Images of purified Shaker potassium channels

Selectivity changes during activation of mutant Shaker potassium channels

Mutations of the pore-region residue T442 in Shaker channels result in large effects on channel kinetics. We studied mutations at this position in the backgrounds of NH2-terminal-truncated Shaker H4 and a Shaker -NGK2 chimeric channel having high conductance (Lopez, G.A., Y.N. Jan, and L.Y. Jan. 1994. Nature (Lond.). 367: 179-182). While mutations of T442 to C, D, H, V, or Y resulted in undetectable expression in Xenopus oocytes, S and G mutants yielded functional channels having deactivation time constants and channel open times two to three orders of magnitude longer than those of the parental channel. Activation time courses at depolarized potentials were unaffected by the mutations, as were first-latency distributions in the T442S chimeric channel. The mutant channels show two subconductance levels, 37 and 70% of full conductance. From single-channel analysis, we concluded that channels always pass through the larger subconductance state on the way to and from the open state. The smaller subconductance state is traversed in approximately 40% of activation time courses. These states apparently represent kinetic intermediates in channel gating having voltage-dependent transitions with apparent charge movements of approximately 1.6 e0. The fully open T442S chimeric channel has the conductance sequence Rb+ > NH4+ > K+. The opposite conductance sequence, K+ > NH4+ > Rb+, is observed in each of the subconductance states, with the smaller subconductance state discriminating most strongly against Rb+.

Tetramerization of the AKT1 plant potassium channel involves its C-terminal cytoplasmic domain

All plant channels identified so far show high conservation throughout the polypeptide sequence except in the ankyrin domain which is present only in those closely related to AKT1. In this study, the architecture of the AKT1 protein has been investigated. AKT1 polypeptides expressed in the baculovirus/Sf9 cells system were found to assemble into tetramers as observed with animal Shaker-like potassium channel subunits. The AKT1 C-terminal intracytoplasmic region (downstream from the transmembrane domain) alone formed tetrameric structures when expressed in Sf9 cells, revealing a tetramerization process different from that of Shaker channels. Tests of subfragments from this sequence in the two-hybrid system detected two kinds of interaction. The first, involving two identical segments (amino acids 371-516), would form a contact between subunits, probably via their putative cyclic nucleotide-binding domains. The second interaction was found between the last 81 amino acids of the protein and a region lying between the channel hydrophobic core and the putative cyclic nucleotide-binding domain. As the interacting regions are highly conserved in all known plant potassium channels, the structural organization of AKT1 is likely to extend to these channels. The significance of this model with respect to animal cyclic nucleotide-gated channels is also discussed.

Subunit stoichiometry of the pancreatic beta-cell ATP-sensitive K+ channel

We have investigated the subunit stoichiometry of the pancreatic beta-cell ATP-sensitive K+ (KATP) channel (SUR1/Kir6.2 channel) by constructing cDNA encoding a single polypeptide (beta alpha polypeptide) consisting of a SUR1 (beta) subunit and a Kir6.2 (alpha) subunit. 86Rb+ efflux and single-channel properties of COS1 cells expressing beta alpha polypeptides were similar to those of COS1 cells coexpressing alpha monomers and beta monomers. Coexpression of beta alpha polypeptides with alpha monomers inhibited the K+ currents, while coexpression with beta monomers did not. We then constructed another single polypeptide (beta alpha2) consisting of a beta subunit and a dimeric repeat of the alpha subunit. 86Rb+ efflux from COS1 cells expressing beta alpha2 polypeptides was small, but was restored by supplementation with beta monomers. These results indicate that the activity of K(ATP) channels is optimized when the alpha and beta subunits are coexpressed with a molar ratio of 1:1. Since inward rectifier K+ channels are thought to function as homo- or hetero-tetramers, this suggests that the K(ATP) channel functions as a multimeric protein, most likely a hetero-octamer composed of a tetramer of the Kir6.2 subunit and a tetramer of the SUR1 subunit.

Shaker pore structure as predicted by annealed atomic simulation using symmetry and novel geometric restraints

Recent studies making use of channel-blocking peptides as molecular calipers have revealed the architecture of the pore-forming region of Shaker-type potassium channels. Here we show that the low-resolution, experimentally derived geometric information can be incorporated as restraints within the context of an annealed molecular dynamics simulation to predict an atomic structure for the channel pore which, by virtue of restraints, conforms to the experimental evidence. The simulation is reminiscent of the computational method employed by nuclear magnetic resonance (NMR) spectroscopists to resolve solution structures of biological macromolecules, but in lieu of restraints conventionally derived from NMR spectra, novel restraints are developed that include side-chain orientation of amino acid residues and assumed symmetry of protein subunits. The method presented here offers the possibility of expanding cooperation between simulation and experiment in developing structural models, especially for systems such as ion channels whose three-dimensional structures may not be amenable to determination by direct methods at the present time.

Kvbeta2 inhibits the Kvbeta1-mediated inactivation of K+ channels in transfected mammalian cells

Cloned auxiliary beta-subunits (e.g. Kvbeta1) modulate the kinetic properties of the pore-forming alpha-subunits of a subset of Shaker-like potassium channels. Coexpression of the alpha-subunit and Kvbeta2, however, induces little change in channel properties. Since more than one beta-subunit has been found in individual K+ channel complexes and expression patterns of different beta-subunits overlap in vivo, it is important to test the possible physical and/or functional interaction(s) between different beta-subunits. In this report, we show that both Kvbeta2 and Kvbeta1 recognize the same region on the pore-forming alpha-subunits of the Kv1 Shaker-like potassium channels. In the absence of alpha-subunits the Kvbeta2 polypeptide interacts with additional beta-subunit(s) to form either a homomultimer with Kvbeta2 or a heteromultimer with Kvbeta1. When coexpressing alpha-subunits and Kvbeta1 in the presence of Kvbeta2, we find that Kvbeta2 is capable of inhibiting the Kvbeta1-mediated inactivation. Using deletion analysis, we have localized the minimal interaction region that is sufficient for Kvbeta2 to associate with both alpha-subunits and Kvbeta1. This mapped minimal interaction region is necessary and sufficient for inhibiting the Kvbeta1-mediated inactivation, consistent with the notion that the inhibitory activity of Kvbeta2 results from the coassembly of Kvbeta2 with compatible alpha-subunits and possibly with Kvbeta1. Together, these results provide biochemical evidence that Kvbeta2 may profoundly alter the inactivation activity of another beta-subunit by either differential subunit assembly or by competing for binding sites on alpha-subunits, which indicates that Kvbeta2 is capable of serving as an important determinant in regulating the kinetic properties of K+ currents.

Electrostatic interactions between transmembrane segments mediate folding of Shaker K+ channel subunits

In voltage-dependent Shaker K+ channels, charged residues E293 in transmembrane segment S2 and R365, R368, and R371 in S4 contribute significantly to the gating charge movement that accompanies activation. Using an intragenic suppression strategy, we have now probed for structural interaction between transmembrane segments S2, S3, and S4 in Shaker channels. Charge reversal mutations of E283 in S2 and K374 in S4 disrupt maturation of the protein. Maturation was specifically and efficiently rescued by second-site charge reversal mutations, indicating that electrostatic interactions exist between E283 in S2 and R368 and R371 in S4, and between K374 in S4 and E293 in S2 and D316 in S3. Rescued subunits were incorporated into functional channels, demonstrating that a native structure was restored. Our data indicate that K374 interacts with E293 and D316 within the same subunit. These electrostatic interactions mediate the proper folding of the protein and are likely to persist in the native structure. Our results raise the possibility that the S4 segment is tilted relative to S2 and S3 in the voltage-sensing domain of Shaker channels. Such an arrangement might provide solvent access to voltage-sensing residues, which we find to be highly tolerant of mutations.

Mapping the receptor site for hanatoxin, a gating modifier of voltage-dependent K+ channels

Hanatoxin (HaTx) binds to multiple sites on the surface of the drk1 voltage-gated K+ channel and modifies channel gating. We set out to identify channel residues that contribute to form these HaTx binding sites. Chimeras constructed using the drk1 and shaker K+ channels suggest that the S3-S4 linker may contain influential residues. Alanine scanning mutagenesis of the region extending from the C terminal end of S3 through S4 identified a number of residues that likely contribute to form the HaTx binding sites. The pore blocker Agitoxin2 and the gating modifier HaTx can simultaneously bind to individual K+ channels. These results suggest that residues near the outer edges of S3 and S4 form the HaTx binding sites and are eccentrically located at least 15 A from the central pore axis on the surface of voltage-gated K+ channels.

Stabilization of ion selectivity filter by pore loop ion pairs in an inwardly rectifying potassium channel

Ion selectivity is critical for the biological functions of voltage-dependent cation channels and is achieved by specific ion binding to a pore region called the selectivity filter. In voltage-gated K+, Na+ and Ca2+ channels, the selectivity filter is formed by a short polypeptide loop (called the H5 or P region) between the fifth and sixth transmembrane segments, donated by each of the four subunits or internal homologous domains forming the channel. While mutagenesis studies on voltage-gated K+ channels have begun to shed light on the structural organization of this pore region, little is known about the physical and chemical interactions that maintain the structural stability of the selectivity filter. Here we show that in an inwardly rectifying K+ (IRK) channel, IRK1, short range interactions of an ion pair in the H5 pore loop are crucial for pore structure and ion permeation. The two residues, a glutamate and an arginine, appear to form exposed salt bridges in the tetrameric channel. Alteration or disruption of such ion pair interactions dramatically alters ion selectivity and permeation. Since this ion pair is conserved in all IRK channels, it may constitute a general mechanism for maintaining the stability of the pore structure in this channel superfamily.

Activation-dependent subconductance levels in the drk1 K channel suggest a subunit basis for ion permeation and gating

Ion permeation and channel opening are two fundamental properties of ion channels, the molecular bases of which are poorly understood. Channels can exist in two permeability states, open and closed. The relative amount of time a channel spends in the open conformation depends on the state of activation. In voltage-gated ion channels, activation involves movement of a charged voltage sensor, which is required for channel opening. Single-channel recordings of drk1 K channels expressed in Xenopus oocytes suggested that intermediate current levels (sublevels) may be associated with transitions between the closed and open states. Because K channels are formed by four identical subunits, each contributing to the lining of the pore, it was hypothesized that these sublevels resulted from heteromeric pore conformations. A formal model based on this hypothesis predicted that sublevels should be more frequently observed in partially activated channels, in which some but not all subunits have undergone voltage-dependent conformational changes required for channel opening. Experiments using the drk1 K channel, as well as drk1 channels with mutations in the pore and in the voltage sensor, showed that the probability of visiting a sublevel correlated with voltage- and time-dependent changes in activation. A subunit basis is proposed for channel opening and permeation in which these processes are coupled.

Potassium channel alpha and beta subunits assemble in the endoplasmic reticulum

We have characterized the maturation of Shaker K+ channel protein and the cellular site of assembly of pore-forming alpha and cytoplasmic beta subunits in a transfected mammalian cell line. Shaker protein is made as a partially glycosylated, immature precursor that is converted to a fully glycosylated, mature product. Shaker protein did not mature when transport from the endoplasmic reticulum (ER) to the Golgi apparatus was blocked. Consistent with this finding, only the immature form was sensitive to digestion with endoglycosidase H. These results indicate that the immature protein is core-glycosylated in the ER, whereas the oligosaccharides of the mature protein have been further processed in the Golgi compartment. After inhibiting ER-to-Golgi transport, the oligomeric state of Shaker subunits was assessed by cross-linking in intact cells or by solubilization and sucrose gradient sedimentation. The results indicate that Shaker subunits assemble with each other in the ER. When co-expressed, the Kvbeta2 subunit also associated with Shaker in the ER. Assembly with the beta2 subunit did not increase the rate or extent of Shaker protein maturation. Our results indicate that the biogenesis of Shaker K+ channels in vivo involves core glycosylation and subunit assembly in the ER, followed by efficient transfer to the Golgi apparatus where the oligosaccharides are modified.

Purification, visualization, and biophysical characterization of Kv1.3 tetramers

The voltage-gated K+ channel of T-lymphocytes, Kv1.3, was heterologously expressed in African Green Monkey kidney cells (CV-1) using a vaccinia virus/T7 hybrid expression system; each infected cell exhibited 10(4) to 5 x 10(5) functional channels on the cell surface. The protein, solubilized with detergent (3-[cholamidopropyl)dimethylammonio]-1-propanesulfonic acid or cholate), was purified to near-homogeneity by a single nickel-chelate chromatography step. The Kv1.3 protein expressed in vaccinia virus-infected cells and its purified counterpart are both modified by a approximately 2-kDa core-sugar moiety, most likely at a conserved N-glycosylation site in the external S1-S2 loop; absence of the sugar does not alter the biophysical properties of the channel nor does it affect expression levels. Purified Kv1.3 has an estimated size of approximately 64 kDa in denaturing SDS-polyacrylamide electrophoresis gels, consistent with its predicted size based on the amino acid sequence. By sucrose gradient sedimentation, purified Kv1.3 is seen primarily as a single peak with an approximate mass of 270 kDa, compatible with its being a homotetrameric complex of the approximately 64-kDa subunits. When reconstituted in the presence of lipid and visualized by negative-staining electron microscopy, the purified Kv1.3 protein forms small crystalline domains consisting of tetramers with dimensions of approximately 65 x 65 A. The center of each tetramer contains a stained depression which may represent the ion conduction pathway. Functional reconstitution of the Kv1.3 protein into lipid bilayers produces voltage-dependent K+-selective currents that can be blocked by two high affinity peptide antagonists of Kv1.3, margatoxin and stichodactylatoxin.

The human delta1261 mutation of the HERG potassium channel results in a truncated protein that contains a subunit interaction domain and decreases the channel expression

HERG (human eag-related gene) encodes an inward-rectifier potassium channel formed by the assembly of four subunits. Since the truncated HERG protein in patients with long QT syndrome induces a dominant phenotype, that is, cardiac sudden death, the assembly of nonfunctional complexes between wild-type and mutated subunits was implicated in causing the disease. To understand HERG-mediated cardiac sudden death at the molecular level, it is important to determine which regions in the HERG protein participate in subunit interaction. We therefore report the identification of a subunit interaction domain, NAB(HERG), that is localized at the hydrophilic cytoplasmic N terminus and can form a tetramer in the absence of the rest of the HERG protein. Truncated HERG proteins containing NAB(HERG), including one that resulted from the delta1261 human mutation, inhibit the functional expression of the HERG channel in transfected cells. Together, these results support the notion that the expression of HERG in the human heart may be decreased in the presence of the truncated subunit. Such a decrease of potassium channel expression can contribute to the longer QT intervals observed in the patients with the HERG mutation.

Inner ear defects induced by null mutation of the isk gene

The isk gene is expressed in many tissues. Pharmacological evidence from the inner ear suggests that isk mediates potassium secretion into the endolymph. To examine the consequences of IsK null mutation on inner ear function, and to produce a system useful for examining the role(s) IsK plays elsewhere, we have produced a mouse strain that carries a disrupted isk locus. Knockout mice exhibit classic shaker/waltzer behavior. Hair cells degenerate, but those of different inner ear organs degenerate at different times. Functionally, we show that in mice lacking isk, the strial marginal cells and the vestibular dark cells of the inner ear are unable to generate an equivalent short circuit current in vitro, indicating a lack of transepithelial potassium secretion.

Heteromultimeric CLC chloride channels with novel properties

The skeletal muscle chloride channel CLC-1 and the ubiquitous volume-activated chloride channel CLC-2 belong to a large gene family whose members often show overlapping expression patterns. CLC-1 and CLC-2 are coexpressed in skeletal and smooth muscle and in the heart. By coexpressing CLC-1 and CLC-2 in Xenopus oocytes, we now show the formation of novel CLC-1/CLC-2 heterooligomers that yield time-independent linear chloride currents with a chloride-->bromide-->iodide selectivity sequence. Formation of heterooligomeric CLC channels increases the number and possible functions of chloride channels.

A study of the oligomeric state of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-preferring glutamate receptors in the synaptic junctions of porcine brain

The number of the subunits in an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-preferring L-glutamate receptor in the synaptic junctions of porcine brain was investigated in this study. Upon incubation of the synaptic junctions with three cross-linking regents, dimethyl adipimidate (DMA), dimethyl suberimidate (DMS) and N-succinimidyl-(4-azidophenyl)-1,3'-dithiopropionate (SADP), AMPA receptor subunits in higher-molecular-mass aggregates were detected by immunoblotting. These aggregates migrated as proteins of approx. 200, 300 and 400 kDa. The number and identity of the subunits in a solubilized AMPA receptor were also investigated here. Two samples, W1 and W2, enriched in AMPA receptors were prepared from synaptic junctions by a combination of detergent-solubilization, anion-exchange chromatography and wheatgerm agglutinin affinity chromatography. Hydrodynamic behaviour analyses revealed that the majority of the AMPA receptors in either one of these samples were asymmetrical detergent-surrounded particles with a protein mass around 350 kDa. SDS/PAGE analysis revealed that the majority of AMPA receptors in the W1 sample were comprised of dimers of 106 kDa subunits which were covalently linked by disulphide bonds. Cross-linking these receptors with SADP yielded a new band of approx. 400 kDa. The results obtained here, either from the studies of AMPA receptors embedding in synaptic junctions or from those of detergent-solubilized and partially purified receptors, suggest that AMPA receptors contain a basic core structure comprising of four 106 kDa subunits.

Surface Charges of K channels. Effects of strontium on five cloned channels expressed in Xenopus oocytes

The effects of strontium (Sr2+; 7-50 mM) on five different cloned rat K channels (Kv1.1, Kv1.5, Kv1.6, Kv2.1, and Kv3.4), expressed in oocytes of Xenopus laevis, were investigated with a two-electrode voltage clamp technique. The main effect was a shift of the Gk(V) curve along the potential axis, different in size for the different channels. Kv1.1 was shifted most and Kv3.4 least, 21 and 8 mV, respectively, at 50 mM. The effect was interpreted in terms of screening of fixed surface charges. The estimated charge densities ranged from -0.37 (Kv1.1) to -0.11 (Kv3.4) e nm-2 and showed good correlation with the total net charge of the extracellularly located amino acid residues of the channel as well as with the charge of a specific region (the loop between the S5 segment and the pore forming segment). The estimated surface potentials were found to be linearly related to the activation midpoint potential, suggesting a functional role for the surface charges.

The baculovirus/insect cell system as an alternative to Xenopus oocytes. First characterization of the AKT1 K+ channel from Arabidopsis thaliana

Two plant (Arabidopsis thaliana) K+ transport systems, KAT1 and AKT1, have been expressed in insect cells (Sf9 cell line) using recombinant baculoviruses. Microscopic observation after immunogold staining revealed that the expressed AKT1 and KAT1 polypeptides were mainly associated with internal membranes, but that a minute fraction was targeted to the cell membrane. KAT1 was known, from earlier electrophysiological characterization in Xenopus oocytes, to be an inwardly rectifying voltage-gated channel highly selective for K+, while similar experiments had failed to characterize AKT1. Insect cells expressing KAT1 displayed an exogenous inwardly rectifying K+ conductance reminiscent of that described previously in Xenopus oocytes expressing KAT1. Under similar conditions, cells expressing AKT1 showed a disturbed cell membrane electrical stability that precluded electrophysiological analysis. Use of a baculovirus transfer vector designed so as to decrease the expression level allowed the first electrophysiological characterization of AKT1. The baculovirus system can thus be used as an alternative method when expression in Xenopus oocytes is unsuccessful for electrophysiological characterization of the ion channel of interest. The plant AKT1 protein has been shown in this way to be an inwardly rectifying voltage-gated channel highly selective for K+ ions and sensitive to cGMP.

Voltage-sensing residues in the S2 and S4 segments of the Shaker K+ channel

The activation of Shaker K+ channels is steeply voltage dependent. To determine whether conserved charged amino acids in putative transmembrane segments S2, S3, and S4 contribute to the gating charge of the channel, the total gating charge movement per channel was measured in channels containing neutralization mutations. Of eight residues tested, four contributed significantly to the gating charge: E293, an acidic residue in S2, and R365, R368, and R371, three basic residues in the S4 segment. The results indicate that these residues are a major component of the voltage sensor. Furthermore, the S4 segment is not solely responsible for gating charge movement in Shaker K+ channels.

Contribution of the S4 segment to gating charge in the Shaker K+ channel

Voltage-activated ion channels respond to changes in membrane voltage by coupling the movement of charges to channel opening. A K+ channel-specific radioligand was designed and used to determine the origin of these gating charges in the Shaker K+ channel. Opening of a Shaker K+ channel is associated with a displacement of 13.6 electron charge units. Gating charge contributions were determined for six of the seven positive charges in the S4 segment, an unusual amino acid sequence in voltage-activated cation channels consisting of repeating basic residues at every third position. Charge-neutralizing mutations of the first four positive charges led to large decreases (approximately 4 electron charge units each) in the gating charge; however, the gating charge of Shaker delta 10, a Shaker K+ channel with 10 altered nonbasic residues in its S4 segment, was found to be identical to the wild-type channel. These findings show that movement of the NH2-terminal half but not the CO2H-terminal end of the S4 segment underlies gating charge, and that this portion of the S4 segment appears to move across the entire transmembrane voltage difference in association with channel activation.

Molecular properties of voltage-gated K+ channels

Subfamilies of voltage-activated K+ channels (Kv1-4) contribute to controlling neuron excitability and the underlying functional parameters. Genes encoding the multiple alpha subunits from each of these protein groups have been cloned, expressed and the resultant distinct K+ currents characterized. The predicted amino acid sequences showed that each alpha subunit contains six putative membrane-spanning alpha-helical segments (S1-6), with one (S4) being deemed responsible for the channels' voltage sensing. Additionally, there is an H5 region, of incompletely defined structure, that traverses the membrane and forms the ion pore; residues therein responsible for K+ selectively have been identified. Susceptibility of certain K+ currents produced by the Shaker-related subfamily (Kv1) to inhibition by alpha-dendrotoxin has allowed purification of authentic K+ channels from mammalian brain. These are large (M(r) approximately 400 kD), octomeric sialoglycoproteins composed of alpha and beta subunits in a stoichiometry of (alpha)4(beta)4, with subtypes being created by combinations of subunit isoforms. Subsequent cloning of the genes for beta 1, beta 2 and beta 3 subunits revealed novel sequences for these hydrophilic proteins that are postulated to be associated with the alpha subunits on the inner side of the membrane. Coexpression of beta 1 and Kv1.4 subunits demonstrated that this auxiliary beta protein accelerates the inactivation of the K+ current, a striking effect mediate by an N-terminal moiety. Models are presented that indicate the functional domains pinpointed in the channel proteins.

Peptide models for membrane channels

Peptides may be synthesized with sequences corresponding to putative transmembrane domains and/or pore-lining regions that are deduced from the primary structures of ion channel proteins. These can then be incorporated into lipid bilayer membranes for structural and functional studies. In addition to the ability to invoke ion channel activity, critical issues are the secondary structures adopted and the mode of assembly of these short transmembrane peptides in the reconstituted systems. The present review concentrates on results obtained with peptides from ligand-gated and voltage-gated ion channels, as well as proton-conducting channels. These are considered within the context of current molecular models and the limited data available on the structure of native ion channels and natural channel-forming peptides.

Functional expression of the plant K+ channel KAT1 in insect cells

Following the biophysical analysis of plant K+ channels in their natural environment, three members from the green branch of the evolutionary tree of life KAT1, AKT1, and KST1 have recently been identified on the molecular level. Among them, we focussed on the expression and characterization of the Arabidopsis thaliana K+ channel KAT1 in the insect cell line Sf9. The infection of Sf9 cells with KAT1-recombinant baculovirus resulted in functional expression of KAT1 channels, which was monitored by inward-rectifying, K+-selective (impermeable to Na+ and even NH4+) ionic conductance in whole-cell patch-clamp recordings. A voltage threshold as low as -60 to -80mV for voltage activation compared to other plant inward rectifiers in vivo, and to in vitro expression of KAT1 in Xenopus oocytes or yeast, may be indicative for channel modulation by the expression system. A rise in cytoplasmic Ca2+ concentration (up to 1 mM), a regulator of the inward rectifier in Vicia faba guard cells, did not modify the voltage dependence of KAT1 in Sf9 cells. The access to channel function on one side and channel protein on the other make Sf9 cells a suitable heterologous system for studies on the biophysical properties, post-traditional modification and assembly of a green inward rectifier.

NAB domain is essential for the subunit assembly of both alpha-alpha and alpha-beta complexes of shaker-like potassium channels

There are at least five subfamilies of Shaker-like K+ channels. The diverse function of K+ channels are thought to be further modulated by hydrophilic beta subunits. Here we report that Kv beta 1 inactivates RCK4 and Shaker B K+ channels of the Kv1 subfamily, but not Shal2 of the Kv4 subfamily. This correlates the subfamily-specific bindings of Kv beta 1 to the cytoplasmic N-terminal domains of Kv1 alpha subunits. We map the Kv beta 1-binding site to a region overlapping NABKv1, a domain that specifies different Kv1 alpha subunits to form heterotetramers. Using chimeric alpha subunits, we demonstrate that NABKv1 is essential for the Kv beta 1-mediated inactivation. These results suggest that Kv beta 1 modulates a subset of K+ channels through the specific assembly of alpha-beta complexes and reveal the dual function of the NAB domain in mediating the assembly of both alpha-alpha and alpha-beta complexes.

Three distinct structural environments of a transmembrane domain in the inwardly rectifying potassium channel ROMK1 defined by perturbation

To probe the protein environment of an ion channel, we have perturbed the structure of a transmembrane domain by substituting side chains with those of two different sizes by using site-specific mutagenesis. We have used Trp and Ala as a high- and a low-impact perturbation probe, respectively, to replace each of 18 consecutive residues within the putative second transmembrane segment, M2, of an inwardly rectifying potassium channel, ROMK1. Our rationale is that a change in the channel function as a consequence of these mutations at a particular position will reflect the structural environment of the altered side chain. Each position can then be assigned to one of three classes of environments, as grated by different levels of perturbation: very tolerant (channel functions with both Trp and Ala substitutions), tolerant (function preserved with Ala but not with Trp substitution), and intolerant (either Ala or Trp substitution destroys function). We identify the very tolerant environment as being lipid-facing, tolerant as protein-interior-facing, and intolerant as pore-facing. We observe a strikingly ordered pattern of perturbation of all three environmental classes. This result indicates that M2 is a straight alpha-helix.

Determination of the subunit stoichiometry of an inwardly rectifying potassium channel

Inwardly rectifying K+ channels are distantly related to their voltage-gated counterparts and possess a structural motif of only two putative transmembrane segments in each subunit. They are formed by the assembly of an unknown number of subunits. We have examined the subunit stoichiometry of a strongly rectifying K+ channel, IRK1, by linking together the coding sequence of three or four subunits and distinguishing channels with different numbers of subunits carrying a double mutation that alters inward rectification and single-channel properties. We find that IRK1 channels, like voltage-gated K+ channels, are tetrameric channels. Interestingly, the high sensitivity to Mg2+ and polyamines, cations that produce inward rectification by blocking the channel pore from the cytoplasmic side is largely retained in a channel containing only one wild-type subunit and three subunits bearing mutations that abolish high affinity Mg2+ and polyamine block.

A method for determining the unitary functional capacity of cloned channels and transporters expressed in Xenopus laevis oocytes

The Xenopus laevis oocyte is widely used to express exogenous channels and transporters and is well suited for functional measurements including currents, electrolyte and nonelectrolyte fluxes, water permeability and even enzymatic activity. It is difficult, however, to transform functional measurements recorded in whole oocytes into the capacity of a single channel or transporter because their number often cannot be estimated accurately. We describe here a method of estimating the number of exogenously expressed channels and transporters inserted in the plasma membrane of oocytes. The method is based on the facts that the P (protoplasmic) face in water-injected control oocytes exhibit an extremely low density of endogenous particles (212 +/- 48 particles/microns2, mean, SD) and that exogenously expressed channels and transporters increased the density of particles (up to 5,000/microns2) only on the P face. The utility and generality of the method were demonstrated by estimating the "gating charge" per particle of the Na+/glucose cotransporter (SGLT1) and a nonconducting mutant of the Shaker K+ channel proteins, and the single molecule water permeability of CHIP (Channel-like In-tramembrane Protein) and MIP (Major Intrinsic Protein). We estimated a "gating charge" of approximately 3.5 electronic charges for SGLT1 and approximately 9 for the mutant Shaker K+ channel from the ratio of Qmax to density of particles measured on the same oocytes. The "gating charges" were 3-fold larger than the "effective valences" calculated by fitting a Boltzmann equation to the same charge transfer data suggesting that the charge movement in the channel and cotransporter occur in several steps. Single molecule water permeabilities (pfs) of 1.4 x 10(-14) cm3/sec for CHIP and of 1.5 x 10(-16) cm3/sec for MIP were estimated from the ratio of the whole-oocyte water permeability (Pf) to the density of particles. Therefore, MIP is a water transporter in oocytes, albeit approximately 100-fold less effective than CHIP.

Assembly of voltage-gated potassium channels. Conserved hydrophilic motifs determine subfamily-specific interactions between the alpha-subunits

Voltage-gated potassium (K+) channels are assembled by four identical or homologous alpha-subunits to form a tetrameric complex with a central conduction pore for potassium ions. Most of the cloned genes for the alpha-subunits are classified into four subfamilies: Kv1 (Shaker), Kv2 (Shab), Kv3 (Shaw), and Kv4 (Shal). Subfamily-specific assembly of heteromeric K+ channel complexes has been observed in vitro and in vivo, which contributes to the diversity of K+ currents. However, the molecular codes that mediate the subfamily-specific association remain unknown. To understand the molecular basis of the subfamily-specific assembly, we tested the protein-protein interactions of different regions of alpha-subunits. We report here that the cytoplasmic NH2-terminal domains of Kv1, Kv2, Kv3, and Kv4 subfamilies each associate to form homomultimers. Using the yeast two-hybrid system and eight K+ channel genes, two genes (one isolated from rat and one from Drosophila) from each subfamily, we demonstrated that the associations to form heteromultimers by the NH2-terminal domains are strictly subfamily-specific. These subfamily-specific associations suggest a molecular basis for the selective formation of heteromultimeric channels in vivo.

Slow inactivation conserved in heteromultimeric voltage-dependent K+ channels between Shaker (Kv1) and Shaw (Kv3) subfamilies

Single K+ channels were recorded under the cell-attached mode in Xenopus oocytes injected with an equal amount of mRNAs coding for NGK1 (Kv1.2) and NGK2 (Kv3.1a) voltage-dependent K+ channels. A new class of channels of 20 pS in conductance with three degrees of inactivation was observed. The results suggest that voltage-dependent NGK1 Shaker and NGK2 Shaw K+ channels, from different subfamilies, assemble to form heteromultimeric K+ channels in Xenopus oocytes and show characteristics inherited from two parental channels.

Structural conservation of ion conduction pathways in K channels and glutamate receptors

Single channel recordings demonstrate that ion channels switch stochastically between an open and a closed pore conformation. In search of a structural explanation for this universal open/close behavior, we have uncovered a striking degree of amino acid homology across the pore-forming regions of voltage-gated K channels and glutamate receptors. This suggested that the pores of these otherwise unrelated classes of channels could be structurally conserved. Strong experimental evidence supports a hairpin structure for the pore-forming region of K channels. Consequently, we hypothesized the existence of a similar structure for the pore of glutamate receptors. In ligand-gated channels, the pore is formed by M2, the second of four putative transmembrane segments. A hairpin structure for M2 would affect the subsequent membrane topology, inverting the proposed orientation of the next segments, M3. We have tested this idea for the NR1 subunit of the N-methyl-D-aspartate receptor. Mutations that affected the glycosylation pattern of the NR1 subunit localize both extremes of the M3-M4 linker to the extracellular space. Whole cell currents and apparent agonist affinities were not affected by these mutations. Therefore it can be assumed that they represent the native transmembrane topology. The extracellular assignment of the M3-M4 linker challenged the current topology model by inverting M3. Taken together, the amino acid homology and the new topology suggest that the pore-forming M2 segment of glutamate receptors does not transverse the membrane but, rather, forms a hairpin structure, similar to that found in K channels.

Truncated K+ channel DNA sequences specifically suppress lymphocyte K+ channel gene expression

We have constructed a series of deletion mutants of Kv1.3, a Shaker-like, voltage-gated K+ channel, and examined the ability of these truncated mutants to form channels and to specifically suppress full-length Kv1.3 currents. These constructs were expressed heterologously in both Xenopus oocytes and a mouse cytotoxic T cell line. Our results show that a truncated mutant Kv1.3 must contain both the amino terminus and the first transmembrane-spanning segment, S1, to suppress full-length Kv1.3 currents. Amino-terminal-truncated DNA sequences from one subfamily suppress K+ channel expression of members of only the same subfamily. The first 141 amino acids of the amino-terminal of Kv1.3 are not necessary for channel formation. Deletion of these amino acids yields a current identical to that of full-length Kv1.3, except that it cannot be suppressed by a truncated Kv1.3 containing the amino terminus and S1. To test the ability of truncated Kv1.3 to suppress endogenous K+ currents, we constructed a plasmid that contained both truncated Kv1.3 and a selection marker gene (mouse CD4). Although constitutively expressed K+ currents in Jurkat (a human T cell leukemia line) and GH3 (an anterior pituitary cell line) cells cannot be suppressed by this double-gene plasmid, stimulated (up-regulated) Shaker-like K+ currents in GH3 cells can be suppressed.

The kinetics of voltage-gated ion channels

When Hodgkin & Huxley (1952) first embarked on the analysis of their voltageclamp data on the ionic currents in the squid giant axon, they hoped to be able to deduce a mechanism from it, but it soon became clear that the electrical data would by themselves yield only very general information about the class of system likely to be involved.

Towards the elucidation of the structural-functional relationship of inward rectifying K+ channel family

With recent cDNA cloning of members of the inward rectifying K+ channel family, it was revealed that they have only 2 putative transmembrane regions with no voltage-sensor element. Based on the deduced primary structure, possible schematic models to explain their characteristic features are proposed in this article. The features are (1) blocking by intracellular Mg2+, (2) intrinsic gating, (3) the triple barrel structure of the inward rectifier K+ channel and (4) the activation by the direct interaction with G-protein subunits of the muscarinic K+ channel. The recent findings of the mutagenesis study of voltage-gated K+ channels, which provide a clue for the structural-functional study of the inward rectifying K+ channels, are also looked at.

Calcium-calmodulin modulation of the olfactory cyclic nucleotide-gated cation channel

Although several ion channels have been reported to be directly modulated by calcium-calmodulin, they have not been conclusively shown to bind calmodulin, nor are the modulatory mechanisms understood. Study of the olfactory cyclic nucleotide-activated cation channel, which is modulated by calcium-calmodulin, indicates that calcium-calmodulin directly binds to a specific domain on the amino terminus of the channel. This binding reduces the effective affinity of the channel for cyclic nucleotides, apparently by acting on channel gating, which is tightly coupled to ligand binding. The data reveal a control mechanism that resembles those underlying the regulation of enzymes by calmodulin. The results also point to the amino-terminal part of the olfactory channel as an element for gating, which may have general significance in the operation of ion channels with similar overall structures.

New structural motif for ligand-gated ion channels defined by an ionotropic ATP receptor

The adenosine-5'-triphosphate (ATP) molecule is an extracellular messenger in neural and non-neural tissues, where it activates several cell-surface-receptor subtypes, including G-protein-coupled receptors and ligand-gated ion channels. ATP-gated channels (termed P2x receptors) have been characterized on smooth muscle cells and autonomic and sensory neurons, where they mediate membrane depolarization and, in some cases, Ca2+ entry. P2x receptors are functionally heterogeneous, but resemble acetylcholine- and serotonin-gated channels with respect to ion selectivity and kinetic parameters of channel gating. We report here that despite such close functional similarities, the deduced sequence of a cloned P2x receptor predicts an unusual subunit structure resembling voltage-insensitive cation channels. Thus, the P2x receptor provides a striking example of convergent evolution, whereby proteins have been fashioned with similar functional properties from subunits having very different structural characteristics. There is sequence similarity between the ATP receptor and RP-2, a gene activated in thymocytes undergoing programmed cell death. RP-2 may encode a receptor for ATP or another metabolite released during apoptosis.

High-level expression and functional reconstitution of Shaker K+ channels

Voltage-gated K+ channels were expressed in COS cells transiently transfected with a plasmid carrying a cDNA for an inactivation-removed Shaker K+ channel driven by an adenovirus promoter. Channel expression was followed by immunological detection, binding of radioactive charybdotoxin (CTX), and functional reconstitution into planar lipid bilayers. About 10(7) channels per transfected cell are expressed on the plasma membrane. The expressed channels are glycosylated and competent to bind CTX with the expected characteristics. Channels observed after insertion into planar lipid bilayers displayed the voltage-dependent gating, conduction, and ion selectivity behavior expected for this channel. Channels were solubilized in several detergents without loss of CTX binding activity. The results make plausible a systematic attack on the purification of milligram-level amounts of functional K+ channels from a heterologous expression system.

Structural basis of ion channel permeation and selectivity

There has been rapid progress in understanding the structural basis of ion selectivity and permeation in both ligand- and voltage-gated channels. Recognition of similarities in overall architecture within a channel class has led to an increasing focus on the specific molecular determinants that endow a channel with its own distinctive character. It has been possible in some cases to identify individual amino acids essential for ion selectivity.