Fusion of bacterial spheroplasts by electric fields

DNA-assisted selective electrofusion (DASE) of Escherichia coli and giant lipid vesicles

Synthetic biology and cellular engineering require chemical and physical alterations, which are typically achieved by fusing target cells with each other or with payload-carrying vectors. On one hand, electrofusion can efficiently induce the merging of biological cells and/or synthetic analogues via the application of intense DC pulses, but it lacks selectivity and often leads to uncontrolled fusion. On the other hand, synthetic DNA-based constructs, inspired by natural fusogenic proteins, have been shown to induce a selective fusion between membranes, albeit with low efficiency. Here we introduce DNA-assisted selective electrofusion (DASE) which relies on membrane-anchored DNA constructs to bring together the objects one seeks to merge, and applying an electric impulse to trigger their fusion. The DASE process combines the efficiency of standard electrofusion and the selectivity of fusogenic nanostructures, as we demonstrate by inducing and characterizing the fusion of spheroplasts derived from Escherichia coli bacteria with cargo-carrying giant lipid vesicles.

Preparation of Giant Escherichia coli spheroplasts for Electrophysiological Recordings

Prokaryotic ion channels have been instrumental in furthering our understanding of many fundamental aspects of ion channels' structure and function. However, characterizing the biophysical properties of a prokaryotic ion channel in a native membrane system using patch-clamp electrophysiology is technically challenging. Patch-clamp is regarded as a gold standard technique to study ion channel properties in both native and heterologous expression systems. The presence of a cell wall and the small size of bacterial cells makes it impossible to directly patch clamp using microelectrodes. Here, we describe a method for the preparation of giant E. coli spheroplasts in order to investigate the electrophysiological properties of bacterial cell membranes. Spheroplasts are formed by first inhibiting bacterial cell wall synthesis, followed by enzymatic digestion of the outer cell wall in the presence of a permeabilizing agent. This protocol can be used to characterize the function of any heterologous ion channels or ion transporters expressed in E. coli membranes.

Development of Brucella melitensis Rev.1 ΔOmp19 mutants with DIVA feature and comparison of their efficacy against three commercial vaccines in a mouse model

Brucella is an intracellular zoonotic pathogen that can affect many hosts. Brucella melitensis Rev.1 is a live attenuated, is one of the most effective vaccine strain against brucellosis. It can be used safely in sheep, goats, and even cattle. Although many studies are available on this topic, there is no effective vaccine strain for sheep and goats that distinguishes the antibody titer produced between the field infections and vaccinations. Outer membrane protein 19 (Omp 19) is both virulent and a protective antigen found on the cell-wall of the Brucella strain. In this study, used the suicide plasmid pJQ200KS, which contained homologous region without Omp19 Open Reading Frame (ORF) that was transferred to B. melitensis Rev.1 and further transformed into spheroplasts along with penicillin, ampicillin, and glycine by electroporation. To obtain a mutant vector from Escherichia coli, we used the heat shock transformation method along with the blue-white colony screening using X-gal media, whereas for the gene transfer in Brucella, we used electroporation. A scanning electron microscope (S.E.M) was used to observe the spheroplast transformation while the mutant vector and deletion mutants were confirmed through PCR and sequence analysis. In the mouse model efficacy trials, three commercial vaccines were found to comply with the OIE standards. Although the deletion mutants 19 and 44/10 had similar efficiency as the commercial vaccines in terms of stimulation power, the ELISA test with Omp19 protein showed the same results as the negative control. The Rev.1 Omp19 deletion mutants obtained in this study contained sufficient residual virulence, and their protective immunity was similar to the commercial vaccines. The study showed that a vaccine prepared using a B. melitensis Rev.1 ΔOmp19 can act as a marker vaccine or differentiate infected from vaccinated animals (DIVA) through the ELISA test that detects the Omp19 protein.

Action of antimicrobial peptides and cell-penetrating peptides on membrane potential revealed by the single GUV method

Membrane potential plays various key roles in live bacterial and eukaryotic cells. So far, the effects of membrane potential on action of antimicrobial peptides (AMPs) and cell-penetrating peptides (CPPs) have been examined using cells and small lipid vesicles. However, due to the technical drawbacks of these experiments, the effect of membrane potential on the actions of AMPs and CPPs and the elementary processes of interactions of these peptides with cell membranes and vesicle membranes are not well understood. In this short review, we summarize the results of the effect of membrane potential on the action of an AMP, lactoferricin B (LfcinB), and a CPP, transportan 10 (TP10), in vesicle membranes revealed by the single giant unilamellar vesicle (GUV) method. Parts of the actions and their elementary steps of AMPs and CPPs interacting vesicle membranes under membrane potential are clearly revealed using the single GUV method. The experimental methods and their analysis described here can be used to elucidate the effects of membrane potential on various activities of peptides such as AMPs, CPPs, and proteins. Moreover, GUVs with membrane potential are more suitable as a model of cells or artificial cells, as well as GUVs containing small vesicles.

Life with Bacterial Mechanosensitive Channels, from Discovery to Physiology to Pharmacological Target

General principles in biology have often been elucidated from the study of bacteria. This is true for the bacterial mechanosensitive channel of large conductance, MscL, the channel highlighted in this review. This channel functions as a last-ditch emergency release valve discharging cytoplasmic solutes upon decreases in osmotic environment. Opening the largest gated pore, MscL passes molecules up to 30 Å in diameter; exaggerated conformational changes yield advantages for study, including in vivo assays. MscL contains structural/functional themes that recur in higher organisms and help elucidate how other, structurally more complex, channels function. These features of MscL include (i) the ability to directly sense, and respond to, biophysical changes in the membrane, (ii) an α helix ("slide helix") or series of charges ("knot in a rope") at the cytoplasmic membrane boundary to guide transmembrane movements, and (iii) important subunit interfaces that, when disrupted, appear to cause the channel to gate inappropriately. MscL may also have medical applications: the modality of the MscL channel can be changed, suggesting its use as a triggered nanovalve in nanodevices, including those for drug targeting. In addition, recent studies have shown that the antibiotic streptomycin opens MscL and uses it as one of the primary paths to the cytoplasm. Moreover, the recent identification and study of novel specific agonist compounds demonstrate that the channel is a valid drug target. Such compounds may serve as novel-acting antibiotics and adjuvants, a way of permeabilizing the bacterial cell membrane and, thus, increasing the potency of commonly used antibiotics.

Using fluorescence microscopy to shed light on the mechanisms of antimicrobial peptides

Antimicrobial peptides (AMPs) are promising in the fight against increasing bacterial resistance, but the development of AMPs with enhanced activity requires a thorough understanding of their mechanisms of action. Fluorescence microscopy is one of the most flexible and effective tools to characterize AMPs, particularly in its ability to measure the membrane interactions and cellular localization of peptides. Recent advances have increased the scope of research questions that can be addressed via microscopy through improving spatial and temporal resolution. Unique combinations of fluorescent labels and dyes can simultaneously consider different aspects of peptide-membrane interaction mechanisms. This review emphasizes the central role that fluorescence microscopy will continue to play in the interrogation of AMP structure-function relationships and the engineering of more potent peptides.

Single-Cell Bacterial Electrophysiology Reveals Mechanisms of Stress-Induced Damage

An electrochemical gradient of protons, or proton motive force (PMF), is at the basis of bacterial energetics. It powers vital cellular processes and defines the physiological state of the cell. Here, we use an electric circuit analogy of an Escherichia coli cell to mathematically describe the relationship between bacterial PMF, electric properties of the cell membrane, and catabolism. We combine the analogy with the use of bacterial flagellar motor as a single-cell "voltmeter" to measure cellular PMF in varied and dynamic external environments (for example, under different stresses). We find that butanol acts as an ionophore and functionally characterize membrane damage caused by the light of shorter wavelengths. Our approach coalesces noninvasive and fast single-cell voltmeter with a well-defined mathematical framework to enable quantitative bacterial electrophysiology.

Membrane potential is vital for rapid permeabilization of plasma membranes and lipid bilayers by the antimicrobial peptide lactoferricin B

Lactoferricin B (LfcinB) is a cationic antimicrobial peptide, and its capacity to damage the bacterial plasma membrane is suggested to be a main factor in LfcinB's antimicrobial activity. However, the specific processes and mechanisms in LfcinB-induced membrane damage are unclear. In this report, using confocal laser-scanning microscopy, we examined the interaction of LfcinB with single Escherichia coli cells and spheroplasts containing the water-soluble fluorescent probe calcein in the cytoplasm. LfcinB induced rapid calcein leakage from single E. coli cells and from single spheroplasts, indicating that LfcinB interacts directly with the plasma membrane and induces its rapid permeabilization. The proton ionophore carbonyl cyanide m-chlorophenylhydrazone suppressed this leakage. Next, we used the single giant unilamellar vesicle (GUV) method to examine LfcinB's interaction with GUVs comprising polar lipid extracts of E. coli containing a water-soluble fluorescent probe, Alexa Fluor 647 hydrazide (AF647). We observed that LfcinB stochastically induces local rupture in single GUVs, causing rapid AF647 leakage; however, higher LfcinB concentrations were required for AF647 leakage from GUVs than from E. coli cells and spheroplasts. To identify the reason for this difference, we examined the effect of membrane potential on LfcinB-induced pore formation, finding that the rate of LfcinB-induced local rupture in GUVs increases greatly with increasing negative membrane potential. These results indicate that membrane potential plays an important role in LfcinB-induced local rupture of lipid bilayers and rapid permeabilization of E. coli plasma membranes. On the basis of these results, we discuss the mode of action of LfcinB's antimicrobial activity.

Production and Visualization of Bacterial Spheroplasts and Protoplasts to Characterize Antimicrobial Peptide Localization

The use of confocal microscopy as a method to assess peptide localization patterns within bacteria is commonly inhibited by the resolution limits of conventional light microscopes. As the resolution for a given microscope cannot be easily enhanced, we present protocols to transform the small rod-shaped gram-negative Escherichia coli (E. coli) and gram-positive Bacillus megaterium (B. megaterium) into larger, easily imaged spherical forms called spheroplasts or protoplasts. This transformation allows observers to rapidly and clearly determine whether peptides lodge themselves into the bacterial membrane (i.e., membrane localizing) or cross the membrane to enter the cell (i.e., translocating). With this approach, we also present a systematic method to characterize peptides as membrane localizing or translocating. While this method can be used for a variety of membrane-active peptides and bacterial strains, we demonstrate the utility of this protocol by observing the interaction of Buforin II P11A (BF2 P11A), an antimicrobial peptide (AMP), with E. coli spheroplasts and B. megaterium protoplasts.

Understanding membrane-active antimicrobial peptides

Bacterial membranes represent an attractive target for the design of new antibiotics to combat widespread bacterial resistance to traditional inhibitor-based antibiotics. Understanding how antimicrobial peptides (AMPs) and other membrane-active agents attack membranes could facilitate the design of new, effective antimicrobials. AMPs, which are small, gene-encoded host defense proteins, offer a promising basis for the study of membrane-active antimicrobial agents. These peptides are cationic and amphipathic, spontaneously binding to bacterial membranes and inducing transmembrane permeability to small molecules. Yet there are often confusions surrounding the details of the molecular mechanisms of AMPs. Following the doctrine of structure-function relationship, AMPs are often viewed as the molecular scaffolding of pores in membranes. Instead we believe that the full mechanism of AMPs is understandable if we consider the interactions of AMPs with the whole membrane domain, where interactions induce structural transformations of the entire membrane, rather than forming localized molecular structures. We believe that it is necessary to consider the entire soft matter peptide-membrane system as it evolves through several distinct states. Accordingly, we have developed experimental techniques to investigate the state and structure of the membrane as a function of the bound peptide to lipid ratio, exactly as AMPs in solution progressively bind to the membrane and induce structural changes to the entire system. The results from these studies suggest that global interactions of AMPs with the membrane domain are of fundamental importance to understanding the antimicrobial mechanisms of AMPs.

Bacterial Spheroplasts as a Model for Visualizing Membrane Translocation of Antimicrobial Peptides

Studies attempting to characterize the membrane translocation of antimicrobial and cell-penetrating peptides are frequently limited by the resolution of conventional light microscopy. This study shows that spheroplasts provide a valuable approach to overcome these limits. Spheroplasts produce less ambiguous images and allow for more systematic analyses of localization. Data collected with spheroplasts are consistent with studies using normal bacterial cells and imply that a particular peptide may not always follow the same mechanism of action.

Colanic Acid Intermediates Prevent De Novo Shape Recovery of Escherichia coli Spheroplasts, Calling into Question Biological Roles Previously Attributed to Colanic Acid

After losing their protective peptidoglycan, bacterial spheroplasts can resynthesize a cell wall to recreate their normal shape. In Escherichia coli, this process requires the Rcs response. In its absence, spheroplasts do not revert to rod shapes but instead form enlarged spheroids and lyse. Here, we investigated the reason for this Rcs requirement. Rcs-deficient spheroids exhibited breaks and bulges in their periplasmic spaces and failed to synthesize a complete peptidoglycan cell wall, indicating that the bacterial envelope was defective. To determine the Rcs-dependent gene(s) required for shape recovery, we tested spheroplasts lacking selected RcsB-regulated genes and found that colanic acid (CA) biosynthesis appeared to be involved. Surprisingly, though, extracellular CA was not required for recovery. Instead, lysis was caused by mutations that interrupted CA biosynthesis downstream of the initial glycosyl transferase, WcaJ. Deleting wcaJ prevented lysis of spheroplasts lacking ensuing steps in the pathway, and providing WcaJ in trans to a mutant lacking the entire CA operon triggered spheroplast enlargement and lysis. Thus, CA is not required for spheroplast recovery. Instead, CA intermediates accumulate as dead-end products which inhibit recovery of wall-less cells. The results strongly imply that CA may not be required for the survival E. coli L-forms. More broadly, these findings mandate that previous conclusions about the role of colanic acid in biofilm formation or virulence must be reevaluated.

IMPORTANCE

Wall-less bacteria can resynthesize their walls and recreate a normal shape, which in Escherichia coli requires the Rcs response. While attempting to identify the Rcs-dependent gene required for shape recovery, we found that colanic acid (CA) biosynthesis appeared to be involved. Surprisingly, though, cell death was caused by mutations that interrupted CA biosynthesis downstream of the initial step in the pathway, creating dead-end compounds that inhibited recovery of wall-less cells. When testing for the biological role of CA, most previous experiments used mutants that would accumulate these deadly intermediates, meaning that all prior conclusions must be reexamined to determine if the results were caused by these lethal side effects instead of accurately reflecting the biological purpose of CA itself.

Physical properties of Escherichia coli spheroplast membranes

We investigated the physical properties of bacterial cytoplasmic membranes by applying the method of micropipette aspiration to Escherichia coli spheroplasts. We found that the properties of spheroplast membranes are significantly different from that of laboratory-prepared lipid vesicles or that of previously investigated animal cells. The spheroplasts can adjust their internal osmolality by increasing their volumes more than three times upon osmotic downshift. Until the spheroplasts are swollen to their volume limit, their membranes are tensionless. At constant external osmolality, aspiration increases the surface area of the membrane and creates tension. What distinguishes spheroplast membranes from lipid bilayers is that the area change of a spheroplast membrane by tension is a relaxation process. No such time dependence is observed in lipid bilayers. The equilibrium tension-area relation is reversible. The apparent area stretching moduli are several times smaller than that of stretching a lipid bilayer. We conclude that spheroplasts maintain a minimum surface area without tension by a membrane reservoir that removes the excessive membranes from the minimum surface area. Volume expansion eventually exhausts the membrane reservoir; then the membrane behaves like a lipid bilayer with a comparable stretching modulus. Interestingly, the membranes cease to refold when spheroplasts lost viability, implying that the membrane reservoir is metabolically maintained.

Electrophysiology of bacteria

Bacteria secrete and harbor in their membranes a number of pore-forming proteins. Some of these are bona fide ion channels that may respond to changes in membrane tension, voltage, or pH. Others may be large translocons used for the secretion of folded or unfolded polypeptide substrates. Additionally, many secreted toxins insert into target cell membranes and form pores that either collapse membrane electrochemical gradients or provide conduits for the delivery of virulence factors. In all cases, electrophysiological approaches have yielded much progress in past decades in understanding the functional mechanisms of these pores. By monitoring the changes in current due to ion flow through the pores, these techniques are used as high-resolution tools to gather detailed information on the kinetic and permeation properties of these proteins, including those whose physiological role is not ion flux. This review highlights some of the electrophysiological studies that have advanced the field of transport by pore-forming proteins of bacterial origin.

Bacterial mechanosensitive channels: models for studying mechanosensory transduction

SIGNIFICANCE

Sensations of touch and hearing are manifestations of mechanical contact and air pressure acting on touch receptors and hair cells of the inner ear, respectively. In bacteria, osmotic pressure exerts a significant mechanical force on their cellular membrane. Bacteria have evolved mechanosensitive (MS) channels to cope with excessive turgor pressure resulting from a hypo-osmotic shock. MS channel opening allows the expulsion of osmolytes and water, thereby restoring normal cellular turgor and preventing cell lysis.

RECENT ADVANCES

As biological force-sensing systems, MS channels have been identified as the best examples of membrane proteins coupling molecular dynamics to cellular mechanics. The bacterial MS channel of large conductance (MscL) and MS channel of small conductance (MscS) have been subjected to extensive biophysical, biochemical, genetic, and structural analyses. These studies have established MscL and MscS as model systems for mechanosensory transduction.

CRITICAL ISSUES

In recent years, MS ion channels in mammalian cells have moved into focus of mechanotransduction research, accompanied by an increased awareness of the role they may play in the pathophysiology of diseases, including cardiac hypertrophy, muscular dystrophy, or Xerocytosis.

FUTURE DIRECTIONS

A recent exciting development includes the molecular identification of Piezo proteins, which function as nonselective cation channels in mechanosensory transduction associated with senses of touch and pain. Since research on Piezo channels is very young, applying lessons learned from studies of bacterial MS channels to establishing the mechanism by which the Piezo channels are mechanically activated remains one of the future challenges toward a better understanding of the role that MS channels play in mechanobiology.

The mechanoelectrical response of the cytoplasmic membrane of Vibrio cholerae

Persistence of Vibrio cholerae in waters of fluctuating salinity relies on the capacity of this facultative enteric pathogen to adapt to varying osmotic conditions. In an event of osmotic downshift, osmolytes accumulated inside the bacterium can be quickly released through tension-activated channels. With the newly established procedure of giant spheroplast preparation from V. cholerae, we performed the first patch-clamp characterization of its cytoplasmic membrane and compared tension-activated currents with those in Esherichia coli. Saturating pressure ramps revealed two waves of activation belonging to the ∼1-nS mechanosensitive channel of small conductance (MscS)-like channels and ∼3-nS mechanosensitive channel of large conductance (MscL)-like channels, with a pressure midpoint ratio p_0.5MscS/p_0.5MscL of 0.48. We found that MscL-like channels in V. cholerae present at a density three times higher than in E. coli, and yet, these vibrios were less tolerant to large osmotic downshocks. The Vibrio MscS-like channels exhibit characteristic inward rectification and subconductive states at depolarizing voltages; they also adapt and inactivate at subsaturating tensions and recover within 2 s upon tension release, just like E. coli MscS. Trehalose, a compatible internal osmolyte accumulated under hypertonic conditions, significantly shifts activation curves of both MscL- and MscS-like channels toward higher tensions, yet does not freely partition into the channel pore. Direct electrophysiology of V. cholerae offers new avenues for the in situ analysis of membrane components critical for osmotic survival and electrogenic transport in this pathogen.

MscS-like mechanosensitive channels in plants and microbes

The challenge of osmotic stress is something all living organisms must face as a result of environmental dynamics. Over the past three decades, innovative research and cooperation across disciplines have irrefutably established that cells utilize mechanically gated ion channels to release osmolytes and prevent cell lysis during hypoosmotic stress. Early electrophysiological analysis of the inner membrane of Escherichia coli identified the presence of three distinct mechanosensitive activities. The subsequent discoveries of the genes responsible for two of these activities, the mechanosensitive channels of large (MscL) and small (MscS) conductance, led to the identification of two diverse families of mechanosensitive channels. The latter of these two families, the MscS family, consists of members from bacteria, archaea, fungi, and plants. Genetic and electrophysiological analysis of these family members has provided insight into how organisms use mechanosensitive channels for osmotic regulation in response to changing environmental and developmental circumstances. Furthermore, determining the crystal structure of E. coli MscS and several homologues in several conformational states has contributed to our understanding of the gating mechanisms of these channels. Here we summarize our current knowledge of MscS homologues from all three domains of life and address their structure, proposed physiological functions, electrophysiological behaviors, and topological diversity.

Lightning-triggered electroporation and electrofusion as possible contributors to natural horizontal gene transfer

Phylogenetic studies show that horizontal gene transfer (HGT) is a significant contributor to genetic variability of prokaryotes, and was perhaps even more abundant during the early evolution. Hitherto, research of natural HGT has mainly focused on three mechanisms of DNA transfer: conjugation, natural competence, and viral transduction. This paper discusses the feasibility of a fourth such mechanism--cell electroporation and/or electrofusion triggered by atmospheric electrostatic discharges (lightnings). A description of electroporation as a phenomenon is followed by a review of experimental evidence that electroporation of prokaryotes in aqueous environments can result in release of non-denatured DNA, as well as uptake of DNA from the surroundings and transformation. Similarly, a description of electrofusion is followed by a review of experiments showing that prokaryotes devoid of cell wall can electrofuse into hybrids expressing the genes of their both precursors. Under sufficiently fine-tuned conditions, electroporation and electrofusion are efficient tools for artificial transformation and hybridization, respectively, but the quantitative analysis developed here shows that conditions for electroporation-based DNA release, DNA uptake and transformation, as well as for electrofusion are also present in many natural aqueous environments exposed to lightnings. Electroporation is thus a plausible contributor to natural HGT among prokaryotes, and could have been particularly important during the early evolution, when the other mechanisms might have been scarcer or nonexistent. In modern prokaryotes, natural absence of the cell wall is rare, but it is reasonable to assume that the wall has formed during a certain stage of evolution, and at least prior to this, electrofusion could also have contributed to natural HGT. The concluding section outlines several guidelines for assessment of the feasibility of lightning-triggered HGT.

The Rcs stress response and accessory envelope proteins are required for de novo generation of cell shape in Escherichia coli

Interactions with immune responses or exposure to certain antibiotics can remove the peptidoglycan wall of many Gram-negative bacteria. Though the spheroplasts thus created usually lyse, some may survive by resynthesizing their walls and shapes. Normally, bacterial morphology is generated by synthetic complexes directed by FtsZ and MreBCD or their homologues, but whether these classic systems can recreate morphology in the absence of a preexisting template is unknown. To address this question, we treated Escherichia coli with lysozyme to remove the peptidoglycan wall while leaving intact the inner and outer membranes and periplasm. The resulting lysozyme-induced (LI) spheroplasts recovered a rod shape after four to six generations. Recovery proceeded via a series of cell divisions that produced misshapen and branched intermediates before later progeny assumed a normal rod shape. Importantly, mutants defective in mounting the Rcs stress response and those lacking penicillin binding protein 1B (PBP1B) or LpoB could not divide or recover their cell shape but instead enlarged until they lysed. LI spheroplasts from mutants lacking the Lpp lipoprotein or PBP6 produced spherical daughter cells that did not recover a normal rod shape or that did so only after a significant delay. Thus, to regenerate normal morphology de novo, E. coli must supplement the classic FtsZ- and MreBCD-directed cell wall systems with activities that are otherwise dispensable for growth under normal laboratory conditions. The existence of these auxiliary mechanisms implies that they may be required for survival in natural environments, where bacterial walls can be damaged extensively or removed altogether.

Patch clamp electrophysiology for the study of bacterial ion channels in giant spheroplasts of E. coli

Ion channel studies have been focused on ion channels from animal and human cells over many years. Based on the knowledge acquired, predominantly over the last 20 years, a large diversity of ion channels exists in cellular membranes of prokaryotes as well. Paradoxically, most of what is known about the structure of eukaryotic ion channels is based on the structure of bacterial channels. This is largely due to the suitability of bacterial cells for functional and structural studies of biological macromolecules in a laboratory environment. Development of the "giant spheroplast" preparation from E. coli cells was instrumental for functional studies of ion channels in the bacterial cell membrane. Here we describe detailed protocols used for the preparation of giant spheroplasts as well as protocols used for the patch-clamp recording of native or heterologously expressed ion channels in E. coli spheroplast membrane.

The protective effect of osmoprotectant TMAO on bacterial mechanosensitive channels of small conductance MscS/MscK under high hydrostatic pressure

Activity of the bacterial mechanosensitive channels of small conductance MscS/MscK of E. coli was investigated under high hydrostatic pressure (HHP) using the "flying-patch" patch-clamp technique. The channels were gated by negative pipette voltage and their open probability was measured at HHP of 0.1 to 80 MPa. The channel open probability decreased with increasing HHP. When the osmolyte methylamine N-oxide (TMAO) was applied to the cytoplasmic side of the inside-out excised membrane patches of E. coli giant spheroplasts the inhibitory effect of HHP on the channel activity was suppressed at pressures of up to 40 MPa. At 40 MPa and above the channel open probability decreased in a similar fashion with or without TMAO. Our study suggests that TMAO helps to counteract the effect of HHP up to 40 MPa on the MscS/MscK open state by "shielding" the cytoplasmic domain of the channels.

Bacterial mechanosensitive channels--MscS: evolution's solution to creating sensitivity in function

The discovery of mechanosensing channels has changed our understanding of bacterial physiology. The mechanosensitive channel of small conductance (MscS) is perhaps the most intensively studied of these channels. MscS has at least two states: closed, which does not allow solutes to exit the cytoplasm, and open, which allows rapid efflux of solvent and solutes. The ability to appropriately open or close the channel (gating) is critical to bacterial survival. We briefly review the science that led to the isolation and identification of MscS. We concentrate on the structure-function relationship of the channel, in particular the structural and biochemical approaches to understanding channel gating. We highlight the troubling discrepancies between the various models developed to understand MscS gating.

Bacterial mechanosensitive channels as a paradigm for mechanosensory transduction

Research on bacterial mechanosensitive (MS) channels has since their discovery been at the forefront of the MS channel field due to extensive studies of the structure and function of MscL and MscS, two of the several different types of MS channels found in bacteria. Just a few years after these two MS channels were cloned their 3D structure was solved by X-ray crystallography. Today, the repertoire of multidisciplinary approaches used in experimental and theoretical studies following the cloning and crystallographic determination of the MscL and MscS structure has expanded by including electronparamagnetic resonance (EPR) and Förster resonance energy transfer (FRET) spectroscopy aided by computational modelling employing molecular dynamics as well as Brownian dynamics simulations, which significantly advanced the understanding of structural determinants of the gating and conduction properties of these two MS channels. These extensive multidisciplinary studies of MscL and MscS have greatly contributed to elucidation of the basic physical principles of MS channel gating by mechanical force. This review summarizes briefly the major experimental and conceptual advancements, which helped in establishing MscL and MscS as a major paradigm of mechanosensory transduction in living cells.

Expression and characterization of the bacterial mechanosensitive channel MscS in Xenopus laevis oocytes

We have successfully expressed and characterized mechanosensitive channel of small conductance (MscS) from Escherichia coli in oocytes of the African clawed frog, Xenopus laevis. MscS expressed in oocytes has the same single-channel conductance and voltage dependence as the channel in its native environment. Two hallmarks of MscS activity, the presence of conducting substates at high potentials and reversible adaptation to a sustained stimulus, are also exhibited by oocyte-expressed MscS. In addition to its ease of use, the oocyte system allows the user to work with relatively large patches, which could be an advantage for the visualization of membrane deformation. Furthermore, MscS can now be compared directly to its eukaryotic homologues or to other mechanosensitive channels that are not easily studied in E. coli.

My life with nature

After a childhood in Germany and being a youth in Grand Forks, North Dakota, I went to Harvard University, then to graduate school in biochemistry at the University of Wisconsin. Then to Washington University and Stanford University for postdoctoral training in biochemistry and genetics. Then at the University of Wisconsin, as a professor in the Department of Biochemistry and the Department of Genetics, I initiated research on bacterial chemotaxis. Here, I review this research by me and by many, many others up to the present moment. During the past few years, I have been studying chemotaxis and related behavior in animals, namely in Drosophila fruit flies, and some of these results are presented here. My current thinking is described.

Coarse-grained model for mechanosensitive ion channels

In this work, a coarse-grained model for a kind of membrane protein, the mechanosensitive channel of small conductance (MscS), is proposed. The basic structure of the MscS is preserved when the protein is coarse-grained. For the coarse-grained model, the channels show two different states, namely the open and closed states, depending on the model parameters. Under the same membrane tension, the state of the ion channel is found to be critically determined by the protein structure, especially the length of the transmembrane alpha-helix. It is also found that for the protein with certain size, the gating transition occurs when the membrane tension is applied, resembling in a real mechanosensitive channel.

Protein localization in Escherichia coli cells: comparison of the cytoplasmic membrane proteins ProP, LacY, ProW, AqpZ, MscS, and MscL

Fluorescence microscopy has revealed that the phospholipid cardiolipin (CL) and FlAsH-labeled transporters ProP and LacY are concentrated at the poles of Escherichia coli cells. The proportion of CL among E. coli phospholipids can be varied in vivo as it is decreased by cls mutations and it increases with the osmolality of the growth medium. In this report we compare the localization of CL, ProP, and LacY with that of other cytoplasmic membrane proteins. The proportion of cells in which FlAsH-labeled membrane proteins were concentrated at the cell poles was determined as a function of protein expression level and CL content. Each tagged protein was expressed from a pBAD24-derived plasmid; tagged ProP was also expressed from the chromosome. The osmosensory transporter ProP and the mechanosensitive channel MscS concentrated at the poles at frequencies correlated with the cellular CL content. The lactose transporter LacY was found at the poles at a high and CL-independent frequency. ProW (a component of the osmoregulatory transporter ProU), AqpZ (an aquaporin), and MscL (a mechanosensitive channel) were concentrated at the poles in a minority of cells, and this polar localization was CL independent. The frequency of polar localization was independent of induction (at arabinose concentrations up to 1 mM) for proteins encoded by pBAD24-derived plasmids. Complementation studies showed that ProW, AqpZ, MscS, and MscL remained functional after introduction of the FlAsH tag (CCPGCC). These data suggest that CL-dependent polar localization in E. coli cells is not a general characteristic of transporters, channels, or osmoregulatory proteins. Polar localization can be frequent and CL independent (as observed for LacY), frequent and CL dependent (as observed for ProP and MscS), or infrequent (as observed for AqpZ, ProW, and MscL).

Ion channels in microbes

Studies of ion channels have for long been dominated by the animalcentric, if not anthropocentric, view of physiology. The structures and activities of ion channels had, however, evolved long before the appearance of complex multicellular organisms on earth. The diversity of ion channels existing in cellular membranes of prokaryotes is a good example. Although at first it may appear as a paradox that most of what we know about the structure of eukaryotic ion channels is based on the structure of bacterial channels, this should not be surprising given the evolutionary relatedness of all living organisms and suitability of microbial cells for structural studies of biological macromolecules in a laboratory environment. Genome sequences of the human as well as various microbial, plant, and animal organisms unambiguously established the evolutionary links, whereas crystallographic studies of the structures of major types of ion channels published over the last decade clearly demonstrated the advantage of using microbes as experimental organisms. The purpose of this review is not only to provide an account of acquired knowledge on microbial ion channels but also to show that the study of microbes and their ion channels may also hold a key to solving unresolved molecular mysteries in the future.

Bacterial mechanosensitive channels: Experiment and theory

Since their discovery in Escherichia coli some 20 years ago, studies of bacterial mechanosensitive (MS) ion channels have been at the forefront of the MS channel research field. Two major events greatly advanced the research on bacterial MS channels: (i) cloning of MscL and MscS, the MS channels of Large and Small conductance, and (ii) solving their 3D crystal structure. These events enabled further experimental studies employing EPR and FRET spectroscopy in addition to patch clamp and molecular biological techniques that have successfully been used in characterization of the structure and function of bacterial MS channels. In parallel with the experimental studies computational modelling has been applied to elucidate the molecular dynamics of MscL and MscS, which has significantly contributed to our understanding of basic physical principles of the mechanosensory transduction in living organisms.

Gating prokaryotic mechanosensitive channels

Prokaryotic mechanosensitive channels function as molecular switches that transduce bilayer deformations into protein motion. These protein structural rearrangements generate large non-selective pores that function as a prokaryotic 'last line of defence' to sudden osmotic challenges. Once considered an electrophysiological artefact, recent structural, spectroscopic and functional data have placed this class of protein at the centre of efforts to understand the molecular basis of lipid-protein interactions and their influence on protein function.

Effect of high hydrostatic pressure on the bacterial mechanosensitive channel MscS

We have investigated the effect of high hydrostatic pressure on MscS, the bacterial mechanosensitive channel of small conductance. Pressure affected channel kinetics but not conductance. At negative pipette voltages (corresponding to membrane depolarization in the inside-out patch configuration used in our experiments) the channel exhibited a reversible reduction in activity with increasing hydrostatic pressure between 0 and 900 atm (90 MPa) at 23 degrees C. The reduced activity was characterized by a significant reduction in the channel opening probability resulting from a shortening of the channel openings with increasing pressure. Thus high hydrostatic pressure generally favoured channel closing. Cooling the patch by approximately 10 degrees C, intended to order the bilayer component of the patch by an amount similar to that caused by 50 MPa at 23 degrees C, had relatively little effect. This implies that pressure does not affect channel kinetics via bilayer order. Accordingly we postulate that lateral compression of the bilayer, under high hydrostatic pressure, is responsible. These observations also have implications for our understanding of the adaptation of mechanosensitive channels in deep-sea bacteria.

Effect of cellular filamentation on adventurous and social gliding motility of Myxococcus xanthus

Filamentous bacterial cells often provide biological information that is not readily evident in normal-size cells. In this study, the effect of cellular filamentation on gliding motility of Myxococcus xanthus, a Gram-negative social bacterium, was investigated. Elongation of the cell body had different effects on adventurous and social motility of M. xanthus. The rate of A-motility was insensitive to cell-body elongation whereas the rate of S-motility was reduced dramatically as the cell body got longer, indicating that these two motility systems work in different ways. The study also showed that filamentous wild-type cells glide smoothly with relatively straight, long cell bodies. However, filamentous cells of certain social motility mutants showed zigzag, tangled cell bodies on a solid surface, apparently a result of a lack of coordination between different fragments within the filaments. Further genetic and biochemical analyses indicated that the uncoordinated movements of these mutant filaments were correlated with the absence of cell surface fibril materials, indicating a possible new function for fibrils.

Patch clamp studies on ion pumps of the cytoplasmic membrane of Escherichia coli. Formation, preparation, and utilization of giant vacuole-like structures consisting of everted cytoplasmic membrane

Formation of giant protoplasts from normal Escherichia coli cells resulted in the formation of giant vacuole-type structures (which we designate as provacuoles) in the protoplasts. Electron microscopic observation revealed that these provacuoles were surrounded by a single membrane. We detected inner (cytoplasmic) membrane proteins in the provacuolar membrane but not outer membrane proteins. Biochemical analyses revealed that the provacuoles consist of everted cytoplasmic membranes. We applied the patch clamp method to the giant provacuoles. We have succeeded in measuring current that represents inward movement of H+ because of respiration and to ATP hydrolysis by the FoF1-ATPase. Such current was inhibited by inhibitors of the respiratory chain or FoF1-ATPase. This method is applicable for analyses of ion channels, ion pumps, or ion transporters in E. coli or other microorganisms.

Mutations in a bacterial mechanosensitive channel change the cellular response to osmotic stress

MscL is a channel found in bacterial plasma membranes that opens a large pore in response to mechanical stress. Here we demonstrate that some mutations within this channel protein (K31D and K31E) evoke a cellular phenotype in which the growth rate is severely depressed. Increasing the osmolarity of the growth medium partially rescues this "slowed growth" phenotype and decreases an abnormal cytosolic potassium loss observed in cells expressing the mutants. In addition, upon sudden decrease in osmolarity (osmotic downshock) more cytoplasmic potassium is released from cells expressing the mutants than cells expressing wild-type MscL. After osmotic downshock, all cells remained viable; hence, the differences in potassium efflux observed are not due to cell lysis but instead appear to be an exaggeration of the normal response to this sudden change in environmental osmolarity. Patch clamp studies in native bacterial membranes substantiate the hypothesis that these mutant channels are more sensitive to mechanical stresses, especially at voltages approaching those estimated for bacterial membrane potentials. These data are consistent with a crucial role for MscL in the adaptation to large osmotic downshock and suggest that if the normally tight regulation of MscL gating is disrupted, cell growth can be severely inhibited.

Mechanosensitive channels of Escherichia coli: the MscL gene, protein, and activities

Although mechanosensory responses are ubiquitous and diverse, the molecular bases of mechanosensation in most cases remain mysterious MscL, a mechanosensitive channel of large conductance of Escherichia coli and its bacterial homologues are the first and currently only channel molecules shown to directly sense mechanical stretch of the membrane. In response to the tension conveyed via the lipid bilayer, MscL increases its open probability by several orders of magnitude. In the present review we describe the identification, cloning, and first sets of biophysical and structural data on this simplest mechanosensory molecule. We discovered a 2.5-ns mechanosensitive conductance in giant E. coli spheroplasts. Using chromatographies to enrich the target and patch clamp to assay the channel activity in liposome-reconstituted fractions, we identified the MscL protein and cloned the mscL gene. MscL comprises 136 amino acid residues (15 kDa), with two highly hydrophobic regions, and resides in the inner membrane of the bacterium. PhoA-fusion experiments indicate that the protein spans the membrane twice with both termini in the cytoplasm. Spectroscopic techniques show that it is highly helical. Expression of MscL tandems and covalent cross-linking suggest that the active channel complex is a homo-hexamer. We have identified several residues, which when deleted or substituted, affect channel kinetics or mechanosensitivity. Although unique when discovered, highly conserved MscL homologues in both gram-negative and gram-positive bacteria have been found, suggesting their ubiquitous importance among bacteria.

Single residue substitutions that change the gating properties of a mechanosensitive channel in Escherichia coli

MscL is a channel that opens a large pore in the Escherichia coli cytoplasmic membrane in response to mechanical stress. Previously, we highly enriched the MscL protein by using patch clamp as a functional assay and cloned the corresponding gene. The predicted protein contains a largely hydrophobic core spanning two-thirds of the molecule and a more hydrophilic carboxyl terminal tail. Because MscL had no homology to characterized proteins, it was impossible to predict functional regions of the protein by simple inspection. Here, by mutagenesis, we have searched for functionally important regions of this molecule. We show that a short deletion from the amino terminus (3 amino acids), and a larger deletion of 27 amino acids from the carboxyl terminus of this protein, had little if any effect in channel properties. We have thus narrowed the search of the core mechanosensitive mechanism to 106 residues of this 136-amino acid protein. In contrast, single residue substitutions of a lysine in the putative first transmembrane domain or a glutamine in the periplasmic loop caused pronounced shifts in the mechano-sensitivity curves and/or large changes in the kinetics of channel gating, suggesting that the conformational structure in these regions is critical for normal mechanosensitive channel gating.

Generation of giant protoplasts of Escherichia coli and an inner-membrane anion selective conductance

Established methods were modified and combined to generate unilamellar giant protoplasts in order to study the electric events on the cytoplasmic membrane of Escherichia coli with patch-clamp technique. The activities of many types of conductances were encountered, one of them is characterized here. This channel conductance is 109 pS in 150 mM KCl; it prefers anions, it is highly voltage dependent and is blocked by micromolar concentrations of anthracene-9-carboxylic acid.

Surface shape change during fusion of erythrocyte membranes is sensitive to membrane skeleton agents

We previously reported that the induction of membrane fusion between pairs of erythrocyte ghosts is accompanied by the formation of a multipore fusion zone that undergoes an area expansion with condition-dependent characteristics. These characteristics allowed us to hypothesize substantial, if not major, involvement of the spectrin-based membrane skeleton in controlling this expansion. It was also found that the fusion zone, which first appears in phase optics as a flat diaphragm, has a lifetime that is also highly condition-dependent. We report here that 2,3-diphosphoglycerate, wheat germ agglutinin, diamide, and N-ethylmaleimide, all known to have binding sites primarily on skeleton components (including spectrin), have condition-dependent effects on specific components of the fusion zone diameter versus time expansion curve and the flat diaphragm lifetime. We also report a pH/ionic strength condition that causes a dramatic stabilization of flat diaphragms in a manner consistent with the known pH/ionic strength dependence of the spectrin calorimetric transition, thus further supporting the hypothesis of spectrin involvement. Our data suggest that the influence of the membrane skeleton on cell fusion is to restrain the rounding up that takes place after membrane fusion and that it may have variable, rather than fixed, mechanical properties. Data show that WGA, a known ligand for sialic acid, and DPG, a known metabolite, influences the flat diaphragm stability and late period expansion rates, raising the possibility that some of these mechanical properties are biologically regulated.

Two types of mechanosensitive channels in the Escherichia coli cell envelope: solubilization and functional reconstitution

Mechanosensitive ion channels (MSCs) which could provide for fast osmoregulatory responses in bacteria, remain unidentified as molecular entities. MSCs from Escherichia coli (strain AW740) were examined using the patch-clamp technique, either (a) in giant spheroplasts, (b) after reconstitution by fusing native membrane vesicles with asolectin liposomes, or (c) by reassembly of octylglucoside-solubilized membrane extract into asolectin liposomes. MSC activities were similar in all three preparations, consisting of a large nonselective MSC of 3-nS conductance (in 200 mM KCl) that was activated by high negative pressures, and a small weakly anion-selective MSC of 1 nS activated by lower negative pressures. Both channels appeared more sensitive to suction in liposomes than in spheroplasts. After gel filtration of the solubilized membrane extract and reconstituting the fractions, both large MSC and small MSC activities were retrieved in liposomes. The positions of the peaks of channel activity in the column eluate, assayed by patch sampling of individual fractions reconstituted in liposomes, showed an apparent molecular mass under nondenaturing conditions of about 60-80 kDa for the large and 200-400 kDa for the small MSC. We conclude that (a) the large MSC and the small MSC are distinct molecular entities, (b) the fact that both MSCs were functional in liposomes following chromatography strongly suggests that these channels are gated by tension transduced via lipid bilayer, and (c) chromatographic fractionation of detergent-solubilized membrane proteins with subsequent patch sampling of reconstituted fractions can be used to identify and isolate these MS channel proteins.