MscS, the bacterial mechanosensitive channel of small conductance

"Force-From-Lipids" mechanosensation in Corynebacterium glutamicum

Since the mechanosensitive channel MscCG has been identified as the major glutamate efflux system in Corynebacterium glutamicum, studies of mechanotransduction processes in this bacterium have helped to unpuzzle a long-unresolved mystery of glutamate efflux that has been utilised for industrial monosodium glutamate production. The patch clamp recording from C. glutamicum giant spheroplasts revealed the existence of three types of mechanosensitive (MS) channels in the cell membrane of this bacterium. The experiments demonstrated that the MS channels could be activated by membrane tension, indicating that the channel gating by mechanical force followed the "Force-From-Lipids (FFL)" principle characteristic of ion channels inherently sensitive to transbilayer pressure profile changes in the mechanically stressed membrane bilayer. Mechanical properties of the C. glutamicum membrane are characteristics of very soft membranes, which in the C. glutamicum membrane are due to negatively charged lipids as its exclusive constituents. Given that membrane lipids are significantly altered during the fermentation process in the monosodium glutamate production, MS channels seem to respond to changes in force transmission through the membrane bilayer due to membrane lipid dynamics. In this review, we describe the recent results describing corynebacterial FFL-dependent mechanosensation originating from the particular lipid composition of the C. glutamicum membrane and unique structure of MscCG-type channels.

Identification and Characterization of the Neisseria gonorrhoeae MscS-Like Mechanosensitive Channel

Mechanosensitive channels are ubiquitous in bacteria and provide an essential mechanism to survive sudden exposure to a hypo-osmotic environment by the sensing and release of increased turgor pressure. No mechanosensitive channels have thus far been identified and characterized for the human-specific bacterial pathogen Neisseria gonorrhoeae In this study, we identified and characterized the N. gonorrhoeae MscS-like mechanosensitive channel (Ng-MscS). Electrophysiological analyses by the patch clamp method showed that Ng-MscS is stretch activated and contains pressure-dependent gating properties. Further mutagenesis studies of critical residues forming the hydrophobic vapor lock showed that gain-of-function mutations in Ng-MscS inhibited bacterial growth. Subsequent analysis of the function of Ng-MscS in N. gonorrhoeae by osmotic down-shock assays revealed that the survival of Ng-mscS deletion mutants was significantly reduced compared with that of wild-type strains, while down-shock survival was restored upon the ectopic complementation of mscS Finally, to investigate whether Ng-MscS is important for N. gonorrhoeae during infections, competition assays were performed by using a murine vaginal tract infection model. Ng-mscS deletion mutants were outcompeted by N. gonorrhoeae wild-type strains for colonization and survival in this infection model, highlighting that Ng-MscS contributes to in vivo colonization and survival. Therefore, Ng-MscS might be a promising target for the future development of novel antimicrobials.

Lipid-protein interactions: Lessons learned from stress

Biological membranes are essential for normal function and regulation of cells, forming a physical barrier between extracellular and intracellular space and cellular compartments. These physical barriers are subject to mechanical stresses. As a consequence, nature has developed proteins that are able to transpose mechanical stimuli into meaningful intracellular signals. These proteins, termed Mechanosensitive (MS) proteins provide a variety of roles in response to these stimuli. In prokaryotes these proteins form transmembrane spanning channels that function as osmotically activated nanovalves to prevent cell lysis by hypoosmotic shock. In eukaryotes, the function of MS proteins is more diverse and includes physiological processes such as touch, pain and hearing. The transmembrane portion of these channels is influenced by the physical properties such as charge, shape, thickness and stiffness of the lipid bilayer surrounding it, as well as the bilayer pressure profile. In this review we provide an overview of the progress to date on advances in our understanding of the intimate biophysical and chemical interactions between the lipid bilayer and mechanosensitive membrane channels, focusing on current progress in both eukaryotic and prokaryotic systems. These advances are of importance due to the increasing evidence of the role the MS channels play in disease, such as xerocytosis, muscular dystrophy and cardiac hypertrophy. Moreover, insights gained from lipid-protein interactions of MS channels are likely relevant not only to this class of membrane proteins, but other bilayer embedded proteins as well. This article is part of a Special Issue entitled: Lipid-protein interactions.

The evolutionary 'tinkering' of MscS-like channels: generation of structural and functional diversity

The mechanosensitive channel of small conductance (MscS)-like channel superfamily is present in cell-walled organisms throughout all domains of life (Bacteria, Archaea and Eukarya). In bacteria, members of this channel family play an integral role in the protection of cells against acute downward shifts in environmental osmolarity. In this review, we discuss how evolutionary 'tinkering' has taken MscS-like channels from their currently accepted physiological function in bacterial osmoregulation to potential roles in processes as diverse as amino acid efflux, Ca(2+) regulation and cell division. We also illustrate how this structurally and functionally diverse family of channels represents an essential industrial component in the production of monosodium glutamate, an attractive antibiotic target and a rich source of membrane proteins for the studies of molecular evolution.

Plant mechanosensing and Ca2+ transport

Mechanical stimuli generate Ca(2+) signals and influence growth and development in plants. Recently, candidates for Ca(2+)-permeable mechanosensitive (MS) channels have been identified. These channels are thought to be responsible for sensing osmotic shock, touch, and gravity. One candidate is the MscS-like (MSL) protein family, a homolog of the typical bacterial MS channels. Some of the MSL proteins are localized to plastids to maintain their shape and size. Another candidate is the mid1-complementing activity (MCA) protein family, which is structurally unique to the plant kingdom. MCA proteins are localized in the plasma membrane and are suggested to be involved in mechanosensing and to be functionally related to reactive oxygen species (ROS) signaling. Here, we review their structural features and role in planta.

Controlled delivery of bioactive molecules into live cells using the bacterial mechanosensitive channel MscL

Bacterial mechanosensitive channels are some of the largest pores in nature. In particular, MscL, with a pore diameter >25 Å, allows passage of large organic ions and small proteins. Functional MscL reconstitution into lipids has been proposed for applications in vesicular-based drug release. Here we show that these channels can be functionally expressed in mammalian cells to afford rapid controlled uptake of membrane-impermeable molecules. We first demonstrate that MscL gating in response to increased membrane tension is preserved in mammalian cell membranes. Molecular delivery is controlled by adopting an established method of MscL charge-induced activation. We then determine pore size limitations using fluorescently labelled model cargoes. Finally, we activate MscL to introduce the cell-impermeable bi-cyclic peptide phalloidin, a specific marker for actin filaments, into cells. We propose that MscL will be a useful tool for gated and controlled delivery of bioactive molecules into cells.

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 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.

Defining the role of the tension sensor in the mechanosensitive channel of small conductance

Mutations that alter the phenotypic behavior of the Escherichia coli mechanosensitive channel of small conductance (MscS) have been identified; however, most of these residues play critical roles in the transition between the closed and open states of the channel and are not directly involved in lipid interactions that transduce the tension response. In this study, we use molecular dynamic simulations to predict critical lipid interacting residues in the closed state of MscS. The physiological role of these residues was then investigated by performing osmotic downshock assays on MscS mutants where the lipid interacting residues were mutated to alanine. These experiments identified seven residues in the first and second transmembrane helices as lipid-sensing residues. The majority of these residues are hydrophobic amino acids located near the extracellular interface of the membrane. All of these residues interact strongly with the lipid bilayer in the closed state of MscS, but do not face the bilayer directly in structures associated with the open and desensitized states of the channel. Thus, the position of these residues relative to the lipid membrane appears related to the ability of the channel to sense tension in its different physiological states.

Bacterial community assembly based on functional genes rather than species

The principles underlying the assembly and structure of complex microbial communities are an issue of long-standing concern to the field of microbial ecology. We previously analyzed the community membership of bacterial communities associated with the green macroalga Ulva australis, and proposed a competitive lottery model for colonization of the algal surface in an attempt to explain the surprising lack of similarity in species composition across different algal samples. Here we extend the previous study by investigating the link between community structure and function in these communities, using metagenomic sequence analysis. Despite the high phylogenetic variability in microbial species composition on different U. australis (only 15% similarity between samples), similarity in functional composition was high (70%), and a core of functional genes present across all algal-associated communities was identified that were consistent with the ecology of surface- and host-associated bacteria. These functions were distributed widely across a variety of taxa or phylogenetic groups. This observation of similarity in habitat (niche) use with respect to functional genes, but not species, together with the relative ease with which bacteria share genetic material, suggests that the key level at which to address the assembly and structure of bacterial communities may not be "species" (by means of rRNA taxonomy), but rather the more functional level of genes.

Coastal strains of marine Synechococcus species exhibit increased tolerance to copper shock and a distinctive transcriptional response relative to those of open-ocean strains

Copper appears to be influencing the distribution and abundance of phytoplankton in marine environments, and cyanobacteria are thought to be the most sensitive of the phytoplankton groups to copper toxicity. By using growth assays of phylogenetically divergent clades, we found that coastal strains of marine Synechococcus species were more tolerant to copper shock than open-ocean strains. The global transcriptional response to two levels of copper shock were determined for both a coastal strain and an open-ocean strain of marine Synechococcus species using whole-genome expression microarrays. Both strains showed an osmoregulatory-like response, perhaps as a result of increasing membrane permeability. This could have implications for marine carbon cycling if copper shock leads to dissolved organic carbon leakage in Synechococcus species. The two strains additionally showed a common reduction in levels of photosynthesis-related gene transcripts. Contrastingly, the open-ocean strain showed a general stress response, whereas the coastal strain exhibited a more specifically oxidative or heavy-metal acclimation response that may be conferring tolerance. In addition, the coastal strain activated more regulatory elements and transporters, many of which are not conserved in other marine Synechococcus strains and may have been acquired by horizontal gene transfer. Thus, tolerance to copper shock in some marine Synechococcus strains may in part be a result of a generally increased ability to sense and respond in a more stress-specific manner.

Mechanical force and cytoplasmic Ca(2+) activate yeast TRPY1 in parallel

The ability to sense mechanical and osmotic stimuli is vital to all organisms from mammals to bacteria. Members of the transient receptor potential (TRP) ion-channel family have attracted intense attention for their involvement in mechanosensation. The yeast homologue TRPY1 can clearly be activated by hypertonic shock in vivo and by stretch force under patch clamp. Like its animal counterparts, TRPY1 is polymodal, being gated by membrane stretch force and by cytoplasmic Ca(2+). Here, we investigated how these two gating principles interact. We found that stretch force can induce some channel activation without cytoplasmic Ca(2+). Tens of micromolar Ca(2+) greatly enhance the observed force-induced activities, with open probabilities following well the Boltzmann distribution, in which the two gating energies are summed as exponents. To map this formalism to structures, we found Ca(2+)-binding proteins such as calmodulin or calcineurin to be unnecessary. However, removing a dense cluster of negative charges in the C-terminal cytoplasmic domain of TRPY1 greatly diminishes the Ca(2+) activation as well as its influence on force activation. We also found a strategic point upstream of this charge cluster, at which insertion of amino acids weakens Ca(2+) activation considerably but leaves the mechanosensitivity nearly intact. These results led to a structure-function model in which Ca(2+) binding to the cytoplasmic domain and stretching of the membrane-embedded domain both generate gating force, reaching the gate in parallel.

Insight into the proteome of the hyperthermophilic Crenarchaeon Ignicoccus hospitalis: the major cytosolic and membrane proteins

Ignicoccus hospitalis, a hyperthermophilic, chemolithoautotrophic Crenarchaeon, is the host of Nanoarchaeum equitans. Together, they form an intimate association, the first among Archaea. Membranes are of fundamental importance for the interaction of I. hospitalis and N. equitans, as they harbour the proteins necessary for the transport of macromolecules like lipids, amino acids, and cofactors between these organisms. Here, we investigated the protein inventory of I. hospitalis cells, and were able to identify 20 proteins in total. Experimental evidence and predictions let us conclude that 11 are soluble cytosolic proteins, eight membrane or membrane-associated proteins, and a single one extracellular. The quantitatively dominating proteins in the cytoplasm (peroxiredoxin; thermosome) antagonize oxidative and temperature stress which I. hospitalis cells are exposed to at optimal growth conditions. Three abundant membrane protein complexes are found: the major protein of the outer membrane, which might protect the cell against the hostile environment, forms oligomeric complexes with pores of unknown selectivity; two other complexes of the cytoplasmic membrane, the hydrogenase and the ATP synthase, play a key role in energy production and conversion.