Visualization of the mechanosensitive ion channel MscS under membrane tension

Nematocida displodere mechanosensitive ion channel of small conductance 2 assembles into a unique 6-channel super-structure in vitro

Mechanosensitive ion channels play an essential role in reacting to environmental signals and sustaining cell integrity by facilitating ion flux across membranes. For obligate intracellular pathogens like microsporidia, adapting to changes in the host environment is crucial for survival and propagation. Despite representing a eukaryote of extreme genome reduction, microsporidia have expanded the gene family of mechanosensitive ion channels of small conductance (mscS) through repeated gene duplication and horizontal gene transfer. All microsporidian genomes characterized to date contain mscS genes of both eukaryotic and bacterial origin. Here, we investigated the cryo-electron microscopy structure of the bacterially derived mechanosensitive ion channel of small conductance 2 (MscS2) from Nematocida displodere, an intracellular pathogen of Caenorhabditis elegans. MscS2 is the most compact MscS-like channel known and assembles into a unique superstructure in vitro with six heptameric MscS2 channels. Individual MscS2 channels are oriented in a heterogeneous manner to one another, resembling an asymmetric, flexible six-way cross joint. Finally, we show that microsporidian MscS2 still forms a heptameric membrane channel, however the extreme compaction suggests a potential new function of this MscS-like protein.

Monitoring the conformational ensemble and lipid environment of a mechanosensitive channel under cyclodextrin-induced membrane tension

Membrane forces shift the equilibria of mechanosensitive channels enabling them to convert mechanical cues into electrical signals. Molecular tools to stabilize and methods to capture their highly dynamic states are lacking. Cyclodextrins can mimic tension through the sequestering of lipids from membranes. Here we probe the conformational ensemble of MscS by EPR spectroscopy, the lipid environment with NMR, and function with electrophysiology under cyclodextrin-induced tension. We show the extent of MscS activation depends on the cyclodextrin-to-lipid ratio, and that lipids are depleted slower when MscS is present. This has implications in MscS' activation kinetics when distinct membrane scaffolds such as nanodiscs or liposomes are used. We find MscS transits from closed to sub-conducting state(s) before it desensitizes, due to the lack of lipid availability in its vicinity required for closure. Our approach allows for monitoring tension-sensitive states in membrane proteins and screening molecules capable of inducing molecular tension in bilayers.

A monomeric structure of human TMEM63A protein

OSCA/TMEM63 is a newly identified family of mechanically activated (MA) ion channels in plants and animals, respectively, which convert physical forces into electrical signals or trigger intracellular cascades and are essential for eukaryotic physiology. OSCAs and related TMEM16s and transmembrane channel-like (TMC) proteins form homodimers with two pores. However, the molecular architecture of the mammalian TMEM63 proteins remains unclear. Here we elucidate the structure of human TMEM63A in the presence of calcium by single particle cryo-EM, revealing a distinct monomeric architecture containing eleven transmembrane helices. It has structural similarity to the single subunit of the Arabidopsis thaliana OSCA proteins. We locate the ion permeation pathway within the monomeric configuration and observe a nonprotein density resembling lipid. These results lay a foundation for understanding the structural organization of OSCA/TMEM63A family proteins.

The Variety of Mechanosensitive Ion Channels in Retinal Neurons

Alterations in intraocular and external pressure critically involve the pathogenesis of glaucoma, traumatic retinal injury (TRI), and other retinal disorders, and retinal neurons have been reported to express multiple mechanical-sensitive channels (MSCs) in recent decades. However, the role of MSCs in visual functions and pressure-related retinal conditions has been unclear. This review will focus on the variety and functional significance of the MSCs permeable to K+, Na+, and Ca2+, primarily including the big potassium channel (BK); the two-pore domain potassium channels TRAAK and TREK; Piezo; the epithelial sodium channel (ENaC); and the transient receptor potential channels vanilloid TRPV1, TRPV2, and TRPV4 in retinal photoreceptors, bipolar cells, horizontal cells, amacrine cells, and ganglion cells. Most MSCs do not directly mediate visual signals in vertebrate retinas. On the other hand, some studies have shown that MSCs can open in physiological conditions and regulate the activities of retinal neurons. While these data reasonably predict the crossing of visual and mechanical signals, how retinal light pathways deal with endogenous and exogenous mechanical stimulation is uncertain.

Evidence for partial functional overlap of KEA and MSL transport proteins in the chloroplast inner envelope of Arabidopsis thaliana

Arabidopsis thaliana possesses two different ion-export mechanisms in the plastid inner envelope membrane. Due to a genome duplication, the transport proteins are encoded by partly redundant loci: K+-efflux antiporter1 (KEA1) and KEA2 and mechanosensitive channel of small conductance-like2 (MSL2) and MSL3. Thus far, a functional link between these two mechanisms has not been established. Here, we show that kea1msl2 loss-of-function mutants exhibit phenotypes such as slow growth, reduced photosynthesis and changes in chloroplast morphology, several of which are distinct from either single mutants and do not resemble kea1kea2 or msl2msl3 double mutants. Our data suggest that KEA1 and MSL2 function in concert to maintain plastid ion homeostasis and osmoregulation. Their interplay is critical for proper chloroplast development, organelle function, and plant performance.

Capture of endogenous lipids in peptidiscs and effect on protein stability and activity

Compared to protein-protein and protein-nucleic acid interactions, our knowledge of protein-lipid interactions remains limited. This is primarily due to the inherent insolubility of membrane proteins (MPs) in aqueous solution. The traditional use of detergents to overcome the solubility barrier destabilizes MPs and strips away certain lipids that are increasingly recognized as crucial for protein function. Recently, membrane mimetics have been developed to circumvent the limitations. In this study, using the peptidisc, we find that MPs in different lipid states can be isolated based on protein purification and reconstitution methods, leading to observable effects on MP activity and stability. Peptidisc also enables re-incorporating specific lipids to fine-tune the protein microenvironment and assess the impact on downstream protein associations. This study offers a first look at the illusive protein-lipid interaction specificity, laying the path for a systematic evaluation of lipid identity and contributions to membrane protein function.

Structure-guided mutagenesis of OSCAs reveals differential activation to mechanical stimuli

The dimeric two-pore OSCA/TMEM63 family has recently been identified as mechanically activated ion channels. Previously, based on the unique features of the structure of OSCA1.2, we postulated the potential involvement of several structural elements in sensing membrane tension (Jojoa-Cruz et al., 2018). Interestingly, while OSCA1, 2, and 3 clades are activated by membrane stretch in cell-attached patches (i.e. they are stretch-activated channels), they differ in their ability to transduce membrane deformation induced by a blunt probe (poking). Here, in an effort to understand the domains contributing to mechanical signal transduction, we used cryo-electron microscopy to solve the structure of Arabidopsis thaliana (At) OSCA3.1, which, unlike AtOSCA1.2, only produced stretch- but not poke-activated currents in our initial characterization (Murthy et al., 2018). Mutagenesis and electrophysiological assessment of conserved and divergent putative mechanosensitive features of OSCA1.2 reveal a selective disruption of the macroscopic currents elicited by poking without considerable effects on stretch-activated currents (SAC). Our results support the involvement of the amphipathic helix and lipid-interacting residues in the membrane fenestration in the response to poking. Our findings position these two structural elements as potential sources of functional diversity within the family.

Mechanical activation opens a lipid-lined pore in OSCA ion channels

OSCA/TMEM63 channels are the largest known family of mechanosensitive channels1-3, playing critical roles in plant4-7 and mammalian8,9 mechanotransduction. Here we determined 44 cryogenic electron microscopy structures of OSCA/TMEM63 channels in different environments to investigate the molecular basis of OSCA/TMEM63 channel mechanosensitivity. In nanodiscs, we mimicked increased membrane tension and observed a dilated pore with membrane access in one of the OSCA1.2 subunits. In liposomes, we captured the fully open structure of OSCA1.2 in the inside-in orientation, in which the pore shows a large lateral opening to the membrane. Unusually for ion channels, structural, functional and computational evidence supports the existence of a 'proteo-lipidic pore' in which lipids act as a wall of the ion permeation pathway. In the less tension-sensitive homologue OSCA3.1, we identified an 'interlocking' lipid tightly bound in the central cleft, keeping the channel closed. Mutation of the lipid-coordinating residues induced OSCA3.1 activation, revealing a conserved open conformation of OSCA channels. Our structures provide a global picture of the OSCA channel gating cycle, uncover the importance of bound lipids and show that each subunit can open independently. This expands both our understanding of channel-mediated mechanotransduction and channel pore formation, with important mechanistic implications for the TMEM16 and TMC protein families.

Structure-guided mutagenesis of OSCAs reveals differential activation to mechanical stimuli

The dimeric two-pore OSCA/TMEM63 family has recently been identified as mechanically activated ion channels. Previously, based on the unique features of the structure of OSCA1.2, we postulated the potential involvement of several structural elements in sensing membrane tension1. Interestingly, while OSCA1, 2, and 3 clades are activated by membrane stretch in cell-attached patches (i.e., they are stretch-activated channels), they differ in their ability to transduce membrane deformation induced by a blunt probe (poking). In an effort to understand the domains contributing to mechanical signal transduction, we used cryo-electron microscopy to solve the structure of Arabidopsis thaliana (At) OSCA3.1, which, unlike AtOSCA1.2, only produced stretch- but not poke-activated currents in our initial characterization2. Mutagenesis and electrophysiological assessment of conserved and divergent putative mechanosensitive features of OSCA1.2 reveal a selective disruption of the macroscopic currents elicited by poking without considerable effects on stretch-activated currents (SAC). Our results support the involvement of the amphipathic helix and lipid-interacting residues in the membrane fenestration in the response to poking. Our findings position these two structural elements as potential sources of functional diversity within the family.

Piezo channels in the intestinal tract

The intestine is the largest mechanosensitive organ in the human body whose epithelial cells, smooth muscle cells, neurons and enteroendocrine cells must sense and respond to various mechanical stimuli such as motility, distension, stretch and shear to regulate physiological processes including digestion, absorption, secretion, motility and immunity. Piezo channels are a newly discovered class of mechanosensitive ion channels consisting of two subtypes, Piezo1 and Piezo2. Piezo channels are widely expressed in the intestine and are involved in physiological and pathological processes. The present review summarizes the current research progress on the expression, function and regulation of Piezo channels in the intestine, with the aim of providing a reference for the future development of therapeutic strategies targeting Piezo channels.

Polymer-extracted structure of the mechanosensitive channel MscS reveals the role of protein-lipid interactions in the gating cycle

Membrane protein structure determination is not only technically challenging but is further complicated by the removal or displacement of lipids, which can result in non-native conformations or a strong preference for certain states at the exclusion of others. This is especially applicable to mechanosensitive channels (MSC's) that evolved to gate in response to subtle changes in membrane tension transmitted through the lipid bilayer. E. coli MscS, a model bacterial system, is an ancestral member of the large family of MSCs found across all phyla of walled organisms. As a tension sensor, MscS is very sensitive and highly adaptive; it readily opens under super-threshold tension and closes under no tension, but under lower tensions, it slowly inactivates and can only recover when tension is released. However, existing cryo-EM structures do not explain the entire functional gating cycle of open, closed, and inactivated states. A central question in the field has been the assignment of the frequently observed non-conductive conformation to either a closed or inactivated state. Here, we present a 3 Å MscS structure in native nanodiscs obtained with Glyco-DIBMA polymer extraction, eliminating the lipid removal step that is common to all previous structures. Besides the protein in the non-conductive conformation, we observe well-resolved densities of four endogenous phospholipid molecules intercalating between the lipid-facing and pore-lining helices in preferred orientations. Mutations of positively charged residues coordinating these lipids inhibit MscS inactivation, whereas removal of a negative charge near the lipid-filled crevice increases inactivation. The functional data allows us to assign this class of structures to the inactivated state. This structure reveals preserved lipids in their native locations, and the functional effects of their destabilization illustrate a novel inactivation mechanism based on an uncoupling of the peripheral tension-sensing helices from the gate.

Structure of mechanically activated ion channel OSCA2.3 reveals mobile elements in the transmembrane domain

Members of the OSCA/TMEM63 family are mechanically activated ion channels and structures of some OSCA members have revealed the architecture of these channels and structural features that are potentially involved in mechanosensation. However, these structures are all in a similar state and information about the motion of different elements of the structure is limited, preventing a deeper understanding of how these channels work. Here, we used cryoelectron microscopy to determine high-resolution structures of Arabidopsis thaliana OSCA1.2 and OSCA2.3 in peptidiscs. The structure of OSCA1.2 matches previous structures of the same protein in different environments. Yet, in OSCA2.3, the TM6a-TM7 linker adopts a different conformation that constricts the pore on its cytoplasmic side. Furthermore, coevolutionary sequence analysis uncovered a conserved interaction between the TM6a-TM7 linker and the beam-like domain (BLD). Our results reveal conformational heterogeneity and differences in conserved interactions between the TMD and BLD among members of the OSCA family.

[The Dynamic Model of the Active-Inactive Cell Interface]

Objective

To explore the morphodynamics of the active-inactive cell monolayer interfaces by using the active liquid crystal model.

Methods

A continuum mechanical model was established based on the active liquid crystal theory and the active-inactive cell monolayer interfaces were established by setting the activity difference of cell monolayers. The theoretical equations were solved numerically by the finite difference and the lattice Boltzmann method.

Results

The active-inactive cell interfaces displayed three typical morphologies, namely, flat interface, wavy interface, and finger-like interface. On the flat interfaces, the cells were oriented perpendicular to the interface, the -1/2 topological defects were clustered in the interfaces, and the interfaces were negatively charged. On the wavy interfaces, cells showed no obvious preference for orientation at the interfaces and the interfaces were neutrally charged. On the finger-like interfaces, cells were tangentially oriented at the interfaces, the +1/2 topological defects were collected at the interfaces, driving the growth of the finger-like structures, and the interfaces were positively charged.

Conclusion

The orientation of the cell alignment at the interface can significantly affect the morphologies of the active-inactive cell monolayer interfaces, which is closely associated with the dynamics of topological defects at the interfaces.

MscS inactivation and recovery are slow voltage-dependent processes sensitive to interactions with lipids

Mechanosensitive channel MscS, the major bacterial osmolyte release valve, shows a characteristic adaptive behavior. With a sharp onset of activating tension the channel population readily opens, but under prolonged action of moderate tension it inactivates. The inactivated state is non-conductive and tension insensitive, which suggests that the gate becomes uncoupled from the lipid-facing domains. Because the distinct opening and inactivation transitions are both driven from the closed state by tension transmitted through the lipid bilayer, here we explore how mutations of two conserved positively charged lipid anchors, R46 and R74, affect 1) the rates of opening and inactivation and 2) the voltage dependences of these transitions. Previously estimated kinetic rates for opening-closing transitions in wild-type MscS at low voltages were 3-6 orders of magnitude higher than the rates for inactivation and recovery. Here we show that MscS activation exhibits a shallow nearly symmetric dependence on voltage, whereas inactivation is substantially augmented and recovery is slowed down by depolarization. Conversely, hyperpolarization impedes inactivation and speeds up recovery. Mutations of R46 and R74 anchoring the lipid-facing helices to the inner interface to an aromatic residue (W) do not substantially change the activation energy and closing rates, but instead change the kinetics of both inactivation and recovery and essentially eliminate their voltage dependence. Uncharged polar substitutions (S or Q) for these anchors produce functional channels but increase the inactivation and reduce the recovery rates. The data clearly delineate the activation-closing and the inactivation-recovery pathways and strongly suggest that only the latter involves extensive rearrangements of the protein-lipid boundary associated with the uncoupling of the lipid-facing helices from the gate. The discovery that hyperpolarization robustly assists MscS recovery suggests that membrane potential is one of the factors that regulates osmolyte release valves by putting them either on "ready" or "standby" based on the cell's metabolic state.

TMEM63 mechanosensitive ion channels: Activation mechanisms, biological functions and human genetic disorders

The transmembrane 63 (TMEM63) family of proteins are originally identified as homologs of the osmosensitive calcium-permeable (OSCA) channels in plants. Mechanosensitivity of OSCA and TMEM63 proteins are recently demonstrated in addition to their proposed activation mechanism by hyper/hypo-osmolarity. TMEM63 proteins exist in all animals, with a single member in Drosophila (TMEM63) and three members in mammals (TMEM63 A/B/C). In humans, monoallelic variants of TMEM63A have been reported to cause transient hypomyelination during infancy, or severe hypomyelination and global developmental delay. Heterozygous variants of TMEM63B are found in patients with intellectual disability and abnormal motor function and brain morphology. Biallelic variants of TMEM63C are associated with hereditary spastic paraplegias accompanied by mild or no intellectual disability. Physiological functions of TMEM63 proteins clearly recognized so far include detecting food grittiness and environmental humidity in Drosophila, and supporting hearing in mice by regulating survival of cochlear hair cells. In this review, we summarize current knowledge about the activation mechanisms and biological functions of TMEM63 channels, and provide a concise reference for researchers interested in investigating more physiological and pathogenic roles of this family of proteins with ubiquitous expression in the body.

Open structure and gating of the Arabidopsis mechanosensitive ion channel MSL10

Plants are challenged by drastically different osmotic environments during growth and development. Adaptation to these environments often involves mechanosensitive ion channels that can detect and respond to mechanical force. In the model plant Arabidopsis thaliana, the mechanosensitive channel MSL10 plays a crucial role in hypo-osmotic shock adaptation and programmed cell death induction, but the molecular basis of channel function remains poorly understood. Here, we report a structural and electrophysiological analysis of MSL10. The cryo-electron microscopy structures reveal a distinct heptameric channel assembly. Structures of the wild-type channel in detergent and lipid environments, and in the absence of membrane tension, capture an open conformation. Furthermore, structural analysis of a non-conductive mutant channel demonstrates that reorientation of phenylalanine side chains alone, without main chain rearrangements, may generate the hydrophobic gate. Together, these results reveal a distinct gating mechanism and advance our understanding of mechanotransduction.

Mechanosensitive aquaporins

Cellular systems must deal with mechanical forces to satisfy their physiological functions. In this context, proteins with mechanosensitive properties play a crucial role in sensing and responding to environmental changes. The discovery of aquaporins (AQPs) marked a significant breakthrough in the study of water transport. Their transport capacity and regulation features make them key players in cellular processes. To date, few AQPs have been reported to be mechanosensitive. Like mechanosensitive ion channels, AQPs respond to tension changes in the same range. However, unlike ion channels, the aquaporin's transport rate decreases as tension increases, and the molecular features of the mechanism are unknown. Nevertheless, some clues from mechanosensitive ion channels shed light on the AQP-membrane interaction. The GxxxG motif may play a critical role in the water permeation process associated with structural features in AQPs. Consequently, a possible gating mechanism triggered by membrane tension changes would involve a conformational change in the cytoplasmic extreme of the single file region of the water pathway, where glycine and histidine residues from loop B play a key role. In view of their transport capacity and their involvement in relevant processes related to mechanical forces, mechanosensitive AQPs are a fundamental piece of the puzzle for understanding cellular responses.

A mechanical-coupling mechanism in OSCA/TMEM63 channel mechanosensitivity

Mechanosensitive (MS) ion channels are a ubiquitous type of molecular force sensor sensing forces from the surrounding bilayer. The profound structural diversity in these channels suggests that the molecular mechanisms of force sensing follow unique structural blueprints. Here we determine the structures of plant and mammalian OSCA/TMEM63 proteins, allowing us to identify essential elements for mechanotransduction and propose roles for putative bound lipids in OSCA/TMEM63 mechanosensation. Briefly, the central cavity created by the dimer interface couples each subunit and modulates dimeric OSCA/TMEM63 channel mechanosensitivity through the modulating lipids while the cytosolic side of the pore is gated by a plug lipid that prevents the ion permeation. Our results suggest that the gating mechanism of OSCA/TMEM63 channels may combine structural aspects of the 'lipid-gated' mechanism of MscS and TRAAK channels and the calcium-induced gating mechanism of the TMEM16 family, which may provide insights into the structural rearrangements of TMEM16/TMC superfamilies.

Bragg-Williams Theory for Particles with a Size-Modulating Internal Degree of Freedom

The field of soft matter teems with molecules and aggregates of molecules that have internal size-modulating degrees of freedom. Proteins, peptides, microgels, polymers, micelles, and even some colloids can exist in multiple-often just two dominating-states with different effective sizes, where size can refer to the volume or to the cross-sectional area for particles residing on surfaces. The size-dependence of their accessible states renders the behavior of these particles pressure-sensitive. The Bragg-Williams model is among the most simple mean-field methods to translate the presence of inter-particle interactions into an approximate phase diagram. Here, we extend the Bragg-Williams model to account for the presence of particles that are immersed in a solvent and exist in two distinct states, one occupying a smaller and the other one a larger size. The basis of the extension is a lattice-sublattice approximation that we use to host the two size-differing states. Our model includes particle-solvent interactions that act as an effective surface tension between particles and solvent and are ignorant of the state in which the particles reside. We analyze how the energetic preference of the particles for one or the other state affects the phase diagrams. The possibility of a single phase-two phases-single phase sequence of phase transitions as a function of increasing temperature is demonstrated.

Long-term menopause exacerbates vaginal wall support injury in ovariectomized rats by regulating amino acid synthesis and glycerophospholipid metabolism

Purpose

Menopause is a risk factor for pelvic organ prolapse (POP) and is frequently associated with diminished vaginal wall support. To uncover relevant molecular mechanisms and provide potential therapeutic targets, we evaluated changes in the transcriptome and metabolome of the vaginal wall in ovariectomized rats to identify important molecular changes.

Methods

Sixteen adult female Sprague-Dawley rats were randomly assigned to either the control or menopause group. Seven months after the operation, hematoxylin and eosin (H&E) staining and Masson trichrome staining were used to observe changes in the rat vaginal wall structure. Differentially expressed genes (DEGs) and metabolites (DEMs) in the vaginal wall were detected by RNA-sequencing and LC-MS, respectively. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses of DEGs and DEMs were performed.

Results

We verified that long-term menopause causes vaginal wall injury by H&E and Masson trichrome staining. From the multiomics analyses, 20,669 genes and 2193 metabolites were identified. Compared with the control group, 3255 DEGs were found in the vaginal wall of long-term menopausal rats. Bioinformatics analysis showed that the DEGs were mainly enriched in mechanistic pathways, including cell-cell junction, extracellular matrix, muscle tissue developments, the PI3K-Akt signaling pathway, the MAPK signaling pathway, tight junctions and the Wnt signaling pathway. Additionally, 313 DEMs were found, and they consisted mostly of amino acids and their metabolites. DEMs were also enriched in mechanistic pathways, such as glycine, serine and threonine metabolism, glycerophospholipid metabolism, gap junctions and ferroptosis. Coexpression analysis of DEGs and DEMs revealed that biosynthesis of amino acids (isocitric acid and PKM) and glycerophospholipid metabolism (1-(9Z-hexadecenoyl)-sn-glycero-3-phosphocholine and PGS1) are critical metabolic pathways, suggesting that POP induced by menopause may be associated with the regulation of these processes.

Conclusion

The findings showed that long-term menopause greatly exacerbated vaginal wall support injury by decreasing the biosynthesis of amino acids and interfering with glycerophospholipid metabolism, which may result in POP. This study not only clarified that long-term menopause exacerbates damage to the vaginal wall but also provided insight into the potential molecular mechanisms by which long-term menopause induces POP.

Molecular Biophysics of Class A G Protein Coupled Receptors-Lipids Interactome at a Glance-Highlights from the A2A Adenosine Receptor

G protein-coupled receptors (GPCRs) are embedded in phospholipid membrane bilayers with cholesterol representing 34% of the total lipid content in mammalian plasma membranes. Membrane lipids interact with GPCRs structures and modulate their function and drug-stimulated signaling through conformational selection. It has been shown that anionic phospholipids form strong interactions between positively charged residues in the G protein and the TM5-TM6-TM 7 cytoplasmic interface of class A GPCRs stabilizing the signaling GPCR-G complex. Cholesterol with a high content in plasma membranes can be identified in more specific sites in the transmembrane region of GPCRs, such as the Cholesterol Consensus Motif (CCM) and Cholesterol Recognition Amino Acid Consensus (CRAC) motifs and other receptor dependent and receptor state dependent sites. Experimental biophysical methods, atomistic (AA) MD simulations and coarse-grained (CG) molecular dynamics simulations have been applied to investigate these interactions. We emphasized here the impact of phosphatidyl inositol-4,5-bisphosphate (PtdIns(4,5)P₂ or PIP₂), a minor phospholipid component and of cholesterol on the function-related conformational equilibria of the human A_2A adenosine receptor (A_2AR), a representative receptor in class A GPCR. Several GPCRs of class A interacted with PIP₂ and cholesterol and in many cases the mechanism of the modulation of their function remains unknown. This review provides a helpful comprehensive overview for biophysics that enter the field of GPCRs-lipid systems.

MscS inactivation and recovery are slow voltage-dependent processes sensitive to interactions with lipids

Mechanosensitive channel MscS, the major bacterial osmolyte release valve, shows a characteristic adaptive behavior. With a sharp onset of activating tension, the channel population readily opens, but under prolonged action of moderate near-threshold tension, it inactivates. The inactivated state is non-conductive and tension-insensitive, which suggests that the gate gets uncoupled from the lipid-facing domains. The kinetic rates for tension-driven opening-closing transitions are 4-6 orders of magnitude higher than the rates for inactivation and recovery. Here we show that inactivation is augmented and recovery is slowed down by depolarization. Hyperpolarization, conversely, impedes inactivation and speeds up recovery. We then address the question of whether protein-lipid interactions may set the rates and influence voltage dependence of inactivation and recovery. Mutations of conserved arginines 46 and 74 anchoring the lipid-facing helices to the inner membrane leaflet to tryptophans do not change the closing transitions, but instead change the kinetics of both inactivation and recovery and essentially eliminate their voltage-dependence. Uncharged polar substitutions (S or Q) for these anchors produce functional channels but increase the inactivation and reduce the recovery rates. The data suggest that it is not the activation and closing transitions, but rather the inactivation and recovery pathways that involve substantial rearrangements of the protein-lipid boundary associated with the separation of the lipid-facing helices from the gate. The discovery that hyperpolarization robustly assists MscS recovery indicates that membrane potential can regulate osmolyte release valves by putting them either on the 'ready' or 'standby' mode depending on the cell's metabolic state.

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.

Mechanosensitive membrane proteins: Usual and unusual suspects in mediating mechanotransduction

This Viewpoint, which accompanies a Special Issue focusing on membrane mechanosensors, discusses unifying and unique features of both established and emerging mechanosensitive (MS) membrane proteins, their distribution across protein families and phyla, and current and future challenges in the study of these important proteins and their partners. MS membrane proteins are essential for tissue development, cellular motion, osmotic homeostasis, and sensing external and self-generated mechanical cues like those responsible for touch and proprioception. Though researchers' attention and this Viewpoint focus on a few famous ion channels that are considered the usual suspects as MS mechanosensors, we also discuss some of the more unusual suspects, such as G-protein coupled receptors. As the field continues to grow, so too will the list of proteins suspected to function as mechanosensors and the diversity of known MS membrane proteins.

Novel imaging methods and force probes for molecular mechanobiology of cytoskeleton and adhesion

Detection and conversion of mechanical forces into biochemical signals is known as mechanotransduction. From cells to tissues, mechanotransduction regulates migration, proliferation, and differentiation in processes such as immune responses, development, and cancer progression. Mechanosensitive structures such as integrin adhesions, the actin cortex, ion channels, caveolae, and the nucleus sense and transmit forces. In vitro approaches showed that mechanosensing is based on force-dependent protein deformations and reorganizations. However, the mechanisms in cells remained unclear since cell imaging techniques lacked molecular resolution. Thanks to recent developments in super-resolution microscopy (SRM) and molecular force sensors, it is possible to obtain molecular insight of mechanosensing in live cells. We discuss how understanding of molecular mechanotransduction was revolutionized by these innovative approaches, focusing on integrin adhesions, actin structures, and the plasma membrane.

Membrane phospholipids control gating of the mechanosensitive potassium leak channel TREK1

Tandem pore domain (K2P) potassium channels modulate resting membrane potentials and shape cellular excitability. For the mechanosensitive subfamily of K2Ps, the composition of phospholipids within the bilayer strongly influences channel activity. To examine the molecular details of K2P lipid modulation, we solved cryo-EM structures of the TREK1 K2P channel bound to either the anionic lipid phosphatidic acid (PA) or the zwitterionic lipid phosphatidylethanolamine (PE). At the extracellular face of TREK1, a PA lipid inserts its hydrocarbon tail into a pocket behind the selectivity filter, causing a structural rearrangement that recapitulates mutations and pharmacology known to activate TREK1. At the cytoplasmic face, PA and PE lipids compete to modulate the conformation of the TREK1 TM4 gating helix. Our findings demonstrate two distinct pathways by which anionic lipids enhance TREK1 activity and provide a framework for a model that integrates lipid gating with the effects of other mechanosensitive K2P modulators.

Cholesterol Regulation of Membrane Proteins Revealed by Two-Color Super-Resolution Imaging

Cholesterol and phosphatidyl inositol 4,5-bisphosphate (PIP₂) are hydrophobic molecules that regulate protein function in the plasma membrane of all cells. In this review, we discuss how changes in cholesterol concentration cause nanoscopic (<200 nm) movements of membrane proteins to regulate their function. Cholesterol is known to cluster many membrane proteins (often palmitoylated proteins) with long-chain saturated lipids. Although PIP₂ is better known for gating ion channels, in this review, we will discuss a second independent function as a regulator of nanoscopic protein movement that opposes cholesterol clustering. The understanding of the movement of proteins between nanoscopic lipid domains emerged largely through the recent advent of super-resolution imaging and the establishment of two-color techniques to label lipids separate from proteins. We discuss the labeling techniques for imaging, their strengths and weakness, and how they are used to reveal novel mechanisms for an ion channel, transporter, and enzyme function. Among the mechanisms, we describe substrate and ligand presentation and their ability to activate enzymes, gate channels, and transporters rapidly and potently. Finally, we define cholesterol-regulated proteins (CRP) and discuss the role of PIP₂ in opposing the regulation of cholesterol, as seen through super-resolution imaging.

State-specific morphological deformations of the lipid bilayer explain mechanosensitive gating of MscS ion channels

The force-from-lipids hypothesis of cellular mechanosensation posits that membrane channels open and close in response to changes in the physical state of the lipid bilayer, induced for example by lateral tension. Here, we investigate the molecular basis for this transduction mechanism by studying the mechanosensitive ion channel MscS from Escherichia coli and its eukaryotic homolog MSL1 from Arabidopsis thaliana. First, we use single-particle cryo-electron microscopy to determine the structure of a novel open conformation of wild-type MscS, stabilized in a thinned lipid nanodisc. Compared with the closed state, the structure shows a reconfiguration of helices TM1, TM2, and TM3a, and widening of the central pore. Based on these structures, we examined how the morphology of the membrane is altered upon gating, using molecular dynamics simulations. The simulations reveal that closed-state MscS causes drastic protrusions in the inner leaflet of the lipid bilayer, both in the absence and presence of lateral tension, and for different lipid compositions. These deformations arise to provide adequate solvation to hydrophobic crevices under the TM1-TM2 hairpin, and clearly reflect a high-energy conformation for the membrane, particularly under tension. Strikingly, these protrusions are largely eradicated upon channel opening. An analogous computational study of open and closed MSL1 recapitulates these findings. The gating equilibrium of MscS channels thus appears to be dictated by opposing conformational preferences, namely those of the lipid membrane and of the protein structure. We propose a membrane deformation model of mechanosensation, which posits that tension shifts the gating equilibrium towards the conductive state not because it alters the mode in which channel and lipids interact, but because it increases the energetic cost of the morphological perturbations in the membrane required by the closed state.

Designer Nanodiscs to Probe and Reprogram Membrane Biology in Synapses

Signal transduction at the synapse is mediated by a variety of protein-lipid interactions, which are vital for the spatial and temporal regulation of synaptic vesicle biogenesis, neurotransmitter release, and postsynaptic receptor activation. Therefore, our understanding of synaptic transmission cannot be completed until the elucidation of these critical protein-lipid interactions. On this front, recent advances in nanodiscs have vastly expanded our ability to probe and reprogram membrane biology in synapses. Here, we summarize the progress of the nanodisc toolbox and discuss future directions in this exciting field.

"Force-From-Lipids" Dependence of the MscCG Mechanosensitive Channel Gating on Anionic Membranes

Mechanosensory transduction in Corynebacterium glutamicum plays a major role in glutamate efflux for industrial MSG, whose production depends on the activation of MscCG-type mechanosensitive channels. Dependence of the MscCG channel activation by membrane tension on the membrane lipid content has to date not been functionally characterized. Here, we report the MscCG channel patch clamp recording from liposomes fused with C. glutamicum membrane vesicles as well as from proteoliposomes containing the purified MscCG protein. Our recordings demonstrate that mechanosensitivity of MscCG channels depends significantly on the presence of negatively charged lipids in the proteoliposomes. MscCG channels in liposome preparations fused with native membrane vesicles exhibited the activation threshold similar to the channels recorded from C. glutamicum giant spheroplasts. In comparison, the activation threshold of the MscCG channels reconstituted into azolectin liposomes was higher than the activation threshold of E. coli MscL, which is gated by membrane tension close to the bilayer lytic tension. The spheroplast-like activation threshold was restored when the MscCG channels were reconstituted into liposomes made of E. coli polar lipid extract. In liposomes made of polar lipids mixed with synthetic phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin, the activation threshold of MscCG was significantly reduced compared to the activation threshold recorded in azolectin liposomes, which suggests the importance of anionic lipids for the channel mechanosensitivity. Moreover, the micropipette aspiration technique combined with patch fluorometry demonstrated that membranes containing anionic phosphatidylglycerol are softer than membranes containing only polar non-anionic phosphatidylcholine and phosphatidylethanolamine. The difference in mechanosensitivity between C. glutamicum MscCG and canonical MscS of E. coli observed in proteoliposomes explains the evolutionary tuning of the force from lipids sensing in various bacterial membrane environments.

Lipid-membrane protein interaction visualised by cryo-EM: A review

Membrane proteins reside at interfaces between aqueous and lipid media and solving their molecular structure relies most of the time on removing them from the membrane using detergent. Luckily, this solubilization process does not strip them from all the associated lipids and single-particle cryo-transmission electron microscopy (SP-TEM) has proved a very good tool to visualise both protein high-resolution structure and, often, many of its associated lipids. In this review, we observe membrane protein structures from the Protein DataBank and their associated maps in the Electron Microscopy DataBase and determine how the SP-TEM maps allow lipid visualization, the type of binding sites, the influence of symmetry, resolution and other factors. We illustrate lipid visualization around and inside the protein core, show that some lipid bilayers in the core can be shifted with respect to the membrane and how some proteins can actively bend the lipid bilayer that binds to them. We conclude that resolution improvement in SP-TEM will likely enable many more discoveries regarding the role of lipids bound to proteins.

How Functional Lipids Affect the Structure and Gating of Mechanosensitive MscS-like Channels

The ability to cope with and adapt to changes in the environment is essential for all organisms. Osmotic pressure is a universal threat when environmental changes result in an imbalance of osmolytes inside and outside the cell which causes a deviation from the normal turgor. Cells have developed a potent system to deal with this stress in the form of mechanosensitive ion channels. Channel opening releases solutes from the cell and relieves the stress immediately. In bacteria, these channels directly sense the increased membrane tension caused by the enhanced turgor levels upon hypoosmotic shock. The mechanosensitive channel of small conductance, MscS, from Escherichia coli is one of the most extensively studied examples of mechanically stimulated channels. Different conformational states of this channel were obtained in various detergents and membrane mimetics, highlighting an intimate connection between the channel and its lipidic environment. Associated lipids occupy distinct locations and determine the conformational states of MscS. Not all these features are preserved in the larger MscS-like homologues. Recent structures of homologues from bacteria and plants identify common features and differences. This review discusses the current structural and functional models for MscS opening, as well as the influence of certain membrane characteristics on gating.

Genetic and cellular characterization of MscS-like putative channels in the filamentous fungus Aspergillus nidulans

Mechanosensitive ion channels are integral membrane proteins ubiquitously present in bacteria, archaea, and eukarya. They act as molecular sensors of mechanical stress to serve vital functions such as touch, hearing, osmotic pressure, proprioception and balance, while their malfunction is often associated with pathologies. Amongst them, the structurally distinct MscL and MscS channels from bacteria are the most extensively studied. MscS-like channels have been found in plants and Schizosaccharomyces pombe, where they regulate intracellular Ca²⁺ and cell volume under hypo-osmotic conditions. Here we characterize two MscS-like putative channels, named MscA and MscB, from the model filamentous fungus Aspergillus nidulans. Orthologues of MscA and MscB are present in most fungi, including relative plant and animal pathogens. MscA/MscB and other fungal MscS-like proteins share the three transmembrane helices and the extended C-terminal cytosolic domain that form the structural fingerprint of MscS-like channels with at least three additional transmembrane segments than Escherichia coli MscS. We show that MscA and MscB localize in Endoplasmic Reticulum and the Plasma Membrane, respectively, whereas their overexpression leads to increased CaCl₂ toxicity or/and reduction of asexual spore formation. Our findings contribute to understanding the role of MscS-like channels in filamentous fungi and relative pathogens.

Structural basis for mechanotransduction in a potassium-dependent mechanosensitive ion channel

Mechanosensitive channels of small conductance, found in many living organisms, open under elevated membrane tension and thus play crucial roles in biological response to mechanical stress. Amongst these channels, MscK is unique in that its activation also requires external potassium ions. To better understand this dual gating mechanism by force and ligand, we elucidate distinct structures of MscK along the gating cycle using cryo-electron microscopy. The heptameric channel comprises three layers: a cytoplasmic domain, a periplasmic gating ring, and a markedly curved transmembrane domain that flattens and expands upon channel opening, which is accompanied by dilation of the periplasmic ring. Furthermore, our results support a potentially unifying mechanotransduction mechanism in ion channels depicted as flattening and expansion of the transmembrane domain.

Root osmotic sensing from local perception to systemic responses

Plants face a constantly changing environment, requiring fine tuning of their growth and development. Plants have therefore developed numerous mechanisms to cope with environmental stress conditions. One striking example is root response to water deficit. Upon drought (which causes osmotic stress to cells), plants can among other responses alter locally their root system architecture (hydropatterning) or orientate their root growth to optimize water uptake (hydrotropism). They can also modify their hydraulic properties, metabolism and development coordinately at the whole root and plant levels. Upstream of these developmental and physiological changes, plant roots must perceive and transduce signals for water availability. Here, we review current knowledge on plant osmotic perception and discuss how long distance signaling can play a role in signal integration, leading to the great phenotypic plasticity of roots and plant development.

Accelerated 2D Classification With ISAC Using GPUs

A widely used approach to analyze single particles in electron microscopy data is 2D classification. This process is very computationally expensive, especially when large data sets are analyzed. In this paper we present GPU ISAC, a newly developed, GPU-accelerated version of the established Iterative Stable Alignment and Clustering (ISAC) algorithm for 2D images and generating class averages. While the previously existing implementation of ISAC relied on a computer cluster, GPU ISAC enables users to produce high quality 2D class averages from large-scale data sets on a single desktop machine equipped with affordable, consumer-grade GPUs such as Nvidia GeForce GTX 1080 TI cards. With only two such cards GPU ISAC matches the performance of twelve high end cluster nodes and, using high performance GPUs, is able to produce class averages from a million particles in between six to thirteen hours, depending on data set quality and box size. We also show GPU ISAC to scale linearly in all input dimensions, and thereby capable of scaling well with the increasing data load demand of future data sets. Further user experience improvements integrate GPU ISAC seamlessly into the existing SPHIRE GUI, as well as the TranSPHIRE on-the-fly processing pipeline. It is open source and can be downloaded at https://gitlab.gwdg.de/mpi-dortmund/sphire/cuISAC/

Physics of mechanotransduction by Piezo ion channels

Piezo ion channels are sensors of mechanical forces and mediate a wide range of physiological mechanotransduction processes. More than a decade of intense research has elucidated much of the structural and mechanistic principles underlying Piezo gating and its roles in physiology, although wide gaps of knowledge continue to exist. Here, we review the forces and energies involved in mechanical activation of Piezo ion channels and their functional modulation by other chemical and physical stimuli including lipids, voltage, and temperature. We compare the three predominant mechanisms likely to explain Piezo activation—the force-from-lipids mechanism, the tether model, and the membrane footprint theory. Additional sections shine light on how Piezo ion channels may affect each other through spatial clustering and functional cooperativity, and how substantial functional heterogeneity of Piezo ion channels arises as a byproduct of the precise physical environment each channel experiences. Finally, our review concludes by pointing out major research questions and technological limitations that future research can address.

Native-like environments afford novel mechanistic insights into membrane proteins

Advances in cryogenic electron microscopy (cryo-EM) enabled routine near-atomic structure determination of membrane proteins, while nanodisc technology has provided a way to provide membrane proteins with a native or native-like lipid environment. After giving a brief history of membrane mimetics, we present example structures of membrane proteins in nanodiscs that revealed information not provided by structures obtained in detergent. We describe how the lipid environment surrounding the membrane protein can be custom designed during nanodisc assembly and how it can be modified after assembly to test functional hypotheses. Because nanodiscs most closely replicate the physiologic environment of membrane proteins and often afford novel mechanistic insights, we propose that nanodiscs ought to become the standard for structural studies on membrane proteins.

Affinities and Kinetics Detection of Protein-Small Molecule Interactions with a Monolayer MoS2 -Based Optical Imaging Platform

Quantifying the binding kinetics and affinities of protein-small molecule interactions is critical for biomarker validation, drug discovery, and deep understanding of various biological processes at the molecular-scale. Novel approaches are demanded as most common label-free techniques are mass-sensitive, which are not suitable for the detection of small molecule interactions. Here, an optical imaging platform is developed to measure the binding kinetics of both protein-small molecules and protein-ions based on monolayer MoS₂ , an ultra-thin 2D material whose optical absorption is extremely sensitive to charge. A model is established to calibrate the optical response due to the charged analyte binding and it is applied to quantify the interactions between abl1 kinase and different small-molecule inhibitors. Such a presented method is capable of distinguishing different inhibitors binding to a wild or mutated kinase, which provides guidance for drug evaluation and drug mechanism exploration. The binding kinetics of calcium ions to calmodulin is also measured, further broadening the application field of the method. In addition, the imaging capability allows mapping the local binding kinetics of the molecular interactions with a high resolution, which reveals visible spatial variability and offers a promising tool for studying heterogeneous local interfacial interactions.

Supramolecular Mechanosensitive Potassium Channel Formed by Fluorinated Amphiphilic Cyclophane

Inspired by mechanosensitive potassium channels found in nature, we developed a fluorinated amphiphilic cyclophane composed of fluorinated rigid aromatic units connected via flexible hydrophilic octa(ethylene glycol) chains. Microscopic and emission spectroscopic studies revealed that the cyclophane could be incorporated into the hydrophobic layer of the lipid bilayer membranes and self-assembled to form a supramolecular transmembrane ion channel. Current recording measurements using cyclophane-containing planer lipid bilayer membranes successfully demonstrated an efficient transmembrane ion transport. We also demonstrated that the ion transport property was sensitive to the mechanical forces applied to the membranes. In addition, ion transport assays using pH-sensitive fluorescence dye revealed that the supramolecular channel possesses potassium ion selectivity. We also performed all-atom hybrid quantum-mechanical/molecular mechanical simulations to assess the channel structures at atomic resolution and the mechanism of selective potassium ion transport. This research demonstrated the first example of a synthetic mechanosensitive potassium channel, which would open a new door to sensing and manipulating biologically important processes and purification of key materials in industries.

Roles of Bacterial Mechanosensitive Channels in Infection and Antibiotic Susceptibility

Bacteria accumulate osmolytes to prevent cell dehydration during hyperosmotic stress. A sudden change to a hypotonic environment leads to a rapid water influx, causing swelling of the protoplast. To prevent cell lysis through osmotic bursting, mechanosensitive channels detect changes in turgor pressure and act as emergency-release valves for the ions and osmolytes, restoring the osmotic balance. This adaptation mechanism is well-characterized with respect to the osmotic challenges bacteria face in environments such as soil or an aquatic habitat. However, mechanosensitive channels also play a role during infection, e.g., during host colonization or release into environmental reservoirs. Moreover, recent studies have proposed roles for mechanosensitive channels as determinants of antibiotic susceptibility. Interestingly, some studies suggest that they serve as entry gates for antimicrobials into cells, enhancing antibiotic efficiency, while others propose that they play a role in antibiotic-stress adaptation, reducing susceptibility to certain antimicrobials. These findings suggest different facets regarding the relevance of mechanosensitive channels during infection and antibiotic exposure as well as illustrate that they may be interesting targets for antibacterial chemotherapy. Here, we summarize the recent findings on the relevance of mechanosensitive channels for bacterial infections, including transitioning between host and environment, virulence, and susceptibility to antimicrobials, and discuss their potential as antibacterial drug targets.

Asymmetric effects of amphipathic molecules on mechanosensitive channels

Mechanosensitive (MS) ion channels are primary transducers of mechanical force into electrical and/or chemical intracellular signals. Many diverse MS channel families have been shown to respond to membrane forces. As a result of this intimate relationship with the membrane and proximal lipids, amphipathic compounds exert significant effects on the gating of MS channels. Here, we performed all-atom molecular dynamics (MD) simulations and employed patch-clamp recording to investigate the effect of two amphipaths, Fluorouracil (5-FU) a chemotherapy agent, and the anaesthetic trifluoroethanol (TFE) on structurally distinct mechanosensitive channels. We show that these amphipaths have a profound effect on the bilayer order parameter as well as transbilayer pressure profile. We used bacterial mechanosensitive channels (MscL/MscS) and a eukaryotic mechanosensitive channel (TREK-1) as force-from-lipids reporters and showed that these amphipaths have differential effects on these channels depending on the amphipaths' size and shape as well as which leaflet of the bilayer they incorporate into. 5-FU is more asymmetric in shape and size than TFE and does not penetrate as deep within the bilayer as TFE. Thereby, 5-FU has a more profound effect on the bilayer and channel activity than TFE at much lower concentrations. We postulate that asymmetric effects of amphipathic molecules on mechanosensitive membrane proteins through the bilayer represents a general regulatory mechanism for these proteins.

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.

High-fidelity biosensing of dNTPs and nucleic acids by controllable subnanometer channel PaMscS

Current tools for dNTP analysis mainly rely on expensive fluorescent labeling, mass spectrometry or electrochemistry. Single-molecule assay by protein nanopores with an internal diameter of ca. 1-3.6 nm provides a useful tool for dNTP sensing. However, the most commonly used protein nanopores require additional modifications to enable dNTP detection. In this study, the PaMscS channel (mechanosensitive channel of small conductance from Pseudomonas aeruginosa) embedded in the bilayer lipid membrane (BLM) of E. coli polar lipid extract was applied as a nanopore for single molecular sensing. Two mutants of PaMscS nanopores on the side portal region (PaMscS W130A and PaMscS K180R) were selected for direct dNTP or pyrophosphoric acid (PPi) detection without aptamer or protein modification. Notably, the PaMscS mutant pore can be adjusted by regulation of osmolarity differences, which is crucial for the optimal detection of specific molecules. In addition, we established a PaMscS-based diagnosis method for the rapid sensing of disease-associated nucleic acids by monitoring the consumption of dNTPs, with 86% specificity and 100% sensitivity among 22 clinical samples. This protein nanopore, without aptamer or modification, paves a new way for dNTPs, PPi direct sensing and nucleic acid detection with low cost but high versatility.

Structural insights into the Venus flytrap mechanosensitive ion channel Flycatcher1

Flycatcher1 (FLYC1), a MscS homolog, has recently been identified as a candidate mechanosensitive (MS) ion channel involved in Venus flytrap prey recognition. FLYC1 is a larger protein and its sequence diverges from previously studied MscS homologs, suggesting it has unique structural features that contribute to its function. Here, we characterize FLYC1 by cryo-electron microscopy, molecular dynamics simulations, and electrophysiology. Akin to bacterial MscS and plant MSL1 channels, we find that FLYC1 central core includes side portals in the cytoplasmic cage that regulate ion preference and conduction, by identifying critical residues that modulate channel conductance. Topologically unique cytoplasmic flanking regions can adopt ‘up’ or ‘down’ conformations, making the channel asymmetric. Disruption of an up conformation-specific interaction severely delays channel deactivation by 40-fold likely due to stabilization of the channel open state. Our results illustrate novel structural features and likely conformational transitions that regulate mechano-gating of FLYC1.

Flycatcher1 (FLYC1) is a candidate mechanosensitive channel involved in Venus flytrap touch-induced prey capture. Here, the authors report structural and functional details of FLYC1, with insights into gating conformational transitions.