Association and colocalization of the Kvbeta1 and Kvbeta2 beta-subunits with Kv1 alpha-subunits in mammalian brain K+ channel complexes

Xenopus embryonic spinal neurons express potassium channel Kvbeta subunits

Developmental regulation of voltage-dependent delayed rectifier potassium current (I(Kv)) of Xenopus primary spinal neurons regulates the waveform of the action potential. I(Kv) undergoes a tripling in density and acceleration of it activation kinetics during the initial day of its appearance. Another voltage-dependent potassium current, the A current, is acquired during the subsequent day and contributes to further shortening of the impulse duration. To decipher the molecular mechanisms underlying this functional differentiation, we are identifying potassium channel genes expressed in the embryonic amphibian nervous system. Potassium channels consist of pore-forming (alpha) as well as auxiliary (beta) subunits. Here, we report the primary sequence, developmental localization, and functional properties of two Xenopus Kvbeta genes. On the basis of primary sequence, one of these (xKvbeta2) is highly conserved with Kvbeta2 genes identified in other species, whereas the other (xKvbeta4) appears to identify a new member of the Kvbeta family. Both are expressed in developing spinal neurons during the period of impulse maturation but in different neuronal populations. In a heterologous system, coexpression of xKvbeta subunits modulates properties of potassium current that are developmentally regulated in the endogenous I(Kv). Consistent with xKvbeta4's unique primary sequence, the repertoire of functional effects it has on coexpressed Kv1alpha subunits is novel. Taken together, the results implicate auxiliary subunits in regulation of potassium current function and action potential waveforms in subpopulations of embryonic primary spinal neurons.

Developmental changes in the expression of Shaker- and Shab-related K(+) channels in neurons of the rat trigeminal ganglion

We have investigated properties of voltage-gated K(+) channels in neurons of the pre- and postnatal rat trigeminal ganglion (TG). To correlate functional data with information on gene expression of Shaker- and Shab-related channels in these pseudo-unipolar neurons, the patch-clamp technique was combined with the single-cell reverse transcription-polymerase chain reaction (RT-PCR). A majority (80%) of prenatal TG neurons possessed only sustained delayed rectifier currents with half-maximal current inactivation at -30 mV. In the postnatal cells, steady-state inactivation of sustained currents occurred at more negative voltages (half-maximal inactivation at -58 mV). About 65% of the postnatal cells displayed a transient outward component in addition to the sustained currents. With increasing age, the sensitivity of sustained currents to 4-aminopyridine (4-AP) decreased significantly. The Shaker channel toxins, alpha-dendrotoxin and agitoxin-2 (50 and 10 nM), were much less effective. Discrimination between both stages with tetraethylammonium chloride (5 mM) was not possible since the currents were reduced generally by about 50%. After recording, the cell content was harvested and single-cell RT-PCR was performed to compare K(+) current properties and mRNA expression within the same cell. Most cells simultaneously expressed several different Shaker- and Shab-like transcripts. At postnatal day 14, the frequency of cells carrying transcripts encoding Kv1.1 decreased. Detailed analysis revealed a higher 4-AP sensitivity of TG neurons expressing Kv1.1 transcripts.

Caspr2, a new member of the neurexin superfamily, is localized at the juxtaparanodes of myelinated axons and associates with K+ channels

Rapid conduction in myelinated axons depends on the generation of specialized subcellular domains to which different sets of ion channels are localized. Here, we describe the identification of Caspr2, a mammalian homolog of Drosophila Neurexin IV (Nrx-IV), and show that this neurexin-like protein and the closely related molecule Caspr/Paranodin demarcate distinct subdomains in myelinated axons. While contactin-associated protein (Caspr) is present at the paranodal junctions, Caspr2 is precisely colocalized with Shaker-like K+ channels in the juxtaparanodal region. We further show that Caspr2 specifically associates with Kv1.1, Kv1.2, and their Kvbeta2 subunit. This association involves the C-terminal sequence of Caspr2, which contains a putative PDZ binding site. These results suggest a role for Caspr family members in the local differentiation of the axon into distinct functional subdomains.

Evaluation of functional interaction between K(+) channel alpha- and beta-subunits and putative inactivation gating by Co-expression in Xenopus laevis oocytes

Animal K(+) channel alpha- (pore-forming) subunits form native proteins by association with beta-subunits, which are thought to affect channel function by modifying electrophysiological parameters of currents (often by inducing fast inactivation) or by stabilizing the protein complex. We evaluated the functional association of KAT1, a plant K(+) channel alpha-subunit, and KAB1 (a putative homolog of animal K(+) channel beta-subunits) by co-expression in Xenopus laevis oocytes. Oocytes expressing KAT1 displayed inward-rectifying, non-inactivating K(+) currents that were similar in magnitude to those reported in prior studies. K(+) currents recorded from oocytes expressing both KAT1 and KAB1 had similar gating kinetics. However, co-expression resulted in greater total current, consistent with the possibility that KAB1 is a beta-subunit that stabilizes and therefore enhances surface expression of K(+) channel protein complexes formed by alpha-subunits such as KAT1. K(+) channel protein complexes formed by alpha-subunits such as KAT1 that undergo (voltage-dependent) inactivation do so by means of a "ball and chain" mechanism; the ball portion of the protein complex (which can be formed by the N terminus of either an alpha- or beta-subunit) occludes the channel pore. KAT1 was co-expressed in oocytes with an animal K(+) channel alpha-subunit (hKv1.4) known to contain the N-terminal ball and chain. Inward currents through heteromeric hKv1. 4:KAT1 channels did undergo typical voltage-dependent inactivation. These results suggest that inward currents through K(+) channel proteins formed at least in part by KAT1 polypeptides are capable of inactivation, but the structural component facilitating inactivation is not present when channel complexes are formed by either KAT1 or KAB1 in the absence of additional subunits.

Developmental expression of voltage-gated potassium channel beta subunits

Expression of potassium channel beta subunits (Kvbeta) was determined in the developing mouse CNS using an antiserum against an amino acid sequence present in the C-terminus of Kvbeta1, Kvbeta2, and Kvbeta3. Using the anti-Kvbeta antiserum, we determined that Kvbeta expression is restricted to the spinal cord and dorsal root ganglia in the embryonic CNS. At birth, Kvbeta expression is detected in brainstem and midbrain nuclei, but was not detected in the hippocampus, cerebellum or cerebral cortex. During the first postnatal week, Kvbeta expression is present in hippocampal and cortical pyramidal cells and in cerebellar Purkinje cells. Expression of Kvbeta subunits reaches adult levels by the third postnatal week in all of the brain regions examined. A rabbit antiserum directed against a unique peptide sequence in the N-terminus of the Kvbeta1 protein demonstrates that this subunit displays a novel expression pattern in the developing mouse brain. Kvbeta1 expression is high at birth in all brain regions examined and decreases with age. In contrast, Kvbeta2 expression is low at birth and increases with age to reach adult levels by the third postnatal week. These findings support the notion that the differential regulation of distinct potassium channel beta subunits, in the developing mouse nervous system, may confer the functional diversity required to mediate both neuronal survival and maturation.

Voltage-gated ion channels and hereditary disease

By the introduction of technological advancement in methods of structural analysis, electronics, and recombinant DNA techniques, research in physiology has become molecular. Additionally, focus of interest has been moving away from classical physiology to become increasingly centered on mechanisms of disease. A wonderful example for this development, as evident by this review, is the field of ion channel research which would not be nearly as advanced had it not been for human diseases to clarify. It is for this reason that structure-function relationships and ion channel electrophysiology cannot be separated from the genetic and clinical description of ion channelopathies. Unique among reviews of this topic is that all known human hereditary diseases of voltage-gated ion channels are described covering various fields of medicine such as neurology (nocturnal frontal lobe epilepsy, benign neonatal convulsions, episodic ataxia, hemiplegic migraine, deafness, stationary night blindness), nephrology (X-linked recessive nephrolithiasis, Bartter), myology (hypokalemic and hyperkalemic periodic paralysis, myotonia congenita, paramyotonia, malignant hyperthermia), cardiology (LQT syndrome), and interesting parallels in mechanisms of disease emphasized. Likewise, all types of voltage-gated ion channels for cations (sodium, calcium, and potassium channels) and anions (chloride channels) are described together with all knowledge about pharmacology, structure, expression, isoforms, and encoding genes.

Inhibition of hKv2.1, a major human neuronal voltage-gated K+ channel, by meclofenamic acid

Using the standard patch clamp whole cell recording method, we assessed the pharmacological activity of four fenamate nonsteroidal anti-inflammatory drugs, meclofenamic acid, flufenamic acid, mefenamic acid and niflumic acid, on hKv2.1, a major human neuronal voltage-gated potassium channel stably expressed heterologously in Chinese hamster ovary cells. Meclofenamic acid inhibited hKv2.1 in a concentration-dependent manner whereas the other three fenamates had weaker or no effect on these channels at a concentration of 100 microM. The estimated IC50 of meclofenamic acid was 56.0 microM for hKv2.1 compared an IC50 of 155.9 microM for another human neuronal K channel (hKv1.1). Meclofenamic acid reached its maximum inhibition within 5 min of bath application and its effect was readily reversed upon wash. Kinetic analysis revealed that this drug did not alter the channel activation or deactivation time courses. Moreover, the effect of meclofenamic acid on hKv2.1 channels was not voltage-dependent. Indomethacin, another inhibitor of the cyclooxygenase that catalyses the synthesis of prostaglandin from arachidonic acid, had no effect on either hKv2.1 or hKv1.1. These results indicate that meclofenamic acid inhibits hKv2.1 more potently than hKv1.1 and it is likely that this compound acts directly on the channel proteins.

Subunit composition of Kv1 channels in human CNS

The alpha subunits of Shaker-related K+ channels (Kv1.X) show characteristic distributions in mammalian brain and restricted coassembly. Despite the functional importance of these voltage-sensitive K+ channels and involvement in a number of diseases, little progress has been achieved in deciphering the subunit composition of the (alpha)4(beta)4 oligomers occurring in human CNS. Thus, the association of alpha and beta subunits was investigated in cerebral grey and white matter and spinal cord from autopsy samples. Immunoblotting established the presence of Kv1.1, 1.2, and 1.4 in all the tissues, with varying abundance. Sequential immunoprecipitations identified the subunits coassembled. A putative tetramer of Kv1.3/1.4/1.1/1.2 was found in grey matter. Both cerebral white matter and spinal cord contained the heterooligomers Kv1.1/1.4 and Kv1.1/1.2, similar to grey matter, but both lacked Kv1.3 and the Kv1.4/1.2 combination. An apparent Kv1.4 homooligomer was detected in all the samples, whereas only the brain tissue possessed a putative Kv1.2 homomer. In grey matter, Kvbeta2.1 was coassociated with the Kv1.1/1.2 combination and Kv1.2 homooligomer. In white matter, Kvbeta2.1 was associated with Kv1.2 only, whereas Kvbeta1.1 coprecipitated with all the alpha subunits present. This represents the first description of Kv1 subunit complexes in the human CNS and demonstrates regional variations, indicative of functional specialisation.

Delayed rectifier currents in rat globus pallidus neurons are attributable to Kv2.1 and Kv3.1/3.2 K(+) channels

The symptoms of Parkinson disease are thought to result in part from increased burst activity in globus pallidus neurons. To gain a better understanding of the factors governing this activity, we studied delayed rectifier K(+) conductances in acutely isolated rat globus pallidus (GP) neurons, using whole-cell voltage-clamp and single-cell RT-PCR techniques. From a holding potential of -40 mV, depolarizing voltage steps in identified GP neurons evoked slowly inactivating K(+) currents. Analysis of the tail currents revealed rapidly and slowly deactivating currents of similar amplitude. The fast component of the current deactivated with a time constant of 11. 1 +/- 0.8 msec at -40 mV and was blocked by micromolar concentrations of 4-AP and TEA (K(D) approximately 140 microM). The slow component of the current deactivated with a time constant of 89 +/- 10 microseconds at -40 mV and was less sensitive to TEA (K(D) = 0.8 mM) and 4-AP (K(D) approximately 6 mM). Organic antagonists of Kv1 family channels had little or no effect on somatic currents. These properties are consistent with the hypothesis that the rapidly deactivating current is attributable to Kv3.1/3.2 channels and the slowly deactivating current to Kv2.1-containing channels. Semiquantitative single-cell RT-PCR analysis of Kv3 and Kv2 family mRNAs supported this conclusion. An alteration in the balance of these two channel types could underlie the emergence of burst firing after dopamine-depleting lesions.

Expression in mammalian cells and electrophysiological characterization of two mutant Kv1.1 channels causing episodic ataxia type 1 (EA-1)

Episodic ataxia type 1 (EA-1) is a rare neurological disorder and was the first ionic channel disease to be associated with defects in a potassium channel. Until now 10 different point mutations in the KCNA1-gene have been reported to cause this disorder. We have investigated the functional consequences of two mutations leading to amino acid substitutions in the first and sixth transmembrane segments of a Kv1.1 channel subunit, by means of the patch-clamp technique; we injected cRNA coding for, respectively, F184C and V408A mutant Kv1.1 channels into mammalian cells and compared the resulting currents with those in the wild-type. The expression levels of F184C and V408A mutant channels relative to that of the wild-type was 38 and 68%, respectively. Since the single-channel conductance of the F184C mutant was similar to that of the wild-type (12 pS) without an apparent change in the maximum open probability, we conclude that the lower expression level in the F184C mutant channels is due to a reduced number of functional channels on the cell surface. F184C activated slower, and at more depolarized potentials, and deactivated faster compared with the wild-type. V408A channels deactivated and inactivated faster compared with the wild-type. Studies with different extracellular cations and tetraethylammonium gave no indication that the pore structure was changed in the mutant channels. Acetazolamide, that is helpful in some patients suffering from EA-1, was without effect on Kv1.1 wild-type or mutant channels. This study confirms and extends earlier studies on the functional consequences of Kv1.1 mutations associated with EA-1, in an attempt to understand the pathophysiology of the disease.

alpha subunit compositions of Kv1.1-containing K+ channel subtypes fractionated from rat brain using dendrotoxins

K+ channels from the Kv1 subfamily contain four alpha-subunits and the combinations (from Kv1.1-1.6) determine susceptibility to dendrotoxin (DTX) homologues. The subunit composition of certain subtypes in rat brain was investigated using DTXk which only interacts with Kv1.1-containing channels and alphaDTX (and its closely related homologue DTXi) that binds preferentially to Kv1. 2-possessing homo- or hetero-oligomers. Covalent attachment of [125I]DTXk bound to channels in synaptic membranes unveiled subunits of Mr = 78 000 and 96 000. Immunoprecipitation of these solubilized and dissociated cross-linked proteins with IgG specific for each of the alpha-subunits identified Kv1.1, 1.2 and 1.4; this led to assemblies of Kv1.1/1.2 and 1.1/1.4 being established. Kv1. 2-enriched channels, purified from rat brain by chromatography on immobilized DTXi, contained Kv1.1, 1.2 and 1.6 confirming one of the above-noted pairs and indicating an additional Kv1.1-containing oligomer (Kv1.1/1.2/1.6); the notable lack of Kv1.4 excludes a Kv1. 1/1.2/1.4 combination. On the other hand, channels with Kv1.1 as a constituent, isolated using DTXk, possessed Kv1.4 in addition to those found in the DTXi-purified oligomers; this provides convergent support for the occurrence of the three combinations established above but adds a possible fourth (Kv1.1/1.4/1.6), though this was not confirmed. Moreover, sequential purification on DTXi and DTXk resins yielded channels containing only Kv1.1/1.2 but with an apparent predominance of Kv1.1, reaffirming the latter multimer.

Molecular diversity of K(V) alpha- and beta-subunit expression in canine gastrointestinal smooth muscles

Voltage-activated K(+) (K(V)) channels play an important role in regulating the membrane potential in excitable cells. In gastrointestinal (GI) smooth muscles, these channels are particularly important in modulating spontaneous electrical activities. The purpose of this study was to identify the molecular components that may be responsible for the K(V) currents found in the canine GI tract. In this report, we have examined the qualitative expression of eighteen different K(V) channel genes in canine GI smooth muscle cells at the transcriptional level using RT-PCR analysis. Our results demonstrate the expression of K(V)1.4, K(V)1.5, K(V)1.6, K(V)2.2, and K(V)4.3 transcripts in all regions of the GI tract examined. Transcripts encoding K(V)1.2, K(V)beta1.1, and K(V)beta1.2 subunits were differentially expressed. K(V)1.1, K(V)1.3, K(V)2.1, K(V)3.1, K(V)3.2, K(V)3.4, K(V)4.1, K(V)4.2, and K(V)beta2.1 transcripts were not detected in any GI smooth muscle cells. We have also determined the protein expression for a subset of these K(V) channel subunits using specific antibodies by immunoblotting and immunohistochemistry. Immunoblotting and immunohistochemistry demonstrated that K(V)1.2, K(V)1.4, K(V)1.5, and K(V)2.2 are expressed at the protein level in GI tissues and smooth muscle cells. K(V)2.1 was not detected in any regions of the GI tract examined. These results suggest that the wide array of electrical activity found in different regions of the canine GI tract may be due in part to the differential expression of K(V) channel subunits.

Voltage-gated potassium channels: from hyperexcitability to excitement

The superfamily of voltage-activated potassium channels may express structurally and functionally diverse voltage-activated potassium channels in the nervous system. The roles of some voltage-activated potassium channel types, e.g. rapidly inactivating (transiently active type) channels and muscarine sensitive muscarine sensitive channels, are beginning to be understood. They may significantly influence dendritic action-potential back-propagation, signal to noise ratios in presynaptic excitability or the responsiveness of a neuron to synaptic input. Inherited disorders related to changes in excitability (episodic ataxia, epilepsy, heart arrhythmia) or to defects in sensory perception (hearing loss) have been associated with mutations in a few voltage-activated potassium channel genes. Most likely, more voltage-activated potassium channel genes will be linked to related disorders in the near future.

Kvbeta1.2 subunit coexpression in HEK293 cells confers O2 sensitivity to kv4.2 but not to Shaker channels

Voltage-gated K+ (KV) channels are protein complexes composed of ion-conducting integral membrane alpha subunits and cytoplasmic modulatory beta subunits. The differential expression and association of alpha and beta subunits seems to contribute significantly to the complexity and heterogeneity of KV channels in excitable cells, and their functional expression in heterologous systems provides a tool to study their regulation at a molecular level. Here, we have studied the effects of Kvbeta1.2 coexpression on the properties of Shaker and Kv4.2 KV channel alpha subunits, which encode rapidly inactivating A-type K+ currents, in transfected HEK293 cells. We found that Kvbeta1.2 functionally associates with these two alpha subunits, as well as with the endogenous KV channels of HEK293 cells, to modulate different properties of the heteromultimers. Kvbeta1.2 accelerates the rate of inactivation of the Shaker currents, as previously described, increases significantly the amplitude of the endogenous currents, and confers sensitivity to redox modulation and hypoxia to Kv4.2 channels. Upon association with Kvbeta1.2, Kv4.2 can be modified by DTT (1,4 dithiothreitol) and DTDP (2,2'-dithiodipyridine), which also modulate the low pO2 response of the Kv4.2+beta channels. However, the physiological reducing agent GSH (reduced glutathione) did not mimic the effects of DTT. Finally, hypoxic inhibition of Kv4.2+beta currents can be reverted by 70% in the presence of carbon monoxide and remains in cell-free patches, suggesting the presence of a hemoproteic O2 sensor in HEK293 cells and a membrane-delimited mechanism at the origin of hypoxic responses. We conclude that beta subunits can modulate different properties upon association with different KV channel subfamilies; of potential relevance to understanding the molecular basis of low pO2 sensitivity in native tissues is the here described acquisition of the ability of Kv4. 2+beta channels to respond to hypoxia.

Molecular diversity of K+ channels

K+ channel principal subunits are by far the largest and most diverse of the ion channels. This diversity originates partly from the large number of genes coding for K+ channel principal subunits, but also from other processes such as alternative splicing, generating multiple mRNA transcripts from a single gene, heteromeric assembly of different principal subunits, as well as possible RNA editing and posttranslational modifications. In this chapter, we attempt to give an overview (mostly in tabular format) of the different genes coding for K+ channel principal and accessory subunits and their genealogical relationships. We discuss the possible correlation of different principal subunits with native K+ channels, the biophysical and pharmacological properties of channels formed when principal subunits are expressed in heterologous expression systems, and their patterns of tissue expression. In addition, we devote a section to describing how diversity of K+ channels can be conferred by heteromultimer formation, accessory subunits, alternative splicing, RNA editing and posttranslational modifications. We trust that this collection of facts will be of use to those attempting to compare the properties of new subunits to the properties of others already known or to those interested in a comparison between native channels and cloned candidates.

Determinants of potassium channel assembly localised within the cytoplasmic C-terminal domain of Kv2.1

The C-terminal domain of the voltage-gated potassium channel Kv2.1 is shown to have a role in channel assembly using dominant negative experiments in Xenopus oocytes. Kv2.1 channel polypeptides were co-expressed with a number of polypeptide fragments of the cytosolic C-terminus and the assembly of functional channel homotetramers quantified electrophysiologically using the two electrode voltage clamp technique. Co-expression of C-terminal polypeptides corresponding to the final 440, 318, 220 and 150 amino acid residues of Kv2.1 all resulted in a significant reduction in the functional expression of the full-length channel. A truncated version of Kv2.1 lacking the final 318 amino acids of the C-terminal domain (Kv2. 11-535) exhibited similar electrophysiological properties to the full-length channel. Co-expression with either the 440 or 318 residue polypeptides resulted in a reduction in the activity of the truncated channel. In contrast, the 220 and 150 residue C-terminal fragments had no effect on Kv2.11-535 activity. These data demonstrate that C-terminal interactions are important for driving Kv2.1 channel assembly and that distinct regions of the C-terminal domain may have differential effects on the formation of functional tetramers.

Cloning and expression of Shaker alpha- and beta-subunits during inner ear development

Sensory cells of the chicken cochlea exhibit different ion channels relative to their position along the epithelium. One of these channels conducts an A-type potassium current which is found primarily in 'short' hair cells. Here, we report the first full length cloning and developmental expression of Shaker genes from this endorgan. Clones were obtained by screening a chicken (Gallus gallus) cochlea cDNA library, using probes made from RHK1 (i.e., Kvalpha1.4) cDNA, a Shaker homologue isolated from rat heart, and hKvbeta1.2 cDNA, a beta homologue isolated from human heart. Sequence analysis revealed a chick homologue of Kvalpha1.4, with a deduced amino acid similarity of 76-79% to mammalian Kvalpha1.4, and a chick homologue of Kvbeta1.1, with a similarity of 95% to mammalian Kvbeta1.1. In addition, we isolated a variant of cKvalpha1. 4 (cKvalpha1.4(m)) that differs in its untranslated regions and shows complete similarity in its coding region, except for the deletion of a single nucleotide. During development of the inner ear, reverse transcription-polymerase chain reaction (RT-PCR) studies show that the beta-subunit is expressed as early as embryonic day 3, whereas alpha- and beta-subunits are coexpressed on embryonic days 7 to 10, 14, and in adult.

A new ER trafficking signal regulates the subunit stoichiometry of plasma membrane K(ATP) channels

Proper ion channel function often requires specific combinations of pore-forming alpha and regulatory beta subunits, but little is known about the mechanisms that regulate the surface expression of different channel combinations. Our studies of ATP-sensitive K+ channel (K(ATP)) trafficking reveal an essential quality control function for a trafficking motif present in each of the alpha (Kir6.1/2) and beta (SUR1) subunits of the K(ATP) complex. We show that this novel motif for endoplasmic reticulum (ER) retention/retrieval is required at multiple stages of K(ATP) assembly to restrict surface expression to fully assembled and correctly regulated octameric channels. We conclude that exposure of a three amino acid motif (RKR) can explain how assembly of an ion channel complex is coupled to intracellular trafficking.

Identification of the Kv2.1 K+ channel as a major component of the delayed rectifier K+ current in rat hippocampal neurons

Molecular cloning studies have revealed the existence of a large family of voltage-gated K+ channel genes expressed in mammalian brain. This molecular diversity underlies the vast repertoire of neuronal K+ channels that regulate action potential conduction and neurotransmitter release and that are essential to the control of neuronal excitability. However, the specific contribution of individual K+ channel gene products to these neuronal K+ currents is poorly understood. We have shown previously, using an antibody, "KC, " specific for the Kv2.1 K+ channel alpha-subunit, the high-level expression of Kv2.1 protein in hippocampal neurons in situ and in culture. Here we show that KC is a potent blocker of K+ currents expressed in cells transfected with the Kv2.1 cDNA, but not of currents expressed in cells transfected with other highly related K+ channel alpha-subunit cDNAs. KC also blocks the majority of the slowly inactivating outward current in cultured hippocampal neurons, although antibodies to two other K+ channel alpha-subunits known to be expressed in these cells did not exhibit blocking effects. In all cases the blocking effects of KC were eliminated by previous incubation with a recombinant fusion protein containing the KC antigenic sequence. Together these studies show that Kv2.1, which is expressed at high levels in most mammalian central neurons, is a major contributor to the delayed rectifier K+ current in hippocampal neurons and that the KC antibody is a powerful tool for the elucidation of the role of the Kv2.1 K+ channel in regulating neuronal excitability.

Dynamic potassium channel distributions during axonal development prevent aberrant firing patterns

The distribution and function of Shaker-related K+ channels were studied with immunofluorescence and electrophysiology in sciatic nerves of developing rats. At nodes of Ranvier, Na+ channel clustering occurred very early (postnatal days 1-3). Although K+ channels were not yet segregated at most of these sites, they were directly involved in action potential generation, reducing duration, and the refractory period. At approximately 1 week, K+ channel clusters were first seen but were within the nodal gap and in paranodes, and only later (weeks 2-4) were they shifted to juxtaparanodal regions. K+ channel function was most dramatic during this transition period, with block producing repetitive firing in response to single stimuli. As K+ channels were increasingly sequestered in juxtaparanodes, conduction became progressively insensitive to K+ channel block. Over the first 3 weeks, K+ channel clustering was often asymmetric, with channels exclusively in the distal paranode in approximately 40% of cases. A computational model suggested a mechanism for the firing patterns observed, and the results provide a role for K+ channels in the prevention of aberrant excitation as myelination proceeds during development.

Cloning and functional expression of rat ether-à-go-go-like K+ channel genes

1. Screening of rat cortex cDNA resulted in cloning of two complete and one partial orthologue of the Drosophila ether-à-go-go-like K+ channel (elk). 2. Northern blot and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed predominant expression of rat elk mRNAs in brain. Each rat elk mRNA showed a distinct, but overlapping expression pattern in different rat brain areas. 3. Transient transfection of Chinese hamster ovary (CHO) cells with rat elk1 or rat elk2 cDNA gave rise to voltage-activated K+ channels with novel properties. 4. RELK1 channels mediated slowly activating sustained potassium currents. The threshold for activation was at -90 mV. Currents were insensitive to tetraethylammonium (TEA) and 4-aminopyridine (4-AP), but were blocked by micromolar concentrations of Ba2+. RELK1 activation kinetics were not dependent on prepulse potential like REAG-mediated currents. 5. RELK2 channels produced currents with a fast inactivation component and HERG-like tail currents. RELK2 currents were not sensitive to the HERG channel blocker E4031.

Differential interaction of voltage-gated K+ channel beta-subunits with cytoskeleton is mediated by unique amino terminal domains

To define the molecular characteristics of K+ channel beta-subunit polypeptides, we have studied their biochemical properties and subcellular distribution in transfected mammalian cells. We find that the recombinant voltage-dependent K+ (Kv) beta1.1 and Kvbeta2 polypeptides have distinct detergent solubility properties owing to a novel association of Kvbeta1.1 with the actin-based cytoskeleton. Mutational and chimeric protein analyses show that the unique aminoterminus of Kvbeta1.1 is both necessary and sufficient for mediating the association of beta-subunits with cytoskeleton. Thus, the interaction with cytoskeleton is mediated through the amino-terminal domain previously shown to be necessary for modulating alpha-subunit inactivation, but not necessary for interaction with alpha-subunit polypeptides. These data reveal that different domains of beta-subunit polypeptides mediate interactions with cytoskeleton and with alpha-subunits, and provide a structural basis for previous reports that linked the extent of beta-subunit-induced inactivation to the state of the actin cytoskeleton.

Structure-activity relationships of the Kvbeta1 inactivation domain and its putative receptor probed using peptide analogs of voltage-gated potassium channel alpha- and beta-subunits

Certain beta-subunits exert profound effects on the kinetics of voltage-gated (Kv) potassium channel inactivation through an interaction between the amino-terminal "inactivation domain" of the beta-subunit and a "receptor" located at or near the cytoplasmic mouth of the channel pore. Here we used a bacterial random peptide library to examine the structural requirements for this interaction. To identify peptides that bind Kv1.1 we screened the library against a synthetic peptide corresponding to the predicted S4-S5 cytoplasmic loop of the Kv1.1 alpha-subunit (residues 313-328). Among the highest affinity interactors were peptides with significant homology to the amino terminus of Kvbeta1. We performed a second screen using a peptide from the amino terminus of Kvbeta1 (residues 2-31) as "bait" and identified peptide sequences with significant homology to the S4-S5 loop of Kv1.1. A series of synthetic peptides containing mutations of the wild-type Kvbeta1 and Kv1.1 sequences were examined for their ability to inhibit Kvbeta1/Kv1.1 binding. Amino acids Arg20 and Leu21 in Kvbeta1 and residues Arg324 and Leu328 in Kv1.1 were found to be important for the interaction. Taken together, these data provide support for the contention that the S4-S5 loop of the Kv1.1 alpha subunit is the likely acceptor for the Kvbeta1 inactivation domain and provide information about residues that may underlie the protein-protein interactions responsible for beta-subunit mediated Kv channel inactivation.

Separable effects of human Kvbeta1.2 N- and C-termini on inactivation and expression of human Kv1.4

1. The Kvbeta subunits of voltage-gated K+ channels alter the functional expression and gating of non- or slowly inactivating Kvalpha1 subunits via two separate domains. To determine how Kvbeta subunits modulate a rapidly inactivating Kvalpha1 subunit, we did two-microelectrode voltage clamp experiments on human Kv1.4 voltage-gated K+ channels expressed heterologously in Xenopus oocytes. In addition we tested a slowly inactivating mutant of Kv1.4 lacking amino acids 2-146 of the N-terminal alpha-ball domain (Kv1. 4DeltaN2-146). Kv1.4 or Kv1.4DeltaN2-146 were co-expressed with either rat Kvbeta2 or human Kvbeta1.2. To separate domain effects, we also used a mutant of Kvbeta1.2 lacking the unique 79 amino acid N-terminal beta-ball domain (Kvbeta1-C). 2. For the mutant Kv1.4DeltaN2-146 we found that Kvbeta1-C or Kvbeta2 increased current amplitude without altering activation or inactivation. By contrast Kvbeta1.2 produced rapid inactivation and slowed deactivation due to block produced by the beta-ball. The beta-ball also increased the rate of C-type inactivation in 5 mM, but not 50 mM, external K+ consistent with an effect of blockade on K+ efflux. 3. For Kv1.4, Kvbeta1-C produced a voltage-independent increase in the rate of inactivation and shifted the inactivation curve to more hyperpolarized potentials, but had no effect on deactivation. Kvbeta1-C, Kvbeta2 and Kvbeta1.2 slowed recovery from inactivation similarly, thereby excluding involvement of the beta-ball. Kvbeta1.2 produced an additional more rapid, voltage-dependent component of inactivation, significantly reduced peak outward current and shifted steady-state inactivation towards hyperpolarized potentials. 4. Yeast two-hybrid studies showed that alpha-beta interaction was restricted to the N-terminus of Kv1.4 and the C-terminus of Kvbeta1. 2 or Kvbeta2. Direct interaction with the alpha-ball did not occur. Our interpretation is that Kvbeta1-C and Kvbeta2 enhanced N-type inactivation produced by the Kv1.4 alpha-ball allosterically. 5. We propose that Kvbeta1.2 has three effects on Kv1.4, the first two of which it shares with Kvbeta2. First, Kvbeta1-C and Kvbeta2 have a current-enhancing effect. Second, Kvbeta1-C and Kvbeta2 increase block by the alpha-ball allosterically. Third, the beta-ball of Kbeta1.2 directly blocks both Kv1.4 and Kv1.4DeltaN2-146. When both alpha- and beta-balls are present, competition for their respective binding sites slows the block produced by either ball.

Tetrameric subunit structure of the native brain inwardly rectifying potassium channel Kir 2.2

Strongly inwardly rectifying potassium channels of the Kir 2 subfamily (IRK1, IRK2, and IRK3) are involved in maintenance and modulation of cell excitability in brain and heart. Electrophysiological studies of channels expressed in heterologous systems have suggested that the pore-conducting pathway contains four subunits. However, inferences from electrophysiological studies have not been tested on native channels and do not address the possibility of nonconducting auxiliary subunits. Here, we investigate the subunit stoichiometry of endogenous inwardly rectifying potassium channel Kir 2.2 (IRK2) from rat brain. Using chemical cross-linking, immunoprecipitiation, and velocity sedimentation, we report physical evidence demonstrating the tetrameric organization of the native channel. Kir 2.2 was sequentially cross-linked to produce bands on SDS-polyacrylamide gel electrophoresis corresponding in size to monomer, dimer, trimer, and three forms of tetramer. Fully cross-linked channel was present as a single band of tetrameric size. Immunoprecipitation of biotinylated membranes revealed a single band corresponding to Kir 2.2, suggesting that the channel is composed of a single type of subunit. Hydrodynamic properties of 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonic acid-solubilized channel were used to calculate the molecular mass of the channel. Velocity sedimentation in H2O or D2O gave a sharp peak with a sedimentation coefficient of 17.3 S. Gel filtration yielded a Stokes radius of 5.92 nm. These data indicate a multisubunit protein with a molecular mass of 193 kDa, calculated to contain 3.98 subunits. Together, these results demonstrate that Kir 2.2 channels are formed by the homotetrameric association of Kir 2.2 subunits and do not contain tightly associated auxiliary subunits. These studies suggest that Kir 2.2 channels differ in structure from related heterooctomeric ATP-sensitive K channels and heterotetrameric G-protein-regulated inward rectifier K channels.

Regulation of ion channel expression by cytoplasmic subunits

Neuronal and cardiac voltage-gated ion channels contain auxiliary subunits that can profoundly affect the gating of the pore-forming and voltage-sensing alpha subunits. Recent studies on the structurally similar cytoplasmic beta subunits of Ca2+ and K+ channels reveal that these subunits can also exert profound effects on the expression of the integral membrane protein channel components. The mechanisms by which these effects occur are now being elucidated through a combined approach that employs biophysical, pharmacological, cell biological and biochemical techniques.

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

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