Multiple mRNA isoforms encoding the mouse cardiac Kv1-5 delayed rectifier K+ channel

Mechanisms contributing to myocardial potassium channel diversity, regulation and remodeling

In the mammalian heart, multiple types of K(+) channels contribute to the control of cardiac electrical and mechanical functioning through the regulation of resting membrane potentials, action potential waveforms and refractoriness. There are similarly vast arrays of K(+) channel pore-forming and accessory subunits that contribute to the generation of functional myocardial K(+) channel diversity. Maladaptive remodeling of K(+) channels associated with cardiac and systemic diseases results in impaired repolarization and increased propensity for arrhythmias. Here, we review the diverse transcriptional, post-transcriptional, post-translational, and epigenetic mechanisms contributing to regulating the expression, distribution, and remodeling of cardiac K(+) channels under physiological and pathological conditions.

Cardiac potassium channel subtypes: new roles in repolarization and arrhythmia

About 10 distinct potassium channels in the heart are involved in shaping the action potential. Some of the K+ channels are primarily responsible for early repolarization, whereas others drive late repolarization and still others are open throughout the cardiac cycle. Three main K+ channels drive the late repolarization of the ventricle with some redundancy, and in atria this repolarization reserve is supplemented by the fairly atrial-specific KV1.5, Kir3, KCa, and K2P channels. The role of the latter two subtypes in atria is currently being clarified, and several findings indicate that they could constitute targets for new pharmacological treatment of atrial fibrillation. The interplay between the different K+ channel subtypes in both atria and ventricle is dynamic, and a significant up- and downregulation occurs in disease states such as atrial fibrillation or heart failure. The underlying posttranscriptional and posttranslational remodeling of the individual K+ channels changes their activity and significance relative to each other, and they must be viewed together to understand their role in keeping a stable heart rhythm, also under menacing conditions like attacks of reentry arrhythmia.

Isoenzyme-specific regulation of cardiac Kv1.5/Kvβ1.2 ion channel complex by protein kinase C: central role of PKCβII

The ultrarapidly activating delayed rectifier current, I(Kur), is a main determinant of atrial repolarization in humans. I(Kur) and the underlying ion channel complex Kv1.5/Kvβ1.2 are negatively regulated by protein kinase C. However, the exact mode of action is only incompletely understood. We therefore analyzed isoenzyme-specific regulation of the Kv1.5/Kvβ1.2 ion channel complex by PKC. Cloned ion channel subunits were heterologously expressed in Xenopus oocytes, and measurements were performed using the double-electrode voltage-clamp technique. Activation of PKC with phorbol 12-myristate 13-acetate (PMA) resulted in a strong reduction of Kv1.5/Kvβ1.2 current. This effect could be prevented using the PKC inhibitor staurosporine. Using the bisindolylmaleimide Ro-31-8220 as an inhibitor and ingenol as an activator of the conventional PKC isoforms, we were able to show that the Kv1.5/Kvβ1.2 ion channel complex is mainly regulated by conventional isoforms. Whereas pharmacological inhibition of PKCα with HBDDE did not attenuate the PMA-induced effect, current reduction could be prevented using inhibitors of PKCβ. Here, we show the isoform βII plays a central role in the PKC-dependent regulation of Kv1.5/Kvβ1.2 channels. These results add to the current understanding of isoenzyme-selective regulation of cardiac ion channels by protein kinases.

Molecular physiology of cardiac repolarization

The heart is a rhythmic electromechanical pump, the functioning of which depends on action potential generation and propagation, followed by relaxation and a period of refractoriness until the next impulse is generated. Myocardial action potentials reflect the sequential activation and inactivation of inward (Na(+) and Ca(2+)) and outward (K(+)) current carrying ion channels. In different regions of the heart, action potential waveforms are distinct, owing to differences in Na(+), Ca(2+), and K(+) channel expression, and these differences contribute to the normal, unidirectional propagation of activity and to the generation of normal cardiac rhythms. Changes in channel functioning, resulting from inherited or acquired disease, affect action potential repolarization and can lead to the generation of life-threatening arrhythmias. There is, therefore, considerable interest in understanding the mechanisms that control cardiac repolarization and rhythm generation. Electrophysiological studies have detailed the properties of the Na(+), Ca(2+), and K(+) currents that generate cardiac action potentials, and molecular cloning has revealed a large number of pore forming (alpha) and accessory (beta, delta, and gamma) subunits thought to contribute to the formation of these channels. Considerable progress has been made in defining the functional roles of the various channels and in identifying the alpha-subunits encoding these channels. Much less is known, however, about the functioning of channel accessory subunits and/or posttranslational processing of the channel proteins. It has also become clear that cardiac ion channels function as components of macromolecular complexes, comprising the alpha-subunits, one or more accessory subunit, and a variety of other regulatory proteins. In addition, these macromolecular channel protein complexes appear to interact with the actin cytoskeleton and/or the extracellular matrix, suggesting important functional links between channel complexes, as well as between cardiac structure and electrical functioning. Important areas of future research will be the identification of (all of) the molecular components of functional cardiac ion channels and delineation of the molecular mechanisms involved in regulating the expression and the functioning of these channels in the normal and the diseased myocardium.

Expression and characterization of delayed rectifying K+channels in anterior rat taste buds

Delayed rectifying K+(DRK) channels in taste cells have been implicated in the regulation of cell excitability and as potential targets for direct and indirect modulation by taste stimuli. In the present study, we have used patch-clamp recording to determine the biophysical properties and pharmacological sensitivity of DRK channels in isolated rat fungiform taste buds. Molecular biological assays at the taste bud and single-cell levels are consistent with the interpretation that taste cells express a variety of DRK channels, including members from each of the three major subfamilies: KCNA, KCNB, and KCNC. Real-time PCR assays were used to quantify expression of the nine DRK channel subtypes. While taste cells express a number of DRK channels, the electrophysiological and molecular biological assays indicate that the Shaker Kv1.5 channel (KCNA5) is the major functional DRK channel expressed in the anterior rat tongue.

Separation of P/C- and U-type inactivation pathways in Kv1.5 potassium channels

P/C-type inactivation of Kv channels is thought to involve conformational changes in the outer pore of the channel, culminating in a partial constriction of the selectivity filter. Recent studies have identified a number of phenotypic differences in the inactivation properties of different Kv channels, including different sensitivities to elevation of extracellular K+ concentration, and different state dependencies of inactivation. We have demonstrated that an alternatively spliced short form of Kv1.5, resulting in disruption of the T1 domain, exhibits a shift in the state dependence of inactivation in this channel, and in the current study we have examined this further to contrast the properties of inactivation from open versus closed states. In a TEA+-sensitive mutant of Kv1.5 (Kv1.5 R487T), 10 mM extracellular TEA+ inhibits inactivation in both full-length and T1-deleted channels, but does not inhibit closed-state inactivation in T1-deleted channel forms. Similarly, substitution of K+ and Na+ with Cs+ ions in the recording medium inhibits inactivation of both full-length and T1-deleted channel forms, but fails to inhibit closed-state inactivation of T1-deleted channels. Collectively, these data distinguish between open-state and closed-state inactivation, and suggest the presence of multiple possible mechanisms of inactivation coexisting in Kv1 channels.

PDGF upregulates delayed rectifier via Src family kinases and sphingosine kinase in oligodendroglial progenitors

An increase in the expression of the delayed rectifier current (I(K)) has been shown to correlate with mitogenesis in many cell types. However, pathways involved in the upregulation of I(K) by growth factors in oligodendroglial progenitors (OPs) have not been well-elucidated. In this study, we found that treatment with platelet-derived growth factor (PDGF) and basic fibroblast growth factor but not ciliary neurotrophic factor resulted in increased I(K) density and upregulation of Kv1.5 and Kv1.6 mRNA transcripts. The effect of PDGF on I(K) was blocked by mimosine, a cell cycle inhibitor, and by genistein, a tyrosine kinase inhibitor. Using inhibitors of PDGF-activated pathways, we found that PDGF-induced upregulation of Kv1.5 and I(K) density involves Src family tyrosine kinases, sphingosine kinase, and intracellular Ca(2+) but not ERK1/2 or phosphatidylinositol 3-kinase pathways. Furthermore, agents that were effective inhibitors of PDGF-induced I(K) upregulation also attenuated OP proliferation, supporting the concept that I(K) is an important link between PDGF-activated signaling cascades and cell cycle progression.

Amino-terminal determinants of U-type inactivation of voltage-gated K+ channels

The T1 domain is a cytosolic NH2-terminal domain present in all Kv (voltage-dependent potassium) channels, and is highly conserved between Kv channel subfamilies. Our characterization of a truncated form of Kv1.5 (Kv1.5deltaN209) expressed in myocardium demonstrated that deletion of the NH2 terminus of Kv1.5 imparts a U-shaped inactivation-voltage relationship to the channel, and prompted us to investigate the NH2 terminus as a regulatory site for slow inactivation of Kv channels. We examined the macroscopic inactivation properties of several NH2-terminal deletion mutants of Kv1.5 expressed in HEK 293 cells, demonstrating that deletion of residues up to the T1 boundary (Kv1.5deltaN19, Kv1.5deltaN91, and Kv1.5deltaN119) did not alter Kv1.5 inactivation, however, deletion mutants that disrupted the T1 structure consistently exhibited inactivation phenotypes resembling Kv1.5deltaN209. Chimeric constructs between Kv1.5 and the NH2 termini of Kv1.1 and Kv1.3 preserved the inactivation kinetics observed in full-length Kv1.5, again suggesting that the Kv1 T1 domain influences slow inactivation. Furthermore, disruption of intersubunit T1 contacts by mutation of residues Glu(131) and Thr(132) to alanines resulted in channels exhibiting a U-shaped inactivation-voltage relationship. Fusion of the NH2 terminus of Kv2.1 to the transmembrane segments of Kv1.5 imparted a U-shaped inactivation-voltage relationship to Kv1.5, whereas fusion of the NH2 terminus of Kv1.5 to the transmembrane core of Kv2.1 decelerated Kv2.1 inactivation and abolished the U-shaped voltage dependence of inactivation normally observed in Kv2.1. These data suggest that intersubunit T1 domain interactions influence U-type inactivation in Kv1 channels, and suggest a generalized influence of the T1 domain on U-type inactivation between Kv channel subfamilies.

Modulation of the human Kv1.5 channel by protein kinase C activation: role of the Kvbeta1.2 subunit

Kv1.5 is the principal molecular component of I(Kur), an atrial-specific K(+) current in human myocytes that is suppressed by activation of protein kinase C (PKC). We examined the effect of phorbol 12-myristate 13-acetate (PMA), a direct activator of PKC, on Kv1.5 current. Although PMA had minimal effect when Kv1.5 was expressed alone, K(+) currents derived from coexpression of Kvbeta1.2 (but not another closely related beta subunit, Kvbeta1.3) with Kv1.5 were markedly reduced by PMA, associated with a small depolarizing shift in the voltage dependence of channel activation. Additional experiments with an inactive stereoisomer, 4alpha-PMA, and the PKC inhibitor chelerythrine indicated that the effects of PMA were mediated by PKC activation. Assembly of Kv1.5 in vivo with both beta subunits was demonstrated, and all three K(+) channel proteins were substrates for phosphorylation by PKC. These results demonstrate that coexpression of Kvbeta1.2 enhances the response of Kv1.5 to PKC activation and that direct phosphorylation of K(+) channel subunits is a potential molecular basis for the effect. Furthermore, they suggest that Kvbeta1.2 may be a component of the I(Kur) complex in human atrium.

Potassium channel ontogeny

Potassium channels are multi-subunit complexes, often composed of several polytopic membrane proteins and cytosolic proteins. The formation of these oligomeric structures, including both biogenesis and trafficking, is the subject of this review. The emphasis is on events in the endoplasmic reticulum (ER), particularly on how, where, and when K(+) channel polypeptides translocate and integrate into the bilayer, oligomerize and fold to form pore-forming units, and associate with auxiliary subunits to create the mature channel complex. Questions are raised with respect to the sequence of these events, when biogenic decisions are made, models for integration of K(+) channel transmembrane segments, crosstalk between the cell surface and ER, and recognition of compatible partner subunits. Also considered are determinants of subunit composition and stoichiometry, their consequence for trafficking, mechanisms for ER retention and export, and sequence motifs that direct channels to the cell surface. It is these mechanistic issues that govern the differential distributions of K(+) conductances at the cell surface, and hence the electrical activity of cells and tissues underlying both the physiology and pathophysiology of an organism.

Expression pattern and functional characteristics of two novel splice variants of the two-pore-domain potassium channel TREK-2

Two novel alternatively spliced isoforms of the human two-pore-domain potassium channel TREK-2 were isolated from cDNA libraries of human kidney and fetal brain. The cDNAs of 2438 base pairs (bp) (TREK-2b) and 2559 bp (TREK-2c) encode proteins of 508 amino acids each. RT-PCR showed that TREK-2b is strongly expressed in kidney (primarily in the proximal tubule) and pancreas, whereas TREK-2c is abundantly expressed in brain. In situ hybridization revealed a very distinct expression pattern of TREK-2c in rat brain which partially overlapped with that of TREK-1. Expression of TREK-2b and TREK-2c in human embryonic kidney (HEK) 293 cells showed that their single-channel characteristics were similar. The slope conductance at negative potentials was 163 +/- 5 pS for TREK-2b and 179 +/- 17 pS for TREK-2c. The mean open and closed times of TREK-2b at -84 mV were 133 +/- 16 and 109 +/- 11 micros, respectively. Application of forskolin decreased the whole-cell current carried by TREK-2b and TREK-2c. The sensitivity to forskolin was abolished by mutating a protein kinase A phosphorylation site at position 364 of TREK-2c (construct S364A). Activation of protein kinase C (PKC) by application of phorbol-12-myristate-13-acetate (PMA) also reduced whole-cell current. However, removal of the putative TREK-2b-specific PKC phosphorylation site (construct T7A) did not affect inhibition by PMA. Our results suggest that alternative splicing of TREK-2 contributes to the diversity of two-pore-domain K+ channels.

Functional expression of a GFP-tagged Kv1.5 alpha-subunit in mouse ventricle

The experiments here were undertaken to determine the feasibility of increasing the cell surface expression of voltage-gated ion channels in cardiac cells in vivo and to explore the functional consequences of ectopic channel expression. Transgenic mice expressing a green fluorescent protein (GFP)-tagged, voltage-gated K+ (Kv) channel alpha-subunit, Kv1.5-GFP, driven by the cardiac-specific alpha-MHC promoter, were generated. In recent studies, Kv1.5 has been shown to encode the micromolar 4-aminopyridine (4-AP)-sensitive delayed rectifier K+ current (I(K,slow)) in mouse myocardium. Unexpectedly, Kv1.5-GFP expression is heterogeneous in the ventricles of these animals. Although no electrocardiographic abnormalities were evident, expression of Kv1.5-GFP results in marked decreases in action potential durations in GFP-positive ventricular myocytes. In voltage-clamp recordings from GFP-positive ventricular myocytes, peak outward K+ currents are significantly higher, and their waveforms are distinct from those recorded from wild-type cells. Pharmacological experiments revealed a selective increase in a micromolar 4-AP-sensitive current, similar to the 4-AP-sensitive component of I(K,slow) in wild-type cells. The inactivation rate of the "overexpressed" current, however, is significantly slower than the Kv1.5-encoded component of I(K,slow) in wild-type cells, suggesting differences in association with accessory subunits and/or posttranslational processing.

K(V)alpha1 channels in murine arterioles: differential cellular expression and regulation of diameter

The primary objectives of this study were to reveal cell-specific expression patterns and functions of voltage-gated K(+) channel (K(V)alpha1) subunits in precapillary arterioles of the murine cerebral circulation. K(V)alpha1 were detected using peptide-specific antibodies in immunofluorescence and Western blotting assays. K(V)1.2 was localized almost exclusively to endothelial cells, whereas K(V)1.5 was discretely localized to the nerves and nerve terminals that innervate the arterioles. K(V)1.5 also localized specifically to arteriolar nerves in human pial membrane. K(V)1.5 was notable for its absence from smooth muscle cells. K(V)1.3, K(V)1.4, and K(V)1.6 were localized to endothelial and smooth muscle cells, although K(V)1.4 had a low expression level. K(V)1.1 was not expressed. Therefore, we show that different cell types of pial arterioles have distinct physiological expression profiles of K(V)alpha1, conferring the possibility of differential modulation by extracellular and second messengers. Furthermore, we show recombinant agitoxin-2 and margatoxin are potent vasoconstrictors, suggesting that K(V)alpha1 subunits have a major function in determining arteriolar resistance to blood flow.

Altered state dependence of c-type inactivation in the long and short forms of human Kv1.5

Evidence from both human and murine cardiomyocytes suggests that truncated isoforms of Kv1.5 can be expressed in vivo. Using whole-cell patch-clamp recordings, we have characterized the activation and inactivation properties of Kv1.5DeltaN209, a naturally occurring short form of human Kv1.5 that lacks roughly 75% of the T1 domain. When expressed in HEK 293 cells, this truncated channel exhibited a V(1/2) of -19.5 +/- 0.9 mV for activation and -35.7 +/- 0.7 mV for inactivation, compared with a V(1/2) of -11.2 +/- 0.3 mV for activation and -0.9 +/- 1.6 mV for inactivation in full-length Kv.15. Kv1.5DeltaN209 channels exhibited several features rarely observed in voltage-gated K(+) channels and absent in full-length Kv1.5, including a U-shaped voltage dependence of inactivation and "excessive cumulative inactivation," in which a train of repetitive depolarizations resulted in greater inactivation than a continuous pulse. Kv1.5DeltaN209 also exhibited a stronger voltage dependence to recovery from inactivation, with the time to half-recovery changing e-fold over 30 mV compared with 66 mV in full-length Kv1.5. During trains of human action potential voltage clamps, Kv1.5DeltaN209 showed 30-35% greater accumulated inactivation than full-length Kv1.5. These results can be explained with a model based on an allosteric model of inactivation in Kv2.1 (Klemic, K.G., C.-C. Shieh, G.E. Kirsch, and S.W. Jones. 1998. Biophys. J. 74:1779-1789) in which an absence of the NH(2) terminus results in accelerated inactivation from closed states relative to full-length Kv1.5. We suggest that differential expression of isoforms of Kv1.5 may contribute to K(+) current diversity in human heart and many other tissues.

Expression of Shal potassium channel subunits in the adult and developing cochlear nucleus of the mouse

The pattern of expression of potassium (K(+)) channel subunits is thought to contribute to the establishment of the unique discharge characteristics exhibited by cochlear nucleus (CN) neurons. This study describes the developmental distribution of mRNA for the three Shal channel subunits Kv4.1, Kv4.2 and Kv4.3 within the mouse CN, as assessed with in situ hybridization and RT-PCR techniques. Kv4.1 was not present in CN at any age. Kv4.2 mRNA was detectable as early as postnatal day 2 (P2) in all CN subdivisions, and continued to be constitutively expressed throughout development. Kv4.2 was abundantly expressed in a variety of CN cell types, including all of the major projection neuron classes (i.e., octopus, bushy, stellate, fusiform, and giant cells). In contrast, Kv4.3 was expressed at lower levels and by fewer cell types. Kv4.3-labeled cells were more prevalent in ventral subdivisions than in the dorsal CN. Kv4.3 expression was significantly delayed developmentally in comparison to Kv4.2, as it was detectable only after P14. Although the techniques employed in this study detect mRNA and not protein, it can be inferred from the differential distribution of Kv4 transcripts that CN neurons selectively regulate the expression of Shal K(+) channels among individual neurons throughout development.

Use of serial analysis of gene expression to generate kidney expression libraries

Chronic renal disease initiation and progression remain incompletely understood. Genome-wide expression monitoring should clarify mechanisms that cause progressive renal disease by determining how clusters of genes coordinately change their activity. Serial analysis of gene expression (SAGE) is a technique of expression profiling, which permits simultaneous, comparative, and quantitative analysis of gene-specific, 9- to 13-bp sequence tags. Using SAGE, we have constructed a tag expression library from ROP-+/+ mouse kidney. Tag sequences were sorted by abundance, and identity was determined by sequence homology searching. Analyses of 3,868 tags yielded 1,453 unique kidney transcripts. Forty-two percent of these transcripts matched mRNA sequence entries with known function, 35% of the transcripts corresponded to expressed sequence tag (EST) entries or cloned genes, whose function has not been established, and 23% represented unidentified genes. Previously characterized transcripts were clustered into functional groups, and those encoding metabolic enzymes, plasma membrane proteins (transporters/receptors), and ribosomal proteins were most abundant (39, 14, and 12% of known transcripts, respectively). The most common, kidney-specific transcripts were kidney androgen-regulated protein (4% of all transcripts), sodium-phosphate cotransporter (0.3%), renal cytochrome P-450 (0.3%), parathyroid hormone receptor (0.1%), and kidney-specific cadherin (0.1%). Comprehensively characterizing and contrasting gene expression patterns in normal and diseased kidneys will provide an alternative strategy to identify candidate pathways, which regulate nephropathy susceptibility and progression, and novel targets for therapeutic intervention.

Regulation of the transient outward K(+) current by Ca(2+)/calmodulin-dependent protein kinases II in human atrial myocytes

Ca(2+)/calmodulin-dependent protein kinases II (CaMKII) have important functions in regulating cardiac excitability and contractility. In the present study, we examined whether CaMKII regulated the transient outward K(+) current (I(to)) in whole-cell patch-clamped human atrial myocytes. We found that a specific CaMKII inhibitor, KN-93 (20 micromol/L), but not its inactive analog, KN-92, accelerated the inactivation of I(to) (tau(fast): 66.9+/-4.4 versus 43.0+/-4.4 ms, n=35; P<0.0001) and inhibited its maintained component (at +60 mV, 4.9+/-0.4 versus 2.8+/-0.4 pA/pF, n = 35; P<0. 0001), leading to an increase in the extent of its inactivation. Similar effects were observed by dialyzing cells with a peptide corresponding to CaMKII residues 281 to 309 or with autocamtide-2-related inhibitory peptide and by external application of the calmodulin inhibitor calmidazolium, which also suppressed the effects of KN-93. Furthermore, the phosphatase inhibitor okadaic acid (500 nmol/L) slowed I(to) inactivation, increased I(sus), and inhibited the effects of KN-93. Changes in [Ca(2+)](i) by dialyzing cells with approximately 30 nmol/L Ca(2+) or by using the fast Ca(2+) buffer BAPTA had opposite effects on I(to). In BAPTA-loaded myocytes, I(to) was less sensitive to KN-93. In myocytes from patients in chronic atrial fibrillation, characterized by a prominent I(sus), KN-93 still increased the extent of inactivation of I(to). Western blot analysis of atrial samples showed that delta-CaMKII expression was enhanced during chronic atrial fibrillation. In conclusion, CaMKII control the extent of inactivation of I(to) in human atrial myocytes, a process that could contribute to I(to) alterations observed during chronic atrial fibrillation.

Identification, cloning and expression of rabbit vascular smooth muscle Kv1.5 and comparison with native delayed rectifier K+ current

1. The molecular basis of voltage-gated, delayed rectifier K+ (KDR) channels in vascular smooth muscle cells is poorly defined. In this study we employed (i) an antibody against Kv1.5 and (ii) a cDNA clone encoding Kv1.5 derived from rabbit portal vein (RPV) to demonstrate Kv1.5 expression in RPV and to compare the properties of RPVKv1.5 expressed in mammalian cells with those of native RPV KDR current. 2. Expression of Kv1.5 channel protein in RPV was demonstrated by (i) immunocytolocalization of an antibody raised against a C-terminal epitope of mouse cardiac Kv1.5 in permeabilized, freshly isolated RPV smooth muscle cells and (ii) isolation of a cDNA clone encoding RPVKv1.5 by reverse transcription-polymerase chain reaction (RT-PCR) using mRNA derived from endothelium-denuded and adventitia-free RPV. 3. RPVKv1.5 cDNA was expressed in mammalian L cells and human embryonic kidney (HEK293) cells and the properties of the expressed channels compared with those of native KDR channels of freshly dispersed myocytes under identical conditions. 4. The kinetics and voltage dependence of activation of L cell-expressed RPVKv1.5 and native KDR current were identical, as were the kinetics of recovery from inactivation and single channel conductance. In contrast, there was little similarity between HEK293 cell-expressed RPVKv1.5 and native KDR current. 5. Inactivation occurred with the same voltage for half-maximal availability, but the kinetics and slope constant for the voltage dependence of inactivation for L cell-expressed RPVKv1.5 and the native current were different: slow time constants were 6.5 +/- 0.6 and 3.5 +/- 0.4 s and slope factors were 4.7 +/- 0.2 and 7.0 +/- 0.8 mV, respectively. 6. This study provides immunofluorescence and functional evidence that Kv1.5 alpha-subunits are a component of native KDR channels of vascular smooth muscle cells of RPV. However, the differences in kinetics and voltage sensitivity of inactivation between L cell- and HEK293 cell-expressed channels and native KDR channels provide functional evidence that vascular KDR current is not due to homomultimers of RPV Kv1.5 alone. The channel structure may be more complex, involving heteromultimers and modulatory Kvbeta-subunits, and/or native KDR current may have other components involving Kvalpha-subunits of other families.

Overexpression of nerve growth factor in the heart alters ion channel activity and beta-adrenergic signalling in an adult transgenic mouse

1. The electrophysiological and pharmacological properties of cardiac myocytes from the hearts of adult transgenic mice engineered to overexpress nerve growth factor (NGF) in the heart were studied. 2. There was a 12% increase in the ventricular myocyte capacitance in NGF myocytes consistent with cardiac hypertrophy, and action potential duration at 90% repolarization (APD90) was prolonged by 142 % compared with wild-type (WT) myocytes. This was due, at least in part, to a decrease in the density of two K+ currents, Ito and IK(ur), which were significantly reduced in NGF mice with no change in their electrophysiological characteristics. We found no change in the current density or electrophysiological properties of the L-type Ca2+ current. 3. The effect on Ito and IK(ur) of TEA and 4-aminopyridine (4-AP) was not different in cells isolated from WT and NGF mice. The prolongation of APD observed in NGF cells was mimicked in WT cells by exposure to 1 mM 4-AP, which partially blocked Ito, completely blocked IK(ur) and increased APD90 by 157%. 4. The isoprenaline-induced increase in ICa was significantly smaller in NGF myocytes than in WT myocytes. This was not due to a decrease in beta-adrenergic receptor (beta-AR) density, as this was increased in NGF tissue by 55%. Analysis of beta-AR subtypes showed that this increase was entirely due to an increase in beta2-AR density with no change in beta1-ARs. 5. The response of the beta-AR-coupled adenylyl cyclase system to isoprenaline, Gpp(NH)p and forskolin was studied by measuring cAMP production. In NGF tissue, isoprenaline elicited a significantly smaller response than in WT myoyctes and this was not due to reduced adenylyl cyclase activity as the responses of NGF tissue to guanylylimidodiphosphate (Gpp(NH)p) and forskolin were unaffected. 6. In conclusion, the overexpression of NGF in the mouse heart resulted in a decrease in the current density of two K+ channels, which contributed to the prolongation of the cardiac action potential. Despite an increase in beta2-AR density in the hearts of the NGF mice, the response to isoprenaline was diminished, and this was due to an uncoupling of the beta-ARs from the intracellular signalling cascade. These potentially pathological changes may be involved in the occurrence of ventricular arrhythmias in cardiac hypertrophy and failure, and this mouse provides a novel model in which to study such changes.

Characterization of a slowly inactivating outward current in adult mouse ventricular myocytes

We recently have reported that suppression of the slowly inactivating component of the outward current, Islow, in ventricular myocytes of transgenic mice (long QT mice) overexpressing the N-terminal fragment and S1 segment of Kv1.1 resulted in a significant prolongation of action potential duration and the QT interval. Here we describe the detailed biophysical properties and physiological role of Islow by applying the whole-cell patch-clamp technique at both room temperature and 37 degreesC. This current activates rapidly with time constants ranging from 3.8+/-0.8 ms at -20 mV to 2.1+/-0.5 ms at 50 mV at room temperature. The half-activation voltage and slope factor are -12.5+/-2.6 mV and 7. 7+/-1.0 mV, respectively. The inactivation of this current is slow compared with the fast inactivating component Ito, with time constants of approximately 100 ms at 37 degreesC. The steady-state inactivation of Islow is not temperature-dependent, with half-inactivation voltages and slope factors of -35.1+/-1.3 and -5. 4+/-0.4 mV at 37 degreesC, and -37.6+/-1.8 and -5.8+/-0.6 mV at room temperature. Double exponentials were required to describe the time-dependent recovery of Islow from steady-state inactivation, with time constants of 233+/-34 and 3730+/-702 ms at 37 degreesC, and 830+/-240 and 8680+/-2410 ms at room temperature. Islow is highly sensitive to 4-aminopyridine but is insensitive to tetraethylammonium, alpha-dendrotoxin, and E-4031. Stimulation with action-potential waveforms under voltage-clamp mode revealed that this current plays an important role in the early and middle phases of repolarization of the cardiac action potential. We conclude that the biophysical properties and pharmacological profiles of Islow are similar to those of Kv1.5-encoded currents.

The 1997 Stevenson Award Lecture. Cardiac K+channel gating: cloned delayed rectifier mechanisms and drug modulation

K+ channels are ubiquitous membrane proteins, which have a central role in the control of cell excitability. In the heart, voltage-gated delayed rectifier K+ channels, like Kv1.5, determine repolarization and the cardiac action potential plateau duration. Here we review the broader properties of cloned voltage-gated K+ channels with specific reference to the hKv1.5 channel in heart. We discuss the basic structural components of K+ channels such as the pore, voltage sensor, and fast inactivation, all of which have been extensively studied. Slow, or C-type, inactivation and the structural features that control pore opening are less well understood, although recent studies have given new insight into these problems. Information about channel transitions that occur prior to opening is provided by gating currents, which reflect charge-carrying transitions between kinetic closed states. By studying modulation of the gating properties of K+ channels by cations and with drugs, we can make a more complete interpretation of the state dependence of drug and ion interactions with the channel. In this way we can uncover the detailed mechanisms of action of K+ channel blockers such as tetraethylammonium ions and 4-aminopyridine, and antiarrhythmic agents such as nifedipine and quinidine.Key words: potassium channel, Kv1.5, channel gating, inactivation, pore region, gating currents.

A rapidly activating sustained K+ current modulates repolarization and excitation-contraction coupling in adult mouse ventricle

1. The K+ currents which control repolarization in adult mouse ventricle, and the effects of changes in action potential duration on excitation-contraction coupling in this tissue, have been studied with electrophysiological methods using single cell preparations and by recording mechanical parameters from an in vitro working heart preparation. 2. Under conditions where Ca(2+)-dependent currents were eliminated by buffering intracellular Ca2+ with EGTA, depolarizing voltage steps elicited two rapidly activating outward K+ currents: (i) a transient outward current, and (ii) a slowly inactivating or 'sustained' delayed rectifier. 3. These two currents were separated pharmacologically by the K+ channel blocker 4-amino-pyridine (4-AP). 4-AP at concentrations between 3 and 200 microM resulted in (i) a marked increase in action potential duration and a large decrease in the sustained K+ current at plateau potentials, as well as (ii) a significant increase in left ventricular systolic pressure in the working heart preparation. 4. The current-voltage (I-V) relation, kinetics, and block by low concentrations of 4-AP strongly suggest that the rapid delayed rectifier in adult mouse ventricles is the same K+ current (Kv1.5) that has been characterized in detail in human and canine atria. 5. These results show that the 4-AP-sensitive rapid delayed rectifier is a very important repolarizing current in mouse ventricle. The enhanced contractility produced by 4-AP (50 microM) in the working heart preparation demonstrates that modulation of the action potential duration, by blocking a K+ current, is a very significant inotropic variable.

New modulatory alpha subunits for mammalian Shab K+ channels

Two novel K+ channel alpha subunits, named Kv9.1 and Kv9.2, have been cloned. The Kv9.2 gene is situated in the 8q22 region of the chromosome. mRNAs for these two subunits are highly and selectively expressed in the nervous system. High levels of expressions are found in the olfactory bulb, cerebral cortex, hippocampal formation, habenula, basolateral amygdaloid nuclei, and cerebellum. Interestingly Kv9.1 and Kv9.2 colocalized with Kv2.1 and/or Kv2.2 alpha subunits in several regions of the brain. Neither Kv9.1 nor Kv9.2 have K+ channel activity by themselves, but both modulate the activity of Kv2.1 and Kv2.2 channels by changing kinetics and levels of expression and by shifting the half-inactivation potential to more polarized values. This report also analyzes the changes in electrophysiological properties of Kv2 subunits induced by Kv5.1 and Kv6.1, two other modulatory subunits. Each modulatory subunit has its own specific properties of regulation of the functional Kv2 subunits, and they can lead to extensive inhibitions, to large changes in kinetics, and/or to large shifts in the voltage dependencies of the inactivation process. The increasing number of modulatory subunits for Kv2.1 and Kv2.2 provides an amazingly new capacity of functional diversity.

Modes of regulation of shab K+ channel activity by the Kv8.1 subunit

The Kv8.1 subunit is unable to generate K+ channel activity in Xenopus oocytes or in COSm6 cells. The Kv8.1 subunit expressed at high levels acts as a specific suppressor of the activity of Kv2 and Kv3 channels in Xenopus oocytes (Hugnot, J. P., Salinas, M., Lesage, F., Guillemare, E., Weille, J., Heurteaux, C., Mattéi, M. G., and Lazdunski, M. (1996) EMBO J. 15, 3322-3331). At lower levels, Kv8.1 associates with Kv2.1 and Kv2.2 to form hybrid Kv8.1/Kv2 channels, which have new biophysical properties and more particularly modified properties of the inactivation process as compared with homopolymers of Kv2.1 or Kv2.2 channels. The same effects have been seen by coexpressing the Kv8.1 subunit and the Kv2.2 subunit in COSm6 cells. In these cells, Kv8.1 expressed alone remains in intracellular compartments, but it can reach the plasma membrane when it associates with Kv2.2, and it then also forms new types of Kv8.1/Kv2. 2 channels. Present results indicate that Kv8.1 when expressed at low concentrations acts as a modifier of Kv2.1 and Kv2.2 activity, while when expressed at high concentrations in oocytes it completely abolishes Kv2.1, Kv2.2, or Kv3.4 K+ channel activity. The S6 segment of Kv8.1 is atypical and contains the structural elements that modify inactivation of Kv2 channels.

K(V)LQT1 and lsK (minK) proteins associate to form the I(Ks) cardiac potassium current

In mammalian cardiac cells, a variety of transient or sustained K+ currents contribute to the repolarization of action potentials. There are two main components of the delayed-rectifier sustained K+ current, I(Kr) (rapid) and I(Ks), (slow). I(Kr) is the product of the gene HERG, which is altered in the long-QT syndrome, LQT2. A channel with properties similar to those of the I(Ks) channel is produced when the cardiac protein IsK is expressed in Xenopus oocytes. However, it is a small protein with a very unusual structure for a cation channel. The LQT1 gene is another gene associated with the LQT syndrome, a disorder that causes sudden death from ventricular arrhythmias. Here we report the cloning of the full-length mouse K(V)LQT1 complementary DNA and show that K(V)LQT1 associates with IsK to form the channel underlying the I(Ks) cardiac current, which is a target of class-III anti-arrhythmic drugs and is involved in the LQT1 syndrome.

A new K+ channel beta subunit to specifically enhance Kv2.2 (CDRK) expression

Cloned K+ channel beta subunits are hydrophilic proteins which associate to pore-forming alpha subunits of the Shaker subfamily. The resulting alphabeta heteromultimers K+ channels have inactivation kinetics significantly more rapid than those of the corresponding alpha homomultimers. This paper reports the cloning and the brain localization of mKvbeta4 (m for mouse), a new beta subunit. This new beta subunit is highly expressed in the nervous system but is also present in other tissues such as kidney. In contrast with other beta subunits, coexpression of the mKvbeta4 subunit with alpha subunits of Shaker-type K+ channel does not modify the kinetic properties or voltage-dependence of these channels in Xenopus oocytes. Instead, mKvbeta4 associates to Kv2.2 (CDRK), a Shab K+ channel, to specifically enhance (a factor of up to 6) its expression level without changing its elementary conductance or kinetics. It is without effect on another closely related Shab K+ channel Kv2.1 (DRK1). Chimeras between Kv2.1 and Kv2. 2 indicate that the COOH-terminal end of the Kv2.2 protein is essential for its mKvbeta4 sensitivity. The functional results associated with the observation of the co-localization of mKvbeta4 and Kv2.2 transcripts in most brain areas strongly suggest that both subunits interact in vivo to form a slowly-inactivating K+ channel. A chaperone-like effect of mKvbeta4 seems to permit the integration of a larger number of Kv2.2 channels at the plasma membrane.

Chronic morphine administration enhances the expression of Kv1.5 and Kv1.6 voltage-gated K+ channels in rat spinal cord

Prolonged opiate administration leads to the development of tolerance and dependence. These phenomena are accompanied by selective regulation of distant cellular proteins and mRNAs, including ionic channels. Acute opiate administration differentially affects voltage-dependent K+ currents. Whereas, opiate activation of K+ channels is well established opioid-induced inhibition of K+ conductance has also been studied. In this study, we focused on the effect of chronic morphine exposure on voltage-dependent Shaker-related Kv1.5 and Kv1.6 K+ channel gene expression and on Kv1.5 protein levels in the rat spinal cord. Several experimental approaches including in-situ hybridization, RNAse protection, reverse transcriptase-polymerase chain reaction (RT-PCR), Western blotting and immunohistochemistry were employed. We found that motor neurons are highly enriched in Kv1.5 and Kv1.6 mRNA and in Kv1.5 channel protein. Moreover, we found significant increases in the amount of mRNA encoding for these two K+ channels and in Kv1.5 channel protein in the spinal cord of morphine-treated rats, compared with controls. For example, quantitative in-situ hybridization, revealed a 2.1 +/- 0.15- and 2.3 +/- 0.5-fold increase in Kv1.5 and Kv1.6 channel mRNA levels, respectively. Similar results were obtained by semiquantitative RT-PCR analyses. Kv1.5 protein level was increased by 1.9-fold in the spinal cord or morphine-treated rats. Our results suggest that Kv1.5 and Kv1.6 Shaker K+ channels play an important role in regulating motor activity that increases in mRNA and protein levels of the spinal cord K+ channels after chronic morphine exposure could be viewed as a cellular adaptation which compensates for a persistent opioid-induced inhibition of K+ channel activity. These alterations may account, in part, for the cellular events leading to opiate tolerance and dependence.

Expression and function of voltage-dependent potassium channel genes in human airway smooth muscle

Patch clamp and RNA-polymerase chain reaction methods were used to determine the expression of voltage-dependent potassium channel currents and mRNAs in human airway smooth muscle cells, and tension measurements were used to examine the functional role of specific potassium channel gene products in human bronchial smooth muscle. RNA from airway smooth muscle tissue revealed the presence of Kv1.2 (11 kilobases (kb)) and Kv1.5 (3.5 and 4.4 kb) transcripts, as well as Kv1.1 mRNA (9.5 kb), which has not previously been reported in smooth muscle; transcripts from other gene families were not detected. RNA-polymerase chain reaction from cultured human myocytes confirmed that the identified transcripts were expressed by smooth muscle cells. The available voltage-dependent potassium current in human airway myocytes was insensitive to charybdotoxin (200 nM) but blocked by 4-aminopyridine. Dendrotoxin (1-300 nM; inhibits Kv1.1 and Kv1.2 channels), charybdotoxin (10 nM to 1 microM; inhibits KCa and Kv1.2 channels), and glybenclamide (0.1-100 microM; inhibits KATP channels) had no effect on resting tone. Conversely, 4-aminopyridine increased resting tension with an EC50 (1.8 mM) equivalent to that observed for current inhibition (1.9 mM). Human airway myocytes express mRNA from several members of the Kv1 gene family; the channel that underlies the predominate voltage-dependent current and the regulation of basal tone appears to be Kv1.5.

Functional differences in Kv1.5 currents expressed in mammalian cell lines are due to the presence of endogenous Kv beta 2.1 subunits

The voltage-sensitive currents observed following hKv1.5 alpha subunit expression in HEK 293 and mouse L-cells differ in the kinetics and voltage dependence of activation and slow inactivation. Molecular cloning, immunopurification, and Western blot analysis demonstrated that an endogenous L-cell Kv beta 2.1 subunit assembled with transfected hKv 1.5 protein. In contrast, both mRNA and protein analysis failed to detect a beta subunit in the HEK 293 cells, suggesting that functional differences observed between these two systems are due to endogenous L-cell Kv beta 2.1 expression. In the absence of Kv beta 2.1, midpoints for activation and inactivation of hKv1.5 in HEK 293 cells were -0.2 +/- 2.0 and -9.6 +/- 1.8 mV, respectively. In the presence of Kv beta 2.1 these values were -14.1 +/- 1.8 and -22.1 +/- 3.7 mV, respectively. The beta subunit also caused a 1.5-fold increase in the extent of slow inactivation at 50 mV, thus completely reconstituting the L-cell current phenotype in the HEK 293 cells. These results indicate that 1) the Kv beta 2.1 subunit can alter Kv1.5 alpha subunit function, 2) beta subunits are not required for alpha subunit expression, and 3) endogenous beta subunits are expressed in heterologous expression systems used to study K+ channel function.

Dominant negative chimeras provide evidence for homo and heteromultimeric assembly of inward rectifier K+ channel proteins via their N-terminal end

Chimeras have been constructed using three different fragments (N-terminal, central and C-terminal) of IRK3, a constitutive inward rectifier K+ channel subunit, and GIRK2, a G-protein activated inward rectifier K+ channel subunit and have been coinjected into Xenopus oocytes together with IRK3 or IRK1 (another constitutive inward rectifier) cRNA. Both IRK1 and IRK3 expression was inhibited by coinjection with chimeras containing a N-terminal fragment of IRK3 suggesting that subunits of K+ channels in the IRK family form a functional multimeric assembly where the N-terminal end has an important role. In situ hybridization shows that IRK1 and IRK3 are coexpressed in the same areas of the brain and probably in the same cells. Taken together both the localization and the oocyte expression results suggest that not only homomultimeric IRK1 or homomultimeric IRK3 assemblies take place but that heteromultimeric IRK1/IRK3 assemblies are also formed.

Shaker K+ channel T1 domain self-tetramerizes to a stable structure

The potassium channel T1 domain plays an important role in the regulated assembly of subunit proteins. We have examined the assembly properties of the Shaker channel T1 domain to determine if the domain can self-assemble, the number of subunits in a multimer, Ns and the mechanism of assembly. High pressure liquid chromatography (HPLC) size exclusion chromotography (SEC) separates T1 domain proteins into two peaks. By co-assembly assays, these peaks are identified to be a high molecular weight assembled form and a low molecular weight monomeric form. To determine the Ns of the assembled protein peak on HPLC SEC, we first cross-linked the T1 domain proteins and then separated them on HPLC. Four evenly spaced bands co-migrate with the assembled protein peak; thus, the T1 domain assembles to form a tetramer. The absence of separate dimeric and trimeric peaks of assembled T1 domain protein suggests that the tetramer is the stable assembled state, most probably a closed ring structure.

Identification and localization of K+ channels in the mouse retina

Voltage-gated potassium channels are differentially expressed in the brain, and recent studies have shown that K+ channels show subcellular localization. We characterized the distribution of five different K+ channels in the mouse retina. Each channel was distributed in a unique pattern in the retina and was localized to specific subcellular domains within a given retinal neuron. Kv1.4 and Kv4.2 were consistently found in axonal and somatodendritic portions, respectively, consistent with previous studies in brain. In contrast, Kv1.2, Kv1.3, and Kv2.1 showed variable subcellular distribution depending upon cellular context. These results suggest that no one K+ channel is distributed over the entire length of the neuron to provide a "housekeeping" level of membrane potential stabilization. Instead, we propose that each K+ channel is associated with a specific subcellular functional module, and each local K+ conductance responds uniquely to local voltage and second messenger signals.

Immunochemical characterization and developmental expression of Shaker potassium channels from the nervous system of Drosophila

We have raised antisera against recombinant peptides expressed from cDNAs fragments common to all splicing variants generated at the Shaker locus of Drosophila and used them as a tool to biochemically characterize these channel proteins. This antisera succeeded in detecting the expression of multiple Shaker potassium channels (Sh Kch), proteins with variable molecular mass (65-85 kDa) and pI (5.5-7). Additionally, for first time, specific Sh proteins of 40-45 kDa most probably corresponding to some of the so-called short Sh cDNAs previously isolated by others have been identified. Using genetic criteria, it has been determined that at least a good part of this variety of proteins is generated by alternative splicing. Developmental experiments show a double wave of Sh Kch channel expression with a first pick at the third instar larvae stage, a minimum at the beginning of puparation, and the highest plateau 36 h after hatching of adult flies. The pattern of Sh splice variants changes dramatically throughout development. A detergent-resistant fraction with about 50% of Sh Kch which seems to be anchored to submembranous structures has been found. Finally, other biochemical properties of Sh Kch, like membrane fractionation and glycosylation, are also described.

The voltage-dependent K+ channel (Kv1.5) cloned from rabbit heart and facilitation of inactivation of the delayed rectifier current by the rat beta subunit

We have isolated a cDNA coding for a delayed rectifier K+ channel (RBKV1.5) from rabbit heart. The amino acid sequence of RBKV1.5 displays a homology to that of other K+ channels of Kv1.5 class. Overall amino acid identity between RBKV1.5 channel and Kv1.5 channel of other species is about 85%. RNA blot analysis revealed the expression of the primary transcript in various rabbit tissues, at the highest level in both the atrium and ventricle. When expressed in Xenopus oocytes, RBKV1.5 current showed a delayed rectifier type characteristics, which was converted to rapidly inactivating currents upon coexpression with a beta subunit.

alpha-Bungarotoxin-sensitive nicotinic receptors on bovine chromaffin cells: molecular cloning, functional expression and alternative splicing of the alpha 7 subunit

Chromaffin cells from the bovine adrenal medulla express alpha-bungarotoxin-sensitive acetylcholine receptors whose subunit composition is unknown. Northern blot analysis showed that the alpha 7 subunit, a main component of these alpha-bungarotoxin-sensitive acetylcholine receptors in avian and rat brain, is expressed in chromaffin cells. The cDNA of this bovine alpha 7 subunit was cloned by polymerase chain reaction amplification of adrenal medulla RNA for detailed characterization of structure and function. The protein-coding region revealed 92% amino acid sequence identity to rat alpha 7 and 89% to chicken alpha 7 subunits. The alpha-bungarotoxin affinity of alpha 7 homomers expressed in Xenopus oocytes was similar to that observed previously with native chromaffin alpha-bungarotoxin-sensitive acetylcholine receptors. Cross-linking and sucrose gradient experiments suggested that, like the muscular and neuronal acetylcholine receptors; the alpha 7 receptor has a pentameric structure. Upon activation with nicotinic agonists the alpha 7 receptor exhibited rapidly desensitizing cation currents that were blocked by nicotinic antagonists and showed inward rectification. The amplification of adrenal medulla RNA by reverse transcription-polymerase chain reaction methods revealed an alternatively spliced isoform of the bovine alpha 7 subunit, where the exon that codes for the M2 transmembrane segment was skipped during mRNA processing. Oocyte expression of this isoform does not yield functional channels. However, this alternative mRNA exhibits dose-dependent inhibition of alpha 7 homomer expression when coinjected with the undeleted isoform.

Cloning and functional expression of a Shaker-related voltage-gated potassium channel gene from Schistosoma mansoni (Trematoda: Digenea)

We have isolated a cDNA (SKv1.1) encoding a Shaker-related K+ channel from an adult cDNA library of the human parasitic trematode Schistosoma mansoni. The deduced amino acid sequence (512 aa, 56.5 kDa) contains 6 putative membrane-spanning domains (S1-S6) and a pore-forming domain (H5). SKv1.1 is grouped in the Shaker family, but forms a unique branch within this family, on the basis of dendrogram analysis. SKv1.1 shows significant sequence identity with most other Shaker channels, with 64-74% identity in the core region (S1-S6). It has the highest sequence identity with the K+ channel (Ak01a) from Aplysia. Northern blot analysis detected a single primary transcript of 2.8 kb. Southern blot analysis indicated that SKv1.1 is present as a single copy in the genomic DNA of S. mansoni. Expression of SKv1.1 in Xenopus oocytes produced a rapidly activating and inactivating outward K+ current which is highly sensitive to 4-aminopyridine, but is insensitive to tetraethylammonium, mast cell degranulating peptide, dendrotoxin and charybdotoxin. The presence of a Shaker homologue in Schistosoma suggests that Sh subfamilies may exist in other lower invertebrates as well as platyhelminths.

Molecular biology of voltage-gated K+ channels in heart

The recent cloning of numerous voltage-activated K+ channels provides new information concerning the architecture of K+ channel proteins. The combination of molecular genetic and biophysical methods gives us a new insight into the molecular mechanisms of K+ channel pharmacology.

Assembly of mammalian voltage-gated potassium channels: evidence for an important role of the first transmembrane segment

Three different experimental approaches were used to establish that the first transmembrane segment (S1) is important for K+ channel assembly. First, hydrodynamic analyses of in vitro translated Kv1.1 N-terminal domain containing the S1 segment coassembles to form homotetrameric complexes, whereas deletion of the S1 segment abolishes coassembly. Second, coimmunoprecipitation experiments of cotranslated Kv1.1 and Kv1.5 truncated polypeptides show that the S1 segment is essential for coimmunoprecipitation. Third, dominant negative experiments in Xenopus oocytes confirm that over-expression of either the S1 segment or the N-terminal domain is sufficient for abolishing the expression of functional Kv1.1 and Kv1.5 K+ channels. These data indicate that S1 segment plays an important role in the coassembly of homo- and heterotetrameric K+ channels.