Beta subunits promote K+ channel surface expression through effects early in biosynthesis

KCR1, a membrane protein that facilitates functional expression of non-inactivating K+ currents associates with rat EAG voltage-dependent K+ channels

Cerebellar granule neurons possess a non-inactivating K+ current, which controls resting membrane potentials and modulates the firing rate by means of muscarinic agonists. kcr1 was cloned from the cerebellar cDNA library by suppression cloning. KCR1 is a novel protein with 12 putative transmembrane domains and enhances the functional expression of the cerebellar non-inactivating K+ current in Xenopus oocytes. KCR1 also accelerates the activation of rat EAG K+ channels expressed in Xenopus oocytes or in COS-7 cells. Far-Western blotting revealed that KCR1 and EAG proteins interacted with each other by means of their C-terminal regions. These results suggest that KCR1 is the regulatory component of non-inactivating K+ channels.

Reduced K+ Channel Inactivation, Spike Broadening, and After-Hyperpolarization in Kvβ1.1-Deficient Mice with Impaired Learning

A-type K+ channels are known to regulate neuronal firing, but their role in repetitive firing and learning in mammals is not well characterized. To determine the contribution of the auxiliary K+ channel subunit Kvβ1.1 to A-type K+ currents and to study the physiological role of A-type K+ channels in repetitive firing and learning, we deleted the Kvβ1.1 gene in mice. The loss of Kvβ1.1 resulted in a reduced K+ current inactivation in hippocampal CA1 pyramidal neurons. Furthermore, in the mutant neurons, frequency-dependent spike broadening and the slow afterhyperpolarization (sAHP) were reduced. This suggests that Kvβ1.1-dependent A-type K+ channels contribute to frequency-dependent spike broadening and may regulate the sAHP by controlling Ca2+ influx during action potentials. The Kvβ1.1-deficient mice showed normal synaptic plasticity but were impaired in the learning of a water maze test and in the social transmission of food preference task, indicating that the Kvβ1.1 subunit contributes to certain types of learning and memory.

Cyclic AMP regulates potassium channel expression in C6 glioma by destabilizing Kv1.1 mRNA

The tissue distributions and physiological properties of a variety of cloned voltage-gated potassium channel genes have been characterized extensively, yet relatively little is known about the mechanisms controlling expression of these genes. Here, we report studies on the regulation of Kv1.1 expressed endogenously in the C6 glioma cell line. We demonstrate that elevation of intracellular cAMP leads to the accelerated degradation of Kv1.1 RNA. The cAMP-induced decrease in Kv1.1 RNA is followed by a decrease in Kv1. 1 protein and a decrease in the whole cell sustained K+ current amplitude. Dendrotoxin-I, a relatively specific blocker of Kv1.1, blocks 96% of the sustained K+ current in glioma cells, causing a shift in the resting membrane potential from -40 mV to -7 mV. These data suggest that expression of Kv1.1 contributes to setting the resting membrane potential in undifferentiated glioma cells. We therefore suggest that receptor-mediated elevation of cAMP reduces outward K+ current density by acting at the translational level to destabilize Kv1.1 RNA, an additional mechanism for regulating potassium channel gene expression.

MOLECULAR BIOLOGY OF CATION TRANSPORT IN PLANTS

This review summarizes current knowledge about genes whose products function in the transport of various cationic macronutrients (K, Ca) and micronutrients (Cu, Fe, Mn, and Zn) in plants. Such genes have been identified on the basis of function, via complementation of yeast mutants, or on the basis of sequence similarity, via database analysis, degenerate PCR, or low stringency hybridization. Not surprisingly, many of these genes belong to previously described transporter families, including those encoding Shaker-type K+ channels, P-type ATPases, and Nramp proteins. ZIP, a novel cation transporter family first identified in plants, also seems to be ubiquitous; members of this family are found in protozoa, yeast, nematodes, and humans. Emerging information on where in the plant each transporter functions and how each is controlled in response to nutrient availability may allow creation of food crops with enhanced mineral content as well as crops that bioaccumulate or exclude toxic metals.

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.

Cloning and expression of a novel K+ channel regulatory protein, KChAP

Voltage-gated K+ (Kv) channels are important in the physiology of both excitable and nonexcitable cells. The diversity in Kv currents is reflected in multiple Kv channel genes whose products may assemble as multisubunit heteromeric complexes. Given the fundamental importance and diversity of Kv channels, surprisingly little is known regarding the cellular mechanisms regulating their synthesis, assembly, and metabolism. To begin to dissect these processes, we have used the yeast two-hybrid system to identify cytoplasmic regulatory molecules that interact with Kv channel proteins. Here we report the cloning of a novel gene encoding a Kv channel binding protein (KChAP, for K+ channel-associated protein), which modulates the expression of Kv2 channels in heterologous expression system assays. KChAP interacts with the N termini of Kvalpha2 subunits, as well as the N termini of Kvalpha1 and the C termini of Kvbeta subunits. Kv2.1 and KChAP were coimmunoprecipitated from in vitro translation reactions supporting a direct interaction between the two proteins. The amplitudes of Kv2. 1 and Kv2.2 currents are enhanced dramatically in Xenopus oocytes coexpressing KChAP, but channel kinetics and gating are unaffected. Although KChAP binds to Kv1.5, it has no effect on Kv1.5 currents. We suggest that KChAP may act as a novel type of chaperone protein to facilitate the cell surface expression of Kv2 channels.

dSLo interacting protein 1, a novel protein that interacts with large-conductance calcium-activated potassium channels

Large-conductance calcium-activated potassium channels (BK channels) are activated by depolarized membrane potential and elevated levels of intracellular calcium. BK channel activity underlies the fast afterhyperpolarization that follows an action potential and attenuates neurotransmitter and hormone secretion. Using a modified two-hybrid approach, the interaction trap, we have identified a novel protein from Drosophila, dSLIP1 (dSLo interacting protein), which specifically interacts with Drosophila and human BK channels and has partial homology to the PDZ domain of alpha1 syntrophin. The dSLIP1 and dSlo mRNAs are expressed coincidently throughout the Drosophila nervous system, the two proteins interact in vitro, and they may be coimmunoprecipitated from transfected cells. Coexpression of dSLIP1 with dSlo or hSlo BK channels in Xenopus oocytes results in reduced currents as compared with expression of BK channels alone; current amplitudes may be rescued by coexpression with the channel domain that interacts with dSLIP1. Single-channel recordings and immunostaining of transfected tissue culture cells suggest that dSLIP1 selectively reduces Slo BK currents by reducing the number of BK channels in the plasma membrane.

Subunit interaction sites in voltage-dependent Ca2+ channels: role in channel function

Voltage-dependent Ca2+ channels are heteromeric complexes found in the plasma membrane of virtually all cell types and show a high level of electrophysiological and pharmacological diversity. Associated with the pore-forming alpha 1 subunit are the membrane anchored, largely extracellular alpha2-delta, the cytoplasmic beta and sometimes a transmembrane gamma subunit; these subunits dramatically influence the properties and surface expression of these channels. Effects vary depending on subunit isoforms, suggesting that functional diversity of native channels reflects heterogeneity of combinations. Interaction sites between subunits have been identified and advances have been made in our understanding of the molecular basis of functional effects of the auxiliary subunits, their capacity to be regulated by G proteins, and their interaction with related cellular systems.

Distinct functional stoichiometry of potassium channel beta subunits

Shaker-type potassium channels play important roles in determining the electrical excitability of cells. The native channel complex is thought to be formed by four pore-forming alpha subunits that provide four interaction sites for auxiliary modulatory Kvbeta subunits. Because Kvbeta subunits possess diverse modulatory activities including either up-regulation or down-regulation of potassium currents, differential assembly of the alpha-beta complex could give rise to diverse current properties. However, the detailed physical and functional stoichiometry of the alpha-beta complex remains unknown. Kvbeta1 subunits reduce potassium currents through inactivation, whereas Kvbeta2 subunits enhance potassium currents by inhibiting the Kvbeta1-mediated inactivation and at the same time by promoting the surface expression of certain potassium channels. In this report we show that Kvbeta1 and Kvbeta2 of the Shaker-type potassium channels display distinct functional stoichiometry to interact with the Kv1 alpha subunits, a subfamily of Shaker-type potassium channels. The interaction of Kvbeta1 subunits with alpha subunits is consistent with the alpha4betan model, where n equals 0, 1, 2, 3, or 4, depending upon the relative concentration of alpha and beta subunits. The alpha4betan stoichiometry allows for gradual changes of the Kvbeta1-mediated inactivation. In contrast, Kvbeta2 subunits self-associate to form oligomers and interact with the alpha subunits via alpha4beta4 stoichiometry, which permits effective multivalent associations with alpha subunits. Such distinct functional stoichiometry of Kvbeta1 and Kvbeta2 provides a molecular mechanism that is well suited to their contrasting activities of up-regulation or down-regulation of potassium currents.

Independent trafficking of KATP channel subunits to the plasma membrane

KATP channels are unique in requiring two distinct subunits (Kir6.2, a potassium channel subunit) and SUR1 (an ABC protein) for generation of functional channels. To examine the cellular trafficking of KATP channel subunits, green fluorescent protein (GFP) was tagged to the cytoplasmic N or C terminus of SUR1 and Kir6. 2 subunits and to the C terminus of a dimeric fusion between SUR1 and Kir6.2 (SUR1-Kir6.2). All tagged constructs generated functional channels with essentially normal properties when coexpressed with the relevant other subunit. GFP-tagged Kir6.2 (Kir6.2-GFP) showed perinuclear and plasma membrane fluorescence patterns when expressed alone or with SUR1, and a very similar pattern was observed when channel-forming SUR1-Kir6.2-GFP was expressed on its own. In contrast, whereas SUR1 (SUR1-GFP) also showed a perinuclear and plasma membrane fluorescence pattern when expressed alone, an apparently cytoplasmic fluorescence was observed when coexpressed with Kir6.2 subunits. The results indicate that Kir6.2 subunits traffic to the plasma membrane in the presence or absence of SUR1, in contradiction to the hypothesis that homomeric Kir6.2 channels are not observed because SUR1 is required as a chaperone to guide Kir6.2 subunits through the secretory pathway.

Potassium channel distribution, clustering, and function in remyelinating rat axons

The K+ channel alpha-subunits Kv1.1 and Kv1.2 and the cytoplasmic beta-subunit Kvbeta2 were detected by immunofluorescence microscopy and found to be colocalized at juxtaparanodes in normal adult rat sciatic nerve. After demyelination by intraneural injection of lysolecithin, and during remyelination, the subcellular distributions of Kv1.1, Kv1.2, and Kvbeta2 were reorganized. At 6 d postinjection (dpi), axons were stripped of myelin, and K+ channels were found to be dispersed across zones that extended into both nodal and internodal regions; a few days later they were undetectable. By 10 dpi, remyelination was underway, but Kv1.1 immunoreactivity was absent at newly forming nodes of Ranvier. By 14 dpi, K+ channels were detected but were in the nodal gap between Schwann cells. By 19 dpi, most new nodes had Kv1.1, Kv1.2, and Kvbeta2, which precisely colocalized. However, this nodal distribution was transient. By 24 dpi, the majority of K+ channels was clustered within paranodal regions of remyelinated axons, leaving a gap that overlapped with Na+ channel immunoreactivity. Inhibition of Schwann cell proliferation delayed both remyelination and the development of the K+ channel distributions described. Conduction studies indicate that neither 4-aminopyridine (4-AP) nor tetraethylammonium alters normal nerve conduction. However, during remyelination, 4-AP profoundly increased both compound action potential amplitude and duration. The level of this effect matched closely the nodal presence of these voltage-dependent K+ channels. Our results suggest that K+ channels may have a significant effect on conduction during remyelination and that Schwann cells are important in K+ channel redistribution and clustering.

A caspase-activated DNase that degrades DNA during apoptosis, and its inhibitor ICAD

The homeostasis of animals is regulated not only by the growth and differentiation of cells, but also by cell death through a process known as apoptosis. Apoptosis is mediated by members of the caspase family of proteases, and eventually causes the degradation of chromosomal DNA. A caspase-activated deoxyribonuclease (CAD) and its inhibitor (ICAD) have now been identified in the cytoplasmic fraction of mouse lymphoma cells. CAD is a protein of 343 amino acids which carries a nuclear-localization signal; ICAD exists in a long and a short form. Recombinant ICAD specifically inhibits CAD-induced degradation of nuclear DNA and its DNase activity. When CAD is expressed with ICAD in COS cells or in a cell-free system, CAD is produced as a complex with ICAD: treatment with caspase 3 releases the DNase activity which causes DNA fragmentation in nuclei. ICAD therefore seems to function as a chaperone for CAD during its synthesis, remaining complexed with CAD to inhibit its DNase activity; caspases activated by apoptotic stimuli then cleave ICAD, allowing CAD to enter the nucleus and degrade chromosomal DNA.

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.

Voltage-gated and inwardly rectifying potassium channels

This lecture is dedicated to Max Delbrück and Seymour Benzer. Max Delbrück was our graduate advisor. He introduced us to a variety of biophysical problems, and taught us ways of thinking about these problems by example. Potassium channels was one of the topics included in his journal club in the early seventies; Max also carefully considered the feasibility of purifying potassium channels then. It was in Seymour Benzer's laboratory that we began to look for Drosophila mutants that affect synaptic transmission at the larval neuromuscular junction. Shaker was the first behavioural mutant we tested that gave a robust phenotype, a phenotype that could be mimicked by treating wild-type preparations with a potassium channel blocker. This mutant fly has led us to our subsequent molecular studies of potassium channels. Since we settled in the University of California, San Francisco, and began to study neural development as well as potassium channels, we have settled into the pattern of each attending meetings and presenting our studies on one of these two areas so as to avoid both being away from home and our children at the same time. In following this pattern, I will be presenting the studies of potassium channels as part of our long-term collaboration. In this talk I will first briefly take you through the path that led us to the molecular studies of potassium channels and then discuss the diversity and modulation of these potassium channels at the molecular and physiological level.

Differences in the fate of neuronal acetylcholine receptor protein expressed in neurons and stably transfected cells

Ligand-gated ion channels are structurally complex transmembrane proteins that all neurons must synthesize for rapid chemical synaptic transmission. The most abundant nicotinic acetylcholine receptor serving as a ligand-gated ion channel in the nervous system is a species that contains alpha7 subunits, binds alpha-bungarotoxin, and has a high relative permeability to calcium. The ability of neurons to make such receptors was compared with that of non-neuronal cells stably transfected with an alpha7 cDNA to determine whether neuron-specific machinery is likely to aid in their assembly or stabilization. Transfected cells expressed alpha7 protein and assembled it into a species that was indistinguishable in size and pharmacology from native receptors, but much of the alpha7 protein they synthesized was rapidly degraded without becoming receptor. Neurons were not only more efficient than the best transfectants at assembling the receptors but also produced a subpopulation of receptors on the cell surface that was relatively stable and resistant to solubilization. This subpopulation, which was absent from transfected cells, may be tethered to cytoskeletal elements in the neurons. The results support the contention that neurons contain components that facilitate the production and stabilization of ligand-gated ion channels.

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

The differential expression and association of cytoplasmic beta-subunits with pore-forming alpha-subunits may contribute significantly to the complexity and heterogeneity of voltage-gated K+ channels in excitable cells. Here we examined the association and colocalization of two mammalian beta-subunits, Kvbeta1 and Kvbeta2, with the K+ channel alpha-subunits Kv1.1, Kv1.2, Kv1.4, Kv1.6, and Kv2.1 in adult rat brain. Reciprocal coimmunoprecipitation experiments using subunit-specific antibodies indicated that Kvbeta1 and Kvbeta2 associate with all the Kv1 alpha-subunits examined, and with each other, but not with Kv2.1. A much larger portion of the total brain pool of Kv1-containing channel complexes was found associated with Kvbeta2 than with Kvbeta1. Single- and multiple-label immunohistochemical staining indicated that Kvbeta1 codistributes extensively with Kv1.1 and Kv1.4 in cortical interneurons, in the hippocampal perforant path and mossy fiber pathways, and in the globus pallidus and substantia nigra. Kvbeta2 codistributes extensively with Kv1.1 and Kv1.2 in all brain regions examined and was strikingly colocalized with these alpha-subunits in the juxtaparanodal region of nodes of Ranvier as well as in the axons and terminals of cerebellar basket cells. Taken together, these data provide a direct demonstration that Kvbeta1 and Kvbeta2 associate and colocalize with Kv1 alpha-subunits in native tissues and provide a biochemical and neuroanatomical basis for the differential contribution of Kv1 alpha- and beta-subunits to electrophysiologically diverse neuronal K+ currents.

Interactions among inactivating and noninactivating Kvbeta subunits, and Kvalpha1.2, produce potassium currents with intermediate inactivation

Experiments were carried out to determine whether coinjection of Kvalpha1.2 with inactivating and noninactivating Kvbeta subunits would produce currents with intermediate kinetics and channel complexes containing a mixture of these subunits. Upon coexpression with a saturating amount of Kvbeta1.2 and increasing levels of a noninactivating deletion mutant of Kvbeta1.2, we show that macroscopic Kvalpha1.2 currents have levels of fractional inactivation and inactivation time constants that are intermediate between those obtained with either the inactivating Kvbeta1.2 or the noninactivating Kvbeta1.2 mutant. We also find that coexpression of Kvalpha1.2 with saturating amounts of Kvbeta1.2 and the deletion mutant produces a population of single channels with properties intermediate to either the inactivating or noninactivating parental phenotype. Our data can best be explained by the presence of an intermediate population of heterooligomeric channels consisting of Kvalpha1.2 with different combinations of both types of subunits. Since Kvalpha1.2 subunits coexist in cells with inactivating and noninactivating Kvbeta subunits, our findings suggest that heterooligomeric assembly of these subunits occurs to increase the range of K+ current kinetics and expression levels.

Mutations in the hminK gene cause long QT syndrome and suppress IKs function

Ion-channel beta-subunits are ancillary proteins that co-assemble with alpha-subunits to modulate the gating kinetics and enhance stability of multimeric channel complexes. Despite their functional importance, dysfunction of potassium-channel beta-subunits has not been associated with disease. Recent physiological studies suggest that KCNE1 encodes beta-subunits (hminK) that co-assemble with KvLQT1 alpha-subunits to form the slowly activating delayed rectifier K+ (IKs) channel. Because KVLQT1 mutations cause arrhythmia susceptibility in the long QT syndrome (LQT), we hypothesized that mutations in KCNE1 also cause this disorder. Here, we define KCNE1 missense mutations in affected members of two LQT families. Both mutations (S74L, D76N) reduced IKs by shifting the voltage dependence of activation and accelerating channel deactivation. D76N hminK also had a strong dominant-negative effect. The functional consequences of these mutations would be delayed cardiac repolarization and an increased risk of arrhythmia. This is the first description of KCNE1 as an LQT gene and confirms that hminK is an integral protein of the IKs channel.

Separable Kvbeta subunit domains alter expression and gating of potassium channels

Kvbeta subunits have been shown to affect kinetic properties of voltage-gated K+ channel Kv1alpha subunits and increase the number of cell surface dendrotoxin-binding sites when coexpressed with Kv1. 2. Here, we show that Kvbeta1.2 alters both current expression and gating of Kvalpha1 channels and that each effect is mediated by a distinct Kvbeta1.2 domain. The Kvbeta1.2 N terminus or Kvalpha1-blocking domain introduced steady state current block, an apparent negative shift in steady state activation, and a slowing of deactivation along with a dramatic reduction in single channel open probability. N-terminal deletions of Kvbeta1.2 no longer altered channel kinetics but promoted dramatic increases in Kv1.2 current. The conserved Kvbeta1 C terminus or Kvalpha1 expression domain alone was sufficient to increase the number of functional channels. The same effect was observed with the normally noninactivating subunit, Kvbeta2. By contrast, Kv1.5 currents were reduced when coexpressed with either the Kvbeta1 C terminus or Kvbeta2, indicating that the Kvalpha1 expression domain has Kvalpha1 isoform-specific effects. Our results demonstrate that Kvbeta subunits consist of two domains that are separable on the basis of both primary structure and functional modulation of voltage-gated K+ channels.

Structural diversity among subtypes of small-conductance Ca2+-activated potassium channels

125I-Apamin and photolabile derivatives of the toxin have been used to investigate the binding properties and subunit composition of small conductance Ca2+-activated potassium channels (SK(Ca) channels) expressed on plasma membranes from rat brain, rabbit liver, or rat pheochromocytoma (PC12) cells. On all preparations, 125I-apamin recognized single classes of acceptor binding sites with similar high affinity (Kd approximately 3-6 pM). Gallamine, however, was found to readily discriminate between 125I-apamin acceptors present in these preparations, showing a maximal approx nine-fold difference in affinity for acceptors expressed by rabbit liver or PC12 cells. Affinity-labeling patterns revealed the expression of different hetero-oligomeric combinations of high (86 or 59 kDa) and low (33 or 30 kDa) molecular mass 125I-apamin-binding polypeptides, consistent with pharmacological differences. Alternative expression of either 86- or 59-kDa polypeptides appeared to be the most important factor influencing gallamine's affinity for SK(Ca) channel subtypes. Both high- and low-molecular-mass polypeptides are integral membrane proteins, the latter being glycosylated in a tissue-specific manner.

Quantitative single-cell-reverse transcription-PCR demonstrates that A-current magnitude varies as a linear function of shal gene expression in identified stomatogastric neurons

Different Shaker family alpha-subunit genes generate distinct voltage-dependent K+ currents when expressed in heterologous expression systems. Thus it generally is believed that diverse neuronal K+ current phenotypes arise, in part, from differences in Shaker family gene expression among neurons. It is difficult to evaluate the extent to which differential Shaker family gene expression contributes to endogenous K+ current diversity, because the specific Shaker family gene or genes responsible for a given K+ current are still unknown for nearly all adult neurons. In this paper we explore the role of differential Shaker family gene expression in creating transient K+ current (IA) diversity in the 14-neuron pyloric network of the spiny lobster, Panulirus interruptus. We used two-electrode voltage clamp to characterize the somatic IA in each of the six different cell types of the pyloric network. The size, voltage-dependent properties, and kinetic properties of the somatic IA vary significantly among pyloric neurons such that the somatic IA is unique in each pyloric cell type. Comparing these currents with the IAs obtained from oocytes injected with Panulirus shaker and shal cRNA (lobster Ishaker and lobster Ishal, respectively) reveals that the pyloric cell IAs more closely resemble lobster Ishal than lobster Ishaker. Using a novel, quantitative single-cell-reverse transcription-PCR method to count the number of shal transcripts in individual identified pyloric neurons, we found that the size of the somatic IA varies linearly with the number of endogenous shal transcripts. These data suggest that the shal gene contributes substantially to the peak somatic IA in all neurons of the pyloric network.

Molecular mechanism and functional significance of the MinK control of the KvLQT1 channel activity

The very slowly activating delayed rectifier K+ channel IKs is essential for controlling the repolarization phase of cardiac action potentials and K+ homeostasis in the inner ear. The IKs channel is formed via the assembly of two transmembrane proteins, KvLQT1 and MinK. Mutations in KvLQT1 are associated with a long QT syndrome that causes syncope and sudden death and also with deafness. Here, we show a new mode of association between ion channel forming subunits in that the cytoplasmic C-terminal end of MinK interacts directly with the pore region of KvLQT1. This interaction reduces KvLQT1 channel conductance from 7.6 to 0.58 picosiemens. However, because MinK also reveals a large number of previously silent KvLQT1 channels (x 60), the overall effect is a large increase (x 4) in the macroscopic K+ current. Conformational changes associated with the KvLQT1/MinK association create very slow and complex activation kinetics without much alteration in the deactivation process. Changes induced by MinK have an essential regulatory role in the development of this K+ channel activity upon repetitive electrical stimulation with a particular interest in tachycardia.

EGL-36 Shaw channels regulate C. elegans egg-laying muscle activity

The C. elegans egl-36 gene encodes a Shaw-type potassium channel that regulates egg-laying behavior. Gain of function [egl-36(gf)] and dominant negative [egl-36(dn)] mutations in egl-36 cause reciprocal defects in egg laying. An egl-36::gfp reporter is expressed in the egg-laying muscles and in a few other tissues. Expression of an egl-36(gf) cDNA in the egg-laying muscles causes behavioral defects similar to those observed in egl-36(gf) mutants. Gain of function EGL-36 subunits form channels that are active at more negative potentials than wild-type channels. The egl-36(gf) alleles correspond to missense mutations in an amino terminal subunit assembly domain (E138K) and in the S6 transmembrane domain (P435S), neither of which were previously implicated in the voltage dependence of channel activation. Altogether, these results suggest that EGL-36 channels regulate the excitability of the egg-laying muscles.

Regional localization of ERG, the channel protein responsible for the rapid component of the delayed rectifier, K+ current in the ferret heart

Repolarization of the cardiac action potential varies widely throughout the heart. This could be due to the differential distribution of ion channels responsible for repolarization, especially the K+ channels. We have therefore studied the cardiac localization of ERG, a channel protein known to play an important role in generation of the rapid component of the delayed rectifier K+ current (IKr), an important determinant of the repolarization waveform, Cryosections of the ferret atrium and ventricle were prepared to determine the localization of ERG by fluorescence in situ hybridization (FISH) and immunofluorescence. We found that in the ferret, ERG transcript and protein expression was most abundant in the epicardial cell layers throughout most of the ventricle, except at the base. In the atrium, we found that ERG is most abundant in the medial right atrium, especially in the trabeculae and the crista terminalis of the right atrial appendage. It also is present in areas within the sinoatrial node. In all regions studied, FISH and immunofluorescence showed concordant localization patterns. These data suggest that repolarization mediated by IKr is not uniform throughout the ferret heart and provide a molecular explanation for heterogeneity in action potential repolarization throughout the mammalian heart.

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.

Importance of the different beta subunits in the membrane expression of the alpha1A and alpha2 calcium channel subunits: studies using a depolarization-sensitive alpha1A antibody

The plasma membrane expression of the rat brain calcium channel subunits alpha1A, alpha2-delta and the beta subunits beta1b, beta2a, beta3b and beta4 was examined by transient expression in COS-7 cells. Neither alpha1A nor alpha2-delta localized to the plasma membrane, either alone or when coexpressed. However, coexpression of alpha1A or alpha2-delta/alpha1A with any of the beta subunits caused alpha1A and alpha2 to be targetted to the plasma membrane. The alpha1A antibody is directed against an exofacial epitope at the mouth of the pore, which is not exposed unless cells are depolarized, both for native alpha1A channels in dorsal root ganglion neurons and for alpha1A expressed with a beta subunit. This subsidiary result provides evidence that either channel opening or inactivation causes a conformational change at the mouth of the pore of alpha1A. Immunostaining for alpha1A was obtained in depolarized non-permeabilized cells, indicating correct orientation in the membrane only when it was coexpressed with a beta subunit. In contrast, beta1b and beta2a were associated with the plasma membrane when expressed alone. However, this is not a prerequisite to target alpha1A to the membrane since beta3 and beta4 alone showed no differential localization, but did direct the translocation of alpha1A to the plasma membrane, suggesting a chaperone role for the beta subunits.

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

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

Potassium channel alpha and beta subunits assemble in the endoplasmic reticulum

We have characterized the maturation of Shaker K+ channel protein and the cellular site of assembly of pore-forming alpha and cytoplasmic beta subunits in a transfected mammalian cell line. Shaker protein is made as a partially glycosylated, immature precursor that is converted to a fully glycosylated, mature product. Shaker protein did not mature when transport from the endoplasmic reticulum (ER) to the Golgi apparatus was blocked. Consistent with this finding, only the immature form was sensitive to digestion with endoglycosidase H. These results indicate that the immature protein is core-glycosylated in the ER, whereas the oligosaccharides of the mature protein have been further processed in the Golgi compartment. After inhibiting ER-to-Golgi transport, the oligomeric state of Shaker subunits was assessed by cross-linking in intact cells or by solubilization and sucrose gradient sedimentation. The results indicate that Shaker subunits assemble with each other in the ER. When co-expressed, the Kvbeta2 subunit also associated with Shaker in the ER. Assembly with the beta2 subunit did not increase the rate or extent of Shaker protein maturation. Our results indicate that the biogenesis of Shaker K+ channels in vivo involves core glycosylation and subunit assembly in the ER, followed by efficient transfer to the Golgi apparatus where the oligosaccharides are modified.

Voltage-gated K+ channel beta subunits: expression and distribution of Kv beta 1 and Kv beta 2 in adult rat brain

Recent cloning of K+ channel beta subunits revealed that these cytoplasmic polypeptides can dramatically alter the kinetics of current inactivation and promote efficient glycosylation and surface expression of the channel-forming alpha subunits. Here, we examined the expression, distribution, and association of two of these beta subunits, Kv beta 1 and Kv beta 2, in adult rat brain. In situ hybridization using cRNA probes revealed that these beta-subunit genes are heterogeneously expressed, with high densities of Kv beta 1 mRNA in the striatum, CA1 subfield of the hippocampus, and cerebellar Purkinje cells, and high densities of Kv beta 2 mRNA in the cerebral cortex, cerebellum, and brainstem. Immunohistochemical staining using subunit-specific monoclonal and affinity-purified polyclonal antibodies revealed that the Kv beta 1 and Kv beta 2 polypeptides frequently co-localize and are concentrated in neuronal perikarya, dendrites, and terminal fields, and in the juxtaparanodal region of myelinated axons. Immunoblot and reciprocal co-immunoprecipitation analyses indicated that Kv beta 2 is the major beta subunit present in rat brain membranes, and that most K+ channel complexes containing Kv beta 1 also contain Kv beta 2. Taken together, these data suggest that Kv beta 2 is a component of almost all K+ channel complexes containing Kv 1 alpha subunits, and that individual channels may contain two or more biochemically and functionally distinct beta-subunit polypeptides.

In vivo functional role of the Drosophila hyperkinetic beta subunit in gating and inactivation of Shaker K+ channels

The physiological roles of the beta, or auxiliary, subunits of voltage-gated ion channels, including Na+, Ca2+, and K+ channels, have not been demonstrated directly in vivo. Drosophila Hyperkinetic (Hk) mutations alter a gene encoding a homolog of the mammalian K+ channel beta subunit, providing a unique opportunity to delineate the in vivo function of auxiliary subunits in K+ channels. We found that the Hk beta subunit modulates a wide range of the Shaker (Sh) K+ current properties, including its amplitude, activation and inactivation, temperature dependence, and drug sensitivity. Characterizations of the existing mutants in identified muscle cells enabled an analysis of potential mechanisms of subunit interactions and their functional consequences. The results are consistent with the idea that via hydrophobic interaction, Hk beta subunits modulate Sh channel conformation in the cytoplasmic pore region. The modulatory effects of the Hk beta subunit appeared to be specific to the Sh alpha subunit because other voltage- and Ca(2+)-activated K+ currents were not affected by Hk mutations. The mutant effects were especially pronounced near the voltage threshold of IA activation, which can disrupt the maintenance of the quiescent state and lead to the striking neuromuscular and behavioral hyperexcitability previously reported.

Absence of the beta subunit (cchb1) of the skeletal muscle dihydropyridine receptor alters expression of the alpha 1 subunit and eliminates excitation-contraction coupling

The multisubunit (alpha 1s, alpha 2/delta, beta 1, and gamma) skeletal muscle dihydropyridine receptor transduces transverse tubule membrane depolarization into release of Ca2+ from the sarcoplasmic reticulum, and also acts as an L-type Ca2+ channel. The alpha 1s subunit contains the voltage sensor and channel pore, the kinetics of which are modified by the other subunits. To determine the role of the beta 1 subunit in channel activity and excitation-contraction coupling we have used gene targeting to inactivate the beta 1 gene. beta 1-null mice die at birth from asphyxia. Electrical stimulation of beta 1-null muscle fails to induce twitches, however, contractures are induced by caffeine. In isolated beta 1-null myotubes, action potentials are normal, but fail to elicit a Ca2+ transient. L-type Ca2+ current is decreased 10- to 20-fold in the beta 1-null cells compared with littermate controls. Immunohistochemistry of cultured myotubes shows that not only is the beta 1 subunit absent, but the amount of alpha 1s in the membrane also is undetectable. In contrast, the beta 1 subunit is localized appropriately in dysgenic, mdg/mdg, (alpha 1s-null) cells. Therefore, the beta 1 subunit may not only play an important role in the transport/insertion of the alpha 1s subunit into the membrane, but may be vital for the targeting of the muscle dihydropyridine receptor complex to the transverse tubule/sarcoplasmic reticulum junction.

Comparison of binding and block produced by alternatively spliced Kvbeta1 subunits

Voltage-gated K+ (Kv) channels consist of alpha subunits complexed with cytoplasmic Kvbeta subunits. Kvbeta1 subunits enhance the inactivation of currents expressed by the Kv1 alpha subunit subfamily. Binding has been demonstrated between the C terminus of Kvbeta1.1 and a conserved segment of the N terminus of Kv1.4, Kv1.5, and Shaker alpha subunits. Here we have examined the interaction and functional properties of two alternatively spliced human Kvbeta subunits, 1.2 and 1.3, with Kvalpha subunits 1.1, 1.2, 1.4, and 1.5. In the yeast two-hybrid assay, we found that both Kvbeta subunits interact specifically through their conserved C-terminal domains with the N termini of each Kvalpha subunit. In functional experiments, we found differences in modulation of Kv1alpha subunit currents that we attribute to the unique N-terminal domains of the two Kvbeta subunits. Both Kvbeta subunits act as open channel blockers at physiological membrane potentials, but hKvbeta1.2 is a more potent blocker than hKvbeta1.3 of Kv1.1, Kv1.2, Kv1.4, and Kv1. 5. Moreover, hKvbeta1.2 is sensitive to redox conditions, whereas hKvbeta1.3 is not. We suggest that different Kvbeta subunits extend the range over which distinct Kv1alpha subunits are modulated and may provide a variable mechanism for adjusting K+ currents in response to alterations in cellular conditions.

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

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

Ion channel associated proteins

Neuronal ion channels are directly associated in vivo with a wide variety of proteins. New classes of channel-associated proteins have been identified recently, including the PSD-95/SAP90 family of channel-clustering molecules and components of the synaptic vesicle release machinery. Recent findings suggest that non-pore-forming subunits of ion channels may also have cell biological functions independent of their effects on channel electrophysiological properties.

Generation and characterization of subtype-specific monoclonal antibodies to K+ channel alpha- and beta-subunit polypeptides

Molecular characterization of mammalian voltage-sensitive K+ channel genes and their expression became possible with the cloning of the Shaker locus of Drosophila. However, analysis of the expression patterns and subunit composition of native K+ channel protein complexes requires immunological probes specific for the individual K+ channel gene products expressed in excitable tissue. Here, we describe the generation and characterization of monoclonal antibodies (mAbs) against eight distinct mammalian K+ channel polypeptides; the Kv1.1, Kv1.2, Kv1.4, Kv1.5 and Kv1.6 Shaker-related alpha-subunits, the Kv2.1 Shab-related alpha-subunit, and the Kv beta 1 and Kv beta 2 beta-subunits. We characterized the subtype-specificity of these mAbs against native K+ channels in mammalian brain and against recombinant K+ channels expressed in transfected mammalian cells. In addition, we used these mAbs to investigate the cellular and subcellular distribution of the corresponding polypeptides in rat cerebral cortex, as well as their expression levels across brain regions.

Localization and interaction of epitope-tagged GIRK1 and CIR inward rectifier K+ channel subunits

GIRK1 and CIR are G-protein activated inward rectifier K+ channel subunits that combine to form the heteromultimer IKACh, the G beta gamma-activated atrial K channel responsible for the vagal slowing of heart rate. Epitope-tagged channel subunits were constructed by the introduction of distinct six amino acid epitopes into the C-termini or putative extracellular domains of GIRK1 and CIR. Carboxyl-terminal tagged subunits were activated by purified G beta gamma subunits in inside-out patches when expressed in Cos cells. Interestingly, insertion of three amino acids into the putative extracellular domain of GIRK1 resulted in an inactive subunit that acted as a dominant negative subunit when coexpressed with wild type GIRK1 and CIR in Xenopus oocytes. The epitope-tagged CIR-AU1 subunit coimmunoprecipitated GIRK1-AU5 from metabolically labeled Cos cells. Immunofluorescence labeling of Cos cells localized GIRK1-AU5 to internal cytoskeletal structures that co-stained with antibodies against the intermediate filament protein, vimentin. CIR-AU1 localized primarily to the plasma membrane. Double immunofluorescence labeling showed that GIRK1-AU5 plasma membrane staining was detectable only when coexpressed with CIR-AU1.