New insights on the voltage dependence of the KCa3.1 channel block by internal TBA

Aromatic-aromatic interactions between residues in KCa3.1 pore helix and S5 transmembrane segment control the channel gating process

Interactions between aromatic amino acid residues in the pore helix and S5 transmembrane domain control gating of the Ca²⁺-activated potassium channel KCa3.1.

The Ca²⁺-activated potassium channel KCa3.1 is emerging as a therapeutic target for a large variety of health disorders. One distinguishing feature of KCa3.1 is that the channel open probability at saturating Ca²⁺ concentrations (Pomax) is low, typically 0.1–0.2 for KCa3.1 wild type. This observation argues for the binding of Ca²⁺ to the calmodulin (CaM)–KCa3.1 complex, promoting the formation of a preopen closed-state configuration leading to channel opening. We have previously shown that the KCa3.1 active gate is most likely located at the level of the selectivity filter. As Ca²⁺-dependent gating of KCa3.1 originates from the binding of Ca²⁺ to CaM in the C terminus, the hypothesis of a gate located at the level of the selectivity filter requires that the conformational change initiated in the C terminus be transmitted to the S5 and S6 transmembrane helices, with a resulting effect on the channel pore helix directly connected to the selectivity filter. A study was thus undertaken to determine to what extent the interactions between the channel pore helix with the S5 and S6 transmembrane segments contribute to KCa3.1 gating. Molecular dynamics simulations first revealed that the largest contact area between the pore helix and the S5 plus S6 transmembrane helices involves residue F248 at the C-terminal end of the pore helix. Unitary current recordings next confirmed that modulating aromatic–aromatic interactions between F248 and W216 of the S5 transmembrane helical segment and/or perturbing the interactions between F248 and residues in S6 surrounding the glycine hinge G274 cause important changes in Pomax. This work thus provides the first evidence for a key contribution of the pore helix in setting Pomax by stabilizing the channel closed configuration through aromatic–aromatic interactions involving F248 of the pore helix. We propose that the interface pore helix/S5 constitutes a promising site for designing KCa3.1 potentiators.

Contribution of the KCa3.1 channel-calmodulin interactions to the regulation of the KCa3.1 gating process

The Ca(2+)-activated potassium channel of intermediate conductance, KCa3.1, is now emerging as a therapeutic target for a large variety of health disorders. The Ca(2+) sensitivity of KCa3.1 is conferred by the Ca(2+)-binding protein calmodulin (CaM), with the CaM C-lobe constitutively bound to an intracellular domain of the channel C terminus. It was proposed on the basis of the crystal structure obtained for the C-terminal region of the rat KCa2.2 channel (rSK2) with CaM that the binding of Ca(2+) to the CaM N-lobe results in CaM interlocking the C-terminal regions of two adjacent KCa3.1 subunits, leading to the formation of a dimeric structure. A study was thus undertaken to identify residues of the CaM N-lobe-KCa3.1 complex that either contribute to the channel activation process or control the channel open probability at saturating Ca(2+) (Pomax). A structural homology model of the KCa3.1-CaM complex was first generated using as template the crystal structure of the C-terminal region of the rat KCa2.2 channel with CaM. This model was confirmed by cross-bridging residues R362 of KCa3.1 and K75 of CaM. Patch-clamp experiments were next performed, demonstrating that the solvation energy of the residue at position 367 in KCa3.1 is a key determinant to the channel Pomax and deactivation time toff. Mutations of residues M368 and Q364 predicted to form anchoring points for CaM binding to KCa3.1 had little impact on either toff or Pomax. Finally, our results show that channel activation depends on electrostatic interactions involving the charged residues R362 and E363, added to a nonpolar energy contribution coming from M368. We conclude that electrostatic interactions involving residues R362 and E363 and hydrophobic effects at M368 play a prominent role in KCa3.1 activation, whereas hydrophobic interactions at S367 are determinant to the stability of the CaM-KCa3.1 complex throughout gating.

Characterizing the fatty acid binding site in the cavity of potassium channel KcsA

We show that interactions of fatty acids with the central cavity of potassium channel KcsA can be characterized using the fluorescence probe 11-dansylaminoundecanoic acid (Dauda). The fluorescence emission spectrum of Dauda bound to KcsA in bilayers of dioleoylphosphatidylcholine contains three components, which can be attributed to KcsA-bound and lipid-bound Dauda together with unbound Dauda. The binding of Dauda to KcsA was characterized by a dissociation constant of 0.47 ± 0.10 μM with 0.94 ± 0.06 binding site per KcsA tetramer. Displacement of KcsA-bound Dauda by the tetrabutylammonium (TBA) ion confirmed that the Dauda binding site was in the central cavity of KcsA. Dissociation constants for a range of fatty acids were determined by displacement of Dauda: binding of fatty acids increased in strength with an increasing chain length from C14 to C20 but then decreased in strength from C20 to C22. Increasing the number of double bonds in the chain from one to four had little effect on binding, dissociation constants for oleic acid and arachidonic acid, for example, being 2.9 ± 0.2 and 3.0 ± 0.4 μM, respectively. Binding of TBA to KcsA was very slow, whereas binding of Dauda was fast, suggesting that TBA can enter the cavity only through an open channel whereas Dauda can bind to the closed channel, presumably entering the cavity via the lipid bilayer.

Inhibition of the KCa3.1 channels by AMP-activated protein kinase in human airway epithelial cells

The vectorial transport of ions and water across epithelial cells depends to a large extent on the coordination of the apical and basolateral ion fluxes with energy supply. In this work we provide the first evidence for a regulation by the 5'-AMP-activated protein kinase (AMPK) of the calcium-activated potassium channel KCa3.1 expressed at the basolateral membrane of a large variety of epithelial cells. Inside-out patch-clamp experiments performed on human embryonic kidney (HEK) cells stably transfected with KCa3.1 first revealed a decrease in KCa3.1 activity following the internal addition of AMP at a fixed ATP concentration. This effect was dose dependent with half inhibition at 140 muM AMP in 1 mM ATP. Evidence for an interaction between the COOH-terminal region of KCa3.1 and the gamma1-subunit of AMPK was next obtained by two-hybrid screening and pull-down experiments. Our two-hybrid analysis confirmed in addition that the amino acids extending from Asp(380) to Ala(400) in COOH-terminal were essential for the interaction AMPK-gamma1/KCa3.1. Inside-out experiments on cells coexpressing KCa3.1 with the dominant negative AMPK-gamma1-R299G mutant showed a reduced sensitivity of KCa3.1 to AMP, arguing for a functional link between KCa3.1 and the gamma1-subunit of AMPK. More importantly, coimmunoprecipitation experiments carried out on bronchial epithelial NuLi cells provided direct evidence for the formation of a KCa3.1/AMPK-gamma1 complex at endogenous AMPK and KCa3.1 expression levels. Finally, treating NuLi monolayers with the membrane permeant AMPK activator 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) caused a significant decrease of the KCa3.1-mediated short-circuit currents, an effect reversible by coincubation with the AMPK inhibitor Compound C. These observations argue for a regulation of KCa3.1 by AMPK in a functional epithelium through protein/protein interactions involving the gamma1-subunit of AMPK.

Hydrophobic interactions as key determinants to the KCa3.1 channel closed configuration. An analysis of KCa3.1 mutants constitutively active in zero Ca2+

In this study we present evidence that residue Val282 in the S6 transmembrane segment of the calcium-activated KCa3.1 channel constitutes a key determinant of channel gating. A Gly scan of the S6 transmembrane segment first revealed that the substitutions A279G and V282G cause the channel to become constitutively active in zero Ca2+. Constitutive activity was not observed when residues extending from Cys276 to Ala286, other than Ala279 and Val282, were substituted to Gly. The accessibility of Cys engineered at Val275 deep in the channel cavity was next investigated for the ion-conducting V275C/V282G mutant and closed V275C channel in zero Ca2+ using Ag+ as probe. These experiments demonstrated that internal Ag+ ions have free access to the channel cavity independently of the channel conducting state, arguing against an activation gate located at the S6 segment C-terminal end. Experiments were also conducted where Val282 was substituted by residues differing in size and/or hydrophobicity. We found a strong correlation between constitutive activity in zero Ca2+ and the hydrophobic energy for side chain burial. Single channel recordings showed finally that constitutive activation in zero Ca2+ is better explained by a model where the channel is locked in a low conducting state with a high open probability rather than resulting from a change in the open/closed energy balance that would favor channel openings to a full conducting state in the absence of Ca2+. We conclude that hydrophobic interactions involving Val282 constitute key determinants to KCa3.1 gating by modulating the ion conducting state of the selectivity filter through an effect on the S6 transmembrane segment.

Accessibility of four arginine residues on the S4 segment of the Bacillus halodurans sodium channel

The voltage-gated Na(+) channel of Bacillus halodurans (NaChBac) is composed of six transmembrane segments (S1-S6), with a pore-forming region composed of segments S5 and S6 and a voltage-sensing domain composed of segments S1-S4. The S4 segment forms the core of the voltage sensor. We explored the accessibility of four arginine residues on the S4 segment of NaChBac, which are positioned at every third position from each other. These arginine residues on the S4 segment were replaced with cysteines using site-directed mutagenesis. Na(+) currents were recorded using the whole-cell configuration of the patch-clamp technique. We tested the effect of the sulfhydryl reagents applied from inside and outside the cellular space in the open and closed conformations. Structural models of the voltage sensor of NaChBac were constructed based on the recently crystallized KvAP and Kv1.2 K(+) channels to visualize arginine residue accessibility. Our results suggest that arginine accessibility did not change significantly between the open and closed conformations, supporting the idea of a small movement of the S4 segment during gating. Molecular modeling of the closed conformation also supported a small movement of S4, which is mainly characterized by a rotation and a tilt along the periphery of the pore. Interestingly, the second arginine residue of the S4 segment (R114) was accessible to sulfhydryl reagents from both sides of the membrane in the closed conformation and, based on our model, seemed to be at the junction of the intracellular and extracellular water crevices.

Structural determinants of the closed KCa3.1 channel pore in relation to channel gating: results from a substituted cysteine accessibility analysis

In this work we address the question of the KCa3.1 channel pore structure in the closed configuration in relation to the contribution of the C-terminal end of the S6 segments to the Ca(2+)-dependent gating process. Our results based on SCAM (substituted cysteine accessibility method) experiments first demonstrate that the S6 transmembrane segment of the open KCa3.1 channel contains two distinct functional domains delimited by V282 with MTSEA and MTSET binding leading to a total channel inhibition at positions V275, T278, and V282 and to a steep channel activation at positions A283 and A286. The rates of modification by MTSEA (diameter 4.6 A) of the 275C (central cavity) and 286C residues (S6 C-terminal end) for the closed channel configuration were found to differ by less than sevenfold, whereas experiments performed with the larger MTSET reagent (diameter 5.8 A) resulted in modification rates 10(3)-10(4) faster for cysteines at 286 compared with 275. Consistent with these results, the modification rates of the cavity lining 275C residue by MTSEA, Et-Hg(+), and Ag(+) appeared poorly state dependent, whereas modification rates by MTSET were 10(3) faster for the open than the closed configuration. A SCAM analysis of the channel inner vestibule in the closed state revealed in addition that cysteine residues at 286 were accessible to MTS reagents as large as MTS-PtrEA, a result supported by the observation that binding of MTSET to cysteines at positions 283 or 286 could neither sterically nor electrostatically block the access of MTSEA to the closed channel cavity (275C). It follows that the closed KCa3.1 structure can hardly be accountable by an inverted teepee-like structure as described for KcsA, but is better represented by a narrow passage centered at V282 (equivalent to V474 in Shaker) connecting the channel central cavity to the cytosolic medium. This passage would not be however restrictive to the diffusion of small reagents such as MTSEA, Et-Hg(+), and Ag(+), arguing against the C-terminal end of S6 forming an obstructive barrier to the diffusion of K(+) ions for the closed channel configuration.