Taking apart the gating of voltage-gated K+ channels

Ataxia and myoclonic epilepsy due to a heterozygous new mutation in KCNA2: proposal for a new channelopathy

We have recently performed exome analysis in a 7 year boy who presented in infancy with an encephalopathy characterized by ataxia and myoclonic epilepsy. Parents were not consanguineous and there was no family history of the disease. Exome analysis did not show any pathogenic variants in genes known to be associated with seizures and/or ataxia in children, including all known human channelopathies. However, we have identified a mutation in KCNA2 that we believe to be responsible for the disease in our patient. This gene, which encodes a member of the potassium channel, voltage-gated, shaker-related subfamily, has not been previously described as a cause of disease in humans, but mutations of the orthologous gene in mice (Kcna2) are known to cause both ataxia and convulsions. The mutation is c.890C>A, leading to the amino acid substitution p.Arg297Gln, which involves the second of the critical arginines in the S4 voltage sensor. This mutation is characterized as pathogenic by five different prediction programs. RFLP analysis and Sanger sequencing confirmed the presence of the mutation in the patient, but not in his parents, characterizing it as de novo. We believe that this discovery characterizes a new channelopathy.

Gating charge interactions with the S1 segment during activation of a Na+ channel voltage sensor

Voltage-gated Na(+) channels initiate action potentials during electrical signaling in excitable cells. Opening and closing of the pore of voltage-gated ion channels are mechanically linked to voltage-driven outward movement of the positively charged S4 transmembrane segment in their voltage sensors. Disulfide locking of cysteine residues substituted for the outermost T0 and R1 gating-charge positions and a conserved negative charge (E43) at the extracellular end of the S1 segment of the bacterial Na(+) channel NaChBac detects molecular interactions that stabilize the resting state of the voltage sensor and define its conformation. Upon depolarization, the more inward gating charges R2 and R3 engage in these molecular interactions as the S4 segment moves outward to its intermediate and activated states. The R4 gating charge does not disulfide-lock with E43, suggesting an outer limit to its transmembrane movement. These molecular interactions reveal how the S4 gating charges are stabilized in the resting state and how their outward movement is catalyzed by interaction with negatively charged residues to effect pore opening and initiate electrical signaling.

Voltage-gated calcium channels

Voltage-gated calcium (Ca(2+)) channels are key transducers of membrane potential changes into intracellular Ca(2+) transients that initiate many physiological events. There are ten members of the voltage-gated Ca(2+) channel family in mammals, and they serve distinct roles in cellular signal transduction. The Ca(V)1 subfamily initiates contraction, secretion, regulation of gene expression, integration of synaptic input in neurons, and synaptic transmission at ribbon synapses in specialized sensory cells. The Ca(V)2 subfamily is primarily responsible for initiation of synaptic transmission at fast synapses. The Ca(V)3 subfamily is important for repetitive firing of action potentials in rhythmically firing cells such as cardiac myocytes and thalamic neurons. This article presents the molecular relationships and physiological functions of these Ca(2+) channel proteins and provides information on their molecular, genetic, physiological, and pharmacological properties.

Tetramerization domain mutations in KCNA5 affect channel kinetics and cause abnormal trafficking patterns

The activity of voltage-gated K(+) (K(V)) channels plays an important role in regulating pulmonary artery smooth muscle cell (PASMC) contraction, proliferation, and apoptosis. The highly conserved NH(2)-terminal tetramerization domain (T1) of K(V) channels is important for proper channel assembly, association with regulatory K(V) beta-subunits, and localization of the channel to the plasma membrane. We recently reported two nonsynonymous mutations (G182R and E211D) in the KCNA5 gene of patients with idiopathic pulmonary arterial hypertension, which localize to the T1 domain of KCNA5. To study the electrophysiological properties and expression patterns of the mutants compared with the wild-type (WT) channel in vitro, we transfected HEK-293 cells with WT KCNA5, G182R, E211D, or the double mutant G182R/E211D channel. The mutants form functional channels; however, whole cell current kinetic differences between WT and mutant channels exist. Steady-state inactivation curves of the G182R and G182R/E211D channels reveal accelerated inactivation; the mutant channels inactivated at more hyperpolarized potentials compared with the WT channel. Channel protein expression was also decreased by the mutations. Compared with the WT channel, which was present in its mature glycosylated form, the mutant channels are present in greater proportion in their immature form in HEK-293 cells. Furthermore, G182R protein level is greatly reduced in COS-1 cells compared with WT. Immunostaining data support the hypothesis that, while WT protein localizes to the plasma membrane, mutant protein is mainly retained in intracellular packets. Overall, these data support a role for the T1 domain in channel kinetics as well as in KCNA5 channel subcellular localization.

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.

The gate of the influenza virus M2 proton channel is formed by a single tryptophan residue

The influenza virus M2 proton-selective ion channel is known to be essential for acidifying the interior of virions during virus uncoating in the lumen of endosomes. The M2 protein is a homotetramer that contains four 19-residue transmembrane (TM) domains. These TM domains are multifunctional, because they contain the channel pore and also anchor the protein in membranes. The M2 protein is gated by pH, and thus we have measured pH-gated currents, the accessibility of the pore to Cu2+, and the effect of a protein-modifying reagent for a series of TM domain mutant M2 proteins. The results indicate that gating of the M2 ion channel is governed by a single side chain at residue 41 of the TM domain and that this property is mediated by an indole moiety. Unlike many ion channels where the gate is formed by a whole segment of a protein, our data suggest a model of striking simplicity for the M2 ion channel protein, with the side chain of Trp(41) blocking the pore of the M2 channel when pH(out) is high and with this side chain leaving the pore when pH(out) is low. Thus, the Trp(41) side chain acts as the gate that opens and closes the pore.

The alpha-helix and the organization and gating of channels

The structures of an increasing number of channels and other alpha-helical membrane proteins have been determined recently, including the KcsA potassium channel, the MscL mechanosensitive channel, and the AQP1 and GlpF members of the aquaporin family. In this chapter, the orientation and packing characteristics of bilayer-spanning helices are surveyed in integral membrane proteins. In the case of channels, alpha-helices create the sealed barrier that separates the hydrocarbon region of the bilayer from the permeation pathway for solutes. The helices surrounding the permeation pathway tend to be rather steeply tilted relative to the membrane normal and are consistently arranged in a right-handed bundle. The helical framework further provides a supporting scaffold for nonmembrane-spanning structures associated with channel selectivity. Although structural details remain scarce, the conformational changes associated with gating transitions between closed and open states of channels are reviewed, emphasizing the potential roles of helix-helix interactions in this process.

The 25-kDa synaptosome-associated protein (SNAP-25) binds and inhibits delayed rectifier potassium channels in secretory cells

Delayed-rectifier K(+) channels (K(DR)) are important regulators of membrane excitability in neurons and neuroendocrine cells. Opening of these voltage-dependent K(+) channels results in membrane repolarization, leading to the closure of the Ca(2+) channels and cessation of insulin secretion in neuroendocrine islet beta cells. Using patch clamp techniques, we have demonstrated that the activity of the K(DR) channel subtype, K(V)1.1, identified by its specific blocker dendrodotoxin-K, is inhibited by SNAP-25 in insulinoma HIT-T15 beta cells. A co-precipitation study of rat brain confirmed that SNAP-25 interacts with the K(V)1.1 protein. Cleavage of SNAP-25 by expression of botulinum neurotoxin A in HIT-T15 cells relieved this SNAP-25-mediated inhibition of K(DR). This inhibitory effect of SNAP-25 is mediated by the N terminus of K(V)1.1, likely by direct interactions with K(Valpha)1.1 and/or K(V)beta subunits, as revealed by co-immunoprecipitation performed in the Xenopus oocyte expression system and in vitro binding. Taken together we have concluded that SNAP-25 mediates secretion not only through its participation in the exocytotic SNARE complex but also by regulating membrane potential and calcium entry through its interaction with K(DR) channels.

Functional stoichiometry of glutamate receptor desensitization

Potassium (K+) channels and ionotropic glutamate receptors (iGluRs) fulfill divergent roles in vertebrate nervous systems. Despite this, however, recent work suggests that these ion channels are structurally homologous, sharing an ancestral protein, architectural design, and tetrameric subunit stoichiometry. Their gating mechanisms also are speculated to have overlapping features. Here we show that the mechanism of iGluR desensitization is unique. Unlike K+ channels, AMPA- and kainate-type iGluR subunits desensitize in several ordered conformational steps. AMPA receptors operate as dimers, whereas the functional stoichiometry of kainate receptor desensitization is dependent on external ions. Contrary to conventional understanding, kinetic models suggest that partially desensitized AMPA and kainate receptors conduct ions and are likely participants in synaptic signaling. Although sharing many structural correlates with K+ channels, iGluRs have evolved unique subunit-subunit interactions, tailoring their gating behavior to fulfill distinct roles in neuronal signaling.

Two adaptor proteins differentially modulate the phosphorylation and biophysics of Kv1.3 ion channel by SRC kinase

The Shaker family K(+) channel protein, Kv1.3, is tyrosine phosphorylated by v-Src kinase at Tyr(137) and Tyr(449) to modulate current magnitude and kinetic properties. Despite two proline rich sequences and these phosphotyrosines contained in the carboxyl and amino terminals of the channel, v-Src kinase fails to co-immunoprecipitate with Kv1.3 as expressed in HEK 293 cells, indicating a lack of direct Src homology 3- or Src homology 2-mediated protein-protein interaction between the channel and the kinase. We show that the adaptor proteins, n-Shc and Grb10, are expressed in the olfactory bulb, a region of the brain where Kv1.3 is highly expressed. In HEK 293 cells, co-expression of Kv1.3 plus v-Src with Grb10 causes a decrease in v-Src-induced Kv1.3 tyrosine phosphorylation and a reversal of v-Src-induced Kv1.3 current suppression, increase in inactivation time constant (tau(inact)), and disruption of cumulative inactivation properties. Co-expression of Kv1.3 plus v-Src with n-Shc did not significantly alter v-Src-induced Kv1.3 current suppression but reversed v-Src induced increased tau(inact) and restored the right-shifted voltage at half-activation (V(1/2)) induced by v-Src. The v-Src-induced shift in V(1/2) and increased tau(inact) was retained when Tyr(220), Tyr(221), and Tyr(304) in the CH domain of n-Shc were mutated to Phe (triple Shc mutant) but was reversed back to control values when either wild-type Shc or the family member Sck, which is not a substrate for Src kinase, was substituted for the triple Shc mutant. Thus the portion of the CH domain that includes Tyr(220), Tyr(221), and Tyr(304) may regulate a shift in Kv1.3 voltage dependence and inactivation kinetics produced by n-Shc in the presence of v-Src. Collectively these data indicate that Grb10 and n-Shc adaptor molecules differentially modulate the degree of Kv1.3 tyrosine phosphorylation, the channel's biophysical properties, and the physical complexes associated with Kv1.3 in the presence of Src kinase.

External anions and cations distinguish between AMPA and kainate receptor gating mechanisms

Experiments were designed to examine if ion-flow through alpha-amino-3-hydroxy-5-methyl-isoxazolepropionic acid (AMPA) or kainate receptors interferes with protein structures associated with the gating machinery. Gating was studied using ultra-fast drug perfusion of outside-out patches containing rat GluR-A or GluR6 subunits excised from transfected human embryonic kidney cells. Deactivation rates of GluR6 kainate receptors observed following brief L-glutamate (10 mM Glu, 1 ms) applications differed by two to threefold in high (405 mM symmetrical Na(+), tau(decay) = 2.7 ms at -100 mV) and low ionic strength (55 mM, tau(decay) = 1.1 ms) solutions. In comparison, GluR-A AMPA receptors were much less sensitive. Ion effects on GluR6 receptors did not reflect surface potential screening or ion-agonist competition at the agonist-binding site since deactivation rates were slower in high ionic strength solutions. Moreover, the apparent agonist affinity did not decrease with increasing ionic strength (e.g. 55 mM, EC(50) = 110 microM vs. 405 mM, EC(50) = 61 microM). GluR6 responses were strongly dependent on ions present on the external, but not the internal, side of the plasma membrane. Decay kinetics was regulated by the type of ion present suggesting that the chemical nature of the solution, not its ionic strength, governed channel behaviour. Both external anions and cations modulated the amplitude and decay kinetics of GluR6 responses in a concomitant manner. AMPA receptor responses recorded in identical ionic conditions did not exhibit this behaviour. These results identify a novel mechanism that distinguishes AMPA and kainate receptors. External ions regulate the gating machinery of kainate receptors through an allosteric mechanism that involves both anions and cations.

The S4-S5 linker couples voltage sensing and activation of pacemaker channels

Voltage-gated channels are normally opened by depolarization and closed by repolarization of the membrane. Despite sharing significant sequence homology with voltage-gated K(+) channels, the gating of hyperpolarization-activated, cyclic-nucleotide-gated (HCN) pacemaker channels has the opposite dependence on membrane potential: hyperpolarization opens, whereas depolarization closes, these channels. The mechanism and structural basis of the process that couples voltage sensor movement to HCN channel opening and closing is not understood. On the basis of our previous studies of a mutant HERG (human ether-a-go-go-related gene) channel, we hypothesized that the intracellular linker that connects the fourth and fifth transmembrane domains (S4-S5 linker) of HCN channels might be important for channel gating. Here, we used alanine-scanning mutagenesis of the HCN2 S4-S5 linker to identify three residues, E324, Y331, and R339, that when mutated disrupted normal channel closing. Mutation of a basic residue in the S4 domain (R318Q) prevented channel opening, presumably by disrupting S4 movement. However, channels with R318Q and Y331S mutations were constitutively open, suggesting that these channels can open without a functioning S4 domain. We conclude that the S4-S5 linker mediates coupling between voltage sensing and HCN channel activation. Our findings also suggest that opening of HCN and related channels corresponds to activation of a gate located near the inner pore, rather than recovery of channels from a C-type inactivated state.