A novel beta subunit increases rate of inactivation of specific voltage-gated potassium channel alpha subunits

Cloning, expression, and distribution of a Ca(2+)-activated K+ channel beta-subunit from human brain

We have cloned and expressed a Ca(2+)-activated K+ channel beta-subunit from human brain. The open reading frame encodes a 191-amino acid protein possessing significant homology to a previously described subunit cloned from bovine muscle. The gene for this subunit is located on chromosome 5 at band q34 (hslo-beta). There is no evidence for alternative RNA splicing of this gene product. hslo-beta mRNA is abundantly expressed in smooth muscle, but expression levels are low in most other tissues, including brain. Brain subregions in which beta-subunit mRNA expression is relatively high are the hippocampus and corpus callosum. The coexpression of hslo-beta mRNA together with hslo-alpha subunits in either Xenopus oocytes or stably transfected HEK 293 cells give rise to Ca(2+)-activated potassium currents with a much increased calcium and/or voltage sensitivity. These data indicate that the beta-subunit shows a tissue distribution different to that of the alpha-subunit, and in many tissues there may be no association of alpha-subunits with beta-subunits. These beta-subunits can play a functional role in the regulation of neuronal excitability by tuning the Ca2+ and/or the voltage dependence of alpha-subunits.

Layer-specific properties of the transient K current (IA) in piriform cortex

Piriform cortex in the rat is highly susceptible to induction of epileptiform activity. Experiments in vivo and in vitro indicate that this activity originates in endopiriform nucleus (EN). In slices, EN neurons are more excitable than layer II (LII) pyramidal cells, with more positive resting potentials and lower spike thresholds. We investigated potassium currents in EN and LII to evaluate their contribution to these differences in excitability. Whole-cell currents were recorded from identified cells in brain slices. A rapidly inactivating outward current (IA) had distinct properties in LII (IA,LII) versus EN (IA,EN). The peak amplitude of IA,EN was 45% smaller than IA,LII, and the kinetics of activation and inactivation was significantly slower for IA,EN. The midpoint of steady-state inactivation was hyperpolarized by 10 mV for IA,EN versus IA,LII, whereas activation was similar in the two cell groups. Other voltage-dependent potassium currents were indistinguishable between EN and LII. Simulations using a compartmental model of LII cells argue that different cellular distributions of IA channels in EN versus LII cells cannot account for these differences. Thus, at least some of the differences are intrinsic to the channels themselves. Current-clamp simulations suggest that the differences between IA,LII and IA,EN can account for the observed difference in resting potentials between the two cell groups. Simulations show that this difference in resting potential leads to longer first spike latencies in response to depolarizing stimuli. Thus, these differences in the properties of IA could make EN more susceptible to induction and expression of epileptiform activity.

In situ hybridization reveals extensive diversity of K+ channel mRNA in isolated ferret cardiac myocytes

The molecular basis of K+ currents that generate repolarization in the heart is uncertain. In part, this reflects the similar functional properties different K+ channel clones display when heterologously expressed, in addition to the molecular diversity of the voltage-gated K+ channel family. To determine the identity, regional distribution, and cellular distribution of voltage-sensitive K+ channel mRNA subunits expressed in ferret heart, we used fluorescent labeled oligonucleotide probes to perform in situ hybridization studies on enzymatically isolated myocytes from the sinoatrial (SA) node, right and left atria, right and left ventricles, and interatrial and interventricular septa. The most widely distributed K+ channel transcripts in the ferret heart were Kv1.5 (present in 69.3% to 85.6% of myocytes tested, depending on the anatomic region from which myocytes were isolated) and Kv1.4 (46.1% to 93.7%), followed by kv1.2, Kv2.1, and Kv4.2. Surprisingly, many myocytes contain transcripts for Kv1.3, Kv2.2, Kv4.1, Kv5.1, and members of the Kv3 family. Kv1.1, Kv1.6, and Kv6.1, which were rarely expressed in working myocytes, were more commonly expressed in SA nodal cells. IRK was expressed in ventricular (84.3% to 92.8%) and atrial (52.4% to 64.0%) cells but was nearly absent (6.6%) in SA nodal cells; minK was most frequently expressed in SA nodal cells (33.7%) as opposed to working myocytes (10.3% to 29.3%). Two gene products implicated in long-QT syndrome, ERG and KvLQT1, were common in all anatomic regions (41.1% to 58.2% and 52.1% to 71.8%, respectively). These results show that the diversity of K+ channel mRNA in heart is greater than previously suspected and that the molecular basis of K+ channels may vary from cell to cell within distinct regions of the heart and also between major anatomic regions.

Molecular properties of voltage-gated K+ channels

Subfamilies of voltage-activated K+ channels (Kv1-4) contribute to controlling neuron excitability and the underlying functional parameters. Genes encoding the multiple alpha subunits from each of these protein groups have been cloned, expressed and the resultant distinct K+ currents characterized. The predicted amino acid sequences showed that each alpha subunit contains six putative membrane-spanning alpha-helical segments (S1-6), with one (S4) being deemed responsible for the channels' voltage sensing. Additionally, there is an H5 region, of incompletely defined structure, that traverses the membrane and forms the ion pore; residues therein responsible for K+ selectively have been identified. Susceptibility of certain K+ currents produced by the Shaker-related subfamily (Kv1) to inhibition by alpha-dendrotoxin has allowed purification of authentic K+ channels from mammalian brain. These are large (M(r) approximately 400 kD), octomeric sialoglycoproteins composed of alpha and beta subunits in a stoichiometry of (alpha)4(beta)4, with subtypes being created by combinations of subunit isoforms. Subsequent cloning of the genes for beta 1, beta 2 and beta 3 subunits revealed novel sequences for these hydrophilic proteins that are postulated to be associated with the alpha subunits on the inner side of the membrane. Coexpression of beta 1 and Kv1.4 subunits demonstrated that this auxiliary beta protein accelerates the inactivation of the K+ current, a striking effect mediate by an N-terminal moiety. Models are presented that indicate the functional domains pinpointed in the channel proteins.

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

Voltage-gated K+ channels are protein complexes composed of ion-conducting integral membrane alpha subunits and cytoplasmic beta subunits. Here, we show that, in transfected mammalian cells, the predominant beta subunit isoform in brain, Kv beta 2, associates with the Kv1.2 alpha subunit early in channel biosynthesis and that Kv beta 2 exerts multiple chaperone-like effects on associated Kv1.2 including promotion of cotranslational N-linked glycosylation of the nascent Kv1.2 polypeptide, increased stability of Kv beta 2/Kv1.2 complexes, and increased efficiency of cell surface expression of Kv1.2. Taken together, these results indicate that while some cytoplasmic K+ channel beta subunits affect the inactivation kinetics of alpha subunits, a more general, and perhaps more fundamental, role is to mediate the biosynthetic maturation and surface expression of voltage-gated K+ channel complexes. These findings provide a molecular basis for recent genetic studies indicating that beta subunits are key determinants of neuronal excitability.

Selective interaction of voltage-gated K+ channel beta-subunits with alpha-subunits

To begin to study the molecular bases that determine the selective interaction of the beta-subunits of voltage-gated K+ channels with alpha-subunits observed in situ, we have expressed these polypeptides in transfected mammalian cells. Analysis of the specificity of alpha/bet a-subunit interaction indicates that both the Kvbeta1 and Kvbeta2 beta-subunits display robust and selective interaction with the five members of the Shaker-related (Kv1) alpha-subunit subfamily tested. The interaction of these beta-subunits with Kv1 alpha-subunits does not require the beta-subunit N-terminal domains. Thus, the previously observed failure of N-terminal mutants of Kv beta1 to modulate inactivation kinetics of Kv1 family members is not simply due to a lack of subunit interaction. Interaction of these beta-subunits with members of two other subfamilies (Shab- and Shaw-related) could not be detected. Somewhat surprisingly, a member of the Shal-related subfamily was found to interact with beta-subunits; however, this interaction had biochemical characteristics distinct from the beta-subunit interaction with Kv1 family members. In all cases, Kvbeta1 and Kvbeta2 exhibited indistinguishable alpha-subunit selectivity. These studies point to a selective interaction between K+ channel alpha- and beta-subunits mediated through conserved domains in the respective subunits.

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

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

NAB domain is essential for the subunit assembly of both alpha-alpha and alpha-beta complexes of shaker-like potassium channels

There are at least five subfamilies of Shaker-like K+ channels. The diverse function of K+ channels are thought to be further modulated by hydrophilic beta subunits. Here we report that Kv beta 1 inactivates RCK4 and Shaker B K+ channels of the Kv1 subfamily, but not Shal2 of the Kv4 subfamily. This correlates the subfamily-specific bindings of Kv beta 1 to the cytoplasmic N-terminal domains of Kv1 alpha subunits. We map the Kv beta 1-binding site to a region overlapping NABKv1, a domain that specifies different Kv1 alpha subunits to form heterotetramers. Using chimeric alpha subunits, we demonstrate that NABKv1 is essential for the Kv beta 1-mediated inactivation. These results suggest that Kv beta 1 modulates a subset of K+ channels through the specific assembly of alpha-beta complexes and reveal the dual function of the NAB domain in mediating the assembly of both alpha-alpha and alpha-beta complexes.

The cardiac ion channels: relevance to management of arrhythmias

The electrical activity of cardiac tissue is determined by the highly regulated flow of ions across the cell membrane during the cardiac action potential. Ion channels are pore-forming proteins through which these electric currents flow. In this review, the ion currents that underlie the action potential are first described. Then, the way in which expression of individual ion-channel genes results in such ion currents is discussed. Finally, the concept that arrhythmias may be due to abnormalities of structure, function, or number of ion channels, or the way in which they respond to abnormalities in their environment (such as acute ischemia), is reviewed. Further understanding of the molecular mechanisms underlying normal and abnormal cardiac electrophysiologic behavior should allow the development of safer and more effective antiarrhythmic interventions.

Kv beta 1 subunit binding specific for shaker-related potassium channel alpha subunits

Voltage-activated potassium (Kv) channels from mammalian brain are hetero-oligomers containing alpha and beta subunits. Coexpression of Kv1 alpha and Kv beta 1 subunits confers rapid A-type inactivation on noninactivating potassium channels (delayed rectifiers) in expression systems in vitro. We have delineated a Kv1.5 aminoterminal region of up to 90 amino acids (residues 112-201) that is sufficient for interactions of Kv1.5 alpha and Kv beta 1 subunits. Within this region of the Kv1.5 amino terminus (residues 193-201), a Kv beta 1 interaction site necessary for Kv beta 1-mediated rapid inactivation of Kv1.5 currents was detected. This interaction site motif (FYE/QLGE/DEAM/L) is found exclusively in the Shaker-related subfamily (Kv1). The results show that hetero-oligomerization between alpha and Kv beta 1 subunits is restricted to Shaker-related potassium channel alpha subunits.

The modulation of the rate of inactivation of the mKv1.1 K+ channel by the beta subunit, Kv beta 1 and lack of effect of a Kv beta 1 N-terminal peptide

The coexpression of the rat Kv beta 1 subunit with the mouse Kv1.1 (mKv1.1) K+ channel in Chinese hamster ovary cells caused an increase in the rate of inactivation of whole-cell current. Current decayed in a bi-exponential fashion with a fast voltage-dependent and a slower voltage-independent component. The inactivating current component accounted for around 40% of the total outward current. In contrast to previous studies using K+ channel alpha subunits, peptides based on the N-terminal of the Kv beta 1 subunit were unable to mimic the action of the entire subunit. The findings indicate differences between the inactivation induced by the Kv beta 1 subunit and the N-type inactivation mechanism associated with certain rapidly-inactivating cloned K+ channel alpha subunits.

Structural and functional characterization of human potassium channel subunit beta 1 (KCNA1B)

Voltage-activated Shaker-related potassium channels (kv1) consist of alpha and beta subunits. We have analysed the structure of the human KCNA1B (hKv beta 1) gene. KCNA1B is > 250 kb in size and encodes at least three Kv beta 1 splice variants. The Kv beta 1 open reading frame is divided into 14 exons. In contrast, genes coding for family members of KCNA (Kv 1 alpha) subunits are markedly smaller and have intronless open reading frames. The expression of Kv 1 alpha and Kv beta mRNA was compared in Northern blots of poly(A+) RNA isolated from various human brain tissues. The results suggest an intricate and cell-specific regulation of Kv 1 alpha and Kv beta mRNA synthesis such that distinct combinations of alpha and beta subunits would occur in different nuclei of the brain. The splice variants hKv beta 1.1 and hKv beta 1.2 were functionally characterized in coexpression studies with hKv 1.5 alpha subunits in 293 cells. It is shown that the confer rapid inactivation on hKv 1.5 channels with different potencies. This may be due to differences in their amino terminal sequences and/or inactivating domains. It is also shown that the amino terminal Kv beta 1.1 and Kv 1.4 alpha inactivating domains compete with each other, probably for the binding to the same receptor site(s) on Kv 1 alpha-subunits.

Molecular and functional characterization of a rat brain Kv beta 3 potassium channel subunit

A novel potassium channel beta-subunit (Kv beta 3) was cloned from rat brain being the third member of a Kv beta subunit gene family. It is a protein of 403 amino acid residues with a 68% amino acid sequence homology to Kv beta 1.1. Kv beta 3 is primarily expressed in rat brain having a distribution distinct to those of Kv beta 1.1 and Kv beta 2. This subunit also has a long N-terminal structure and induces inactivation in N-terminal deleted Kv1.4 but not in other members of the Kv1 channel family. Similarly to Kv beta 1.1, the Kv beta 3-induced inactivation is regulated by the intracellular redox potential.

A novel K+ channel beta-subunit (hKv beta 1.3) is produced via alternative mRNA splicing

Voltage-gated K+ channels can form multimeric complexes with accessory beta-subunits. We report here a novel K+ channel beta-subunit cloned from human heart, hKv beta 1.3, that has 74-83% overall identity with previously cloned beta-subunits. Comparison of hKv beta 1.3 with the previously cloned hKv beta 3 and rKv beta 1 proteins indicates that the carboxyl-terminal 328 amino acids are identical, while unique variable length amino termini exist. Analysis of human beta-subunit cDNA and genomic nucleotide sequences confirm that these three beta-subunits are alternatively spliced from a common beta-subunit gene. Co-expression of hKv beta 1.3 in Xenopus oocytes with the delayed rectifier hKv1.5 indicated that hKv beta 1.3 has unique functional effects. This novel beta-subunit induced a time-dependent inactivation during membrane voltage steps to positive potentials, induced a 13-mV hyperpolarizing shift in the activation curve, and slowed deactivation (tau = 13 +/- 0.5 ms versus 35 +/- 1.7 ms at -40 mV). Most notably, hKv beta 1.3 converted the Kv1.5 outwardly rectifying current voltage relationship to one showing strong inward rectification. These data suggest that Kv channel current diversity may arise from association with alternatively spliced Kv beta-subunits. A simplified nomenclature for the K+ channel beta-subunit subfamilies is suggested.

Assembly of voltage-gated potassium channels. Conserved hydrophilic motifs determine subfamily-specific interactions between the alpha-subunits

Voltage-gated potassium (K+) channels are assembled by four identical or homologous alpha-subunits to form a tetrameric complex with a central conduction pore for potassium ions. Most of the cloned genes for the alpha-subunits are classified into four subfamilies: Kv1 (Shaker), Kv2 (Shab), Kv3 (Shaw), and Kv4 (Shal). Subfamily-specific assembly of heteromeric K+ channel complexes has been observed in vitro and in vivo, which contributes to the diversity of K+ currents. However, the molecular codes that mediate the subfamily-specific association remain unknown. To understand the molecular basis of the subfamily-specific assembly, we tested the protein-protein interactions of different regions of alpha-subunits. We report here that the cytoplasmic NH2-terminal domains of Kv1, Kv2, Kv3, and Kv4 subfamilies each associate to form homomultimers. Using the yeast two-hybrid system and eight K+ channel genes, two genes (one isolated from rat and one from Drosophila) from each subfamily, we demonstrated that the associations to form heteromultimers by the NH2-terminal domains are strictly subfamily-specific. These subfamily-specific associations suggest a molecular basis for the selective formation of heteromultimeric channels in vivo.

Alternative splicing of the human Shaker K+ channel beta 1 gene and functional expression of the beta 2 gene product

Mammalian voltage-activated Shaker K+ channels associate with at least three cytoplasmic proteins: Kv beta 1, Kv beta 2 and Kv beta 3. These beta subunits contain variable N-termini, which can modulate the inactivation of Shaker alpha subunits, but are homologous throughout an aldo-keto reductase core. Human and ferret beta 3 proteins are identical with rat beta 1 throughout the core while beta 2 proteins are not; beta 2 also contains a shorter N-terminus and has no reported physiological role. We report that human beta 1 and beta 3 are derived from the same gene and that beta 2 modulates the inactivation properties of Kv1.4 alpha subunits.

Characterization of a voltage-gated K+ channel beta subunit expressed in human heart

Voltage-gated K+ channels are important modulators of the cardiac action potential. However, the correlation of endogenous myocyte currents with K+ channels cloned from human heart is complicated by the possibility that heterotetrameric alpha-subunit combinations and function-altering beta subunits exist in native tissue. Therefore, a variety of subunit interactions may generate cardiac K+ channel diversity. We report here the cloning of a voltage-gated K+ channel beta subunit, hKv beta 3, from adult human left ventricle that shows 84% and 74% amino acid sequence identity with the previously cloned rat Kv beta 1 and Kv beta 2 subunits, respectively. Together these three Kv beta subunits share > 82% identity in the carboxyl-terminal 329 aa and show low identity in the amino-terminal 79 aa. RNA analysis indicated that hKv beta 3 message is 2-fold more abundant in human ventricle than in atrium and is expressed in both healthy and diseased human hearts. Coinjection of hKv beta 3 with a human cardiac delayed rectifier, hKv1.5, in Xenopus oocytes increased inactivation, induced an 18-mV hyperpolarizing shift in the activation curve, and slowed deactivation (tau = 8.0 msec vs. 35.4 msec at -50 mV). hKv beta 3 was localized to human chromosome 3 by using a human/rodent cell hybrid mapping panel. These data confirm the presence of functionally important K+ channel beta subunits in human heart and indicate that beta-subunit composition must be accounted for when comparing cloned channels with endogenous cardiac currents.