different targets have been explored and small molecules developed that show promising activity in vitro. However, these endeavours have met limited success in clinical testing and the polymyxins, discovered more than 70 years ago, remain the only LPS-targeting drugs to enter the clinic thus far. In this review, we will discuss efforts to develop therapeutic inhibitors of LPS synthesis and transport and the reasons for limited success, and explore new developments in understanding polymyxin mode of action and the identification of new analogues with reduced toxicity and enhanced activity.

Plasma Membrane-Cell Wall Feedback in Bacteria

Most bacteria have cell wall peptidoglycan surrounding their plasma membranes. The essential cell wall provides a scaffold for the envelope, protection against turgor pressure and is a proven drug target. Synthesis of the cell wall involves reactions that span cytoplasmic and periplasmic compartments. Bacteria carry out the last steps of cell wall synthesis along their plasma membrane. The plasma membrane in bacteria is heterogeneous and contains membrane compartments. Here, I outline findings that highlight the emerging notion that plasma membrane compartments and the cell wall peptidoglycan are functionally intertwined. I start by providing models of cell wall synthesis compartmentalization within the plasma membrane in mycobacteria, Escherichia coli, and Bacillus subtilis. Then, I revisit literature that supports a role for the plasma membrane and its lipids in modulating enzymatic reactions that synthesize cell wall precursors. I also elaborate on what is known about bacterial lateral organization of the plasma membrane and the mechanisms by which organization is established and maintained. Finally, I discuss the implications of cell wall partitioning in bacteria and highlight how targeting plasma membrane compartmentalization serves as a way to disrupt cell wall synthesis in diverse species.

Of zones, bridges and chaperones - phospholipid transport in bacterial outer membrane assembly and homeostasis

The outer membrane (OM) is a formidable permeability barrier that protects Gram-negative bacteria from detergents and antibiotics. It possesses exquisite lipid asymmetry, requiring the placement and retention of lipopolysaccharides (LPS) in the outer leaflet, and phospholipids (PLs) in the inner leaflet. To establish OM lipid asymmetry, LPS are transported from the inner membrane (IM) directly to the outer leaflet of the OM. In contrast, mechanisms for PL trafficking across the cell envelope are much less understood. In this review, we summarize and discuss recent advances in our understanding of PL transport, making parallel comparisons to well-established pathways for OM lipoprotein (Lol) and LPS (Lpt). Insights into putative PL transport systems highlight possible connections back to the 'Bayer bridges', adhesion zones between the IM and the OM that had been observed more than 50 years ago, and proposed as passages for export of OM components, including LPS and PLs.

The Dynamic SecYEG Translocon

The spatial and temporal coordination of protein transport is an essential cornerstone of the bacterial adaptation to different environmental conditions. By adjusting the protein composition of extra-cytosolic compartments, like the inner and outer membranes or the periplasmic space, protein transport mechanisms help shaping protein homeostasis in response to various metabolic cues. The universally conserved SecYEG translocon acts at the center of bacterial protein transport and mediates the translocation of newly synthesized proteins into and across the cytoplasmic membrane. The ability of the SecYEG translocon to transport an enormous variety of different substrates is in part determined by its ability to interact with multiple targeting factors, chaperones and accessory proteins. These interactions are crucial for the assisted passage of newly synthesized proteins from the cytosol into the different bacterial compartments. In this review, we summarize the current knowledge about SecYEG-mediated protein transport, primarily in the model organism Escherichia coli, and describe the dynamic interaction of the SecYEG translocon with its multiple partner proteins. We furthermore highlight how protein transport is regulated and explore recent developments in using the SecYEG translocon as an antimicrobial target.

Countering Gram-Negative Antibiotic Resistance: Recent Progress in Disrupting the Outer Membrane with Novel Therapeutics

Gram-negative bacteria shield themselves from antibiotics by producing an outer membrane (OM) that forms a formidable permeability barrier. Multidrug resistance among these organisms is a particularly acute problem that is exacerbated by the OM. The poor penetrance of many available antibiotics prevents their clinical use, and efforts to discover novel classes of antibiotics against Gram-negative bacteria have been unsuccessful for almost 50 years. Recent insights into how the OM is built offer new hope. Several essential multiprotein molecular machines (Bam, Lpt, and Lol) work in concert to assemble the barrier and offer a swathe of new targets for novel therapeutic development. Murepavadin has been at the vanguard of these efforts, but its recently reported phase III clinical trial toxicity has tempered the anticipation of imminent new clinical options. Nonetheless, the many concerted efforts aimed at breaking down the OM barrier provide a source of ongoing optimism for what may soon come through the development pipeline. We will review the current state of drug development against the OM assembly targets, highlighting insightful new discovery approaches and strategies.

The Way It Was

Mary Osborn was a native Californian. She was an undergraduate at the University of California, Berkeley, where she worked in the laboratory of I.L. Chaikoff. She received her PhD at the University of Washington, where her work on the role of folic acid coenzymes in one-carbon metabolism revealed the mechanism of action of methotrexate. After postdoctoral training with Bernard Horecker in the Department of Microbiology at New York University (NYU), she embarked on her research career as a faculty member in the NYU Department of Microbiology and in the Department of Molecular Biology at Albert Einstein College of Medicine. In 1968 she moved as one of the founding faculty of the new medical school of the University of Connecticut, where she remained until her retirement in 2014. Her research was focused on the biosynthesis of the endotoxin lipopolysaccharide (LPS) of gram-negative bacteria and on the assembly of the bacterial cell envelope. She made seminal contributions in these areas. She was the recipient of numerous honors and served as president of several important scientific organizations. Later in her career she served as chair of the National Research Council Committee on Space Biology and Medicine, advisory to the National Aeronautics and Space Administration (NASA), which produced an influential report that plotted the path for NASA's space biology research program in the first decade of the twenty-first century. Dr. Osborn died on Jan. 17, 2019.

Enhanced Methodologies for Detecting Phenotypic Resistance in Mycobacteria

Lipid droplets found in algae and other microscopic organisms have become of interest to many researchers partially because they carry the capacity to produce bio-oil for the mass market. They are of importance in biology and clinical practice because their presence can be a phenotypic marker of an altered metabolism, including reversible resistance to antibiotics, prompting intense research.A useful stain for detecting lipid bodies in the lab is Nile red. It is a dye that exhibits solvatochromism; its absorption band varies in spectral position, shape and intensity with the nature of its solvent environment, it will fluoresce intensely red in polar environment and blue shift with the changing polarity of its solvent. This makes it ideal for the detection of lipid bodies within Mycobacterium spp. This is because mycobacterial lipid bodies' primary constituents are nonpolar lipids such as triacylglycerols but bacterial cell membranes are primarily polar lipid species. In this chapter we describe an optimal method for using Nile red to distinguish lipid containing (Lipid rich or LR cells) from those without lipid bodies (Lipid Poor or LP). As part of the process we have optimized a method for separating LP and LR cells that does not require the use of an ultracentrifuge or complex separation media. We believe that these methods will facilitate further research in these enigmatic, transient and important cell states.

Lipopolysaccharide transport and assembly at the outer membrane: the PEZ model

Gram-negative bacteria have a double-membrane cellular envelope that enables them to colonize harsh environments and prevents the entry of many clinically available antibiotics. A main component of most outer membranes is lipopolysaccharide (LPS), a glycolipid containing several fatty acyl chains and up to hundreds of sugars that is synthesized in the cytoplasm. In the past two decades, the proteins that are responsible for transporting LPS across the cellular envelope and assembling it at the cell surface in Escherichia coli have been identified, but it remains unclear how they function. In this Review, we discuss recent advances in this area and present a model that explains how energy from the cytoplasm is used to power LPS transport across the cellular envelope to the cell surface.

Bacterial membranes are the target for antimicrobial polysiloxane-methacrylate copolymer

Antibacterial polysiloxane polymers with pending tert-butylamine groups are a novel class of compounds that are compatible with silicone elastomers, but their mechanism of action is not well understood. The research into their action mechanism was conducted on a polysiloxane copolymer grafted with tert-butylaminoethyl methacrylate and covalently attached fluorescein. Fluorometric measurements results suggest that the polymer forms a stable link with bacteria. The results of β-galactosidase enzyme assay with the use of ortho-nitrophenyl-β-galactoside as a substrate show that the polymer has a damaging effect on bacterial membranes. The scanning and transmission electron micrographs of Escherichia coli cells incubated with the polymer prove further that the polymer's site of action is bacterial cell membranes. In order to investigate the polymer interaction with bacterial membranes the fluorescein labelled polymer was incubated with bacterial cells and membranes isolation and identification method was next applied. The E. coli membrane fractions were identified by light scattering, protein content, oxidase NADH activity and N-phenylnaphtylamine fluorescence measurements, as well as electron microscopy. Oxidase NADH and N-phenylnaphtylamine were the inner membrane markers. The bacterial membranes were then tested for the presence of the polymer. The experiments gave evidence that the copolymer binds to the inner bacterial membrane. Further studies, where the copolymer was incubated with isolated mixed (inner and outer) membrane fractions, proved that the copolymer exerts more destructive effect on E. coli outer membrane. The damaging effect on the membranes is concentration dependent.

Assembly of Outer Membrane β-Barrel Proteins: the Bam Complex

The major class of integral proteins found in the outer membrane (OM) of E. coli and Salmonella adopt a β-barrel conformation (OMPs). OMPs are synthesized in the cytoplasm with a typical signal sequence at the amino terminus, which directs them to the secretion machinery (SecYEG) located in the inner membrane for translocation to the periplasm. Chaperones such as SurA, or DegP and Skp, escort these proteins across the aqueous periplasm protecting them from aggregation. The chaperones then deliver OMPs to a highly conserved outer membrane assembly site termed the Bam complex. In E. coli, the Bam complex is composed of an essential OMP, BamA, and four associated OM lipoproteins, BamBCDE, one of which, BamD, is also essential. Here we provide an overview of what we know about the process of OMP assembly and outline the various hypotheses that have been proposed to explain how proteins might be integrated into the asymmetric OM lipid bilayer in an environment that lacks obvious energy sources. In addition, we describe the envelope stress responses that ensure the fidelity of OM biogenesis and how factors, such as phage and certain toxins, have coopted this essential machine to gain entry into the cell.

The lipopolysaccharide export pathway in Escherichia coli: structure, organization and regulated assembly of the Lpt machinery

The bacterial outer membrane (OM) is a peculiar biological structure with a unique composition that contributes significantly to the fitness of Gram-negative bacteria in hostile environments. OM components are all synthesized in the cytosol and must, then, be transported efficiently across three compartments to the cell surface. Lipopolysaccharide (LPS) is a unique glycolipid that paves the outer leaflet of the OM. Transport of this complex molecule poses several problems to the cells due to its amphipatic nature. In this review, the multiprotein machinery devoted to LPS transport to the OM is discussed together with the challenges associated with this process and the solutions that cells have evolved to address the problem of LPS biogenesis.

Membrane association via an amino-terminal amphipathic helix is required for the cellular organization and function of RNase II

The subcellular localization of the exoribonuclease RNase II is not known despite the advanced biochemical characterization of the enzyme. Here we report that RNase II is organized into cellular structures that appear to coil around the Escherichia coli cell periphery and that RNase II is associated with the cytoplasmic membrane by its amino-terminal amphipathic helix. The helix also acts as an autonomous transplantable membrane binding domain capable of directing normally cytoplasmic proteins to the membrane. Assembly of the organized cellular structures of RNase II required the RNase II amphipathic membrane binding domain. Co-immunoprecipitation of the protein from cell extracts indicated that RNase II interacts with itself. The RNase II self-interaction and the ability of the protein to assemble into organized cellular structures required the membrane binding domain. The ability of RNase II to maintain cell viability in the absence of the exoribonuclease polynucleotide phosphorylase was markedly diminished when the RNase II cellular structures were lost due to changes in the amphipathicity of the amino-terminal helix, suggesting that membrane association and assembly of RNase II into organized cellular structures play an important role in the normal function of the protein within the bacterial cell.

New role for the ibeA gene in H2O2 stress resistance of Escherichia coli

ibeA is a virulence factor found in some extraintestinal pathogenic Escherichia coli (ExPEC) strains from the B2 phylogenetic group and particularly in newborn meningitic and avian pathogenic strains. It was shown to be involved in the invasion process of the newborn meningitic strain RS218. In a previous work, we showed that in the avian pathogenic E. coli (APEC) strain BEN2908, isolated from a colibacillosis case, ibeA was rather involved in adhesion to eukaryotic cells by modulating type 1 fimbria synthesis (M. A. Cortes et al., Infect. Immun. 76:4129-4136, 2008). In this study, we demonstrate a new role for ibeA in oxidative stress resistance. We showed that an ibeA mutant of E. coli BEN2908 was more sensitive than its wild-type counterpart to H(2)O(2) killing. This phenotype was also observed in a mutant deleted for the whole GimA genomic region carrying ibeA and might be linked to alterations in the expression of a subset of genes involved in the oxidative stress response. We also showed that RpoS expression was not altered by the ibeA deletion. Moreover, the transfer of an ibeA-expressing plasmid into an E. coli K-12 strain, expressing or not expressing type 1 fimbriae, rendered it more resistant to an H(2)O(2) challenge. Altogether, these results show that ibeA by itself is able to confer increased H(2)O(2) resistance to E. coli. This feature could partly explain the role played by ibeA in the virulence of pathogenic strains.

YghG (GspSβ) is a novel pilot protein required for localization of the GspSβ type II secretion system secretin of enterotoxigenic Escherichia coli

The enterotoxigenic Escherichia coli (ETEC) pathotype, characterized by the prototypical strain H10407, is a leading cause of morbidity and mortality in the developing world. A major virulence factor of ETEC is the type II secretion system (T2SS) responsible for secretion of the diarrheagenic heat-labile enterotoxin (LT). In this study, we have characterized the two type II secretion systems, designated alpha (T2SS(α)) and beta (T2SS(β)), encoded in the H10407 genome and describe the prevalence of both systems in other E. coli pathotypes. Under laboratory conditions, the T2SS(β) is assembled and functional in the secretion of LT into culture supernatant, whereas the T2SS(α) is not. Insertional inactivation of the three genes located upstream of gspC(β) (yghJ, pppA, and yghG) in the atypical T2SS(β) operon revealed that YghJ is not required for assembly of the GspD(β) secretin or secretion of LT, that PppA is likely the prepilin peptidase required for the function of T2SS(β), and that YghG is required for assembly of the GspD(β) secretin and thus function of the T2SS(β). Mutational and physiological analysis further demonstrated that YghG (redesignated GspS(β)) is a novel outer membrane pilotin protein that is integral for assembly of the T2SS(β) by localizing GspD(β) to the outer membrane, whereupon GspD(β) forms the macromolecular secretin multimer through which T2SS(β) substrates are translocated.

Translation levels control multi-spanning membrane protein expression

Attempts to express eukaryotic multi-spanning membrane proteins at high-levels have been generally unsuccessful. In order to investigate the cause of this limitation and gain insight into the rate limiting processes involved, we have analyzed the effect of translation levels on the expression of several human membrane proteins in Escherichia coli (E. coli). These results demonstrate that excessive translation initiation rates of membrane proteins cause a block in protein synthesis and ultimately prevent the high-level accumulation of these proteins. Moderate translation rates allow coupling of peptide synthesis and membrane targeting, resulting in a significant increase in protein expression and accumulation over time. The current study evaluates four membrane proteins, CD20 (4-transmembrane (TM) helixes), the G-protein coupled receptors (GPCRs, 7-TMs) RA1c and EG-VEGFR1, and Patched 1 (12-TMs), and demonstrates the critical role of translation initiation rates in the targeting, insertion and folding of integral membrane proteins in the E. coli membrane.

The host outer membrane proteins OmpA and OmpC are associated with the Shigella phage Sf6 virion

Assembly of dsDNA bacteriophage is a precisely programmed process. Potential roles of host cell components in phage assembly haven't been well understood. It was previously reported that two unidentified proteins were present in bacteriophage Sf6 virion (Casjens et al, 2004, J.Mol.Biol. 339, 379-394, Fig. 2A). Using tandem mass spectrometry, we have identified the two proteins as outer membrane proteins (OMPs) OmpA and OmpC from its host Shigella flexneri. The transmission electron cryo-microscopy structure of Sf6 shows significant density at specific sites at the phage capsid inner surface. This density fit well with the characteristic beta-barrel domains of OMPs, thus may be due to the two host proteins. Locations of this density suggest a role in Sf6 morphogenesis reminiscent of phage-encoded cementing proteins. These data indicate a new, OMP-related phage:host linkage, adding to previous knowledge that some lambdoid bacteriophage genomes contain OmpC-like genes that express phage-encoded porins in the lysogenic state.

Proteins required for lipopolysaccharide assembly in Escherichia coli form a transenvelope complex

The viability of Gram-negative organisms is dependent on the proper placement of lipopolysaccharide (LPS) in the outer leaflet of its outer membrane. LPS is synthesized inside the cell and transported to the surface by seven essential lipopolysaccharide transport (Lpt) proteins. How these proteins cooperate to transport LPS is unknown. We show that these Lpt proteins can be found in a membrane fraction that contains inner and outer membranes and that they copurify. This constitutes the first evidence that the Lpt proteins form a transenvelope complex. We suggest that this protein bridge provides a route for LPS transport across the cell envelope.

Purification of bacteriophages and SDS-PAGE analysis of phage structural proteins from ghost particles

Concentration and purification of infectious particles are prerequisites for structural and functional characterization of bacteriophages. The methods detailed in the first part of this chapter outline the protocols commonly used to obtain purified phages: the concentration of phage particles by precipitation with polyethylene glycol and their purification by centrifugation in CsCl step gradients and subsequently by equilibrium centrifugation. This sequence of procedures, if carried out as a whole, ensures a purification of high quality, which is well suited for most analytical techniques used to characterize bacteriophage particles. The second part of this chapter describes the preparation of "ghosts" or DNA-less bacteriophages. These particles should be preferred to the entire bacteriophages for one-dimensional SDS-PAGE analysis of phage structural proteins, since running of the phage proteins through the gel is not disturbed by the presence of the phage DNA. This allows an optimal resolution, which is necessary for proteomic approaches such as N-terminal protein sequencing or mass spectrometry using proteins isolated from distinct gel bands.

Functional analysis of the protein machinery required for transport of lipopolysaccharide to the outer membrane of Escherichia coli

Lipopolysaccharide (LPS) is an essential component of the outer membrane (OM) in most gram-negative bacteria, and its structure and biosynthetic pathway are well known. Nevertheless, the mechanisms of transport and assembly of this molecule at the cell surface are poorly understood. The inner membrane (IM) transport protein MsbA is responsible for flipping LPS across the IM. Additional components of the LPS transport machinery downstream of MsbA have been identified, including the OM protein complex LptD/LptE (formerly Imp/RlpB), the periplasmic LptA protein, the IM-associated cytoplasmic ATP binding cassette protein LptB, and LptC (formerly YrbK), an essential IM component of the LPS transport machinery characterized in this work. Here we show that depletion of any of the proteins mentioned above leads to common phenotypes, including (i) the presence of abnormal membrane structures in the periplasm, (ii) accumulation of de novo-synthesized LPS in two membrane fractions with lower density than the OM, and (iii) accumulation of a modified LPS, which is ligated to repeating units of colanic acid in the outer leaflet of the IM. Our results suggest that LptA, LptB, LptC, LptD, and LptE operate in the LPS assembly pathway and, together with other as-yet-unidentified components, could be part of a complex devoted to the transport of LPS from the periplasmic surface of the IM to the OM. Moreover, the location of at least one of these five proteins in every cellular compartment suggests a model for how the LPS assembly pathway is organized and ordered in space.

Insight from TonB hybrid proteins into the mechanism of iron transport through the outer membrane

We created hybrid proteins to study the functions of TonB. We first fused the portion of Escherichia coli tonB that encodes the C-terminal 69 amino acids (amino acids 170 to 239) of TonB downstream from E. coli malE (MalE-TonB69C). Production of MalE-TonB69C in tonB(+) bacteria inhibited siderophore transport. After overexpression and purification of the fusion protein on an amylose column, we proteolytically released the TonB C terminus and characterized it. Fluorescence spectra positioned its sole tryptophan (W213) in a weakly polar site in the protein interior, shielded from quenchers. Affinity chromatography showed the binding of the TonB C-domain to other proteins: immobilized TonB-dependent (FepA and colicin B) and TonB-independent (FepADelta3-17, OmpA, and lysozyme) proteins adsorbed MalE-TonB69C, revealing a general affinity of the C terminus for other proteins. Additional constructions fused full-length TonB upstream or downstream of green fluorescent protein (GFP). TonB-GFP constructs had partial functionality but no fluorescence; GFP-TonB fusion proteins were functional and fluorescent. The activity of the latter constructs, which localized GFP in the cytoplasm and TonB in the cell envelope, indicate that the TonB N terminus remains in the inner membrane during its biological function. Finally, sequence analyses revealed homology in the TonB C terminus to E. coli YcfS, a proline-rich protein that contains the lysin (LysM) peptidoglycan-binding motif. LysM structural mimicry occurs in two positions of the dimeric TonB C-domain, and experiments confirmed that it physically binds to the murein sacculus. Together, these findings infer that the TonB N terminus remains associated with the inner membrane, while the downstream region bridges the cell envelope from the affinity of the C terminus for peptidoglycan. This architecture suggests a membrane surveillance model of action, in which TonB finds occupied receptor proteins by surveying the underside of peptidoglycan-associated outer membrane proteins.

Biogenesis of the gram-negative bacterial outer membrane

The cell envelope of gram-negative bacteria consists of two membranes, the inner and the outer membrane, that are separated by the periplasm. The outer membrane consists of phospholipids, lipopolysaccharides, integral membrane proteins, and lipoproteins. These components are synthesized in the cytoplasm or at the inner leaflet of the inner membrane and have to be transported across the inner membrane and through the periplasm to assemble eventually in the correct membrane. Recent studies in Neisseria meningitidis and Escherichia coli have led to the identification of several machineries implicated in these transport and assembly processes.

Colicin biology

Colicins are proteins produced by and toxic for some strains of Escherichia coli. They are produced by strains of E. coli carrying a colicinogenic plasmid that bears the genetic determinants for colicin synthesis, immunity, and release. Insights gained into each fundamental aspect of their biology are presented: their synthesis, which is under SOS regulation; their release into the extracellular medium, which involves the colicin lysis protein; and their uptake mechanisms and modes of action. Colicins are organized into three domains, each one involved in a different step of the process of killing sensitive bacteria. The structures of some colicins are known at the atomic level and are discussed. Colicins exert their lethal action by first binding to specific receptors, which are outer membrane proteins used for the entry of specific nutrients. They are then translocated through the outer membrane and transit through the periplasm by either the Tol or the TonB system. The components of each system are known, and their implication in the functioning of the system is described. Colicins then reach their lethal target and act either by forming a voltage-dependent channel into the inner membrane or by using their endonuclease activity on DNA, rRNA, or tRNA. The mechanisms of inhibition by specific and cognate immunity proteins are presented. Finally, the use of colicins as laboratory or biotechnological tools and their mode of evolution are discussed.

Characterization of lptA and lptB, two essential genes implicated in lipopolysaccharide transport to the outer membrane of Escherichia coli

The outer membrane (OM) of gram-negative bacteria is an asymmetric lipid bilayer that protects the cell from toxic molecules. Lipopolysaccharide (LPS) is an essential component of the OM in most gram-negative bacteria, and its structure and biosynthesis are well known. Nevertheless, the mechanisms of transport and assembly of this molecule in the OM are poorly understood. To date, the only proteins implicated in LPS transport are MsbA, responsible for LPS flipping across the inner membrane, and the Imp/RlpB complex, involved in LPS targeting to the OM. Here, we present evidence that two Escherichia coli essential genes, yhbN and yhbG, now renamed lptA and lptB, respectively, participate in LPS biogenesis. We show that mutants depleted of LptA and/or LptB not only produce an anomalous LPS form, but also are defective in LPS transport to the OM and accumulate de novo-synthesized LPS in a novel membrane fraction of intermediate density between the inner membrane (IM) and the OM. In addition, we show that LptA is located in the periplasm and that expression of the lptA-lptB operon is controlled by the extracytoplasmic sigma factor RpoE. Based on these data, we propose that LptA and LptB are implicated in the transport of LPS from the IM to the OM of E. coli.

YfiO stabilizes the YaeT complex and is essential for outer membrane protein assembly inEscherichia coli

Recent advances in the study of bacterial membranes have led to the identification of a multicomponent YaeT complex in the outer membrane (OM) of Gram-negative bacteria that is involved in the targeting and folding of beta-barrel outer membrane proteins (OMPs). In Escherichia coli, this complex consists of an essential OMP, YaeT, and three OM lipoproteins, YfgL, NlpB and YfiO. YfiO is the only essential lipoprotein component of the complex. We show that this lipoprotein is required for the proper assembly and/or targeting of OMPs to the OM but not the assembly of lipopolysaccharides (LPS). Depletion of YfiO causes similar phenotypes as does the depletion of YaeT, and we conclude that YfiO plays a critical role in YaeT-mediated OMP folding. We demonstrate that YfiO and YfgL directly interact with YaeT in vitro, while NlpB interacts directly with YfiO. Genetic analysis verifies the importance of YfiO and its interactions with NlpB in maintaining the functional integrity of the YaeT complex.

Isolation of Escherichia coli inner membranes by metal affinity two-phase partitioning

As reduction of sample complexity is a central issue in membrane proteomic research, the need for new pre-fractionation methods is significant. Here we present a method for fast and efficient enrichment of Escherichia coli inner membranes expressing a His-tagged integral membrane L-fucose-proton symporter (FucP). An enriched inner membrane fraction was obtained from a crude membrane mixture using affinity two-phase partitioning in combination with nickel-nitrilotriacetic acid (Ni-NTA) immobilized on agarose beads. Due to interaction between the beads and FucP, inner membranes were selectively partitioned to the bottom phase of a polymer/polymer aqueous two-phase system consisting of poly(ethylene glycol) (PEG) and dextran. The partitioning of membranes was monitored by assaying the activity of an inner membrane marker protein and measuring the total protein content in both phases. The enrichment of inner membrane proteins in the dextran phase was also investigated by proteomic methodology, including sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), trypsin digestion and liquid chromatography in combination with tandem mass spectrometry (LC-MS/MS). Using a high level of significance (99.95%) in the subsequent database search, 36 proteins assigned to the inner membrane were identified in the bottom phase, compared to 29 when using the standard sucrose gradient centrifugation method for inner membrane isolation. Furthermore, metal affinity two-phase partitioning was up to 10 times faster than sucrose gradient centrifugation. The separation conditions in these model experiments provide a basis for the selective isolation of E. coli membranes expressing His-tagged proteins and can therefore facilitate research on such membrane proteomes.

Advances in understanding bacterial outer-membrane biogenesis

The outer membrane of gram-negative bacteria such as Escherichia coli serves as a protective barrier that controls the influx and efflux of solutes. This allows the bacteria to inhabit several different, and often hostile, environments. The assembly of the E. coli outer membrane has been difficult to study using traditional genetic and biochemical methods, and how all its components reach the outer membrane after being synthesized in the cytoplasm and cytoplasmic membrane, how they are assembled in an environment that is devoid of an obvious energy source, and how assembly proceeds without disrupting the integrity of this essential cellular structure are all fundamental questions that remain unanswered. Here, we review the new approaches that have led to the recent discovery of components of the machinery involved in the biogenesis of this distinctive cellular organelle.

Cell division defects in Escherichia coli deficient in the multidrug efflux transporter AcrEF-TolC

The Escherichia coli chromosome contains several operons encoding confirmed and predicted multidrug transporters. Among these transporters only the inactivation of components of the AcrAB-TolC complex leads to substantial changes in susceptibility to multiple drugs. This observation prompted a conclusion that other transporters are silent or expressed at levels insufficient to contribute to multidrug resistance phenotype. We found that increased expression of AcrA, the periplasmic membrane fusion protein, is toxic only in cells lacking the multidrug efflux transporter AcrEF. AcrEF-deficient cells with increased expression of AcrA have a severe cell division defect that results in cell filamentation (>50 microm). Similar defects were obtained in cells lacking the outer membrane channel TolC, which acts with AcrEF, suggesting that cell filamentation is caused by the loss of AcrEF function. Green fluorescent protein-AcrA fusion studies showed that in normal and filamentous cells AcrA is associated with membranes in a confined manner and that this localization is not affected by the lack of AcrEF. Similarly, the structure and composition of membranes were normal in filamentous cells. Fluorescence microscopy showed that the filamentous AcrEF-deficient E. coli cells are defective in chromosome condensation and segregation. Our results suggest that the E. coli AcrEF transporter is expressed under standard laboratory conditions and plays an important role in the normal maintenance of cell division.

Towards a comprehensive view of the bacterial cell wall

Direct in vivo visualization, in full atomic detail, of the microbial cell wall and its stress-bearing structural architecture remains one of the prime challenges in microbiology. In the meantime, molecular modeling can provide a framework for explaining and predicting mechanisms involved in morphogenesis, bacterial cell growth and cell division, during which the wall and its major structural component--murein--have to protect the cell from osmotic pressure and multiple tensile forces. Here, we illustrate why the scaffold concept of murein architecture provides a more comprehensive representation of bacterial cell wall physiology than previous models.

Isolation and Characterization of the B-Cell Marker CD20

The integral membrane protein CD20 has been identified as an important therapeutic target in the treatment of non-Hodgkin's lymphoma (NHL). CD20 binding of many antibodies including the therapeutic antibody, rituximab, has been shown to be critically dependent upon the conformation of a loop structure between the third and fourth helical transmembrane regions. In this work, human and murine CD20 proteins expressed in Escherichia coli are shown to be localized with the cell membrane and are purified in nondenaturing detergent solutions. The purified human and murine CD20 proteins have a substantial helical structure as measured by circular dichroism spectroscopy. Only small changes in the secondary structure are observed following the reduction of CD20, with the addition of SDS, or after heating. The rituximab antibody is shown to bind to purified human CD20 with nanomolar affinity. Rituximab binding is abolished by reduction and alkylation of CD20, with data consistent with the proposed antibody epitope being within the disulfide-bonded loop formed between cysteine residues 167 and 183. Disulfide-bond-dependent antibody binding is partially recovered following reoxidation of reduced CD20. Antibody binding is unaffected by mutations of cysteines proposed to be in the intracellular domain of CD20. The affinities of intact rituximab and its Fab fragment to the isolated and purified CD20 are similar to the observed affinity of rituximab Fab for CD20 on the surface of B cells. However, the intact rituximab antibody shows much higher affinity for CD20 on B cells. This suggests that B cells display CD20 in such a way that allows for marked avidity effects to be observed, perhaps through cross-linking of CD20 monomers into lipid rafts, which limits receptor diffusion in the membrane. Such cross-linking may play a role in partitioning CD20 into lipid rafts and in enhancing antibody-dependent B-cell depletion activities of rituximab and other therapeutic anti-CD20 antibodies.

AcrA, AcrB, and TolC of Escherichia coli Form a Stable Intermembrane Multidrug Efflux Complex

Many transporters of Gram-negative bacteria involved in the extracellular secretion of proteins and the efflux of toxic molecules operate by forming intermembrane complexes. These complexes are proposed to span both inner and outer membranes and create a bridge across the periplasm. In this study, we analyzed interactions between the inner and outer membrane components of the tri-partite multidrug efflux pump AcrAB-TolC from Escherichia coli. We found that, once assembled, the intermembrane AcrAB-TolC complex is stable during the separation of the inner and outer membranes and subsequent purification. All three components of the complex co-purify when the affinity tag is attached to either of the proteins suggesting bi-partite interactions between AcrA, AcrB, and TolC. We show that antibiotics, the substrates of AcrAB-TolC, stabilize interactions within the complex. However, the formation of the AcrAB-TolC complex does not require an input of energy.

Modulation of the bilayer thickness of exocytic pathway membranes by membrane proteins rather than cholesterol

A biological membrane is conceptualized as a system in which membrane proteins are naturally matched to the equilibrium thickness of the lipid bilayer. Cholesterol, in addition to lipid composition, has been suggested to be a major regulator of bilayer thickness in vivo because measurements in vitro have shown that cholesterol can increase the thickness of simple phospholipid/cholesterol bilayers. Using solution x-ray scattering, we have directly measured the average bilayer thickness of exocytic pathway membranes, which contain increasing amounts of cholesterol. The bilayer thickness of membranes of the endoplasmic reticulum, the Golgi, and the basolateral and apical plasma membranes, purified from rat hepatocytes, were determined to be 37.5 +/- 0.4 A, 39.5 +/- 0.4 A, 35.6 +/- 0.6 A, and 42.5 +/- 0.3 A, respectively. After cholesterol depletion using cyclodextrins, Golgi and apical plasma membranes retained their respective bilayer thicknesses whereas the bilayer thickness of the endoplasmic reticulum and the basolateral plasma membrane decreased by 1.0 A. Because cholesterol was shown to have a marginal effect on the thickness of these membranes, we measured whether membrane proteins could modulate thickness. Protein-depleted membranes demonstrated changes in thickness of up to 5 A, suggesting that (i) membrane proteins rather than cholesterol modulate the average bilayer thickness of eukaryotic cell membranes, and (ii) proteins and lipids are not naturally hydrophobically matched in some biological membranes. A marked effect of membrane proteins on the thickness of Escherichia coli cytoplasmic membranes, which do not contain cholesterol, was also observed, emphasizing the generality of our findings.

Mapping the MinE site involved in interaction with the MinD division site selection protein of Escherichia coli

Interactions between the MinD and MinE proteins are required for proper placement of the Escherichia coli division septum. The site within MinE that is required for interaction with MinD was mapped by studying the effects of site-directed minE mutations on MinD-MinE interactions in yeast two-hybrid and three-hybrid experiments. This confirmed that the MinE N-terminal domain is responsible for the interaction of MinE with MinD. Mutations that interfered with the interaction defined an extended surface on one face of the alpha-helical region of the MinE N-terminal domain, consistent with the idea that the MinE-MinD interaction involves formation of a coiled-coil structure by interaction with a complementary helical surface within MinD.

Salmonella type III secretion-associated protein InvE controls translocation of effector proteins into host cells

Salmonella enterica encodes a type III secretion system (TTSS) within a pathogenicity island located at centisome 63 (SPI-1), which is essential for its pathogenicity. This system mediates the transfer of a battery of bacterial proteins into the host cell with the capacity to modulate cellular functions. The transfer process is dependent on the function of protein translocases SipB, SipC, and SipD. We report here that Salmonella protein InvE, which is also encoded within SPI-1, is essential for the translocation of bacterial proteins into host cells. An S. enterica serovar Typhimurium mutant carrying a loss-of-function mutation in invE shows reduced secretion of SipB, SipC, and SipD while exhibiting increased secretion of other TTSS effector proteins. We also demonstrate that InvE interacts with a protein complex formed by SipB, SipC, and their cognate chaperone, SicA. We propose that InvE controls protein translocation by regulating the function of the Sip protein translocases.

Imp/OstA is required for cell envelope biogenesis in Escherichia coli

In Gram-negative bacteria, all components of the outer membrane are synthesized in the cytoplasm or the cytoplasmic leaflet of the inner membrane and must thus traverse the inner membrane and the periplasm on the way to their final destination. In this study, we show Imp/OstA to have characteristics typical for proteins involved in envelope biogenesis. Imp is essential and forms a high-molecular-weight disulphide-bonded complex in the outer membrane. Upon depletion of Imp, lipids and outer membrane proteins appear in a novel membrane fraction with higher density than the outer membrane. We propose Imp to be part of a targeting/usher system for components of the outer membrane.

Expression of the ExeAB complex of Aeromonas hydrophila is required for the localization and assembly of the ExeD secretion port multimer

Aeromonas hydrophila secretes protein toxins via the type II pathway, involving the products of at least two operons, exeAB (gspAB) and exeC-N (gspC-N). In the studies reported here, aerolysin secretion was restored to C5.84, an exeA::Tn5-751 mutant, by overexpression of exeD alone in trans. Expression studies indicated that these results did not reflect a role of ExeAB in the regulation of the exeC-N operon. Instead, immunoblot analysis showed that ExeD did not multimerize in C5.84, and fractionation of the membranes showed that the monomeric ExeD remained in the inner membrane. Expression of ExeAB, but not either protein alone, from a plasmid in C5.84 resulted in increases in the amount of multimeric ExeD, which correlated with increases in aerolysin secretion. Pulse-chase analysis also suggested that the induction of ExeAB allowed multimerization of previously accumulated monomer ExeD. In C5.84 cells overproducing ExeD, it multimerized even in the absence of ExeAB and, although most remained in the inner membrane, an amount similar to that in wild-type outer membranes fractionated with the outer membrane of the overproducing cells. These results indicate that the secretion defect of exeAB mutants is a result of an inability to assemble the ExeD secretin in the outer membrane. The localization and multimerization of overproduced ExeD in these mutants further suggests that the ExeAB complex plays either a direct or indirect role in the transport of ExeD into the outer membrane.

Analysis of a putative voltage-gated prokaryotic potassium channel

Most of the completely sequenced prokaryotic genomes contain genes of potassium channel homologues, but there is still not much known about the role of these proteins in prokaryotes. Here we describe the large-scale overproduction and purification of a prokaryotic voltage-gated potassium channel homologue, Kch, from Escherichia coli. After successful overproduction of the protein, a specific increase in the potassium permeability of the cells was found. Kch could be purified in large amounts using classical purification methods to prevent aggregation of the protein. The physiological state of the protein was revealed to be a homotetramer and the protein was shown to be localized to the cytoplasmic membrane of the cells. In the course of the localization studies, we found a specific increase in the density of the cytoplasmic membrane on Kch production. This was linked to the observed increase in the protein to lipid ratio in the membranes. Another observed change in the membrane composition was an increase in the cardiolipin to phosphatidylglycerol ratio, which may indicate a specific cardiolipin requirement of Kch. On the basis of some of our results, we discuss a function for Kch in the maintenance of the membrane potential in E. coli.

The Tol-Pal proteins of the Escherichia coli cell envelope: an energized system required for outer membrane integrity?

The outer membrane of gram-negative bacteria acts as a barrier against harmful lipophilic compounds and larger molecules unable to diffuse freely through the porins. However, outer membrane proteins together with the Tol-Pal and TonB systems have been exploited for the entry of macromolecules such as bacteriocins and phage DNA through the Escherichia coli cell envelope. The TonB system is involved in the active transport of iron siderophores and vitamin B12, while no more precise physiological role of the Tol-Pal system has yet been defined than its requirement for cell envelope integrity. These two systems, containing an energized inner membrane protein interacting with outer membrane proteins, share similarities.

Formation of the glycan chains in the synthesis of bacterial peptidoglycan

The main structural features of bacterial peptidoglycan are linear glycan chains interlinked by short peptides. The glycan chains are composed of alternating units of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc), all linkages between sugars being beta,1-->4. On the outside of the cytoplasmic membrane, two types of activities are involved in the polymerization of the peptidoglycan monomer unit: glycosyltransferases that catalyze the formation of the linear glycan chains and transpeptidases that catalyze the formation of the peptide cross-bridges. Contrary to the transpeptidation step, for which there is an abundant literature that has been regularly reviewed, the transglycosylation step has been studied to a far lesser extent. The aim of the present review is to summarize and evaluate the molecular and cellullar data concerning the formation of the glycan chains in the synthesis of peptidoglycan. Early work concerned the use of various in vivo and in vitro systems for the study of the polymerization steps, the attachment of newly made material to preexisting peptidoglycan, and the mechanism of action of antibiotics. The synthesis of the glycan chains is catalyzed by the N-terminal glycosyltransferase module of class A high-molecular-mass penicillin-binding proteins and by nonpenicillin-binding monofunctional glycosyltransferases. The multiplicity of these activities in a given organism presumably reflects a variety of in vivo functions. The topological localization of the incorporation of nascent peptidoglycan into the cell wall has revealed that bacteria have at least two peptidoglycan-synthesizing systems: one for septation, the other one for elongation or cell wall thickening. Owing to its location on the outside of the cytoplasmic membrane and its specificity, the transglycosylation step is an interesting target for antibacterials. Glycopeptides and moenomycins are the best studied antibiotics known to interfere with this step. Their mode of action and structure-activity relationships have been extensively studied. Attempts to synthesize other specific transglycosylation inhibitors have recently been made.

Identification of phospholipids as new components that assist in the in vitro trimerization of a bacterial pore protein

The in vitro trimerization of folded monomers of the bacterial pore protein PhoE, into its native-like, heat- and SDS-stable form requires incubations with isolated cell envelopes and Triton X-100. The possibility that membranes could be isolated that are enriched in assembly factors required for assembly of the pore protein was now investigated. Fractionation of total cell envelopes of Escherichia coli via various techniques indeed revealed the existence of membrane fractions with different capacities to support assembly in vitro. Fractions containing mainly inner membrane vesicles supported the formation of trimers that were associated with these membrane vesicles. However, only a proportion of these trimers were heat- and SDS-stable and these were formed with slow kinetics. In contrast, fractions containing mainly outer membrane vesicles supported formation of high amounts of heat-stable trimers with fast kinetics. We identified phospholipids as active assembly components in these membranes that support trimerization of folded monomers in a process with similar characteristics as observed with inner membrane vesicles. Furthermore, phospholipids strongly stimulate the kinetics of trimerization and increase the final yield of heat-stable trimers in the context of outer membranes. We propose that lipopolysaccharides stabilize the assembly competent state of folded monomers as a lipochaperone. Phospholipids are involved in converting the folded monomer into new assembly competent intermediate with a short half-life that will form heat-stable trimers most efficiently in the context of outer membrane vesicles. These results provide biochemical evidence for the involvement of different lipidic components at distinct stages of the porin assembly process.

Surface expression of O-specific lipopolysaccharide in Escherichia coli requires the function of the TolA protein

We investigated the involvement of Tol proteins in the surface expression of lipopolysaccharide (LPS). tolQ, -R, -A and -B mutants of Escherichia coli K-12, which do not form a complete LPS-containing O antigen, were transformed with the O7+ cosmid pJHCV32. The tolA and tolQ mutants showed reduced O7 LPS expression compared with the respective isogenic parent strains. No changes in O7 LPS expression were found in the other tol mutants. The O7-deficient phenotype in the tolQ and tolA mutants was complemented with a plasmid encoding the tolQRA operon, but not with a similar plasmid containing a frameshift mutation inactivating tolA. Therefore, the reduction in O7 LPS was attributed to the lack of a functional tolA gene, caused either by a direct mutation of this gene or by a polar effect on tolA gene expression exerted by the tolQ mutation. Reduced surface expression of O7 LPS was not caused by changes in lipid A-core structure or downregulation of the O7 LPS promoter. However, an abnormal accumulation of radiolabelled mannose was detected in the plasma membrane. As mannose is a sugar unique to the O7 subunit, this result suggested the presence of accumulated O7 LPS biosynthesis intermediates. Attempts to construct a tolA mutant in the E. coli O7 wild-type strain VW187 were unsuccessful, suggesting that this mutation is lethal. In contrast, a polar tolQ mutation affecting tolA expression in VW187 caused slow growth rate and serum sensitivity in addition to reduced O7 LPS production. VW187 tolQ cells showed an elongated morphology and became permeable to the membrane-impermeable dye propidium iodide. All these phenotypes were corrected upon complementation with cloned tol genes but were not restored by complementation with the tolQRA operon containing the frameshift mutation in tolA. Our results demonstrate that the TolA protein plays a critical role in the surface expression of O antigen subunits by an as yet uncharacterized involvement in the processing of O antigen.

Identification of a potential membrane-targeting region of the replication initiator protein (TrfA) of broad-host-range plasmid RK2

Plasmid RK2 codes for two species of the replication initiator protein TrfA (33 and 44 kDa). Both polypeptides are strongly associated with membrane fractions of Escherichia coli host cells (W. Firshein and P. Kim, Mol. Microbiol. 23, 1-10, 1997). We investigated the role of a 12-amino-acid hydrophobic region (HR) in the membrane association of TrfA. Epitope-tagged polypeptide fragments of TrfA that contained HR were expressed and found to be associated with membrane fractions. Site-directed mutagenesis of trfA revealed that changes of specific amino acids in HR can affect both TrfA association with the membrane and its ability to support replication of an RK2 oriV plasmid in vivo. These results are consistent with the hypothesis that membrane association of TrfA is functionally relevant and that the HR region of TrfA is involved in membrane association and DNA replication in vivo.

Bacteriophage lambda lysis gene product modified and inserted into Escherichia coli outer membrane: Rz1 lipoprotein

Lysis proteins of bacteriophage lambda were localized in different parts of the host envelope: S in the inner membrane,36 Rz in the membrane adhesion sites,14 and Rz1 in the outer membrane. The R gene product, the transglycosylase destroying bacterial murein, is a soluble protein. Computer-assisted analysis of the Rz1 protein amino acids sequence revealed that its N-terminal part contained the site 15VVVG [symbol: see text] C20, which could be recognizable for the SPase II and cleaved leaving lipid modified C20 as the N-terminal amino acid of the mature protein. Microsequencing of the Rz1 protein isolated from the expression products of E. coli [pSB54] carrying the Rz1 gene showed that the N-terminal part of the protein was cleaved as predicted. Lipid labeling with [3H]palmitate confirmed the expectation that Rz1 was a lipoprotein. E. coli [pSB54] treated with globomycin accumulated prolipoprotein, the Rz1 precursor, which was detectable by the anti-Rz1 serum on electropherograms as the 6.5-kDa protein, larger than mature protein. Physiological function of the Rz1 protein remains to be discovered, but as a first hint we noticed that it evokes increase of the fraction of adhesion sites of outer and inner membranes when overproduced from pSB54. The same effect was observed in induced E. coli (lambda) just before the lysis onset, however, one should be cautious in interpreting the results obtained in conditions of the overproduction of the Rz1 lipoprotein.

Membrane association of the Escherichia coli enterobactin synthase proteins EntB/G, EntE, and EntF

The cytosolic proteins EntE, EntF, and EntB/G, which are Escherichia coli enzymes necessary for the final stage of enterobactin synthesis, are released by osmotic shock. Here, consistent with the idea that cytoplasmic proteins found in shockates have an affinity for membranes, a small fraction of each was found in membrane preparations. Two procedures demonstrated that the enzymes were enriched in a minor membrane fraction of buoyant density intermediate between that of cytoplasmic and outer membranes, providing indirect support for the notion that these proteins have a role in enterobactin excretion as well as synthesis.

Isolation of an inner membrane-derived subfraction that supports in vitro replication of a mini-RK2 plasmid in Escherichia coli

Previous results have demonstrated that the inner, but not the outer, membrane fraction of Escherichia coli is the site of membrane-associated DNA replication of plasmid RK2, a broad-host-range plasmid capable of replication in a wide variety of gram-negative hosts (K. Michaels, J. Mei, and W. Firshein, Plasmid 32:19-31, 1994). To resolve the inner membrane replication site further, the procedure of Ishidate et al. (K. Ishidate, E. S. Creeger, J. Zrike, S. Deb, G. Glauner, T. J. MacAlister, and L. I. Rothfield, J. Biol. Chem. 261:428-443, 1986) was used to separate the inner membrane into a number of subfractions, of which only one, a small subfraction containing only 10% of the entire membrane, was found to synthesize DNA inhibited by antibody prepared against the plasmid-encoded initiation protein TrfA. This is the same subfraction that was also found to bind oriV and TrfA to the greatest extent in filter binding assays (J. Mei, S. Benashski, and W. Firshein, J. Bacteriol. 177:6766-6772, 1995).

Components of the RP4 conjugative transfer apparatus form an envelope structure bridging inner and outer membranes of donor cells: implications for related macromolecule transport systems

During bacterial conjugation, the single-stranded DNA molecule is transferred through the cell envelopes of the donor and the recipient cell. A membrane-spanning transfer apparatus encoded by conjugative plasmids has been proposed to facilitate protein and DNA transport. For the IncPalpha plasmid RP4, a thorough sequence analysis of the gene products of the transfer regions Tra1 and Tra2 revealed typical features of mainly inner membrane proteins. We localized essential RP4 transfer functions to Escherichia coli cell fractions by immunological detection with specific polyclonal antisera. Each of the gene products of the RP4 mating pair formation (Mpf) system, specified by the Tra2 core region and by traF of the Tra1 region, was found in the outer membrane fraction with one exception, the TrbB protein, which behaved like a soluble protein. The membrane preparation from Mpf-containing cells had an additional membrane fraction whose density was intermediate between those of the cytoplasmic and outer membranes, suggesting the presence of attachment zones between the two E. coli membranes. The Tra1 region is known to encode the components of the RP4 relaxosome. Several gene products of this transfer region, including the relaxase TraI, were detected in the soluble fraction, but also in the inner membrane fraction. This indicates that the nucleoprotein complex is associated with and/or assembled facing the cytoplasmic site of the E. coli cell envelope. The Tra1 protein TraG was predominantly localized to the cytoplasmic membrane, supporting its potential role as an interface between the RP4 Mpf system and the relaxosome.

Membrane fusion by proline-rich Rz1 lipoprotein, the bacteriophage lambda Rz1 gene product

The fusogenic properties of Rz1, the proline-rich lipoprotein that is the bacteriophage lambda Rz1 gene product, were studied. Light scattering was used to monitor Rz1-induced aggregation of artificial neutral (dipalmitoylphosphatidylcholine/cholesterol) and negatively charged (dipalmitoylphosphatidylcholine/cholesterol/dioleoylphosphatidylserin e) liposomes. Fluorescence assays [the resonance energy transfer between N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine and N-(lissamine rhodamine B sulfonyl)dihexadecanol-sn-glycero-3-phosphoethanolamine lipid fluorescent probes, as well as fluorescent complex formation between terbium ions and dipicolinic acid encapsulated in two liposome populations and calcein fluorescence] were used to monitor Rz1-induced lipid mixing, contents mixing and leakage of neutral and negatively charged liposomes. The results demonstrated that Rz1 caused adhesion of neutral and negatively charged liposomes with concomitant lipid mixing; membrane distortion, leading to the fusion of liposomes and hence their internal content mixing; and local destruction of the membrane accompanied by leakage of the liposome contents. The use of artificial membranes showed that Rz1 induced the fusion of membranes devoid of any proteins. This might mean that the proline stretch of Rz1 allowed interaction with membrane lipids. It is suggested that Rz1-induced liposome fusion was mediated primarily by the generation of local perturbation in the bilayer lipid membrane and to a lesser extent by electrostatic forces.