Mechanical coupling of the multiple structural elements of the large-conductance mechanosensitive channel during expansion

An efficient method for detecting membrane protein oligomerization and complex using 05SAR-PAGE

Oligomerization is an important feature of proteins, which gives a defined quaternary structure to complete the biological functions. Although frequently observed in membrane proteins, characterizing the oligomerization state remains complicated and time-consuming. In this study, 0.05% (w/v) sarkosyl-polyacrylamide gel electrophoresis (05SAR-PAGE) was used to identify the oligomer states of the membrane proteins CpxA, EnvZ, and Ma-Mscl with high sensitivity. Furthermore, two-dimensional electrophoresis (05SAR/sodium dodecyl sulfate-PAGE) combined with western blotting and liquid chromatography-tandem mass spectrometry was successfully applied to study the complex of CpxA/OmpA in cell lysate. The results indicated that 05SAR-PAGE is an efficient, economical, and practical gel method that can be widely used for the identification of membrane protein oligomerization and the analysis of weak protein interactions.

Mechanosensitive channel MscL gating transitions coupling with constriction point shift

The mechanosensitive channel of large conductance (MscL) acts as an "emergency release valve" that protects bacterial cells from acute hypoosmotic stress, and it serves as a paradigm for studying the mechanism underlying the transduction of mechanical forces. MscL gating is proposed to initiate with an expansion without opening, followed by subsequent pore opening via a number of intermediate substates, and ends in a full opening. However, the details of gating process are still largely unknown. Using in vivo viability assay, single channel patch clamp recording, cysteine cross-linking, and tryptophan fluorescence quenching approach, we identified and characterized MscL mutants with different occupancies of constriction region in the pore domain. The results demonstrated the shifts of constriction point along the gating pathway towards cytoplasic side from residue G26, though G22, to L19 upon gating, indicating the closed-expanded transitions coupling of the expansion of tightly packed hydrophobic constriction region to conduct the initial ion permeation in response to the membrane tension. Furthermore, these transitions were regulated by the hydrophobic and lipidic interaction with the constricting "hot spots". Our data reveal a new resolution of the transitions from the closed to the opening substate of MscL, providing insights into the gating mechanisms of MscL.

Membrane Protein Structures in Native Cellular Membranes Revealed by Solid-State NMR Spectroscopy

The structural characterization of membrane proteins within the cellular membrane environment is critical for understanding the molecular mechanism in their native functional context. However, conducting residue site-specific structural analysis of membrane proteins in native membranes by solid-state NMR faces challenges due to poor spectral sensitivity and serious interference from background protein signals. In this study, we present a new protocol that combines various strategies for cellular membrane sample preparations, enabling us to reveal the secondary structure of the mechanosensitive channel of large conductance from Methanosarcina acetivorans (MaMscL) in Escherichia coli inner membranes. Our findings demonstrate the feasibility of achieving complete resonance assignments and the potential for determining the 3D structures of membrane proteins within cellular membranes. We find that the use of the BL21(DE3) strain in this protocol is crucial for effectively suppressing background protein labeling without compromising the sensitivity of the target protein. Furthermore, our data reveal that the structures of different proteins exhibit varying degrees of sensitivity to the membrane environment. These results underscore the significance of studying membrane proteins within their native cellular membranes when performing structural characterizations. Overall, this study opens up a new avenue for achieving the atomic-resolution structural characterization of membrane proteins within their native cellular membranes, providing valuable insights into the nativeness of membrane proteins.

Solid-state NMR structure determination of a membrane protein in E. coli cellular inner membrane

Structure determination of membrane proteins in native cellular membranes is critical to precisely reveal their structures in physiological conditions. However, it remains challenging for solid-state nuclear magnetic resonance (ssNMR) due to the low sensitivity and high complexity of ssNMR spectra of cellular membranes. Here, we present the structure determination of aquaporin Z (AqpZ) by ssNMR in Escherichia coli inner membranes. To enhance the signal sensitivity of AqpZ, we optimized protein overexpression and removed outer membrane components. To suppress the interference of background proteins, we used a "dual-media" expression approach and antibiotic treatment. Using 1017 distance restraints obtained from two-dimensional 13C-13C spectra based on the complete chemical shift assignments, the 1.7-Å ssNMR structure of AqpZ is determined in E. coli inner membranes. This cellular ssNMR structure determination paves the way for analyzing the atomic structural details for membrane proteins in native cellular membranes.

Approaches for the modulation of mechanosensitive MscL channel pores

MscL was the first mechanosensitive ion channel identified in bacteria. The channel opens its large pore when the turgor pressure of the cytoplasm increases close to the lytic limit of the cellular membrane. Despite their ubiquity across organisms, their importance in biological processes, and the likelihood that they are one of the oldest mechanisms of sensory activation in cells, the exact molecular mechanism by which these channels sense changes in lateral tension is not fully understood. Modulation of the channel has been key to understanding important aspects of the structure and function of MscL, but a lack of molecular triggers of these channels hindered early developments in the field. Initial attempts to activate mechanosensitive channels and stabilize functionally relevant expanded or open states relied on mutations and associated post-translational modifications that were often cysteine reactive. These sulfhydryl reagents positioned at key residues have allowed the engineering of MscL channels for biotechnological purposes. Other studies have modulated MscL by altering membrane properties, such as lipid composition and physical properties. More recently, a variety of structurally distinct agonists have been shown bind to MscL directly, close to a transmembrane pocket that has been shown to have an important role in channel mechanical gating. These agonists have the potential to be developed further into antimicrobial therapies that target MscL, by considering the structural landscape and properties of these pockets.

Dynamic Interactions Between Brilliant Green and MscL Investigated by Solid-State NMR Spectroscopy and Molecular Dynamics Simulations

The mechanosensitive ion channel of large conductance (MscL) is a promising template for the development of new antibiotics due to its high conservation and uniqueness to microbes. Brilliant green (BG), a triarylmethane dye, has been identified as a new antibiotic targeted MscL. However, the detailed binding sites to MscL and the dynamic pathway of BG through the MscL channel remain unknown. Here, the dynamic interactions between BG and MscL were investigated using solid-state NMR spectroscopy and molecule dynamics (MD) simulations. Residue site-specific binding sites of BG to the MscL channel were identified by solid-state NMR. In addition, MD simulations revealed that BG conducts through the MscL channel via residues along the inner surface of the pore sequentially, in which the strong hydrophobic interactions between BG and hydrophobic residues F23 and I27 in the hydrophobic gate region of the MscL channel are major restrictions. Particularly, it was demonstrated that BG activates the MscL channel by reducing the hydrophobicity of the F23 in the gate region by water molecules that are bound to BG. Taken together, these simulations and experimental data provide novel insights into the dynamic interactions between BG and MscL, based on which new hydrophobic antibiotics and adjuvants targeting MscL can be developed.

Activation of a Bacterial Mechanosensitive Channel, MscL, Underlies the Membrane Permeabilization of Dual-Targeting Antibacterial Compounds

Resistance to antibiotics is a serious and worsening threat to human health worldwide, and there is an urgent need to develop new antibiotics that can avert it. One possible solution is the development of compounds that possess multiple modes of action, requiring at least two mutations to acquire resistance. Compound SCH-79797 both avoids resistance and has two mechanisms of action: one inhibiting the folate pathway, and a second described as “membrane permeabilization”; however, the mechanism by which membranes from bacterial cells, but not the host, are disrupted has remained mysterious. The opening of the bacterial mechanosensitive channel of large conductance, MscL, which ordinarily serves the physiological role of osmotic emergency release valves gated by hypoosmotic shock, has been previously demonstrated to affect bacterial membrane permeabilization. MscL allows the rapid permeabilization of both ions and solutes through the opening of the largest known gated pore, which has a diameter of 30 Å. We found that SCH-79797 and IRS-16, a more potent derivative, directly bind to the MscL channel and produce membrane permeabilization as a result of its activation. These findings suggest that possessing or adding an MscL-activating component to an antibiotic compound could help to lower toxicity and evade antibiotic resistance.

Pocket delipidation induced by membrane tension or modification leads to a structurally analogous mechanosensitive channel state

The mechanosensitive ion channel of large conductance MscL gates in response to membrane tension changes. Lipid removal from transmembrane pockets leads to a concerted structural and functional MscL response, but it remains unknown whether there is a correlation between the tension-mediated state and the state derived by pocket delipidation in the absence of tension. Here, we combined pulsed electron paramagnetic resonance spectroscopy and hydrogen-deuterium exchange mass spectrometry, coupled with molecular dynamics simulations under membrane tension, to investigate the structural changes associated with the distinctively derived states. Whether it is tension- or modification-mediated pocket delipidation, we find that MscL samples a similar expanded subconducting state. This is the final step of the delipidation pathway, but only an intermediate stop on the tension-mediated path, with additional tension triggering further channel opening. Our findings hint at synergistic modes of regulation by lipid molecules in membrane tension-activated mechanosensitive channels.

In Silico Screen Identifies a New Family of Agonists for the Bacterial Mechanosensitive Channel MscL

MscL is a highly conserved mechanosensitive channel found in the majority of bacterial species, including pathogens. It functions as a biological emergency release valve, jettisoning solutes from the cytoplasm upon acute hypoosmotic stress. It opens the largest known gated pore and has been heralded as an antibacterial target. Although there are no known endogenous ligands, small compounds have recently been shown to specifically bind to and open the channel, leading to decreased cell growth and viability. Their binding site is at the cytoplasmic/membrane and subunit interfaces of the protein, which has been recently been proposed to play an essential role in channel gating. Here, we have targeted this pocket using in silico screening, resulting in the discovery of a new family of compounds, distinct from other known MscL-specific agonists. Our findings extended the study of this functional region, the progression of MscL as a viable drug target, and demonstrated the power of in silico screening for identifying and improving the design of MscL agonists.

Mechanosensitive Channels Mediate Hypoionic Shock-Induced Aminoglycoside Potentiation against Bacterial Persisters by Enhancing Antibiotic Uptake

Improving the efficacy of existing antibiotics is a promising strategy for combating antibiotic-resistant/tolerant bacterial pathogens that have become a severe threat to human health. We previously reported that aminoglycoside antibiotics could be dramatically potentiated against stationary-phase Escherichia coli cells under hypoionic shock conditions (i.e., treatment with ion-free solutions), but the underlying molecular mechanism remains unknown. Here, we show that mechanosensitive (MS) channels, a ubiquitous protein family sensing mechanical forces of cell membrane, mediate such hypoionic shock-induced aminoglycoside potentiation. Two-minute treatment under conditions of hypoionic shock (e.g., in pure water) greatly enhances the bactericidal effects of aminoglycosides against both spontaneous and triggered E. coli persisters, numerous strains of Gram-negative pathogens in vitro, and Pseudomonas aeruginosa in mice. Such potentiation is achieved by hypoionic shock-enhanced bacterial uptake of aminoglycosides and is linked to hypoionic shock-induced destabilization of the cytoplasmic membrane in E. coli. Genetic and biochemical analyses reveal that MscS-family channels directly and redundantly mediate aminoglycoside uptake upon hypoionic shock and thus potentiation, with MscL channel showing reduced effect. Molecular docking and site-directed mutagenesis analyses reveal a putative streptomycin-binding pocket in MscS, critical for streptomycin uptake and potentiation. These results suggest that hypoionic shock treatment destabilizes the cytoplasmic membrane and thus changes the membrane tension, which immediately activates MS channels that are able to effectively transport aminoglycosides into the cytoplasm for downstream killing. Our findings reveal the biological effects of hypoionic shock on bacteria and can help to develop novel adjuvants for aminoglycoside potentiation to combat bacterial pathogens via activating MS channels.

Nanchung and Inactive define pore properties of the native auditory transduction channel in Drosophila

Auditory transduction is mediated by chordotonal (Cho) neurons in Drosophila larvae, but the molecular identity of the mechanotransduction (MET) channel is elusive. Here, we established a whole-cell recording system of Cho neurons and showed that two transient receptor potential vanilloid (TRPV) channels, Nanchung (NAN) and Inactive (IAV), are essential for MET currents in Cho neurons. NAN and IAV form active ion channels when expressed simultaneously in S2 cells. Point mutations in the pore region of NAN-IAV change the reversal potential of the MET currents. Particularly, residues 857 through 990 in the IAV carboxyl terminus regulate the kinetics of MET currents in Cho neurons. In addition, TRPN channel NompC contributes to the adaptation of auditory transduction currents independent of its ion-conduction function. These results indicate that NAN-IAV, rather than NompC, functions as essential pore-forming subunits of the native auditory transduction channel in Drosophila and provide insights into the gating mechanism of MET currents in Cho neurons.

Model of Environmental Membrane Field for Transmembrane Proteins

The water environment determines the activity of biological processes. The role of such an environment interpreted in the form of an external field expressed by the 3D Gaussian distribution in the fuzzy oil drop model directs the folding process towards the generation of a centrally located hydrophobic core with the simultaneous exposure of polar residues on the surface. In addition to proteins soluble in the water environment, there is a significant group of membrane proteins that act as receptors or channels, including ion channels in particular. The change of the polar (water) environment into a highly hydrophobic (membrane) environment is quite radical, resulting in a different hydrophobicity distribution within the membrane protein. Modification of the notation of the force field expressing the presence of the hydrophobic environment has been proposed in this work. A modified fuzzy oil drop model with its adaptation to membrane proteins was used to interpret the structure of membrane proteins–mechanosensitive channel. The modified model was also used to describe the so-called negative cases—i.e., for water-soluble proteins with a clear distribution consistent with the fuzzy oil drop model.

Discoveries in structure and physiology of mechanically activated ion channels

The ability to sense physical forces is conserved across all organisms. Cells convert mechanical stimuli into electrical or chemical signals via mechanically activated ion channels. In recent years, the identification of new families of mechanosensitive ion channels-such as PIEZO and OSCA/TMEM63 channels-along with surprising insights into well-studied mechanosensitive channels have driven further developments in the mechanotransduction field. Several well-characterized mechanosensory roles such as touch, blood-pressure sensing and hearing are now linked with primary mechanotransducers. Unanticipated roles of mechanical force sensing continue to be uncovered. Furthermore, high-resolution structures representative of nearly every family of mechanically activated channel described so far have underscored their diversity while advancing our understanding of the biophysical mechanisms of pressure sensing. Here we summarize recent discoveries in the physiology and structures of known mechanically activated ion channel families and discuss their implications for understanding the mechanisms of mechanical force sensing.

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.

Voltage vs. Ligand II: Structural insights of the intrinsic flexibility in cyclic nucleotide-gated channels

In the preceding article, we present a flexibility analysis of the voltage-gated ion channel (VGIC) superfamily. In this study, we describe in detail the flexibility profile of the voltage-sensor domain (VSD) and the pore domain (PD) concerning the evolution of 6TM ion channels. In particular, we highlight the role of flexibility in the emergence of CNG channels and describe a significant level of sequence similarity between the archetypical VSD and the TolQ proteins. A highly flexible S4-like segment exhibiting Lys instead Arg for these membrane proteins is reported. Sequence analysis indicates that, in addition to this S4-like segment, TolQ proteins also show similarity with specific motifs in S2 and S3 from typical V-sensors. Notably, S3 flexibility profiles from typical VSDs and S3-like in TolQ proteins are also similar. Interestingly, TolQ from early divergent prokaryotes are comparatively more flexible than those in modern counterparts or true V-sensors. Regarding the PD, we also found that 2TM K+-channels in early prokaryotes are considerably more flexible than the ones in modern microbes, and such flexibility is comparable to the one present in CNG channels. Voltage dependence is mainly exhibited in prokaryotic CNG channels whose VSD is rigid whereas the eukaryotic CNG channels are considerably more flexible and poorly V-dependent. The implication of the flexibility present in CNG channels, their sensitivity to cyclic nucleotides and the cation selectivity are discussed. Finally, we generated a structural model of the putative cyclic nucleotide-modulated ion channel, which we coined here as AqK, from the thermophilic bacteria Aquifex aeolicus, one of the earliest diverging prokaryotes known. Overall, our analysis suggests that V-sensors in CNG-like channels were essentially rigid in early prokaryotes but raises the possibility that this module was probably part of a very flexible stator protein of the bacterial flagellum motor complex.

Voltage vs. Ligand I: Structural basis of the intrinsic flexibility of S3 segment and its significance in ion channel activation

We systematically predict the internal flexibility of the S3 segment, one of the most mobile elements in the voltage-sensor domain. By analyzing the primary amino acid sequences of V-sensor containing proteins, including Hv1, TPC channels and the voltage-sensing phosphatases, we established correlations between the local flexibility and modes of activation for different members of the VGIC superfamily. Taking advantage of the structural information available, we also assessed structural aspects to understand the role played by the flexibility of S3 during the gating of the pore. We found that S3 flexibility is mainly determined by two specific regions: (1) a short NxxD motif in the N-half portion of the helix (S3a), and (2) a short sequence at the beginning of the so-called paddle motif where the segment has a kink that, in some cases, divide S3 into two distinct helices (S3a and S3b). A good correlation between the flexibility of S3 and the reported sensitivity to temperature and mechanical stretch was found. Thus, if the channel exhibits high sensitivity to heat or membrane stretch, local S3 flexibility is low. On the other hand, high flexibility of S3 is preferentially associated to channels showing poor heat and mechanical sensitivities. In contrast, we did not find any apparent correlation between S3 flexibility and voltage or ligand dependence. Overall, our results provide valuable insights into the dynamics of channel-gating and its modulation.

Allosteric activation of an ion channel triggered by modification of mechanosensitive nano-pockets

Lipid availability within transmembrane nano-pockets of ion channels is linked with mechanosensation. However, the effect of hindering lipid-chain penetration into nano-pockets on channel structure has not been demonstrated. Here we identify nano-pockets on the large conductance mechanosensitive channel MscL, the high-pressure threshold channel. We restrict lipid-chain access to the nano-pockets by mutagenesis and sulfhydryl modification, and monitor channel conformation by PELDOR/DEER spectroscopy. For a single site located at the entrance of the nano-pockets and distal to the channel pore we generate an allosteric response in the absence of tension. Single-channel recordings reveal a significant decrease in the pressure activation threshold of the modified channel and a sub-conducting state in the absence of applied tension. Threshold is restored to wild-type levels upon reduction of the sulfhydryl modification. The modification associated with the conformational change restricts lipid access to the nano-pocket, interrupting the contact between the membrane and the channel that mediates mechanosensitivity.

What are the effects of the serine triad on proton conduction of an influenza B M2 channel? An investigation by molecular dynamics simulations

The tetrameric influenza B M2 channel (BM2), an acid activated proton channel, is important in the influenza virus B lifecycle. A conserved HxxxW motif is responsible for proton conduction and channel gating. In this study, to explore the effects of the serine triad (S9, S12 and S16) on proton conduction, we performed classical molecular dynamics (CMD) simulations and adaptive steered molecular dynamics (ASMD) simulations at different protonation states of the H19 tetrad. The results of the pore radius and the C-terminal tilt angle show that the electrostatic repulsion induced by protonated H19 is the key driving force for opening the BM2 channel. The open states could be stabilized by the hydrogen bonds between S16 and protonated H19. The solvent accessible surface area and water density indicate that the polar hydrophilic environment provided by the serine triad facilitates the formation of a water wire, and then exhibits favourable effects on proton conduction. The mutant research verifies and supports these views. Our work clarifies the effects of the serine triad on proton conduction in the BM2 channel, which would help us deeply understand the proton conduction mechanism in BM2 and provides a new perspective for antiviral drug design against BM2.

Transcriptomic Responses to Thermal Stress and Varied Phosphorus Conditions in Fugacium kawagutii

Coral reef-associated Symbiodiniaceae live in tropical and oligotrophic environments and are prone to heat and nutrient stress. How their metabolic pathways respond to pulses of warming and phosphorus (P) depletion is underexplored. Here, we conducted RNA-seq analysis to investigate transcriptomic responses to thermal stress, phosphate deprivation, and organic phosphorus (OP) replacement in Fugacium kawagutii. Using dual-algorithm (edgeR and NOIseq) to remedy the problem of no replicates, we conservatively found 357 differentially expressed genes (DEGs) under heat stress, potentially regulating cell wall modulation and the transport of iron, oxygen, and major nutrients. About 396 DEGs were detected under P deprivation and 671 under OP utilization, both mostly up-regulated and potentially involved in photosystem and defensome, despite different KEGG pathway enrichments. Additionally, we identified 221 genes that showed relatively stable expression levels across all conditions (likely core genes), mostly catalytic and binding proteins. This study reveals a wide range of, and in many cases previously unrecognized, molecular mechanisms in F. kawagutii to cope with heat stress and phosphorus-deficiency stress. Their quantitative expression dynamics, however, requires further verification with triplicated experiments, and the data reported here only provide clues for generating testable hypotheses about molecular mechanisms underpinning responses and adaptation in F. kawagutii to temperature and nutrient stresses.

Elucidating a role for the cytoplasmic domain in the Mycobacterium tuberculosis mechanosensitive channel of large conductance

Microbial survival in dynamic environments requires the ability to successfully respond to abrupt changes in osmolarity. The mechanosensitive channel of large conductance (MscL) is a ubiquitous channel that facilitates the survival of bacteria and archaea under severe osmotic downshock conditions by relieving excess turgor pressure in response to increased membrane tension. A prominent structural feature of MscL, the cytoplasmic C-terminal domain, has been suggested to influence channel assembly and function. In this report, we describe the X-ray crystal structure and electrophysiological properties of a C-terminal domain truncation of the Mycobacterium tuberculosis MscL (MtMscLΔC). A crystal structure of MtMscLΔC solubilized in the detergent n-dodecyl-β-D-maltopyranoside reveals the pentameric, closed state-like architecture for the membrane spanning region observed in the previously solved full-length MtMscL. Electrophysiological characterization demonstrates that MtMscLΔC retains mechanosensitivity, but with conductance and tension sensitivity more closely resembling full length EcMscL than MtMscL. This study establishes that the C-terminal domain of MtMscL is not required for oligomerization of the full-length channel, but rather influences the tension sensitivity and conductance properties of the channel. The collective picture that emerges from these data is that each MscL channel structure has characteristic features, highlighting the importance of studying multiple homologs.

Engineering a pH-Sensitive Liposomal MRI Agent by Modification of a Bacterial Channel

MscL is a bacterial mechanosensitive channel that serves as a cellular emergency release valve, protecting the cell from lysis upon a drop in external osmolarity. The channel has an extremely large pore (30 Å) and can be purified and reconstituted into artificial membranes. Moreover, MscL is modified to open in response to alternative external stimuli including changes in pH. These properties suggest this channel's potential as a triggered "nanopore" for localized release of vesicular contents such as magnetic resonance imaging (MRI) contrast agents and drugs. Toward this end, several variants of pH-triggered MscL nanovalves are engineered. Stealth vesicles previously been shown to evade normal in vivo clearance and passively accumulate in inflamed and malignant tissues are reconstituted. These vesicles are loaded with 1,4,7,10-tetraazacyclododecane tetraacetic acid gadolinium complex (Gd-DOTA), an MRI contrast reagent, and the resulting nanodevices tested for their ability to release Gd-DOTA as evidenced by enhancement of the longitudinal relaxation rate (R₁ ) of the bulk water proton spins. Nanovalves that are responsive to physiological pH changes are identified, but differ in sensitivity and efficacy, thus giving an array of nanovalves that could potentially be useful in different settings. These triggered nanodevices may be useful in delivering both diagnostic and therapeutic agents.

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.

Thermodynamic secrets of multidrug resistance: A new take on transport mechanisms of secondary active antiporters

Multidrug resistance (MDR) presents a growing challenge to global public health. Drug extrusion transporters play a critical part in MDR; thus, their mechanisms of substrate recognition are being studied in great detail. In this work, we review common structural features of key transporters involved in MDR. Based on our membrane potential-driving hypothesis, we propose a general energy-coupling mechanism for secondary-active antiporters. This putative mechanism provides a common framework for understanding poly-specificity of most-if not all-MDR transporters.

Structural Dynamics of the MscL C-terminal Domain

The large conductance mechanosensitive channel (MscL), acts as an osmoprotective emergency valve in bacteria by opening a large, water-filled pore in response to changes in membrane tension. In its closed configuration, the last 36 residues at the C-terminus form a bundle of five α-helices co-linear with the five-fold axis of symmetry. Here, we examined the structural dynamics of the C-terminus of EcMscL using site-directed spin labelling electron paramagnetic resonance (SDSL EPR) spectroscopy. These experiments were complemented with computational modelling including molecular dynamics (MD) simulations and finite element (FE) modelling. Our results show that under physiological conditions, the C-terminus is indeed an α-helical bundle, located near the five-fold symmetry axis of the molecule. Both experiments and computational modelling demonstrate that only the top part of the C-terminal domain (from the residue A110 to E118) dissociates during the channel gating, while the rest of the C-terminus stays assembled. This result is consistent with the view that the C-terminus functions as a molecular sieve and stabilizer of the oligomeric MscL structure as previously suggested.

Bacterial Mechanosensors

Bacteria represent one of the most evolutionarily successful groups of organisms to inhabit Earth. Their world is awash with mechanical cues, probably the most ancient form of which are osmotic forces. As a result, they have developed highly robust mechanosensors in the form of bacterial mechanosensitive (MS) channels. These channels are essential in osmoregulation, and in this setting, provide one of the simplest paradigms for the study of mechanosensory transduction. We explore the past, present, and future of bacterial MS channels, including the alternate mechanosensory roles that they may play in complex microbial communities. Central to all of these functions is their ability to change conformation in response to mechanical stimuli. We discuss their gating according to the force-from-lipids principle and its applicability to eukaryotic MS channels. This includes the new paradigms emerging for bilayer-mediated channel mechanosensitivity and how this molecular detail may provide advances in both industry and medicine.

How do mechanosensitive channels sense membrane tension?

Mechanosensitive (MS) channels provide protection against hypo-osmotic shock in bacteria whereas eukaryotic MS channels fulfil a multitude of important functions beside osmoregulation. Interactions with the membrane lipids are responsible for the sensing of mechanical force for most known MS channels. It emerged recently that not only prokaryotic, but also eukaryotic, MS channels are able to directly sense the tension in the membrane bilayer without any additional cofactor. If the membrane is solely viewed as a continuous medium with specific anisotropic physical properties, the sensitivity towards tension changes can be explained as result of the hydrophobic coupling between membrane and transmembrane (TM) regions of the channel. The increased cross-sectional area of the MS channel in the active conformation and elastic deformations of the membrane close to the channel have been described as important factors. However, recent studies suggest that molecular interactions of lipids with the channels could play an important role in mechanosensation. Pockets in between TM helices were identified in the MS channel of small conductance (MscS) and YnaI that are filled with lipids. Less lipids are present in the open state of MscS than the closed according to MD simulations. Thus it was suggested that exclusion of lipid fatty acyl chains from these pockets, as a consequence of increased tension, would trigger gating. Similarly, in the eukaryotic MS channel TRAAK it was found that a lipid chain blocks the conducting path in the closed state. The role of these specific lipid interactions in mechanosensation are highlighted in this review.

Ion- and water-binding sites inside an occluded hourglass pore of a trimeric intracellular cation (TRIC) channel

BACKGROUND

Trimeric intracellular cation (TRIC) channels are crucial for Ca2+ handling in eukaryotes and are involved in K+ uptake in prokaryotes. Recent studies on the representative members of eukaryotic and prokaryotic TRIC channels demonstrated that they form homotrimeric units with the ion-conducting pores contained within each individual monomer.

RESULTS

Here we report detailed insights into the ion- and water-binding sites inside the pore of a TRIC channel from Sulfolobus solfataricus (SsTRIC). Like the mammalian TRIC channels, SsTRIC is permeable to both K+ and Na+ with a slight preference for K+, and is nearly impermeable to Ca2+, Mg2+, or Cl-. In the 2.2-Å resolution K+-bound structure of SsTRIC, ion/water densities have been well resolved inside the pore. At the central region, a filter-like structure is shaped by the kinks on the second and fifth transmembrane helices and two nearby phenylalanine residues. Below the filter, the cytoplasmic vestibule is occluded by a plug-like motif attached to an array of pore-lining charged residues.

CONCLUSIONS

The asymmetric filter-like structure at the pore center of SsTRIC might serve as the basis for the channel to bind and select monovalent cations. A Velcro-like plug-pore interacting model has been proposed and suggests a unified framework accounting for the gating mechanisms of prokaryotic and eukaryotic TRIC channels.

Spectator no more, the role of the membrane in regulating ion channel function

A pressure gradient across a curved lipid bilayer leads to a lateral force within the bilayer. Following ground breaking work on eukaryotic ion channels, it is now known that many proteins sense this change in the lateral tension and alter their functions in response. It has been proposed that responding to pressure differentials may be one of the oldest signaling mechanisms in biology. The most well characterized mechanosensing ion channels are the bacterial ones which open when the pressure differential hits a threshold. Recent studies on one of these channels, MscS, have developed a simple molecular model for how they sense and adapt to pressure. Biochemical and structural studies on mechanosensitive channels from eukaryotes have disclosed pressure sensing mechanisms. In this review, we highlight these findings and discuss the potential for a general model for pressure sensing.

From membrane tension to channel gating: A principal energy transfer mechanism for mechanosensitive channels

Mechanosensitive (MS) channels are evolutionarily conserved membrane proteins that play essential roles in multiple cellular processes, including sensing mechanical forces and regulating osmotic pressure. Bacterial MscL and MscS are two prototypes of MS channels. Numerous structural studies, in combination with biochemical and cellular data, provide valuable insights into the mechanism of energy transfer from membrane tension to gating of the channel. We discuss these data in a unified two-state model of thermodynamics. In addition, we propose a lipid diffusion-mediated mechanism to explain the adaptation phenomenon of MscS.

The role of MscL amphipathic N terminus indicates a blueprint for bilayer-mediated gating of mechanosensitive channels

The bacterial mechanosensitive channel MscL gates in response to membrane tension as a result of mechanical force transmitted directly to the channel from the lipid bilayer. MscL represents an excellent model system to study the basic biophysical principles of mechanosensory transduction. However, understanding of the essential structural components that transduce bilayer tension into channel gating remains incomplete. Here using multiple experimental and computational approaches, we demonstrate that the amphipathic N-terminal helix of MscL acts as a crucial structural element during tension-induced gating, both stabilizing the closed state and coupling the channel to the membrane. We propose that this may also represent a common principle in the gating cycle of unrelated mechanosensitive ion channels, allowing the coupling of channel conformation to membrane dynamics.

Dihydrostreptomycin Directly Binds to, Modulates, and Passes through the MscL Channel Pore

The primary mechanism of action of the antibiotic dihydrostreptomycin is binding to and modifying the function of the bacterial ribosome, thus leading to decreased and aberrant translation of proteins; however, the routes by which it enters the bacterial cell are largely unknown. The mechanosensitive channel of large conductance, MscL, is found in the vast majority of bacterial species, where it serves as an emergency release valve rescuing the cell from sudden decreases in external osmolarity. While it is known that MscL expression increases the potency of dihydrostreptomycin, it has remained unclear if this effect is due to a direct interaction. Here, we use a combination of genetic screening, MD simulations, and biochemical and mutational approaches to determine if dihydrostreptomycin directly interacts with MscL. Our data strongly suggest that dihydrostreptomycin binds to a specific site on MscL and modifies its conformation, thus allowing the passage of K+ and glutamate out of, and dihydrostreptomycin into, the cell.