Electron microscopic analysis of KvAP voltage-dependent K+ channels in an open conformation

Contributions of natural products to ion channel pharmacology

Covering: Up to 2020Ion channels are a vast super-family of membrane proteins that play critical physiological roles in excitable and non-excitable cells. Their biomedical importance makes them valuable and attractive drug targets for neurological, cardiovascular, gastrointestinal and metabolic diseases, and for cancer therapy and immune modulation. Current therapeutics target only a minor subset of ion channels, leaving a large unexploited space within the ion channel field. Natural products harnessed from the almost unlimited and diverse universe of compounds within the bioenvironment have been used to modulate channels for decades. In this review we highlight the impact made by natural products on ion channel pharmacology, specifically on K⁺, Na_V and Ca_V channels, and use case studies to describe the development of ion channel-modulating drugs from natural sources for the treatment of pain, heart disease and autoimmune diseases.

Structural Studies of Thyroid Peroxidase Show the Monomer Interacting With Autoantibodies in Thyroid Autoimmune Disease

Thyroid peroxidase (TPO) is a critical membrane-bound enzyme involved in the biosynthesis of multiple thyroid hormones, and is a major autoantigen in autoimmune thyroid diseases such as destructive (Hashimoto) thyroiditis. Here we report the biophysical and structural characterization of a novel TPO construct containing only the ectodomain of TPO and lacking the propeptide. The construct was enzymatically active and able to bind the patient-derived TR1.9 autoantibody. Analytical ultracentrifugation data suggest that TPO can exist as both a monomer and a dimer. Combined with negative stain electron microscopy and molecular dynamics simulations, these data show that the TR1.9 autoantibody preferentially binds the TPO monomer, revealing conformational changes that bring together previously disparate residues into a continuous epitope. In addition to providing plausible structural models of a TPO-autoantibody complex, this study provides validated TPO constructs that will facilitate further characterization, and advances our understanding of the structural, functional, and antigenic characteristics of TPO, an autoantigen implicated in some of the most common autoimmune diseases.

Diffracted X-ray tracking method for recording single-molecule protein motions

BACKGROUND

Proteins change their conformation depending on function. Although a vast number of static pictures of proteins have been accumulated, information regarding their dynamics in function is limited. Diffracted X-ray tracking (DXT) is a good candidate to obtain the missing data.

SCOPE OF REVIEW

A gold nanocrystal was attached to the target protein as a probe and the motion of the X-ray diffraction spots from the crystal corresponded to the motion of the target. Although it has advantages of high temporal (sub-millisecond) and spatial (approximately 0.1°) resolutions, it is not extensively utilized. This review focused on its effective application from a user's perspective. We also present an example with the KcsA channel and the status of recent developments to show the future possibilities of the method.

MAJOR CONCLUSIONS

DXT is a powerful method to investigate intramolecular structural changes. For instance, in the KcsA channel, the method revealed a wave of conformational changes transmitted from the gate region to the end of the molecule. The method is continuously being developed, and users can choose an appropriate measurement system depending on the condition of their sample.

GENERAL SIGNIFICANCE

Revealing the protein structural changes with respect to function is an important frontier. The most distinctive feature of the DXT method is that both high temporal and spatial resolutions are achievable, and it is possible to track the motions of multiple molecules at the same time. This feature is an advantage for screening molecules associated with the target proteins (e.g., ligands and medicines).

Investigation of a KcsA Cytoplasmic pH Gate in Lipoprotein Nanodiscs

The bacterial potassium channel KcsA is gated by pH, opening for conduction under acidic conditions. Molecular determinants responsible for this effect have been identified at the extracellular selectivity filter, at the membrane-cytoplasm interface (TM2 gate), and in the cytoplasmic C-terminal domain (CTD), an amphiphilic four-helix bundle mediated by hydrophobic and electrostatic interactions. Here we have employed NMR and EPR to provide a structural view of the pH-induced open-to-closed CTD transition. KcsA was embedded in lipoprotein nanodiscs (LPNs), selectively methyl-protonated at Leu/Val residues to allow observation of both states by NMR, and spin-labeled for the purposes of EPR studies. We observed a pHinduced structural change between an associated structured CTD at neutral pH and a dissociated flexible CTD at acidic pH, with a transition in the 5.0-5.5 range, consistent with a stabilization of the CTD by channel architecture. A double mutant constitutively open at the TM2 gate exhibited reduced stability of associated CTD, as indicated by weaker spin-spin interactions, a shift to higher transition pH values, and a tenfold reduction in the population of the associated "closed" channels. We extended these findings for isolated CTD-derived peptides to full-length KcsA and have established a contribution of the CTD to KcsA pH-controlled gating, which exhibits a strong correlation with the state of the proximal TM2 gate.

Cholesterol-Dependent Gating Effects on Ion Channels

Biomembranes separate a live cell from its environment and keep it in an off-equilibrium, steady state. They contain both phospholipids and nonphospholipids, depending on whether there are phosphate groups in the headgroup regions. Cholesterol (CHOL) is one type of nonphospholipids, and one of the most abundant lipid molecules in humans. Its content in plasma membranes and intracellular membranes varies and is tightly regulated. Voltage-gated ion channels are universally present in every cell and are fairly diversified in the eukaryotic domain of life. Our lipid-dependent gating hypothesis postulates that the controlled switch of the voltage-sensor domains (VSDs) in a voltage-gated potassium (Kv) channel between the "down" and the "up" state (gating) is sensitive to the ratio of phospholipids:nonphospholipids in the annular layer around the channel. High CHOL content is found to exert strong inhibitory effects on Kv channels. Such effects have been observed in in vitro membranes, cultured cells, and animal models for cholesterol metabolic defects. Thermodynamic analysis of the CHOL-dependent gating suggests that the inhibitory effects of CHOL result from collective interactions between annular CHOL molecules and the channel, which appear to be a more generic principle behind the CHOL effects on other ion channels and transporters. We will review the recent progress in the CHOL-dependent gating of voltage-gated ion channels, discuss the current technical limitations, and then expand briefly the learned principles to other ion channels that are known to be sensitive to the CHOL-channel interactions.

Secretory granule protein chromogranin B (CHGB) forms an anion channel in membranes

The CHGB subfamily of secretory granule proteins forms a new family of anion-selective channels by interacting with membranes via two amphipathic α-helices. The channel exhibits higher anion selectivity, larger conductance, higher DIDS-binding affinity, and higher Cl⁻ sensitivity than other known anion channels.

Regulated secretion is an intracellular pathway that is highly conserved from protists to humans. Granin family proteins were proposed to participate in the biogenesis, maturation and release of secretory granules in this pathway. However, the exact molecular mechanisms underlying the intracellular functions of the granin family proteins remain unclear. Here, we show that chromogranin B (CHGB), a secretory granule protein, inserts itself into membrane and forms a chloride-conducting channel. CHGB interacts strongly with phospholipid membranes through two amphipathic α helices. At a high local concentration, CHGB insertion in membrane causes significant bilayer remodeling, producing protein-coated nanoparticles and nanotubules. Fast kinetics and high cooperativity for anion efflux from CHGB vesicles suggest that CHGB tetramerizes to form a functional channel with a single-channel conductance of ∼125 pS (150/150 mM Cl⁻). The CHGB channel is sensitive to an anion channel blocker and exhibits higher anion selectivity than the other six known families of Cl⁻ channels. Our data suggest that the CHGB subfamily of granin proteins forms a new family of organelle chloride channels.

Development of a new protein labeling system to map subunits and domains of macromolecular complexes for electron microscopy

Several gene fusion technologies have been successfully applied to label particular subunits or domains within macromolecular complexes to enable positional mapping of electron microscopy (EM) density maps, but exogenous fusion of a protein domain into the target polypeptide can cause unwanted structural and functional outcomes. Fab fragments from antibodies can be used as labeling reagents during EM visualization without gene manipulation of the target protein, but this method requires a panel of high-affinity antibodies that recognize a wide variety of epitopes. Linear peptide tags and their anti-tag antibodies can be used but they have a limited mapping ability as their placement is usually limited to the terminal regions of a protein. The PA dodecapeptide epitope tag (GVAMPGAEDDVV), forms a tight β-turn in the antigen binding pocket of its antibody (NZ-1). This capability allows for insertion of the PA tag into various surface-exposed loops within a multi-domain cell adhesion receptor, α_IIbβ₃ integrin. We confirmed that the purified PA-tagged integrin ectodomain fragments can form a stable complex with NZ-1 Fab. Negative stain EM of the various integrin-NZ-1 complexes revealed that a majority of the particles exhibited a clear density corresponding to the NZ-1 Fab; and the positions of the bound Fab were in good agreement with the predicted location of the inserted PA tag. The high-affinity and insertion-compatibility of the PA tag system allowed us to develop a new EM labeling methodology applicable to proteins for which good antibodies are not available.

Structural Study of Heterogeneous Biological Samples by Cryoelectron Microscopy and Image Processing

In living organisms, biological macromolecules are intrinsically flexible and naturally exist in multiple conformations. Modern electron microscopy, especially at liquid nitrogen temperatures (cryo-EM), is able to visualise biocomplexes in nearly native conditions and in multiple conformational states. The advances made during the last decade in electronic technology and software development have led to the revelation of structural variations in complexes and also improved the resolution of EM structures. Nowadays, structural studies based on single particle analysis (SPA) suggests several approaches for the separation of different conformational states and therefore disclosure of the mechanisms for functioning of complexes. The task of resolving different states requires the examination of large datasets, sophisticated programs, and significant computing power. Some methods are based on analysis of two-dimensional images, while others are based on three-dimensional studies. In this review, we describe the basic principles implemented in the various techniques that are currently used in the analysis of structural conformations and provide some examples of successful applications of these methods in structural studies of biologically significant complexes.

While the revolution will not be crystallized, biochemistry reigns supreme

Single-particle cryo-electron microscopy (EM) is currently gaining attention for the ability to calculate structures that reach sub-5 Å resolutions; however, the technique is more than just an alternative approach to X-ray crystallography. Molecular machines work via dynamic conformational changes, making structural flexibility the hallmark of function. While the dynamic regions in molecules are essential, they are also the most challenging to structurally characterize. Single-particle EM has the distinct advantage of being able to directly visualize purified molecules without the formation of ordered arrays of molecules locked into identical conformations. Additionally, structures determined using single-particle EM can span resolution ranges from very low- to atomic-levels (>30-1.8 Å), sometimes even in the same structure. The ability to accommodate various resolutions gives single-particle EM the unique capacity to structurally characterize dynamic regions of biological molecules, thereby contributing essential structural information needed for the development of molecular models that explain function. Further, many important molecular machines are intrinsically dynamic and compositionally heterogeneous. Structures of these complexes may never reach sub-5 Å resolutions due to this flexibility required for function. Thus, the biochemical quality of the sample, as well as, the calculation and interpretation of low- to mid-resolution cryo-EM structures (30-8 Å) remains critical for generating insights into the architecture of many challenging biological samples that cannot be visualized using alternative techniques.

bSUM: A bead-supported unilamellar membrane system facilitating unidirectional insertion of membrane proteins into giant vesicles

KvAP conjugated to beads via a C-terminal His-tag seeds formation of a supported bilayer with unidirectional channel orientation for functional studies.

Fused or giant vesicles, planar lipid bilayers, a droplet membrane system, and planar-supported membranes have been developed to incorporate membrane proteins for the electrical and biophysical analysis of such proteins or the bilayer properties. However, it remains difficult to incorporate membrane proteins, including ion channels, into reconstituted membrane systems that allow easy control of operational dimensions, incorporation orientation of the membrane proteins, and lipid composition of membranes. Here, using a newly developed chemical engineering procedure, we report on a bead-supported unilamellar membrane (bSUM) system that allows good control over membrane dimension, protein orientation, and lipid composition. Our new system uses specific ligands to facilitate the unidirectional incorporation of membrane proteins into lipid bilayers. Cryo–electron microscopic imaging demonstrates the unilamellar nature of the bSUMs. Electrical recordings from voltage-gated ion channels in bSUMs of varying diameters demonstrate the versatility of the new system. Using KvAP as a model system, we show that compared with other in vitro membrane systems, the bSUMs have the following advantages: (a) a major fraction of channels are orientated in a controlled way; (b) the channels mediate the formation of the lipid bilayer; (c) there is one and only one bilayer membrane on each bead; (d) the lipid composition can be controlled and the bSUM size is also under experimental control over a range of 0.2–20 µm; (e) the channel activity can be recorded by patch clamp using a planar electrode; and (f) the voltage-clamp speed (0.2–0.5 ms) of the bSUM on a planar electrode is fast, making it suitable to study ion channels with fast gating kinetics. Our observations suggest that the chemically engineered bSUMs afford a novel platform for studying lipid–protein interactions in membranes of varying lipid composition and may be useful for other applications, such as targeted delivery and single-molecule imaging.

Single-particle electron microscopy in the study of membrane protein structure

Single-particle electron microscopy (EM) provides the great advantage that protein structure can be studied without the need to grow crystals. However, due to technical limitations, this approach played only a minor role in the study of membrane protein structure. This situation has recently changed dramatically with the introduction of direct electron detection device cameras, which allow images of unprecedented quality to be recorded, also making software algorithms, such as three-dimensional classification and structure refinement, much more powerful. The enhanced potential of single-particle EM was impressively demonstrated by delivering the first long-sought atomic model of a member of the biomedically important transient receptor potential channel family. Structures of several more membrane proteins followed in short order. This review recounts the history of single-particle EM in the study of membrane proteins, describes the technical advances that now allow this approach to generate atomic models of membrane proteins and provides a brief overview of some of the membrane protein structures that have been studied by single-particle EM to date.

Single-Particle Cryo-EM of the Ryanodine Receptor Channel in an Aqueous Environment

Ryanodine receptors (RyRs) are tetrameric ligand-gated Ca²⁺ release channels that are responsible for the increase of cytosolic Ca²⁺ concentration leading to muscle contraction. Our current understanding of RyR channel gating and regulation is greatly limited due to the lack of a high-resolution structure of the channel protein. The enormous size and unwieldy shape of Ca²⁺ release channels make X-ray or NMR methods difficult to apply for high-resolution structural analysis of the full-length functional channel. Single-particle electron cryo-microscopy (cryo-EM) is one of the only effective techniques for the study of such a large integral membrane protein and its molecular interactions. Despite recent developments in cryo-EM technologies and break-through single-particle cryo-EM studies of ion channels, cryospecimen preparation, particularly the presence of detergent in the buffer, remains the main impediment to obtaining atomic-resolution structures of ion channels and a multitude of other integral membrane protein complexes. In this review we will discuss properties of several detergents that have been successfully utilized in cryo-EM studies of ion channels and the emergence of the detergent alternative amphipol to stabilize ion channels for structure-function characterization. Future structural studies of challenging specimen like ion channels are likely to be facilitated by cryo-EM amenable detergents or alternative surfactants.

Molecular determinants of tetramerization in the KcsA cytoplasmic domain

The cytoplasmic C-terminal domain (CTD) of KcsA, a bacterial homotetrameric potassium channel, is an amphiphilic domain that forms a helical bundle with four-fold symmetry mediated by hydrophobic and electrostatic interactions. Previously we have established that a CTD-derived 34-residue peptide associates into a tetramer in a pH-dependent manner (Kamnesky et al., JMB 2012;418:237-247). Here we further investigate the molecular determinants of tetramer formation in the CTD by characterizing the kinetics of monomer-tetramer equilibrium for 10 alanine mutants using NMR, sedimentation equilibrium (SE) and molecular dynamics simulation. NMR and SE concur in finding single-residue contributions to tetramer stability to be in the 0.5 to 3.5 kcal/mol range. Hydrophobic interactions between residues lining the tetramer core generally contributed more to formation of tetramer than electrostatic interactions between residues R147, D149 and E152. In particular, alanine replacement of residue R147, a key contributor to inter-subunit salt bridges, resulted in only a minor effect on tetramer dissociation. Mutations outside of the inter-subunit interface also influenced tetramer stability by affecting the tetramerization on-rate, possibly by changing the inherent helical propensity of the peptide. These findings are interpreted in the context of established paradigms of protein-protein interactions and protein folding, and lay the groundwork for further studies of the CTD in full-length KcsA channels.

Chemically functionalized carbon films for single molecule imaging

Many biological complexes are naturally low in abundance and pose a significant challenge to their structural and functional studies. Here we describe a new method that utilizes strong oxidation and chemical linkage to introduce a high density of bioactive ligands onto nanometer-thick carbon films and enable selective enrichment of individual macromolecular complexes at subnanogram levels. The introduced ligands are physically separated. Ni-NTA, Protein G and DNA/RNA oligonucleotides were covalently linked to the carbon surface. They embody negligible mass and their stability makes the functionalized films able to survive long-term storage and tolerate variations in pH, temperature, salts, detergents, and solvents. We demonstrated the application of the new method to the electron microscopic imaging of the substrate-bound C3PO, an RNA-processing enzyme important for the RNA interference pathway. On the ssRNA-linked carbon surface, the formation of C3PO oligomers at subnanomolar concentrations likely mimics their assembly onto ssRNA substrates presented by their native partners. Interestingly, the 3D reconstructions by negative stain EM reveal a side port in the C3PO/ssRNA complex, and the 15Å cryoEM map showed extra density right above the side port, which probably represents the ssRNA. These results suggest a new way for ssRNAs to interact with the active sites of the complex. Together our data demonstrate that the surface-engineered carbon films are suitable for selectively enriching low-abundance biological complexes at nanomolar level and for developing novel applications on a large number of surface-presented molecules.

The voltage sensor module in sodium channels

The mechanism by which voltage-gated ion channels respond to changes in membrane polarization during action potential signaling in excitable cells has been the subject of research attention since the original description of voltage-dependent sodium and potassium flux in the squid giant axon. The cloning of ion channel genes and the identification of point mutations associated with channelopathy diseases in muscle and brain has facilitated an electrophysiological approach to the study of ion channels. Experimental approaches to the study of voltage gating have incorporated the use of thiosulfonate reagents to test accessibility, fluorescent probes, and toxins to define domain-specific roles of voltage-sensing S4 segments. Crystallography, structural and homology modeling, and molecular dynamics simulations have added computational approaches to study the relationship of channel structure to function. These approaches have tested models of voltage sensor translocation in response to membrane depolarization and incorporate the role of negative countercharges in the S1 to S3 segments to define our present understanding of the mechanism by which the voltage sensor module dictates gating particle permissiveness in excitable cells.

Antibacterial membrane attack by a pore-forming intestinal C-type lectin

Human body surface epithelia coexist in close association with complex bacterial communities and are protected by a variety of antibacterial proteins. C-type lectins of the RegIII family are bactericidal proteins that limit direct contact between bacteria and the intestinal epithelium and thus promote tolerance to the intestinal microbiota^1,2. RegIII lectins recognize their bacterial targets by binding peptidoglycan carbohydrate^1,3 but the mechanism by which they kill bacteria is unknown. Here we elucidate the mechanistic basis for RegIII bactericidal activity. Here we show that human RegIIIα (hRegIIIα, also known as HIP/PAP) binds membrane phospholipids and kills bacteria by forming a hexameric membrane-permeabilizing oligomeric pore. We derive a three-dimensional model of the hRegIIIα pore by docking the hRegIIIα crystal structure into a cryo-electron microscopic map of the pore complex, and show that the model accords with experimentally determined properties of the pore. Lipopolysaccharide inhibits hRegIIIα pore-forming activity, explaining why hRegIIIα is bactericidal for Gram-positive but not Gram-negative bacteria. Our findings identify C-type lectins as mediators of membrane attack in the mucosal immune system, and provide detailed insight into an antibacterial mechanism that promotes mutualism with the resident microbiota.

Voltage sensor ring in a native structure of a membrane-embedded potassium channel

Voltage-gated ion channels support electrochemical activity in cells and are largely responsible for information flow throughout the nervous systems. The voltage sensor domains in these channels sense changes in transmembrane potential and control ion flux across membranes. The X-ray structures of a few voltage-gated ion channels in detergents have been determined and have revealed clear structural variations among their respective voltage sensor domains. More recent studies demonstrated that lipids around a voltage-gated channel could directly alter its conformational state in membrane. Because of these disparities, the structural basis for voltage sensing in native membranes remains elusive. Here, through electron-crystallographic analysis of membrane-embedded proteins, we present the detailed view of a voltage-gated potassium channel in its inactivated state. Contrary to all known structures of voltage-gated ion channels in detergents, our data revealed a unique conformation in which the four voltage sensor domains of a voltage-gated potassium channel from Aeropyrum pernix (KvAP) form a ring structure that completely surrounds the pore domain of the channel. Such a structure is named the voltage sensor ring. Our biochemical and electrophysiological studies support that the voltage sensor ring represents a physiological conformation. These data together suggest that lipids exert strong effects on the channel structure and that these effects may be changed upon membrane disruption. Our results have wide implications for lipid-protein interactions in general and for the mechanism of voltage sensing in particular.

Interaction between soluble and membrane-embedded potassium channel peptides monitored by Fourier transform infrared spectroscopy

Recent studies have explored the utility of Fourier transform infrared spectroscopy (FTIR) in dynamic monitoring of soluble protein-protein interactions. Here, we investigated the applicability of FTIR to detect interaction between synthetic soluble and phospholipid-embedded peptides corresponding to, respectively, a voltage-gated potassium (Kv) channel inactivation domain (ID) and S4–S6 of the Shaker Kv channel (KV1; including the S4–S5 linker “pre-inactivation” ID binding site). KV1 was predominantly α-helical at 30°C when incorporated into dimyristoyl-l-α-phosphatidylcholine (DMPC) bilayers. Cooling to induce a shift in DMPC from liquid crystalline to gel phase reversibly decreased KV1 helicity, and was previously shown to partially extrude a synthetic S4 peptide. While no interaction was detected in liquid crystalline DMPC, upon cooling to induce the DMPC gel phase a reversible amide I peak (1633 cm⁻¹) consistent with novel hydrogen bond formation was detected. This spectral shift was not observed for KV1 in the absence of ID (or vice versa), nor when the non-inactivating mutant V7E ID was applied to KV1 under similar conditions. Alteration of salt or redox conditions affected KV1-ID hydrogen bonding in a manner suggesting electrostatic KV1-ID interaction favored by a hairpin conformation for the ID and requiring extrusion of one or more KV1 domains from DMPC, consistent with ID binding to S4–S5. These findings support the utility of FTIR in detecting reversible interactions between soluble and membrane-embedded proteins, with lipid state-sensitivity of the conformation of the latter facilitating control of the interaction.

Structural interactions between lipids, water and S1-S4 voltage-sensing domains

Membrane proteins serve crucial signaling and transport functions, yet relatively little is known about their structures in membrane environments or how lipids interact with these proteins. For voltage-activated ion channels, X-ray structures suggest that the mobile voltage-sensing S4 helix would be exposed to the membrane, and functional studies reveal that lipid modification can profoundly alter channel activity. Here, we use solid-state NMR to investigate structural interactions of lipids and water with S1-S4 voltage-sensing domains and to explore whether lipids influence the structure of the protein. Our results demonstrate that S1-S4 domains exhibit extensive interactions with lipids and that these domains are heavily hydrated when embedded in a membrane. We also find evidence for preferential interactions of anionic lipids with S1-S4 domains and that these interactions have lifetimes on the timescale of ≤ 10(-3)s. Arg residues within S1-S4 domains are well hydrated and are positioned in close proximity to lipids, exhibiting local interactions with both lipid headgroups and acyl chains. Comparative studies with a positively charged lipid lacking a phosphodiester group reveal that this lipid modification has only modest effects on the structure and hydration of S1-S4 domains. Taken together, our results demonstrate that Arg residues in S1-S4 voltage-sensing domains reside in close proximity to the hydrophobic interior of the membrane yet are well hydrated, a requirement for carrying charge and driving protein motions in response to changes in membrane voltage.

The distal C-terminal region of the KcsA potassium channel is a pH-dependent tetramerization domain

The intracellular C-terminal domain (CTD) of KcsA, a bacterial homotetrameric potassium channel, is a 40-residue-long segment that natively adopts a helical bundle conformation with 4-fold symmetry. A hallmark of KcsA behavior is pH-induced conformational change, which leads to the opening of the channel at acidic pH. Previous studies have reached conflicting conclusions as to the role of the CTD in this transition. Here, we investigate the involvement of this domain in pH-mediated channel opening by NMR using a soluble peptide corresponding to residues 128-160 of the CTD (CTD34). At neutral pH, CTD34 exhibits concentration-dependent spectral changes consistent with oligomer formation. We prove this slowly tumbling species to be a tetramer with a dissociation constant of (2.0±0.5)×10(-)(11) M(3) by NMR and sedimentation equilibrium experiments. Whereas monomeric CTD34 is only mildly helical, secondary chemical shifts prove that the tetrameric species adopts a tight native-like helical bundle conformation. The tetrameric species undergoes pH-dependent dissociation, and CTD34 is fully monomeric below pH 5.0. The structural basis for this phenomenon is the destabilization of the tetrameric CTD34 by protonation of residue H145 in the monomeric form of the peptide. We conclude that (i) the CTD34 peptide is independently capable of forming a tetrameric helical bundle, and (ii) this structurally significant conformational shift is modulated by the effects of solution pH on residue H145. Therefore, the involvement of this domain in the pH gating of the channel is strongly suggested.

The influence of lipids on voltage-gated ion channels

Voltage-gated ion channels are responsible for transmitting electrochemical signals in both excitable and non-excitable cells. Structural studies of voltage-gated potassium and sodium channels by X-ray crystallography have revealed atomic details on their voltage-sensor domains (VSDs) and pore domains, and were put in context of disparate mechanistic views on the voltage-driven conformational changes in these proteins. Functional investigation of voltage-gated channels in membranes, however, showcased a mechanism of lipid-dependent gating for voltage-gated channels, suggesting that the lipids play an indispensible and critical role in the proper gating of many of these channels. Structure determination of membrane-embedded voltage-gated ion channels appears to be the next frontier in fully addressing the mechanism by which the VSDs control channel opening. Currently electron crystallography is the only structural biology method in which a membrane protein of interest is crystallized within a complete lipid-bilayer mimicking the native environment of a biological membrane. At a sufficiently high resolution, an electron crystallographic structure could reveal lipids, the channel and their mutual interactions at the atomic level. Electron crystallography is therefore a promising avenue toward understanding how lipids modulate channel activation through close association with the VSDs.

Fabs enable single particle cryoEM studies of small proteins

In spite of its recent achievements, the technique of single particle electron cryomicroscopy (cryoEM) has not been widely used to study proteins smaller than 100 kDa, although it is a highly desirable application of this technique. One fundamental limitation is that images of small proteins embedded in vitreous ice do not contain adequate features for accurate image alignment. We describe a general strategy to overcome this limitation by selecting a fragment antigen binding (Fab) to form a stable and rigid complex with a target protein, thus providing a defined feature for accurate image alignment. Using this approach, we determined a three-dimensional structure of an ∼65 kDa protein by single particle cryoEM. Because Fabs can be readily generated against a wide range of proteins by phage display, this approach is generally applicable to study many small proteins by single particle cryoEM.

Lipid-dependent gating of a voltage-gated potassium channel

Recent studies hypothesized that phospholipids stabilize two voltage-sensing arginine residues of certain voltage-gated potassium channels in activated conformations. It remains unclear how lipids directly affect these channels. Here, by examining the conformations of the KvAP in different lipids, we showed that without voltage change, the voltage-sensor domains switched from the activated to the resting state when their surrounding lipids were changed from phospholipids to nonphospholipids. Such lipid-determined conformational change was coupled to the ion-conducting pore, suggesting that parallel to voltage gating, the channel is gated by its annular lipids. Our measurements recognized that the energetic cost of lipid-dependent gating approaches that of voltage gating, but kinetically it appears much slower. Our data support that a channel and its surrounding lipids together constitute a functional unit, and natural nonphospholipids such as cholesterol should exert strong effects on voltage-gated channels. Our first observation of lipid-dependent gating may have general implications to other membrane proteins.

Lipid phosphodiesters affect the conformation of certain potassium channels, but the details of the lipid-channel interactions are unclear. Here, the KvAP channel is found to switch from an active to a resting state when the channels are transferred from a phospholipid membrane to a bilayer lacking phosphodiesters.

An externalized polypeptide partitions between two distinct sites on genome-released poliovirus particles

During cell entry, native poliovirus (160S) converts to a cell-entry intermediate (135S) particle, resulting in the externalization of capsid proteins VP4 and the amino terminus of VP1 (residues 1 to 53). Externalization of these entities is followed by release of the RNA genome (uncoating), leaving an empty (80S) particle. The antigen-binding fragment (Fab) of a monospecific peptide 1 (P1) antibody, which was raised against a peptide corresponding to amino-terminal residues 24 to 40 of VP1, was utilized to track the location of the amino terminus of VP1 in the 135S and 80S states of poliovirus particles via cryogenic electron microscopy (cryo-EM) and three-dimensional image reconstruction. On 135S, P1 Fabs bind to a prominent feature on the external surface known as the "propeller tip." In contrast, our initial 80S-P1 reconstruction showed P1 Fabs also binding to a second site, at least 50 Å distant, at the icosahedral 2-fold axes. Further analysis showed that the overall population of 80S-P1 particles consisted of three kinds of capsids: those with P1 Fabs bound only at the propeller tips, P1 Fabs bound only at the 2-fold axes, or P1 Fabs simultaneously bound at both positions. Our results indicate that, in 80S particles, a significant fraction of VP1 can deviate from icosahedral symmetry. Hence, this portion of VP1 does not change conformation synchronously when switching from the 135S state. These conclusions are compatible with previous observations of multiple conformations of the 80S state and suggest that movement of the amino terminus of VP1 has a role in uncoating. Similar deviations from icosahedral symmetry may be biologically significant during other viral transitions.

Molecular basis of the tarantula toxin jingzhaotoxin-III (β-TRTX-Cj1α) interacting with voltage sensors in sodium channel subtype Nav1.5

With conserved structural scaffold and divergent electrophysiological functions, animal toxins are considered powerful tools for investigating the basic structure-function relationship of voltage-gated sodium channels. Jingzhaotoxin-III (β-TRTX-Cj1α) is a unique sodium channel gating modifier from the tarantula Chilobrachys jingzhao, because the toxin can selectively inhibit the activation of cardiac sodium channel but not neuronal subtypes. However, the molecular basis of JZTX-III interaction with sodium channels remains unknown. In this study, we showed that JZTX-III was efficiently expressed by the secretory pathway in yeast. Alanine-scanning analysis indicated that 2 acidic residues (Asp1, Glu3) and an exposed hydrophobic patch, formed by 4 Trp residues (residues 8, 9, 28 and 30), play important roles in the binding of JZTX-III to Nav1.5. JZTX-III docked to the Nav1.5 DIIS3-S4 linker. Mutations S799A, R800A, and L804A could additively reduce toxin sensitivity of Nav1.5. We also demonstrated that the unique Arg800, not emerging in other sodium channel subtypes, is responsible for JZTX-III selectively interacting with Nav1.5. The reverse mutation D816R in Nav1.7 greatly increased the sensitivity of the neuronal subtype to JZTX-III. Conversely, the mutation R800D in Nav1.5 decreased JZTX-III's IC₅₀ by 72-fold. Therefore, our results indicated that JZTX-III is a site 4 toxin, but does not possess the same critical residues on sodium channels as other site 4 toxins. Our data also revealed the underlying mechanism for JZTX-III to be highly specific for the cardiac sodium channel.

RNA editing by mammalian ADARs

The main type of RNA editing in mammals is the conversion of adenosine to inosine which is translated as if it were guanosine. The enzymes that catalyze this reaction are ADARs (adenosine deaminases that act on RNA), of which there are four in mammals, two of which are catalytically inactive. ADARs edit transcripts that encode proteins expressed mainly in the CNS and editing is crucial to maintain a correctly functioning nervous system. However, the majority of editing has been found in transcripts encoding Alu repeat elements and the biological role of this editing remains a mystery. This chapter describes in detail the different ADAR enzymes and the phenotype of animals that are deficient in their activity. Besides being enzymes, ADARs are also double-stranded RNA-binding proteins, so by binding alone they can interfere with other processes such as RNA interference. Lack of editing by ADARs has been implicated in disorders such as forebrain ischemia and Amyotrophic Lateral Sclerosis (ALS) and this will also be discussed.

Functional and structural studies of TRP channels heterologously expressed in budding yeast

The transient receptor potential (TRP) superfamily is one of the largest families of cation channels. The metazoan TRP family has been subdivided into major branches: TRPC, TRPA, TRPM, TRPP, TRPV, TRPML, and TRPN, while the TRPY family is found in fungi. They are involved in many physiological processes and in the pathogenesis of various disorders. An efficient high-yield expression system for TRP channels is a necessary step towards biophysical and biochemical characterization and structural analysis of these proteins, and the budding yeast, Saccharomyces cerevisiae has proven to be very useful for this purpose. In addition, genetic screens in this organism can be carried out rapidly to identify amino acid residues important for function and to generate useful mutants. Here we present an overview of current developments towards understanding TRP channel function and structure using Saccharomyces cerevisiae as an expression system. In addition, we will summarize recent progress in understanding gating mechanisms of TRP channels using endogenously expressing TRPY channels in S. cerevisiae, and insights gained from genetic screens for mutants in mammalian channels. The discussion will focus particular attention of the use of cryo-electron microscopy (cryo-EM) to determine TRP channel structure, and outlines a "divide and concur" methodology for combining high resolution structures of TRP channel domains determined by X-ray crystallography with lower resolution techniques including cryo-EM and spectroscopy.

The biochemistry, ultrastructure, and subunit assembly mechanism of AMPA receptors

The AMPA-type ionotropic glutamate receptors (AMPA-Rs) are tetrameric ligand-gated ion channels that play crucial roles in synaptic transmission and plasticity. Our knowledge about the ultrastructure and subunit assembly mechanisms of intact AMPA-Rs was very limited. However, the new studies using single particle EM and X-ray crystallography are revealing important insights. For example, the tetrameric crystal structure of the GluA2cryst construct provided the atomic view of the intact receptor. In addition, the single particle EM structures of the subunit assembly intermediates revealed the conformational requirement for the dimer-to-tetramer transition during the maturation of AMPA-Rs. These new data in the field provide new models and interpretations. In the brain, the native AMPA-R complexes contain auxiliary subunits that influence subunit assembly, gating, and trafficking of the AMPA-Rs. Understanding the mechanisms of the auxiliary subunits will become increasingly important to precisely describe the function of AMPA-Rs in the brain. The AMPA-R proteomics studies continuously reveal a previously unexpected degree of molecular heterogeneity of the complex. Because the AMPA-Rs are important drug targets for treating various neurological and psychiatric diseases, it is likely that these new native complexes will require detailed mechanistic analysis in the future. The current ultrastructural data on the receptors and the receptor-expressing stable cell lines that were developed during the course of these studies are useful resources for high throughput drug screening and further drug designing. Moreover, we are getting closer to understanding the precise mechanisms of AMPA-R-mediated synaptic plasticity.

Protein folding in membranes

Separation of cells and organelles by bilayer membranes is a fundamental principle of life. Cellular membranes contain a baffling variety of proteins, which fulfil vital functions as receptors and signal transducers, channels and transporters, motors and anchors. The vast majority of membrane-bound proteins contain bundles of alpha-helical transmembrane domains. Understanding how these proteins adopt their native, biologically active structures in the complex milieu of a membrane is therefore a major challenge in today's life sciences. Here, we review recent progress in the folding, unfolding and refolding of alpha-helical membrane proteins and compare the molecular interactions that stabilise proteins in lipid bilayers. We also provide a critical discussion of a detergent denaturation assay that is increasingly used to determine membrane-protein stability but is not devoid of conceptual difficulties.

Interactions of H562 in the S5 helix with T618 and S621 in the pore helix are important determinants of hERG1 potassium channel structure and function

hERG1 is a member of the cyclic nucleotide binding domain family of K(+) channels. Alignment of cyclic nucleotide binding domain channels revealed an evolutionary conserved sequence HwX(A/G)C in the S5 domain. We reasoned that histidine 562 in hERG1 could play an important structure-function role. To explore this role, we created in silica models of the hERG1 pore domain based on the KvAP crystal structure with Rosetta-membrane modeling and molecular-dynamics simulations. Simulations indicate that the H562 residue in the S5 helix spans the gap between the S5 helix and the pore helix, stabilizing the pore domain, and that mutation at the H562 residue leads to a disruption of the hydrogen bonding to T618 and S621, resulting in distortion of the selectivity filter. Analysis of the simulated point mutations at positions 562/618/621 showed that the reciprocal double mutations H562W/T618I would partially restore the orientation of the 562 residue. Matching hydrophobic interactions between mutated W562 residue and I618 partially compensate for the disrupted hydrogen bonding. Complementary in vitro electrophysiological studies confirmed the results of the molecular-dynamics simulations on single mutations at positions 562, 618, and 621. Experimentally, mutations of the H562 to tryptophan produced a functional channel, but with slowed deactivation and shifted V(1/2) of activation. Furthermore, the double mutation T618I/H562W rescued the defects seen in activation, deactivation, and potassium selectivity seen with the H562W mutation. In conclusion, interactions between H562 in the S5 helix and amino acids in the pore helix are important determinants of hERG1 potassium channel function, as confirmed by theory and experiment.

Toward structural elucidation of the gamma-secretase complex

Gamma-Secretase is an intramembrane protease complex that mediates the Notch signaling pathway and the production of amyloid beta-proteins. As such, this enzyme has emerged as an important target for development of novel therapeutics for Alzheimer disease and cancer. Great progress has been made in the identification and characterization of the membrane complex and its biological functions. One major challenge now is to illuminate the structure of this fascinating and important protease at atomic resolution. Here, we review recent progress on biochemical and biophysical probing of the structure of the four-component complex and discuss obstacles and potential pathways toward elucidating its detailed structure.

Electromechanical coupling in the membranes of Shaker-transfected HEK cells

Membranes flex with changes in transmembrane potential as a result of changes in interfacial tension, the Lippman effect. We studied the membrane electromotility of Shaker K(+)-transfected HEK-293 cells in real time by using combined patch-clamp atomic force microscopy. In the voltage range where the channels were closed, Shaker expression had little effect on electromotility relative to wild-type cells. Depolarization between -120 and -40 mV resulted in a linear upward cantilever deflection equivalent to an increase in membrane tension. However, when depolarized sufficiently for channel opening, the electromotility saturated and only recovered over 10 s of milliseconds. This remarkable loss of motility was associated with channel opening, not ionic flux or movement of the voltage sensors. The IL mutant of Shaker, in which the voltage dependence of channel opening but not sensor movement is shifted to more positive potentials, caused the loss of electromotility saturation also to shift to more positive potentials. The temporary loss of electromotility associated with channel opening is probably caused by local buckling of the bilayer as the inner half of the channel expands as expected from X-ray structural data.

Bimane fluorescence scanning suggests secondary structure near the S3-S4 linker of BK channels

Gating of large conductance Ca(2+)-activated K(+) channels (BK or maxi-K channels) is controlled by a Ca(2+)-sensor, formed by the channel cytoplasmic C-terminal domain, and a voltage sensor, formed by its S0-S4 transmembrane helices. Here we analyze structural properties of a portion of the BK channel voltage sensing domain, the S3-S4 linker, using fluorescence lifetime spectroscopy. Single residues in the S3-S4 linker region were substituted with cysteine, and the cysteine-substituted mutants were expressed in CHO cells and covalently labeled with the sulfhydryl-reactive fluorophore monobromo-trimethylammonio-bimane (qBBr). qBBr fluorescence is quenched by tryptophan and, to a lesser extent, tyrosine side chains. We found that qBBr fluorescence in several of the labeled cysteine-substituted channels shows position-specific quenching, as indicated by increase of the brief lifetime component of the qBBr fluorescence decay. Quenching was reduced with the mutation W203F (in the S4 segment), suggesting that Trp-203 acts as a quenching group. Our results suggest a working hypothesis for the secondary structure of the BK channel S3-S4 region, and places residues Leu-204, Gly-205, and Leu-206 within the extracellular end of the S4 helix.

Locations of the beta1 transmembrane helices in the BK potassium channel

BK channels are composed of alpha-subunits, which form a voltage- and Ca(2+)-gated potassium channel, and of modulatory beta-subunits. The beta1-subunit is expressed in smooth muscle, where it renders the BK channel sensitive to [Ca(2+)](i) in a voltage range near the smooth-muscle resting potential and slows activation and deactivation. BK channel acts thereby as a damped feedback regulator of voltage-dependent Ca(2+) channels and of smooth muscle tone. We explored the contacts between alpha and beta1 by determining the extent of endogenous disulfide bond formation between cysteines substituted just extracellular to the two beta1 transmembrane (TM) helices, TM1 and TM2, and to the seven alpha TM helices, consisting of S1-S6, conserved in all voltage-dependent potassium channels, and the unique S0 helix, which we previously concluded was partly surrounded by S1-S4. We now find that the extracellular ends of beta1 TM2 and alpha S0 are in contact and that beta1 TM1 is close to both S1 and S2. The extracellular ends of TM1 and TM2 are not close to S3-S6. In almost all cases, cross-linking of TM2 to S0 or of TM1 to S1 or S2 shifted the conductance-voltage curves toward more positive potentials, slowed activation, and speeded deactivation, and in general favored the closed state. TM1 and TM2 are in position to contribute, in concert with the extracellular loop and the intracellular N- and C-terminal tails of beta1, to the modulation of BK channel function.

Voltage-induced morphological modifications in oocyte membranes containing exogenous K+ channels studied by electrochemical scanning force microscopy

We report on a novel use of electrochemical scanning force microscopy (SFM) for the investigation of morphological modifications occurring in plasma membranes containing voltage-gated ion channels, on membrane potential variation. Membrane patches of Xenopus laevis oocytes microinjected with exogenous KAT1 cRNA, deposited by a stripping method at the surface of a derivatized gold film in inside-out configuration, have been imaged by SFM in an electrochemical cell. A potentiostat was used to maintain a desired potential drop across the membrane. Performing imaging at potential values corresponding to open (-120 mV) and closed (+20 mV) states for KAT1, morphological differences in localized sample zones were observed. Particularly, cross-shaped features involving a significant membrane portion appear around putative channel locations. The reported approach constitutes the first demonstration of an SPM-based experimental technique suitable to investigate the rearrangements occurring to the plasma membrane containing voltage-gated channels on transmembrane potential variation.

Double bilayers and transmembrane gradients: a molecular dynamics study of a highly charged peptide

The position and extent of movement of a charged peptide within a membrane bilayer provides much controversy. In our study, we have examined the nature of the highly charged helix-turn-helix motif (S3b and S4) to address how a highly charged peptide is stabilized within a bilayer in the presence of various transmembrane electrical potentials. Our double-bilayer simulation results show how the variation of the salt concentrations between the inner and outer bath establishes a transmembrane potential. Our results also show that important features of the peptide affected by changes in electrical potential are the center of mass depth, the swivel/kink degrees of conformation, and the hydrogen-bonding patterns. As the voltage gradient across the bilayer increased, the center of mass of the peptide shifted in a direction toward the outer bath. The peptide also has a higher percent helical content and the swivel/kink conformation is more rigid for nonpolarized systems where no voltage drop occurred between salt baths. Our results also provide some suggestions for how this domain may be affected by environmental changes as part of the voltage sensor in a K-channel.

Polyunsaturated fatty acids and cerebrospinal fluid from children on the ketogenic diet open a voltage-gated K channel: a putative mechanism of antiseizure action

PURPOSE

Many children with epilepsy do not satisfactorily respond to conventional pharmacological therapy, but to the ketogenic diet, a high-fat, low-carbohydrate diet. This diet increases the concentrations of ketone bodies and polyunsaturated fatty acids (PUFAs) in cerebrospinal fluid (CSF) and plasma. However, its anticonvulsant mechanism is not known.

METHODS

To investigate the mechanism by which the diet protects against seizures, we studied the effects of several PUFAs (docosahexaenoic acid, eicosapentaenoic acid, and linoleic acid), ketone bodies (beta-hydroxybuturic acid and acetoacetic acid), and CSF from patients on the ketogenic diet on the voltage-gated Shaker K channel expressed in Xenopus oocytes.

RESULTS

We found that PUFAs at concentrations down to 21microM clearly increased the K current by shifting the conductance versus voltage curve in negative direction along the voltage axis. CSF from patients on the ketogenic diet has similar but smaller effects. In contrast, high concentrations (1-5mM) of ketone bodies did not affect the K current. Computer simulations showed that the observed shifts for clinically relevant concentrations of PUFAs, and CSF from patients could effectively impair repetitive firing.

CONCLUSIONS

These data suggest that the ketogenic diet could prevent epileptic seizures by PUFA-induced openings of voltage-gated K channels.

The structure of the lipid-embedded potassium channel voltage sensor determined by double-electron-electron resonance spectroscopy

A four-pulse electron paramagnetic resonance experiment was used to measure long-range inter-subunit distances in reconstituted KvAP, a voltage-dependent potassium (Kv) channel. The measurements have allowed us to reach the following five conclusions about the native structure of the voltage sensor of KvAP. First, the S1 helix of the voltage sensor engages in a helix packing interaction with the pore domain. Second, the crystallographically observed antiparallel helix-turn-helix motif of the voltage-sensing paddle is retained in the membrane-embedded voltage sensor. Third, the paddle is oriented in such a way as to expose one face to the pore domain and the opposite face to the membrane. Fourth, the paddle and the pore domain appear to be separated by a gap that is sufficiently wide for lipids to penetrate between the two domains. Fifth, the critical voltage-sensing arginine residues on the paddle appear to be lipid exposed. These results demonstrate the importance of the membrane for the native structure of Kv channels, suggest that lipids are an integral part of their native structure, and place the voltage-sensing machinery into a complex lipid environment near the pore domain.

Bubbles, gating, and anesthetics in ion channels

We suggest that bubbles are the bistable hydrophobic gates responsible for the on-off transitions of single channel currents. In this view, many types of channels gate by the same physical mechanism-dewetting by capillary evaporation-but different types of channels use different sensors to modulate hydrophobic properties of the channel wall and thereby trigger and control bubbles and gating. Spontaneous emptying of channels has been seen in many simulations. Because of the physics involved, such phase transitions are inherently sensitive, unstable threshold phenomena that are difficult to simulate reproducibly and thus convincingly. We present a thermodynamic analysis of a bubble gate using morphometric density functional theory of classical (not quantum) mechanics. Thermodynamic analysis of phase transitions is generally more reproducible and less sensitive to details than simulations. Anesthetic actions of inert gases-and their interactions with hydrostatic pressure (e.g., nitrogen narcosis)-can be easily understood by actions on bubbles. A general theory of gas anesthesia may involve bubbles in channels. Only experiments can show whether, or when, or which channels actually use bubbles as hydrophobic gates: direct observation of bubbles in channels is needed. Existing experiments show thin gas layers on hydrophobic surfaces in water and suggest that bubbles nearly exist in bulk water.

A hydrophobic element secures S4 voltage sensor in position in resting Shaker K+ channels

The S4 transmembrane alpha-helix in voltage-gated channels contains several regularly spaced basic amino acid residues that could be protonated and moved across the membrane electric field in response to membrane potential changes. The translocation of the charge-carrying S4 transduces membrane voltage to gating conformational changes of the channel, but how it is positioned and moved with respect to membrane lipid remains controversial. We found that hydrophilic and especially arginine and lysine substitution for L361 at the external end of S4 causes a large negative shift with shallowed slope of both activation and inactivation curves in Shaker K+ channels. Also, the macroscopic kinetics of activation and inactivation become much faster and barely voltage dependent, especially in the L361R mutant channel. These steady-state and kinetic data suggest that the replacement of one single hydrophobic residue, leucine, with arginine may profoundly destabilize the resting conformation of S4, which therefore takes a partially extruded position (partly activated position) at resting potentials (e.g. -120 mV). Consistently, the L361R point mutation gives rise to an extracellularly exposed R365C that is readily modified by external hydrophilic sulfhydryl-specific agents in the resting channel. Moreover, the extruded S4 in the L361R mutant channel could be retracted by strong hyperpolarizing potentials ( approximately -180 mV), from which the mutant channel is gated with slower kinetics but evidently stronger voltage dependence. We conclude that hydrophobic interaction involving a highly conserved residue at the top of S4 is crucial for properly securing the gating voltage sensor in the resting position and thus appropriate gating control of the voltage-gated channels.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Phospholipids and the origin of cationic gating charges in voltage sensors

Cells communicate with their external environment through physical and chemical processes that take place in the cell-surrounding membrane. The membrane serves as a barrier as well as a special environment in which membrane proteins are able to carry out important processes. Certain membrane proteins have the ability to detect the membrane voltage and regulate ion conduction or enzyme activity. Such voltage-dependent processes rely on the action of protein domains known as voltage sensors, which are embedded inside the cell membrane and contain an excess of positively charged amino acids, which react to an electric field. How does the membrane create an environment suitable for voltage sensors? Here we show under a variety of conditions that the function of a voltage-dependent K+ channel is dependent on the negatively charged phosphodiester of phospholipid molecules. A non-voltage-dependent K+ channel does not exhibit the same dependence. The data lead us to propose that the phospholipid membrane, by providing stabilizing interactions between positively charged voltage-sensor arginine residues and negatively charged lipid phosphodiester groups, provides an appropriate environment for the energetic stability and operation of the voltage-sensing machinery. We suggest that the usage of arginine residues in voltage sensors is an adaptation to the phospholipid composition of cell membranes.