Structural correlates of selectivity and inactivation in potassium channels

Transmembrane allosteric coupling of the gates in a potassium channel

It has been hypothesized that transmembrane allostery is the basis for inactivation of the potassium channel KcsA: opening the intracellular gate is spontaneously followed by ion expulsion at the extracellular selectivity filter. This suggests a corollary: following ion expulsion at neutral pH, a spontaneous global conformation change of the transmembrane helices, similar to the motion involved in opening, is expected. Consequently, both the low potassium state and the low pH state of the system could provide useful models for the inactivated state. Unique NMR studies of full-length KcsA in hydrated bilayers provide strong evidence for such a mutual coupling across the bilayer: namely, upon removing ambient potassium ions, changes are seen in the NMR shifts of carboxylates E118 and E120 in the pH gate in the hinges of the inner transmembrane helix (98-103), and in the selectivity filter, all of which resemble changes seen upon acid-induced opening and inhibition and suggest that ion release can trigger channel helix opening.

Using protein backbone mutagenesis to dissect the link between ion occupancy and C-type inactivation in K+ channels

K(+) channels distinguish K(+) from Na(+) in the selectivity filter, which consists of four ion-binding sites (S1-S4, extracellular to intracellular) that are built mainly using the carbonyl oxygens from the protein backbone. In addition to ionic discrimination, the selectivity filter regulates the flow of ions across the membrane in a gating process referred to as C-type inactivation. A characteristic of C-type inactivation is a dependence on the permeant ion, but the mechanism by which permeant ions modulate C-type inactivation is not known. To investigate, we used amide-to-ester substitutions in the protein backbone of the selectivity filter to alter ion binding at specific sites and determined the effects on inactivation. The amide-to-ester substitutions in the protein backbone were introduced using protein semisynthesis or in vivo nonsense suppression approaches. We show that an ester substitution at the S1 site in the KcsA channel does not affect inactivation whereas ester substitutions at the S2 and S3 sites dramatically reduce inactivation. We determined the structure of the KcsA S2 ester mutant and found that the ester substitution eliminates K(+) binding at the S2 site. We also show that an ester substitution at the S2 site in the KvAP channel has a similar effect of slowing inactivation. Our results link C-type inactivation to ion occupancy at the S2 site. Furthermore, they suggest that the differences in inactivation of K(+) channels in K(+) compared with Rb(+) are due to different ion occupancies at the S2 site.

Semisynthetic K+ channels show that the constricted conformation of the selectivity filter is not the C-type inactivated state

C-type inactivation of K(+) channels plays a key role in modulating cellular excitability. During C-type inactivation, the selectivity filter of a K(+) channel changes conformation from a conductive to a nonconductive state. Crystal structures of the KcsA channel determined at low K(+) or in the open state revealed a constricted conformation of the selectivity filter, which was proposed to represent the C-type inactivated state. However, structural studies on other K(+) channels do not support the constricted conformation as the C-type inactivated state. In this study, we address whether the constricted conformation of the selectivity filter is in fact the C-type inactivated state. The constricted conformation can be blocked by substituting the first conserved glycine in the selectivity filter with the unnatural amino acid d-Alanine. Protein semisynthesis was used to introduce d-Alanine into the selectivity filters of the KcsA channel and the voltage-gated K(+) channel KvAP. For semisynthesis of the KvAP channel, we developed a modular approach in which chemical synthesis is limited to the selectivity filter whereas the rest of the protein is obtained by recombinant means. Using the semisynthetic KcsA and KvAP channels, we show that blocking the constricted conformation of the selectivity filter does not prevent inactivation, which suggests that the constricted conformation is not the C-type inactivated state.

Isotope labeling strategies for analysis of an ion channel cytoplasmic domain by NMR spectroscopy

As large, multimeric, integral membrane proteins, ion channels pose technical challenges to analysis by NMR spectroscopy. Here we present a strategy to overcome some of these technical hurdles, using a representative ion channel modulatory domain, the regulator of K(+) conductance (RCK) domain from a K(+) channel cloned from Thermoplasma volcanium. By introducing a mutation to limit the stoichiometry of the octameric RCK domain "gating ring" complex to its dimeric building block, NMR spectral resolution can be greatly improved. Here we present protocols for efficient production of highly deuterated, uniformly (15)N-labeled protein, as well as protein containing (15)N-labeling to specific amino acid types. These labeling strategies can be applied to improve spectral resolution and facilitate sequential resonance assignments.

The voltage-dependent gate in MthK potassium channels is located at the selectivity filter

Understanding how ion channels open and close their pores is crucial for understanding their physiological roles. We used intracellular quaternary ammonium blockers to locate the voltage-dependent gate in MthK potassium channels from Methanobacterium thermoautotrophicum with electrophysiology and X-ray crystallography. Blockers bind in an aqueous cavity between two putative gates, an intracellular gate and the selectivity filter. Thus, these blockers directly probe gate location: an intracellular gate will prevent binding when closed, whereas a selectivity filter gate will always allow binding. A kinetic analysis of tetrabutylammonium block of single MthK channels combined with X-ray crystallographic analysis of the pore with tetrabutylantimony unequivocally determined that the voltage-dependent gate, like the C-type inactivation gate in eukaryotic channels, is located at the selectivity filter. State-dependent binding kinetics suggests that MthK inactivation leads to conformational changes within the cavity and intracellular pore entrance.

Activation of an essential calcium signaling pathway in Saccharomyces cerevisiae by Kch1 and Kch2, putative low-affinity potassium transporters

In the budding yeast Saccharomyces cerevisiae, mating pheromones activate a high-affinity Ca(2+) influx system (HACS) that activates calcineurin and is essential for cell survival. Here we identify extracellular K(+) and a homologous pair of transmembrane proteins, Kch1 and Kch2 (Prm6), as necessary components of the HACS activation mechanism. Expression of Kch1 and especially Kch2 was strongly induced during the response to mating pheromones. When forcibly overexpressed, Kch1 and Kch2 localized to the plasma membrane and activated HACS in a fashion that depended on extracellular K(+) but not pheromones. They also promoted growth of trk1 trk2 mutant cells in low K(+) environments, suggesting they promote K(+) uptake. Voltage-clamp recordings of protoplasts revealed diminished inward K(+) currents in kch1 kch2 double-mutant cells relative to the wild type. Conversely, heterologous expression of Kch1 in HEK293T cells caused the appearance of inwardly rectifying K(+) currents. Collectively, these findings suggest that Kch1 and Kch2 directly promote K(+) influx and that HACS may electrochemically respond to K(+) influx in much the same way as the homologous voltage-gated Ca(2+) channels in most animal cell types.

High-risk long QT syndrome mutations in the Kv7.1 (KCNQ1) pore disrupt the molecular basis for rapid K(+) permeation

Type 1 long QT syndrome (LQT1) is caused by loss-of-function mutations in the KCNQ1 gene, which encodes the K(+) channel (Kv7.1) that underlies the slowly activating delayed rectifier K(+) current in the heart. Intragenic risk stratification suggests LQT1 mutations that disrupt conserved amino acid residues in the pore are an independent risk factor for LQT1-related cardiac events. The purpose of this study is to determine possible molecular mechanisms that underlie the loss of function for these high-risk mutations. Extensive genotype-phenotype analyses of LQT1 patients showed that T322M-, T322A-, or G325R-Kv7.1 confers a high risk for LQT1-related cardiac events. Heterologous expression of these mutations with KCNE1 revealed they generated nonfunctional channels and caused dominant negative suppression of WT-Kv7.1 current. Molecular dynamics simulations of analogous mutations in KcsA (T85M-, T85A-, and G88R-KcsA) demonstrated that they disrupted the symmetrical distribution of the carbonyl oxygen atoms in the selectivity filter, which upset the balance between the strong attractive and K(+)-K(+) repulsive forces required for rapid K(+) permeation. We conclude high-risk LQT1 mutations in the pore likely disrupt the architectural and physical properties of the K(+) channel selectivity filter.

Functional equilibrium of the KcsA structure revealed by NMR

KcsA is a tetrameric K(+) channel that is activated by acidic pH. Under open conditions of the helix bundle crossing, the selectivity filter undergoes an equilibrium between permeable and impermeable conformations. Here we report that the population of the permeable conformation (p(perm)) positively correlates with the tetrameric stability and that the population in reconstituted high density lipoprotein, where KcsA is surrounded by the lipid bilayer, is lower than that in detergent micelles, indicating that dynamic properties of KcsA are different in these two media. Perturbation of the membrane environment by the addition of 1-3% 2,2,2-trifluoroethanol increases p(perm) and the open probability, revealed by NMR and single-channel recording analyses. These results demonstrate that KcsA inactivation is determined not only by the protein itself but also by the surrounding membrane environments.

How does KCNE1 regulate the Kv7.1 potassium channel? Model-structure, mutations, and dynamics of the Kv7.1-KCNE1 complex

The voltage-gated potassium channel Kv7.1 and its auxiliary subunit KCNE1 are expressed in the heart and give rise to the major repolarization current. The interaction of Kv7.1 with the single transmembrane helix of KCNE1 considerably slows channel activation and deactivation, raises single-channel conductance, and prevents slow voltage-dependent inactivation. We built a Kv7.1-KCNE1 model-structure. The model-structure agrees with previous disulfide mapping studies and enables us to derive molecular interpretations of electrophysiological recordings that we obtained for two KCNE1 mutations. An elastic network analysis of Kv7.1 fluctuations in the presence and absence of KCNE1 suggests a mechanistic perspective on the known effects of KCNE1 on Kv7.1 function: slow deactivation is attributed to the low mobility of the voltage-sensor domains upon KCNE1 binding, abolishment of voltage-dependent inactivation could result from decreased fluctuations in the external vestibule, and amalgamation of the fluctuations in the pore region is associated with enhanced ion conductivity.

Metabolic and thermal stimuli control K(2P)2.1 (TREK-1) through modular sensory and gating domains

The two-pore domain potassium channel K_2P2.1 (TREK-1) responds to extracellular and intracellular stimuli, including pH and temperature. This study elucidates how the intracellular sensor element relays metabolic and thermal stimuli to the extracellular C-type gating element.

K_2P2.1 (TREK-1) is a polymodal two-pore domain leak potassium channel that responds to external pH, GPCR-mediated phosphorylation signals, and temperature through the action of distinct sensors within the channel. How the various intracellular and extracellular sensory elements control channel function remains unresolved. Here, we show that the K_2P2.1 (TREK-1) intracellular C-terminal tail (Ct), a major sensory element of the channel, perceives metabolic and thermal commands and relays them to the extracellular C-type gate through transmembrane helix M4 and pore helix 1. By decoupling Ct from the pore-forming core, we further demonstrate that Ct is the primary heat-sensing element of the channel, whereas, in contrast, the pore domain lacks robust temperature sensitivity. Together, our findings outline a mechanism for signal transduction within K_2P2.1 (TREK-1) in which there is a clear crosstalk between the C-type gate and intracellular Ct domain. In addition, our findings support the general notion of the existence of modular temperature-sensing domains in temperature-sensitive ion channels. This marked distinction between gating and sensory elements suggests a general design principle that may underlie the function of a variety of temperature-sensitive channels.

TWIK1, a unique background channel with variable ion selectivity

TWIK1 belongs to the family of background K(+) channels with two pore domains. In native and transfected cells, TWIK1 is detected mainly in recycling endosomes. In principal cells in the kidney, TWIK1 gene inactivation leads to the loss of a nonselective cationic conductance, an unexpected effect that was attributed to adaptive regulation of other channels. Here, we show that TWIK1 ion selectivity is modulated by extracellular pH. Although TWIK1 is K(+) selective at neutral pH, it becomes permeable to Na(+) at the acidic pH found in endosomes. Selectivity recovery is slow after restoration of a neutral pH. Such hysteresis makes plausible a role of TWIK1 as a background channel in which selectivity and resulting inhibitory or excitatory influences on cell excitability rely on its recycling rate between internal acidic stores and the plasma membrane. TWIK1(-/-) pancreatic β cells are more polarized than control cells, confirming a depolarizing role of TWIK1 in kidney and pancreatic cells.