Differential expression of KvLQT1 and its regulator IsK in mouse epithelia

KCNQ1 is the human gene responsible in most cases for the long QT syndrome, a genetic disorder characterized by anomalies in cardiac repolarization leading to arrhythmias and sudden death. KCNQ1 encodes a pore-forming K+ channel subunit termed KvLQT1 which, in association with its regulatory beta-subunit IsK (also called minK), produces the slow component of the delayed-rectifier cardiac K+ current. We used in situ hybridization to localize KvLQT1 and IsK mRNAs in various tissues from adult mice. We showed that KvLQT1 mRNA expression is widely distributed in epithelial tissues, in the absence (small intestine, lung, liver, thymus) or presence (kidney, stomach, exocrine pancreas) of its regulator IsK. In the kidney and the stomach, however, the expression patterns of KvLQT1 and IsK do not coincide. In many tissues, in situ data obtained with the IsK probe coincide with beta-galactosidase expression in IsK-deficient mice in which the bacterial lacZ gene has been substituted for the IsK coding region. Because expression of KvLQT1 in the presence or absence of its regulator generates a K+ current with different biophysical characteristics, the role of KvLQT1 in epithelial cells may vary depending on the expression of its regulator IsK. The high level of KvLQT1 expression in epithelial tissues is consistent with its potential role in K+ secretion and recycling, in maintaining the resting potential, and in regulating Cl- secretion and/or Na+ absorption.     

Properties of KvLQT1 K+ channel mutations in Romano-Ward and Jervell and Lange-Nielsen inherited cardiac arrhythmias.

Mutations in the delayed rectifier K+ channel subunit KvLQT1 have been identified as responsible for both Romano-Ward (RW) and Jervell and Lange-Nielsen (JLN) inherited long QT syndromes. We report the molecular cloning of a human KvLQT1 isoform that is expressed in several human tissues including heart. Expression studies revealed that the association of KvLQT1 with another subunit, IsK, reconstitutes a channel responsible for the IKs current involved in ventricular myocyte repolarization. Six RW and two JLN mutated KvLQT1 subunits were produced and co-expressed with IsK in COS cells. All the mutants, except R555C, fail to produce functional homomeric channels and reduce the K+ current when co-expressed with the wild-type subunit. Thus, in both syndromes, the main effect of the mutations is a dominant-negative suppression of KvLQT1 function. The JLN mutations have a smaller dominant-negative effect, in agreement with the fact that the disease is recessive. The R555C subunit forms a functional channel when expressed with IsK, but with altered gating properties. The voltage dependence of the activation is strongly shifted to more positive values, and deactivation kinetics are accelerated. This finding indicates the functional importance of a small positively charged cytoplasmic region of the KvLQT structure where two RW and one JLN mutations have been found to take place.     

Contribution of the IsK (MinK) potassium channel subunit to regulatory volume decrease in murine tracheal epithelial cells

The cell volume regulatory response following hypotonic shocks is often achieved by the coordinated activation of K(+) and Cl(-) channels. In this study, we investigate the identity of the K(+) and Cl(-) channels that mediate the regulatory volume decrease (RVD) in ciliated epithelial cells from murine trachea. RVD was inhibited by tamoxifen and 1,9-dideoxyforskolin, two agents that block swelling-activated Cl(-) channels. These data suggest that swelling-activated Cl(-) channels play an important role in cell volume regulation in murine tracheal epithelial cells. Ba(2+) and apamin, inhibitors of K(+) channels, were without effect on RVD, while tetraethylammoniun had little effect on RVD. In contrast, clofilium, an inhibitor of the KvLQT/IsK potassium channel complex potently inhibited RVD, suggesting a role for the KvLQT/IsK channel complex in cell volume regulation by tracheal epithelial cells. To investigate further the role of KvLQT/IsK channels in RVD, we used IsK knock-out mice. When exposed to hypotonic solutions, tracheal cells from IsK(+/+) mice underwent RVD, whereas cells from IsK(-/-) failed to recover their normal size. These data suggest that the IsK potassium subunit plays an important role in RVD in murine tracheal epithelial cells.     

Differential expression of KvLQT1 isoforms across the human ventricular wall

Long Q-T mutant (KvLQT1) K(+) channels associate with their regulatory subunit IsK to produce the slow component of the delayed rectifier potassium (I(Ks)) cardiac current. The amplitude of KvLQT1 current depends on the expression of a KvLQT1 splice variant (isoform 2) that exerts strong dominant negative effects on the full-length KvLQT1 protein (isoform 1). We used RNase protection assays to determine the relative expression of KvLQT1 isoforms 1 and 2 and IsK mRNAs in human ventricular layers. Overall expression of KvLQT1 and IsK genes was similar in the three layers. However, there was a significant difference in the ratio between KvLQT1 isoforms 1 and 2. Isoform 2 represented 25.2 +/- 2.3%, 31.7 +/- 1.2%, and 24.9 +/- 1.7% of total KvLQT1 expression in left ventricular endocardial, midmyocardial, and epicardial tissues, respectively. Similar data were obtained from right ventricular samples. COS-7 cells were intranuclearly injected with KvLQT1 isoforms 1 or 2 plus IsK cDNAs, using two different isoform 2-to-isoform 1 ratios. Cells injected with an isoform 2-to-isoform 1 ratio mimicking that in the midmyocardium showed a K(+) current with approximately 75% reduced amplitude compared with those injected with a ratio mimicking that in the epicardium. Our results suggest that differential expression of KvLQT1 isoform 2 in endocardial, midmyocardial, and epicardial tissues is responsible for differential I(Ks) amplitude and contributes to the regional action potential heterogeneity observed across the ventricular wall.     

AKAP proteins anchor cAMP-dependent protein kinase to KvLQT1/IsK channel complex

In cardiac myocytes, the slow component of the delayed rectifier K(+) current (I(Ks)) is regulated by cAMP. Elevated cAMP increases I(Ks) amplitude, slows its deactivation kinetics, and shifts its activation curve. At the molecular level, I(Ks) channels are composed of KvLQT1/IsK complexes. In a variety of mammalian heterologous expression systems maintained at physiological temperature, we explored cAMP regulation of recombinant KvLQT1/IsK complexes. In these systems, KvLQT1/IsK complexes were totally insensitive to cAMP regulation. cAMP regulation was not restored by coexpression with the dominant negative isoform of KvLQT1 or with the cystic fibrosis transmembrane regulator. In contrast, coexpression of the neuronal A kinase anchoring protein (AKAP)79, a fragment of a cardiac AKAP (mAKAP), or cardiac AKAP15/18 restored cAMP regulation of KvLQT1/IsK complexes inasmuch as cAMP stimulation increased the I(Ks) amplitude, increased its deactivation time constant, and negatively shifted its activation curve. However, in cells expressing an AKAP, the effects of cAMP stimulation on the I(Ks) amplitude remained modest compared with those previously reported in cardiac myocytes. The effects of cAMP stimulation were fully prevented by including the Ht31 peptide (a global disruptor of protein kinase A anchoring) in the intracellular medium. We concluded that cAMP regulation of I(Ks) requires protein kinase A anchoring by AKAPs, which therefore participate with the channel protein complex underlying I(Ks).     

KCNE beta subunits determine pH sensitivity of KCNQ1 potassium channels.

BACKGROUND/AIMS: Heteromeric KCNEx/KCNQ1 (=KvLQT1, Kv7.1) K(+) channels are important for repolarization of cardiac myocytes, endolymph secretion in the inner ear, gastric acid secretion, and transport across epithelia. They are modulated by pH in a complex way: homomeric KCNQ1 is inhibited by external acidification (low pH(e)); KCNE2/KCNQ1 is activated; and for KCNE1/KCNQ1, variable effects have been reported. Methods: The role of KCNE subunits for the effect of pH(e) on KCNQ1 was analyzed in transfected COS cells and cardiac myocytes by the patch-clamp technique. RESULTS: In outside-out patches of transfected cells, hKCNE2/hKCNQ1 current was increased by acidification down to pH 4.5. Chimeras with the acid-insensitive hKCNE3 revealed that the extracellular N-terminus and at least part of the transmembrane domain of hKCNE2 are needed for activation by low pH(e). hKCNE1/hKCNQ1 heteromeric channels exhibited marked changes of biophysical properties at low pH(e): The slowly activating hKCNE1/hKCNQ1 channels were converted into constitutively open, non-deactivating channels. Experiments on guinea pig and mouse cardiac myocytes pointed to an important role of KCNQ1 during acidosis implicating a significant contribution to cardiac repolarization under acidic conditions. CONCLUSION: External pH can modify current amplitude and biophysical properties of KCNQ1. KCNE subunits work as molecular switches by modulating the pH sensitivity of human KCNQ1.     

A recessive C-terminal Jervell and Lange-Nielsen mutation of the KCNQ1 channel impairs subunit assembly

The LQT1 locus (KCNQ1) has been correlated with the most common form of inherited long QT (LQT) syndrome. LQT patients suffer from syncopal episodes and high risk of sudden death. The KCNQ1 gene encodes KvLQT1 alpha-subunits, which together with auxiliary IsK (KCNE1, minK) subunits form IK(s) K(+) channels. Mutant KvLQT1 subunits may be associated either with an autosomal dominant form of inherited LQT, Romano-Ward syndrome, or an autosomal recessive form, Jervell and Lange-Nielsen syndrome (JLNS). We have identified a small domain between residues 589 and 620 in the KvLQT1 C-terminus, which may function as an assembly domain for KvLQT1 subunits. KvLQT1 C-termini do not assemble and KvLQT1 subunits do not express functional K(+) channels without this domain. We showed that a JLN deletion-insertion mutation at KvLQT1 residue 544 eliminates important parts of the C-terminal assembly domain. Therefore, JLN mutants may be defective in KvLQT1 subunit assembly. The results provide a molecular basis for the clinical observation that heterozygous JLN carriers show slight cardiac dysfunctions and that the severe JLNS phenotype is characterized by the absence of KvLQT1 channel.     

Upregulation of KCNQ1/KCNE1 K+ channels by Klotho

Klotho is a transmembrane protein expressed primarily in kidney, parathyroid gland, and choroid plexus. The extracellular domain could be cleaved off and released into the systemic circulation. Klotho is in part effective as β-glucuronidase regulating protein stability in the cell membrane. Klotho is a major determinant of aging and life span. Overexpression of Klotho increases and Klotho deficiency decreases life span. Klotho deficiency may further result in hearing loss and cardiac arrhythmia. The present study explored whether Klotho modifies activity and protein abundance of KCNQ1/KCNE1, a K⁺ channel required for proper hearing and cardiac repolarization. To this end, cRNA encoding KCNQ1/KCNE1 was injected in Xenopus oocytes with or without additional injection of cRNA encoding Klotho. KCNQ1/KCNE1 expressing oocytes were treated with human recombinant Klotho protein (30 ng/ml) for 24 h. Moreover, oocytes which express both KCNQ1/KCNE1 and Klotho were treated with 10 µM DSAL (D-saccharic acid-1,4-lactone), a β-glucuronidase inhibitor. The KCNQ1/KCNE1 depolarization-induced current (I_Ks) was determined utilizing dual electrode voltage clamp, while KCNQ1/KCNE1 protein abundance in the cell membrane was visualized utilizing specific antibody binding and quantified by chemiluminescence. KCNQ1/KCNE1 channel activity and KCNQ1/KCNE1 protein abundance were upregulated by coexpression of Klotho. The effect was mimicked by treatment with human recombinant Klotho protein (30 ng/ml) and inhibited by DSAL (10 µM). In conclusion, Klotho upregulates KCNQ1/KCNE1 channel activity by 'mainly' enhancing channel protein abundance in the plasma cell membrane, an effect at least partially mediated through the β-glucuronidase activity of Klotho protein.     

Inhibition of the heterotetrameric K+ channel KCNQ1/KCNE1 by the AMP-activated protein kinase

The heterotetrameric K(+)-channel KCNQ1/KCNE1 is expressed in heart, skeletal muscle, liver and several epithelia including the renal proximal tubule. In the heart, it contributes to the repolarization of cardiomyocytes. The repolarization is impaired in ischemia. Ischemia stimulates the AMP-activated protein kinase (AMPK), a serine/threonine kinase, sensing energy depletion and stimulating several cellular mechanisms to enhance energy production and to limit energy utilization. AMPK has previously been shown to downregulate the epithelial Na(+) channel ENaC, an effect mediated by the ubiquitin ligase Nedd4-2. The present study explored whether AMPK regulates KCNQ1/KCNE1. To this end, cRNA encoding KCNQ1/KCNE1 was injected into Xenopus oocytes with and without additional injection of wild type AMPK (AMPKα1 + AMPKβ1 + AMPKγ1), of the constitutively active (γR70Q)AMPK (α1β1γ1(R70Q)), of the kinase dead mutant (αK45R)AMPK (α1(K45R)β1γ1), or of the ubiquitin ligase Nedd4-2. KCNQ1/KCNE1 activity was determined in two electrode voltage clamp experiments. Moreover, KCNQ1 abundance in the cell membrane was determined by immunostaining and subsequent confocal imaging. As a result, wild type and constitutively active AMPK significantly reduced KCNQ1/KCNE1-mediated currents and reduced KCNQ1 abundance in the cell membrane. Similarly, Nedd4-2 decreased KCNQ1/KCNE1-mediated currents and KCNQ1 protein abundance in the cell membrane. Activation of AMPK in isolated perfused proximal renal tubules by AICAR (10 mM) was followed by significant depolarization. In conclusion, AMPK is a potent regulator of KCNQ1/KCNE1.     

Upregulation of KCNQ1/KCNE1 K+ channels by Klotho

Klotho is a transmembrane protein expressed primarily in kidney, parathyroid gland, and choroid plexus. The extracellular domain could be cleaved off and released into the systemic circulation. Klotho is in part effective as β-glucuronidase regulating protein stability in the cell membrane. Klotho is a major determinant of aging and life span. Overexpression of Klotho increases and Klotho deficiency decreases life span. Klotho deficiency may further result in hearing loss and cardiac arrhythmia. The present study explored whether Klotho modifies activity and protein abundance of KCNQ1/KCNE1, a K⁺ channel required for proper hearing and cardiac repolarization. To this end, cRNA encoding KCNQ1/KCNE1 was injected in Xenopus oocytes with or without additional injection of cRNA encoding Klotho. KCNQ1/KCNE1 expressing oocytes were treated with human recombinant Klotho protein (30 ng/ml) for 24 h. Moreover, oocytes which express both KCNQ1/KCNE1 and Klotho were treated with 10 µM DSAL (D-saccharic acid-1,4-lactone), a β-glucuronidase inhibitor. The KCNQ1/KCNE1 depolarization-induced current (I_Ks) was determined utilizing dual electrode voltage clamp, while KCNQ1/KCNE1 protein abundance in the cell membrane was visualized utilizing specific antibody binding and quantified by chemiluminescence. KCNQ1/KCNE1 channel activity and KCNQ1/KCNE1 protein abundance were upregulated by coexpression of Klotho. The effect was mimicked by treatment with human recombinant Klotho protein (30 ng/ml) and inhibited by DSAL (10 µM). In conclusion, Klotho upregulates KCNQ1/KCNE1 channel activity by 'mainly' enhancing channel protein abundance in the plasma cell membrane, an effect at least partially mediated through the β-glucuronidase activity of Klotho protein.     

Mutations in a dominant-negative isoform correlate with phenotype in inherited cardiac arrhythmias

The long QT syndrome is characterized by prolonged cardiac repolarization and a high risk of sudden death. Mutations in the KCNQ1 gene, which encodes the cardiac KvLQT1 potassium ion (K+) channel, cause both the autosomal dominant Romano-Ward (RW) syndrome and the recessive Jervell and Lange-Nielsen (JLN) syndrome. JLN presents with cardiac arrhythmias and congenital deafness, and heterozygous carriers of JLN mutations exhibit a very mild cardiac phenotype. Despite the phenotypic differences between heterozygotes with RW and those with JLN mutations, both classes of variant protein fail to produce K+ currents in cultured cells. We have shown that an N-terminus-truncated KvLQT1 isoform endogenously expressed in the human heart exerts strong dominant-negative effects on the full-length KvLQT1 protein. Because RW and JLN mutations concern both truncated and full-length KvLQT1 isoforms, we investigated whether RW or JLN mutations would have different impacts on the dominant-negative properties of the truncated KvLQT1 splice variant. In a mammalian expression system, we found that JLN, but not RW, mutations suppress the dominant-negative effects of the truncated KvLQT1. Thus, in JLN heterozygous carriers, the full-length KvLQT1 protein encoded by the unaffected allele should not be subject to the negative influence of the mutated truncated isoform, leaving some cardiac K+ current available for repolarization. This is the first report of a genetic disease in which the impact of a mutation on a dominant-negative isoform correlates with the phenotype.     

The multifaceted phenotype of the knockout mouse for the KCNE1 potassium channel gene

Mutations of the KCNE1 gene (IsK, minK) are related to hereditary forms of cardiac arrhythmias, so-called long QT syndromes (LQT). Here we review the phenotype of a mouse model for the recessive form of LQT known as Jervell and Lange-Nielsen syndrome. KCNE1 knockout mice exhibit an enhanced QT-RR adaptability, which is probably part of the pathophysiological mechanism leading to life-threatening tachyarrhythmia in patients. Like patients, knockout mice are deaf and show vestibular symptoms due to an impaired endolymph production. Knockout mice show urinary and fecal salt wasting and volume depletion. The renal phenotype is due to diminished reabsorption of Na(+) and glucose. The mice are hypokalemic and have increased aldosterone levels. Besides volume depletion, aldosterone is elevated via a set-point shift for sensing of extracellular K(+) in aldosterone-secreting glomerulosa cells, which physiologically express KCNE1. In conclusion, KCNE1 knockout leads to a complex phenotype resulting from direct loss of KCNE1 and compensatory mechanisms. Murine KCNE1 physiology could be helpful for the pathophysiological understanding and perhaps gene-specific treatment of long QT patients.     

Expression and coassociation of ERG1, KCNQ1, and KCNE1 potassium channel proteins in horse heart

In dogs and in humans, potassium channels formed by ether-a-go-go-related gene 1 protein ERG1 (KCNH2) and KCNQ1 alpha-subunits, in association with KCNE beta-subunits, play a role in normal repolarization and may contribute to abnormal repolarization associated with long QT syndrome (LQTS). The molecular basis of repolarization in horse heart is unknown, although horses exhibit common cardiac arrhythmias and may receive drugs that induce LQTS. In horse heart, we have used immunoblotting and immunostaining to demonstrate the expression of ERG1, KCNQ1, KCNE1, and KCNE3 proteins and RT-PCR to detect KCNE2 message. Peptide N-glycosidase F-sensitive forms of horse ERG1 (145 kDa) and KCNQ1 (75 kDa) were detected. Both ERG1 and KCNQ1 coimmunoprecipitated with KCNE1. Cardiac action potential duration was prolonged by antagonists of either ERG1 (MK-499, cisapride) or KCNQ1/KCNE1 (chromanol 293B). Patch-clamp analysis confirmed the presence of a slow delayed rectifier current. These data suggest that repolarizing currents in horses are similar to those of other species, and that horses are therefore at risk for acquired LQTS. The data also provide unique evidence for coassociation between ERG1 and KCNE1 in cardiac tissue.     

The KCNE2 potassium channel ancillary subunit is essential for gastric acid secretion

Genes in the KCNE family encode single transmembrane domain ancillary subunits that co-assemble with voltage-gated potassium (Kv) channel alpha subunits to alter their function. KCNE2 (also known as MiRP1) is expressed in the heart, is associated with human cardiac arrhythmia, and modulates cardiac Kv alpha subunits hERG and KCNQ1 in vitro. KCNE2 and KCNQ1 are also expressed in parietal cells, leading to speculation they form a native channel complex there. Here, we disrupted the murine kcne2 gene and found that kcne2 (-/-) mice have a severe gastric phenotype with profoundly reduced parietal cell proton secretion, abnormal parietal cell morphology, achlorhydria, hypergastrinemia, and striking gastric glandular hyperplasia arising from an increase in the number of non-acid secretory cells. KCNQ1 exhibited abnormal distribution in gastric glands from kcne2 (-/-) mice, with increased expression in non-acid secretory cells. Parietal cells from kcne2 (+/-) mice exhibited normal architecture but reduced proton secretion, and kcne2 (+/-) mice were hypochlorhydric, indicating a gene-dose effect and a primary defect in gastric acid secretion. These data demonstrate that KCNE2 is essential for gastric acid secretion, the first genetic evidence that a member of the KCNE gene family is required for normal gastrointestinal function.     

Inhibition of the heterotetrameric K+ channel KCNQ1/KCNE1 by the AMP-activated protein kinase

Abstract

The heterotetrameric K⁺-channel KCNQ1/KCNE1 is expressed in heart, skeletal muscle, liver and several epithelia including the renal proximal tubule. In the heart, it contributes to the repolarization of cardiomyocytes. The repolarization is impaired in ischemia. Ischemia stimulates the AMP-activated protein kinase (AMPK), a serine/threonine kinase, sensing energy depletion and stimulating several cellular mechanisms to enhance energy production and to limit energy utilization. AMPK has previously been shown to downregulate the epithelial Na⁺ channel ENaC, an effect mediated by the ubiquitin ligase Nedd4-2. The present study explored whether AMPK regulates KCNQ1/KCNE1. To this end, cRNA encoding KCNQ1/KCNE1 was injected into Xenopus oocytes with and without additional injection of wild type AMPK (AMPKα1 + AMPKβ1 + AMPKγ1), of the constitutively active ^γR70QAMPK (α1β1γ1(R70Q)), of the kinase dead mutant ^αK45RAMPK (α1(K45R)β1γ1), or of the ubiquitin ligase Nedd4-2. KCNQ1/KCNE1 activity was determined in two electrode voltage clamp experiments. Moreover, KCNQ1 abundance in the cell membrane was determined by immunostaining and subsequent confocal imaging. As a result, wild type and constitutively active AMPK significantly reduced KCNQ1/KCNE1-mediated currents and reduced KCNQ1 abundance in the cell membrane. Similarly, Nedd4-2 decreased KCNQ1/KCNE1-mediated currents and KCNQ1 protein abundance in the cell membrane. Activation of AMPK in isolated perfused proximal renal tubules by AICAR (10 mM) was followed by significant depolarization. In conclusion, AMPK is a potent regulator of KCNQ1/KCNE1.     

Maintaining T lymphocyte homeostasis: another duty of autophagy

First identified as a pathway for nutrient recovery during periods of starvation, the role of autophagy has expanded to the clearance of "toxic" intracellular material including ubiquitin-positive protein aggregates, damaged organelles as well as microbial pathogens in various cell types. We have examined the role of autophagy in the development and function of the adaptive immune system. Genes encoding autophagy machinery are expressed in T lymphocytes, and autophagy occurs in primary CD4+ and CD8+ T cells. By generating fetal liver chimeric mice, we found that thymocyte development is largely normal but the mature T cell compartment is severely reduced in the absence of the essential autophagy gene Atg5. Consistent with a critical role for autophagy in promoting T cell survival, Atg5-/- CD8+ T cells display high levels of apoptosis. Surprisingly, Atg5-deficient T cells were also unable to efficiently proliferate after T-cell receptor (TCR) stimulation. These findings suggest that autophagy regulates T lymphocyte homeostasis by promoting both survival and proliferation. In addition, T cells offer a new, physiologically relevant system to study the regulation and function of autophagy pathways in vivo.     

KvLQT1 modulates the distribution and biophysical properties of HERG. A novel alpha-subunit interaction between delayed rectifier currents

Cardiac repolarization is under joint control of the slow (IKs) and rapid (IKr) delayed rectifier currents. Experimental and clinical evidence indicates important functional interactions between these components. We hypothesized that there might be more direct interactions between the KvLQT1 and HERG alpha-subunits of IKs and IKr and tested this notion with a combination of biophysical and biochemical techniques. Co-expression of KvLQT1 with HERG in a mammalian expression system significantly accelerated HERG current deactivation at physiologically relevant potentials by increasing the contribution of the fast component (e.g. upon repolarization from +20 mV to -50 mV: from 20 +/- 3 to 32 +/- 5%, p < 0.05), making HERG current more like native IKr. In addition, HERG current density was approximately doubled (e.g. tail current after a step to +10 mV: 18 +/- 3 versus 39 +/- 7 pA/picofarad, p < 0.01) by co-expression with KvLQT1. KvLQT1 co-expression also increased the membrane immunolocalization of HERG by approximately 2-fold (p < 0.05). HERG and KvLQT1 co-immunolocalized in canine ventricular myocytes and co-immunoprecipitated in cultured Chinese hamster ovary cells as well as in native cardiac tissue, indicating physical interactions between HERG and KvLQT1 proteins in vitro and in vivo. Protein interaction assays also demonstrated binding of KvLQT1 (but not another K+ channel alpha-subunit, Kv3.4) to a C-terminal HERG glutathione S-transferase fusion protein. Co-expression with HERG did not affect the membrane localization or ionic current properties of KvLQT1. This study shows that the alpha-subunit of IKs can interact with and modify the localization and current-carrying properties of the alpha-subunit of IKr, providing potentially novel insights into the molecular function of the delayed rectifier current system.     

Structural determinants of KvLQT1 control by the KCNE family of proteins

KvLQT1 is a Shaker-like voltage-gated potassium channel that when complexed with minK (KCNE1) produces the slowly activating delayed rectifier I(ks). The emerging family of KCNE1-related peptides includes KCNE1 and KCNE3, both of which complex with KvLQT1 to produce functionally distinct currents. Namely I(ks), the slowly activating delayed rectifier current, is produced by KvLQT1/KCNE1, whereas KvLQT1/KCNE3 yields a more rapidly activating current with a distinct constitutively active component. We exploited these functional differences and the general structural similarities of KCNE1 and KCNE3 to study which physical regions are critical for control of KvLQT1 by making chimerical constructs of KCNE1 and KCNE3. By using this approach, we have found that a three-amino acid stretch within the transmembrane domain is necessary and sufficient to confer specificity of control of activation kinetics by KCNE1 and KCNE3. Moreover, chimera analysis showed that different regions within the transmembrane domain control deactivation rates. Our results help to provide a basis for understanding the mechanism by which KCNE proteins control K(+) channel activity.     

KCNE4 can co-associate with the I(Ks) (KCNQ1-KCNE1) channel complex

Voltage-gated potassium (K(V)) channels can form heteromultimeric complexes with a variety of accessory subunits, including KCNE proteins. Heterologous expression studies have demonstrated diverse functional effects of KCNE subunits on several K(V) channels, including KCNQ1 (K(V)7.1) that, together with KCNE1, generates the slow-delayed rectifier current (I(Ks)) important for cardiac repolarization. In particular, KCNE4 exerts a strong inhibitory effect on KCNQ1 and other K(V) channels, raising the possibility that this accessory subunit is an important potassium current modulator. A polyclonal KCNE4 antibody was developed to determine the human tissue expression pattern and to investigate the biochemical associations of this protein with KCNQ1. We found that KCNE4 is widely and variably expressed in several human tissues, with greatest abundance in brain, liver and testis. In heterologous expression experiments, immunoprecipitation followed by immunoblotting was used to establish that KCNE4 directly associates with KCNQ1, and can co-associate together with KCNE1 in the same KCNQ1 complex to form a 'triple subunit' complex (KCNE1-KCNQ1-KCNE4). We also used cell surface biotinylation to demonstrate that KCNE4 does not impair plasma membrane expression of either KCNQ1 or the triple subunit complex, indicating that biophysical mechanisms probably underlie the inhibitory effects of KCNE4. The observation that multiple KCNE proteins can co-associate with and modulate KCNQ1 channels to produce biochemically diverse channel complexes has important implications for understanding K(V) channel regulation in human physiology.