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

Exercise-induced expression of cardiac ATP-sensitive potassium channels promotes action potential shortening and energy conservation

Physical activity is one of the most important determinants of cardiac function. The ability of the heart to increase delivery of oxygen and metabolic fuels relies on an array of adaptive responses necessary to match bodily demand while avoiding exhaustion of cardiac resources. The ATP-sensitive potassium (K(ATP)) channel has the unique ability to adjust cardiac membrane excitability in accordance with ATP and ADP levels, and up-regulation of its expression that occurs in response to exercise could represent a critical element of this adaption. However, the mechanism by which K(ATP) channel expression changes result in a beneficial effect on cardiac excitability and function remains to be established. Here, we demonstrate that an exercise-induced rise in K(ATP) channel expression enhanced the rate and magnitude of action potential shortening in response to heart rate acceleration. This adaptation in membrane excitability promoted significant reduction in cardiac energy consumption under escalating workloads. Genetic disruption of normal K(ATP) channel pore function abolished the exercise-related changes in action potential duration adjustment and caused increased cardiac energy consumption. Thus, an expression-driven enhancement in the K(ATP) channel-dependent membrane response to alterations in cardiac workload represents a previously unrecognized mechanism for adaptation to physical activity and a potential target for cardioprotection.

N-terminal transmembrane domain of SUR1 controls gating of Kir6.2 by modulating channel sensitivity to PIP2

Functional integrity of pancreatic adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels depends on the interactions between the pore-forming potassium channel subunit Kir6.2 and the regulatory subunit sulfonylurea receptor 1 (SUR1). Previous studies have shown that the N-terminal transmembrane domain of SUR1 (TMD0) interacts with Kir6.2 and is sufficient to confer high intrinsic open probability (P(o)) and bursting patterns of activity observed in full-length K(ATP) channels. However, the nature of TMD0-Kir6.2 interactions that underlie gating modulation is not well understood. Using two previously described disease-causing mutations in TMD0 (R74W and E128K), we performed amino acid substitutions to study the structural roles of these residues in K(ATP) channel function in the context of full-length SUR1 as well as TMD0. Our results revealed that although R74W and E128K in full-length SUR1 both decrease surface channel expression and reduce channel sensitivity to ATP inhibition, they arrive there via distinct mechanisms. Mutation of R74 uniformly reduced TMD0 protein levels, suggesting that R74 is necessary for stability of TMD0. In contrast, E128 mutations retained TMD0 protein levels but reduced functional coupling between TMD0 and Kir6.2 in mini-K(ATP) channels formed by TMD0 and Kir6.2. Importantly, E128K full-length channels, despite having a greatly reduced P(o), exhibit little response to phosphatidylinositol 4,5-bisphosphate (PIP(2)) stimulation. This is reminiscent of Kir6.2 channel behavior in the absence of SUR1 and suggests that TMD0 controls Kir6.2 gating by modulating Kir6.2 interactions with PIP(2). Further supporting this notion, the E128W mutation in full-length channels resulted in channel inactivation that was prevented or reversed by exogenous PIP(2). These results identify a critical determinant in TMD0 that controls Kir6.2 gating by controlling channel sensitivity to PIP(2). Moreover, they uncover a novel mechanism of K(ATP) channel inactivation involving aberrant functional coupling between SUR1 and Kir6.2.

K(ATP) channels process nucleotide signals in muscle thermogenic response

Uniquely gated by intracellular adenine nucleotides, sarcolemmal ATP-sensitive K(+) (K(ATP)) channels have been typically assigned to protective cellular responses under severe energy insults. More recently, K(ATP) channels have been instituted in the continuous control of muscle energy expenditure under non-stressed, physiological states. These advances raised the question of how K(ATP) channels can process trends in cellular energetics within a milieu where each metabolic system is set to buffer nucleotide pools. Unveiling the mechanistic basis of the K(ATP) channel-driven thermogenic response in muscles thus invites the concepts of intracellular compartmentalization of energy and proteins, along with nucleotide signaling over diffusion barriers. Furthermore, it requires gaining insight into the properties of reversibility of intrinsic ATPase activity associated with K(ATP) channel complexes. Notwithstanding the operational paradigm, the homeostatic role of sarcolemmal K(ATP) channels can be now broadened to a wider range of environmental cues affecting metabolic well-being. In this way, under conditions of energy deficit such as ischemic insult or adrenergic stress, the operation of K(ATP) channel complexes would result in protective energy saving, safeguarding muscle performance and integrity. Under energy surplus, downregulation of K(ATP) channel function may find potential implications in conditions of energy imbalance linked to obesity, cold intolerance and associated metabolic disorders.

The ATP-sensitive K(+)-channel (K(ATP)) controls early left-right patterning in Xenopus and chick embryos

Consistent left-right asymmetry requires specific ion currents. We characterize a novel laterality determinant in Xenopus laevis: the ATP-sensitive K(+)-channel (K(ATP)). Expression of specific dominant-negative mutants of the Xenopus Kir6.1 pore subunit of the K(ATP) channel induced randomization of asymmetric organ positioning. Spatio-temporally controlled loss-of-function experiments revealed that the K(ATP) channel functions asymmetrically in LR patterning during very early cleavage stages, and also symmetrically during the early blastula stages, a period when heretofore largely unknown events transmit LR patterning cues. Blocking K(ATP) channel activity randomizes the expression of the left-sided transcription of Nodal. Immunofluorescence analysis revealed that XKir6.1 is localized to basal membranes on the blastocoel roof and cell-cell junctions. A tight junction integrity assay showed that K(ATP) channels are required for proper tight junction function in early Xenopus embryos. We also present evidence that this function may be conserved to the chick, as inhibition of K(ATP) in the primitive streak of chick embryos randomizes the expression of the left-sided gene Sonic hedgehog. We propose a model by which K(ATP) channels control LR patterning via regulation of tight junctions.

Pancreatic β-cell KATP channels: Hypoglycaemia and hyperglycaemia

The pancreatic β-cell ATP-sensitive K(+) channel (K(ATP) channel) plays a critical role in glucose homeostasis by linking glucose metabolism to electrical excitability and insulin secretion. Changes in the intracellular ratio of ATP/ADP mediate the metabolic regulation of channel activity. The β-cell K(ATP) channel is a hetero-octameric complex composed of two types of subunits: four inward-rectifying potassium channel pore-forming (Kir6.2) subunits and four high-affinity sulfonylurea receptor 1 (SUR1) subunits. Kir6.2 and SUR1 are encoded by the genes KCNJ11 and ABCC8, respectively. Mutations in these genes can result in congenital hyperinsulinism and permanent neonatal diabetes. This review highlights the important role of the β-cell K(ATP) channel in glucose physiology and provides an introduction to some of the other review articles in this special edition of the Reviews in Endocrine and Metabolic Disorders.

Trafficking of neuronal two pore domain potassium channels

The activity of two pore domain potassium (K2P) channels regulates neuronal excitability and cell firing. Post-translational regulation of K2P channel trafficking to the membrane controls the number of functional channels at the neuronal membrane affecting the functional properties of neurons. In this review, we describe the general features of K channel trafficking from the endoplasmic reticulum (ER) to the plasma membrane via the Golgi apparatus then focus on established regulatory mechanisms for K2P channel trafficking. We describe the regulation of trafficking of TASK channels from the ER or their retention within the ER and consider the competing hypotheses for the roles of the chaperone proteins 14-3-3, COP1 and p11 in these processes and where these proteins bind to TASK channels. We also describe the localisation of TREK channels to particular regions of the neuronal membrane and the involvement of the TREK channel binding partners AKAP150 and Mtap2 in this localisation. We describe the roles of other K2P channel binding partners including Arf6, EFA6 and SUMO for TWIK1 channels and Vpu for TASK1 channels. Finally, we consider the potential importance of K2P channel trafficking in a number of disease states such as neuropathic pain and cancer and the protection of neurons from ischemic damage. We suggest that a better understanding of the mechanisms and regulations that underpin the trafficking of K2P channels to the plasma membrane and to localised regions therein may considerably enhance the probability of future therapeutic advances in these areas.

Electrophysiological remodeling in heart failure

Heart failure affects nearly 6 million Americans, with a half-million new cases emerging each year. Whereas up to 50% of heart failure patients die of arrhythmia, the diverse mechanisms underlying heart failure-associated arrhythmia are poorly understood. As a consequence, effectiveness of antiarrhythmic pharmacotherapy remains elusive. Here, we review recent advances in our understanding of heart failure-associated molecular events impacting the electrical function of the myocardium. We approach this from an anatomical standpoint, summarizing recent insights gleaned from pre-clinical models and discussing their relevance to human heart failure.

Regulation of G-protein coupled receptor traffic by an evolutionary conserved hydrophobic signal

Plasma membrane (PM) expression of G-protein coupled receptors (GPCRs) is required for activation by extracellular ligands; however, mechanisms that regulate PM expression of GPCRs are poorly understood. For some GPCRs, such as alpha2c-adrenergic receptors (alpha(2c)-ARs), heterologous expression in non-native cells results in limited PM expression and extensive endoplasmic reticulum (ER) retention. Recently, ER export/retentions signals have been proposed to regulate cellular trafficking of several GPCRs. By utilizing a chimeric alpha(2a)/alpha(2c)-AR strategy, we identified an evolutionary conserved hydrophobic sequence (ALAAALAAAAA) in the extracellular amino terminal region that is responsible in part for alpha(2c)-AR subtype-specific trafficking. To our knowledge, this is the first luminal ER retention signal reported for a GPCR. Removal or disruption of the ER retention signal dramatically increased PM expression and decreased ER retention. Conversely, transplantation of this hydrophobic sequence into alpha(2a)-ARs reduced their PM expression and increased ER retention. This evolutionary conserved hydrophobic trafficking signal within alpha(2c)-ARs serves as a regulator of GPCR trafficking.

Impact of disease-causing SUR1 mutations on the KATP channel subunit interface probed with a rhodamine protection assay

The function of the ATP-sensitive potassium (K(ATP)) channel relies on the proper coupling between its two subunits: the pore-forming Kir6.2 and the regulator SUR. The conformation of the interface between these two subunits can be monitored using a rhodamine 123 (Rho) protection assay because Rho blocks Kir6.2 with an efficiency that depends on the relative position of transmembrane domain (TMD) 0 of the associated SUR (Hosy, E., Dérand, R., Revilloud, J., and Vivaudou, M. (2007) J. Physiol. 582, 27-39). Here we find that the natural and synthetic K(ATP) channel activators MgADP, zinc, and SR47063 induced a Rho-insensitive conformation. The activating mutation F132L in SUR1, which causes neonatal diabetes, also rendered the channel resistant to Rho block, suggesting that it stabilized an activated conformation by uncoupling TMD0 from the rest of SUR1. At a nearby residue, the SUR1 mutation E128K impairs trafficking, thereby reducing surface expression and causing hyperinsulinism. To augment channel density at the plasma membrane to investigate the effect of mutating this residue on channel function, we introduced the milder mutation E126A at the matching residue of SUR2A. Mutation E126A imposed a hypersensitive Rho phenotype indicative of a functional uncoupling between TMD0 and Kir6.2. These results suggest that the TMD0-Kir6.2 interface is mobile and that the gating modes of Kir6.2 correlate with distinct positions of TMD0. They further demonstrate that the second intracellular loop of SUR, which contains the two residues studied here, is a key structural element of the TMD0-Kir6.2 interface.

N terminus is key to the dominant negative suppression of Ca(V)2 calcium channels: implications for episodic ataxia type 2

Expression of the calcium channels Ca_V2.1 and Ca_V2.2 is markedly suppressed by co-expression with truncated constructs containing Domain I. This is the basis for the phenomenon of dominant negative suppression observed for many of the episodic ataxia type 2 mutations in Ca_V2.1 that predict truncated channels. The process of dominant negative suppression has been shown previously to stem from interaction between the full-length and truncated channels and to result in downstream consequences of the unfolded protein response and endoplasmic reticulum-associated protein degradation. We have now identified the specific domain that triggers this effect. For both Ca_V2.1 and Ca_V2.2, the minimum construct producing suppression was the cytoplasmic N terminus. Suppression was enhanced by tethering the N terminus to the membrane with a CAAX motif. The 11-amino acid motif (including Arg⁵² and Arg⁵⁴) within the N terminus, which we have previously shown to be required for G protein modulation, is also essential for dominant negative suppression. Suppression is prevented by addition of an N-terminal tag (XFP) to the full-length and truncated constructs. We further show that suppression of Ca_V2.2 currents by the N terminus-CAAX construct is accompanied by a reduction in Ca_V2.2 protein level, and this is also prevented by mutation of Arg⁵² and Arg⁵⁴ to Ala in the truncated construct. Taken together, our evidence indicates that both the extreme N terminus and the Arg⁵², Arg⁵⁴ motif are involved in the processes underlying dominant negative suppression.

Dual role of K ATP channel C-terminal motif in membrane targeting and metabolic regulation

The coordinated sorting of ion channels to specific plasma membrane domains is necessary for excitable cell physiology. K(ATP) channels, assembled from pore-forming (Kir6.x) and regulatory sulfonylurea receptor subunits, are critical electrical transducers of the metabolic state of excitable tissues, including skeletal and smooth muscle, heart, brain, kidney, and pancreas. Here we show that the C-terminal domain of Kir6.2 contains a motif conferring membrane targeting in primary excitable cells. Kir6.2 lacking this motif displays aberrant channel targeting due to loss of association with the membrane adapter ankyrin-B (AnkB). Moreover, we demonstrate that this Kir6.2 C-terminal AnkB-binding motif (ABM) serves a dual role in K(ATP) channel trafficking and membrane metabolic regulation and dysfunction in these pathways results in human excitable cell disease. Thus, the K(ATP) channel ABM serves as a previously unrecognized bifunctional touch-point for grading K(ATP) channel gating and membrane targeting and may play a fundamental role in controlling excitable cell metabolic regulation.

Presynaptic CaV2 calcium channel traffic requires CALF-1 and the alpha(2)delta subunit UNC-36

Presynaptic voltage-gated calcium channels provide calcium for synaptic vesicle exocytosis. We show here that a green fluorescent protein-tagged alpha(1) subunit of the Caenorhabditis elegans CaV2 channel, UNC-2, is localized to presynaptic active zones of sensory and motor neurons. Synaptic localization of CaV2 requires the alpha(2)delta subunit UNC-36 and CALF-1 (Calcium Channel Localization Factor-1), a neuronal transmembrane protein that localizes to the endoplasmic reticulum. In calf-1 mutants, UNC-2 is retained in the endoplasmic reticulum, but other active-zone components and synaptic vesicles are delivered to synapses. Acute induction of calf-1 mobilizes preexisting UNC-2 for delivery to synapses, consistent with a direct trafficking role. The alpha(2)delta subunit UNC-36 is likewise required for exit of UNC-2 from endoplasmic reticulum but has additional functions. Genetic and cell biological interactions suggest that CALF-1 couples intracellular traffic to functional maturation of CaV2 presynaptic calcium channels.

Cardiac sarcolemmal K(ATP) channels: Latest twists in a questing tale!

Reconstitution of K(ATP) channel activity from coexpression of members of the pore-forming inward rectifier gene family (Kir6.1, KCNJ8, and Kir6.2 KCNJ11) with sulfonylurea receptors (SUR1, ABCC8, and SUR2, ABCC9) of the ABCC protein sub-family, has led to the elucidation of many details of channel gating and pore properties, as well as the essential roles of Kir6.2 and SUR2 subunits in generating cardiac ventricular K(ATP). However, despite this extensive body of knowledge, there remain significant holes in our understanding of the physiological role of the cardiac K(ATP) channel, and surprising new findings keep emerging. Recent findings from genetically modified animals include the apparent insensitivity of cardiac sarcolemmal channels to nucleotide levels, and unenvisioned complexities of the subunit make-up of the cardiac channels. This topical review focuses on these new findings and considers their implications.

A comprehensive guide to the ROMK potassium channel: form and function in health and disease

The discovery of the renal outer medullary K+ channel (ROMK, K(ir)1.1), the founding member of the inward-rectifying K+ channel (K(ir)) family, by Ho and Hebert in 1993 revolutionized our understanding of potassium channel biology and renal potassium handling. Because of the central role that ROMK plays in the regulation of salt and potassium homeostasis, considerable efforts have been invested in understanding the underlying molecular mechanisms. Here we provide a comprehensive guide to ROMK, spanning from the physiology in the kidney to the organization and regulation by intracellular factors to the structural basis of its function at the atomic level.

Modulation of chemokine receptor activity through dimerization and crosstalk

Chemokines are small, secreted proteins that bind to the chemokine receptor subfamily of class A G protein-coupled receptors. Collectively, these receptor-ligand pairs are responsible for diverse physiological responses including immune cell trafficking, development and mitogenic signaling, both in the context of homeostasis and disease. However, chemokines and their receptors are not isolated entities, but instead function in complex networks involving homo- and heterodimer formation as well as crosstalk with other signaling complexes. Here the functional consequences of chemokine receptor activity, from the perspective of both direct physical associations with other receptors and indirect crosstalk with orthogonal signaling pathways, are reviewed. Modulation of chemokine receptor activity through these mechanisms has significant implications in physiological and pathological processes, as well as drug discovery and drug efficacy. The integration of signals downstream of chemokine and other receptors will be key to understanding how cells fine-tune their response to a variety of stimuli, including therapeutics.

Review. SUR1: a unique ATP-binding cassette protein that functions as an ion channel regulator

SUR1 is an ATP-binding cassette (ABC) transporter with a novel function. In contrast to other ABC proteins, it serves as the regulatory subunit of an ion channel. The ATP-sensitive (KATP) channel is an octameric complex of four pore-forming Kir6.2 subunits and four regulatory SUR1 subunits, and it links cell metabolism to electrical activity in many cell types. ATPase activity at the nucleotide-binding domains of SUR results in an increase in KATP channel open probability. Conversely, ATP binding to Kir6.2 closes the channel. Metabolic regulation is achieved by the balance between these two opposing effects. Precisely how SUR1 talks to Kir6.2 remains unclear, but recent studies have identified some residues and domains that are involved in both physical and functional interactions between the two proteins. The importance of these interactions is exemplified by the fact that impaired regulation of Kir6.2 by SUR1 results in human disease, with loss-of-function SUR1 mutations causing congenital hyperinsulinism and gain-of-function SUR1 mutations leading to neonatal diabetes. This paper reviews recent data on the regulation of Kir6.2 by SUR1 and considers the molecular mechanisms by which SUR1 mutations produce disease.

Coassembly of different sulfonylurea receptor subtypes extends the phenotypic diversity of ATP-sensitive potassium (KATP) channels

K(ATP) channels are metabolic sensors and targets of potassium channel openers (KCO; e.g., diazoxide and pinacidil). They comprise four sulfonylurea receptors (SUR) and four potassium channel subunits (Kir6) and are critical in regulating insulin secretion. Different SUR subtypes (SUR1, SUR2A, SUR2B) largely determine the metabolic sensitivities and the pharmacological profiles of K(ATP) channels. SUR1- but not SUR2-containing channels are highly sensitive to metabolic inhibition and diazoxide, whereas SUR2 channels are sensitive to pinacidil. It is generally believed that SUR1 and SUR2 are incompatible in channel coassembly. We used triple tandems, T1 and T2, each containing one SUR (SUR1 or SUR2A) and two Kir6.2Delta26 (last 26 residues are deleted) to examine the coassembly of different SUR. When T1 or T2 was expressed in Xenopus laevis oocytes, small whole-cell currents were activated by metabolic inhibition (induced by azide) plus a KCO (diazoxide for T1, pinacidil for T2). When coexpressed with any SUR subtype, the activated-currents were increased by 2- to 13-fold, indicating that different SUR can coassemble. Consistent with this, heteromeric SUR1+SUR2A channels were sensitive to azide, diazoxide, and pinacidil, and their single-channel burst duration was 2-fold longer than that of the T1 channels. Furthermore, SUR2A was coprecipitated with SUR1. Using whole-cell recording and immunostaining, heteromeric channels could also be detected when T1 and SUR2A were coexpressed in mammalian cells. Finally, the response of the SUR1+SUR2A channels to azide was found to be intermediate to those of the homomeric channels. Therefore, different SUR subtypes can coassemble into K(ATP) channels with distinct metabolic sensitivities and pharmacological profiles.

Role for SUR2A ED domain in allosteric coupling within the K(ATP) channel complex

Allosteric regulation of heteromultimeric ATP-sensitive potassium (K_ATP) channels is unique among protein systems as it implies transmission of ligand-induced structural adaptation at the regulatory SUR subunit, a member of ATP-binding cassette ABCC family, to the distinct pore-forming K⁺ (Kir6.x) channel module. Cooperative interaction between nucleotide binding domains (NBDs) of SUR is a prerequisite for K_ATP channel gating, yet pathways of allosteric intersubunit communication remain uncertain. Here, we analyzed the role of the ED domain, a stretch of 15 negatively charged aspartate/glutamate amino acid residues (948–962) of the SUR2A isoform, in the regulation of cardiac K_ATP channels. Disruption of the ED domain impeded cooperative NBDs interaction and interrupted the regulation of K_ATP channel complexes by MgADP, potassium channel openers, and sulfonylurea drugs. Thus, the ED domain is a structural component of the allosteric pathway within the K_ATP channel complex integrating transduction of diverse nucleotide-dependent states in the regulatory SUR subunit to the open/closed states of the K⁺-conducting channel pore.

Three C-terminal residues from the sulphonylurea receptor contribute to the functional coupling between the K(ATP) channel subunits SUR2A and Kir6.2

Cardiac ATP-sensitive potassium (K(ATP)) channels are metabolic sensors formed by the association of the inward rectifier potassium channel Kir6.2 and the sulphonylurea receptor SUR2A. SUR2A adjusts channel gating as a function of intracellular ATP and ADP and is the target of pharmaceutical openers and blockers which, respectively, up- and down-regulate Kir6.2. In an effort to understand how effector binding to SUR2A translates into Kir6.2 gating modulation, we examined the role of a 65-residue SUR2A fragment linking transmembrane domain TMD2 and nucleotide-binding domain NBD2 that has been shown to interact with Kir6.2. This fragment of SUR2A was replaced by the equivalent residues of its close homologue, the multidrug resistance protein MRP1. The chimeric construct was expressed in Xenopus oocytes and characterized using the patch-clamp technique. We found that activation by MgADP and synthetic openers was greatly attenuated although apparent affinities were unchanged. Further chimeragenetic and mutagenetic studies showed that mutation of three residues, E1305, I1310 and L1313 (rat numbering), was sufficient to confer this defective phenotype. The same mutations had no effects on channel block by the sulphonylurea glibenclamide or by ATP, suggesting a role for these residues in activatory--but not inhibitory--transduction processes. These results indicate that, within the K(ATP) channel complex, the proximal C-terminal of SUR2A is a critical link between ligand binding to SUR2A and Kir6.2 up-regulation.

Neonatal diabetes mellitus

An explosion of work over the last decade has produced insight into the multiple hereditary causes of a nonimmunological form of diabetes diagnosed most frequently within the first 6 months of life. These studies are providing increased understanding of genes involved in the entire chain of steps that control glucose homeostasis. Neonatal diabetes is now understood to arise from mutations in genes that play critical roles in the development of the pancreas, of beta-cell apoptosis and insulin processing, as well as the regulation of insulin release. For the basic researcher, this work is providing novel tools to explore fundamental molecular and cellular processes. For the clinician, these studies underscore the need to identify the genetic cause underlying each case. It is increasingly clear that the prognosis, therapeutic approach, and genetic counseling a physician provides must be tailored to a specific gene in order to provide the best medical care.

cAMP-dependent protein kinase phosphorylation produces interdomain movement in SUR2B leading to activation of the vascular KATP channel

Vascular ATP-sensitive K(+) channels are activated by multiple vasodilating hormones and neurotransmitters via PKA. A critical PKA phosphorylation site (Ser-1387) is found in the second nucleotide-binding domain (NBD(2)) of the SUR2B subunit. To understand how phosphorylation at Ser-1387 leads to changes in channel activity, we modeled the SUR2B using a newly crystallized ABC protein SAV1866. The model showed that Ser-1387 was located on the interface of NBD2 with TMD1 and physically interacted with Tyr-506 in TMD1. A positively charged residue (Arg-1462) in NBD2 was revealed in the close vicinity of Ser-1387. Mutation of either of these three residues abolished PKA-dependent channel activation. Molecular dynamics simulations suggested that Ser-1387, Tyr-506, and Arg-1462 formed a compact triad upon Ser-1387 phosphorylation, leading to reshaping of the NBD2 interface and movements of NBD2 and TMD1. Restriction of the interdomain movements by engineering a disulfide bond between TMD1 and NBD2 prevented the channel activation in a redox-dependent manner. Thus, a channel-gating mechanism is suggested through enhancing the NBD-TMD coupling efficiency following Ser-1387 phosphorylation, which is shared by multiple vasodilators.

H,K-ATPase protein localization and Kir4.1 function reveal concordance of three axes during early determination of left-right asymmetry

Consistent laterality is a fascinating problem, and study of the Xenopus embryo has led to molecular characterization of extremely early steps in left-right patterning: bioelectrical signals produced by ion pumps functioning upstream of asymmetric gene expression. Here, we reveal a number of novel aspects of the H+/K+-ATPase module in chick and frog embryos. Maternal H+/K+-ATPase subunits are asymmetrically localized along the left-right, dorso-ventral, and animal-vegetal axes during the first cleavage stages, in a process dependent on cytoskeletal organization. Using a reporter domain fused to molecular motors, we show that the cytoskeleton of the early frog embryo can provide asymmetric, directional information for subcellular transport along all three axes. Moreover, we show that the Kir4.1 potassium channel, while symmetrically expressed in a dynamic fashion during early cleavages, is required for normal LR asymmetry of frog embryos. Thus, Kir4.1 is an ideal candidate for the K+ ion exit path needed to allow the electroneutral H+/K+-ATPase to generate voltage gradients. In the chick embryo, we show that H+/K+-ATPase and Kir4.1 are expressed in the primitive streak, and that the known requirement for H+/K+-ATPase function in chick asymmetry does not function through effects on the circumferential expression pattern of Connexin43. These data provide details crucial for the mechanistic modeling of the physiological events linking subcellular processes to large-scale patterning and suggest a model where the early cytoskeleton sets up asymmetric ion flux along the left-right axis as a system of planar polarity functioning orthogonal to the apical-basal polarity of the early blastomeres.

Expression of functional Kir6.1 channels regulates glutamate release at CA3 synapses in generation of epileptic form of seizures

The Kir6.1 channels are a subtype of ATP-sensitive inwardly rectifying potassium (K(ATP)) channels that play an essential role in coupling the cell's metabolic events to electrical activity. In this study, we show that functional Kir6.1 channels are located at excitatory pre-synaptic terminals as a complex with type-1 Sulfonylurea receptors (SUR1) in the hippocampus. The mutant mice with deficiencies in expressing the Kir6.1 or the SUR1 gene are more vulnerable to generation of epileptic form of seizures, compared to wild-type controls. Whole-cell patch clamp recordings demonstrate that genetic deletion of the Kir6.1/SUR1 channels enhances glutamate release at CA3 synapses. Hence, expression of functional Kir6.1/SUR1 channels inhibits seizure responses and possibly acts via limiting excitatory glutamate release.

ATP-sensitive potassium channels: metabolic sensing and cardioprotection

The cardiovascular system operates under a wide scale of demands, ranging from conditions of rest to extreme stress. How the heart muscle matches rates of ATP production with utilization is an area of active investigation. ATP-sensitive potassium (K(ATP)) channels serve a critical role in the orchestration of myocardial energetic well-being. K(ATP) channel heteromultimers consist of inwardly-rectifying K(+) channel 6.2 and ATP-binding cassette sulfonylurea receptor 2A that translates local ATP/ADP levels, set by ATPases and phosphotransfer reactions, to the channel pore function. In cells in which the mobility of metabolites between intracellular microdomains is limited, coupling of phosphotransfer pathways with K(ATP) channels permits a high-fidelity transduction of nucleotide fluxes into changes in membrane excitability, matching energy demands with metabolic resources. This K(ATP) channel-dependent optimization of cardiac action potential duration preserves cellular energy balance at varying workloads. Mutations of K(ATP) channels result in disruption of the nucleotide signaling network and generate a stress-vulnerable phenotype with excessive susceptibility to injury, development of cardiomyopathy, and arrhythmia. Solving the mechanisms underlying the integration of K(ATP) channels into the cellular energy network will advance the understanding of endogenous cardioprotection and the development of strategies for the management of cardiovascular injury and disease progression.

Activation of inwardly rectifying potassium (Kir) channels by phosphatidylinosital-4,5-bisphosphate (PIP2): Interaction with other regulatory ligands

All members of the inwardly rectifying potassium channels (Kir1-7) are regulated by the membrane phospholipid, phosphatidylinosital-4,5-bisphosphate (PIP(2)). Some are also modulated by other regulatory factors or ligands such as ATP and G-proteins, which give them their common names, such as the ATP sensitive potassium (K(ATP)) channel and the G-protein gated potassium channel. Other more non-specific regulators include polyamines, kinases, pH and Na(+) ions. Recent studies have demonstrated that PIP(2) acts cooperatively with other regulatory factors to modulate Kir channels. Here we review how PIP(2) and co-factors modulate channel activities in each subfamily of the Kir channels.

Studies of the ATPase activity of the ABC protein SUR1

The ATP-sensitive potassium (K(ATP)) channel couples glucose metabolism to insulin secretion in pancreatic beta-cells. It comprises regulatory sulfonylurea receptor 1 and pore-forming Kir6.2 subunits. Binding and/or hydrolysis of Mg-nucleotides at the nucleotide-binding domains of sulfonylurea receptor 1 stimulates channel opening and leads to membrane hyperpolarization and inhibition of insulin secretion. We report here the first purification and functional characterization of sulfonylurea receptor 1. We also compared the ATPase activity of sulfonylurea receptor 1 with that of the isolated nucleotide-binding domains (fused to maltose-binding protein to improve solubility). Electron microscopy showed that nucleotide-binding domains purified as ring-like complexes corresponding to approximately 8 momomers. The ATPase activities expressed as maximal turnover rate [in nmol P(i).s(-1).(nmol protein)(-1)] were 0.03, 0.03, 0.13 and 0.08 for sulfonylurea receptor 1, nucleotide-binding domain 1, nucleotide-binding domain 2 and a mixture of nucleotide-binding domain 1 and nucleotide-binding domain 2, respectively. Corresponding K(m) values (in mm) were 0.1, 0.6, 0.65 and 0.56, respectively. Thus sulfonylurea receptor 1 has a lower K(m) than either of the isolated nucleotide-binding domains, and a lower maximal turnover rate than nucleotide-binding domain 2. Similar results were found with GTP, but the K(m) values were lower. Mutation of the Walker A lysine in nucleotide-binding domain 1 (K719A) or nucleotide-binding domain 2 (K1385M) inhibited the ATPase activity of sulfonylurea receptor 1 by 60% and 80%, respectively. Beryllium fluoride (K(i) 16 microm), but not MgADP, inhibited the ATPase activity of sulfonylurea receptor 1. In contrast, both MgADP and beryllium fluoride inhibited the ATPase activity of the nucleotide-binding domains. These data demonstrate that the ATPase activity of sulfonylurea receptor 1 differs from that of the isolated nucleotide-binding domains, suggesting that the transmembrane domains may influence the activity of the protein.

Defining a binding pocket for sulfonylureas in ATP-sensitive potassium channels

Sulfonylurea receptors SUR1 and SUR2 are the regulatory subunits of K(ATP) channels. Their differential affinity for hypoglycemic sulfonylureas provides a basis for the selectivity of these compounds for different K(ATP) channel isoforms. Sulfonylureas have a 100- to 1000-fold greater affinity for SUR1 vs. SUR2. Structure-activity studies suggested a bipartite binding pocket. Chimeric SUR1 approximately SUR2 receptors have shown TMD2, the third bundle of transmembrane helices, to be part of an "A" site that confers SUR1 selectivity for sulfonylureas. The purpose of this study is to determine the position of the "B" site. Previous photoaffinity labeling studies have placed the B site on the amino-terminal third of SUR and colabeled the associated K(IR). In our study, deletion of TMD0, the first bundle of transmembrane helices, did not compromise labeling. Further deletions into the cytoplasmic linker, L0, eliminated binding and labeling. Alanine substitutions in L0 identified a limited number of conserved residues, Y230 and W232, important for affinity labeling. A fragment of K(IR)6.2, missing M2 and the entire carboxyl terminal, assembles with SUR1 and is affinity labeled, while deletion of 10 or more amino-terminal residues compromises labeling. These studies indicate that the B site involves L0 and the K(IR) amino terminus, elements that are critical for control of channel gating.

Hyperinsulinism in mice with heterozygous loss of K(ATP) channels

AIMS/HYPOTHESIS

ATP-sensitive K(+) (K(ATP)) channels couple glucose metabolism to insulin secretion in pancreatic beta cells. In humans, loss-of-function mutations of beta cell K(ATP) subunits (SUR1, encoded by the gene ABCC8, or Kir6.2, encoded by the gene KCNJ11) cause congenital hyperinsulinaemia. Mice with dominant-negative reduction of beta cell K(ATP) (Kir6.2[AAA]) exhibit hyperinsulinism, whereas mice with zero K(ATP) (Kir6.2(-/-)) show transient hyperinsulinaemia as neonates, but are glucose-intolerant as adults. Thus, we propose that partial loss of beta cell K(ATP) in vivo causes insulin hypersecretion, but complete absence may cause insulin secretory failure.

MATERIALS AND METHODS

Heterozygous Kir6.2(+/-) and SUR1(+/-) animals were generated by backcrossing from knockout animals. Glucose tolerance in intact animals was determined following i.p. loading. Glucose-stimulated insulin secretion (GSIS), islet K(ATP) conductance and glucose dependence of intracellular Ca(2+) were assessed in isolated islets.

RESULTS

In both of the mechanistically distinct models of reduced K(ATP) (Kir6.2(+/-) and SUR1(+/-)), K(ATP) density is reduced by approximately 60%. While both Kir6.2(-/-) and SUR1(-/-) mice are glucose-intolerant and have reduced glucose-stimulated insulin secretion, heterozygous Kir6.2(+/-) and SUR1(+/-) mice show enhanced glucose tolerance and increased GSIS, paralleled by a left-shift in glucose dependence of intracellular Ca(2+) oscillations.

CONCLUSIONS/INTERPRETATION

The results confirm that incomplete loss of beta cell K(ATP) in vivo underlies a hyperinsulinaemic phenotype, whereas complete loss of K(ATP) underlies eventual secretory failure.

ABCC8 and ABCC9: ABC transporters that regulate K+ channels

The sulfonylurea receptors (SURs) ABCC8/SUR1 and ABCC9/SUR2 are members of the C-branch of the transport adenosine triphosphatase superfamily. Unlike their brethren, the SURs have no identified transport function; instead, evolution has matched these molecules with K(+) selective pores, either K(IR)6.1/KCNJ8 or K(IR)6.2/KCNJ11, to assemble adenosine triphosphate (ATP)-sensitive K(+) channels found in endocrine cells, neurons, and both smooth and striated muscle. Adenine nucleotides, the major regulators of ATP-sensitive K(+) (K(ATP)) channel activity, exert a dual action. Nucleotide binding to the pore reduces the activity or channel open probability, whereas Mg-nucleotide binding and/or hydrolysis in the nucleotide-binding domains of SUR antagonize this inhibitory action to stimulate channel openings. Mutations in either subunit can alter this balance and, in the case of the SUR1/KIR6.2 channels found in neurons and insulin-secreting pancreatic beta cells, are the cause of monogenic forms of hyperinsulinemic hypoglycemia and neonatal diabetes. Additionally, the subtle dysregulation of K(ATP) channel activity by a K(IR)6.2 polymorphism has been suggested as a predisposing factor in type 2 diabetes mellitus. Studies on K(ATP) channel null mice are clarifying the roles of these metabolically sensitive channels in a variety of tissues.

Overexpression of SUR2A generates a cardiac phenotype resistant to ischemia

ATP-sensitive K+ (K(ATP)) channels are present in the sarcolemma of cardiac myocytes where they link membrane excitability with the cellular bioenergetic state. These channels are in vivo composed of Kir6.2, a pore-forming subunit, SUR2A, a regulatory subunit, and at least four accessory proteins. In the present study, real-time RT-PCR has demonstrated that of all six sarcolemmal K(ATP) channel-forming proteins, SUR2A was probably the least expressed protein. We have generated mice where the SUR2A was under the control of a cytomegalovirus promoter, a promoter that is more efficient than the native promoter. These mice had an increase in SUR2A mRNA/protein levels in the heart whereas levels of mRNAs of other channel-forming proteins were not affected at all. Imunoprecipitation/Western blot and patch clamp electrophysiology has shown an increase in K(ATP) channel numbers in the sarcolemma of transgenic mice. Cardiomyocytes from transgenic mice responded to hypoxia with shortening of action membrane potential and were significantly more resistant to this insult than cardiomyocytes from the wild-type. The size of myocardial infarction in response to ischemia-reperfusion was much smaller in hearts from transgenic mice compared to those in wild-type. We conclude that overexpression of SUR2A generates cardiac phenotype resistant to hypoxia/ischemia/reperfusion injury due at least in part to increase in levels of sarcolemmal K(ATP) channels.

Selective mobility and sensitivity to SNAREs is exhibited by the Arabidopsis KAT1 K+ channel at the plasma membrane

Recent findings indicate that proteins in the SNARE superfamily are essential for cell signaling, in addition to facilitating vesicle traffic in plant cell homeostasis, growth, and development. We previously identified SNAREs SYP121/Syr1 from tobacco (Nicotiana tabacum) and the Arabidopsis thaliana homolog SYP121 associated with abscisic acid and drought stress. Disrupting tobacco SYP121 function by expressing a dominant-negative Sp2 fragment had severe effects on growth, development, and traffic to the plasma membrane, and it blocked K(+) and Cl(-) channel responses to abscisic acid in guard cells. These observations raise questions about SNARE control in exocytosis and endocytosis of ion channel proteins and their organization within the plane of the membrane. We have used a dual, in vivo tagging strategy with a photoactivatable green fluorescent protein and externally exposed hemagglutinin epitopes to monitor the distribution and trafficking dynamics of the KAT1 K(+) channel transiently expressed in tobacco leaves. KAT1 is localized to the plasma membrane within positionally stable microdomains of approximately 0.5 microm in diameter; delivery of the K(+) channel, but not of the PMA2 H(+)-ATPase, to the plasma membrane is suppressed by Sp2 fragments of tobacco and Arabidopsis SYP121, and Sp2 expression leads to profound changes in KAT1 distribution and mobility within the plane of the plasma membrane. These results offer direct evidence for SNARE-mediated traffic of the K(+) channel and a role in its distribution within subdomains of the plasma membrane, and they implicate a role for SNAREs in positional anchoring of the K(+) channel protein.

Regulation of surface localization of the small conductance Ca2+-activated potassium channel, Sk2, through direct phosphorylation by cAMP-dependent protein kinase

Small conductance, Ca2+-activated voltage-independent potassium channels (SK channels) are widely expressed in diverse tissues; however, little is known about the molecular regulation of SK channel subunits. Direct alteration of ion channel subunits by kinases is a candidate mechanism for functional modulation of these channels. We find that activation of cyclic AMP-dependent protein kinase (PKA) with forskolin (50 microm) causes a dramatic decrease in surface localization of the SK2 channel subunit expressed in COS7 cells due to direct phosphorylation of the SK2 channel subunit. PKA phosphorylation studies using the intracellular domains of the SK2 channel subunit expressed as glutathione S-transferase fusion protein constructs showed that both the amino-terminal and carboxyl-terminal regions are PKA substrates in vitro. Mutational analysis identified a single PKA phosphorylation site within the amino-terminal of the SK2 subunit at serine 136. Mutagenesis and mass spectrometry studies identified four PKA phosphorylation sites: Ser465 (minor site) and three amino acid residues Ser568, Ser569, and Ser570 (major sites) within the carboxyl-terminal region. A mutated SK2 channel subunit, with the three contiguous serines mutated to alanines to block phosphorylation at these sites, shows no decrease in surface expression after PKA stimulation. Thus, our findings suggest that PKA phosphorylation of these three sites is necessary for PKA-mediated reorganization of SK2 surface expression.

Carvedilol blocks cardiac KATP and KG but not IK1 channels by acting at the bundle-crossing regions

We examined the effects of carvedilol on cardiac inwardly rectifying K(+) (Kir) channels, i.e., ATP-sensitive (K(ATP)), G-protein-activated (K(G)) and background (I(K1)) Kir channels. We found that carvedilol effectively inhibits K(ATP) and K(G), but not I(K1) channels. Carvedilol inhibits K(ATP) channels reconstituted in HEK293 cells with Kir6.2 lacking the C-terminal 26 amino acids (Kir6.2DeltaC26), suggesting that carvedilol acts in the channel pore. A sequence comparison of the three channels revealed that a cysteine residue, C166, in the inner helix of Kir6.2 is conserved in both Kir6.xs (K(ATP)) and Kir3.xs (K(G)), but not in Kir2.xs (I(K1)). The mutation of this residue (C166A) made Kir6.2DeltaC26 resistant to the drug. Homology modeling and docking simulation suggested that interaction between carvedilol and the pore could be located at the cytosolic portion of the inner helix (bundle-crossing region) containing C166. This study shows that carvedilol blocks specific groups of Kir channels by interacting with the bundle-crossing region.

3-D structural and functional characterization of the purified KATP channel complex Kir6.2-SUR1

ATP-sensitive potassium (K(ATP)) channels conduct potassium ions across cell membranes and thereby couple cellular energy metabolism to membrane electrical activity. Here, we report the heterologous expression and purification of a functionally active K(ATP) channel complex composed of pore-forming Kir6.2 and regulatory SUR1 subunits, and determination of its structure at 18 A resolution by single-particle electron microscopy. The purified channel shows ATP-ase activity similar to that of ATP-binding cassette proteins related to SUR1, and supports Rb(+) fluxes when reconstituted into liposomes. It has a compact structure, with four SUR1 subunits embracing a central Kir6.2 tetramer in both transmembrane and cytosolic domains. A cleft between adjacent SUR1s provides a route by which ATP may access its binding site on Kir6.2. The nucleotide-binding domains of adjacent SUR1 appear to interact, and form a large docking platform for cytosolic proteins. The structure, in combination with molecular modelling, suggests how SUR1 interacts with Kir6.2.

A Phosphorylation-dependent Export Structure in ROMK (Kir 1.1) Channel Overrides an Endoplasmic Reticulum Localization Signal

The cell surface density of functional Kir1.1 (ROMK, KCNJ1) channels in the renal collecting duct is precisely regulated to maintain potassium balance. Here, we explore the mechanism by which phosphorylation of Kir1.1a serine 44 controls plasmalemma expression. Studies in Xenopus oocytes, expressing wild-type, phosphorylation mimic (S44D), or phosphorylation null (S44A) Kir1.1a, revealed that phosphorylation of serine 44 is required to stimulate traffic of newly synthesized channels to the plasma membrane through a brefeldin A-sensitive pathway. ROMK channels were found to acquire mature glycosylation in a serine 44 phosphorylation-dependent manner, consistent with a phosphorylation-dependent trafficking step within the endoplasmic reticulum/Golgi. Serine 44 neighbors a string of three "RXR" motifs, reminiscent of basic trafficking signals involved in directing early transport steps within the secretory pathway. Replacement of the arginine residues with alanine (R35A, R37A, R39A, R41A, or all Arg to Ala) did not restore cell surface expression of the phospho-null S44A channel, making it unlikely that phosphorylation abrogates a nearby RXR-type endoplasmic reticulum (ER) localization signal. Instead, analysis of the compound S44D phospho-mimic mutants revealed that the neighboring arginine residues are also necessary for cell surface expression, identifying a structure that determines export in the biosynthetic pathway. Suppressor mutations in a putative dibasic ER retention signal, located within the cytoplasmic C terminus (K370A, R371A), restored cell surface expression of the phospho-null S44A channel to levels exhibited by the phospho-mimic S44D channel. Taken together, these studies indicate that phosphorylation of Ser44 drives an export step within the secretory pathway to override an independent endoplasmic reticulum localization signal.

Folding of CFTR Is Predominantly Cotranslational

The folding process for newly synthesized, multispanning membrane proteins in the endoplasmic reticulum (ER) is largely unknown. Here, we describe early folding events of the cystic fibrosis transmembrane conductance regulator (CFTR), a member of the ABC-transporter family. In vitro translation of CFTR in the presence of semipermeabilized cells allowed us to investigate this protein during nascent chain elongation. We found that CFTR folds mostly during synthesis as determined by protease susceptibility. C-terminally truncated constructs showed that individual CFTR domains formed well-defined structures independent of C-terminal parts. We conclude that the multidomain protein CFTR folds mostly cotranslationally, domain by domain.

Genetic isolation of transport signals directing cell surface expression

Membrane proteins represent approximately 30% of the proteome in both prokaryotes and eukaryotes. The spatial localization of membrane-bound proteins is often determined by specific sequence motifs that may be regulated in response to physiological changes, such as protein interactions and receptor signalling. Identification of signalling motifs is therefore important for understanding membrane protein expression, function and transport mechanisms. We report a genetic isolation of novel motifs that confer surface expression. Further characterization showed that SWTY, one class of these isolated motifs with homology to previously reported forward transport motifs, has the ability to both override the RKR endoplasmic reticulum localization signal and potentiate steady-state surface expression. The genetically isolated SWTY motif is functionally interchangeable with a known motif in cardiac potassium channels and an identified motif in an HIV coreceptor, and operates by recruiting 14-3-3 proteins. This study expands the repertoire of and enables a screening method for membrane trafficking signals.

Role of ubiquitin-proteasome degradation pathway in biogenesis efficiency of {beta}-cell ATP-sensitive potassium channels

ATP-sensitive potassium (K(ATP)) channels of pancreatic beta-cells mediate glucose-induced insulin secretion by linking glucose metabolism to membrane excitability. The number of plasma membrane K(ATP) channels determines the sensitivity of beta-cells to glucose stimulation. The K(ATP) channel is formed in the endoplasmic reticulum (ER) on coassembly of four inwardly rectifying potassium channel Kir6.2 subunits and four sulfonylurea receptor 1 (SUR1) subunits. Little is known about the cellular events that govern the channel's biogenesis efficiency and expression. Recent studies have implicated the ubiquitin-proteasome pathway in modulating surface expression of several ion channels. In this work, we investigated whether the ubiquitin-proteasome pathway plays a role in the biogenesis efficiency and surface expression of K(ATP) channels. We provide evidence that, when expressed in COS cells, both Kir6.2 and SUR1 undergo ER-associated degradation via the ubiquitin-proteasome system. Moreover, treatment of cells with proteasome inhibitors MG132 or lactacystin leads to increased surface expression of K(ATP) channels by increasing the efficiency of channel biogenesis. Importantly, inhibition of proteasome function in a pancreatic beta-cell line, INS-1, that express endogenous K(ATP) channels also results in increased channel number at the cell surface, as assessed by surface biotinylation and whole cell patch-clamp recordings. Our results support a role of the ubiquitin-proteasome pathway in the biogenesis efficiency and surface expression of beta-cell K(ATP) channels.

Cardiac KATP channels in health and disease

ATP-sensitive potassium (K(ATP)) channels are evolutionarily conserved plasma-membrane protein complexes, widely represented in tissue beds with high metabolic activity. There, they are formed through physical association of the inwardly rectifying potassium channel pore, most typically Kir6.2, and the regulatory sulfonylurea receptor subunit, an ATP-binding cassette protein. Energetic signals, received via tight integration with cellular metabolic pathways, are processed by the sulfonylurea receptor subunit that in turn gates the nucleotide sensitivity of the channel pore thereby controlling membrane potential dependent cellular functions. Recent findings, elicited from genetic disruption of channel proteins, have established in vivo the requirement of intact K(ATP) channels in the proper function of cardiac muscle under stress. In the heart, where K(ATP) channels were originally discovered, channel ablation compromises cardioprotection under ischemic insult. New data implicate the requirement of intact K(ATP) channels for the cardiac adaptive response to acute stress. K(ATP) channels have been further implicated in the adaptive cardiac response to chronic (patho)physiologic hemodynamic load, with K(ATP) channel deficiency affecting structural remodeling, rendering the heart vulnerable to calcium-dependent maladaptation and predisposing to heart failure. These findings are underscored by the identification in humans that defective K(ATP) channels induced by mutations in ABCC9, the gene encoding the cardiac sulfonylurea receptor subunit, confer susceptibility to dilated cardiomyopathy. Thus, in parallel with the developed understanding of the molecular identity and mode of action of K(ATP) channels since their discovery, there is now an expanded understanding of their critical significance in the cardiac stress response in health and disease.

ATP-sensitive K+ channel channel/enzyme multimer: metabolic gating in the heart

Cardiac ATP-sensitive K(+) (K(ATP)) channels, gated by cellular metabolism, are formed by association of the inwardly rectifying potassium channel Kir6.2, the potassium conducting subunit, and SUR2A, the ATP-binding cassette protein that serves as the regulatory subunit. Kir6.2 is the principal site of ATP-induced channel inhibition, while SUR2A regulates K(+) flux through adenine nucleotide binding and catalysis. The ATPase-driven conformations within the regulatory SUR2A subunit of the K(ATP) channel complex have determinate linkage with the states of the channel's pore. The probability and life-time of ATPase-induced SUR2A intermediates, rather than competitive nucleotide binding alone, defines nucleotide-dependent K(ATP) channel gating. Cooperative interaction, instead of independent contribution of individual nucleotide binding domains within the SUR2A subunit, serves a decisive role in defining K(ATP) channel behavior. Integration of K(ATP) channels with the cellular energetic network renders these channel/enzyme heteromultimers high-fidelity metabolic sensors. This vital function is facilitated through phosphotransfer enzyme-mediated transmission of controllable energetic signals. By virtue of coupling with cellular energetic networks and the ability to decode metabolic signals, K(ATP) channels set membrane excitability to match demand for homeostatic maintenance. This new paradigm in the operation of an ion channel multimer is essential in providing the basis for K(ATP) channel function in the cardiac cell, and for understanding genetic defects associated with life-threatening diseases that result from the inability of the channel complex to optimally fulfill its physiological role.

Genetics of type 2 diabetes mellitus: status and perspectives

Throughout the last decade, molecular genetic studies of non-autoimmune diabetes mellitus have contributed significantly to our present understanding of this disease's complex aetiopathogenesis. Monogenic forms of diabetes (maturity-onset diabetes of the young, MODY) have been identified and classified into MODY1-6 according to the mutated genes that by being expressed in the pancreatic beta-cells confirm at the molecular level the clinical presentation of MODY as a predominantly insulin secretory deficient form of diabetes mellitus. Genomewide linkage studies of presumed polygenic type 2 diabetic populations indicate that loci on chromosomes 1q, 5q, 8p, 10q, 12q and 20q contain susceptibility genes. Yet, so far, the only susceptibility gene, calpain-10 (CAPN10), which has been identified using genomewide linkage studies, is located on chromosome 2q37. Mutation analyses of selected 'candidate' susceptibility genes in various populations have also identified the widespread Pro12Ala variant of the peroxisome proliferator-activated receptor-gamma and the common Glu23Lys variant of the ATP-sensitive potassium channel, Kir6.2 (KCNJ11). These variants may contribute significantly to the risk type 2 diabetes conferring insulin resistance of liver, muscle and fat (Pro12Ala) and a relative insulin secretory deficiency (Glu23Lys). It is likely that, in the near future, the recent more detailed knowledge of the human genome and insights into its haploblocks together with the developments of high-throughput and cheap genotyping will facilitate the discovery of many more type 2 diabetes gene variants in study materials, which are statistically powered and phenotypically well characterized. The results of these efforts are likely to be the platform for major progress in the development of personalized antidiabetic drugs with higher efficacy and few side effects.

Toward linking structure with function in ATP-sensitive K+ channels

Advances in understanding the overall structural features of inward rectifiers and ATP-binding cassette (ABC) transporters are providing novel insight into the architecture of ATP-sensitive K+ channels (KATP channels) (KIR6.0/SUR)4. The structure of the K(IR) pore has been modeled on bacterial K+ channels, while the lipid-A exporter, MsbA, provides a template for the MDR-like core of sulfonylurea receptor (SUR)-1. TMD0, an NH2-terminal bundle of five alpha-helices found in SURs, binds to and activates KIR6.0. The adjacent cytoplasmic L0 linker serves a dual function, acting as a tether to link the MDR-like core to the KIR6.2/TMD0 complex and exerting bidirectional control over channel gating via interactions with the NH2-terminus of the KIR. Homology modeling of the SUR1 core offers the possibility of defining the glibenclamide/sulfonylurea binding pocket. Consistent with 30-year-old studies on the pharmacology of hypoglycemic agents, the pocket is bipartite. Elements of the COOH-terminal half of the core recognize a hydrophobic group in glibenclamide, adjacent to the sulfonylurea moiety, to provide selectivity for SUR1, while the benzamido group appears to be in proximity to L0 and the KIR NH2-terminus.

Localization and functional analyses of the MLC1 protein involved in megalencephalic leukoencephalopathy with subcortical cysts

Mutations in the MLC1 gene are responsible for one form of the neurological disorder megalencephalic leukoencephalopathy with subcortical cysts (MLC). The disease is a type of vacuolating myelinopathy. The biochemical properties and the function of the MLC1 protein are unknown. To characterize MLC1, we generated polyclonal antibodies. The MLC1 protein was detected in the brain, assembled into higher molecular complexes, as assessed by assembly-dependent trafficking assays. In situ hybridization and immunohistochemistry were used to determine MLC1 localization within the adult mouse brain. MLC1 was expressed in neurons, detected preferentially in particular axonal tracts. This expression pattern correlates with the major phenotype observed in the disease. In addition, it was expressed in some astrocytes, concentrating in Bergmann glia, the astrocyte end-feet membranes adjacent to blood vessels and in astrocyte-astrocyte membrane contact regions. Other neuronal barriers, such as the ependyma and the pia mater, were also positive for MLC1 expression. MLC1 was detected in vivo and in heterologous systems at the plasma membrane. MLC mutations impaired folding, and the defect was corrected in vitro by addition of curcumin, a Ca(2+)-ATPase inhibitor. In summary, this study provides an explanation as to why mutations in MLC1 provoke the disease and points to a possible therapy for some patients.

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.

Identification of a trafficking motif involved in the stabilization and polarization of P2X receptors

Extracellular ATP-gated channels (P2X receptors) define the third major family of ionotropic receptors, and they are expressed widely in nerve cells, muscles, and endocrine and exocrine glands. P2X subunits have two membrane-spanning domains, and a receptor is thought to be formed by oligomerization of three subunits. We have identified a conserved motif in the cytoplasmic C termini of P2X subunits that is necessary for their surface expression; mutations in this motif result in a marked reduction of the receptors at the plasma membrane because of a rapid internalization. Transfer of the motif to a reporter protein (CD(4)) enhances the surface expression of the chimera, indicating that this motif is likely involved in the stabilization of P2X receptor at the cell surface. In neurons, mutated P2X(2) subunits showed reduced membrane expression and an altered axodendritic distribution. This motif is also present in intracellular regions of other membrane proteins, such as in the third intracellular loop of some G protein-coupled receptors, suggesting that it might be involve in their cellular stabilization and polarization.

ABCC9 mutations identified in human dilated cardiomyopathy disrupt catalytic KATP channel gating

Stress tolerance of the heart requires high-fidelity metabolic sensing by ATP-sensitive potassium (K(ATP)) channels that adjust membrane potential-dependent functions to match cellular energetic demand. Scanning of genomic DNA from individuals with heart failure and rhythm disturbances due to idiopathic dilated cardiomyopathy identified two mutations in ABCC9, which encodes the regulatory SUR2A subunit of the cardiac K(ATP) channel. These missense and frameshift mutations mapped to evolutionarily conserved domains adjacent to the catalytic ATPase pocket within SUR2A. Mutant SUR2A proteins showed aberrant redistribution of conformations in the intrinsic ATP hydrolytic cycle, translating into abnormal K(ATP) channel phenotypes with compromised metabolic signal decoding. Defective catalysis-mediated pore regulation is thus a mechanism for channel dysfunction and susceptibility to dilated cardiomyopathy.

Assembly, Maturation, and Turnover of KATP Channel Subunits

ATP-sensitive K(+), or K(ATP), channels are comprised of K(IR)6.x and sulfonylurea receptor (SUR) subunits that assemble as octamers, (K(IR)/SUR)(4). The assembly pathway is unknown. Pulse-labeling studies show that when K(IR)6.2 is expressed individually, its turnover is biphasic; approximately 60% is lost with t((1/2)) approximately 36 min. The remainder converts to a long-lived species (t((1/2)) approximately 26 h) with an estimated half-time of 1.2 h. Expressed alone, SUR1 has a long half-life, approximately 25.5 h. When K(IR)6.2 and SUR1 are co-expressed, they associate rapidly and the fast degradation of K(IR)6.2 is eliminated. Based on changes in the glycosylation state of SUR1, the half-time for the maturation of K(ATP) channels, including completion of assembly, transit to the Golgi, and glycosylation, is approximately 2.2 h. Estimation of the turnover rates of mature, fully glycosylated SUR1 associated with K(IR)6.2 and of K(IR)6.2 associated with Myc-tagged SUR1 gave similar values for the half-life of K(ATP) channels, a mean value of approximately 7.3 h. K(ATP) channel subunits in INS-1 beta-cells displayed qualitatively similar kinetics. The results imply the octameric channels are stable. Two mutations, K(IR)6.2 W91R and SUR1 DeltaF1388, identified in patients with the severe form of familial hyperinsulinism, profoundly alter the rate of K(IR)6.2 and SUR1 turnover, respectively. Both mutant subunits associate with their respective partners but dissociate freely and degrade rapidly. The data support models of channel formation in which K(IR)6.2-SUR1 heteromers assemble functional channels and are inconsistent with models where SUR1 can only assemble with K(IR)6.2 tetramers.

Molecular analysis of the subtype-selective inhibition of cloned KATP channels by PNU-37883A

1. In this study, we have used Kir6.1/Kir6.2 chimeric proteins and current recordings to investigate the molecular basis of PNU-37883A inhibition of cloned K(ATP) channels. 2. Rat Kir6.1, Kir6.2 and Kir6.1/Kir6.2 chimeras were co-expressed with either SUR2B or SUR1, following RNA injection into Xenopus oocytes, and fractional inhibition of K(ATP) currents by 10 microm PNU-37883A reported. 3. Channels containing Kir6.1/SUR2B were more sensitive to inhibition by PNU-37883A than those containing Kir6.2/SUR2B (mean fractional inhibition: 0.70, cf. 0.07). 4. On expression with SUR2B, a chimeric channel with the Kir6.1 pore and the Kir6.2 amino- and carboxy-terminal domains was PNU-37883A insensitive (0.06). A chimera with the Kir6.1 carboxy-terminus and Kir6.2 amino-terminus and pore was inhibited (0.48). These results, and those obtained with other chimeras, suggest that the C-terminus is an important determinant of PNU-37883A inhibition of Kir6.1. Similar results were seen when constructs were co-expressed with SUR1. Further chimeric constructs localised PNU-37883A sensitivity to an 81 amino-acid residue section in the Kir6.1 carboxy-terminus. 5. Our data show that structural differences between Kir6.1 and Kir6.2 are important in determining sensitivity to PNU-37883A. This compound may prove useful in probing the structural and functional differences between the two channel subtypes.