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

Nucleotide inhibition of the pancreatic ATP-sensitive K+ channel explored with patch-clamp fluorometry

Pancreatic ATP-sensitive K⁺ channels (K_ATP) comprise four inward rectifier subunits (Kir6.2), each associated with a sulphonylurea receptor (SUR1). ATP/ADP binding to Kir6.2 shuts K_ATP. Mg-nucleotide binding to SUR1 stimulates K_ATP. In the absence of Mg²⁺, SUR1 increases the apparent affinity for nucleotide inhibition at Kir6.2 by an unknown mechanism. We simultaneously measured channel currents and nucleotide binding to Kir6.2. Fits to combined data sets suggest that K_ATP closes with only one nucleotide molecule bound. A Kir6.2 mutation (C166S) that increases channel activity did not affect nucleotide binding, but greatly perturbed the ability of bound nucleotide to inhibit K_ATP. Mutations at position K205 in SUR1 affected both nucleotide affinity and the ability of bound nucleotide to inhibit K_ATP. This suggests a dual role for SUR1 in K_ATP inhibition, both in directly contributing to nucleotide binding and in stabilising the nucleotide-bound closed state.

ATP binding without hydrolysis switches sulfonylurea receptor 1 (SUR1) to outward-facing conformations that activate KATP channels

Neuroendocrine-type ATP-sensitive K⁺ (K_ATP) channels are metabolite sensors coupling membrane potential with metabolism, thereby linking insulin secretion to plasma glucose levels. They are octameric complexes, (SUR1/Kir6.2)₄, comprising sulfonylurea receptor 1 (SUR1 or ABCC8) and a K⁺-selective inward rectifier (Kir6.2 or KCNJ11). Interactions between nucleotide-, agonist-, and antagonist-binding sites affect channel activity allosterically. Although it is hypothesized that opening these channels requires SUR1-mediated MgATP hydrolysis, we show here that ATP binding to SUR1, without hydrolysis, opens channels when nucleotide antagonism on Kir6.2 is minimized and SUR1 mutants with increased ATP affinities are used. We found that ATP binding is sufficient to switch SUR1 alone between inward- or outward-facing conformations with low or high dissociation constant, K_D , values for the conformation-sensitive channel antagonist [³H]glibenclamide ([³H]GBM), indicating that ATP can act as a pure agonist. Assembly with Kir6.2 reduced SUR1's K_D for [³H]GBM. This reduction required the Kir N terminus (KNtp), consistent with KNtp occupying a "transport cavity," thus positioning it to link ATP-induced SUR1 conformational changes to channel gating. Moreover, ATP/GBM site coupling was constrained in WT SUR1/WT Kir6.2 channels; ATP-bound channels had a lower K_D for [³H]GBM than ATP-bound SUR1. This constraint was largely eliminated by the Q1179R neonatal diabetes-associated mutation in helix 15, suggesting that a "swapped" helix pair, 15 and 16, is part of a structural pathway connecting the ATP/GBM sites. Our results suggest that ATP binding to SUR1 biases K_ATP channels toward open states, consistent with SUR1 variants with lower K_D values causing neonatal diabetes, whereas increased K_D values cause congenital hyperinsulinism.

Cryo-electron microscopy structures and progress toward a dynamic understanding of KATP channels

Puljung reviews recent cryo-EM K_ATP channel structures and proposes a mechanism by which ligand binding results in channel opening.

Adenosine triphosphate (ATP)–sensitive K⁺ (K_ATP) channels are molecular sensors of cell metabolism. These hetero-octameric channels, comprising four inward rectifier K⁺ channel subunits (Kir6.1 or Kir6.2) and four sulfonylurea receptor (SUR1 or SUR2A/B) subunits, detect metabolic changes via three classes of intracellular adenine nucleotide (ATP/ADP) binding site. One site, located on the Kir subunit, causes inhibition of the channel when ATP or ADP is bound. The other two sites, located on the SUR subunit, excite the channel when bound to Mg nucleotides. In pancreatic β cells, an increase in extracellular glucose causes a change in oxidative metabolism and thus turnover of adenine nucleotides in the cytoplasm. This leads to the closure of K_ATP channels, which depolarizes the plasma membrane and permits Ca²⁺ influx and insulin secretion. Many of the molecular details regarding the assembly of the K_ATP complex, and how changes in nucleotide concentrations affect gating, have recently been uncovered by several single-particle cryo-electron microscopy structures of the pancreatic K_ATP channel (Kir6.2/SUR1) at near-atomic resolution. Here, the author discusses the detailed picture of excitatory and inhibitory ligand binding to K_ATP that these structures present and suggests a possible mechanism by which channel activation may proceed from the ligand-binding domains of SUR to the channel pore.

Anti-diabetic drug binding site in a mammalian KATP channel revealed by Cryo-EM

Sulfonylureas are anti-diabetic medications that act by inhibiting pancreatic K_ATP channels composed of SUR1 and Kir6.2. The mechanism by which these drugs interact with and inhibit the channel has been extensively investigated, yet it remains unclear where the drug binding pocket resides. Here, we present a cryo-EM structure of a hamster SUR1/rat Kir6.2 channel bound to a high-affinity sulfonylurea drug glibenclamide and ATP at 3.63 Å resolution, which reveals unprecedented details of the ATP and glibenclamide binding sites. Importantly, the structure shows for the first time that glibenclamide is lodged in the transmembrane bundle of the SUR1-ABC core connected to the first nucleotide binding domain near the inner leaflet of the lipid bilayer. Mutation of residues predicted to interact with glibenclamide in our model led to reduced sensitivity to glibenclamide. Our structure provides novel mechanistic insights of how sulfonylureas and ATP interact with the K_ATP channel complex to inhibit channel activity.

Parametrisation of the free energy of ATP binding to wild-type and mutant Kir6.2 potassium channels

ATP-sensitive K(+) (K(ATP)) channels, comprised of pore-forming Kir6.x and regulatory SURx subunits, play important roles in many cellular functions; because of their sensitivity to inhibition by intracellular ATP, K(ATP) channels provide a link between cell metabolism and membrane electrical activity. We constructed structural homology models of Kir6.2 and a series of Kir6.2 channels carrying mutations within the putative ATP-binding site. Computational docking was carried out to determine the conformation of ATP in its binding site. The Linear Interaction Energy (LIE) method was used to estimate the free-energy of ATP binding to wild-type and mutant Kir6.2 channels. Comparisons of the theoretical binding free energies for ATP with those determined from mutational experiments enabled the identification of the most probable conformation of ATP bound to the Kir6.2 channel. A set of LIE parameters was defined that may enable prediction of the effects of additional Kir6.2 mutations within the ATP binding site on the affinity for ATP.

Current understanding of K ATP channels in neonatal diseases: focus on insulin secretion disorders

ATP-sensitive potassium (K(ATP)) channels are cell metabolic sensors that couple cell metabolic status to electric activity, thus regulating many cellular functions. In pancreatic beta cells, K(ATP) channels modulate insulin secretion in response to fluctuations in plasma glucose level, and play an important role in glucose homeostasis. Recent studies show that gain-of-function and loss-of-function mutations in K(ATP) channel subunits cause neonatal diabetes mellitus and congenital hyperinsulinism respectively. These findings lead to significant changes in the diagnosis and treatment for neonatal insulin secretion disorders. This review describes the physiological and pathophysiological functions of K(ATP) channels in glucose homeostasis, their specific roles in neonatal diabetes mellitus and congenital hyperinsulinism, as well as future perspectives of K(ATP) channels in neonatal diseases.

Muscle KATP channels: recent insights to energy sensing and myoprotection

ATP-sensitive potassium (K(ATP)) channels are present in the surface and internal membranes of cardiac, skeletal, and smooth muscle cells and provide a unique feedback between muscle cell metabolism and electrical activity. In so doing, they can play an important role in the control of contractility, particularly when cellular energetics are compromised, protecting the tissue against calcium overload and fiber damage, but the cost of this protection may be enhanced arrhythmic activity. Generated as complexes of Kir6.1 or Kir6.2 pore-forming subunits with regulatory sulfonylurea receptor subunits, SUR1 or SUR2, the differential assembly of K(ATP) channels in different tissues gives rise to tissue-specific physiological and pharmacological regulation, and hence to the tissue-specific pharmacological control of contractility. The last 10 years have provided insights into the regulation and role of muscle K(ATP) channels, in large part driven by studies of mice in which the protein determinants of channel activity have been deleted or modified. As yet, few human diseases have been correlated with altered muscle K(ATP) activity, but genetically modified animals give important insights to likely pathological roles of aberrant channel activity in different muscle types.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Modeling K(ATP) channel gating and its regulation

ATP-sensitive potassium (K(ATP)) channels couple cell metabolism to plasmalemmal potassium fluxes in a variety of cell types. The activity of these channels is primarily determined by intracellular adenosine nucleotides, which have both inhibitory and stimulatory effects. The role of K(ATP) channels has been studied most extensively in pancreatic beta-cells, where they link glucose metabolism to insulin secretion. Many mutations in K(ATP) channel subunits (Kir6.2, SUR1) have been identified that cause either neonatal diabetes or congenital hyperinsulinism. Thus, a mechanistic understanding of K(ATP) channel behavior is necessary for modeling beta-cell electrical activity and insulin release in both health and disease. Here, we review recent advances in the K(ATP) channel structure and function. We focus on the molecular mechanisms of K(ATP) channel gating by adenosine nucleotides, phospholipids and sulphonylureas and consider the advantages and limitations of various mathematical models of macroscopic and single-channel K(ATP) currents. Finally, we outline future directions for the development of more realistic models of K(ATP) channel gating.

A mutation in the ATP-binding site of the Kir6.2 subunit of the KATP channel alters coupling with the SUR2A subunit

Mutations in the pore-forming subunit of the ATP-sensitive K(+) (K(ATP)) channel Kir6.2 cause neonatal diabetes. Understanding the molecular mechanism of action of these mutations has provided valuable insight into the relationship between the structure and function of the K(ATP) channel. When Kir6.2 containing a mutation (F333I) in the putative ATP-binding site is coexpressed with the cardiac type of regulatory K(ATP) channel subunit, SUR2A, the channel sensitivity to ATP inhibition is reduced and the intrinsic open probability (P(o)) is increased. However, the extent of macroscopic current activation by MgADP was unaffected. Here we examine rundown and MgADP activation of wild-type and Kir6.2-F333I/SUR2A channels using single-channel recording, noise analysis and spectral analysis. We also compare the effect of mutating the adjacent residue, G334, on rundown and MgADP activation. All three approaches indicated that rundown of Kir6.2-F333I/SUR2A channels is due to a reduction in the number of active channels in the patch and that MgADP reactivation involves recruitment of inactive channels. In contrast, rundown and MgADP reactivation of wild-type and Kir6.2-G334D/SUR2A channels, and of Kir6.2-F333I/SUR1 channels, involve a gradual change in P(o). Our results suggest that F333 in Kir6.2 interacts functionally with SUR2A to modulate channel rundown and MgADP activation. This interaction is fairly specific as it is not disturbed when the adjacent residue (G334) is mutated. It is also not a consequence of the enhanced P(o) of Kir6.2-F333I/SUR2A channels, as it is not found for other mutant channels with high P(o) (Kir6.2-I296L/SUR2A).

Functional effects of mutations at F35 in the NH2-terminus of Kir6.2 (KCNJ11), causing neonatal diabetes, and response to sulfonylurea therapy

Heterozygous mutations in the human Kir6.2 gene (KCNJ11), the pore-forming subunit of the ATP-sensitive K(+) channel (K(ATP) channel), cause neonatal diabetes. To date, all mutations increase whole-cell K(ATP) channel currents by reducing channel inhibition by MgATP. Here, we provide functional characterization of two mutations (F35L and F35V) at residue F35 of Kir6.2, which lies within the NH(2)-terminus. We further show that the F35V patient can be successfully transferred from insulin to sulfonylurea therapy. The patient has been off insulin for 24 months and shows improved metabolic control (mean HbA(1c) 7.58 before and 6.18% after sulfonylurea treatment; P < 0.007). Wild-type and mutant Kir6.2 were heterologously coexpressed with SUR1 in Xenopus oocytes. Whole-cell K(ATP) channel currents through homomeric and heterozygous F35V and F35L channels were increased due to a reduced sensitivity to inhibition by MgATP. The mutation also increased the open probability (P(O)) of homomeric F35 mutant channels in the absence of ATP. These effects on P(O) and ATP sensitivity were abolished in the absence of SUR1. Our results suggest that mutations at F35 cause permanent neonatal diabetes by affecting K(ATP) channel gating and thereby, indirectly, ATP inhibition. Heterozygous F35V channels were markedly inhibited by the sulfonylurea tolbutamide, accounting for the efficacy of sulfonylurea therapy in the patient.

KATP channels as molecular sensors of cellular metabolism

In responding to cytoplasmic nucleotide levels, ATP-sensitive potassium (K(ATP)) channel activity provides a unique link between cellular energetics and electrical excitability. Over the past ten years, a steady drumbeat of crystallographic and electrophysiological studies has led to detailed structural and kinetic models that define the molecular basis of channel activity. In parallel, the uncovering of disease-causing mutations of K(ATP) has led to an explanation of the molecular basis of disease and, in turn, to a better understanding of the structural basis of channel function.

Ligand-dependent linkage of the ATP site to inhibition gate closure in the KATP channel

Major advances have been made on the inhibition gate and ATP site of the K(ir)6.2 subunit of the K(ATP) channel, but little is known about conformational coupling between the two. ATP site mutations dramatically disrupt ATP-dependent gating without effect on ligand-independent gating, observed as interconversions between active burst and inactive interburst conformations in the absence of ATP. This suggests that linkage between site and gate is conditionally dependent on ATP occupancy. We studied all substitutions at position 334 of the ATP site in K(ir)6.2deltaC26 that express in Xenopus oocytes. All substitutions disrupted ATP-dependent gating by 10-fold or more. Only positive-charged arginine or lysine at 334, however, slowed ligand-independent gating from the burst, and this was in some but not all patches. Moreover, the polycationic peptide protamine reversed the slowed gating from the burst of 334R mutant channels, and speeded the slow gating from the burst of wild-type SUR1/K(ir)6.2 in the absence of ATP. Our results support a two-step ligand-dependent linkage mechanism for K(ir)6.2 channels in which ATP-occupied sites function to electrostatically dissociate COOH-terminal domains from the membrane, then as in all K(ir) channels, free COOH-terminal domains and inner M2 helices transit to a lower energy state for gate closure.

Diabetes and insulin secretion: the ATP-sensitive K+ channel (K ATP) connection

The ATP-sensitive K+ channel (K ATP channel) senses metabolic changes in the pancreatic beta-cell, thereby coupling metabolism to electrical activity and ultimately to insulin secretion. When K ATP channels open, beta-cells hyperpolarize and insulin secretion is suppressed. The prediction that K ATP channel "overactivity" should cause a diabetic state due to undersecretion of insulin has been dramatically borne out by recent genetic studies implicating "activating" mutations in the Kir6.2 subunit of K ATP channel as causal in human diabetes. This article summarizes the emerging picture of K ATP channel as a major cause of neonatal diabetes and of a polymorphism in K ATP channel (E23K) as a type 2 diabetes risk factor. The degree of K ATP channel "overactivity" correlates with the severity of the diabetic phenotype. At one end of the spectrum, polymorphisms that result in a modest increase in K ATP channel activity represent a risk factor for development of late-onset diabetes. At the other end, severe "activating" mutations underlie syndromic neonatal diabetes, with multiple organ involvement and complete failure of glucose-dependent insulin secretion, reflecting K ATP channel "overactivity" in both pancreatic and extrapancreatic tissues.

ATP and sulfonylurea sensitivity of mutant ATP-sensitive K+ channels in neonatal diabetes: implications for pharmacogenomic therapy

The prediction that overactivity of the pancreatic ATP-sensitive K(+) channel (K(ATP) channel) underlies reduced insulin secretion and causes a diabetic phenotype in humans has recently been borne out by genetic studies implicating "activating" mutations in the Kir6.2 subunit of K(ATP) as causal in both permanent and transient neonatal diabetes. Here we characterize the channel properties of Kir6.2 mutations that underlie transient neonatal diabetes (I182V) or more severe forms of permanent neonatal diabetes (V59M, Q52R, and I296L). In all cases, the mutations result in a significant decrease in sensitivity to inhibitory ATP, which correlates with channel "overactivity" in intact cells. Mutations can be separated into those that directly affect ATP affinity (I182V) and those that stabilize the open conformation of the channel and indirectly reduce ATP sensitivity (V59M, Q52R, and I296L). With respect to the latter group, alterations in channel gating are also reflected in a functional "uncoupling" of sulfonylurea (SU) block: SU sensitivity of I182V is similar to that of wild-type mutants, but the SU sensitivity of all gating mutants is reduced, with the I296L mutant being resistant to block by tolbutamide (</=10 mmol/l). These results have important implications for the use of insulinotropic SU drugs as an alternative therapy to insulin injections.

Kir6.2 mutations causing neonatal diabetes provide new insights into Kir6.2-SUR1 interactions

ATP-sensitive K(+) (K(ATP)) channels, comprised of pore-forming Kir6.2 and regulatory SUR1 subunits, play a critical role in regulating insulin secretion. Binding of ATP to Kir6.2 inhibits, whereas interaction of MgATP with SUR1 activates, K(ATP) channels. We tested the functional effects of two Kir6.2 mutations (Y330C, F333I) that cause permanent neonatal diabetes mellitus, by heterologous expression in Xenopus oocytes. Both mutations reduced ATP inhibition and increased whole-cell currents, which in pancreatic beta-cells is expected to reduce insulin secretion and precipitate diabetes. The Y330C mutation reduced ATP inhibition both directly, by impairing ATP binding (and/or transduction), and indirectly, by stabilizing the intrinsic open state of the channel. The F333I mutation altered ATP binding/transduction directly. Both mutations also altered Kir6.2/SUR1 interactions, enhancing the stimulatory effect of MgATP (which is mediated via SUR1). This effect was particularly dramatic for the Kir6.2-F333I mutation, and was abolished by SUR1 mutations that prevent MgATP binding/hydrolysis. Further analysis of F333I heterozygous channels indicated that at least three SUR1 must bind/hydrolyse MgATP to open the mutant K(ATP) channel.

Relapsing diabetes can result from moderately activating mutations in KCNJ11

Neonatal diabetes can either remit and hence be transient or else may be permanent. These two phenotypes were considered to be genetically distinct. Abnormalities of 6q24 are the commonest cause of transient neonatal diabetes (TNDM). Mutations in KCNJ11, which encodes Kir6.2, the pore-forming subunit of the ATP-sensitive potassium channel (K(ATP)), are the commonest cause of permanent neonatal diabetes (PNDM). In addition to diabetes, some KCNJ11 mutations also result in marked developmental delay and epilepsy. These mutations are more severe on functional characterization. We investigated whether mutations in KCNJ11 could also give rise to TNDM. We identified the three novel heterozygous mutations (G53S, G53R, I182V) in three of 11 probands with clinically defined TNDM, who did not have chromosome 6q24 abnormalities. The mutations co-segregated with diabetes within families and were not found in 100 controls. All probands had insulin-treated diabetes diagnosed in the first 4 months and went into remission by 7-14 months. Functional characterization of the TNDM associated mutations was performed by expressing the mutated Kir6.2 with SUR1 in Xenopus laevis oocytes. All three heterozygous mutations resulted in a reduction in the sensitivity to ATP when compared with wild-type (IC(50) approximately 30 versus approximately 7 microM, P-value for is all <0.01); however, this was less profoundly reduced than with the PNDM associated mutations. In conclusion, mutations in KCNJ11 are the first genetic cause for remitting as well as permanent diabetes. This suggests that a fixed ion channel abnormality can result in a fluctuating glycaemic phenotype. The multiple phenotypes associated with activating KCNJ11 mutations may reflect their severity in vitro.

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.

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.

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.

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.

Hyperinsulinism in Infancy: From Basic Science to Clinical Disease

Dunne, Mark J., Karen E. Cosgrove, Ruth M. Shepherd, Albert Aynsley-Green, and Keith J. Lindley. Hyperinsulinism in Infancy: From Basic Science to Clinical Disease. Physiol Rev 84: 239–275, 2004; 10.1152/physrev.00022.2003.—Ion channelopathies have now been described in many well-characterized cell types including neurons, myocytes, epithelial cells, and endocrine cells. However, in only a few cases has the relationship between altered ion channel function, cell biology, and clinical disease been defined. Hyperinsulinism in infancy (HI) is a rare, potentially lethal condition of the newborn and early childhood. The causes of HI are varied and numerous, but in almost all cases they share a common target protein, the ATP-sensitive K+channel. From gene defects in ion channel subunits to defects in β-cell metabolism and anaplerosis, this review describes the relationship between pathogenesis and clinical medicine. Until recently, HI was generally considered an orphan disease, but as parallel defects in ion channels, enzymes, and metabolic pathways also give rise to diabetes and impaired insulin release, the HI paradigm has wider implications for more common disorders of the endocrine pancreas and the molecular physiology of ion transport.

Verapamil, a Ca2+ entry blocker, targets the pore-forming subunit of cardiac type KATP channel (Kir6.2)

This study investigated the mechanism by which verapamil, which blocks 10R1, l-type Ca2+ channel and the HERG channel, blocks ATP-sensitive K+ (K(ATP)) channels. In whole cell patch experiments, verapamil reversibly inhibited cardiac type K(ATP) (Kir6.2/SUR2A) channels previously activated by 100-micromol/L pinacidil. In inside-out patch experiments, verapamil inhibited the C-terminal truncated form of Kir6.2 (Kir6.2DeltaC36) in a concentration-dependent manner; half-maximal inhibition (IC(50)) was obtained at 11.5 +/- 2.8 micromol/L when Kir6.2DeltaC36 was expressed without SUR2A. Verapamil also inhibited Kir6.2/SUR2A with a similar potency; IC(50) was 8.9 +/- 2.1 micromol/L for Kir6.2/SUR2A (not statistically different from the value for Kir6.2DeltaC36 alone). Thus, verapamil appeared to target the pore-forming subunit Kir6.2 rather than SUR2A, a member of ABC superfamily. Verapamil did not decrease the single-channel conductance, but increased the closed time of Kir6.2/SUR2A. The mutations of Kir6.2DeltaC36 (Kir6.2DeltaC36-R50G, -K185Q, -G334D), which have much lower ATP sensitivity, had no significant effect on verapamil block, suggesting that the site at which verapamil mediates K(ATP) channel inhibition is not identical with that involved in ATP block.

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.

Phosphatidic acid stimulates cardiac KATP channels like phosphatidylinositols, but with novel gating kinetics

Membrane-bound anionic phospholipids such as phosphatidylinositols have the capacity to modulate ATP-sensitive potassium (K(ATP)) channels through a mechanism involving long-range electrostatic interaction between the lipid headgroup and channel. However, it has not yet been determined whether the multiple effects of phosphatidylinositols reported in the literature all result from this general electrostatic interaction or require a specific headgroup structure. The present study investigated whether phosphatidic acid (PA), an anionic phospholipid substantially different in structure from phosphatidylinositols, evokes effects similar to phosphatidylinositols on native K(ATP) channels of rat heart and heterogeneous Kir6.2/SUR2A channels. Channels treated with PA (0.2-1 mg/ml applied to the cytoplasmic side of the membrane) exhibited higher activity, lower sensitivity to ATP inhibition, less Mg(2+)-dependent nucleotide stimulation, and poor sulfonylurea inhibition. These effects match the spectrum of phosphatidylinositols' effects, but, in addition, PA also induced a novel pattern in gating kinetics, represented by a decreased mean open time (from 12.2 +/- 2.0 to 3.3 +/- 0.7 ms). This impact on gating kinetics clearly distinguishes PA's effects from those of phosphatidylinositols. Results indicate that multiple effects of anionic phospholipids on K(ATP) channels are related phenomena and can likely be attributed to a common mechanism, but additional specific effects due to other mechanisms may also coincide.

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.

Diabetes and insulin secretion: whither KATP?

The critical involvement of ATP-sensitive potassium (KATP) channels in insulin secretion is confirmed both by the demonstration that mutations that reduce KATP channel activity underlie many if not most cases of persistent hyperinsulinemia, and by the ability of sulfonylureas, which inhibit KATP channels, to enhance insulin secretion in type II diabetics. By extrapolation, we contend that mutations that increase beta-cell KATP channel activity should inhibit glucose-dependent insulin secretion and underlie, or at least predispose to, a diabetic phenotype. In transgenic animal models, this prediction seems to be borne out. Although earlier genetic studies failed to demonstrate a linkage between KATP mutations and diabetes in humans, recent studies indicate significant association of KATP channel gene mutations or polymorphisms and type II diabetes. We suggest that further efforts to understand the involvement of KATP channels in diabetes are warranted.

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.

K(ATP) channels and insulin secretion disorders

ATP-sensitive potassium (K(ATP)) channels are inhibited by intracellular ATP and activated by ADP. Nutrient oxidation in beta-cells leads to a rise in [ATP]-to-[ADP] ratios, which in turn leads to reduced K(ATP) channel activity, depolarization, voltage-dependent Ca(2+) channel activation, Ca(2+) entry, and exocytosis. Persistent hyperinsulinemic hypoglycemia of infancy (HI) is a genetic disorder characterized by dysregulated insulin secretion and, although rare, causes severe mental retardation and epilepsy if left untreated. The last five or six years have seen rapid advance in understanding the molecular basis of K(ATP) channel activity and the molecular genetics of HI. In the majority of cases for which a genotype has been uncovered, causal HI mutations are found in one or the other of the two genes, SUR1 and Kir6.2, that encode the K(ATP) channel. This article will review studies that have defined the link between channel activity and defective insulin release and will consider implications for future understanding of the mechanisms of control of insulin secretion in normal and diseased states.

Open state destabilization by ATP occupancy is mechanism speeding burst exit underlying KATP channel inhibition by ATP

The ATP-sensitive potassium (K(ATP)) channel is named after its characteristic inhibition by intracellular ATP. The inhibition is a centerpiece of how the K(ATP) channel sets electrical signaling to the energy state of the cell. In the beta cell of the endocrine pancreas, for example, ATP inhibition results from high blood glucose levels and turns on electrical activity leading to insulin release. The underlying gating mechanism (ATP inhibition gating) includes ATP stabilization of closed states, but the action of ATP on the open state of the channel is disputed. The original models of ATP inhibition gating proposed that ATP directly binds the open state, whereas recent models indicate a prerequisite transition from the open to a closed state before ATP binds and inhibits activity. We tested these two classes of models by using kinetic analysis of single-channel currents from the cloned mouse pancreatic K(ATP) channel expressed in Xenopus oocytes. In particular, we combined gating models based on fundamental rate law and burst gating kinetic considerations. The results demonstrate open-state ATP dependence as the major mechanism by which ATP speeds exit from the active burst state underlying inhibition of the K(ATP) channel by ATP.

Signaling in channel/enzyme multimers: ATPase transitions in SUR module gate ATP-sensitive K+ conductance

ATP-sensitive potassium (K(ATP)) channels are bifunctional multimers assembled by an ion conductor and a sulfonylurea receptor (SUR) ATPase. Sensitive to ATP/ADP, K(ATP) channels are vital metabolic sensors. However, channel regulation by competitive ATP/ADP binding would require oscillations in intracellular nucleotides incompatible with cell survival. We found that channel behavior is determined by the ATPase-driven engagement of SUR into discrete conformations. Capture of the SUR catalytic cycle in prehydrolytic states facilitated pore closure, while recruitment of posthydrolytic intermediates translated in pore opening. In the cell, channel openers stabilized posthydrolytic states promoting K(ATP) channel activation. Nucleotide exchange between intrinsic ATPase and ATP/ADP-scavenging systems defined the lifetimes of specific SUR conformations gating K(ATP) channels. Signal transduction through the catalytic module provides a paradigm for channel/enzyme operation and integrates membrane excitability with metabolic cascades.

ATP interaction with the open state of the K(ATP) channel

The mechanism of ATP-sensitive potassium (K(ATP)) channel closure by ATP is unclear, and various kinetic models in which ATP binds to open or to closed states have previously been presented. Effects of phosphatidylinositol bisphosphate (PIP2) and multiple Kir6.2 mutations on ATP inhibition and open probability in the absence of ATP are explainable in kinetic models where ATP stabilizes a closed state and interaction with an open state is not required. Evidence that ATP can in fact interact with the open state of the channel is presented here. The mutant Kir6.2[L164C] is very sensitive to Cd2+ block, but very insensitive to ATP, with no significant inhibition in 1 mM ATP. However, 1 mM ATP fully protects the channel from Cd2+ block. Allosteric kinetic models in which the channel can be in either open or closed states with or without ATP bound are considered. Such models predict a pedestal in the ATP inhibition, i.e., a maximal amount of inhibition at saturating ATP concentrations. This pedestal is predicted to occur at >50 mM ATP in the L164C mutant, but at >1 mM in the double mutant L164C/R176A. As predicted, ATP inhibits Kir6.2[L164C/R176A] to a maximum of approximately 40%, with a clear plateau beyond 2 mM. These results indicate that ATP acts as an allosteric ligand, interacting with both open and closed states of the channel.