Lactococcus lactis uses MscL as its principal mechanosensitive channel

Lactococcus lactis, an Attractive Cell Factory for the Expression of Functional Membrane Proteins

Membrane proteins play key roles in most crucial cellular processes, ranging from cell-to-cell communication to signaling processes. Despite recent improvements, the expression of functionally folded membrane proteins in sufficient amounts for functional and structural characterization remains a challenge. Indeed, it is still difficult to predict whether a protein can be overproduced in a functional state in some expression system(s), though studies of high-throughput screens have been published in recent years. Prokaryotic expression systems present several advantages over eukaryotic ones. Among them, Lactococcus lactis (L. lactis) has emerged in the last two decades as a good alternative expression system to E. coli. The purpose of this chapter is to describe L. lactis and its tightly inducible system, NICE, for the effective expression of membrane proteins from both prokaryotic and eukaryotic origins.

Exploring the diversity of mechanosensitive channels in bacterial genomes

Mechanosensitive ion channels are responsible for touch sensation and proprioception in higher level organisms such as humans and recovery after osmotic stress in bacteria. Bacterial mechanosensitive channels are homologous to either the mechanosensitive channel of large conductance (MscL) or the mechanosensitive channel of small conductance (MscS). In the E. coli genome there are seven unique mechanosensitive channels, a single MscL homologue, and six MscS homologues. The six MscS homologues are members of the diverse MscS superfamily of ion channels, and these channels show variation on both the N and C termini when compared to E. coli MscS. In bacterial strains with phenotypic analysis of the endogenous mechanosensors, the quantity of MscS superfamily members in the genome range from 2 to 6 and all of the strains contain a copy of MscL. Here, we show an in-depth analysis of over 150 diverse bacterial genomes, encompassing nine phyla, to determine the number of genomes that contain an MscL homologue and the average number of MscS superfamily members per genome. We determined that the average genome contains 4 ± 3 MscS homologues and 67% of bacterial genomes encode for a MscL homologue.

In-Lipid Structure of Pressure-Sensitive Domains Hints Mechanosensitive Channel Functional Diversity

The mechanosensitive channel of large conductance (MscL) from Mycobacterium tuberculosis has been used as a structural model for rationalizing functional observations in multiple MscL orthologs. Although these orthologs adopt similar structural architectures, they reportedly present significant functional differences. Subtle structural discrepancies on mechanosensitive channel nanopockets are known to affect mechanical gating and may be linked to large variability in tension sensitivity among these membrane channels. Here, we modify the nanopocket regions of MscL from Escherichia coli and M. tuberculosis and employ PELDOR/DEER distance and 3pESEEM deuterium accessibility measurements to interrogate channel structure within lipids, in which both channels adopt a closed conformation. Significant in-lipid structural differences between the two constructs suggest a more compact E. coli MscL at the membrane inner-leaflet, as a consequence of a rotated TM2 helix. Observed differences within lipids could explain E. coli MscL’s higher tension sensitivity and should be taken into account in extrapolated models used for MscL gating rationalization.

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.

An agonist of the MscL channel affects multiple bacterial species and increases membrane permeability and potency of common antibiotics

The bacterial MscL channel normally functions as an emergency release valve discharging cytoplasmic solutes upon osmotic stress. The channel opens and passes molecules up to 30 Å and its pore is the largest of any gated channel. Opening the MscL pore inappropriately is detrimental to the bacterial cell, suggesting MscL as a potential novel drug target. A small-molecule compound, 011A, has been shown to increase sensitivity of the Escherichia coli MscL channel, slow growth, and even decrease viability of quiescent cultures. The mscL gene is highly conserved and found in the vast majority of bacterial species, including pathogens. Here, we test the hypothesis that 011A can influence the growth and viability of other bacterial species, specifically Staphylococcus aureus and Mycobacterium smegmatis, in a MscL-dependent manner. Furthermore, we demonstrate that the 011A compound can increase potency of other antibiotics, presumably by permeabilizing the membrane and allowing easier access of the antibiotic into the cytoplasm. Thus, MscL activators have potential as novel broad-spectrum antibiotics or adjuvants that work with antibiotics to selectively allow passage across bacterial membranes.

Bacterial Mechanosensitive Channels

Mechanosensitive (MS) channels protect bacteria against hypo-osmotic shock and fulfil additional functions. Hypo-osmotic shock leads to high turgor pressure that can cause cell rupture and death. MS channels open under these conditions and release unspecifically solutes and consequently the turgor pressure. They can recognise the raised pressure via the increased tension in the cell membrane. Currently, a better understanding how MS channels can sense tension on molecular level is developing because the interaction of the lipid bilayer with the channel is being investigated in detail. The MS channel of large conductance (MscL) and of small conductance (MscS) have been distinguished and studied in molecular detail. In addition, larger channels were found that contain a homologous region corresponding to MscS so that MscS represents a family of channels. Often several members of this family are present in a species. The importance of this family is underlined by the fact that members can be found not only in bacteria but also in higher organisms. While MscL and MscS have been studied for years in particular by electrophysiology, mutagenesis, molecular dynamics, X-ray crystallography and other biophysical techniques, only recently more details are emerging about other members of the MscS-family.

Stress Physiology of Lactic Acid Bacteria

Lactic acid bacteria (LAB) are important starter, commensal, or pathogenic microorganisms. The stress physiology of LAB has been studied in depth for over 2 decades, fueled mostly by the technological implications of LAB robustness in the food industry. Survival of probiotic LAB in the host and the potential relatedness of LAB virulence to their stress resilience have intensified interest in the field. Thus, a wealth of information concerning stress responses exists today for strains as diverse as starter (e.g., Lactococcus lactis), probiotic (e.g., several Lactobacillus spp.), and pathogenic (e.g., Enterococcus and Streptococcus spp.) LAB. Here we present the state of the art for LAB stress behavior. We describe the multitude of stresses that LAB are confronted with, and we present the experimental context used to study the stress responses of LAB, focusing on adaptation, habituation, and cross-protection as well as on self-induced multistress resistance in stationary phase, biofilms, and dormancy. We also consider stress responses at the population and single-cell levels. Subsequently, we concentrate on the stress defense mechanisms that have been reported to date, grouping them according to their direct participation in preserving cell energy, defending macromolecules, and protecting the cell envelope. Stress-induced responses of probiotic LAB and commensal/pathogenic LAB are highlighted separately due to the complexity of the peculiar multistress conditions to which these bacteria are subjected in their hosts. Induction of prophages under environmental stresses is then discussed. Finally, we present systems-based strategies to characterize the "stressome" of LAB and to engineer new food-related and probiotic LAB with improved stress tolerance.

Mechanisms of mechanosensing - mechanosensitive channels, function and re-engineering

Sensing and responding to mechanical stimuli is an ancient behavior and ubiquitous to all forms of life. One of its players 'mechanosensitive ion channels' are involved in processes from osmosensing in bacteria to pain in humans. However, the mechanism of mechanosensing is yet to be elucidated. This review describes recent developments in the understanding of a bacterial mechanosensitive channel. Force from the lipid principle of mechanosensation, new methods to understand protein-lipid interactions, the role of water in the gating, the use of engineered mechanosensitive channels in the understanding of the gating mechanism and application of the accumulated knowledge in the field of drug delivery, drug design and sensor technologies are discussed.

The activation mode of the mechanosensitive ion channel, MscL, by lysophosphatidylcholine differs from tension-induced gating

One of the best-studied mechanosensitive channels is the mechanosensitive channel of large conductance (MscL). MscL senses tension in the membrane evoked by an osmotic down shock and directly couples it to large conformational changes leading to the opening of the channel. Spectroscopic techniques offer unique possibilities to monitor these conformational changes if it were possible to generate tension in the lipid bilayer, the native environment of MscL, during the measurements. To this end, asymmetric insertion of l-α-lysophosphatidylcholine (LPC) into the lipid bilayer has been effective; however, how LPC activates MscL is not fully understood. Here, the effects of LPC on tension-sensitive mutants of a bacterial MscL and on MscL homologs with different tension sensitivities are reported, leading to the conclusion that the mode of action of LPC is different from that of applied tension. Our results imply that LPC shifts the free energy of gating by interfering with MscL-membrane coupling. Furthermore, we demonstrate that the fine-tuned addition of LPC can be used for controlled activation of MscL in spectroscopic studies.

Bacterial mechanosensitive channels: progress towards an understanding of their roles in cell physiology

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Multiple mechanosensitive channels are found in most bacteria and archaea.

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Channels are required to prevent loss of structural integrity during transitions from high to low osmolarity.

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Channel diversity feeds into the detailed response of cells to hypo-osmotic stress.

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There is growing evidence that organisms have evolved MS channels that reflect their niche.

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Structural diversity may reflect roles additional to the observed function of protection of structural integrity.

Bacterial mechanosensitive channels sense the changes in lateral tension in the bilayer of the cytoplasmic membrane generated by rapid water flow into the cell. Two major structural families are found widely distributed across bacteria and archaea: MscL and MscS. Our understanding of the mechanisms of gating has advanced rapidly through genetic analysis, structural biology and electrophysiology. It is only recently that the analysis of the physiological roles of the channels has kept pace with mechanistic studies. Recent advances have increased our understanding of the role of the channels in preventing structural perturbation during osmotic transitions and its relationship to water flow across the membrane. It is to these recent developments that this review is dedicated.

Mechanosensitive channels and bacterial cell wall integrity: does life end with a bang or a whimper?

Mechanogated channels are fundamental components of bacterial cells that enable retention of physical integrity during extreme increases in cell turgor. Optical tweezers combined with microfluidics have been used to study the fate of individual Escherichia coli cells lacking such channels when subjected to a bursting stress caused by increased turgor. Fluorescence-activated cell sorting and electron microscopy complement these studies. These analyses show that lysis occurs with a high probability, but the precise path differs between individual cells. By monitoring the loss of cytoplasmic green fluorescent protein, we have determined that some cells release this protein but remain phase dark (granular) consistent with the retention of the majority of large proteins. By contrast, most cells suffer cataclysmic wall failure leading to loss of granularity but with the retention of DNA and overall cell shape (protein-depleted ghosts). The time span of these events induced by hypo-osmotic shock varies but is of the order of milliseconds. The data are interpreted in terms of the timing of mechanosensitive channel gating relative to osmotically induced water influx.

Expanding the diversity of oenococcal bacteriophages: insights into a novel group based on the integrase sequence

Temperate bacteriophages are a contributor of the genetic diversity in the lactic acid bacterium Oenococcus oeni. We used a classification scheme for oenococcal prophages based on integrase gene polymorphism, to analyze a collection of Oenococcus strains mostly isolated in the area of Bordeaux, which represented the major lineages identified through MLST schemes in the species. Genome sequences of oenococcal prophages were clustered into four integrase groups (A to D) which were related to the chromosomal integration site. The prevalence of each group was determined and we could show that members of the intB- and intC-prophage groups were rare in our panel of strains. Our study focused on the so far uncharacterized members of the intD-group. Various intD viruses could be easily isolated from wine samples, while intD lysogens could be induced to produce phages active against two permissive O. oeni isolates. These data support the role of this prophage group in the biology of O. oeni. Global alignment of three relevant intD-prophages revealed significant conservation and highlighted a number of unique ORFs that may contribute to phage and lysogen fitness.

Chimeras reveal a single lipid-interface residue that controls MscL channel kinetics as well as mechanosensitivity

MscL, the highly conserved bacterial mechanosensitive channel of large conductance, serves as an osmotic "emergency release valve," is among the best-studied mechanosensors, and is a paradigm of how a channel senses and responds to membrane tension. Although all homologs tested thus far encode channel activity, many show functional differences. We tested Escherichia coli and Staphylococcus aureus chimeras and found that the periplasmic region of the protein, particularly E. coli I49 and the equivalent S. aureus F47 at the periplasmic lipid-aqueous interface of the first transmembrane domain, drastically influences both the open dwell time and the threshold of channel opening. One mutant shows a severe hysteresis, confirming the importance of this residue in determining the energy barriers for channel gating. We propose that this site acts similarly to a spring for a clasp knife, adjusting the resistance for obtaining and stabilizing an open or closed channel structure.

Functional analysis of conserved motifs in the mechanosensitive channel homolog MscS-Like2 from Arabidopsis thaliana

The Mechanosensitive channel of Small conductance (MscS) of Escherichia coli has become an excellent model system for the structural, biophysical, and functional study of mechanosensitive ion channels. MscS, a complex channel with multiple states, contributes to protection against lysis upon osmotic downshock. MscS homologs are widely and abundantly dispersed among the bacterial and plant lineages, but are not found in animals. Investigation into the eukaryotic branch of the MscS family is in the beginning stages, and it remains unclear how much MscS homologs from eukaryotes resemble E. coli MscS with respect to structure, function, and regulation. Here we test the effect of mutating three conserved motifs on the function of MscS-Like (MSL)2, a MscS homolog localized to the plastids of Arabidopsis thaliana. We show that 1) a motif at the top of the cytoplasmic domain, referred to here as the PN(X)₉N motif, is essential for MSL2 function and for its proper intraplastidic localization; 2) substituting polar residues for two large hydrophobic residues located in the predicted pore-lining transmembrane helix of MSL2 produces a likely gain-of-function allele, as previously shown for MscS; and 3) mis-expression of this allele causes severe defects in leaf growth, loss of chloroplast integrity, and abnormal starch accumulation. Thus, two of the three conserved motifs we analyzed are critical for MSL2 function, consistent with the conservation of structure and function between MscS family members in bacteria and plants. These results underscore the importance of plastidic mechanosensitive channels in the maintenance of normal plastid and leaf morphology.

The MscS and MscL families of mechanosensitive channels act as microbial emergency release valves

Single-celled organisms must survive exposure to environmental extremes. Perhaps one of the most variable and potentially life-threatening changes that can occur is that of a rapid and acute decrease in external osmolarity. This easily translates into several atmospheres of additional pressure that can build up within the cell. Without a protective mechanism against such pressures, the cell will lyse. Hence, most microbes appear to possess members of one or both families of bacterial mechanosensitive channels, MscS and MscL, which can act as biological emergency release valves that allow cytoplasmic solutes to be jettisoned rapidly from the cell. While this is undoubtedly a function of these proteins, the discovery of the presence of MscS homologues in plant organelles and MscL in fungus and mycoplasma genomes may complicate this simplistic interpretation of the physiology underlying these proteins. Here we compare and contrast these two mechanosensitive channel families, discuss their potential physiological roles, and review some of the most relevant data that underlie the current models for their structure and function.

Genetic complementation of the obligate marine actinobacterium Salinispora tropica with the large mechanosensitive channel gene mscL rescues cells from osmotic downshock

Marine actinomycetes in the genus Salinispora fail to grow when seawater is replaced with deionized (DI) water in complex growth media. While bioinformatic analyses have led to the identification of a number of candidate marine adaptation genes, there is currently no experimental evidence to support the genetic basis for the osmotic requirements associated with this taxon. One hypothesis is that the lineage-specific loss of mscL is responsible for the failure of strains to grow in media prepared with DI water. The mscL gene encodes a conserved transmembrane protein that reduces turgor pressure under conditions of acute osmotic downshock. In the present study, the mscL gene from a Micromonospora strain capable of growth on media prepared with DI water was transformed into S. tropica strain CNB-440. The single-copy, chromosomal genetic complementation yielded a recombinant Salinispora mscL(+) strain that demonstrated an increased capacity to survive osmotic downshock. The enhanced survival of the S. tropica transformant provides experimental evidence that the loss of mscL is associated with the failure of Salinispora spp. to grow in low-osmotic-strength media.

Engineering trehalose synthesis in Lactococcus lactis for improved stress tolerance

Trehalose accumulation is a common cell defense strategy against a variety of stressful conditions. In particular, our team detected high levels of trehalose in Propionibacterium freudenreichii in response to acid stress, a result that led to the idea that endowing Lactococcus lactis with the capacity to synthesize trehalose could improve the acid tolerance of this organism. To this end, we took advantage of the endogenous genes involved in the trehalose catabolic pathway of L. lactis, i.e., trePP and pgmB, encoding trehalose 6-phosphate phosphorylase and β-phosphoglucomutase, respectively, which enabled the synthesis of trehalose 6-phosphate. Given that L. lactis lacks trehalose 6-phosphate phosphatase, the respective gene, otsB, from the food-grade organism P. freudenreichii was used to provide the required activity. The trehalose yield was approximately 15% in resting cells and in mid-exponential-phase cells grown without pH control. The intracellular concentration of trehalose reached maximal values of approximately 170 mM, but at least 67% of the trehalose produced was found in the growth medium. The viability of mutant and control strains was examined after exposure to heat, cold or acid shock, and freeze-drying. The trehalose-producing strains showed improved tolerance (5- to 10-fold-higher survivability) to acid (pH 3) and cold shock (4°C); there was also a strong improvement in cell survival in response to heat shock (45°C), and no protection was rendered against dehydration. The insight provided by this work may help the design of food-grade strains optimized for the dairy industry as well as for oral drug delivery.

Cloning and functional expression of an MscL ortholog from Rhizobium etli: characterization of a mechanosensitive channel

Rhizobium etli is equipped with several systems to handle both hyper- and hypo-osmotic stress. For adaptation to hypo-osmotic stress, R. etli possesses a single gene with clear homology to MscS, four MscS-like channels and one ortholog of MscL (ReMscL, identity approximately 44% compared to Escherichia coli MscL). We subcloned and expressed the ReMscL channel ortholog from R. etli in E. coli to examine its activity by patch clamp in giant spheroplasts and characterized it at the single-channel level. We obtained evidence that ReMscL prevents the lysis of E. coli null mutant log-phase cells upon a rapid, osmotic downshock and identified a slight pH dependence for ReMscL activation. Here, we describe the facilitation of ReMscL activation by arachidonic acid (AA) and a reversible inhibitory effect of Gd(3+). The results obtained in these experiments suggest a stabilizing effect of micromolar AA and traces of Gd(3+) ions in the partially expanded conformation of the protein. Finally, we discuss a possible correlation between the number of gene paralogs for MS channels and the habitats of several microorganisms. Taken together, our data show that ReMscL may play an important role in free-living rhizobacteria during hypo-osmotic shock in the rhizosphere.

Levels and localization of mechanosensitive channel proteins in Bacillus subtilis

Log phase Bacillus subtilis cells lacking the mscL gene encoding the mechanosensitive (MS) channel of large conductance are sensitive to an osmotic downshock > or =0.5 M. However, B. subtilis mscL cells develop osmotic downshock resistance in late log and early stationary phase growth that is partially dependent on three likely MS channel proteins of small conductance (MscS), YfkC, YhdY, and YkuT. Bacillus subtilis MS proteins were fused with green fluorescent protein (GFP) at their C termini; at least the MscL-, YfkC-, and YkuT-GFP fusions were functional and overexpression of YkuT-GFP, or YkuT alone abolished log phase mscL cells' osmotic downshock sensitivity. Western blot analysis found high levels of MscL-GFP in early exponential phase cells with levels subsequently decreasing greatly. MscS-GFP proteins were present in exponential phase cells, but again disappeared almost completely in stationary phase cells and these proteins were not detected in spores. Western blot analyses further showed that MS-GFP proteins were associated with the plasma membrane, as expected. Fluorescence microscopy confirmed the localization of MscL-GFP and YhdY-GFP to the plasma membrane, with non-uniform distribution of these proteins along this membrane consistent with but by no means proving that these proteins are present in a helical array.

Responses of Bacillus subtilis to hypotonic challenges: physiological contributions of mechanosensitive channels to cellular survival

Mechanosensitive channels are thought to function as safety valves for the release of cytoplasmic solutes from cells that have to manage a rapid transition from high- to low-osmolarity environments. Subsequent to an osmotic down-shock of cells grown at high osmolarity, Bacillus subtilis rapidly releases the previously accumulated compatible solute glycine betaine in accordance with the degree of the osmotic downshift. Database searches suggest that B. subtilis possesses one copy of a gene for a mechanosensitive channel of large conductance (mscL) and three copies of genes encoding proteins that putatively form mechanosensitive channels of small conductance (yhdY, yfkC, and ykuT). Detailed mutational analysis of all potential channel-forming genes revealed that a quadruple mutant (mscL yhdY yfkC ykuT) has no growth disadvantage in high-osmolarity media in comparison to the wild type. Osmotic down-shock experiments demonstrated that the MscL channel is the principal solute release system of B. subtilis, and strains with a gene disruption in mscL exhibited a severe survival defect upon an osmotic down-shock. We also detected a minor contribution of the SigB-controlled putative MscS-type channel-forming protein YkuT to cellular survival in an mscL mutant. Taken together, our data revealed that mechanosensitive channels of both the MscL and MscS types play pivotal roles in managing the transition of B. subtilis from hyper- to hypo-osmotic environments.

Growth, osmotic downshock resistance and differentiation of Bacillus subtilis strains lacking mechanosensitive channels

Previous work has shown that the mechanosensitive (MS) channel of large conductance (MscL) is essential for preventing lysis of Bacillus subtilis log phase cells upon a rapid, severe osmotic downshock. Growing cells of B. subtilis strains lacking MscL and one or more putative MS channel proteins of small conductance (YhdY, YkuT and YfkC) showed even higher sensitivity to an osmotic downshock. The effect was greatest for a strain lacking MscL and YkuT, and a strain lacking all four MS channel proteins had a similar phenotype. These defects were complemented by expression of either MscL or YkuT in trans. All MS channel mutant strains ultimately became resistant to osmotic downshock in stationary phase but at varying times, with mscL ykuT strains taking the longest time to become resistant. Expression of beta-galactosidase from gene fusions to lacZ showed modest expression of ykuT and lower levels of expression of yhdY and yfkC when strains were grown in medium containing high salt. Sporulation of all MS channel mutant strains was normal, and the mutant spores germinated normally with L-alanine or dodecylamine.

Mechanosensitive channels in bacteria: signs of closure?

Bacterial mechanosensitive channels are activated by increases in tension in the lipid bilayer of the cytoplasmic membrane, where they transiently create large pores in a controlled manner. Mechanosensitive channel research has benefited from advances in electrophysiology, genomics and molecular genetics as well as from the application of biophysical techniques. Most recently, new analytical methods have been used to complement existing knowledge and generate insights into the molecular interactions that take place between mechanosensitive channel proteins and the surrounding membrane lipids. This article reviews the latest developments.

Transport capabilities of eleven gram-positive bacteria: comparative genomic analyses

The genomes of eleven Gram-positive bacteria that are important for human health and the food industry, nine low G+C lactic acid bacteria and two high G+C Gram-positive organisms, were analyzed for their complement of genes encoding transport proteins. Thirteen to 18% of their genes encode transport proteins, larger percentages than observed for most other bacteria. All of these bacteria possess channel proteins, some of which probably function to relieve osmotic stress. Amino acid uptake systems predominate over sugar and peptide cation symporters, and of the sugar uptake porters, those specific for oligosaccharides and glycosides often outnumber those for free sugars. About 10% of the total transport proteins are constituents of putative multidrug efflux pumps with Major Facilitator Superfamily (MFS)-type pumps (55%) being more prevalent than ATP-binding cassette (ABC)-type pumps (33%), which, however, usually greatly outnumber all other types. An exception to this generalization is Streptococcus thermophilus with 54% of its drug efflux pumps belonging to the ABC superfamily and 23% belonging each to the Multidrug/Oligosaccharide/Polysaccharide (MOP) superfamily and the MFS. These bacteria also display peptide efflux pumps that may function in intercellular signalling, and macromolecular efflux pumps, many of predictable specificities. Most of the bacteria analyzed have no pmf-coupled or transmembrane flow electron carriers. The one exception is Brevibacterium linens, which in addition to these carriers, also has transporters of several families not represented in the other ten bacteria examined. Comparisons with the genomes of organisms from other bacterial kingdoms revealed that lactic acid bacteria possess distinctive proportions of recognized transporter types (e.g., more porters specific for glycosides than reducing sugars). Some homologues of transporters identified had previously been identified only in Gram-negative bacteria or in eukaryotes. Our studies reveal unique characteristics of the lactic acid bacteria such as the universal presence of genes encoding mechanosensitive channels, competence systems and large numbers of sugar transporters of the phosphotransferase system. The analyses lead to important physiological predictions regarding the preferred signalling and metabolic activities of these industrially important bacteria.

Physiological analysis of bacterial mechanosensitive channels

Bacterial mechanosensitive (MS) channels play a significant role in protecting cells against hypoosmotic shock. Bacteria that have been diluted from high osmolarity medium into dilute solution are required to cope with sudden water influx associated with an osmotic imbalance equivalent to 10 to 14 atm. The cell wall is only poorly expansive and the cytoplasmic membrane even less so. Thus, swelling is not an option and the cell must rapidly eject solutes to diminish the osmotic gradient and thereby preserve structural integrity. This chapter describes cellular assays of MS channel function and their interpretation.

Dual-color fluorescence-burst analysis to probe protein efflux through the mechanosensitive channel MscL

The mechanosensitive channel protein of large conductance, MscL, from Escherichia coli has been implicated in protein efflux, but the passage of proteins through the channel has never been demonstrated. We used dual-color fluorescence-burst analysis to evaluate the efflux of fluorescent labeled compounds through MscL. The method correlates the fluctuations in intensity of fluorescent labeled membranes and encapsulated (macro)molecules (labeled with second fluorophore) for each liposome diffusing through the observation volume. The analysis provides quantitative information on the concentration of macromolecules inside the liposomes and the fraction of functional channel proteins. For MscL, reconstituted in large unilamellar vesicles, we show that insulin, bovine pancreas trypsin inhibitor, and other compounds smaller than 6.5 kDa can pass through MscL, whereas larger macromolecules cannot.

The synthesis and role of the mechanosensitive channel of large conductance in growth and differentiation of Bacillus subtilis

A translational lacZ fusion of the Bacillus subtilis mscL gene that encodes the mechanosensitive channel of large conductance (MscL) was expressed at significant levels during log phase growth of B. subtilis, and the level of mscL-lacZ expression was increased 1.5-fold by growth in medium with high salt (1 M NaCl). However, in growth media with either low or high salt, mscL-lacZ expression fell drastically beginning in the late log phase of growth, and fell to even lower levels during sporulation, although a significant amount of beta-galactosidase from mscL to lacZ was accumulated in the developing spore. Deletion of mscL had no effect on B. subtilis growth, sporulation or subsequent spore germination. The DeltamscL strain also grew as well as the wild-type parental strain in medium with 1.2 M NaCl. While log phase wild-type cells grown with 1.2 M NaCl survived a rapid 0.9 M osmotic downshift, log phase DeltamscL cells rapidly lost viability and lysed when subjected to this same osmotic downshift. However, by the early stationary phase of growth, DeltamscL cells had become resistant to a 0.9 M osmotic downshift.

Engineering covalent oligomers of the mechanosensitive channel of large conductance from Escherichia coli with native conductance and gating characteristics

To obtain a gene construct for making single substitutions per channel and to determine the quaternary structure of the mechanosensitive channel MscL from Escherichia coli, covalent oligomers (monomer to hexamer) were engineered by gene fusion; up to six copies of the mscL gene were fused in tandem. All the multimeric tandem constructs yielded functional channels with wild-type conductance and dwell times. Importantly, only the covalent pentamer opened at the same relative pressure (compared to the pressure required to open MscS) as the wild-type MscL channel. The in vivo data strongly suggest that pentameric MscL represents the functional state of the channel.