The selectivity, voltage-dependence and acid sensitivity of the tandem pore potassium channel TASK-1 : contributions of the pore domains

We have investigated the contribution to ionic selectivity of residues in the selectivity filter and pore helices of the P1 and P2 domains in the acid sensitive potassium channel TASK-1. We used site directed mutagenesis and electrophysiological studies, assisted by structural models built through computational methods. We have measured selectivity in channels expressed in Xenopus oocytes, using voltage clamp to measure shifts in reversal potential and current amplitudes when Rb+ or Na+ replaced extracellular K+. Both P1 and P2 contribute to selectivity, and most mutations, including mutation of residues in the triplets GYG and GFG in P1 and P2, made channels nonselective. We interpret the effects of these—and of other mutations—in terms of the way the pore is likely to be stabilised structurally. We show also that residues in the outer pore mouth contribute to selectivity in TASK-1. Mutations resulting in loss of selectivity (e.g. I94S, G95A) were associated with slowing of the response of channels to depolarisation. More important physiologically, pH sensitivity is also lost or altered by such mutations. Mutations that retained selectivity (e.g. I94L, I94V) also retained their response to acidification. It is likely that responses both to voltage and pH changes involve gating at the selectivity filter

Artificial Modulation of the Gating Behavior of a K+ Channel in a KvAP-DNA Chimera

We present experiments where the gating behavior of a voltage-gated ion channel is modulated by artificial ligand binding. We construct a channel-DNA chimera with the KvAP potassium channel reconstituted in an artificial membrane. The channel is functional and the single channel ion conductivity unperturbed by the presence of the DNA. However, the channel opening probability vs. bias voltage, i.e., the gating, can be shifted considerably by the electrostatic force between the charges on the DNA and the voltage sensing domain of the protein. Different hybridization states of the chimera DNA thus lead to different response curves of the channel

Non-equivalent role of TM2 gating hinges in heteromeric Kir4.1/Kir5.1 potassium channels

Comparison of the crystal structures of the KcsA and MthK potassium channels suggests that the process of opening a K+ channel involves pivoted bending of the inner pore-lining helices at a highly conserved glycine residue. This bending motion is proposed to splay the transmembrane domains outwards to widen the gate at the “helix-bundle crossing”. However, in the inwardly rectifying (Kir) potassium channel family, the role of this “hinge” residue in the second transmembrane domain (TM2) and that of another putative glycine gating hinge at the base of TM2 remain controversial. We investigated the role of these two positions in heteromeric Kir4.1/Kir5.1 channels, which are unique amongst Kir channels in that both subunits lack a conserved glycine at the upper hinge position. Contrary to the effect seen in other channels, increasing the potential flexibility of TM2 by glycine substitutions at the upper hinge position decreases channel opening. Furthermore, the contribution of the Kir4.1 subunit to this process is dominant compared to Kir5.1, demonstrating a non-equivalent contribution of these two subunits to the gating process. A homology model of heteromeric Kir4.1/Kir5.1 shows that these upper “hinge” residues are in close contact with the base of the pore α-helix that supports the selectivity filter. Our results also indicate that the highly conserved glycine at the “lower” gating hinge position is required for tight packing of the TM2 helices at the helix-bundle crossing, rather than acting as a hinge residue

Unambiguous observation of blocked states reveals altered, blocker-induced, cardiac ryanodine receptor gating

The flow of ions through membrane channels is precisely regulated by gates. The architecture and function of these elements have been studied extensively, shedding light on the mechanisms underlying gating. Recent investigations have focused on ion occupancy of the channel’s selectivity filter and its ability to alter gating, with most studies involving prokaryotic K+ channels. Some studies used large quaternary ammonium blocker molecules to examine the effects of altered ionic flux on gating. However, the absence of blocking events that are visibly distinct from closing events in K+ channels makes unambiguous interpretation of data from single channel recordings difficult. In this study, the large K+ conductance of the RyR2 channel permits direct observation of blocking events as distinct subconductance states and for the first time demonstrates the differential effects of blocker molecules on channel gating. This experimental platform provides valuable insights into mechanisms of blocker-induced modulation of ion channel gating

The Ankyrin Repeat Domain of the TRPA Protein Painless Is Important for Thermal Nociception but Not Mechanical Nociception

The Drosophila TRPA channel Painless is required for the function of polymodal nociceptors which detect noxious heat and noxious mechanical stimuli. These functions of Painless are reminiscent of mammalian TRPA channels that have also been implicated in thermal and mechanical nociception. A popular hypothesis to explain the mechanosensory functions of certain TRP channels proposes that a string of ankyrin repeats at the amino termini of these channels acts as an intracellular spring that senses force. Here, we describe the identification of two previously unknown Painless protein isoforms which have fewer ankyrin repeats than the canonical Painless protein. We show that one of these Painless isoforms, that essentially lacks ankyrin repeats, is sufficient to rescue mechanical nociception phenotypes of painless mutant animals but does not rescue thermal nociception phenotypes. In contrast, canonical Painless, which contains Ankyrin repeats, is sufficient to largely rescue thermal nociception but is not capable of rescuing mechanical nociception. Thus, we propose that in the case of Painless, ankryin repeats are important for thermal nociception but not for mechanical nociception

Calmodulin is responsible for Ca2+-dependent regulation of TRPA1 channels

TRPA1 is a Ca2+-permeable ion channel involved in many sensory disorders such as pain, itch and neuropathy. Notably, the function of TRPA1 depends on Ca2+, with low Ca2+ potentiating and high Ca2+ inactivating TRPA1. However, it remains unknown how Ca2+ exerts such contrasting effects. Here, we show that Ca2+ regulates TRPA1 through calmodulin, which binds to TRPA1 in a Ca2+-dependent manner. Calmodulin binding enhanced TRPA1 sensitivity and Ca2+-evoked potentiation of TRPA1 at low Ca2+, but inhibited TRPA1 sensitivity and promoted TRPA1 desensitization at high Ca2+. Ca2+-dependent potentiation and inactivation of TRPA1 were selectively prevented by disrupting the interaction of the carboxy-lobe of calmodulin with a calmodulin-binding domain in the C-terminus of TRPA1. Calmodulin is thus a critical Ca2+ sensor enabling TRPA1 to respond to diverse Ca2+ signals distinctly

Asymmetric Switching in a Homodimeric ABC Transporter: A Simulation Study

ABC transporters are a large family of membrane proteins involved in a variety of cellular processes, including multidrug and tumor resistance and ion channel regulation. Advances in the structural and functional understanding of ABC transporters have revealed that hydrolysis at the two canonical nucleotide-binding sites (NBSs) is co-operative and non-simultaneous. A conserved core architecture of bacterial and eukaryotic ABC exporters has been established, as exemplified by the crystal structure of the homodimeric multidrug exporter Sav1866. Currently, it is unclear how sequential ATP hydrolysis arises in a symmetric homodimeric transporter, since it implies at least transient asymmetry at the NBSs. We show by molecular dynamics simulation that the initially symmetric structure of Sav1866 readily undergoes asymmetric transitions at its NBSs in a pre-hydrolytic nucleotide configuration. MgATP-binding residues and a network of charged residues at the dimer interface are shown to form a sequence of putative molecular switches that allow ATP hydrolysis only at one NBS. We extend our findings to eukaryotic ABC exporters which often consist of two non-identical half-transporters, frequently with degeneracy substitutions at one of their two NBSs. Interestingly, many residues involved in asymmetric conformational switching in Sav1866 are substituted in degenerate eukaryotic NBS. This finding strengthens recent suggestions that the interplay of a consensus and a degenerate NBS in eukaroytic ABC proteins pre-determines the sequence of hydrolysis at the two NBSs

TRPA1 Is a Polyunsaturated Fatty Acid Sensor in Mammals

Fatty acids can act as important signaling molecules regulating diverse physiological processes. Our understanding, however, of fatty acid signaling mechanisms and receptor targets remains incomplete. Here we show that Transient Receptor Potential Ankyrin 1 (TRPA1), a cation channel expressed in sensory neurons and gut tissues, functions as a sensor of polyunsaturated fatty acids (PUFAs) in vitro and in vivo. PUFAs, containing at least 18 carbon atoms and three unsaturated bonds, activate TRPA1 to excite primary sensory neurons and enteroendocrine cells. Moreover, behavioral aversion to PUFAs is absent in TRPA1-null mice. Further, sustained or repeated agonism with PUFAs leads to TRPA1 desensitization. PUFAs activate TRPA1 non-covalently and independently of known ligand binding domains located in the N-terminus and 5th transmembrane region. PUFA sensitivity is restricted to mammalian (rodent and human) TRPA1 channels, as the drosophila and zebrafish TRPA1 orthologs do not respond to DHA. We propose that PUFA-sensing by mammalian TRPA1 may regulate pain and gastrointestinal functions

Pore-opening mechanism in trimeric P2X receptor channels

The opening of ion channels in response to ligand binding, voltage or membrane stretch underlies electrical and chemical signalling throughout biology. Two structural classes of pore-opening mechanisms have been established, including bending of pore-lining helices in the case of tetrameric cation channels, or tilting of such helices in mechanosensitive channels. In this paper, we explore how the structure of the pore changes during opening in P2X receptors by measuring the modification of introduced cysteine residues in transmembrane helices by thiol-reactive reagents, and by engineering metal bridges. Our results are consistent with the X-ray structure of the closed state, and demonstrate that expansion of the gate region in the external pore is accompanied by a significant narrowing of the inner pore, indicating that pore-forming helices straighten on ATP binding to open the channel. This unique pore-opening mechanism has fundamental implications for the role of subunit interfaces in the gating mechanism of P2X receptors and points to a role of the internal pore in ion permeation

Cooperative Transition between Open and Closed Conformations in Potassium Channels

Potassium (K+) ion channels switch between open and closed conformations. The nature of this important transition was revealed by comparing the X-ray crystal structures of the MthK channel from Methanobacterium thermoautotrophicum, obtained in its open conformation, and the KcsA channel from Streptomyces lividans, obtained in its closed conformation. We analyzed the dynamic characteristics and energetics of these homotetrameric structures in order to study the role of the intersubunit cooperativity in this transition. For this, elastic models and in silico alanine-scanning mutagenesis were used, respectively. Reassuringly, the calculations manifested motion from the open (closed) towards the closed (open) conformation. The calculations also revealed a network of dynamically and energetically coupled residues. Interestingly, the network suggests coupling between the selectivity filter and the gate, which are located at the two ends of the channel pore. Coupling between these two regions was not observed in calculations that were conducted with the monomer, which emphasizes the importance of the intersubunit interactions within the tetrameric structure for the cooperative gating behavior of the channel