The neutral, hydrophobic isoleucine at position I521 in the extracellular S4 domain of hERG contributes to channel gating equilibrium

An LQT2-related mutation in the voltage-sensing domain is involved in switching the gating polarity of hERG

BACKGROUND

Cyclic Nucleotide-Binding Domain (CNBD)-family channels display distinct voltage-sensing properties despite sharing sequence and structural similarity. For example, the human Ether-a-go-go Related Gene (hERG) channel and the Hyperpolarization-activated Cyclic Nucleotide-gated (HCN) channel share high amino acid sequence similarity and identical domain structures. hERG conducts outward current and is activated by positive membrane potentials (depolarization), whereas HCN conducts inward current and is activated by negative membrane potentials (hyperpolarization). The structural basis for the "opposite" voltage-sensing properties of hERG and HCN remains unknown.

RESULTS

We found the voltage-sensing domain (VSD) involves in modulating the gating polarity of hERG. We identified that a long-QT syndrome type 2-related mutation within the VSD, K525N, mediated an inwardly rectifying non-deactivating current, perturbing the channel closure, but sparing the open state and inactivated state. K525N rescued the current of a non-functional mutation in the pore helix region (F627Y) of hERG. K525N&F627Y switched hERG into a hyperpolarization-activated channel. The reactivated inward current induced by hyperpolarization mediated by K525N&F627Y can be inhibited by E-4031 and dofetilide quite well. Moreover, we report an extracellular interaction between the S1 helix and the S5-P region is crucial for modulating the gating polarity. The alanine substitution of several residues in this region (F431A, C566A, I607A, and Y611A) impaired the inward current of K525N&F627Y.

CONCLUSIONS

Our data provide evidence that a potential cooperation mechanism in the extracellular vestibule of the VSD and the PD would determine the gating polarity in hERG.

Environment-Sensitive Fluorescent Labelling of Peptides by Luciferin Analogues

Environment-sensitive fluorophores are very valuable tools in the study of molecular and cellular processes. When used to label proteins and peptides, they allow for the monitoring of even small variations in the local microenvironment, thus acting as reporters of conformational variations and binding events. Luciferin and aminoluciferin, well known substrates of firefly luciferase, are environment-sensitive fluorophores with unusual and still-unexploited properties. Both fluorophores show strong solvatochromism. Moreover, luciferin fluorescence is influenced by pH and water abundance. These features allow to detect local variations of pH, solvent polarity and local water concentration, even when they occur simultaneously, by analyzing excitation and emission spectra. Here, we describe the characterization of (amino)luciferin-labeled derivatives of four bioactive peptides: the antimicrobial peptides GKY20 and ApoB_L, the antitumor peptide p53pAnt and the integrin-binding peptide RGD. The two probes allowed for the study of the interaction of the peptides with model membranes, SDS micelles, lipopolysaccharide micelles and Escherichia coli cells. K_d values and binding stoichiometries for lipopolysaccharide were also determined. Aminoluciferin also proved to be very well-suited to confocal laser scanning microscopy. Overall, the characterization of the labeled peptides demonstrates that luciferin and aminoluciferin are previously neglected environment-sensitive labels with widespread potential applications in the study of proteins and peptides.

Functional characterization of Kv11.1 (hERG) potassium channels split in the voltage-sensing domain

Voltage-dependent KCNH family potassium channel functionality can be reconstructed using non-covalently linked voltage-sensing domain (VSD) and pore modules (split channels). However, the necessity of a covalent continuity for channel function has not been evaluated at other points within the two functionally independent channel modules. We find here that by cutting Kv11.1 (hERG, KCNH2) channels at the different loops linking the transmembrane spans of the channel core, not only channels split at the S4–S5 linker level, but also those split at the intracellular S2–S3 and the extracellular S3–S4 loops, yield fully functional channel proteins. Our data indicate that albeit less markedly, channels split after residue 482 in the S2–S3 linker resemble the uncoupled gating phenotype of those split at the C-terminal end of the VSD S4 transmembrane segment. Channels split after residues 514 and 518 in the S3–S4 linker show gating characteristics similar to those of the continuous wild-type channel. However, breaking the covalent link at this level strongly accelerates the voltage-dependent accessibility of a membrane impermeable methanethiosulfonate reagent to an engineered cysteine at the N-terminal region of the S4 transmembrane helix. Thus, besides that of the S4–S5 linker, structural integrity of the intracellular S2–S3 linker seems to constitute an important factor for proper transduction of VSD rearrangements to opening and closing the cytoplasmic gate. Furthermore, our data suggest that the short and probably rigid characteristics of the extracellular S3–S4 linker are not an essential component of the Kv11.1 voltage sensing machinery.

Gating mechanism of Kv11.1 (hERG) K+ channels without covalent connection between voltage sensor and pore domains

Kv11.1 (hERG, KCNH2) is a voltage-gated potassium channel crucial in setting the cardiac rhythm and the electrical behaviour of several non-cardiac cell types. Voltage-dependent gating of Kv11.1 can be reconstructed from non-covalently linked voltage sensing and pore modules (split channels), challenging classical views of voltage-dependent channel activation based on a S4–S5 linker acting as a rigid mechanical lever to open the gate. Progressive displacement of the split position from the end to the beginning of the S4–S5 linker induces an increasing negative shift in activation voltage dependence, a reduced z_g value and a more negative ΔG₀ for current activation, an almost complete abolition of the activation time course sigmoid shape and a slowing of the voltage-dependent deactivation. Channels disconnected at the S4–S5 linker near the S4 helix show a destabilization of the closed state(s). Furthermore, the isochronal ion current mode shift magnitude is clearly reduced in the different splits. Interestingly, the progressive modifications of voltage dependence activation gating by changing the split position are accompanied by a shift in the voltage-dependent availability to a methanethiosulfonate reagent of a Cys introduced at the upper S4 helix. Our data demonstrate for the first time that alterations in the covalent connection between the voltage sensor and the pore domains impact on the structural reorganizations of the voltage sensor domain. Also, they support the hypothesis that the S4–S5 linker integrates signals coming from other cytoplasmic domains that constitute either an important component or a crucial regulator of the gating machinery in Kv11.1 and other KCNH channels.

The Fast Component of hERG Gating Charge: An Interaction between D411 in the S1 and S4 Residues

Kv11.1 (hERG) is a voltage-gated potassium channel that shows very slow ionic current activation kinetics, and an unusual underlying biphasic gating charge movement with fast and slow components that differ greatly in time course. The structural basis and role of the fast component of gating charge (Q_fast) is unclear, and its relationship to the slow activation of hERG channels is not understood. In this study we have used the cut-open oocyte voltage-clamp technique to investigate the relationship of fast gating charge movement-to-residue interactions between D411 at the bottom of the S1, and lower S4 domain charged and uncharged residues. Neutralization of D411 or K538 and V535A prevented Q_fast and greatly accelerated overall charge movement. Voltage-clamp fluorometry showed a loss of a fast component of S4 fluorescence in D411N, V535A, and K538Q upon depolarization, whereas [2-(trimethyl ammonium) ethyl] methanethiosulfonate chloride modification of I521C in the outer S4 was enhanced at more negative potentials and at earlier times in these same mutants. A functional interaction between these regions during activation was suggested by ΔΔG_o values >4.2 kJ/mol obtained from double mutant cycle analysis. The data indicate that interactions of S1 residue D411 with lower S4 residues stabilizes early closed states of the channel, and that disruption of these interactions results in both faster rates of activation gating and an elimination of the fast component of gating charge movement and of fluorescence. We propose that the Q_fast charge movement during activation accompanies transitions through early closed states of the hERG activation pathway, and that the weak voltage dependence of these transitions limits the overall activation rate of hERG channels. Disruption of the D411-S4 interactions destabilizes these early closed states, leaving hERG channels able to activate at a rate similar to conventional potassium channels.

NS1643 interacts around L529 of hERG to alter voltage sensor movement on the path to activation

Activators of hERG1 such as NS1643 are being developed for congenital/acquired long QT syndrome. Previous studies identify the neighborhood of L529 around the voltage-sensor as a putative interacting site for NS1643. With NS1643, the V1/2 of activation of L529I (-34 ± 4 mV) is similar to wild-type (WT) (-37 ± 3 mV; P > 0.05). WT and L529I showed no difference in the slope factor in the absence of NS1643 (8 ± 0 vs. 9 ± 0) but showed a difference in the presence of NS1643 (9 ± 0.3 vs. 22 ± 1; P < 0.01). Voltage-clamp-fluorimetry studies also indicated that in L529I, NS1643 reduces the voltage-sensitivity of S4 movement. To further assess mechanism of NS1643 action, mutations were made in this neighborhood. NS1643 shifts the V1/2 of activation of both K525C and K525C/L529I to hyperpolarized potentials (-131 ± 4 mV for K525C and -120 ± 21 mV for K525C/L529I). Both K525C and K525C/K529I had similar slope factors in the absence of NS1643 (18 ± 2 vs. 34 ± 5, respectively) but with NS1643, the slope factor of K525C/L529I increased from 34 ± 5 to 71 ± 10 (P < 0.01) whereas for K525C the slope factor did not change (18 ± 2 at baseline and 16 ± 2 for NS1643). At baseline, K525R had a slope factor similar to WT (9 vs. 8) but in the presence of NS1643, the slope factor of K525R was increased to 24 ± 4 vs. 9 ± 0 mV for WT (P < 0.01). Molecular modeling indicates that L529I induces a kink in the S4 voltage-sensor helix, altering a salt-bridge involving K525. Moreover, docking studies indicate that NS1643 binds to the kinked structure induced by the mutation with a higher affinity. Combining biophysical, computational, and electrophysiological evidence, a mechanistic principle governing the action of some activators of hERG1 channels is proposed.