Inhibition of the hERG Potassium Channel by a Methanesulphonate-Free E-4031 Analogue

hERG (human Ether-à-go-go Related Gene)-encoded potassium channels underlie the cardiac rapid delayed rectifier (IKr) potassium current, which is a major target for antiarrhythmic agents and diverse non-cardiac drugs linked to the drug-induced form of long QT syndrome. E-4031 is a high potency hERG channel inhibitor from the methanesulphonanilide drug family. This study utilized a methanesulphonate-lacking E-4031 analogue, "E-4031-17", to evaluate the role of the methanesulphonamide group in E-4031 inhibition of hERG. Whole-cell patch-clamp measurements of the hERG current (IhERG) were made at physiological temperature from HEK 293 cells expressing wild-type (WT) and mutant hERG constructs. For E-4031, WT IhERG was inhibited by a half-maximal inhibitory concentration (IC50) of 15.8 nM, whilst the comparable value for E-4031-17 was 40.3 nM. Both compounds exhibited voltage- and time-dependent inhibition, but they differed in their response to successive applications of a long (10 s) depolarisation protocol, consistent with greater dissociation of E-4031-17 than the parent compound between applied commands. Voltage-dependent inactivation was left-ward voltage shifted for E-4031 but not for E-4031-17; however, inhibition by both compounds was strongly reduced by attenuated-inactivation mutations. Mutations of S6 and S5 aromatic residues (F656V, Y652A, F557L) greatly attenuated actions of both drugs. The S624A mutation also reduced IhERG inhibition by both molecules. Overall, these results demonstrate that the lack of a methanesulphonate in E-4031-17 is not an impediment to high potency inhibition of IhERG.

Pro-arrhythmic effects of gain-of-function potassium channel mutations in the short QT syndrome

The congenital short QT syndrome (SQTS) is a rare condition characterized by abbreviated rate-corrected QT (QTc) intervals on the electrocardiogram and by increased susceptibility to both atrial and ventricular arrhythmias and sudden death. Although mutations to multiple genes have been implicated in the SQTS, evidence of causality is particularly strong for the first three (SQT1-3) variants: these result from gain-of-function mutations in genes that encode K⁺ channel subunits responsible, respectively, for the I_Kr, I_Ks and I_K1 cardiac potassium currents. This article reviews evidence for the impact of SQT1-3 missense potassium channel gene mutations on the electrophysiological properties of I_Kr, I_Ks and I_K1 and of the links between these changes and arrhythmia susceptibility. Data from experimental and simulation studies and future directions for research in this field are considered. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Evolutionary coupling analysis guides identification of mistrafficking-sensitive variants in cardiac K+ channels: Validation with hERG

Loss of function (LOF) mutations of voltage sensitive K+ channel proteins hERG (Kv11.1) and KCNQ1 (Kv7.1) account for the majority of instances of congenital Long QT Syndrome (cLQTS) with the dominant molecular phenotype being a mistrafficking one resulting from protein misfolding. We explored the use of Evolutionary Coupling (EC) analysis, which identifies evolutionarily conserved pairwise amino acid interactions that may contribute to protein structural stability, to identify regions of the channels susceptible to misfolding mutations. Comparison with published experimental trafficking data for hERG and KCNQ1 showed that the method strongly predicts "scaffolding" regions of the channel membrane domains and has useful predictive power for trafficking phenotypes of individual variants. We identified a region in and around the cytoplasmic S2-S3 loop of the hERG Voltage Sensor Domain (VSD) as susceptible to destabilising mutation, and this was confirmed using a quantitative LI-COR ® based trafficking assay that showed severely attenuated trafficking in eight out of 10 natural hERG VSD variants selected using EC analysis. Our analysis highlights an equivalence in the scaffolding structures of the hERG and KCNQ1 membrane domains. Pathogenic variants of ion channels with an underlying mistrafficking phenotype are likely to be located within similar scaffolding structures that are identifiable by EC analysis.

Electrophysiological characterization of the modified hERGT potassium channel used to obtain the first cryo-EM hERG structure

The voltage-gated hERG (human-Ether-à-go-go Related Gene) K+ channel plays a fundamental role in cardiac action potential repolarization. Loss-of-function mutations or pharmacological inhibition of hERG leads to long QT syndrome, whilst gain-of-function mutations lead to short QT syndrome. A recent open channel cryo-EM structure of hERG represents a significant advance in the ability to interrogate hERG channel structure-function. In order to suppress protein aggregation, a truncated channel construct of hERG (hERGT ) was used to obtain this structure. In hERGT cytoplasmic domain residues 141 to 350 and 871 to 1,005 were removed from the full-length channel protein. There are limited data on the electrophysiological properties of hERGT channels. Therefore, this study was undertaken to determine how hERGT influences channel function at physiological temperature. Whole-cell measurements of hERG current (IhERG ) were made at 37°C from HEK 293 cells expressing wild-type (WT) or hERGT channels. With a standard +20 mV activating command protocol, neither end-pulse nor tail IhERG density significantly differed between WT and hERGT . However, the IhERG deactivation rate was significantly slower for hERGT . Half-maximal activation voltage (V0.5 ) was positively shifted for hERGT by ~+8 mV (p < .05 versus WT), without significant change to the activation relation slope factor. Neither the voltage dependence of inactivation, nor time course of development of inactivation significantly differed between WT and hERGT , but recovery of IhERG from inactivation was accelerated for hERGT (p < .05 versus WT). Steady-state "window" current was positively shifted for hERGT with a modest increase in the window current peak. Under action potential (AP) voltage clamp, hERGT IhERG showed modestly increased current throughout the AP plateau phase with a significant increase in current integral during the AP. The observed consequences for hERGT IhERG of deletion of the two cytoplasmic regions may reflect changes to electrostatic interactions influencing the voltage sensor domain.

Serine mutation of a conserved threonine in the hERG K+ channel S6-pore region leads to loss-of-function through trafficking impairment

The human Ether-à-go-go Related Gene (hERG) encodes a potassium channel responsible for the cardiac rapid delayed rectifier K⁺ current, I_Kr, which regulates ventricular repolarization. Loss-of-function hERG mutations underpin the LQT2 form of congenital long QT syndrome. This study was undertaken to elucidate the functional consequences of a variant of uncertain significance, T634S, located at a highly conserved position at the top of the S6 helix of the hERG channel. Whole-cell patch-clamp recordings were made at 37 °C of hERG current (I_hERG) from HEK 293 cells expressing wild-type (WT) hERG, WT+T634S and hERG-T634S alone. When the T634S mutation was expressed alone little or no I_hERG could be recorded. Co-expressing WT and hERG-T634S suppressed I_hERG tails by ∼57% compared to WT alone, without significant alteration of voltage dependent activation of I_hERG. A similar suppression of I_hERG was observed under action potential voltage clamp. Comparable reduction of I_Kr in a ventricular AP model delayed repolarization and led to action potential prolongation. A LI-COR® based On/In-Cell Western assay showed that cell surface expression of hERG channels in HEK 293 cells was markedly reduced by the T634S mutation, whilst total cellular hERG expression was unaffected, demonstrating impaired trafficking of the hERG-T634S mutant. Incubation with E−4031, but not lumacaftor, rescued defective hERG-T634S channel trafficking and I_hERG density. In conclusion, these data identify hERG-T634S as a rescuable trafficking defective mutation that reduces I_Kr sufficiently to delay repolarization and, thereby, potentially produce a LQT2 phenotype.

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hERG potassium channel variants can cause dangerous ventricular arrhythmias.

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An S6 helix threonine in hERG, T634, is highly conserved amongst potassium channels.

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The T634S mutation reduces hERG current and its contribution to ventricular repolarization.

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The T634S mutation decreases hERG channel surface expression but not synthesis.

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T634S-induced hERG trafficking impairment is pharmacologically rescuable.

The macrolide drug erythromycin does not protect the hERG channel from inhibition by thioridazine and terfenadine

The macrolide antibiotic erythromycin has been associated with QT interval prolongation and inhibition of the hERG-encoded channels responsible for the rapid delayed rectifier K⁺ current I(_Kr ). It has been suggested that low concentrations of erythromycin may have a protective effect against hERG block and associated drug-induced arrhythmia by reducing the affinity of the pore-binding site for high potency hERG inhibitors. This study aimed to explore further the notion of a potentially protective effect of erythromycin. Whole-cell patch-clamp experiments were performed in which hERG-expressing mammalian (Human Embryonic Kidney; HEK) cells were preincubated with low to moderate concentrations of erythromycin (3 or 30 µM) prior to whole-cell patch clamp recordings of hERG current (I_hERG ) at 37°C. In contrast to a previous report, exposure to low concentrations of erythromycin did not reduce pharmacological sensitivity of hERG to the antipsychotic thioridazine and antihistamine terfenadine. The IC₅₀ value for I_hERG tail inhibition by terfenadine was decreased by ~32-fold in the presence of 3 µM erythromycin (p < .05 vs. no preincubation). Sensitivity to thioridazine remained unchanged (p > .05 vs. no preincubation). The effects of low concentrations of erythromycin were investigated for a series of pore blocking drugs, and the results obtained were consistent with additive and/or synergistic effects. Experiments with the externally acting blocker BeKm-1 on WT hERG and a pore mutant (F656V) were used to explore the location of the binding site for erythromycin. Our data are inconsistent with the use of erythromycin for the management of drug-induced QT prolongation.

An Update on the Structure of hERG

The human voltage-sensitive K⁺ channel hERG plays a fundamental role in cardiac action potential repolarization, effectively controlling the QT interval of the electrocardiogram. Inherited loss- or gain-of-function mutations in hERG can result in dangerous “long” (LQTS) or “short” QT syndromes (SQTS), respectively, and the anomalous susceptibility of hERG to block by a diverse range of drugs underlies an acquired LQTS. A recent open channel cryo-EM structure of hERG should greatly advance understanding of the molecular basis of hERG channelopathies and drug-induced LQTS. Here we describe an update of recent research that addresses the nature of the particular gated state of hERG captured in the new structure, and the insight afforded by the structure into the molecular basis for high affinity drug block of hERG, the binding of hERG activators and the molecular basis of hERG's peculiar gating properties. Interpretation of the pharmacology of natural SQTS mutants in the context of the structure is a promising approach to understanding the molecular basis of hERG inactivation, and the structure suggests how voltage-dependent changes in the membrane domain may be transmitted to an extracellular “turret” to effect inactivation through aromatic side chain motifs that are conserved throughout the KCNH family of channels.

Identification of a proton sensor that regulates conductance and open time of single hERG channels

The hERG potassium channel influences ventricular action potential duration. Extracellular acidosis occurs in pathological states including cardiac ischaemia. It reduces the amplitude of hERG current and speeds up deactivation, which can alter cardiac excitability. This study aimed to identify the site of action by which extracellular protons regulate the amplitude of macroscopic hERG current. Recordings of macroscopic and single hERG1a and 1b channel activity, mutagenesis, and the recent cryoEM structure for hERG were employed. Single hERG1a and 1b channels displayed open times that decreased with membrane depolarization, suggestive of a blocking mechanism that senses approximately 20% of the membrane electric field. This mechanism was sensitive to pH; extracellular acidosis reduced both hERG1a and1b channel open time and conductance. The effects of acidosis on macroscopic current amplitude and deactivation displayed different sensitivities to protons. Point mutation of a pair of residues (E575/H578) in the pore turret abolished the acidosis-induced decrease of current amplitude, without affecting the change in current deactivation. In single hERG1a channel recordings, the conductance of the double-mutant channel was unaffected by extracellular acidosis. These findings identify residues in the outer turret of the hERG channel that act as a proton sensor to regulate open time and channel conductance.

Functional and pharmacological characterization of an S5 domain hERG mutation associated with short QT syndrome

Congenital short QT syndrome (SQTS) is a repolarization disorder characterized by abbreviated QT intervals, atrial and ventricular arrhythmias and a risk of sudden death. This study characterized a missense mutation (I560T) in the S5 domain of the hERG K⁺ channel that has been associated with variant 1 of the SQTS. Whole cell patch clamp recordings of wild-type (WT) and I560T hERG current (I_hERG) were made at 37 °C from hERG expressing HEK 293 cells, and the structural context of the mutation was investigated using a recently reported cryo-EM structure of hERG. Under conventional voltage clamp, the I560T mutation increased I_hERG amplitude without altering the voltage-dependence of activation, although it accelerated activation time-course and also slowed deactivation time-course at some voltages. The voltage dependence of I_hERG inactivation was positively shifted (by ∼24 mV) and the time-course of inactivation was slowed by the I560T mutation. There was also a modest decrease in K⁺ over Na⁺ ion selectivity with the I560T mutation. Under action potential (AP) voltage clamp, the net charge carried by hERG was significantly increased during ventricular, Purkinje fibre and atrial APs, with maximal I_hERG also occurring earlier during the plateau phase of ventricular and Purkinje fibre APs. The I560T mutation exerted only a modest effect on quinidine sensitivity of I_hERG: the IC₅₀ for mutant I_hERG was 2.3 fold that for WT I_hERG under conventional voltage clamp. Under AP voltage clamp the inhibitory effect of 1 μM quinidine was largely retained for I560T hERG and the timing of peak I560T I_hERG was altered towards that of the WT channel. In both the open channel structure and a closed hERG channel model based on the closely-related EAG structure, I560T side-chains were oriented towards membrane lipid and away from adjacent domains of the channel, contrasting with previous predictions based on homology modelling. In summary, the I560T mutation produces multiple effects on hERG channel operation that result in a gain-of-function that is expected to abbreviate ventricular, atrial and Purkinje fibre repolarization. Quinidine is likely to be of value in offsetting the increase in I_hERG and altered I_hERG timing during ventricular APs in SQTS with this mutation.

Analysis of α-Dystroglycan/LG Domain Binding Modes: Investigating Protein Motifs That Regulate the Affinity of Isolated LG Domains

Dystroglycan (DG) is an adhesion complex that links the cytoskeleton to the surrounding extracellular matrix in skeletal muscle and a wide variety of other tissues. It is composed of a highly glycosylated extracellular α-DG associated noncovalently with a transmembrane β-DG whose cytodomain interacts with dystrophin and its isoforms. Alpha-dystroglycan (α-DG) binds tightly and in a calcium-dependent fashion to multiple extracellular proteins and proteoglycans, each of which harbors at least one, or, more frequently, tandem arrays of laminin-globular (LG) domains. Considerable biochemical and structural work has accumulated on the α-DG-binding LG domains, highlighting a significant heterogeneity in ligand-binding properties of domains from different proteins as well as between single and multiple LG domains within the same protein. Here we review biochemical, structural, and functional information on the LG domains reported to bind α-dystroglycan. In addition, we have incorporated bioinformatics and modeling to explore whether specific motifs responsible for α-dystroglycan recognition can be identified within isolated LG domains. In particular, we analyzed the LG domains of slits and agrin as well as those of paradigmatic α-DG non-binders such as laminin-α3. While some stretches of basic residues may be important, no universally conserved motifs could be identified. However, the data confirm that the coordinated calcium atom within the LG domain is needed to establish an interaction with the sugars of α-DG, although it appears that this alone is insufficient to mediate significant α-DG binding. We develop a scenario involving different binding modes of a single LG domain unit, or tandemly repeated units, with α-DG. A variability of binding modes might be important to generate a range of affinities to allow physiological regulation of this interaction, reflecting its crucial biological importance.

Action potential clamp characterization of the S631A hERG mutation associated with short QT syndrome

The hERG potassium channel is critical to normal repolarization of cardiac action potentials (APs) and loss- and gain-of-function hERG mutations are associated, respectively, with long and short QT syndromes, pathological conditions that can lead to arrhythmias and sudden death. hERG current (IhERG ) exhibits uniquely fast inactivation involving conformational changes to the channel pore. The S631A hERG pore mutation was originally engineered to interrogate hERG channel inactivation, but has very recently been found in a family with short QT syndrome (SQTS). Accordingly, this study characterized the effects of the S631A mutation on IhERG profile during ventricular, atrial, and Purkinje fiber (PF) AP waveforms, using patch clamp recording from hERG expressing HEK 293 cells at 37°C. Under conventional voltage clamp, the current-voltage (I-V) relation for IhERG exhibited a marked right-ward shift in the region of negative slope at positive membrane potentials. Under ventricular AP clamp, the S631A mutation resulted in augmented IhERG , which also peaked much earlier during the AP plateau than did wild-type (WT) IhERG . Instantaneous I-V relations showed a marked positive shift in peak repolarizing current during the ventricular AP in the S631A setting, while the instantaneous conductance-voltage relation showed an earlier and more sustained rise in S631A compared to WT IhERG conductance during ventricular repolarization. Experiments with atrial and PF APs in each case also showed augmented and positively shifted IhERG in the S631A setting, indicating that the S631A mutation is likely to accelerate repolarization in all three cardiac regions. Ventricular AP clamp experiments showed retained effectiveness of the class Ia antiarrhythmic drug quinidine (1 μmol/L) against S631A IhERG . Quinidine is thus likely to be effective in reducing excessively fast repolarization in SQTS resulting from the S631A hERG mutation.

Structural implications of hERG K+ channel block by a high-affinity minimally structured blocker

Cardiac potassium channels encoded by human ether-à-go-go–related gene (hERG) are major targets for structurally diverse drugs associated with acquired long QT syndrome. This study characterized hERG channel inhibition by a minimally structured high-affinity hERG inhibitor, Cavalli-2, composed of three phenyl groups linked by polymethylene spacers around a central amino group, chosen to probe the spatial arrangement of side chain groups in the high-affinity drug-binding site of the hERG pore. hERG current (I_hERG) recorded at physiological temperature from HEK293 cells was inhibited with an IC₅₀ of 35.6 nm with time and voltage dependence characteristic of blockade contingent upon channel gating. Potency of Cavalli-2 action was markedly reduced for attenuated inactivation mutants located near (S620T; 54-fold) and remote from (N588K; 15-fold) the channel pore. The S6 Y652A and F656A mutations decreased inhibitory potency 17- and 75-fold, respectively, whereas T623A and S624A at the base of the selectivity filter also decreased potency (16- and 7-fold, respectively). The S5 helix F557L mutation decreased potency 10-fold, and both F557L and Y652A mutations eliminated voltage dependence of inhibition. Computational docking using the recent cryo-EM structure of an open channel hERG construct could only partially recapitulate experimental data, and the high dependence of Cavalli-2 block on Phe-656 is not readily explainable in that structure. A small clockwise rotation of the inner (S6) helix of the hERG pore from its configuration in the cryo-EM structure may be required to optimize Phe-656 side chain orientations compatible with high-affinity block.

Arginine side chain interactions and the role of arginine as a gating charge carrier in voltage sensitive ion channels

Gating charges in voltage-sensing domains (VSD) of voltage-sensitive ion channels and enzymes are carried on arginine side chains rather than lysine. This arginine preference may result from the unique hydration properties of the side chain guanidinium group which facilitates its movement through a hydrophobic plug that seals the center of the VSD, as suggested by molecular dynamics simulations. To test for side chain interactions implicit in this model we inspected interactions of the side chains of arginine and lysine with each of the 19 non-glycine amino acids in proteins in the protein data bank. The arginine guanidinium interacts with non-polar aromatic and aliphatic side chains above and below the guanidinium plane while hydrogen bonding with polar side chains is restricted to in-plane positions. In contrast, non-polar side chains interact largely with the aliphatic part of the lysine side chain. The hydration properties of arginine and lysine are strongly reflected in their respective interactions with non-polar and polar side chains as observed in protein structures and in molecular dynamics simulations, and likely underlie the preference for arginine as a mobile charge carrier in VSD.

Voltage sensor gating charge transfer in a hERG potassium channel model

Relaxation of a hERG K⁺ channel model during molecular-dynamics simulation in a hydrated POPC bilayer was accompanied by transitions of an arginine gating charge across a charge transfer center in two voltage sensor domains. Inspection of the passage of arginine side chains across the charge transfer center suggests that the unique hydration properties of the arginine guanidine cation facilitates charge transfer during voltage sensor responses to changes in membrane potential, and underlies the preference of Arg over Lys as a mobile charge carrier in voltage-sensitive ion channels.

Molecular basis of hERG potassium channel blockade by the class Ic antiarrhythmic flecainide

The class Ic antiarrhythmic drug flecainide inhibits KCNH2-encoded "hERG" potassium channels at clinically relevant concentrations. The aim of this study was to elucidate the underlying molecular basis of this action. Patch clamp recordings of hERG current (IhERG) were made from hERG expressing cells at 37°C. Wild-type (WT) IhERG was inhibited with an IC50 of 1.49μM and this was not significantly altered by reversing the direction of K(+) flux or raising external [K(+)]. The use of charged and uncharged flecainide analogues showed that the charged form of the drug accesses the channel from the cell interior to produce block. Promotion of WT IhERG inactivation slowed recovery from inhibition, whilst the N588K and S631A attenuated-inactivation mutants exhibited IC50 values 4-5 fold that of WT IhERG. The use of pore-helix/selectivity filter (T623A, S624A V625A) and S6 helix (G648A, Y652A, F656A) mutations showed <10-fold shifts in IC50 for all but V625A and F656A, which respectively exhibited IC50s 27-fold and 142-fold their WT controls. Docking simulations using a MthK-based homology model suggested an allosteric effect of V625A, since in low energy conformations flecainide lay too low in the pore to interact directly with that residue. On the other hand, the molecule could readily form π-π stacking interactions with aromatic residues and particularly with F656. We conclude that flecainide accesses the hERG channel from the cell interior on channel gating, binding low in the inner cavity, with the S6 F656 residue acting as a principal binding determinant.

hERG potassium channel blockade by the HCN channel inhibitor bradycardic agent ivabradine

Background

Ivabradine is a specific bradycardic agent used in coronary artery disease and heart failure, lowering heart rate through inhibition of sinoatrial nodal HCN‐channels. This study investigated the propensity of ivabradine to interact with KCNH2‐encoded human Ether‐à‐go‐go–Related Gene (hERG) potassium channels, which strongly influence ventricular repolarization and susceptibility to torsades de pointes arrhythmia.

Methods and Results

Patch clamp recordings of hERG current (I_hERG) were made from hERG expressing cells at 37°C. I_hERG was inhibited with an IC₅₀ of 2.07 μmol/L for the hERG 1a isoform and 3.31 μmol/L for coexpressed hERG 1a/1b. The voltage and time‐dependent characteristics of I_hERG block were consistent with preferential gated‐state‐dependent channel block. Inhibition was partially attenuated by the N588K inactivation‐mutant and the S624A pore‐helix mutant and was strongly reduced by the Y652A and F656A S6 helix mutants. In docking simulations to a MthK‐based homology model of hERG, the 2 aromatic rings of the drug could form multiple π‐π interactions with the aromatic side chains of both Y652 and F656. In monophasic action potential (MAP) recordings from guinea‐pig Langendorff‐perfused hearts, ivabradine delayed ventricular repolarization and produced a steepening of the MAPD₉₀ restitution curve.

Conclusions

Ivabradine prolongs ventricular repolarization and alters electrical restitution properties at concentrations relevant to the upper therapeutic range. In absolute terms ivabradine does not discriminate between hERG and HCN channels: it inhibits I_hERG with similar potency to that reported for native I_f and HCN channels, with S6 binding determinants resembling those observed for HCN4. These findings may have important implications both clinically and for future bradycardic drug design.

Assessing hERG pore models as templates for drug docking using published experimental constraints: the inactivated state in the context of drug block

Many structurally and therapeutically diverse drugs interact with the human heart K⁺ channel hERG by binding within the K⁺ permeation pathway of the open channel, leading to drug-induced ‘long QT syndrome’. Drug binding to hERG is often stabilized by inactivation gating. In the absence of a crystal structure, hERG pore homology models have been used to characterize drug interactions. Here we assess potentially inactivated states of the bacterial K⁺ channel, KcsA, as templates for inactivated state hERG pore models in the context of drug binding using computational docking. Although Flexidock and GOLD docking produced low energy score poses in the models tested, each method selected a MthK K⁺ channel-based model over models based on the putative inactivated state KcsA structures for each of the 9 drugs tested. The variety of docking poses found indicates that an optimal arrangement for drug binding of aromatic side chains in the hERG pore can be achieved in several different configurations. This plasticity of the drug “binding site” is likely to be a feature of the hERG inactivated state. The results demonstrate that experimental data on specific drug interactions can be used as structural constraints to assess and refine hERG homology models.

Interactions between voltage sensor and pore domains in a hERG K+ channel model from molecular simulations and the effects of a voltage sensor mutation

The hERG K(+) channel is important for establishing normal electrical activity in the human heart. The channel's unique gating response to membrane potential changes indicates specific interactions between voltage sensor and pore domains that are poorly understood. In the absence of a crystal structure we constructed a homology model of the full hERG membrane domain and performed 0.5 μs molecular dynamics (MD) simulations in a hydrated membrane. The simulations identify potential interactions involving residues at the extracellular surface of S1 in the voltage sensor and at the N-terminal end of the pore helix in the hERG model. In addition, a diffuse interface involving hydrophobic residues on S4 (voltage sensor) and pore domain S5 of an adjacent subunit was stable during 0.5 μs of simulation. To assess the ability of the model to give insight into the effects of channel mutation we simulated a hERG mutant that contains a Leu to Pro substitution in the voltage sensor S4 helical segment (hERG L532P). Consistent with the retention of gated K(+) conductance, the L532P mutation was accommodated in the S4 helix with little disruption of helical structure. The mutation reduced the extent of interaction across the S4-S5 interface, suggesting a structural basis for the greatly enhanced deactivation rate in hERG L532P. The study indicates that pairwise comparison of wild-type and mutated channel models is a useful approach to interpreting functional data where uncertainty in model structures exist.

High potency inhibition of hERG potassium channels by the sodium-calcium exchange inhibitor KB-R7943

BACKGROUND AND PURPOSE

KB-R7943 is an isothiourea derivative that is used widely as a pharmacological inhibitor of sodium-calcium exchange (NCX) in experiments on cardiac and other tissue types. This study investigated KB-R7943 inhibition of hERG (human ether-à-go-go-related gene) K(+) channels that underpin the cardiac rapid delayed rectifier potassium current, I(Kr) .

EXPERIMENTAL APPROACH

Whole-cell patch-clamp measurements were made of hERG current (I(hERG) ) carried by wild-type or mutant hERG channels and of native rabbit ventricular I(Kr) . Docking simulations utilized a hERG homology model built on a MthK-based template.

KEY RESULTS

KB-R7943 inhibited both I(hERG) and native I(Kr) rapidly on membrane depolarization with IC(50) values of ∼89 and ∼120 nM, respectively, for current tails at -40 mV following depolarizing voltage commands to +20 mV. Marked I(hERG) inhibition also occurred under ventricular action potential voltage clamp. I(hERG) inhibition by KB-R7943 exhibited both time- and voltage-dependence but showed no preference for inactivated over activated channels. Results of alanine mutagenesis and docking simulations indicate that KB-R7943 can bind to a pocket formed of the side chains of aromatic residues Y652 and F656, with the compound's nitrobenzyl group orientated towards the cytoplasmic side of the channel pore. The structurally related NCX inhibitor SN-6 also inhibited I(hERG) , but with a markedly reduced potency.

CONCLUSIONS AND IMPLICATIONS

KB-R7943 inhibits I(hERG) /I(Kr) with a potency that exceeds that reported previously for acute cardiac NCX inhibition. Our results also support the feasibility of benzyloxyphenyl-containing NCX inhibitors with reduced potential, in comparison with KB-R7943, to inhibit hERG.

Molecular determinants of hERG potassium channel inhibition by disopyramide

The Class Ia antiarrhythmic drug disopyramide (DISO) causes QT interval prolongation that is potentially dangerous in acquired Long QT Syndrome but beneficial in short QT syndrome, through inhibition of the hERG-encoded channels responsible for rapid delayed rectifier K(+) current (I(Kr)). In this study, alanine mutants of hERG S6 and pore helix residues and MthK-based homology modelling and ligand docking were used to investigate molecular determinants of DISO binding to hERG. Whole-cell hERG current (I(hERG)) recordings were made at 37°C from HEK-293 cells expressing WT or mutant hERG channels. WT outward I(hERG) tails were inhibited with an IC(50) of 7.3μM, whilst inward I(hERG) tails in a high [K(+)](e) of 94mM were blocked with an IC(50) of 25.7μM. The IC(50) for the Y652A mutation was ~55-fold that of WT I(hERG); this mutation also abolished a leftward shift in voltage-dependent I(hERG) activation present for WT hERG. The IC(50) for F656A I(hERG) was ~51 fold its corresponding WT control. In contrast to previously studied methanesulphonanilide hERG inhibitors, neither the G648A S6 nor the T623A and S624A pore helical mutations modified DISO IC(50). Computational docking with the hERG model showed that DISO did not exhibit a single unique binding pose; instead several low energy binding poses at the lower end of the pore cavity favoured interactions with Y652 and F656. In the WT hERG model DISO did not interact directly with residues at the base of the pore helix, consistent with the minimal effect of mutation of these residues on drug block.

The hERG K(+) channel S4 domain L532P mutation: characterization at 37°C

hERG (human Ether-à-go-go Related Gene) is responsible for ion channels mediating rapid delayed rectifier potassium current, I_Kr, which is key to cardiac action potential repolarization. Gain-of-function hERG mutations give rise to the SQT1 variant of the Short QT Syndrome (SQTS). Reggae mutant zebrafish, with a S4 zERG mutation (Leucine499Proline; L499P), display arrhythmic features analogous to those seen in the SQTS. The affected S4 domain ERG residue is highly conserved. This study was executed to determine how the homologous hERG mutation (L532P) influences channel function at 37 °C. Whole-cell measurements of current (I_hERG) were made from HEK 293 cells expressing WT or L532P hERG. The half maximal activation voltage (V_0.5) of L532P I_hERG was positively shifted by ~+36 mV compared to WT I_hERG; however at negative voltages a pronounced L532P I_hERG was observed. Both activation and deactivation time-courses were accelerated for L532P I_hERG. The inactivation V_0.5 for L532P I_hERG was shifted by ~+32 mV. Under action potential (AP) voltage-clamp, L532P I_hERG exhibited a dome-shaped current peaking at ~+16 mV, compared to ~−31 mV for WT-I_hERG. The L532P mutation produced an ~ 5-fold increase in the IC₅₀ for dronedarone inhibition of I_hERG. Homology modeling indicated that the L532 residue within the S4 helix lies closely apposed to the S5 region of an adjacent hERG subunit. Alterations to the S4 domain structure and, potentially, to interactions between adjacent hERG subunits are likely to account for the functional effects of this mutation.

Glucose interactions with a model peptide

Molecular dynamics simulations have been conducted of the helical polypeptide melittin, in concentrated aqueous solutions of the alpha and beta anomers of D-glucopyranose. Glucose is an osmolyte, and it is expected to be preferentially excluded from the surfaces of proteins. This was indeed found to be the case in the simulations. The results indicate that the observed exclusion may have a contribution from an under-representation of hydrogen bonding interactions between glucose groups and exposed side chains, compared to water. However, glucose was found to bind quite specifically to melittin by stacking its hydrophobic face, consisting of aliphatic protons, against the flat hydrophobic face of the indole group of the tryptophan-19 side chain. Although the binding site for this interaction is localized, the binding is weak for both anomers, with a binding free energy estimated as only ∼0.5 kcal/mol (i.e. near k(B)T). The face of the sugar stacked against the Trp indole ring is different for the two anomers of glucose, due to the disruption of the H1-H3-H5 hydrophobic triad of the beta anomer by the axial C1 hydroxyl group in the alpha anomer. The measurable affinity of the sugar for the Trp side chain is consistent with the very frequent occurrence of this group in the binding sites of proteins that complex with sugars.

Amphipathic antimicrobial peptides--from biophysics to therapeutics?

Amphipathic peptides are accommodated within the diffuse gradient of polarity that characterizes the interfacial regions of phospholipid bilayer membranes. Interfacial membrane interactions are key to the diverse biological functions and activities of these peptides, which encompass a large class of antimicrobial peptides, including the helical peptides magainin, melittin, and RTA3 derived from the commensal bacterium Streptococcus mitis. For these peptides in vitro efficacy (high antimicrobial activity with minimal mammalian cell toxicity, equivalent to high potential therapeutic index; PTI), can be broadly understood in relation to the thermodynamics of interfacial binding and membrane disruption in membranes having surface charges that correspond to bacterial and mammalian cell membranes, respectively. Peptides with disrupted amphipathicity resulting from a positively charged amino acid residue on the non-polar helix face, can have greatly enhanced PTI, although a balance of amphipathicity, hydrophobicity and positive charge is required for retention of high antimicrobial activity. These observations are illustrated with recent examples from the literature, and studies on RTA3 and magainin analogues from our laboratories. Despite the identification and optimisation of peptides with very good PTI, a focus on addressing toxicity upon systemic administration and poor in vivo efficacy is likely to be required to translate growing understanding of the relationships between peptide interfacial activity and effects on cells, into novel systemic therapeutics.

Observation of pyridine aggregation in aqueous solution using neutron scattering experiments and MD simulations

Neutron diffraction with isotopic substitution (NDIS) experiments have been used to examine the structuring of aqueous solutions of pyridine. A new method is described for extracting the structure factors relating to intermolecular correlations from neutron scattering experiments on liquid solutions of complex molecular species. This approach performs experiments at different concentrations and exploits the intramolecular coordination number concentration invariance (ICNCI) to separate the internal and intermolecular contributions to the total intensities. The ability of this method to deconvolute molecular and intermolecular correlations is tested and demonstrated using simulated neutron scattering results predicted from molecular dynamics simulations of aqueous solutions of the polyatomic solute pyridine in which the inter- and intramolecular terms are known. The method is then implemented using neutron scattering measurements on solutions of pyridine. The results confirm that pyridine shows a significant propensity to aggregate in solution and demonstrate the prospects for the future application of the ICNCI approach to the study of large polyatomic solutes using next-generation neutron sources and detectors.

Preferential interactions of guanidinum ions with aromatic groups over aliphatic groups

Small angle neutron scattering (SANS) and molecular dynamics (MD) simulations were used to characterize the long-range structuring (aggregation) of aqueous solutions of isopropanol (IPA) and pyridine and the effect on structuring of guanidinium chloride (GdmCl). These solutes serve as highly soluble analogs of the nonpolar aliphatic (IPA) and aromatic (pyridine) side chains of proteins. SANS data showed that isopropanol and pyridine both form clusters in water resulting from interaction between nonpolar groups of the solutes, with pyridine aggregation producing longer-range structuring than isopropanol in 3 m solutions. Addition of GdmCl at 3 m concentration considerably reduced pyridine aggregation but had no effect on isopropanol aggregation. MD simulations of these solutions support the conclusion that long-range structuring involves hydrophobic solute interactions and that Gdm(+) interacts with the planar pyridine group to suppress pyridine-pyridine interactions in solution. Hydrophobic interactions involving the aliphatic groups of isopropanol were unaffected by GdmCl, indicating that the planar and weakly hydrated Gdm(+) cation cannot make productive interactions with the highly curved or "lumpy" aliphatic groups of this solute. These observations support the conclusion that the effects of Gdm(+) ions on protein-stabilizing interactions involving aromatic amino acid side chains make significant contributions to the denaturant activity of GdmCl, whereas interactions with the "lumpy" aliphatic side chains are likely to be less important.

hERG1a/1b heteromeric currents exhibit amplified attenuation of inactivation in variant 1 short QT syndrome

Potassium channels encoded by hERG (human ether-à-go-go-related gene) underlie the cardiac rapid delayed rectifier K⁺ current (I_Kr) and hERG mutations underpin clinically important repolarization disorders. Virtually all electrophysiological investigations of hERG mutations have studied exclusively the hERG1a isoform; however, recent evidence indicates that native I_Kr channels may be comprised of hERG1a together with the hERG1b variant, which has a shorter N-terminus. Here, for the first time, electrophysiological effects were studied of a gain-of-function hERG mutation (N588K; responsible for the ‘SQT1’ variant of the short QT syndrome) on current (I_hERG1a/1b) carried by co-expressed hERG1a/1b channels. There were no significant effects of N588K on I_hERG1a/1b activation or deactivation, but N588K I_hERG1a/1b showed little inactivation up to highly positive voltages (⩽+80 mV), a more marked effect than seen for hERG1a expressed alone. I_hERG1a/1b under action potential voltage-clamp, and the effects on this of the N588K mutation, also showed differences from those previously reported for hERG1a. The amplified attenuation of I_hERG inactivation for the N588K mutation reported here indicates that the study of co-expressed hERG1a/1b channels should be considered when investigating clinically relevant hERG channel mutations, even if these reside outside of the N-terminus region.

The reversal by sulfate of the denaturant activity of guanidinium

Guanidinium (Gdm+) chloride is a powerful protein denaturant, whereas the sulfate dianion (SO42-) is a strong stabilizer of folded protein states; Gdm2SO4 is effectively neutral in its effects on protein stability. While the "neutralizing" effects of protein-stabilizing solutes on the activity of denaturants can be broadly interpreted in terms of additive effects of the solutes, recent experimental and simulation studies support a role for hetero-ion interactions in the effect of sulfate on Gdm+ denaturation [Mason, P. E.; et al. J. Phys. Chem. B 2005, 109, 24185-24196]. Here we describe an experimental strategy for testing this mechanism that involves spectroscopic analysis of the separate effects of alkali metal sulfates (Na2SO4, Rb2SO4), GdmCl, and Gdm2SO4 on the folded populations of several peptides chosen to dissect specific noncovalent contributions to the conformational stability of proteins [alanine-based helical peptides stabilized by hydrogen bonds, tryptophan zipper (trpzip) peptides stabilized largely by cross-strand indole-indole interactions]. While the trpzip peptides are highly sensitive to GdmCl denaturation, they are unaffected by NaCl, Na2SO4, or Gdm2SO4, indicating that the reversal of the denaturant activity of Gdm+ by sulfate in this case is not due to competing stabilizing (sulfate) and destabilizing (Gdm+) interactions. Gdm2SO4 was found to retain considerable denaturant activity against alanine-based alpha-helical peptides. The differences in the effects of Gdm2SO4 on the two peptide types can be understood in terms of the different mechanisms of Gdm+ denaturation of trpzip peptides and helical peptides, respectively, and the specific nature of Gdm+ and SO42- ionic "clustering" that differentially affects the ability of Gdm+ to make the molecular interactions with the peptides that underlie its denaturant activity.

Nanometer-scale ion aggregates in aqueous electrolyte solutions: guanidinium carbonate

Neutron diffraction with isotopic substitution (NDIS) experiments and molecular dynamics (MD) simulations have been used to characterize the structure of aqueous guanidinium carbonate (Gdm2CO3) solutions. The MD simulations found very strong hetero-ion pairing in Gdm2CO3 solution and were used to determine the best structural experiment to demonstrate this ion pairing. The NDIS experiments confirm the most significant feature of the MD simulation, which is the existence of strong hetero-ion pairing between the Gdm+ and CO3(2-) ions. The neutron structural data also support the most interesting feature of the MD simulation, that the hetero-ion pairing is sufficiently strong as to lead to nanometer-scale aggregation of the ions. The presence of such clustering on the nanometer length scale was then confirmed using small-angle neutron scattering experiments. Taken together, the experiment and simulation suggest a molecular-level explanation for the contrasting denaturant properties of guanidinium salts in solution.

The interaction of guanidinium ions with a model peptide

In addition to promoting unfolded protein states, the denaturants urea and guanidinium (Gdm(+)) accumulate at the surface of folded proteins at subdenaturing concentrations, a phenomenon that correlates with their denaturant activities. The enhanced accumulation of Gdm(+) relative to urea indicates different binding modes, or additional binding sites, for Gdm(+), and we recently proposed potential binding modes to protein functional groups for Gdm(+) based on the determination of the weak hydration properties of this complex cation. Here we describe molecular dynamics simulations of a model helical peptide, melittin, in a 3 M solution of GdmCl, to identify potential interactions with amino-acid side chains in a nondenatured polypeptide surface. The simulations indicate that Gdm(+) can interact with a number of planar amino-acid side chains (Arg, Trp, Gln) in a stacking manner, as well as more weakly with hydrophobic surfaces composed of aliphatic side chains, and that these interactions result in enhanced number densities of Gdm(+) at certain locations on the peptide surface. These observations provide molecular scale insight into the accumulation of Gdm(+) at protein surfaces that has previously been observed experimentally.

Neutron Diffraction and Simulation Studies of CsNO3 and Cs2CO3 Solutions

Neutron diffraction with isotopic substitution (NDIS) experiments and molecular dynamics (MD) simulations have been used to study the structuring in aqueous solution of two cesium salts, cesium carbonate, and cesium nitrate. As was previously found for guanidinium salts of carbonate, mesoscopic-scale clusters were seen to form in the Cs2CO3 solution both in the MD simulations and in the diffraction experiments. No such large scale ion clusters were found in the CsNO3 solutions in either the modeling or experiments. The results are dominated by the strength and geometry of the direct first-neighbor interactions, which explain the differences in the clustering behavior between the two solutions without need to refer to longer-range water-water structuring.

Insight into indole interactions from alkali metal chloride effects on a tryptophan zipper beta-hairpin peptide

Weakly solvated, low charge density, alkali metal cations (K+ and Rb+) destabilize tryptophan zipper (trpzip) peptides with an effectiveness (for Rb+) similar to that of the protein denaturant urea. An analysis of alkali metal cation effects on polypeptides stabilized predominantly either by hydrogen bonds or by the classical hydrophobic effect indicates that the alkali metals attenuate stabilizing interactions involving the tryptophan indole groups. Destabilization does not result from electrolyte screening of the electrostatic component of the indole-indole interaction, but is likely to involve direct interaction of the low charge density cation with the indole group in a cation-pi interaction. The observations highlight a general simplicity in the nature of molecular interactions in solution, in which stabilizing contributions to polypeptide and protein structures are attenuated by solutes of a complementary nature.

Erythromycin block of the HERG K+ channel: Accessibility to F656 and Y652

The HERG potassium channel might have a non-canonical drug binding site, distinct from the channel's inner cavity, that could be responsible for elements of closed-state pharmacological inhibition of the channel. The macrolide antibiotic erythromycin is a drug that may block unconventionally because of its size. Here we used whole-cell patch-clamp recording at 37 degrees C from heterologously expressed HERG channels in a mammalian cell line to show that erythromycin either produces a rapid open-state-dependent HERG channel inhibition, or components of both open-state-dependent and closed-state-dependent inhibition. Alanine-substitution of HERG's canonical determinants of blockade revealed that Y652 was not important as a molecular determinant of blockade, and that mutation of F656 resulted in only weak attenuation of inhibition. In computer models of the channel, erythromycin could make several direct contacts with F656, but not with Y652, in the open-state model, and erythromycin was unable to fit into a closed-state channel model.

Nanometer-Scale Ion Aggregates in Aqueous Electrolyte Solutions: Guanidinium Sulfate and Guanidinium Thiocyanate

Neutron diffraction experiments and molecular dynamics simulations are used to study the structure of aqueous solutions of two electrolytes: guanidinium sulfate (a mild protein conformation stabilizer) and guanidinium thiocyanate (a powerful denaturant). The MD simulations find the unexpected result that in the Gdm2SO4 solution the ions aggregated into mesoscopic (nanometer-scale) clusters, while no such aggregation is found in the GdmSCN solution. The neutron diffraction studies, the most direct experimental probe of solution structure, provide corroborating evidence that the predicted very strong ion pairing does occur in solutions of 1.5 m Gdm2SO4 but not in 3 m solutions of GdmSCN. A mechanism is proposed as to how this mesoscopic solution structure affects solution denaturant properties and suggests an explanation for the Hofmeister ordering of these solutions in terms of this ion pairing and the ability of sulfate to reverse the denaturant power of guanidinium.

Dissecting Contributions to the Denaturant Sensitivities of Proteins

Understanding the molecular basis for protein denaturation by urea and guanidinium chloride (GdmCl) should accommodate the observation that, on a molar basis, GdmCl is generally 2-2.5-fold more effective as a protein denaturant than urea. Previous studies [Smith, J. S., and Scholtz, J. M. (1996) Biochemistry 35, 7292-7297] have suggested that the effects of GdmCl on the stability of alanine-based helical peptides can be separated into denaturant and salt effects, since adding equimolar NaCl to urea enhanced urea-induced unfolding to an extent that was close to that of Gdm. We reinvestigated this observation using an alanine-based helical peptide (alahel) that lacks side chain electrostatic contributions to stability, and compared the relative denaturant sensitivities of this peptide with that of tryptophan zipper peptides (trpzip) whose native conformations are stabilized largely by cross-strand indole ring interactions. In contrast to the observations of Smith and Scholtz, GdmCl was only slightly more powerful as a denaturant of alahel than urea in salt-free buffer (the denaturant m value m(GdmCl)/m(urea) ratio = 1.4), and the denaturation of alahel by urea exhibited only a small dependence on NaCl or KCl. The trpzip peptides were much more sensitive to GdmCl than to urea (m(GdmCl)/m(urea) = 3.5-4). These observations indicate that the m(GdmCl)/m(urea) ratio of 2-2.5 for proteins results from a combination of effects on the multiple contributions to protein stability, for which GdmCl may be only slightly more effective than urea (e.g., hydrogen bonds) or considerably more effective than urea (e.g., indole-indole interactions).

The Structure of Aqueous Guanidinium Chloride Solutions

The combination of neutron diffraction with isotopic substitution (NDIS) experiments and molecular dynamics (MD) simulations to characterize the structuring in an aqueous solution of the denaturant guanidinium chloride is described. The simulations and experiments were carried out at a concentration of 3 m at room temperature, allowing for an examination of any propensity for ion association in a realistic solution environment. The simulations satisfactorily reproduced the principal features of the neutron scattering and indicate a bimodal hydration of the guanidinium ions, with the N-H groups making well-ordered hydrogen bonds in the molecular plane, but with the planar faces relatively deficient in interactions with water. The most striking feature of these solutions is the rich ion-ion ordering observed around the guanidinium ion in the simulations. The marked tendency of the guanidinium ions to stack parallel to their water-deficient surfaces indicates that the efficiency of this ion as a denaturant is due to its ability to simultaneously interact favorably with both water and hydrophobic side chains of proteins.

The low-potency, voltage-dependent HERG blocker propafenone--molecular determinants and drug trapping

The molecular determinants of high-affinity human ether-a-go-go-related gene (HERG) potassium channel blockade by methanesulfonanilides include two aromatic residues (Phe656 and Tyr652) on the inner helices (S6) and residues on the pore helices that face into the inner cavity, but determinants for lower-affinity HERG blockers may be different. In this study, alanine-substituted HERG channel mutants of inner cavity residues were expressed in Xenopus laevis oocytes and were used to characterize the HERG channel binding site of the antiarrhythmic propafenone. Propafenone's blockade of HERG was strongly dependent on residue Phe656 but was insensitive or weakly sensitive to mutation of Tyr652, Thr623, Ser624, Val625, Gly648, or Val659 and did not require functional inactivation. Homology models of HERG based on KcsA and MthK crystal structures, representing the closed and open forms of the channel, respectively, suggest propafenone is trapped in the inner cavity and is unable to interact exclusively with Phe656 in the closed state (whereas exclusive interactions between propafenone and Phe656 are found in the open-channel model). These findings are supported by very slow recovery of wild-type HERG channels from block at -120 mV, but extremely rapid recovery of D540K channels that reopen at this potential. The experiments and modeling suggest that the open-state propafenone binding-site may be formed by the Phe656 residues alone. The binding site for propafenone (which may involve pi-stacking interactions with two or more Phe656 side-chains) is either perturbed or becomes less accessible because of closed-channel gating. This provides further evidence for the existence of gating-induced changes in the spatial location of Phe656 side chains.

Preferential closed channel blockade of HERG potassium currents by chemically synthesised BeKm-1 scorpion toxin

The scorpion toxin peptide BeKm-1 was synthesised by fluorenylmethoxycarbonyl solid phase chemistry and folded by air oxidation. The peptide's effects on heterologous human ether-a-go-go-related gene potassium current (I(HERG)) in HEK293 cells were assessed using 'whole-cell' patch clamp. Blockade of I(HERG) by BeKm-1 was concentration-dependent, temperature-dependent, and rapid in onset and reversibility. Blockade also exhibited inverse voltage dependence, inverse dependence on duration of depolarisation, and reverse use- and frequency-dependence. Blockade by BeKm-1 and recombinant ergtoxin, another scorpion toxin known to block HERG, differed in their recovery from HERG current inactivation elicited by strong depolarisation and in their ability to block HERG when the channels were already activated. We conclude that synthetic BeKm-1 toxin blocks HERG preferentially through a closed (resting) state channel blockade mechanism, although some open channel blockade also occurs.

Effect of protein kinase A-mediated phosphorylation on the structure and association properties of the enteropathogenic Escherichia coli Tir virulence protein

Enteropathogenic Escherichia coli virulence is dependent on delivery of the translocated intimin receptor protein (Tir) into host cells. Tir phosphorylation on a single tyrosine (Tyr-474) is essential in mediating cytoskeletal rearrangement correlated with disease. Tir is also phosphorylated on other residues, with cAMP-dependent kinase (PKA) modification shown to play a role in Tir function. However, the mechanism by which nontyrosine phosphorylation affects Tir function remains unclear. In this study, analytical ultracentrifugation, SDS and native gel electrophoresis revealed that both Tir and its C-terminal domain (residues 385-550 of Tir that include the PKA substrate sites) exist in an equilibrium of monomers, dimers, and in the case of Tir, higher oligomers. PKA phosphorylation (1:300, PKA/C-Tir, mol/mol) shifted the equilibrium of C-Tir, but not Tir, predominantly to the dimeric state, whereas, at 100-fold higher concentrations of PKA the phosphorylated C-Tir was largely monomeric. This monomeric state was also produced at the lower PKA concentration and physiological ionic strength. Phosphorylation-mediated effects were achieved without significant changes in secondary structure as determined by circular dichroism spectroscopy. The data presented indicate that PKA-mediated phosphorylation induces changes in the association properties of the C-terminal domain of Tir that may facilitate Tir-Tir interactions and subsequently C-Tir-host protein interactions in vivo.

The hydration structure of guanidinium and thiocyanate ions: implications for protein stability in aqueous solution

Neutron diffraction experiments were carried out on aqueous solutions containing either guanidinium or thiocyanate ions. The first-order difference method of neutron diffraction and isotopic substitution was applied, and the hydration structures of two of nature's strongest denaturant ions were determined. Each ion is shown to interact weakly with water: Guanidinium has no recognizable hydration shell and is one of the most weakly hydrated cations yet characterized. Hydration of thiocyanate is characterized by a low coordination number involving around one hydrogen-bonded water molecule and approximately two water molecules weakly interacting through "hydration bonds." The weak hydration of these denaturant ions strongly supports suggestions that a major contribution to the denaturant effect is the preferential interaction of the denaturant with the protein surface. By contrast, solute species such as many sugars and related polyols that stabilize proteins are strongly hydrated and are thus preferentially retained in the bulk solvent and excluded from the protein surface.

Enhanced membrane permeabilization and antibacterial activity of a disulfide-dimerized magainin analogue

A cysteine substitution analogue of magainin-2 amide (magainin-F12W, N22C; denoted here as mag-N22C), and a disulfide-linked dimer prepared by air oxidation [(mag-N22C)(2)], were compared in their ability to release carboxyfluorescein (CF) from 100-nm large unilamellar vesicles (LUV) and to kill the Gram negative bacteria Stenotrophomonas maltophilia and Escherichia coli. The disulfide-dimerized peptide showed enhanced permeabilization and antimicrobial activity, when compared with the monomeric peptide, that was particularly marked at very low peptide concentrations. The enhanced CF-releasing activity of the dimer at low concentrations in vesicles results from (i) enhanced binding to negatively charged membrane surfaces and (ii) a low concentration dependence for permeabilization in the dimer compared to the monomer. The unique properties of the dimeric peptide suggest a role for structural diversity of antimicrobial peptides in frog skin, including the recent identification of a heterodimer composed of disulfide-linked amphipathic helical peptides [Batista et al. (2001) FEBS Lett. 494, 85-89]. Disulfide-dimerization of pore-forming, positively charged, amphipathic helical peptides may be a useful general approach to the generation of peptide antimicrobials having activity at very low concentrations.

Structure, location, and lipid perturbations of melittin at the membrane interface

Melittin is arguably the most widely studied amphipathic, membrane-lytic alpha-helical peptide. Although several lines of evidence suggest an interfacial membrane location at low concentrations, melittin's exact position and depth of penetration into the hydrocarbon core are unknown. Furthermore, the structural basis for its lytic action remains largely a matter of conjecture. Using a novel x-ray absolute-scale refinement method, we have now determined the location, orientation, and likely conformation of monomeric melittin in oriented phosphocholine lipid multilayers. Its helical axis is aligned parallel to the bilayer plane at the depth of the glycerol groups, but its average conformation differs from the crystallographic structure. As observed earlier for another amphipathic alpha-helical peptide, the lipid perturbations induced by melittin are remarkably modest. Small bilayer perturbations thus appear to be a general feature of amphipathic helices at low concentrations. In contrast, a dimeric form of melittin causes larger structural perturbations under otherwise identical conditions. These results provide direct structural evidence that self-association of amphipathic helices may be the crucial initial step toward membrane lysis.