Dynamic interaction of S5 and S6 during voltage-controlled gating in a potassium channel

Auto-inhibitory intramolecular S5/S6 interaction in the TRPV6 channel regulates breast cancer cell migration and invasion

TRPV6, a Ca-selective channel, is abundantly expressed in the placenta, intestine, kidney and bone marrow. TRPV6 is vital to Ca homeostasis and its defective expression or function is linked to transient neonatal hyperparathyroidism, Lowe syndrome/Dent disease, renal stone, osteoporosis and cancers. The fact that the molecular mechanism underlying the function and regulation of TRPV6 is still not well understood hampers, in particular, the understanding of how TRPV6 contributes to breast cancer development. By electrophysiology and Ca imaging in Xenopus oocytes and cancer cells, molecular biology and numerical simulation, here we reveal an intramolecular S5/S6 helix interaction in TRPV6 that is functionally autoinhibitory and is mediated by the R532:D620 bonding. Predicted pathogenic mutation R532Q within S5 disrupts the S5/S6 interaction leading to gain-of-function of the channel, which promotes breast cancer cell progression through strengthening of the TRPV6/PI3K interaction, activation of a PI3K/Akt/GSK-3β cascade, and up-regulation of epithelial-mesenchymal transition and anti-apoptosis.

Cooperative nature of gating transitions in K(+) channels as seen from dynamic importance sampling calculations

The growing dataset of K(+) channel x-ray structures provides an excellent opportunity to begin a detailed molecular understanding of voltage-dependent gating. These structures, while differing in sequence, represent either a stable open or closed state. However, an understanding of the molecular details of gating will require models for the transitions and experimentally testable predictions for the gating transition. To explore these ideas, we apply dynamic importance sampling to a set of homology models for the molecular conformations of K(+) channels for four different sets of sequences and eight different states. In our results, we highlight the importance of particular residues upstream from the Pro-Val-Pro (PVP) region to the gating transition. This supports growing evidence that the PVP region is important for influencing the flexibility of the S6 helix and thus the opening of the gating domain. The results further suggest how gating on the molecular level depends on intra-subunit motions to influence the cooperative behavior of all four subunits of the K(+) channel. We hypothesize that the gating process occurs in steps: first sidechain movement, then inter-S5-S6 subunit motions, and lastly the large-scale domain rearrangements.

Potassium channels

Potassium channels are integral membrane proteins that selectively transport K+ across the cell membrane. They are present in all mammalian cells and have a wide variety of roles in both excitable and nonexcitable cells. The phenotypic diversity required to accomplish their various roles is created by differences in conductance, the timecourse and mechanisms of different gating events, and the interaction of channels with a variety of accessory proteins. Through the integration of biophysical, molecular, structural, and theoretical studies, significant progress has been made toward understanding the structural basis of K+ channel function, and diseases associated with K+ channel dysfunction.

Molecular compatibility of the channel gate and the N terminus of S5 segment for voltage-gated channel activity

Voltage-gated ion channels are modular proteins designed by the structural linkage of a voltage sensor and a pore domain. The functional coupling of these two protein modules is a subject of intense research. A major focus has been directed to decipher the role of the S4-S5 linker and the C-end of the inner pore helix in channel gating. However, the contribution of the cytosolic N terminus of S5 remains elusive. To address this issue, we used a chimeric subunit that linked the voltage sensor of the Shaker channel to the prokaryotic KcsA pore domain (denoted as Shaker-KcsA). This chimera preserved the Shaker sequences at both the N terminus of S5 and the C-end of S6. Chimeric Shaker-KcsA subunits did not form functional homomeric channels but were synthesized, folded, and trafficked to the cell surface, as evidenced by their co-assembly with Shaker wild type subunits. Sequential substitution of Shaker amino acids at the C-end of S6 and the N terminus of S5 by the corresponding KcsA created voltage-sensitive channels with voltage-dependent properties that asymptotically approached those of the wild type Shaker channel. Noteworthy, substitution of the region encompassing Phe(401)-Phe(404) at the N-end of Shaker S5 by KcsA residues resulted in a significant gain in voltage sensitivity of the chimeras. Furthermore, analysis of channel function at high [K(+)](o) revealed that the Phe(401)-Phe(404) region is an important molecular determinant for competent coupling of voltage sensing and pore opening. Taken together, these findings indicate that complete replacement of Shaker S5 and S6 by KcsA M1 and M2 is required for voltage-dependent gating of the prokaryotic channel. In addition, our results imply that the region encompassing Phe(401)-Phe(404) in Shaker is involved in protein-protein interactions with the voltage sensor, and signal to the Phe(401) in the S5 segment as a key molecular determinant to pair the voltage sensor and the pore domain.

Mutation of colocalized residues of the pore helix and transmembrane segments S5 and S6 disrupt deactivation and modify inactivation of KCNQ1 K+ channels

KCNQ1 (Kv 7.1) alpha-subunits and KCNE1 beta-subunits co-assemble to form channels that conduct the slow delayed rectifier K+ current (IKs) in the heart. Mutations in either subunit cause long QT syndrome (LQTS), an inherited disorder of cardiac repolarization. Here, the functional consequences of the LQTS-associated missense mutation V310I and several nearby residues were determined. Val310 is located at the base of the pore helix of KCNQ1, two residues below the TIGYG signature sequence that defines the K+ selectivity filter. Channels were heterologously expressed in Xenopus laevis oocytes and currents were recorded using the two-microelectrode voltage-clamp technique. V310I KCNQ1 reduced IKs amplitude when co-expressed with wild-type KCNQ1 and KCNE1 subunits. Val310 was also mutated to Gly, Ala or Leu to explore the importance of amino acid side chain volume at this position. Like V310I, V310L KCNQ1 channels gated normally. Unexpectedly, V310G and V310A KCNQ1 channels inactivated strongly and did not close normally in response to membrane hyperpolarization. Based on a homology model of the KCNQ1 channel pore, we speculate that the side group of residue 310 can interact with specific residues in the S5 and S6 domains to alter channel gating. When volume of the side chain is small, the stability of the closed state is disrupted and the extent of channel inactivation is enhanced. We mutated putative interacting residues in S5 and S6 and found that mutant Leu273 and Phe340 channels also can disrupt close states and modify inactivation. Together these findings indicate the importance of a putative pore helix-S5-S6 interaction for normal KCNQ1 channel deactivation and confirm its role in KCNQ1 inactivation. Disturbance of these interactions might underly LQTS associated with KCNQ1 mutant channels.

Ligand-induced closure of inward rectifier Kir6.2 channels traps spermine in the pore

Small organic amines block open voltage-gated K+ channels and can be trapped by subsequent closure. Such studies provide strong evidence for voltage gating occurring at the intracellular end of the channel. We engineered the necessary properties (long block times with unblock kinetics comparable to, or slower than, the kinetics of gating) into spermine-blocked, ATP-gated (N160D,L157C) mutant KATP channels, in order to test the possibility of "blocker trapping" in ligand-gated Kir channels. Spermine block of these channels is very strongly voltage dependent, such that, at positive voltages, the off-rate of spermine is very low. A brief pulse to negative voltages rapidly relieves the block, but no such relief is observed in ATP-closed channels. The results are well fit by a simple kinetic model that assumes no spermine exit from closed channels. The results incontrovertibly demonstrate that spermine is trapped in channels that are closed by ATP, and implicate the M2 helix bundle crossing, or somewhere lower, as the probable location of the gate.

Molecular coupling between voltage sensor and pore opening in the Arabidopsis inward rectifier K+ channel KAT1

Animal and plant voltage-gated ion channels share a common architecture. They are made up of four subunits and the positive charges on helical S4 segments of the protein in animal K⁺ channels are the main voltage-sensing elements. The KAT1 channel cloned from Arabidopsis thaliana, despite its structural similarity to animal outward rectifier K⁺ channels is, however, an inward rectifier. Here we detected KAT1-gating currents due to the existence of an intrinsic voltage sensor in this channel. The measured gating currents evoked in response to hyperpolarizing voltage steps consist of a very fast (τ = 318 ± 34 μs at −180 mV) and a slower component (4.5 ± 0.5 ms at −180 mV) representing charge moved when most channels are closed. The observed gating currents precede in time the ionic currents and they are measurable at voltages (less than or equal to −60) at which the channel open probability is negligible (≈10⁻⁴). These two observations, together with the fact that there is a delay in the onset of the ionic currents, indicate that gating charge transits between several closed states before the KAT1 channel opens. To gain insight into the molecular mechanisms that give rise to the gating currents and lead to channel opening, we probed external accessibility of S4 domain residues to methanethiosulfonate-ethyltrimethylammonium (MTSET) in both closed and open cysteine-substituted KAT1 channels. The results demonstrate that the putative voltage–sensing charges of S4 move inward when the KAT1 channels open.

Tight coupling of rubidium conductance and inactivation in human KCNQ1 potassium channels

KCNQ1 K+ channels in humans are important for repolarization of cardiac action potentials and for K+ secretion in the inner ear. The pore-forming channel subunits form heteromeric complexes with small regulatory subunits of the KCNE family, in particular with KCNE1 to form channels that conduct a slow delayed rectifier K+ current, IKs. This association leads to alteration of biophysical properties, including a slowing of activation, a suppression of inactivation and an increase of the apparent single-channel conductance. In addition, inward Rb+ currents conducted by homomeric KCNQ1 channels are about threefold larger than K+ currents, whereas heteromeric KCNQ1-KCNE1 channels have smaller inward Rb+ currents compared to K+ currents. We determined inactivation properties and compared K+ vs. Rb+ inward currents for channels formed by co-assembly of KCNQ1 with KCNE1, KCNE3 and KCNE5, and for homomeric KCNQ1 channels with point mutations in the pore helix S5 or S6 transmembrane domains. Several of the channels with point mutations eliminated the apparent inactivation of KCNQ1, as described previously (Seebohm et al. 2001). We found that the extent of inactivation and the ratio of Rb+/K+ currents were positively correlated. Since the effect of Rb+ on the current size has been shown previously to be related to a fast 'flickery' process, our results suggest that inactivation of KCNQ1 channels is related to a fast flicker of the open channel. A kinetic model incorporating two open states, no explicit inactivated state and a fast flicker that is different for the two open states is able to account for the apparent inactivation and the correlation of inactivation and large Rb+ currents. We conclude that an association between KCNQ1 and KCNE subunits or removal of inactivation by mutation of KCNQ1 stabilizes the open conformation of the pore principally by altering an interaction between the pore helix and the selectivity filter and with S5/S6 domains.

Allosteric modulation of a neuronal K+ channel by 1-alkanols is linked to a key residue in the activation gate

The selective inhibition of neuronal Shaw2 K+ channels by 1-alkanols is conferred by the internal S4-S5 loop, a region that also contributes to the gating of voltage-gated K+ channels. Here, we applied alanine scanning mutagenesis to examine the contribution of the S5 and S6 segments to the allosteric modulation of Shaw2 K+ channels by 1-alkanols. The internal section of S6 is the main activation gate of K+ channels. While several mutations in S5 and S6 modulated the inhibition of the channels by 1-butanol and others had no effect, a single mutation at a key site in S6 (P410A) converted this inhibition into a dramatic dose-dependent potentiation (approximately 2-fold at 15 mM and approximately 6-fold at 50 mM). P410 is the second proline in the highly conserved PVP motif that may cause a significant alpha-helix kink. The P410A currents in the presence of 1-butanol also exhibited novel kinetics (faster activation and slow inactivation). Internal application of 15 mM 1-butanol to inside-out patches expressing P410A did not significantly affect the mean unitary currents (approximately 2 pA at 0 mV) or the mean open time (5-6 ms) but clearly increased the opening frequency and open probability (approximately 2- to 4-fold). All effects displayed a fast onset and were fully reversible upon washout. The results suggest that the allosteric modulation of the Shaw2 K+ channel by 1-alkanols depends on a critical link between the PVP motif and activation gating. This study establishes the Shaw2 K+ channel as a robust model to investigate the mechanisms of alcohol intoxication and general anesthesia.

The pore helix is involved in stabilizing the open state of inwardly rectifying K+ channels

Ion channels can be gated by various extrinsic cues, such as voltage, pH, and second messengers. However, most ion channels display extrinsic cue-independent transitions as well. These events represent spontaneous conformational changes of the channel protein. The molecular basis for spontaneous gating and its relation to the mechanism by which channels undergo activation gating by extrinsic cue stimulation is not well understood. Here we show that the proximal pore helix of inwardly rectifying (Kir) channels is partially responsible for determining spontaneous gating characteristics, affecting the open state of the channel by stabilizing intraburst openings as well as the bursting state itself without affecting K(+) ion-channel interactions. The effect of the pore helix on the open state of the channel is qualitatively similar to that of two well-characterized mutations at the second transmembrane domain (TM2), which stabilize the channel in its activated state. However, the effects of the pore helix and the TM2 mutations on gating were additive and independent of each other. Moreover, in sharp contrast to the two TM2 mutations, the pore helix mutation did not affect the functionality of the agonist-responsive gate. Our results suggest that in Kir channels, the bottom of the pore helix and agonist-induced conformational transitions at the TM2 ultimately stabilize via different pathways the open conformation of the same gate.

A cysteine scan of the inner vestibule of cyclic nucleotide-gated channels reveals architecture and rearrangement of the pore

Cyclic nucleotide-gated (CNG) channels belong to the P-loop-containing family of ion channels that also includes KcsA, MthK, and Shaker channels. In this study, we investigated the structure and rearrangement of the CNGA1 channel pore using cysteine mutations and cysteine-specific modification. We constructed 16 mutant channels, each one containing a cysteine mutation at one of the positions between 384 and 399 in the S6 region of the pore. By measuring currents activated by saturating concentrations of the full agonist cGMP and the partial agonists cIMP and cAMP, we show that mutating S6 residues to cysteine caused both favorable and unfavorable changes in the free energy of channel opening. The time course of cysteine modification with 2-aminoethylmethane thiosulfonate hydrochloride (MTSEA) was complex. For many positions we observed decreases in current activated by cGMP and concomitant increases in current activated by cIMP and cAMP. A model where modification affected both gating and permeation successfully reproduced the complex time course of modification for most of the mutant channels. From the model fits to the time course of modification for each mutant channel, we quantified the following: (a) the bimolecular rate constant of modification in the open state, (b) the change in conductance, and (c) the change in the free energy of channel opening for modification of each cysteine. At many S6 cysteines, modification by MTSEA caused a decrease in conductance and a favorable change in the free energy of channel opening. Our results are interpreted within the structural framework of the known structures of KcsA and MthK. We conclude that: (a) MTSEA modification affects both gating and permeation, (b) the open configuration of the pore of CNGA1 channels is consistent with the structure of MthK, and (c) the modification of S6 residues disrupts the helical packing of the closed channel, making it easier for channels to open.

From genes to integrative physiology: ion channel and transporter biology in Caenorhabditis elegans

The stunning progress in molecular biology that has occurred over the last 50 years drove a powerful reductionist approach to the study of physiology. That same progress now forms the foundation for the next revolution in physiological research. This revolution will be focused on integrative physiology, which seeks to understand multicomponent processes and the underlying pathways of information flow from an organism's "parts" to increasingly complex levels of organization. Genetically tractable and genomically defined nonmammalian model organisms such as the nematode Caenorhabditis elegans provide powerful experimental advantages for elucidating gene function and the molecular workings of complex systems. This review has two main goals. The first goal is to describe the experimental utility of C. elegans for investigating basic physiological problems. A detailed overview of C. elegans biology and the experimental tools, resources, and strategies available for its study is provided. The second goal of this review is to describe how forward and reverse genetic approaches and direct behavioral and physiological measurements in C. elegans have generated novel insights into the integrative physiology of ion channels and transporters. Where appropriate, I describe how insights from C. elegans have provided new understanding of the physiology of membrane transport processes in mammals.

Gating dependence of inner pore access in inward rectifier K(+) channels

Cation channel gating may occur either at or below the inner vestibule entrance or at the selectivity filter. To differentiate these possibilities in inward rectifier (Kir) channels, we examined cysteine accessibility in the ATP-gated Kir6.2 channel. MTSEA and MTSET both block channels and modify M2 cysteines with identical voltage dependence. If entry is restricted to open channels, modification rates will slow in ATP-closed channels, but because the reagent can be trapped in the pore following brief openings, this may not be apparent until open probability is extremely low (<0.01). When these conditions are met, modification does slow significantly, indicating gated access and highlighting an important caveat for interpretation of MTS-accessibility measurements: reagent "trapping" in nominally "closed" channels may obscure gated access.

Potassium channels: structures, models, simulations

Potassium channels have been studied intensively in terms of the relationship between molecular structure and physiological function. They provide an opportunity to integrate structural and computational studies in order to arrive at an atomic resolution description of mechanism. We review recent progress in K channel structural studies, focussing on the bacterial channel KcsA. Structural studies can be extended via use of computational (i.e. molecular simulation) approaches in order to provide a perspective on aspects of channel function such as permeation, selectivity, block and gating. Results from molecular dynamics simulations are shown to be in good agreement with recent structural studies of KcsA in terms of the interactions of K(+) ions with binding sites within the selectivity filter of the channel, and in revealing the importance of filter flexibility in channel function. We discuss how the KcsA structure may be used as a template for developing structural models of other families of K channels. Progress in this area is explored via two examples: inward rectifier (Kir) and voltage-gated (Kv) potassium channels. A brief account of structural studies of ancillary domains and subunits of K channels is provided.

Molecular determinants of voltage-dependent slow inactivation of the Ca2+ channel

Ba(2+) current through the L-type Ca(2+) channel inactivates essentially by voltage-dependent mechanisms with fast and slow kinetics. Here we found that slow inactivation is mediated by an annular determinant composed of hydrophobic amino acids located near the cytoplasmic ends of transmembrane segments S6 of each repeat of the alpha(1C) subunit. We have determined the molecular requirements that completely obstruct slow inactivation. Critical interventions include simultaneous substitution of A752T in IIS6, V1165T in IIIS6, and I1475T in IVS6, each preventing in additive manner a considerable fraction of Ba(2+) current from inactivation. In addition, it requires the S405I mutation in segment IS6. The fractional inhibition of slow inactivation in tested mutants caused an acceleration of fast inactivation, suggesting that fast and slow inactivation mechanisms are linked. The channel lacking slow inactivation showed approximately 45% of the sustained Ba(2+) or Ca(2+) current with no indication of decay. The remaining fraction of the current was inactivated with a single-exponential decay (pi(f) approximately 10 ms), completely recovered from inactivation within 100 ms and did not exhibit Ca(2+)-dependent inactivation properties. No voltage-dependent characteristics were significantly changed, consistent with the C-type inactivation model suggesting constriction of the pore as the main mechanism possibly targeted by Ca(2+) sensors of inactivation.