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

Crystal structure and functional analysis of the HERG potassium channel N terminus: a eukaryotic PAS domain

The HERG voltage-dependent K+ channel plays a role in cardiac electrical excitability, and when defective, it underlies one form of the long QT syndrome. We have determined the crystal structure of the HERG K+ channel N-terminal domain and studied its role as a modifier of gating using electrophysiological methods. The domain is similar in structure to a bacterial light sensor photoactive yellow protein and provides the first three-dimensional model of a eukaryotic PAS domain. Scanning mutagenesis of the domain surface has allowed the identification of a hydrophobic "hot spot" forming a putative interface with the body of the K+ channel to which it tightly binds. The presence of the domain attached to the channel slows the rate of deactivation. Given the roles of PAS domains in biology, we propose that the HERG N-terminal domain has a regulatory function.

Regulation of deactivation by an amino terminal domain in human ether-à-go-go-related gene potassium channels

Abnormalities in repolarization of the cardiac ventricular action potential can lead to life-threatening arrhythmias associated with long QT syndrome. The repolarization process depends upon the gating properties of potassium channels encoded by the human ether-à-go-go-related gene (HERG), especially those governing the rate of recovery from inactivation and the rate of deactivation. Previous studies have demonstrated that deletion of the NH2 terminus increases the deactivation rate, but the mechanism by which the NH2 terminus regulates deactivation in wild-type channels has not been elucidated. We tested the hypothesis that the HERG NH2 terminus slows deactivation by a mechanism similar to N-type inactivation in Shaker channels, where it binds to the internal mouth of the pore and prevents channel closure. We found that the regulation of deactivation by the HERG NH2 terminus bears similarity to Shaker N-type inactivation in three respects: (a) deletion of the NH2 terminus slows C-type inactivation; (b) the action of the NH2 terminus is sensitive to elevated concentrations of external K+, as if its binding along the permeation pathway is disrupted by K+ influx; and (c) N-ethylmaleimide, covalently linked to an aphenotypic cysteine introduced within the S4-S5 linker, mimics the N deletion phenotype, as if the binding of the NH2 terminus to its receptor site were hindered. In contrast to N-type inactivation in Shaker, however, there was no indication that the NH2 terminus blocks the HERG pore. In addition, we discovered that separate domains within the NH2 terminus mediate the slowing of deactivation and the promotion of C-type inactivation. These results suggest that the NH2 terminus stabilizes the open state and, by a separate mechanism, promotes C-type inactivation.

A K+ channel splice variant common in human heart lacks a C-terminal domain required for expression of rapidly activating delayed rectifier current

We have cloned HERG USO, a C-terminal splice variant of the human ether-à-go-go-related gene (HERG), the gene encoding the rapid component of the delayed rectifier (IKr), from human heart, and we find that its mRNA is approximately 2-fold more abundant than that for HERG1 (the originally described cDNA). After transfection of HERG USO in Ltk- cells, no current was observed. However, coexpression of HERG USO with HERG1 modified IKr by decreasing its amplitude, accelerating its activation, and shifting the voltage dependence of activation 8.8 mV negative. As with HERG USO, HERGDeltaC (a HERG1 construct lacking the C-terminal 462 amino acids) also produced no current in transfected cells. However, IKr was rescued by ligation of 104 amino acids from the C terminus of HERG1 to the C terminus of HERGDeltaC, indicating that the C terminus of HERG1 includes a domain (</=104 amino acids) that is critical for faithful recapitulation of IKr. The lack of this C-terminal domain not only explains the finding that HERG USO does not generate IKr but also indicates a similar mechanism for hitherto-uncharacterized long QT syndrome HERG mutations that disrupt the splice site or the C-terminal. We suggest that the amplitude and gating of cardiac IKr depends on expression of both HERG1 and HERG USO.

Ion channel mutations affecting muscle and brain

Voltage-gated ion channels control many aspects of signal transduction in both muscle and nerve. These channels are finely tuned, and either increased or decreased channel activity can adversely affect function. In skeletal muscle, mutations in ion channel genes have been shown to cause both myotonic discharges and episodic paralysis. In cardiac muscle, mutations in related ion channels can produce repolarization defects and fatal arrhythmias. During the past 2 years, a new chapter in the channelopathy story has been opened with the identification of ion channel mutations in the brain that cause disorders ranging from episodic ataxia to epilepsy.

Inhibition of cardiac delayed rectifier K+ current by overexpression of the long-QT syndrome HERG G628S mutation in transgenic mice

Mutations in the HERG gene are linked to the LQT2 form of the inherited long-QT syndrome. Transgenic mice were generated expressing high myocardial levels of a particularly severe form of LQT2-associated HERG mutation (G628S). Hearts from G628S mice appeared normal except for a modest enlargement seen only in females. Ventricular myocytes isolated from adult wild-type hearts consistently exhibited an inwardly rectifying E-4031-sensitive K+ current resembling the rapidly activating cardiac delayed rectifier K+ current (Ikr) in its time and voltage dependence; this current was not found in cells isolated from G628S mice. Action potential duration was significantly prolonged in single myocytes from G628S ventricle (cycle length=1 second, 26 degrees C) but not in recordings from intact ventricular strips studied at more physiological rates and temperature (200 to 400 bpm, 37 degrees C). ECG intervals, including QT duration, were unchanged, although minor aberrancies were noted in 20% (16/80) of the G628S mice studied, primarily involving the QRS complex and, more rarely, T-wave morphology. The aberrations were more commonly observed in females than males but could not be correlated with sex-based differences in action potential duration. These results establish the presence of IKr in the adult mouse ventricle and demonstrate the ability of the G628S mutation to exert a dominant negative effect on endogenous IKr in vivo, leading to the expected LQT2 phenotype of prolonged repolarization at the single cell level but not QT prolongation in the intact animal. The model may be useful in dissecting repolarization currents in the mouse heart and as a means of examining the mechanism(s) by which the G628S mutation exerts its dominant negative effect on native cardiac cells in vivo.

Shaker and ether-à-go-go K+ channel subunits fail to coassemble in Xenopus oocytes

Members of different voltage-gated K+ channel subfamilies usually do not form heteromultimers. However, coassembly between Shaker and ether-à-go-go (eag) subunits, members of two distinct K+ channel subfamilies, was suggested by genetic and functional studies (Zhong and Wu. 1991. Science. 252: 1562-1564; Chen, M.-L., T. Hoshi, and C.-F. Wu. 1996. Neuron. 17:535-542). We investigated whether Shaker and eag form heteromultimers in Xenopus laevis oocytes using electrophysiological and biochemical approaches. Coexpression of Shaker and eag subunits produced K+ currents that were virtually identical to the sum of separate Shaker and eag currents, with no change in the kinetics of Shaker inactivation. According to the results of dominant negative and reciprocal coimmunoprecipitation experiments, the Shaker and eag proteins do not interact. We conclude that Shaker and eag do not coassemble to form heteromultimers in Xenopus oocytes.

Novel mechanism of HERG current suppression in LQT2: shift in voltage dependence of HERG inactivation

In a Xenopus oocyte heterologous expression system, we characterized the electrophysiology of 3 novel missense mutations of HERG identified in Japanese LQT2 families: T474I (within the S2-S3 linker), A614V, and V630L (in the outer mouth of pore-forming region). For each of the 3 mutations, injection of mutant cRNA alone did not express detectable currents. Coinjection of wild-type (WT) along with each mutant cRNA (T474I/WT, A614V/WT, and V630L/WT) suppressed HERG current in a dominant-negative manner, and the order of magnitude of current suppression was V630L/WT>A614V/WT>T474I/WT. In addition to decreases in slope conductance for all 3 mutants, the voltage dependence of steady-state inactivation was shifted to negative potentials for V630L/WT and A614V/WT. Consequently, channel availability at positive potentials was diminished, and inward rectification was enhanced for these 2 mutants. Thus, missense mutations of HERG caused dominant-negative suppression through multiple mechanisms. The shift in voltage dependence of HERG inactivation and the resulting enhanced inward rectification in A614V/WT and V630L/WT provide a novel mechanism for suppression of the HERG current carrying outward current during the repolarization phase of the action potential.

Cloning and characterization of a cAMP-specific cyclic nucleotide phosphodiesterase

Cyclic nucleotide phosphodiesterases (PDEs) regulate intracellular levels of cAMP and cGMP by hydrolyzing them to their corresponding 5' monophosphates. We report here the cloning and characterization of a novel cAMP-specific PDE from mouse testis. This unique phosphodiesterase contains a catalytic domain that overall shares <40% sequence identity to the catalytic domain of all other known PDEs. Based on this limited homology, this new PDE clearly represents a previously unknown PDE gene family designated as PDE8. The cDNA for PDE8 is 3,678 nucleotides in length and is predicted to encode an 823 amino acid enzyme. The cDNA includes a full ORF as it contains an in-frame stop codon before the start methionine. PDE8 is specific for the hydrolysis of cAMP and has a Km of 0.15 microM. Most common PDE inhibitors are ineffective antagonists of PDE8, including the nonspecific PDE inhibitor 3-isobutyl-1-methylxanthine. Dipyridamole, however, an inhibitor that is generally considered to be relatively specific for the cGMP selective PDEs, does inhibit PDE8 with an IC50 of 4.5 microM. Tissue distribution studies of 22 different mouse tissues indicates that PDE8 has highest expression in testis, followed by eye, liver, skeletal muscle, heart, 7-day embryo, kidney, ovary, and brain in decreasing order. In situ hybridizations in testis, the tissue of highest expression, shows that PDE8 is expressed in the seminiferous epithelium in a stage-specific manner. Highest levels of expression are seen in stages 7-12, with little or no expression in stages 1-6.

Human ether-a-gogo related gene (HERG) K+ channels as pharmacological targets: present and future implications

Electrophysiological and molecular biology techniques have widely expanded our knowledge of the diverse functions where K+ channels are implicated as potential and proven pharmacological targets. The aim of the present commentary is to review the recent progress in the understanding of the functional role of the K+ channels encoded by the human ether-a-gogo related gene (HERG), with particular emphasis on their direct pharmacological modulation by drugs, or on their regulation by pharmacologically relevant phenomena. About 3 years have passed since the cloning, expression, and description of the pathophysiological role of HERG K+ channels in human cardiac repolarization. Despite this short lapse of time, these K+ channels have already gained considerable attention as pharmacological targets. In fact, interference with HERG K+ channels seems to be the main mechanism explaining both the therapeutic actions of the class III antiarrhythmics and the potential cardiotoxicity of second-generation H1 receptor antagonists such as terfenadine and astemizole, as well as of psychotropic drugs such as some antidepressants and neuroleptics. It seems possible to anticipate that the main tasks for future investigation will be, on the one side, the better understanding of the intimate mechanism of action of HERG K+ channel-blocking drugs in order to elucidate the conditions regulating the delicate balance between antiarrhythmic and proarrhythmic potential and, on the other, to unravel the pathophysiological role of this K+ channel in the function of the brain and of other excitable tissues.

Two distantly positioned PDZ domains mediate multivalent INAD-phospholipase C interactions essential for G protein-coupled signaling

Drosophila INAD, which contains five tandem protein interaction PDZ domains, plays an important role in the G protein-coupled visual signal transduction. Mutations in InaD alleles display mislocalization of signaling molecules of phototransduction which include the essential effector, phospholipase C-beta (PLC-beta), which is also known as NORPA. The molecular and biochemical details of this functional link are unknown. We report that INAD directly binds to NORPA via two terminally positioned PDZ1 and PDZ5 domains. PDZ1 binds to the C-terminus of NORPA, while PDZ5 binds to an internal region overlapping with the G box-homology region (a putative G protein-interacting site). The NORPA proteins lacking binding sites, which display normal basal PLC activity, can no longer associate with INAD in vivo. These truncations cause significant reduction of NORPA protein expression in rhabdomeres and severe defects in phototransduction. Thus, the two terminal PDZ domains of INAD, through intermolecular and/or intramolecular interactions, are brought into proximity in vivo. Such domain organization allows for the multivalent INAD-NORPA interactions which are essential for G protein-coupled phototransduction.

The redox- and fixed nitrogen-responsive regulatory protein NIFL from Azotobacter vinelandii comprises discrete flavin and nucleotide-binding domains

Azotobacter vinelandii NIFL is a nitrogen fixation-specific regulatory flavoprotein that modulates the activity of the transcriptional activator NIFA in response to oxygen and fixed nitrogen in vivo. NIFL is also responsive to ADP in vitro. Limited proteolysis of NIFL indicates that it comprises a relatively stable N-terminal domain and a C-terminal domain that is protected from trypsin digestion in the presence of adenosine nucleotides. ATP protects the protein from cleavage in the vicinity of potential nucleotide-binding sites in the C-terminus, whereas ADP protects the entire C-terminal domain. NIFL has an apparent Kd of 130 microM for ATP and 16 microM for ADP. The purified N-terminal domain has an identical UV/visible absorption spectrum to the wild-type protein and is reduced by sodium dithionite, demonstrating that it is a flavin-binding domain. The isolated N-terminal domain does not inhibit NIFA activity. A subdomain fragment containing 160 residues of the C-terminal region, including the nucleotide-binding sites, is also not competent to inhibit NIFA. Removal of the first 146 residues of NIFL, which includes a conserved S-motif (PAS-like domain), found in a large family of sensory proteins from eubacteria, archea and eukarya eliminates the redox response. However, this truncated protein remains competent to inhibit NIFA activity in response to ADP in vitro and to the level of fixed nitrogen in vivo. The redox and nitrogen-sensing functions of A. vinelandii NIFL are therefore separable and are discrete functions of the protein.

Long QT and ventricular arrhythmias in transgenic mice expressing the N terminus and first transmembrane segment of a voltage-gated potassium channel

Voltage-gated potassium channels control cardiac repolarization, and mutations of K+ channel genes recently have been shown to cause arrhythmias and sudden death in families with the congenital long QT syndrome. The precise mechanism by which the mutations lead to QT prolongation and arrhythmias is uncertain, however. We have shown previously that an N-terminal fragment including the first transmembrane segment of the rat delayed rectifier K+ channel Kv1.1 (Kv1.1N206Tag) coassembles with other K+ channels of the Kv1 subfamily in vitro, inhibits the currents encoded by Kv1.5 in a dominant-negative manner when coexpressed in Xenopus oocytes, and traps Kv1.5 polypeptide in the endoplasmic reticulum of GH3 cells. Here we report that transgenic mice overexpressing Kv1.1N206Tag in the heart have a prolonged QT interval and ventricular tachycardia. Cardiac myocytes from these mice have action potential prolongation caused by a significant reduction in the density of a rapidly activating, slowly inactivating, 4-aminopyridine sensitive outward K+ current. These changes correlate with a marked decrease in the level of Kv1.5 polypeptide. Thus, overexpression of a truncated K+ channel in the heart alters native K+ channel expression and has profound effects on cardiac excitability.

Arabidopsis NPH1: a protein kinase with a putative redox-sensing domain

The NPH1 (nonphototropic hypocotyl 1) gene encodes an essential component acting very early in the signal-transduction chain for phototropism. Arabidopsis NPH1 contains a serine-threonine kinase domain and LOV1 and LOV2 repeats that share similarity (36 to 56 percent) with Halobacterium salinarium Bat, Azotobacter vinelandii NIFL, Neurospora crassa White Collar-1, Escherichia coli Aer, and the Eag family of potassium-channel proteins from Drosophila and mammals. Sequence similarity with a known (NIFL) and a suspected (Aer) flavoprotein suggests that NPH1 LOV1 and LOV2 may be flavin-binding domains that regulate kinase activity in response to blue light-induced redox changes.

Carboxy-terminal domain mediates assembly of the voltage-gated rat ether-à-go-go potassium channel

The specific assembly of subunits to oligomers is an important prerequisite for producing functional potassium channels. We have studied the assembly of voltage-gated rat ether-à-go-go (r-eag) potassium channels with two complementary assays. In protein overlay binding experiments it was shown that a 41-amino-acid domain, close to the r-eag subunit carboxy-terminus, is important for r-eag subunit interaction. In an in vitro expression system it was demonstrated that r-eag subunits lacking this assembly domain cannot form functional potassium channels. Also, a approximately 10-fold molar excess of the r-eag carboxy-terminus inhibited in co-expression experiments the formation of functional r-eag channels. When the r-eag carboxy-terminal assembly domain had been mutated, the dominant-negative effect of the r-eag carboxy-terminus on r-eag channel expression was abolished. The results demonstrate that a carboxy-terminal assembly domain is essential for functional r-eag potassium channel expression, in contrast to the one of Shaker-related potassium channels, which is directed by an amino-terminal assembly domain.

Two isoforms of the mouse ether-a-go-go-related gene coassemble to form channels with properties similar to the rapidly activating component of the cardiac delayed rectifier K+ current

HERG, the human ether-a-go-go-related gene, encodes a K(+)-selective channel with properties similar to the rapidly activating component of the delayed rectifier K+ current (IKr). Mutations of HERG cause the autosomal-dominant long-QT syndrome (LQTS), presumably by disrupting the normal function of IKr. The current produced by HERG is not identical to IKr, however, and the mechanism by which HERG mutations cause LQTS remains uncertain. To better define the role of Erg in the heart, we cloned Merg1 from mouse genomic and cardiac cDNA libraries. Merg1 has 16 exons and maps to mouse chromosome 5 in an area syntenic to human chromosome 7q, the map locus of HERG. We isolated three cardiac isoforms of Merg1: Merg1a is homologous to HERG and is expressed in heart, brain, and testes, Merg1a' lacks the first 59 amino acids of Merg1a and is not expressed abundantly, and Merg1b has a markedly shorter divergent N-terminal cytoplasmic domain and is expressed specifically in the heart. The Merg1 isoforms, like HERG, produce inwardly rectifying E-4031-sensitive currents when heterologously expressed in Xenopus oocytes. Merg1a and HERG produce currents with slow deactivation kinetics, whereas Merg1a' and Merg1b currents deactivate more rapidly. Merg1b coassembles with Merg1a to form channels with deactivation kinetics that are more rapid than those of Merg1a or HERG and nearly identical to IKr. In addition, a homologue of Merg1b is present in human cardiac and smooth muscle. Thus, we have identified a novel N-terminal Erg isoform that is expressed specifically in the heart, has rapid deactivation kinetics, and coassembles with the longer isoform in Xenopus oocytes. This N-terminal Erg isoform may determine the properties of IKr and contribute to the pathogenesis of LQTS.

A cellular model for long QT syndrome. Trapping of heteromultimeric complexes consisting of truncated Kv1.1 potassium channel polypeptides and native Kv1.4 and Kv1.5 channels in the endoplasmic reticulum

We demonstrated that overexpression of a cRNA encoding a truncated potassium channel polypeptide that contains the NH2 terminus and the first transmembrane segment (Kv1.1N206Tag) abolished the expression of Kv1.1 and Kv1.5 outward currents in Xenopus oocytes (Babila, T., Moscucci, A., Wang, H., Weaver, F. E. & Koren, G. (1994) Neuron 12, 615-626). Recently, we showed that expression of Kv1.1N206Tag in the heart of transgenic mice resulted in the creation of mice with prolongation of the surface electrocardiogram's QT interval (London, B., Han, X., Folco, E. & Koren, G. (1996) Biophys. J. 70, A2601). To study the dominant negative mechanism of Kv1.1N206Tag, we overexpressed it in GH3 cells, a pituitary cell line expressing Kv1. 5 and Kv1.4. RNase protection analysis comparing the steady-state levels of native Kv1.5 and Kv1.1N206Tag transcripts revealed an excess of Kv1.1N206Tag transcript. Immunoprecipitation analysis using 12CA5 monoclonal antibody detected a 25-kDa polypeptide in the transfected cells. The half-life of Kv1.1N206Tag was 2.6 h. Subcellular fractionation of cell lysates labeled with [35S]methionine revealed that Kv1.1N206Tag polypeptide is detectable in the particulate (membranous) fraction, but not in the soluble (cytosol) fraction. A series of double immunoprecipitations with 12CA5 and polyclonal antibodies against Kv1.5 and Kv1.4 revealed that Kv1.1N206Tag forms heteromultimeric complexes with the native Kv1.4 and Kv1.5 polypeptides. The steady-state levels of Kv1.5 were not affected by the overexpression of Kv1.1N206Tag. Immunofluorescence colocalization and confocal microscopy analyses revealed that Kv1.1N206TagFlag did not reach the plasma membrane, and its distribution pattern was characteristic to that of a resident endoplasmic reticulum polypeptide. Our observations establish that the negative effect of Kv1.1N206Tag is mediated by the formation of heteromultimeric complexes with the native channels and by the retention of these complexes in the endoplasmic reticulum.

Alternatively spliced forms of the cGMP-gated channel in human keratinocytes

Alternatively spliced forms of the alpha subunit of the cGMP-gated channel have been cloned from human keratinocytes. One form encodes a complete channel which is almost identical to the rod photoreceptor. A second spliced variant would encode a protein missing a portion of the intracellular hydrophilic domain and the putative first transmembrane domain. Both complete and spliced variants of the channel also were found in epidermis. The expression of the complete form of the channel was induced by levels of extracellular calcium which promote keratinocyte differentiation. The cGMP-gated channel may play an important role in calcium induced keratinocyte differentiation by mediating Ca2+ entry.

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

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

PDZ domain of neuronal nitric oxide synthase recognizes novel C-terminal peptide sequences

PDZ domains are multifunctional protein-interaction motifs that often bind to the C-terminus of protein targets. Nitric oxide (NO), an endogenous signaling molecule, plays critical roles in nervous, immune, and cardiovascular function. Although there are numerous physiological functions for neuron-derived NO, produced primarily by the neuronal NO synthase (nNOS), excess nNOS activity mediates brain injury in cerebral ischemia and in animal models of Parkinson's disease. Subcellular localization of nNOS activity must therefore be tightly regulated. To determine ligands for the PDZ domain of nNOS, we screened 13 billion distinct peptides and found that the nNOS-PDZ domain binds tightly to peptides ending Asp-X-Val. This differs from the only known (Thr/Ser)-X-Val consensus that interacts with PDZ domains from PSD-95. Preference for Asp at the -2 peptide position is mediated by Tyr-77 of nNOS. A Y77D78 to H77E78 substitution changes the binding specificity from Asp-X-Val to Thr-X-Val. Guided by the Asp-X-Val consensus, candidate nNOS interacting proteins have been identified including glutamate and melatonin receptors. Our results demonstrate that PDZ domains have distinct peptide binding specificity.