Mutation of conserved negatively charged residues in the S2 and S3 transmembrane segments of a mammalian K+ channel selectively modulates channel gating

Amino acid substitution within the S2 and S4 transmembrane segments in Shaker potassium channel modulates channel gating

To investigate of the gating properties in the voltage-activated potassium channel, we have mutated a variety of S2 and S4 residues in the Shaker potassium protein. Results showed that the R365C and R368C, but not the E283C, R362C, R365S, R368S or the ShB-IR, were sensitive to micromolar concentrations of Cd(2+) ions. This indicates that R365 and R368 play a crucial role in the channel gating due to a conformational modulation of the channel structure. Doubly mutated channels of the E283C/R365E and E283C/R368E caused a transient increase in current amplitude, which reached a peak within a few seconds and then decreased toward initial levels, despite the continual presence of Cd(2+). Taken together, our results suggest that E283, R365, and R368 form a network of strong, local, and electrostatic interactions that relate closely to the mechanism of the channel gating.

Localization and molecular determinants of the Hanatoxin receptors on the voltage-sensing domains of a K(+) channel

Hanatoxin inhibits voltage-gated K(+) channels by modifying the energetics of activation. We studied the molecular determinants and physical location of the Hanatoxin receptors on the drk1 voltage-gated K(+) channel. First, we made multiple substitutions at three previously identified positions in the COOH terminus of S3 to examine whether these residues interact intimately with the toxin. We also examined a region encompassing S1-S3 using alanine-scanning mutagenesis to identify additional determinants of the toxin receptors. Finally, guided by the structure of the KcsA K(+) channel, we explored whether the toxin interacts with the peripheral extracellular surface of the pore domain in the drk1 K(+) channel. Our results argue for an intimate interaction between the toxin and the COOH terminus of S3 and suggest that the Hanatoxin receptors are confined within the voltage-sensing domains of the channel, at least 20-25 A away from the central pore axis.

The voltage sensor in voltage-dependent ion channels

In voltage-dependent Na, K, or Ca channels, the probability of opening is modified by the membrane potential. This is achieved through a voltage sensor that detects the voltage and transfers its energy to the pore to control its gate. We present here the theoretical basis of the energy coupling between the electric field and the voltage, which allows the interpretation of the gating charge that moves in one channel. Movement of the gating charge constitutes the gating current. The properties are described, along with macroscopic data and gating current noise analysis, in relation to the operation of the voltage sensor and the opening of the channel. Structural details of the voltage sensor operation were resolved initially by locating the residues that make up the voltage sensor using mutagenesis experiments and determining the number of charges per channel. The changes in conformation are then analyzed based on the differential exposure of cysteine or histidine-substituted residues. Site-directed fluorescence labeling is then analyzed as another powerful indicator of conformational changes that allows time and voltage correlation of local changes seen by the fluorophores with the global change seen by the electrophysiology of gating currents and ionic currents. Finally, we describe the novel results on lanthanide-based resonance energy transfer that show small distance changes between residues in the channel molecule. All of the electrophysiological and the structural information are finally summarized in a physical model of a voltage-dependent channel in which a change in membrane potential causes rotation of the S4 segment that changes the exposure of the basic residues from an internally connected aqueous crevice at hyperpolarized potentials to an externally connected aqueous crevice at depolarized potentials.

Local movement in the S2 region of the voltage-gated potassium channel hKv2.1 studied using cysteine mutagenesis

The positively charged S4 region of voltage-dependent potassium channels moves outward during depolarization, leading to channel opening, but possible movement of the negatively charged S2 region may be more complex. Here we have studied possible movement of the S2 region of the slowly activating human voltage-dependent potassium channel hKv2.1. For this, cysteine mutants in the S2 region were expressed in Xenopus oocytes by injection of cRNA. Whole-cell currents were measured using the two-electrode voltage-clamp technique, and the effect of the membrane-impermeable cysteine-binding reagent parachloromercuribenzenesulfonate (PCMBS) was studied. For mutant S223C (located just outside the membrane in the S2 region), PCMBS inhibited currents and caused faster deactivation of tail currents. The time course of reactivity of PCMBS on tail current amplitudes was faster at more negative holding potentials. There was no effect of PCMBS on potassium channel currents for mutants D225C, N226C, A230C, and V232C. These data suggest that residue S223 is exposed to the extracellular phase at normal resting potentials, making it accessible to PCMBS, but upon depolarization there is a conformational change, making it less accessible, possibly by a local rather than global movement of S2 residues into the membrane. Voltage-dependent movements of nearby residues could also explain the results.

A localized interaction surface for voltage-sensing domains on the pore domain of a K+ channel

Voltage-gated K+ channels contain a central pore domain and four surrounding voltage-sensing domains. How and where changes in the structure of the voltage-sensing domains couple to the pore domain so as to gate ion conduction is not understood. The crystal structure of KcsA, a bacterial K+ channel homologous to the pore domain of voltage-gated K+ channels, provides a starting point for addressing this question. Guided by this structure, we used tryptophan-scanning mutagenesis on the transmembrane shell of the pore domain in the Shaker voltage-gated K+ channel to localize potential protein-protein and protein-lipid interfaces. Some mutants cause only minor changes in gating and when mapped onto the KcsA structure cluster away from the interface between pore domain subunits. In contrast, mutants producing large changes in gating tend to cluster near this interface. These results imply that voltage-sensing domains interact with localized regions near the interface between adjacent pore domain subunits.

alpha-helical structural elements within the voltage-sensing domains of a K(+) channel

Voltage-gated K(+) channels are tetramers with each subunit containing six (S1-S6) putative membrane spanning segments. The fifth through sixth transmembrane segments (S5-S6) from each of four subunits assemble to form a central pore domain. A growing body of evidence suggests that the first four segments (S1-S4) comprise a domain-like voltage-sensing structure. While the topology of this region is reasonably well defined, the secondary and tertiary structures of these transmembrane segments are not. To explore the secondary structure of the voltage-sensing domains, we used alanine-scanning mutagenesis through the region encompassing the first four transmembrane segments in the drk1 voltage-gated K(+) channel. We examined the mutation-induced perturbation in gating free energy for periodicity characteristic of alpha-helices. Our results are consistent with at least portions of S1, S2, S3, and S4 adopting alpha-helical secondary structure. In addition, both the S1-S2 and S3-S4 linkers exhibited substantial helical character. The distribution of gating perturbations for S1 and S2 suggest that these two helices interact primarily with two environments. In contrast, the distribution of perturbations for S3 and S4 were more complex, suggesting that the latter two helices make more extensive protein contacts, possibly interfacing directly with the shell of the pore domain.

Molecular genetics and mechanism of autosomal dominant polycystic kidney disease

Considerable progress toward understanding pathogenesis of autosomal dominant polycystic disease (ADPKD) has been made during the past 15 years. ADPKD is a heterogeneous human disease resulting from mutations in either of two genes, PKD1 and PKD2. The similarity in the clinical presentation and evidence of direct interaction between the COOH termini of polycystin-1 and polycystin-2, the respective gene products, suggest that both proteins act in the same molecular pathway. The fact that most mutations from ADPKD patients result in truncated polycystins as well as evidence of a loss of heterozygosity mechanism in individual PKD cysts indicate that the loss of the function of either PKD1 or PKD2 is the most likely pathogenic mechanism for ADPKD. A novel mouse model, WS25, has been generated with a targeted mutation at Pkd2 locus in which a mutant exon 1 created by inserting a neo(r) cassette exists in tandem with the wild-type exon 1. This causes an unstable allele that undergoes secondary recombination to produce a true null allele at Pkd2 locus. Therefore, the model Pkd2(WS25/-), which carries the WS25 unstable allele and a true null allele, produces somatic second hits during mouse development or adult life and establishes an extremely faithful model of human ADPKD.

Structural and functional modularity of voltage-gated potassium channels

Sequence similarity among known potassium channels indicates the voltage-gated potassium channels consist of two modules: the N-terminal portion of the channel up to and including transmembrane segment S4, called in this paper the 'sensor' module, and the C-terminal portion from transmembrane segment S5 onwards, called the 'pore' module. We investigated the functional role of these modules by constructing chimeric channels which combine the 'sensor' from one native voltage-gated channel, mKv1.1, with the 'pore' from another, Shaker H4, and vice versa. Functional studies of the wild type and chimeric channels show that these modules can operate outside their native context. Each channel has a unique conductance-voltage relation. Channels incorporating the mKv1.1 sensor module have similar rates of activation while channels having the Shaker pore module show similar rates of deactivation. This observation suggests the mKv1.1 sensor module limits activation and the Shaker pore module determines deactivation. We propose a model that explains the observed equilibrium and kinetic properties of the chimeric constructs in terms of the characteristics of the native modules and a novel type of intrasubunit cooperativity. The properties ascribed to the modules are the same whether the modules function in their native context or have been assembled into a chimera.

Aberrant splicing in the PKD2 gene as a cause of polycystic kidney disease

It is estimated that approximately 15% of families with autosomal dominant polycystic kidney disease (ADPKD) have mutations in PKD2. Identification of these mutations is central to identifying functionally important regions of gene and to understanding the mechanisms underlying the pathogenesis of the disorder. The current study describes mutations in six type 2 ADPKD families. Two single base substitution mutations discovered in the ORF in exon 14 constitute the most COOH-terminal pathogenic variants described to date. One of these mutations is a nonsense change and the other encodes an apparent missense variant. Reverse transcription-PCR from patient lymphoblast RNA showed that, in addition, both mutations resulted in out-of-frame splice variants by activating cryptic splice sites via different mechanisms. The apparent missense variant produced such a strong splicing signal that the processed transcript from the mutant chromosome did not contain any of the normally spliced, missense product. A third mutation, a nonconservative missense change effecting a negatively charged residue in the third transmembrane span, is likely pathogenic and defines a highly conserved residue consistent with a potential channel subunit function for polycystin-2. The remaining three mutations included two frame shifts resulting from deletion of one or two bases in exons 6 and 10, respectively, and a nonsense mutation due to a single base substitution in exon 4. The study also defined a novel intragenic polymorphism in exon 1 that will be useful in analyzing "second hits" in PKD2. Finally, the study demonstrates that there are reduced levels of normal polycystin-2 protein in lymphoblast lines from PKD2-affected individuals and that truncated mutant polycystin-2 cannot be detected in patient lymphoblasts, suggesting that the latter may be unstable in at least some tissues. The mutations described will serve as critical reagents for future functional studies in PKD2.

Towards the three-dimensional structure of voltage-gated potassium channels

Electrical excitability is a fundamental property of the neuromuscular systems of metazoans. The varied response of neurons to electrical excitation is largely accounted for by a diverse set of voltage-gated potassium (KV) channels in the excitable membrane. The complete structure of a KV channel is not yet available. However, recent structural biological experiments have begun to provide new insight into how specific KV channels are formed and regulated, and how they function and interact with other proteins. In particular, the selectivity of KV channels for K+ and suggestions as to how these structural elements might assemble into a functional KV channel are discussed.

The screw-helical voltage gating of ion channels

In the voltage-gated ion channels of every animal, whether they are selective for K+, Na+ or Ca2+, the voltage sensors are the S4 transmembrane segments carrying four to eight positive charges always separated by two uncharged residues. It is proposed that they move across the membrane in a screw-helical fashion in a series of three or more steps that each transfer a single electronic charge. The unit steps are stabilized by ion pairing between the mobile positive charges and fixed negative charges, of which there are invariably two located near the inner ends of segments S2 and S3 and a third near the outer end of either S2 or S3. Opening of the channel involves three such steps in each domain.

Cloning of a novel four repeat protein related to voltage-gated sodium and calcium channels

Cloning has led to the discovery of more ion channels than predicted by functional studies, yet there remain channels that have not been cloned. We report the cloning of a novel protein that contains the four domain structure found in voltage-gated Ca2+ and Na+ channels. Phylogenetic relationships suggested that the protein might have diverged from an ancestral four repeat channel before the divergence of Ca2+ and Na+ channels. Northern blot analysis showed that mRNA transcripts encoding the protein are expressed predominantly in the brain, moderately in the heart, and weakly in the pancreas. Despite extensive expression attempts, currents from the putative channel were not detected. Based on its sequence, we propose that the novel protein might be a voltage-activated cation channel with unique gating properties.

Role of the S4 segment in a voltage-dependent calcium-sensitive potassium (hSlo) channel

We investigated the role of individual charged residues of the S4 region of a MaxiK channel (hSlo) in channel gating. We measured macroscopic currents induced by wild type (WT) and point mutants of hSlo in inside-out membrane patches of Xenopus laevis oocytes. Of all the residues tested, only neutralizations of Arg-210 and Arg-213 were associated with a reduction in the number of gating charges as determined using the limiting slope method. Channel activation in WT and mutant channels was interpreted using an allosteric model. Mutations R207Q, R207E, and R210N facilitated channel opening in the absence of Ca2+; however, this facilitation was not observed in the channels Ca2+-bound state. Mutation R213Q behaved similarly to the WT channel in the absence of Ca2+, but Ca2+ was unable to stabilize the open state to the same extent as it does in the WT. Mutations R207Q, R207E, R210N, and R213Q reduced the coupling between Ca2+ binding and channel opening when compared with the WT. Mutations L204R, L204H, Q216R, E219Q, and E219K in the S4 domain showed a similar phenotype to the WT channel. We conclude that the S4 region in the hSlo channel is part of the voltage sensor and that only two charged amino acid residues in this region (Arg-210 and Arg-213) contribute to the gating valence of the channel.

HERG channel dysfunction in human long QT syndrome. Intracellular transport and functional defects

Mutations in HERG are associated with human chromosome 7-linked congenital long QT (LQT-2) syndrome. We used electrophysiological, biochemical, and immunohistochemical methods to study the molecular mechanisms of HERG channel dysfunction caused by LQT-2 mutations. Wild type HERG and LQT-2 mutations were studied by stable and transient expression in HEK 293 cells. We found that some mutations (Y611H and V822M) caused defects in biosynthetic processing of HERG channels with the protein retained in the endoplasmic reticulum. Other mutations (I593R and G628S) were processed similarly to wild type HERG protein, but these mutations did not produce functional channels. In contrast, the T474I mutation expressed HERG current but with altered gating properties. These findings suggest that the loss of HERG channel function in LQT-2 mutations is caused by multiple mechanisms including abnormal channel processing, the generation of nonfunctional channels, and altered channel gating.

Characterization of three episodic ataxia mutations in the human Kv1.1 potassium channel

Episodic ataxia (EA) is a rare inherited neurological disorder due to mutation in the voltage-dependent Kv1.1 potassium channel. In nine unrelated families, a different missense point mutation at highly conserved positions has been reported. We have previously characterized six of the EA mutants. In this study, three recently identified mutations were introduced into the human Kv1.1 cDNA and expressed in Xenopus oocytes. Compared to wild type, T226A and T226M reduced the current amplitude by > 95%, shifted the voltage dependence by 15 mV, and slowed activation and deactivation kinetics. Currents from G311S were approximately 25% of wild type, less steeply voltage-dependent and had more pronounced C-type inactivation. These altered gating properties will reduce the delayed-rectifier potassium current which may underlie the symptoms of EA.

Role of the S4 in cooperativity of voltage-dependent potassium channel activation

Charged residues in the S4 transmembrane segment of voltage-gated cation channels play a key role in opening channels in response to changes in voltage across the cell membrane. However, the molecular mechanism of channel activation is not well understood. To learn more about the role of the S4 in channel gating, we constructed chimeras in which S4 segments from several divergent potassium channels, Shab, Shal, Shaw, and Kv3.2, were inserted into a Shaker potassium channel background. These S4 donor channels have distinctly different voltage-dependent gating properties and S4 amino acid sequences. None of the S4 chimeras have the gating behavior of their respective S4 donor channels. The conductance-voltage relations of all S4 chimeras are shifted to more positive voltages and the slopes are decreased. There is no consistent correlation between the nominal charge content of the S4 and the slope of the conductance-voltage relation, suggesting that the mutations introduced by the S4 chimeras may alter cooperative interactions in the gating process. We compared the gating behavior of the Shaw S4 chimera with its parent channels, Shaker and Shaw, in detail. The Shaw S4 substitution alters activation gating profoundly without introducing obvious changes in other channel functions. Analysis of the voltage-dependent gating kinetics suggests that the dominant effect of the Shaw S4 substitution is to alter a single cooperative transition late in the activation pathway, making it rate limiting. This interpretation is supported further by studies of channels assembled from tandem heterodimer constructs with both Shaker and Shaw S4 subunits. Activation gating in the heterodimer channels can be predicted from the properties of the homotetrameric channels only if it is assumed that the mutations alter a cooperative transition in the activation pathway rather than independent transitions.

Uncharged S4 residues and cooperativity in voltage-dependent potassium channel activation

Substitution of the S4 of Shaw into Shaker alters cooperativity in channel activation by slowing a cooperative transition late in the activation pathway. To determine the amino acids responsible for the functional changes in Shaw S4, we created several mutants by substituting amino acids from Shaw S4 into Shaker. The S4 amino acid sequences of Shaker and Shaw S4 differ at 11 positions. Simultaneous substitution of just three noncharged residues from Shaw S4 into Shaker (V369I, I372L, S376T; ILT) reproduces the kinetic and voltage-dependent properties of Shaw S4 channel activation. These substitutions cause very small changes in the structural and chemical properties of the amino acid side chains. In contrast, substituting the positively charged basic residues in the S4 of Shaker with neutral or negative residues from the S4 of Shaw S4 does not reproduce the shallow voltage dependence or other properties of Shaw S4 opening. Macroscopic ionic currents for ILT could be fit by modifying a single set of transitions in a model for Shaker channel gating (Zagotta, W.N., T. Hoshi, and R.W. Aldrich. 1994. J. Gen. Physiol. 103:321-362). Changing the rate and voltage dependence of a final cooperative step in activation successfully reproduces the kinetic, steady state, and voltage-dependent properties of ILT ionic currents. Consistent with the model, ILT gating currents activate at negative voltages where the channel does not open and, at more positive voltages, they precede the ionic currents, confirming the existence of voltage-dependent transitions between closed states in the activation pathway. Of the three substitutions in ILT, the I372L substitution is primarily responsible for the changes in cooperativity and voltage dependence. These results suggest that noncharged residues in the S4 play a crucial role in Shaker potassium channel gating and that small steric changes in these residues can lead to large changes in cooperativity within the channel protein.

Ataxia, arrhythmia and ion-channel gene defects

Ion channels are essential to a wide range of physiological functions including neuronal signaling, muscle contraction, cardiac pacemaking, hormone secretion and cell proliferation. The important role that highly regulated ion influx plays in these processes has been underscored by a recent flurry of discoveries linking ion-channel gene mutations to inherited disorders. Ion channels of many different types have been demonstrated as being causative factors in genetic disease. This review discusses the growing number of disorders associated with genes of the voltage-gated ion channel superfamily, with special focus on those characterized by neurological, neuromuscular, or cardiac dysfunction in humans and mice.

Hair cell-specific splicing of mRNA for the alpha1D subunit of voltage-gated Ca2+ channels in the chicken's cochlea

The L-type voltage-gated Ca2+ channels that control tonic release of neurotransmitter from hair cells exhibit unusual electrophysiological properties: a low activation threshold, rapid activation and deactivation, and a lack of Ca2+-dependent inactivation. We have inquired whether these characteristics result from cell-specific splicing of the mRNA for the L-type alpha1D subunit that predominates in hair cells of the chicken's cochlea. The alpha1D subunit in hair cells contains three uncommon exons: one encoding a 26-aa insert in the cytoplasmic loop between repeats I and II, an alternative exon for transmembrane segment IIIS2, and a heretofore undescribed exon specifying a 10-aa insert in the cytoplasmic loop between segments IVS2 and IVS3. We propose that the alternative splicing of the alpha1D mRNA contributes to the unusual behavior of the hair cell's voltage-gated Ca2+ channels.

The real life of voltage-gated K+ channels: more than model behaviour

The past ten years have provided an embarrassment of riches for those interested in cloned voltage-gated K+ (Kv) channels. Details of their physiology and pharmacology in expression systems, and their precise cellular location abound, making them excellent targets for pharmacologists. However, there is still a considerable and important gap in our knowledge between the behaviour of expressed Kv channels and K+ currents in vivo. In this review Brian Robertson focuses on a few of the recent developments in the field of Kv channels, namely modulation of their behaviour by accessory subunits, their control, and localization of identified Kv subunits.

Alternative splicing in the pore-forming region of shaker potassium channels

We have cloned cDNAs for the shaker potassium channel gene from the spiny lobster Panulirus interruptus. As previously found in Drosophila, there is alternative splicing at the 5' and 3' ends of the coding region. However, in Panulirus shaker, alternative splicing also occurs within the pore-forming region of the protein. Three different splice variants were found within the P region, two of which bestow unique electrophysiological characteristics to channel function. Pore I and pore II variants differ in voltage dependence for activation, kinetics of inactivation, current rectification, and drug resistance. The pore 0 variant lacks a P region exon and does not produce a functional channel. This is the first example of alternative splicing within the pore-forming region of a voltage-dependent ion channel. We used a recently identified potassium channel blocker, kappa-conotoxin PVIIA, to study the physiological role of the two pore forms. The toxin selectively blocked one pore form, whereas the other form, heteromers between the two pore forms, and Panulirus shal were not blocked. When it was tested in the Panulirus stomatogastric ganglion, the toxin produced no effects on transient K+ currents or synaptic transmission between neurons.

Alternative splicing in the pore-forming region of shaker potassium channels

We have cloned cDNAs for the shaker potassium channel gene from the spiny lobster Panulirus interruptus. As previously found in Drosophila, there is alternative splicing at the 5' and 3' ends of the coding region. However, in Panulirus shaker, alternative splicing also occurs within the pore-forming region of the protein. Three different splice variants were found within the P region, two of which bestow unique electrophysiological characteristics to channel function. Pore I and pore II variants differ in voltage dependence for activation, kinetics of inactivation, current rectification, and drug resistance. The pore 0 variant lacks a P region exon and does not produce a functional channel. This is the first example of alternative splicing within the pore-forming region of a voltage-dependent ion channel. We used a recently identified potassium channel blocker, kappa-conotoxin PVIIA, to study the physiological role of the two pore forms. The toxin selectively blocked one pore form, whereas the other form, heteromers between the two pore forms, and Panulirus shal were not blocked. When it was tested in the Panulirus stomatogastric ganglion, the toxin produced no effects on transient K+ currents or synaptic transmission between neurons.

Behavioral defects in C. elegans egl-36 mutants result from potassium channels shifted in voltage-dependence of activation

Mutations in the C. elegans egl-36 gene result in defective excitation of egg-laying and enteric muscles. Dominant gain-of-function alleles inhibit enteric and egg-laying muscle contraction, whereas a putative null mutation has no observed phenotype. egl-36 encodes a Shaw-type (Kv3) voltage-dependent potassium channel subunit. In Xenopus oocytes, wild-type egl-36 expresses noninactivating channels with slow activation kinetics. One gain-of-function mutation causes a single amino acid substitution in S6, and the other causes a substitution in the cytoplasmic amino terminal domain. Both mutant alleles produce channels dramatically shifted in their midpoints of activation toward hyperpolarized voltages. An egl-36::gfp fusion is expressed in egg-laying muscles and in a pair of enteric muscle motor neurons. The mutant egl-36 phenotypes can thus be explained by expression in these cells of potassium channels that are inappropriately opened at hyperpolarized potentials, causing decreased excitability due to increased potassium conductance.

Transfer of voltage independence from a rat olfactory channel to the Drosophila ether-à-go-go K+ channel

The S4 segment is an important part of the voltage sensor in voltage-gated ion channels. Cyclic nucleotide-gated channels, which are members of the superfamily of voltage-gated channels, have little inherent sensitivity to voltage despite the presence of an S4 segment. We made chimeras between a voltage-independent rat olfactory channel (rolf) and the voltage-dependent ether-à-go-go K+ channel (eag) to determine the basis of their divergent gating properties. We found that the rolf S4 segment can support a voltage-dependent mechanism of activation in eag, suggesting that rolf has a potentially functional voltage sensor that is silent during gating. In addition, we found that the S3-S4 loop of rolf increases the relative stability of the open conformation of eag, effectively converting eag into a voltage-independent channel. A single charged residue in the loop makes a significant contribution to the relative stabilization of the open stage in eag. Our data suggest that cyclic nucleotide-gated channels such as rolf contain a voltage sensor which, in the physiological voltage range, is stabilized in an activated conformation that is permissive for pore opening.

The structure and organization of synthetic putative membranous segments of ROMK1 channel in phospholipid membranes

The hydropathy plot of ROMK1, an inwardly rectifying K+ channel, suggests that the channel contains two transmembrane domains (M1 and M2) and a linker between them with significant homology to the H5 pore region of voltage-gated K+ channels. To gain structural information on the pore region of the ROMK1 channel, we used a spectrofluorimetric approach and characterized the structure, the organization state, and the ability of the putative membranous domains of the ROMK1 channel to self-assemble and coassemble within lipid membranes. Circular dichroism (CD) spectroscopy revealed that M1 and M2 adopt high alpha-helical structures in egg phosphatidylcholine small unilamellar vesicles and 40% trifluoroethanol (TFE)/water, whereas H5 is not alpha-helical in either egg phosphatidylcholine small unilamellar vesicles or 40% TFE/water. Binding experiments with 4-fluoro-7-nitrobenz-2-oxa-1,3-diazole (NBD)-labeled peptide demonstrated that all of the peptides bind to zwitterionic phospholipid membranes with partition coefficients on the order of 10(5) M-1. Tryptophan quenching experiments using brominated phospholipids revealed that M1 is dipped into the hydrophobic core of the membrane. Resonance energy transfer (RET) measurements between fluorescently labeled pairs of donor (NBD)/acceptor (rhodamine) peptides revealed that H5 and M2 can self-associate in their membrane-bound state, but M1 cannot. Moreover, the membrane-associated nonhelical H5 serving as a donor can coassemble with the alpha-helical M2 but not with M1, and M1 can coassemble with M2. No coassembly was observed between any of the segments and a membrane-embedded alpha-helical control peptide, pardaxin. The results are discussed in terms of their relevance to the proposed topology of the ROMK1 channel, and to general aspects of molecular recognition between membrane-bound polypeptides.

Determinant for beta-subunit regulation in high-conductance voltage-activated and Ca(2+)-sensitive K+ channels: an additional transmembrane region at the N terminus

The pore-forming alpha subunit of large conductance voltage- and Ca(2+)-sensitive K (MaxiK) channels is regulated by a beta subunit that has two membrane-spanning regions separated by an extracellular loop. To investigate the structural determinants in the pore-forming alpha subunit necessary for beta-subunit modulation, we made chimeric constructs between a human MaxiK channel and the Drosophila homologue, which we show is insensitive to beta-subunit modulation, and analyzed the topology of the alpha subunit. A comparison of multiple sequence alignments with hydrophobicity plots revealed that MaxiK channel alpha subunits have a unique hydrophobic segment (S0) at the N terminus. This segment is in addition to the six putative transmembrane segments (S1-S6) usually found in voltage-dependent ion channels. The transmembrane nature of this unique S0 region was demonstrated by in vitro translation experiments. Moreover, normal functional expression of signal sequence fusions and in vitro N-linked glycosylation experiments indicate that S0 leads to an exoplasmic N terminus. Therefore, we propose a new model where MaxiK channels have a seventh transmembrane segment at the N terminus (S0). Chimeric exchange of 41 N-terminal amino acids, including S0, from the human MaxiK channel to the Drosophila homologue transfers beta-subunit regulation to the otherwise unresponsive Drosophila channel. Both the unique S0 region and the exoplasmic N terminus are necessary for this gain of function.

Secondary structure, membrane localization, and coassembly within phospholipid membranes of synthetic segments derived from the N- and C-termini regions of the ROMK1 K+ channel

The hydropathy plot of the inwardly rectifying ROMK1 K+ channel, which reveals two transmembrane and a pore region domains, also reveals areas of intermediate hydrophobicity in the N terminus (M0) and in the C terminus (post-M2). Peptides that correspond to M0, post-M2, and a control peptide, pre-M0, were synthesized and characterized for their structure, affinity to phospholipid membranes, organizational state in membranes, and ability to self-assemble and coassemble in the membrane-bound state. CD spectroscopy revealed that both M0 and post-M2 adopt highly alpha-helical structures in 1% SDS and 40% TFE/water, whereas pre-M0 is not alpha-helical in either 1% SDS or 40% TFE/water. Binding experiments with NBD-labeled peptides demonstrated that both M0 and post-M2, but not pre-M0, bind to zwitterionic phospholipid membranes with partition coefficients of 10(3)-10(5) M-1. A surface localization for both post-M2 and M0 was indicated by NBD shift, tryptophan quenching experiments with brominated phospholipids, and enzymatic cleavage. Resonance energy transfer measurements between fluorescently labeled pairs of donor (NBD)/ acceptor (rhodamine) peptides revealed that M0 and post-M2 can coassemble in their membrane-bound state, but cannot self-associate when membrane-bound. The results are in agreement with recent data indicating that amino acids in the carboxy terminus of inwardly rectifying K+ channels have a major role in specifying the pore properties of the channels (Taglialatela M, Wible BA, Caporaso R, Brown AM, 1994 Science 264:844-847; Pessia M, Bond CT, Kavanaugh MP, Adelman JP, 1995, Neuron 14:1039-1045). The relevance of the results presented herein to the suggested model for the structure of the ROMK1 channel and to general aspects of molecular recognition between membrane-bound polypeptides are also discussed.

Episodic ataxia and myokymia syndrome: a new mutation of potassium channel gene Kv1.1

Episodic ataxia and myokymia syndrome is an autosomal dominant disorder characterized by persistent myokymia and attacks of unsteadiness, slurred speech, and tremulousness. This disease has been associated with point mutations in the potassium channel gene Kv1.1 (KCNA1), located at chromosome 12p13. Here, we describe a novel mutation within this gene in a newly diagnosed family.

Voltage-gated K+ channels contain multiple intersubunit association sites

A domain in the cytoplasmic NH2 terminus of voltage-gated K+ channels supervises the proper assembly of specific tetrameric channels (Li, M., Jan, J. M., and Jan, L. Y.(1992) Science 257, 1225-1230; Shen, N. V., Chen X., Boyer, M. M., and Pfaffinger, P. (1993) Neuron 11, 67-76). It is referred to as a first tetramerization domain, or T1 (Shen, N. V., Chen X., Boyer, M. M., and Pfaffinger, P.(1993) Neuron 11, 67-76). However, a deletion mutant of Kv1.3 that lacks the first 141 amino acids, Kv1.3 (T1(-)) forms functional channels, suggesting that additional association sites in the central core of Kv1.3 mediate oligomerization. To characterize these sites, we have tested the abilities of cRNA Kv1.3 (T1(-)) fragments co-injected with Kv1.3 (T1(-)) to suppress current in Xenopus oocytes. The fragments include portions of the six putative transmembrane segments, S1 through S6, specifically: S1, S1-S2, S1-S2-S3, S2-S3, S2-S3-S4, S3-S4, S3-S4-S5, S2 through COOH, S3 through COOH, S4 through COOH, and S5-S6-COOH. Electrophysiologic experiments show that the fragments S1-S2-S3, S3-S4-S5, S2 through COOH, and S3 through COOH strongly suppress Kv1.3 (T1(-)) current, while others do not. Suppression of expressed current is due to specific effects of the translated peptide Kv1.3 fragments, as validated by in vivo immunoprecipitation studies of a strong suppressor and a nonsuppressor. Pulse-chase experiments indicate that translation of truncated peptide fragments neither prevents translation of Kv1.3 (T1(-)) nor increases its rate of degradation. Co-immunoprecipitation experiments suggest that suppression involves direct association of a peptide fragment with Kv1.3 (T1(-)). Fragments that strongly suppress Kv1.3 (T1(-)) also suppress an analogous NH2-terminal deletion mutant of Kv2.1 (Kv2.1 (DeltaN139)), an isoform belonging to a different subfamily. Our results indicate that sites in the central core of Kv1.3 facilitate intersubunit association and that there are suppression sites in the central core, which are promiscuous across voltage-gated K+ channel subfamilies.

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.