Mapping of the physical interaction between the intracellular domains of an inwardly rectifying potassium channel, Kir6.2

Advancements in the study of inward rectifying potassium channels on vascular cells

Inward rectifier potassium channels (Kir channels) exist in a variety of cells and are involved in maintaining resting membrane potential and signal transduction in most cells, as well as connecting metabolism and membrane excitability of body cells. It is closely related to normal physiological functions of body and the occurrence and development of some diseases. Although the functional expression of Kir channels and their role in disease have been studied, they have not been fully elucidated. In this paper, the functional expression of Kir channels in vascular endothelial cells and smooth muscle cells and their changes in disease states were reviewed, especially the recent research progress of Kir channels in stem cells was introduced, in order to have a deeper understanding of Kir channels in vascular tissues and provide new ideas and directions for the treatment of related ion channel diseases.

Functional mapping of the N-terminal arginine cluster and C-terminal acidic residues of Kir6.2 channel fused to a G protein-coupled receptor

Ion channel-coupled receptors (ICCRs) are original man-made ligand-gated ion channels created by fusion of G protein-coupled receptors (GPCRs) to the inward-rectifier potassium channel Kir6.2. GPCR conformational changes induced by ligand binding are transduced into electrical current by the ion channel. This functional coupling is closely related to the length of the linker region formed by the GPCR C-terminus (C-ter) and Kir6.2N-terminus (N-ter). Manipulating the GPCR C-ter length allows to finely tune the channel regulation, both in amplitude and sign (opening or closing Kir6.2). In this work, we demonstrate that the primary sequence of the channel N-terminal domain is an additional parameter for the functional coupling with GPCRs. As for all Kir channels, a cluster of basic residues is present in the N-terminal domain of Kir6.2 and is composed of 5 arginines which are proximal to the GPCR C-ter in the fusion proteins. Using a functional mapping approach, we demonstrate the role of specific arginines (R27 and R32) for the function of ICCRs, indicating that the position and not the cluster of positively-charged arginines is critical for the channel regulation by the GPCR. Following observations provided by molecular dynamics simulation, we explore the hypothesis of interaction of these arginines with acidic residues, and using site-directed mutagenesis, we identified aspartate D307 and glutamate E308 residues as critical for the function of ICCRs. These results demonstrate the critical role of the N-terminal and C-terminal charged residues of Kir6.2 for its allosteric regulation by the fused GPCR.

Regulation of neuronal M-channel gating in an isoform-specific manner: functional interplay between calmodulin and syntaxin 1A

Whereas neuronal M-type K(+) channels composed of KCNQ2 and KCNQ3 subunits regulate firing properties of neurons, presynaptic KCNQ2 subunits were demonstrated to regulate neurotransmitter release by directly influencing presynaptic function. Two interaction partners of M-channels, syntaxin 1A and calmodulin, are known to act presynaptically, syntaxin serving as a major protein component of the membrane fusion machinery and calmodulin serving as regulator of several processes related to neurotransmitter release. Notably, both partners specifically modulate KCNQ2 but not KCNQ3 subunits, suggesting selective presynaptic targeting to directly regulate exocytosis without interference in neuronal firing properties. Here, having first demonstrated in Xenopus oocytes, using analysis of single-channel biophysics, that both modulators downregulate the open probability of KCNQ2 but not KCNQ3 homomers, we sought to resolve the channel structural determinants that confer the isoform-specific gating downregulation and to get insights into the molecular events underlying this mechanism. We show, using optical, biochemical, electrophysiological, and molecular biology analyses, the existence of constitutive interactions between the N and C termini in homomeric KCNQ2 and KCNQ3 channels in living cells. Furthermore, rearrangement in the relative orientation of the KCNQ2 termini that accompanies reduction in single-channel open probability is induced by both regulators, strongly suggesting that closer N-C termini proximity underlies gating downregulation. Different structural determinants, identified at the N and C termini of KCNQ3, prevent the effects by syntaxin 1A and calmodulin, respectively. Moreover, we show that the syntaxin 1A and calmodulin effects can be additive or blocked at different concentration ranges of calmodulin, bearing physiological significance with regard to presynaptic exocytosis.

Coupling ion channels to receptors for biomolecule sensing

Nanoscale electrical biosensors are promising tools for diagnostics and high-throughput screening systems. The electrical signal allows label-free assays with a high signal-to-noise ratio and fast real-time measurements. The challenge in developing such biosensors lies in functionally connecting a molecule detector to an electrical switch. Advances in this field have relied on synthetic ion-conducting pores and modified ion channels that are not yet suitable for biomolecule screening. Here we report the design and characterization of a novel bioelectric-sensing platform engineered by coupling an ion channel, which serves as the electrical probe, to G-protein-coupled receptors (GPCRs), a family of receptors that detect molecules outside the cell. These ion-channel-coupled receptors may potentially detect a wide range of ligands recognized by natural or altered GPCRs, which are known to be major pharmaceutical targets. This could form a unique platform for label-free drug screening.

Identification of two Novel Frameshift Mutations in the KCNJ11 gene in two Italian patients affected by Congenital Hyperinsulinism of Infancy

Congenital Hyperinsulinism of Infancy (CHI) is a genetically heterogeneous disorder characterized by profound hypoglycemia related to inappropriate insulin secretion. Two histopathologically and genetically distinct groups are recognized among patients with CHI due to ATP-sensitive potassium channel (KATP) defects: a diffuse type (Di-CHI), which involves the whole pancreas, and a focal form (Fo-CHI), which shows adenomatous islet-cell hyperplasia of a particular area within the normal pancreas. The beta-cell KATP channel consists of two essential subunits: Kir6.2 encoded by the KCNJ11 gene which is the pore-forming unit and belongs to the inwardly rectifying potassium channel family, and SUR1 (sulfonylurea receptor 1) encoded by the ABCC8 gene, which belongs to the ATP-binding cassette (ABC) transporter family. The KATP channel is an octameric complex of four Kir6.2 and four SUR1 subunits. More than one hundred mutations have been found in KATP channel genes ABCC8 and KCNJ11, but to date only twenty mutations have been identified in KCNJ11, most of them are missense mutations and only one is a single base deletion. The Fo-CHI has been demonstrated to arise in individuals who have a germline mutation in the paternal allele of ABCC8 or KCNJ11 in addition to a somatic loss of the maternally derived chromosome region 11p15 in adenomatous pancreatic beta-cells, while Di-CHI predominantly arises from the autosomal recessive inheritance of KATP channel gene mutations. Here we describe the molecular findings in nine children who presented, in the neonatal period, with signs and symptoms of hypoglycemia and diagnosed affected by CHI according to international diagnostic criteria. Direct sequencing of the complete coding exon and promoter region of KCNJ11 gene showed, in two Italian patients, two new heterozygous mutations which result in the appearance of premature translation termination codons resulting in the premature end of Kir6.2. Interestingly most of the CHI mutations detected in other population studies are situated in the ABCC8 gene.

Kir6.2 channel gating by intracellular protons: subunit stoichiometry for ligand binding and channel gating

The adenosine triphosphate-sensitive K(+) (K(ATP)) channels are gated by several metabolites, whereas the gating mechanism remains unclear. Kir6.2, a pore-forming subunit of the K(ATP) channels, has all machineries for ligand binding and channel gating. In Kir6.2, His175 is the protonation site and Thr71 and Cys166 are involved in channel gating. Here, we show how individual subunits act in proton binding and channel gating by selectively disrupting functional subunits using these residues. All homomeric dimers and tetramers showed pH sensitivity similar to the monomeric channels. Concatenated construction of wild type with disrupted subunits revealed that none of these residues had a dominant-negative effect on the proton-dependent channel gating. Subunit action in proton binding was almost identical to that for channel gating involving Cys166, suggesting a one-to-one coupling from the C terminus to the M2 helix. This was significantly different from the effect of T71Y heteromultimers, suggesting distinct contributions of M1 and M2 helices to channel gating. Subunits underwent concerted rather than independent action. Two wild-type subunits appeared to act as a functional dimer in both cis and trans configurations. The understanding of K(ATP) channel gating by intracellular pH has a profound impact on cellular responses to metabolic stress as a significant drop in intracellular pH is more frequently seen under a number of physiological and pathophysiological conditions than a sole decrease in intracellular ATP levels.

Role of NH2 and COOH termini in targeting, stability, and activity of sodium bicarbonate cotransporter 1

Sodium bicarbonate cotransporter 1 (NBC1) mediates 80% of bicarbonate reabsorption by the kidney, but the molecular determinants for activity, targeting, and cell membrane stability are poorly understood. We generated truncation mutants involving the entire NH2 (ΔN424) or the entire COOH (ΔC92) terminus and examined the effects of these truncations on targeting, cell membrane stability, and NBC1 activity. ΔN424 and ΔC92 targeted to the plasma membrane of HEK293 cells or to the basolateral membrane of opossum kidney (OK) cells at 24 h but did not display NBC1 activity. Unlike the NBC1 wild-type and the ΔN424, ΔC92 expression was significantly decreased in the basolateral membrane at 48 h and yet the total ΔC92 expression in the cell was constant. We found that decreased ΔC92 expression in the basolateral membrane was due to increased endocytosis and mistargeting to the apical membrane. Increased endocytosis was prevented when both ΔN424 and ΔC92 were cotransfected together and more stable expression of ΔC92 was observed. Immunoprecipitation studies using NBC1 antibody specific for the COOH epitope were able to detect the COOH truncated NBC1 when probed with NH2 epitope-specific antibody or vice versa. Similar findings were observed with Ni-NTA pull-down assay. Cotransfection of both mutants partially restored NBC1 activity. In summary, NBC1 targets to the basolateral membrane of OK cells by a default mechanism and the COOH terminus plays a role on NBC1 stability in the basolateral membrane.

Mutation of critical GIRK subunit residues disrupts N- and C-termini association and channel function

The subfamily of G-protein-linked inwardly rectifying potassium channels (GIRKs) is coupled to G-protein receptors throughout the CNS and in the heart. We used mutational analysis to address the role of a specific hydrophobic region of the GIRK1 subunit. Deletion of the GIRK1 C-terminal residues 330-384, as well as the point mutation I331R, resulted in a decrease in channel function when coexpressed with GIRK4 in oocytes and in COS-7 cells. Surface protein expression of GIRK1 I331R coexpressed with GIRK4 was comparable with wild type, indicating that subunits assemble and are correctly localized to the membrane. Subsequent mutation of homologous residues in both the GIRK4 subunit and Kir2.1 (Gbetagamma-independent inward rectifier) also resulted in a decrease in channel function. Intracellular domain associations resulted in the coimmunoprecipitation of the GIRK1 N and C termini and GIRK4 N and C termini. The point mutation I331R in the GIRK1 C terminus or L337R in the GIRK4 C terminus decreased the association between the N and C termini. Mutation of a GIRK1 N-terminal hydrophobic residue, predicted structurally to interact with the C-terminal domain, also resulted in a decrease in channel function and termini association. We hypothesize that the hydrophobic nature of this GIRK1 subunit region is critical for interaction between adjacent termini and is permissive for channel gating. In addition, the homologous mutation in cytoplasmic domains of Kir2.1 (L330R) did not disrupt association, suggesting that the overall structural integrity of this region is critical for inward rectifier function.

Genotypes of the pancreatic beta-cell K-ATP channel and clinical phenotypes of Japanese patients with persistent hyperinsulinaemic hypoglycaemia of infancy

OBJECTIVE

Persistent hyperinsulinaemic hypoglycaemia of infancy (PHHI) is a disorder of glucose metabolism that is characterized by dysregulated secretion of insulin from pancreatic beta-cells. This disease has been reported to be associated with mutations of the sulfonylurea receptor SUR1 (ABCC8) or the inward-rectifying potassium channel Kir6.2 (KCNJ11), which are two subunits of the pancreatic beta-cell ATP-sensitive potassium channel.

PATIENTS AND METHODS

In 14 Japanese PHHI patients, all exons of SUR1 and Kir6.2 genes were analysed by polymerase chain reaction (PCR) and direct sequencing. Four patients responded to diazoxide, and nine patients underwent a subtotal pancreatectomy. Histologically, seven patients were diagnosed to have a focal form and two a diffuse form of the disease.

RESULTS

We found nine novel mutations in the SUR1 gene and two in the Kir6.2 gene. In the SUR1 gene mutations, three were nonsense mutations (Y512X, Y1354X and G1469X), one was a one-base deletion in exon 7, and two were missense mutations in the nucleotide-binding domain 2 (K1385Q, R1487K). The other three mutations occurred in introns 14, 29 and 36, which might cause aberrant splicing of RNA. Two siblings in one family were heterozygotes for a missense mutation, K1385Q, which was maternally inherited. In Kir6.2 gene screening, one patient was found to be a compound heterozygote of a missense mutation (R34H) and a one-base deletion (C344fs/ter).

CONCLUSION

The novel mutations reported here could be pathological candidates for PHHI in Japan. They also reveal that SUR1 and Kir6.2 mutations in the Japanese population exhibit heterogeneity and that they occurred at a frequency similar to other genetic populations.

Novel nucleotide-binding sites in ATP-sensitive potassium channels formed at gating interfaces

The coupling of cell metabolism to membrane electrical activity is a vital process that regulates insulin secretion, cardiac and neuronal excitability and the responses of cells to ischemia. ATP-sensitive potassium channels (K(ATP); Kir6.x) are a major part of this metabolic-electrical coupling system and translate metabolic signals such as the ATP:ADP ratio to changes in the open or closed state (gate) of the channel. The localization of the nucleotide-binding site (NBS) on Kir6.x channels and how nucleotide binding gates these K(ATP) channels remain unclear. Here, we use fluorescent nucleotide binding to purified Kir6.x proteins to define the peptide segments forming the NBS on Kir6.x channels and show that unique N- and C-terminal interactions from adjacent subunits are required for high-affinity nucleotide binding. The short N- and C-terminal segments comprising the novel intermolecular NBS are next to helices that likely move with channel opening/closing, suggesting a lock-and-key model for ligand gating.

Functional analysis of a structural model of the ATP-binding site of the KATP channel Kir6.2 subunit

ATP-sensitive potassium (KATP) channels couple cell metabolism to electrical activity by regulating K+ flux across the plasma membrane. Channel closure is mediated by ATP, which binds to the pore-forming subunit (Kir6.2). Here we use homology modelling and ligand docking to construct a model of the Kir6.2 tetramer and identify the ATP-binding site. The model is consistent with a large amount of functional data and was further tested by mutagenesis. Ligand binding occurs at the interface between two subunits. The phosphate tail of ATP interacts with R201 and K185 in the C-terminus of one subunit, and with R50 in the N-terminus of another; the N6 atom of the adenine ring interacts with E179 and R301 in the same subunit. Mutation of residues lining the binding pocket reduced ATP-dependent channel inhibition. The model also suggests that interactions between the C-terminus of one subunit and the 'slide helix' of the adjacent subunit may be involved in ATP-dependent gating. Consistent with a role in gating, mutations in the slide helix bias the intrinsic channel conformation towards the open state.

Structural insights into the regulatory mechanism of IP3 receptor

Inositol 1,4,5-trisphosphate receptors (IP(3)R) are intracellular Ca(2+) release channels whose opening requires binding of two intracellular messengers IP(3) and Ca(2+). The regulation of IP(3)R function has also been shown to involve a variety of cellular proteins. Recent biochemical and structural analyses have deepened our understanding of how the IP(3)-operated Ca(2+) channel functions. Specifically, the atomic resolution structure of the IP(3)-binding region has provided a sound structural basis for the receptor interaction with the natural ligand. Electron microscopic studies have also shed light on the overall shape of the tetrameric receptor. This review aims to provide comprehensive overview of the current information available on the structure and function relationship of IP(3)R.

Concerted gating mechanism underlying KATP channel inhibition by ATP

KATP channels assemble from four regulatory SUR1 and four pore-forming Kir6.2 subunits. At the single-channel current level, ATP-dependent gating transitions between the active burst and the inactive interburst conformations underlie inhibition of the KATP channel by intracellular ATP. Previously, we identified a slow gating mutation, T171A in the Kir6.2 subunit, which dramatically reduces rates of burst to interburst transitions in Kir6.2DeltaC26 channels without SUR1 in the absence of ATP. Here, we constructed all possible mutations at position 171 in Kir6.2DeltaC26 channels without SUR1. Only four substitutions, 171A, 171F, 171H, and 171S, gave rise to functional channels, each increasing Ki,ATP for ATP inhibition by >55-fold and slowing gating to the interburst by >35-fold. Moreover, we investigated the role of individual Kir6.2 subunits in the gating by comparing burst to interburst transition rates of channels constructed from different combinations of slow 171A and fast T171 "wild-type" subunits. The relationship between gating transition rate and number of slow subunits is exponential, which excludes independent gating models where any one subunit is sufficient for inhibition gating. Rather, our results support mechanisms where four ATP sites independently can control a single gate formed by the concerted action of all four Kir6.2 subunit inner helices of the KATP channel.

Proximal C-terminal domain of sulphonylurea receptor 2A interacts with pore-forming Kir6 subunits in KATP channels

Functional KATP (ATP-sensitive potassium) channels are hetero-octamers of four Kir6 (inwardly rectifying potassium) channel subunits and four SUR (sulphonylurea receptor) subunits. Possible interactions between the C-terminal domain of SUR2A and Kir6.2 were investigated by co-immunoprecipitation of rat SUR2A C-terminal fragments with full-length Kir6.2 and by analysis of cloned KATP channel function and distribution in HEK-293 cells (human embryonic kidney 293 cells) in the presence of competing rSUR2A fragments. Three maltose-binding protein-SUR2A fusions, rSUR2A-CTA (rSUR2A residues 1254-1545), rSUR2A-CTB (residues 1254-1403) and rSUR2A-CTC (residues 1294-1403), were co-immunoprecipitated with full-length Kir6.2 using a polyclonal anti-Kir6.2 antiserum. A fourth C-terminal domain fragment, rSUR2A-CTD (residues 1358-1545) did not co-immunoprecipitate with Kir6.2 under the same conditions, indicating a direct interaction between Kir6.2 and a 65-amino-acid section of the cytoplasmic C-terminal region of rSUR2A between residues 1294 and 1358. ATP- and glibenclamide-sensitive K+ currents were decreased in HEK-293 cells expressing full-length Kir6 and SUR2 subunits that were transiently transfected with fragments rSUR2A-CTA, rSUR2A-CTC and rSUR2A-CTE (residues 1294-1359) compared with fragment rSUR2A-CTD or mock-transfected cells, suggesting either channel inhibition or a reduction in the number of functional KATP channels at the cell surface. Anti-KATP channel subunit-associated fluorescence in the cell membrane was substantially lower and intracellular fluorescence increased in rSUR2A-CTE expressing cells; thus, SUR2A fragments containing residues 1294-1358 reduce current by decreasing the number of channel subunits in the cell membrane. These results identify a site in the C-terminal domain of rSUR2A, between residues 1294 and 1358, whose direct interaction with full-length Kir6.2 is crucial for the assembly of functional KATP channels.

betaL-betaM loop in the C-terminal domain of G protein-activated inwardly rectifying K(+) channels is important for G(betagamma) subunit activation

The activity of G protein-activated inwardly rectifying K(+) channels (GIRK or Kir3) is important for regulating membrane excitability in neuronal, cardiac and endocrine cells. Although G(betagamma) subunits are known to bind the N- and C-termini of GIRK channels, the mechanism underlying G(betagamma) activation of GIRK is not well understood. Here, we used chimeras and point mutants constructed from GIRK2 and IRK1, a G protein-insensitive inward rectifier, to determine the region within GIRK2 important for G(betagamma) binding and activation. An analysis of mutant channels expressed in Xenopus oocytes revealed two amino acid substitutions in the C-terminal domain of GIRK2, GIRK2(L344E) and GIRK2(G347H), that exhibited decreased carbachol-activated currents but significantly enhanced basal currents with coexpression of G(betagamma) subunits. Combining the two mutations (GIRK2(EH)) led to a more severe reduction in carbachol-activated and G(betagamma)-stimulated currents. Ethanol-activated currents were normal, however, suggesting that G protein-independent gating was unaffected by the mutations. Both GIRK2(L344E) and GIRK2(EH) also showed reduced carbachol activation and normal ethanol activation when expressed in HEK-293T cells. Using epitope-tagged channels expressed in HEK-293T cells, immunocytochemistry showed that G(betagamma)-impaired mutants were expressed on the plasma membrane, although to varying extents, and could not account completely for the reduced G(betagamma) activation. In vitro G(betagamma) binding assays revealed an approximately 60% decrease in G(betagamma) binding to the C-terminal domain of GIRK2(L344E) but no statistical change with GIRK2(EH) or GIRK2(G347H), though both mutants exhibited G(betagamma)-impaired activation. Together, these results suggest that L344, and to a lesser extent, G347 play an important functional role in G(betagamma) activation of GIRK2 channels. Based on the 1.8 A structure of GIRK1 cytoplasmic domains, L344 and G347 are positioned in the betaL-betaM loop, which is situated away from the pore and near the N-terminal domain. The results are discussed in terms of a model for activation in which G(betagamma) alters the interaction between the betaL-betaM loop and the N-terminal domain.

ATP-dependent interaction of the cytosolic domains of the inwardly rectifying K+ channel Kir6.2 revealed by fluorescence resonance energy transfer

ATP-sensitive K(+) (K(ATP)) channels play important roles in the regulation of membrane excitability in many cell types. ATP inhibits channel activity by binding to a specific site formed by the N and C termini of the pore-forming subunit, Kir6.2, but the structural changes associated with this interaction remain unclear. Here, we use fluorescence resonance energy transfer (FRET) to study the ATP-dependent interaction between the N and C termini of Kir6.2 using a construct bearing fused cyan and yellow fluorescent proteins (ECFP-Kir6.2-EYFP). When expressed in human embryonic kidney cells, ECFP-Kir6.2-EYFP/SUR1 channels displayed FRET that was augmented by agonist stimulation and diminished by metabolic poisoning. Addition of ATP to permeabilized cells or isolated plasma membrane sheets increased FRET. FRET changes were abolished by Kir6.2 mutations that altered ATP-dependent channel closure and channel gating. In the wild-type channel, the ATP concentrations, which increased FRET (EC(50) = 1.36 mM), were significantly higher than those causing channel inhibition (IC(50) = 0.29 mM). Demonstrating the existence of intermolecular interactions, a dimeric construct comprising two molecules of Kir6.2 linked head-to-tail (ECFP-Kir6.2-Kir6.2-EYFP) displayed less FRET than the monomer in the absence of nucleotide but still exhibited ATP-dependent FRET increases (EC(50) = 1.52 mM) and channel inhibition. We conclude that binding of ATP to Kir6.2, (i). alters the interaction between the N- and C-terminal domains, (ii). probably involves both intrasubunit and intersubunit interactions, (iii). reflects ligand binding not channel gating, and (iv). occurs in intact cells when subplasmalemmal [ATP] changes in the millimolar range.

The birth of a channel

An ion channel protein begins life as a nascent peptide inside a ribosome, moves to the endoplasmic reticulum where it becomes integrated into the lipid bilayer, and ultimately forms a functional unit that conducts ions in a well-regulated fashion. Here, I discuss the nascent peptide and its tasks as it wends its way through ribosomal tunnels and exit ports, through translocons, and into the bilayer. We are just beginning to explore the sequence of these events, mechanisms of ion channel structure formation, when biogenic decisions are made, and by which participants. These decisions include when to exit the endoplasmic reticulum and with whom to associate. Such issues govern the expression of ion channels at the cell surface and thus the electrical activity of a cell.

Classification and rescue of ROMK mutations underlying hyperprostaglandin E syndrome/antenatal Bartter syndrome

BACKGROUND

Mutations in the renal K+ channel ROMK (Kir 1.1) cause hyperprostaglandin E syndrome/antenatal Bartter syndrome (HPS/aBS), a severe tubular disorder leading to renal salt and water wasting. Several studies confirmed the predominance of alterations of current properties in ROMK mutants. However, in most of these studies, analysis was restricted to nonmammalian cells and electrophysiologic methods. Therefore, for the majority of ROMK mutations, disturbances in protein trafficking remained unclear. The aim of the present study was the evaluation of different pathogenic mechanisms of 20 naturally occurring ROMK mutations with consecutive classification into mutational classes and identification of distinct rescue mechanisms according to the underlying defect.

METHODS

Mutated ROMK potassium channels were expressed in Xenopus oocytes and a human kidney cell line and analyzed by two electrode voltage clamp analysis, immunofluorescence, and Western blot analysis.

RESULTS

We identified 14 out of 20 ROMK mutations that did not reach the cell surface, indicating defective membrane trafficking. High expression levels rescued six out of 14 ROMK mutants, leading to significant K+ currents. In addition, two early inframe stop mutations could be rescued by aminoglycosides, resulting in full-length ROMK and correct trafficking to the plasma membrane in a subset of transfected cells.

CONCLUSION

In contrast to previous reports, most of the investigated ROMK mutations displayed a trafficking defect that might be rescued by pharmacologic agents acting as molecular chaperones. The evaluation of different disease-causing mechanisms will be essential for establishing new and more specific therapeutic strategies for HPS/aBS patients.

Identification of residues contributing to the ATP binding site of Kir6.2

The ATP-sensitive potassium (K(ATP)) channel links cell metabolism to membrane excitability. Intracellular ATP inhibits channel activity by binding to the Kir6.2 subunit of the channel, but the ATP binding site is unknown. Using cysteine-scanning mutagenesis and charged thiol-modifying reagents, we identified two amino acids in Kir6.2 that appear to interact directly with ATP: R50 in the N-terminus, and K185 in the C-terminus. The ATP sensitivity of the R50C and K185C mutant channels was increased by a positively charged thiol reagent (MTSEA), and was reduced by the negatively charged reagent MTSES. Comparison of the inhibitory effects of ATP, ADP and AMP after thiol modification suggests that K185 interacts primarily with the beta-phosphate, and R50 with the gamma-phosphate, of ATP. A molecular model of the C-terminus of Kir6.2 (based on the crystal structure of Kir3.1) was constructed and automated docking was used to identify residues interacting with ATP. These results support the idea that K185 interacts with the beta-phosphate of ATP. Thus both N- and C-termini may contribute to the ATP binding site.

Trafficking of the Ca2+-activated K+ channel, hIK1, is dependent upon a C-terminal leucine zipper

We demonstrate that the C-terminal truncation of hIK1 results in a loss of functional channels. This could be caused by either (i) a failure of the channel to traffic to the plasma membrane or (ii) the expression of non-functional channels. To delineate among these possibilities, a hemagglutinin epitope was inserted into the extracellular loop between transmembrane domains S3 and S4. Surface expression and channel function were measured by immunofluorescence, cell surface immunoprecipitation, and whole-cell patch clamp techniques. Although deletion of the last 14 amino acids of hIK1 (L414STOP) had no effect on plasma membrane expression and function, deletion of the last 26 amino acids (K402STOP) resulted in a complete loss of membrane expression. Mutation of the leucine heptad repeat ending at Leu(406) (L399A/L406A) completely abrogated membrane localization. Additional mutations within the heptad repeat (L385A/L392A, L392A/L406A) or of the a positions (I396A/L403A) resulted in a near-complete loss of membrane-localized channel. In contrast, mutating individual leucines did not compromise channel trafficking or function. Both membrane localization and function of L399A/L406A could be partially restored by incubation at 27 degrees C. Co-immunoprecipitation studies demonstrated that leucine zipper mutations do not compromise multimer formation. In contrast, we demonstrated that the leucine zipper region of hIK1 is capable of co-assembly and that this is dependent upon an intact leucine zipper. Finally, this leucine zipper is conserved in another member of the gene family, SK3. However, mutation of the leucine zipper in SK3 had no effect on plasma membrane localization or function. In conclusion, we demonstrate that the C-terminal leucine zipper is critical to facilitate correct folding and plasma membrane trafficking of hIK1, whereas this function is not conserved in other gene family members.

Interaction of the cytosolic domains of the Kir6.2 subunit of the K(ATP) channel is modulated by sulfonylureas

The NH(2)- and COOH-termini of the ATP-sensitive potassium (K(ATP)) channel pore-forming subunit, Kir6.2, both lie intracellularly and interact with one another. To study this interaction, cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) were fused to the NH(2)- and COOH-termini of Kir6.2, respectively (CFP-Kir6.2-YFP). These fluorescent proteins have sufficient spectral overlap to allow distance-dependent fluorescence resonance energy transfer (FRET). When CFP-Kir6.2-YFP was expressed in human embryonic kidney cells and illuminated at 440 nm to excite CFP, significant fluorescence was recorded at 535 nm, the peak of the YFP emission spectrum. This indicated that FRET was occurring and thus that the NH(2)- and COOH-termini of Kir6.2 lie in close proximity to one another. The emission ratio, F(535)/F(480), was increased by co-expression of SUR2A, but not SUR1, suggesting that SUR2A but not SUR1 influences the Kir6.2 NH(2)- and COOH-terminal interaction. This interaction was reduced by the sulfonylureas tolbutamide and gliclazide, but not by the pore blocker barium. The properties of the tolbutamide response indicate that the drug disrupts the interaction between the NH(2)- and COOH-termini of Kir6.2 by binding to a low-affinity site on Kir6.2.

SUR-dependent modulation of KATP channels by an N-terminal KIR6.2 peptide. Defining intersubunit gating interactions

Ntp and Ctp, synthetic peptides based on the N- and C-terminal sequences of K(IR)6.0, respectively, were used to probe gating of K(IR)6.0/SUR K(ATP) channels. Micromolar Ntp dose-dependently increased the mean open channel probability in ligand-free solution (P(O(max))) and attenuated the ATP inhibition of K(IR)6.2/SUR1, but had no effect on homomeric K(IR)6.2 channels. Ntp (up to approximately 10(-4) m) did not affect significantly the mean open or "fast," K(+) driving force-dependent, intraburst closed times, verifying that Ntp selectively modulates the ratio of mean burst to interburst times. Ctp and Rnp, a randomized Ntp, had no effect, indicating that the effects of Ntp are structure specific. Ntp opened K(IR)6.1/SUR1 channels normally silent in the absence of stimulatory Mg(-) nucleotide(s) and attenuated the coupling of high-affinity sulfonylurea binding with K(ATP) pore closure. These effects resemble those seen with N-terminal deletions (DeltaN) of K(IR)6.0, and application of Ntp to DeltaNK(ATP) channels decreased their P(O(max)) and apparent IC(50) for ATP in the absence of Mg(2+). The results are consistent with a competition between Ntp and the endogenous N terminus for a site of interaction on the cytoplasmic face of the channel or with partial replacement of the deleted N terminus by Ntp, respectively. The K(IR) N terminus and the TMD0-L0 segment of SUR1 are known to control the P(O(max)). The L0 linker has been reported to be required for glibenclamide binding, and DeltaNK(IR)6.2/SUR1 channels exhibit reduced labeling of K(IR) with (125)I-azidoglibenclamide, implying that the K(IR) N terminus and L0 of SUR1 are in proximity. We hypothesize that L0 interacts with the K(IR) N terminus in ligand-inhibited K(ATP) channels and put forward a model, based on the architecture of BtuCD, MsbA, and the KcsA channel, in which TMD0-L0 links the MDR-like core of SUR with the K(IR) pore.

Molecular mechanism of a COOH-terminal gating determinant in the ROMK channel revealed by a Bartter's disease mutation

The ROMK subtypes of inward-rectifier K(+) channels mediate potassium secretion and regulate NaCl reabsorption in the kidney. Loss-of-function mutations in this pH-sensitive K(+) channel cause Bartter's disease, a familial salt wasting nephropathy. One disease-causing mutation truncates the extreme COOH-terminus and induces a closed gating conformation. Here we identify a region within the deleted domain that plays an important role in pH-dependent gating. The domain contains a structural element that functionally interacts with the pH sensor in the cytoplasmic NH(2)-terminus to set a physiological range of pH sensitivity. Removal of the domain shifts the pK(a) towards alkaline pH values, causing channel inactivation under physiological conditions. Suppressor mutations within the pH sensor rescued channel gating and trans addition of the cognate peptide restored pH sensitivity. A specific interdomain interaction was revealed in an in vitro protein-protein binding assay between the NH(2)- and COOH-terminal cytoplasmic domains expressed as bacterial fusion proteins. These results provide new insights into the molecular mechanisms underlying Kir channel regulation and channel gating defects that are associated with Bartter's disease.

The role of NH2-terminal positive charges in the activity of inward rectifier KATP channels

Approximately half of the NH(2) terminus of inward rectifier (Kir) channels can be deleted without significant change in channel function, but activity is lost when more than approximately 30 conserved residues before the first membrane spanning domain (M1) are removed. Systematic replacement of the positive charges in the NH(2) terminus of Kir6.2 with alanine reveals several residues that affect channel function when neutralized. Certain mutations (R4A, R5A, R16A, R27A, R39A, K47A, R50A, R54A, K67A) change open probability, whereas an overlapping set of mutants (R16A, R27A, K39A, K47A, R50A, R54A, K67A) change ATP sensitivity. Further analysis of the latter set differentiates mutations that alter ATP sensitivity as a consequence of altered open state stability (R16A, K39A, K67A) from those that may affect ATP binding directly (K47A, R50A, R54A). The data help to define the structural determinants of Kir channel function, and suggest possible structural motifs within the NH(2) terminus, as well as the relationship of the NH(2) terminus with the extended cytoplasmic COOH terminus of the channel.

Structural and functional determinants of conserved lipid interaction domains of inward rectifying Kir6.2 channels

All members of the inward rectifiier K(+) (Kir) channel family are activated by phosphoinositides and other amphiphilic lipids. To further elucidate the mechanistic basis, we examined the membrane association of Kir6.2 fragments of K(ATP) channels, and the effects of site-directed mutations of these fragments and full-length Kir6.2 on membrane association and K(ATP) channel activity, respectively. GFP-tagged Kir6.2 COOH terminus and GFP-tagged pleckstrin homology domain from phospholipase C delta1 both associate with isolated membranes, and association of each is specifically reduced by muscarinic m1 receptor-mediated phospholipid depletion. Kir COOH termini are predicted to contain multiple beta-strands and a conserved alpha-helix (residues approximately 306-311 in Kir6.2). Systematic mutagenesis of D307-F315 reveals a critical role of E308, I309, W311 and F315, consistent with residues lying on one side of a alpha-helix. Together with systematic mutation of conserved charges, the results define critical determinants of a conserved domain that underlies phospholipid interaction in Kir channels.

Heteromerization of Kir2.x potassium channels contributes to the phenotype of Andersen's syndrome

Andersen's syndrome, an autosomal dominant disorder related to mutations of the potassium channel Kir2.1, is characterized by cardiac arrhythmias, periodic paralysis, and dysmorphic bone structure. The aim of our study was to find out whether heteromerization of Kir2.1 channels with wild-type Kir2.2 and Kir2.3 channels contributes to the phenotype of Andersen's syndrome. The following results show that Kir2.x channels can form functional heteromers: (i) HEK293 cells transfected with Kir2.x-Kir2.y concatemers expressed inwardly rectifying K(+) channels with a conductance of 28-30 pS. (ii) Expression of Kir2.x-Kir2.y concatemers in Xenopus oocytes produced inwardly rectifying, Ba(2+) sensitive currents. (iii) When Kir2.1 and Kir2.2 channels were coexpressed in Xenopus oocytes the IC(50) for Ba(2+) block of the inward rectifier current differed substantially from the value expected for independent expression of homomeric channels. (iv) Coexpression of nonfunctional Kir2.x constructs, in which the GYG region of the pore region was replaced by AAA, with wild-type Kir2.x channels produced both homomeric and heteromeric dominant-negative effects. (v) Kir2.1 and Kir2.3 channels could be coimmunoprecipitated in membrane extracts from isolated guinea pig cardiomyocytes. (vi) Yeast two-hybrid analysis showed interaction between the N- and C-terminal intracellular domains of different Kir2.x subunits. Coexpression of Kir2.1 mutants related to Andersen's syndrome with wild-type Kir2.x channels showed a dominant negative effect, the extent of which varied between different mutants. Our results suggest that differential tetramerization of the mutant allele of Kir2.1 with wild-type Kir2.1, Kir2.2, and Kir2.3 channels represents the molecular basis of the extraordinary pleiotropy of Andersen's syndrome.

Potassium channel ontogeny

Potassium channels are multi-subunit complexes, often composed of several polytopic membrane proteins and cytosolic proteins. The formation of these oligomeric structures, including both biogenesis and trafficking, is the subject of this review. The emphasis is on events in the endoplasmic reticulum (ER), particularly on how, where, and when K(+) channel polypeptides translocate and integrate into the bilayer, oligomerize and fold to form pore-forming units, and associate with auxiliary subunits to create the mature channel complex. Questions are raised with respect to the sequence of these events, when biogenic decisions are made, models for integration of K(+) channel transmembrane segments, crosstalk between the cell surface and ER, and recognition of compatible partner subunits. Also considered are determinants of subunit composition and stoichiometry, their consequence for trafficking, mechanisms for ER retention and export, and sequence motifs that direct channels to the cell surface. It is these mechanistic issues that govern the differential distributions of K(+) conductances at the cell surface, and hence the electrical activity of cells and tissues underlying both the physiology and pathophysiology of an organism.

Intramolecular interaction of SUR2 subtypes for intracellular ADP-Induced differential control of K(ATP) channels

ATP-sensitive K+ (K(ATP)) channels are composed of sulfonylurea receptors (SURs) and inwardly rectifying Kir6.2-channels. The C-terminal 42 amino acid residues (C42) of SURs are responsible for ADP-induced differential activation of K(ATP) channels in SUR-subtypes. By examining ADP-effect on K(ATP) channels containing various chimeras of SUR2A and SUR2B, we identified a segment of 7 residues at central portion of C42 critical for this phenomenon. A 3-D structure model of the region containing the second nucleotide-binding domain (NBD2) of SUR and C42 was developed based on the structure of HisP, a nucleotide-binding protein forming the bacterial Histidine transporter complex. In the model, the polar and charged residues in the critical segment located within a distance that allows their electrostatic interaction with Arg1344 at the Walker-A loop of NBD2. Therefore, the interaction might be involved in the control of ADP-induced differential activation of SUR2-subtype K(ATP) channels.

Multiple sites of interaction between the intracellular domains of an inwardly rectifying potassium channel, Kir6.2

The amino-terminal and carboxy-terminal domains of inwardly rectifying potassium channel (Kir) subunits are both intracellular. A direct physical interaction between these two domains is involved in the response of Kir channels to regulatory factors such as G-proteins, nucleotides and intracellular pH. We have previously mapped the region within the N-terminal domain of Kir6.2 that interacts with the C-terminus. In this study we use a similar in vitro protein-protein interaction assay to map the regions within the C-terminus which interact with the N-terminus. We find that multiple interaction domains exist within the C-terminus: CID1 (amino acids (aa) 279-323), CID2 (aa 214-222) and CID3 (aa 170-204). These domains correlate with regions previously identified as making important contributions to Kir channel assembly and function. The highly conserved nature of the C-terminus suggests that a similar association with the N-terminus may be a feature common to all members of the Kir family of potassium channels, and that it may be involved in gating of Kir channels by intracellular ligands.

Requirement of multiple protein domains and residues for gating K(ATP) channels by intracellular pH

ATP-sensitive K(+) channels (K(ATP)) are regulated by pH in addition to ATP, ADP, and phospholipids. In the study we found evidence for the molecular basis of gating the cloned K(ATP) by intracellular protons. Systematic constructions of chimerical Kir6.2-Kir1.1 channels indicated that full pH sensitivity required the N terminus, C terminus, and M2 region. Three amino acid residues were identified in these protein domains, which are Thr-71 in the N terminus, Cys-166 in the M2 region, and His-175 in the C terminus. Mutation of any of them to their counterpart residues in Kir1.1 was sufficient to completely eliminate the pH sensitivity. Creation of these residues rendered the mutant channels clear pH-dependent activation. Thus, critical players in gating K(ATP) by protons are demonstrated. The pH sensitivity enables the K(ATP) to regulate cell excitability in a number of physiological and pathophysiological conditions when pH is low but ATP concentration is normal.

Mutations in Kir2.1 Cause the Developmental and Episodic Electrical Phenotypes of Andersen's Syndrome

Andersen's syndrome is characterized by periodic paralysis, cardiac arrhythmias, and dysmorphic features. We have mapped an Andersen's locus to chromosome 17q23 near the inward rectifying potassium channel gene KCNJ2. A missense mutation in KCNJ2 (encoding D71V) was identified in the linked family. Eight additional mutations were identified in unrelated patients. Expression of two of these mutations in Xenopus oocytes revealed loss of function and a dominant negative effect in Kir2.1 current as assayed by voltage-clamp. We conclude that mutations in Kir2.1 cause Andersen's syndrome. These findings suggest that Kir2.1 plays an important role in developmental signaling in addition to its previously recognized function in controlling cell excitability in skeletal muscle and heart.

Differential pH sensitivity of Kir4.1 and Kir4.2 potassium channels and their modulation by heteropolymerisation with Kir5.1

1. The inwardly rectifying potassium channel Kir5.1 appears to form functional channels only by coexpression with either Kir4.1 or Kir4.2. Kir4.1-Kir5.1 heteromeric channels have been shown to exist in vivo in renal tubular epithelia. However, Kir5.1 is expressed in many other tissues where Kir4.1 is not found. Using Kir5.1-specific antibodies we have localised Kir5.1 expression in the pancreas, a tissue where Kir4.2 is also highly expressed. 2. Heteromeric Kir5.1-Kir4.1 channels are significantly more sensitive to intracellular acidification than Kir4.1 currents. We demonstrate that this increased sensitivity is primarily due to modulation of the intrinsic Kir4.1 pH sensitivity by Kir5.1. 3. Kir4.2 was found to be significantly more pH sensitive (pK(a) = 7.1) than Kir4.1 (pK(a) = 5.99) due to an additional pH-sensing mechanism involving the C-terminus. As a result, coexpression with Kir5.1 does not cause a major shift in the pH sensitivity of the heteromeric Kir4.2-Kir5.1 channel. 4. Cell-attached single channel analysis of Kir4.2 revealed a channel with a high open probability (P(o) > 0.9) and single channel conductance of approximately 25 pS, whilst coexpression with Kir5.1 produced novel bursting channels (P(o) < 0.3) and a principal conductance of approximately 54 pS with several subconductance states. 5. These results indicate that Kir5.1 may form heteromeric channels with Kir4.2 in tissues where Kir4.1 is not expressed (e.g. pancreas) and that these novel channels are likely to be regulated by changes in intracellular pH. In addition, the extreme pH sensitivity of Kir4.2 has implications for the role of this subunit as a homotetrameric channel.

Interaction of stilbene disulphonates with cloned K(ATP) channels

In this study, we tested the effects of the stilbene disulphonates DIDS and SITS on three different types of cloned K(ATP) channel (Kir6.2/SUR1, Kir6.2/SUR2A and Kir6.2DeltaC) heterologously expressed in Xenopus oocytes, with the aim of identifying the part of the channel which is involved in mediating disulphonate inhibition. We found that the inhibitory site(s) for these drugs lies within the Kir6.2 subunit of the channel, although its properties are further modulated by the sulphonylurea (SUR) subunit. In particular, SUR2A reduces both the rate and extent of block, by impairing the ability of DIDS binding to produce channel closure. The disulphonate-binding site interacts with the ATP inhibitory site on Kir6.2 because ATP is able to protect against irreversible channel inhibition by disulphonates. This effect is not mimicked by tolbutamide (at a concentration that interacts with Kir6.2) and is abolished by mutations that render the channel ATP insensitive. A number of point mutations in both the N and C termini of Kir6.2 reduced the extent and reversibility of channel inhibition by SITS. The results are consistent with the idea that residue C42 of Kir6.2 is likely to be involved in covalently linking of SITS to the channel. Other types of Kir channel (Kir1.1, Kir2.1 and Kir4.1) were also irreversibly blocked by DIDS, suggesting that these channels may share common binding sites for these stilbene disulphonates.

Molecular determinants for the distinct pH sensitivity of Kir1.1 and Kir4.1 channels

Kir1.1 (ROMK1) is inhibited by hypercapnia and intracellular acidosis with midpoint pH for channel inhibition (pK(a)) of approximately 6.7. Another close relative, Kir4.1 (BIR10), is also pH sensitive with much lower pH sensitivity (pK(a) approximately 6. 0), although it shares a high sequence homology with Kir1.1. To find the molecular determinants for the distinct pH sensitivity, we studied the structure-functional relationship using site-directed mutagenesis. An NH(2)-terminal residue (Lys-53) was found to be responsible for the low pH sensitivity in Kir4.1. Mutation of this lysine to valine (K53V), a residue seen at the same position in Kir1. 1, markedly increased channel sensitivity to CO(2)/pH. Reverse mutation on Kir1.1 (V66K) decreased the CO(2)/pH sensitivities. Interestingly, mutation of these residues to glutamate greatly enhanced the pH sensitivity in both channels. Other contributors to the distinct pH sensitivity were histidine residues in the COOH terminus, whose numbers are fewer in Kir4.1 than Kir1.1. Mutation of two of these histidine residues in Kir1.1 (H342Q/H354N) reduced CO(2)/pH sensitivities, whereas the creation of two histidines (S328H/G340H) in Kir4.1 increased the CO(2)/pH sensitivities. Combined mutations of the lysine and histidine residues in Kir4.1 (K53V/S328H/G340H) gave rise to a channel that had CO(2)/pH sensitivities almost identical to those of the wild-type Kir1.1. Thus the residues demonstrated in our current studies are likely the molecular basis for the distinct pH sensitivity between Kir1.1 and Kir4.1.

Direct association of ligand-binding and pore domains in homo- and heterotetrameric inositol 1,4,5-trisphosphate receptors

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of intracellular Ca(2+) channels that exist as homo- or heterotetramers. In order to determine whether the N-terminal ligand-binding domain is in close physical proximity to the C-terminal pore domain, we prepared microsomal membranes from COS-7 cells expressing recombinant type I and type III IP(3)R isoforms. Trypsin digestion followed by cross-linking and co-immunoprecipitation of peptide fragments suggested an inter-subunit N- and C-terminal interaction in both homo- and heterotetramers. This observation was further supported by the ability of in vitro translated C-terminal peptides to interact specifically with an N-terminal fusion protein. Using a (45)Ca(2+) flux assay, we provide functional evidence that the ligand-binding domain of one subunit can gate the pore domain of an adjacent subunit. We conclude that common structural motifs are shared between the type I and type III IP(3)Rs and propose that the gating mechanism of IP(3)R Ca(2+) channels involves the association of the N-terminus of one subunit with the C-terminus of an adjacent subunit in both homo- and heterotetrameric complexes.

Gating of inward rectifier K+ channels by proton-mediated interactions of N- and C-terminal domains

Ion channels play an important role in cellular functions, and specific cellular activity can be produced by gating them. One important gating mechanism is produced by intra- or extracellular ligands. Although the ligand-mediated channel gating is an important cellular process, the relationship between ligand binding and channel gating is not well understood. It is possible that ligands are involved in the interactions of different protein domains of the channel leading to opening or closing. To test this hypothesis, we studied the gating of Kir2.3 (HIR) by intracellular protons. Our results showed that hypercapnia or intracellular acidification strongly inhibited these channels. This effect relied on both the N and C termini. The CO(2)/pH sensitivities were abolished or compromised when one of the intracellular termini was replaced. Using purified N- and C-terminal peptides, we found that the N and C termini bound to each other in vitro. Although their binding was weak at pH 7.4, stronger binding was seen at pH 6.6. Two short sequences in the N and C termini were found to be critical for the N/C-terminal interaction. Interestingly, there was no titratable residue in these motifs. To identify the potential protonation sites, we systematically mutated most histidine residues in the intracellular N and C termini. We found that mutations of several histidine residues in the C but not the N terminus had a major effect on channel sensitivities to CO(2) and pH(i). These results suggest that at acidic pH, protons appear to interact with the C-terminal histidine residues and present the C terminus to the N terminus. Consequentially, these two intracellular termini bound to each other through two short motifs and closed the channel. Thus, a novel mechanism for K(+) channel gating is demonstrated, which involves the N- and C-terminal interaction with protons as the mediator.

Investigation of the molecular assembly of beta-cell K(ATP) channels

We have investigated the protein interactions involved in the assembly of pancreatic beta-cell ATP-sensitive potassium channels. The channels are a heterooligomeric complex of pore-forming Kir6.2 subunits and sulfonylurea receptor (SUR1) subunits. SUR1 belongs to the ATP binding cassette (ABC) family of proteins and has two nucleotide binding domains (NBD1 and NBD2) and 17 putative transmembrane (TM) sequences. Previously we showed that co-expression in a baculovirus expression system of two parts of SUR1 divided at Pro1042 between TM12 and 13 leads to restoration of glibenclamide binding activity, whereas expression of either individual N- or C-terminal domain alone gave no glibenclamide binding activity [M.V. Mikhailov and S.J.H. Ashcroft (2000) J. Biol. Chem. 275, 3360-3364]. Here we show that the two half-molecules formed by division of SUR1 between NBD1 and TM12 or between TM13 and 14 also self-assemble to give glibenclamide binding activity. However, deletion of NBD1 from the N-part of SUR1 abolished SUR1 assembly, indicating a critical role for NBD1 in SUR1 assembly. We found that differences in glibenclamide binding activity obtained after co-expression of different half-molecules are attributable to different amounts of binding sites, but the binding affinities remained nearly the same. Simultaneous expression of Kir6.2 resulted in enhanced glibenclamide binding activity only when the N-half of SUR1 included TM12. We conclude that TM12 and 13 are not essential for SUR1 assembly whereas TM12 takes part in SUR1 Kir6.2 interaction. This interaction is specific for Kir 6.2 because no enhancement of glibenclamide binding was observed when half-molecules were expressed together with Kir4.1. We propose a model of K(ATP) channel organisation based on these data.

The I182 region of k(ir)6.2 is closely associated with ligand binding in K(ATP) channel inhibition by ATP

The ATP-inhibited potassium (K(ATP)) channel is assembled from four inward rectifier potassium (K(ir)6.x) subunits and four sulfonylurea receptor (SURx) subunits. The inhibitory action of ATP is mediated by at least two distinct functional domains within the C-terminal cytoplasmic tail of K(ir)6.2. The G334D mutation of K(ir)6.2 virtually eliminates ATP-dependent gating with no effect on ligand-independent gating, suggesting a role in linkage of the site to the gate or in the ATP binding site, itself. The T171A mutation of K(ir)6.2 strongly disrupts both ATP-dependent and ligand-independent gating, suggesting a role for T171 in the gating step. A neighboring mutation, I182Q, virtually eliminates ATP inhibition, but its effect on ligand-independent gating remained unknown. We have now characterized both the K(i) values for inhibition by ATP and the ligand-independent gating kinetics of 15 substitutions at position 182. All substitutions decreased ATP-dependent inhibition gating as measured by the K(i), many profoundly so, yet had little or no effect on ligand-independent gating kinetics. Thus, substitutions at position 182 are unlikely to act by disrupting inhibition gate movement. Our results indicate an indispensable role for I182 in a step of the ATP binding mechanism, the linkage mechanism coupling the ATP binding site to the inhibition gate, or both.

Molecular basis for K(ATP) assembly: transmembrane interactions mediate association of a K+ channel with an ABC transporter

K(ATP) channels are large heteromultimeric complexes containing four subunits from the inwardly rectifying K+ channel family (Kir6.2) and four regulatory sulphonylurea receptor subunits from the ATP-binding cassette (ABC) transporter family (SUR1 and SUR2A/B). The molecular basis for interactions between these two unrelated protein families is poorly understood. Using novel trafficking-based interaction assays, coimmunoprecipitation, and current measurements, we show that the first transmembrane segment (M1) and the N terminus of Kir6.2 are involved in K(ATP) assembly and gating. Additionally, the transmembrane domains, but not the nucleotide-binding domains, of SUR1 are required for interaction with Kir6.2. The identification of specific transmembrane interactions involved in K(ATP) assembly may provide a clue as to how ABC proteins that transport hydrophobic substrates evolved to regulate other membrane proteins.

The role of lysine 185 in the kir6.2 subunit of the ATP-sensitive channel in channel inhibition by ATP

1. ATP-sensitive potassium (KATP) channels are composed of pore-forming Kir6.2 and regulatory SUR subunits. A truncated isoform of Kir6.2, Kir6.2DeltaC26, forms ATP-sensitive channels in the absence of SUR1, suggesting the ATP-inhibitory site lies on Kir6.2. 2. Previous studies have shown that mutation of the lysine residue at position 185 (K185) in the C-terminus of Kir6.2 to glutamine, decreased the channel sensitivity to ATP without affecting the single-channel conductance or the intrinsic channel kinetics. This mutation also impaired 8-azido[32P]-ATP binding to Kir6.2. 3. To determine if K185 interacts directly with ATP, we made a range of mutations at this position, and examined the effect on the channel ATP sensitivity by recording macroscopic currents in membrane patches excised from Xenopus oocytes expressing wild-type or mutant Kir6.2DeltaC26. 4. Substitution of K185 by a positively charged amino acid (arginine) had no substantial effect on the sensitivity of the channel to ATP. Mutation to a negatively charged residue markedly decreased the channel ATP sensitivity: the Ki for ATP inhibition increased from 85 microM to >30 mM when arginine was replaced with aspartic acid. Substitution of neutral residues had intermediate effects. 5. The inhibitory effects of ADP, ITP and GTP were also reduced when K185 was mutated to glutamine or glutamate. 6. The results indicate that a positively charged amino acid at position 185 is required for high-affinity ATP binding to Kir6.2. Our results demonstrate that ATP does not interact with the side-chain of K185. It remains unclear whether ATP interacts with the backbone of this residue, or whether its mutation influences ATP binding allosterically.