Structural determinant for assembly of mammalian K+ channels

Signalling via the G protein-activated K+ channels

The inwardly rectifying K+ channels of the GIRK (Kir3) family, members of the superfamily of inwardly rectifying K+ channels (Kir), are important physiological tools to regulate excitability in heart and brain by neurotransmitters, and the only ion channels conclusively shown to be activated by a direct interaction with heterotrimeric G protein subunits. During the last decade, especially since their cloning in 1993, remarkable progress has been made in understanding the structure, mechanisms of gating, activation by G proteins, and modulation of these channels. However, much of the molecular details of structure and of gating by G protein subunits and other factors, mechanisms of modulation and desensitization, and determinants of specificity of coupling to G proteins, remain unknown. This review summarizes both the recent advances and the unresolved questions now on the agenda in GIRK studies.

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

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

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.

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.

Identification and functional characterization of a K+ channel alpha-subunit with regulatory properties specific to brain

The physiological diversity of K+ channels mainly depends on the expression of several genes encoding different alpha-subunits. We have cloned a new K+ channel alpha-subunit (Kv2.3r) that is unable to form functional channels on its own but that has a major regulatory function. Kv2.3r can coassemble selectively with other alpha-subunits to form functional heteromultimeric K+ channels with kinetic properties that differ from those of the parent channels. Kv2.3r is expressed exclusively in the brain, being concentrated particularly in neocortical neurons. The functional expression of this regulatory alpha-subunit represents a novel mechanism without precedents in voltage-gated channels, which might contribute to further increase the functional diversity of K+ channels necessary to specify the intrinsic electrical properties of individual neurons.

Tetramerization of the AKT1 plant potassium channel involves its C-terminal cytoplasmic domain

All plant channels identified so far show high conservation throughout the polypeptide sequence except in the ankyrin domain which is present only in those closely related to AKT1. In this study, the architecture of the AKT1 protein has been investigated. AKT1 polypeptides expressed in the baculovirus/Sf9 cells system were found to assemble into tetramers as observed with animal Shaker-like potassium channel subunits. The AKT1 C-terminal intracytoplasmic region (downstream from the transmembrane domain) alone formed tetrameric structures when expressed in Sf9 cells, revealing a tetramerization process different from that of Shaker channels. Tests of subfragments from this sequence in the two-hybrid system detected two kinds of interaction. The first, involving two identical segments (amino acids 371-516), would form a contact between subunits, probably via their putative cyclic nucleotide-binding domains. The second interaction was found between the last 81 amino acids of the protein and a region lying between the channel hydrophobic core and the putative cyclic nucleotide-binding domain. As the interacting regions are highly conserved in all known plant potassium channels, the structural organization of AKT1 is likely to extend to these channels. The significance of this model with respect to animal cyclic nucleotide-gated channels is also discussed.

Identification and functional characterization of a K+ channel alpha-subunit with regulatory properties specific to brain

The physiological diversity of K+ channels mainly depends on the expression of several genes encoding different alpha-subunits. We have cloned a new K+ channel alpha-subunit (Kv2.3r) that is unable to form functional channels on its own but that has a major regulatory function. Kv2.3r can coassemble selectively with other alpha-subunits to form functional heteromultimeric K+ channels with kinetic properties that differ from those of the parent channels. Kv2.3r is expressed exclusively in the brain, being concentrated particularly in neocortical neurons. The functional expression of this regulatory alpha-subunit represents a novel mechanism without precedents in voltage-gated channels, which might contribute to further increase the functional diversity of K+ channels necessary to specify the intrinsic electrical properties of individual neurons.

Molecular determinants for assembly of G-protein-activated inwardly rectifying K+ channels

Kir3.1 and Kir3.2 associate to form G-protein-activated, inwardly rectifying K+ channels. To identify regions involved in the coassembly of these subunits, truncated Kir3.1 polypeptides were coexpressed with epitope-tagged subunits in an in vitro translation system. N-terminal, C-terminal, and core region polypeptides were coimmunoprecipitated with both Kir3.2 and Kir3.1, suggesting that multiple elements distributed throughout the Kir3.1 polypeptide contribute to intersubunit binding interactions. The Kir3.2 C-terminal polypeptide coimmunoprecipitated with the Kir3.1 C-terminal polypeptide, but neither region recognized the N-terminal domain and core region of the Kir3.1 subunit. This suggests that within Kir3 channels the C-terminal domains of neighboring subunits interact. Coexpression of the truncated polypeptides with Kir3.1 and Kir3.2 in Xenopus oocytes reduced functional expression of the heteromeric channels. Constructs encoding the core region plus N-terminal and proximal C-terminal regions competed more effectively than the core region alone, which supports the contribution of all three regions to intersubunit binding interactions. Proximal and distal segments of the C-terminal domain were as effective at inhibiting functional expression as the entire C-terminal domain.

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.

Membrane insertion, glycosylation, and oligomerization of inositol trisphosphate receptors in a cell-free translation system

In order to study the membrane topology, processing, and oligomerization of inositol trisphosphate receptor (IP3R) isoforms, we have utilized RNA templates encoding putative transmembrane domains to program a cell-free translation system of rabbit reticulocyte lysates supplemented with canine pancreas microsomes. In the absence of microsomes, translation of the RNA templates encoding all the putative transmembrane domains present in the C-terminal segment of the type I (1TM) and type III (3TM) IP3R isoforms resulted in a 62- and 59-kDa polypeptide, respectively. In both cases, an additional band approximately 3 kDa larger was observed upon the addition of microsomes. Both bands in the translation doublet were integrated into microsomal membranes and were full-length translation products, as shown by sedimentation through a sucrose cushion and immunoprecipitation with C-terminal isoform-specific antibodies. With both isoforms, N-glycopeptidase F digestion indicates that the upper band in the doublet corresponds to a glycosylated translation product. A 17-kDa protected fragment was observed after proteinase-K digestion of 1TM translated in the presence of microsomes. The pattern and size of protected fragments was consistent with the current six-transmembrane domain model of IP3R topology. Cotranslation of both 1TM and 3TM RNA templates in the presence of microsomes followed by immunoprecipitation with isoform specific antibodies revealed coimmunoprecipitation of translation products. This was not observed when the isoforms were translated separately and then mixed, suggesting that heteroligomerization occurs cotranslationally. A construct encoding only the first putative transmembrane domain of the type I isoform was found to be sufficient for integration into membranes but was unable to oligomerize with either 1TM or 3TM. Cotranslation experiments using additional constructs indicate that the major structural determinant for homoligomerization lies between putative transmembrane domain 5 and the C terminus. A second oligomerization domain involved in stabilization of heteroligomers is present within the first four transmembrane domains.

Kv2.1 and electrically silent Kv6.1 potassium channel subunits combine and express a novel current

Heteromultimer formation between Kv potassium channel subfamilies with the production of a novel current is reported for the first time. Protein-protein interactions between Kv2.1 and electrically silent Kv6.1 alpha-subunits were detected using two microelectrode voltage clamp and yeast two-hybrid measurements. Amino terminal portions of Kv6.1 were unable to form homomultimers but interacted specifically with amino termini of Kv2.1. Xenopus oocytes co-injected with Kv6.1 and Kv2.1 cRNAs exhibited a novel current with decreased rates of deactivation, decreased sensitivity to TEA block, and a hyperpolarizing shift of the half maximal activation potential when compared to Kv2.1. Our results indicate that Kv channel subfamilies can form heteromultimeric channels and, for the first time, suggest a possible functional role for the Kv6 subfamily.

Dimerization of TWIK-1 K+ channel subunits via a disulfide bridge

TWIK-1 is a new type of K+ channel with two P domains and is abundantly expressed in human heart and brain. Here we show that TWIK-1 subunits can self-associate to give dimers containing an interchain disulfide bridge. This assembly involves a 34 amino acid domain that is localized to the extracellular M1P1 linker loop. Cysteine 69 which is part of this interacting domain is implicated in the formation of the disulfide bond. Replacing this cysteine with a serine residue results in the loss of functional K+ channel expression. This is the first example of a covalent association of functional subunits in voltage-sensitive channels via a disulfide bridge.

Voltage-gated K+ channels contain multiple intersubunit association sites

A domain in the cytoplasmic NH2 terminus of voltage-gated K+ channels supervises the proper assembly of specific tetrameric channels (Li, M., Jan, J. M., and Jan, L. Y.(1992) Science 257, 1225-1230; Shen, N. V., Chen X., Boyer, M. M., and Pfaffinger, P. (1993) Neuron 11, 67-76). It is referred to as a first tetramerization domain, or T1 (Shen, N. V., Chen X., Boyer, M. M., and Pfaffinger, P.(1993) Neuron 11, 67-76). However, a deletion mutant of Kv1.3 that lacks the first 141 amino acids, Kv1.3 (T1(-)) forms functional channels, suggesting that additional association sites in the central core of Kv1.3 mediate oligomerization. To characterize these sites, we have tested the abilities of cRNA Kv1.3 (T1(-)) fragments co-injected with Kv1.3 (T1(-)) to suppress current in Xenopus oocytes. The fragments include portions of the six putative transmembrane segments, S1 through S6, specifically: S1, S1-S2, S1-S2-S3, S2-S3, S2-S3-S4, S3-S4, S3-S4-S5, S2 through COOH, S3 through COOH, S4 through COOH, and S5-S6-COOH. Electrophysiologic experiments show that the fragments S1-S2-S3, S3-S4-S5, S2 through COOH, and S3 through COOH strongly suppress Kv1.3 (T1(-)) current, while others do not. Suppression of expressed current is due to specific effects of the translated peptide Kv1.3 fragments, as validated by in vivo immunoprecipitation studies of a strong suppressor and a nonsuppressor. Pulse-chase experiments indicate that translation of truncated peptide fragments neither prevents translation of Kv1.3 (T1(-)) nor increases its rate of degradation. Co-immunoprecipitation experiments suggest that suppression involves direct association of a peptide fragment with Kv1.3 (T1(-)). Fragments that strongly suppress Kv1.3 (T1(-)) also suppress an analogous NH2-terminal deletion mutant of Kv2.1 (Kv2.1 (DeltaN139)), an isoform belonging to a different subfamily. Our results indicate that sites in the central core of Kv1.3 facilitate intersubunit association and that there are suppression sites in the central core, which are promiscuous across voltage-gated K+ channel subfamilies.

Molecular cloning of a glibenclamide-sensitive, voltage-gated potassium channel expressed in rabbit kidney

Shaker genes encode voltage-gated potassium channels (Kv). We have shown previously that genes from Shaker subfamilies Kv1.1, 1.2, 1.4 are expressed in rabbit kidney. Recent functional and molecular evidence indicate that the predominant potassium conductance of the kidney medullary cell line GRB-PAP1 is composed of Shaker-like potassium channels. We now report the molecular cloning and functional expression of a new Shaker-related voltage-gated potassium channel, rabKv1.3, that is expressed in rabbit brain and kidney medulla. The protein, predicted to be 513 amino acids long, is most closely related to the Kv1.3 family although it differs significantly from other members of that family at the amino terminus. In Xenopus oocytes, rabKv1.3 cRNA expresses a voltage activated K current with kinetic characteristics similar to other members of the Kv1.3 family. However, unlike previously described Shaker channels, it is sensitive to glibenclamide and its single channel conductance saturates. This is the first report of the functional expression of a voltage-gated K channel clone expressed in kidney. We conclude that rabKv1.3 is a novel member of the Shaker superfamily that may play an important role in renal potassium transport.

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.

Assembly and suppression of endogenous Kv1.3 channels in human T cells

The predominant K+ channel in human T lymphocytes is Kv1.3, which inactivates by a C-type mechanism. To study assembly of these tetrameric channels in Jurkat, a human T-lymphocyte cell line, we have characterized the formation of heterotetrameric channels between endogenous wild-type (WT) Kv1.3 subunits and heterologously expressed mutant (A413V) Kv1.3 subunits. We use a kinetic analysis of C-type inactivation of currents produced by homotetrameric channels and heterotetrameric channels to determine the distribution of channels with different subunit stoichiometries. The distributions are well-described by either a binomial distribution or a binomial distribution plus a fraction of WT homotetramers, indicating that subunit assembly is a random process and that tetramers expressed in the plasma membrane do not dissociate and reassemble. Additionally, endogenous Kv1.3 current is suppressed by a heterologously expressed truncated Kv1.3 that contains the amino terminus and the first two transmembrane segments. The time course for suppression, which is maximal at 48 h after transfection, overlaps with the time interval for heterotetramer formation between heterologously expressed A413V and endogenous WT channels. Our findings suggest that diversity of K+ channel subtypes in a cell is regulated not by spatial segregation of monomeric pools, but rather by the degree of temporal overlap and the kinetics of subunit expression.

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.

Dominant negative chimeras provide evidence for homo and heteromultimeric assembly of inward rectifier K+ channel proteins via their N-terminal end

Chimeras have been constructed using three different fragments (N-terminal, central and C-terminal) of IRK3, a constitutive inward rectifier K+ channel subunit, and GIRK2, a G-protein activated inward rectifier K+ channel subunit and have been coinjected into Xenopus oocytes together with IRK3 or IRK1 (another constitutive inward rectifier) cRNA. Both IRK1 and IRK3 expression was inhibited by coinjection with chimeras containing a N-terminal fragment of IRK3 suggesting that subunits of K+ channels in the IRK family form a functional multimeric assembly where the N-terminal end has an important role. In situ hybridization shows that IRK1 and IRK3 are coexpressed in the same areas of the brain and probably in the same cells. Taken together both the localization and the oocyte expression results suggest that not only homomultimeric IRK1 or homomultimeric IRK3 assemblies take place but that heteromultimeric IRK1/IRK3 assemblies are also formed.

Probing molecular identity of native single potassium channels by overexpression of dominant negative subunits

Overexpression of dominant negative subunits previously has been shown to affect the whole cell delayed-rectifier potassium current (Ikv) in Xenopus spinal neurons. Here, we show that effects of overexpression of wild-type and dominant negative Kv1 channels are evident at the single channel level. The goal of these studies was to match molecular species of Kv subunits to specific, functionally identified single voltage-dependent potassium channels. In a heterologous system (the Xenopus oocyte), co-expression of wild-type and dominant negative mutant Kv1.1 subunits results in loss of active channels rather than channels of altered conductance. However, in situ overexpression studies are difficult to interpret due to the diversity in the control population of channels. Therefore, identification of endogenous channel populations containing Kv1 subunits is limited. Future work will reduce the endogenous diversity of potassium channels by study of single channels in identified subtypes of neurons.

Shaker K+ channel T1 domain self-tetramerizes to a stable structure

The potassium channel T1 domain plays an important role in the regulated assembly of subunit proteins. We have examined the assembly properties of the Shaker channel T1 domain to determine if the domain can self-assemble, the number of subunits in a multimer, Ns and the mechanism of assembly. High pressure liquid chromatography (HPLC) size exclusion chromotography (SEC) separates T1 domain proteins into two peaks. By co-assembly assays, these peaks are identified to be a high molecular weight assembled form and a low molecular weight monomeric form. To determine the Ns of the assembled protein peak on HPLC SEC, we first cross-linked the T1 domain proteins and then separated them on HPLC. Four evenly spaced bands co-migrate with the assembled protein peak; thus, the T1 domain assembles to form a tetramer. The absence of separate dimeric and trimeric peaks of assembled T1 domain protein suggests that the tetramer is the stable assembled state, most probably a closed ring structure.

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.

Microheterogeneity in heteromultimeric assemblies formed by Shaker (Kv1) and Shaw (Kv3) subfamilies of voltage-gated K+ channels

Single K+ channels were recorded in Xenopus oocytes injected with a 1:1 mixture of mRNAs coding for NGK1 (Kv1.2) and NGK2 (Kv3.1a) voltage-dependent K+ channels. A new class of channels of 18 pS conductance was observed, and was designated as NGK1,2 channels. According to their properties of activation voltages and open life times, four types of NGK1,2 channels with microheterogeneity were detected. The results suggest that voltage-dependent NGK1 Shaker and NGK2 Shaw K+ channels, from different subfamilies, assemble to form heteromultimeric K+ channels, giving rise to a mosaic of characteristics inherited from two parental channels.

Molecular recognition and assembly sequences involved in the subfamily-specific assembly of voltage-gated K+ channel subunit proteins

We are analyzing features of the K+ channel subunit proteins that are critical for function and regulation of these proteins. Our studies show biochemically that subunit proteins from the Shaker and Shaw subfamilies fail to assemble into a heteromultimer. The basis for this incompatibility is the sequences contained within the T1 assembly domain. For a subunit protein to heteromultimerize with a Shaker subunit protein, two regions within the T1 domain, A and B, must be of the Shaker subtype. Finally, we show that the incompatibility of a Shaw A region for assembly with a Shaker protein depends upon the composition of a 30 amino acid conserved sequence in the A region.

Truncated K+ channel DNA sequences specifically suppress lymphocyte K+ channel gene expression

We have constructed a series of deletion mutants of Kv1.3, a Shaker-like, voltage-gated K+ channel, and examined the ability of these truncated mutants to form channels and to specifically suppress full-length Kv1.3 currents. These constructs were expressed heterologously in both Xenopus oocytes and a mouse cytotoxic T cell line. Our results show that a truncated mutant Kv1.3 must contain both the amino terminus and the first transmembrane-spanning segment, S1, to suppress full-length Kv1.3 currents. Amino-terminal-truncated DNA sequences from one subfamily suppress K+ channel expression of members of only the same subfamily. The first 141 amino acids of the amino-terminal of Kv1.3 are not necessary for channel formation. Deletion of these amino acids yields a current identical to that of full-length Kv1.3, except that it cannot be suppressed by a truncated Kv1.3 containing the amino terminus and S1. To test the ability of truncated Kv1.3 to suppress endogenous K+ currents, we constructed a plasmid that contained both truncated Kv1.3 and a selection marker gene (mouse CD4). Although constitutively expressed K+ currents in Jurkat (a human T cell leukemia line) and GH3 (an anterior pituitary cell line) cells cannot be suppressed by this double-gene plasmid, stimulated (up-regulated) Shaker-like K+ currents in GH3 cells can be suppressed.

Glucocorticoid induction of Kv1.5 K+ channel gene expression in ventricle of rat heart

Multiple voltage-gated K+ channels contribute to the repolarization phases of the cardiac action potential and are targets of several antiarrhythmic drugs. The Kv1.5 K+ channel gene is expressed in the heart, and heterologous expression of this gene generates a slowly inactivating K+ current. Previously, we found that glucocorticoids specifically upregulate pituitary Kv1.5 gene expression. To test whether these steroids might also induce Kv1.5 gene expression in the heart, cardiac channel mRNA and protein were measured by RNase protection assay and by immunoblotting with antibody specific for the extracellular domain of Kv1.5 polypeptide. Kv1.5 mRNA and immunoreactive protein appeared to be more abundant in rat ventricle than atrium. Reduction of endogenous glucocorticoids by adrenalectomy decreased ventricular Kv1.5 mRNA approximately 8-fold, which was estimated by using cyclophilin mRNA as an internal control. Kv1.5 immunoreactive protein also decreased approximately 6-fold. Injection of dexamethasone into adrenalectomized rats acted within a day to increase ventricular Kv1.5 mRNA and immunoreactive protein approximately 50-fold and approximately 20-fold, respectively. In contrast, atrial Kv1.5 mRNA expression was unaffected by either adrenalectomy or injection of the glucocorticoid agonist. Furthermore, dexamethasone-induced upregulation was specific for Kv1.5, since whole-heart Kv1.4 and Kv2.1 mRNA levels, as well as ventricular Kv2.1 mRNA expression, were unchanged. Thus, dexamethasone specifically upregulates Kv1.5 K+ channel gene expression in rat ventricle but not atrium. Glucocorticoids may affect excitability of ventricular myocytes and the efficacy of clinically useful drugs by changing the expression of the Kv1.5 K+ channel.