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Frances M Ashcroft - One of the best experts on this subject based on the ideXlab platform.
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Sulfonylureas suppress the stimulatory action of Mg-nucleotides on Kir6.2/SUR1 but not Kir6.2/SUR2A KATP channels: a mechanistic study.
The Journal of general physiology, 2014Co-Authors: Peter Proks, Heidi De Wet, Frances M AshcroftAbstract:Sulfonylureas, which stimulate insulin secretion from pancreatic β-cells, are widely used to treat both type 2 diabetes and neonatal diabetes. These drugs mediate their effects by binding to the sulfonylurea receptor subunit (SUR) of the ATP-sensitive K+ (KATP) channel and inducing channel closure. The mechanism of channel inhibition is unusually complex. First, sulfonylureas act as partial antagonists of channel activity, and second, their effect is modulated by MgADP. We analyzed the molecular basis of the interactions between the sulfonylurea gliclazide and Mg-nucleotides on β-cell and cardiac types of KATP channel (Kir6.2/SUR1 and Kir6.2/SUR2A, respectively) heterologously expressed in Xenopus laevis oocytes. The SUR2A-Y1206S mutation was used to confer gliclazide sensitivity on SUR2A. We found that both MgATP and MgADP increased gliclazide inhibition of Kir6.2/SUR1 channels and reduced inhibition of Kir6.2/SUR2A-Y1206S. The latter effect can be attributed to stabilization of the cardiac channel open state by Mg-nucleotides. Using a Kir6.2 mutation that renders the KATP channel insensitive to nucleotide inhibition (Kir6.2-G334D), we showed that gliclazide abolishes the stimulatory effects of MgADP and MgATP on β-cell KATP channels. Detailed analysis suggests that the drug both reduces nucleotide binding to SUR1 and impairs the efficacy with which nucleotide binding is translated into pore opening. Mutation of one (or both) of the Walker A lysines in the catalytic site of the nucleotide-binding domains of SUR1 may have a similar effect to gliclazide on MgADP binding and transduction, but it does not appear to impair MgATP binding. Our results have implications for the therapeutic use of sulfonylureas.
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an in frame deletion in Kir6.2 kcnj11 causing neonatal diabetes reveals a site of interaction between Kir6.2 and sur1
The Journal of Clinical Endocrinology and Metabolism, 2009Co-Authors: Timothy J. Craig, Sarah E Flanagan, Sian Ellard, Kenju Shimomura, Reinhard W. Holl, Frances M AshcroftAbstract:Context: Activating mutations in genes encoding the Kir6.2 (KCNJ11) and SUR1 (ABCC8) subunits of the pancreatic ATP-sensitive K+ channel are a common cause of permanent neonatal diabetes (PNDM). All Kir6.2 mutations identified to date are missense mutations. We describe here a novel in-frame deletion (residues 28-32) in Kir6.2 in a heterozygous patient with PNDM without neurological problems that are detectable by standard evaluation. Objective: The aim of the study was to identify the mutation responsible for neonatal diabetes in this patient and characterize its functional effects. Design: Wild-type and mutant Kir6.2/SUR1 channels were examined by heterologous expression in Xenopus oocytes. Results: The Kir6.2-28Δ32 mutation produced a significant decrease in ATP inhibition and an increase in whole-cell KATP currents, explaining the diabetes of the patient. Tolbutamide block was only slightly reduced in the simulated heterozygous state, suggesting that the patient should respond to sulfonylurea therapy....
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An in-frame deletion in Kir6.2 (KCNJ11) causing neonatal diabetes reveals a site of interaction between Kir6.2 and SUR1.
The Journal of clinical endocrinology and metabolism, 2009Co-Authors: Timothy J. Craig, Sarah E Flanagan, Sian Ellard, Kenju Shimomura, Reinhard W. Holl, Frances M AshcroftAbstract:Activating mutations in genes encoding the Kir6.2 (KCNJ11) and SUR1 (ABCC8) subunits of the pancreatic ATP-sensitive K(+) channel are a common cause of permanent neonatal diabetes (PNDM). All Kir6.2 mutations identified to date are missense mutations. We describe here a novel in-frame deletion (residues 28-32) in Kir6.2 in a heterozygous patient with PNDM without neurological problems that are detectable by standard evaluation. The aim of the study was to identify the mutation responsible for neonatal diabetes in this patient and characterize its functional effects. Wild-type and mutant Kir6.2/SUR1 channels were examined by heterologous expression in Xenopus oocytes. The Kir6.2-28Delta32 mutation produced a significant decrease in ATP inhibition and an increase in whole-cell K(ATP) currents, explaining the diabetes of the patient. Tolbutamide block was only slightly reduced in the simulated heterozygous state, suggesting that the patient should respond to sulfonylurea therapy. The mutation decreased ATP inhibition indirectly, by increasing the intrinsic (unliganded) channel open probability. Neither effect was observed when Kir6.2 was expressed in the absence of SUR1, suggesting that the mutation impairs coupling between SUR1 and Kir6.2. Coimmunoprecipitation studies further revealed that the mutation disrupted a physical interaction between Kir6.2 and residues 1-288 (but not residues 1-196) of SUR1. We report a novel KCNJ11 mutation causing PNDM. Our results show that residues 28-32 in the N terminus of Kir6.2 interact both physically and functionally with SUR1 and suggest that residues 196-288 of SUR1 are important in this interaction.
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Functional analysis of two Kir6.2 (KCNJ11) mutations, K170T and E322K, causing neonatal diabetes.
Diabetes Obesity and Metabolism, 2007Co-Authors: Andrei I. Tarasov, Sarah E Flanagan, Sian Ellard, Paolo Tammaro, Christophe Girard, Brian Larkin, Frances M AshcroftAbstract:Heterozygous activating mutations in Kir6.2 (KCNJ11), the pore-forming subunit of the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel, are a common cause of neonatal diabetes (ND). We assessed the functional effects of two Kir6.2 mutations associated with ND: K170T and E322K. K(ATP) channels were expressed in Xenopus oocytes, and the heterozygous state was simulated by coexpression of wild-type and mutant Kir6.2 with SUR1 (the beta cell type of sulphonylurea receptor (SUR)). Both mutations reduced the sensitivity of the K(ATP) channel to inhibition by MgATP and enhanced whole-cell K(ATP) currents. In pancreatic beta cells, such an increase in the K(ATP) current is expected to reduce insulin secretion and thereby cause diabetes. The E322K mutation was without effect when Kir6.2 was expressed in the absence of SUR1, suggesting that this residue impairs coupling to SUR1. This is consistent with its predicted location on the outer surface of the tetrameric Kir6.2 pore. The kinetics of K170T channel opening and closing were altered by the mutation, which may contribute to the lower ATP sensitivity. Neither mutation affected the sensitivity of the channel to inhibition by the sulphonylurea tolbutamide, suggesting that patients carrying these mutations may respond to these drugs.
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A mutation in the ATP-binding site of the Kir6.2 subunit of the KATP channel alters coupling with the SUR2A subunit.
The Journal of physiology, 2007Co-Authors: Paolo Tammaro, Frances M AshcroftAbstract: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).
Susumu Seino - One of the best experts on this subject based on the ideXlab platform.
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Age-dependent changes in the exocytotic efficacy in Kir6.2 ablated mouse pancreatic β-cells
2012Co-Authors: Ernest Beaudelaire Tsiaze, Susumu Seino, Shibing Yang, Ya-chi Huang, Lidija Križančić Bombek, Marko Jevšek, Marjan Slak RupnikAbstract:In this study, we aimed to examine the electrophysio- logical properties of β-cells in Kir6.2-/- mice using fresh pancreatic tissue slice preparation. This prepa-ration is advantageous since it preserves socio-cellular context of the β-cells. Using this novel approach we revisited basic morphology and used whole-cell patch-clamp to study electrical excitability as well as to assess the modulation of the late steps of the exocy-totic activity of β-cells by cytosolic [Ca2+] changes in control and Kir6.2-/- mice. We found that young Kir6.2-/- mice (2 - 4 weeks old) were hypoglycaemic while aged Kir6.2-/- mice (5 - 60 weeks old) were normo- or even hyper- glycaemic. Membrane ca-pacitance measurements show- ed more efficient Ca2+-secretion coupling in young Kir6.2-/- mice, but this coupling is significantly reduced in older Kir6.2-/- mice. We have found increased exo- cytotic efficacy induced by repetitive trains of depo- larization pulses which may result from higher cyto- solic [Ca2+] due to hyperexcitability in Kir6.2-/- mice. This condition in turn resulted in the reduced β-cell number and func-tion in the following weeks. Detailed assessment of the efficacy of Ca2+ dependent exocyto- sis in β-cell from Kir6.2-/- mice may contribute to our understanding of the pathophysiology of persistent hyperinsulinemia hypoglycemia of infancy (PHHI) and suggest potential alternative therapeutic approaches for PHHI patients.
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protection conferred by myocardial atp sensitive k channels in pressure overload induced congestive heart failure revealed in kcnj11 Kir6.2 null mutant
The Journal of Physiology, 2006Co-Authors: Satsuki Yamada, Susumu Seino, Takashi Miki, Garvan C Kane, Atta Behfar, Xiao Ke Liu, Roy B Dyer, Randolph S Faustino, Andre TerzicAbstract:Ventricular load can precipitate development of the heart failure syndrome, yet the molecular components that control the cardiac adaptive response to imposed demand remain partly understood. Compromised ATP-sensitive K+ (KATP) channel function renders the heart vulnerable to stress, implicating this metabolic sensor in the homeostatic response that would normally prevent progression of cardiac disease. Here, pressure overload was imposed on the left ventricle by transverse aortic constriction in the wild-type and in mice lacking sarcolemmal KATP channels through Kir6.2 pore knockout (Kir6.2-KO). Despite equivalent haemodynamic loads, within 30 min of aortic constriction, Kir6.2-KO showed an aberrant prolongation of action potentials with intracellular calcium overload and ATP depletion, whereas wild-type maintained ionic and energetic handling. On catheterization, constricted Kir6.2-KO displayed compromised myocardial performance with elevated left ventricular end-diastolic pressure, not seen in the wild-type. Glyburide, a KATP channel inhibitor, reproduced the knockout phenotype in the wild-type, whereas the calcium channel antagonist, verapamil, prevented abnormal outcome in Kir6.2-KO. Within 48 h following aortic constriction, fulminant biventricular congestive heart failure, characterized by exercise intolerance, cardiac contractile dysfunction, hepatopulmonary congestion and ascites, halved the Kir6.2-KO cohort, while no signs of organ failure or mortality were seen in wild-type. Surviving Kir6.2-KO developed premature and exaggerated fibrotic myocardial hypertrophy associated with nuclear up-regulation of calcium-dependent pro-remodelling MEF2 and NF-AT pathways, precipitating chamber dilatation within 3 weeks. Thus, KATP channels appear mandatory in acute and chronic cardiac adaptation to imposed haemodynamic load, protecting against congestive heart failure and death.
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Cerebral glucose transporters expression and spatial learning in the K-ATP Kir6.2 −/− knockout mice
Behavioural brain research, 2006Co-Authors: C. Choeiri, Susumu Seino, Takashi Miki, William A. Staines, J. M. Renaud, K. Teutenberg, Claude MessierAbstract:Abstract K-ATP channels formed of the Sur and Kir subunits are widely distributed in the brain. Sur1–Kir6.2 is the most common combination of K-ATP channel subunits in the brain and Kir6.2 plays an important role in glucose metabolism through pancreatic insulin secretion or hypothalamic glucose sensing. K-ATP channels have also been reported to play a role in memory processing. Therefore, the aim of the present experiment is to assess the gene and protein expression of GLUT1, GLUT3 and GLUT4 in various brain regions of Kir6.2 −/− K-ATP knockout mice and to test their working memory performance. GLUT4 was measured using two antibodies, one recognizing an intracellular epitope and the other, an extracellular epitope. Relative to their corresponding wild type, semi-quantitative immunohistochemistry showed that GLUT4 protein expression as measured by a GLUT4 antibody recognizing an extracellular epitope was increased in the Kir6.2 −/− K-ATP mice. However, there was only a small increase in GLUT4 labeling using the GLUT4 antibody recognizing the intracellular epitope. These results suggest a compensatory higher GLUT4 inclusion at the cellular neuronal membrane in the cerebral cortex, hippocampus and cerebellum of the Kir6.2 −/− K-ATP knockout mice. However, there was no change in GLUT4 gene expression assessed by TaqMan PCR except for a decrease in the cerebellum of these mice. Working memory performance of the Kir6.2 −/− K-ATP mice was disrupted at age of 12 weeks but not at 5 weeks. The mild glucose intolerance that is observed in the Kir6.2 knockout mice is unlikely to have created the memory deficits observed. Rather, in light of the effects of K-ATP channel modulators on memory, the memory deficits in the Kir6.2 −/− K-ATP mice are more likely due to the absence of the Kir6.2 and possible disruption of the GLUT4 activity in the brain.
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Knockout of Kir6.2 negates ischemic preconditioning-induced protection of myocardial energetics.
American journal of physiology. Heart and circulatory physiology, 2003Co-Authors: Richard J. Gumina, Susumu Seino, Takashi Miki, Darko Pucar, Peter Bast, Denice M. Hodgson, Christopher E. Kurtz, Petras P. Dzeja, Andre TerzicAbstract:Although ischemic preconditioning induces bioenergetic tolerance and thereby remodels energy metabolism that is crucial for postischemic recovery of the heart, the molecular components associated with preservation of cellular energy production, transfer, and utilization are not fully understood. Here myocardial bioenergetic dynamics were assessed by (18)O-assisted (31)P-NMR spectroscopy in control or preconditioned hearts from wild-type (WT) or Kir6.2-knockout (Kir6.2-KO) mice that lack metabolism-sensing sarcolemmal ATP-sensitive K(+) (K(ATP)) channels. In WT vs. Kir6.2-KO hearts, preconditioning induced a significantly higher total ATP turnover (232 +/- 20 vs. 155 +/- 15 nmol x mg protein(-1) x min(-1)), ATP synthesis rate (58 +/- 3 vs. 46 +/- 3% (18)O labeling of gamma-ATP), and ATP consumption rate (51 +/- 4 vs. 31 +/- 4% (18)O labeling of P(i)) after ischemia-reperfusion. Moreover, preconditioning preserved cardiac creatine kinase-catalyzed phosphotransfer in WT (234 +/- 26 nmol x mg protein(-1) x min(-1)) but not Kir6.2-KO (133 +/- 18 nmol x mg protein(-1) x min(-1)) hearts. In contrast with WT hearts, preconditioning failed to preserve contractile recovery in Kir6.2-KO hearts, as tight coupling between postischemic performance and high-energy phosphoryl transfer was compromised in the K(ATP)-channel-deficient myocardium. Thus intact K(ATP) channels are integral in ischemic preconditioning-induced protection of cellular energetic dynamics and associated cardiac performance.
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Functional Roles of Cardiac and Vascular ATP-Sensitive Potassium Channels Clarified by Kir6.2-Knockout Mice
Circulation research, 2001Co-Authors: Masashi Suzuki, Susumu Seino, Takashi Miki, Hiroko Uemura, Naoya Sakamoto, Yuki Ohmoto-sekine, Masaji Tamagawa, Takehiko Ogura, Eduardo MarbánAbstract:-ATP-sensitive potassium (K(ATP)) channels were discovered in ventricular cells, but their roles in the heart remain mysterious. K(ATP) channels have also been found in numerous other tissues, including vascular smooth muscle. Two pore-forming subunits, Kir6.1 and Kir6.2, contribute to the diversity of K(ATP) channels. To determine which subunits are operative in the cardiovascular system and their functional roles, we characterized the effects of pharmacological K(+) channel openers (KCOs, ie, pinacidil, P-1075, and diazoxide) in Kir6.2-deficient mice. Sarcolemmal K(ATP) channels could be recorded electrophysiologically in ventricular cells from Kir6.2(+/+) (wild-type [WT]) but not from Kir6.2(-/-) (knockout [KO]) mice. In WT ventricular cells, pinacidil induced an outward current and action potential shortening, effects that were blocked by glibenclamide, a K(ATP) channel blocker. KO ventricular cells exhibited no response to KCOs, but gene transfer of Kir6.2 into neonatal ventricular cells rescued the electrophysiological response to P-1075. In terms of contractile function, pinacidil decreased force generation in WT but not KO hearts. Pinacidil and diazoxide produced concentration-dependent relaxation in both WT and KO aortas precontracted with norepinephrine. In addition, pinacidil induced a glibenclamide-sensitive current of similar magnitude in WT and KO aortic smooth muscle cells and comparable levels of hypotension in anesthetized WT and KO mice. In both WT and KO aortas, only Kir6.1 mRNA was expressed. These findings indicate that the Kir6.2 subunit mediates the depression of cardiac excitability and contractility induced by KCOs; in contrast, Kir6.2 plays no discernible role in the arterial tree.
Peter Proks - One of the best experts on this subject based on the ideXlab platform.
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Sulfonylureas suppress the stimulatory action of Mg-nucleotides on Kir6.2/SUR1 but not Kir6.2/SUR2A KATP channels: a mechanistic study.
The Journal of general physiology, 2014Co-Authors: Peter Proks, Heidi De Wet, Frances M AshcroftAbstract:Sulfonylureas, which stimulate insulin secretion from pancreatic β-cells, are widely used to treat both type 2 diabetes and neonatal diabetes. These drugs mediate their effects by binding to the sulfonylurea receptor subunit (SUR) of the ATP-sensitive K+ (KATP) channel and inducing channel closure. The mechanism of channel inhibition is unusually complex. First, sulfonylureas act as partial antagonists of channel activity, and second, their effect is modulated by MgADP. We analyzed the molecular basis of the interactions between the sulfonylurea gliclazide and Mg-nucleotides on β-cell and cardiac types of KATP channel (Kir6.2/SUR1 and Kir6.2/SUR2A, respectively) heterologously expressed in Xenopus laevis oocytes. The SUR2A-Y1206S mutation was used to confer gliclazide sensitivity on SUR2A. We found that both MgATP and MgADP increased gliclazide inhibition of Kir6.2/SUR1 channels and reduced inhibition of Kir6.2/SUR2A-Y1206S. The latter effect can be attributed to stabilization of the cardiac channel open state by Mg-nucleotides. Using a Kir6.2 mutation that renders the KATP channel insensitive to nucleotide inhibition (Kir6.2-G334D), we showed that gliclazide abolishes the stimulatory effects of MgADP and MgATP on β-cell KATP channels. Detailed analysis suggests that the drug both reduces nucleotide binding to SUR1 and impairs the efficacy with which nucleotide binding is translated into pore opening. Mutation of one (or both) of the Walker A lysines in the catalytic site of the nucleotide-binding domains of SUR1 may have a similar effect to gliclazide on MgADP binding and transduction, but it does not appear to impair MgATP binding. Our results have implications for the therapeutic use of sulfonylureas.
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Functional analysis of six Kir6.2 (KCNJ11) mutations causing neonatal diabetes.
Pflugers Archiv : European journal of physiology, 2006Co-Authors: Christophe Girard, Peter Proks, Kenju Shimomura, Nathan L. Absalom, Luis Castaño, Guiomar Perez De Nanclares, Frances M AshcroftAbstract:ATP-sensitive potassium (KATP) channels, composed of pore-forming Kir6.2 and regulatory sulphonylurea receptor (SUR) subunits, play an essential role in insulin secretion from pancreatic beta cells. Binding of ATP to Kir6.2 inhibits, whereas interaction of Mg-nucleotides with SUR, activates the channel. Heterozygous activating mutations in Kir6.2 (KCNJ11) are a common cause of neonatal diabetes (ND). We assessed the functional effects of six novel Kir6.2 mutations associated with ND: H46Y, N48D, E227K, E229K, E292G, and V252A. KATP channels were expressed in Xenopus oocytes and the heterozygous state was simulated by coexpression of wild-type and mutant Kir6.2 with SUR1 (the beta cell type of SUR). All mutations reduced the sensitivity of the KATP channel to inhibition by MgATP, and enhanced whole-cell KATP currents. Two mutations (E227K, E229K) also enhanced the intrinsic open probability of the channel, thereby indirectly reducing the channel ATP sensitivity. The other four mutations lie close to the predicted ATP-binding site and thus may affect ATP binding. In pancreatic beta cells, an increase in the KATP current is expected to reduce insulin secretion and thereby cause diabetes. None of the mutations substantially affected the sensitivity of the channel to inhibition by the sulphonylurea tolbutamide, suggesting patients carrying these mutations may respond to these drugs.
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effects of mitiglinide s 21403 on Kir6.2 sur1 Kir6.2 sur2a and Kir6.2 sur2b types of atp sensitive potassium channel
British Journal of Pharmacology, 2001Co-Authors: Frank Reimann, Peter Proks, Frances M AshcroftAbstract:We have investigated the mechanism of action of the novel anti-diabetic agent mitiglinide (S 21403) on Kir6.2/SUR1, Kir6.2/SUR2A and Kir6.2/SUR2B types of ATP-sensitive potassium (KATP) channel. These possess a common pore-forming subunit, Kir6.2, and different regulatory sulphonylurea receptor (SUR) subunits. It is believed that they correspond to native KATP channels in pancreatic β-cells, heart and non-vascular smooth muscle, respectively. Kir6.2 was coexpressed with SUR1, SUR2A or SUR2B in Xenopus oocytes and macroscopic currents were recorded in giant inside-out membrane patches. Mitiglinide was added to the intracellular membrane surface. Mitiglinide inhibited Kir6.2/SUR currents at two sites: a low-affinity site on Kir6.2 and a high-affinity site on SUR. Low-affinity inhibition was similar for all three types of KATP channel but high-affinity inhibition was greater for Kir6.2/SUR1 currents (IC50, 4 nM) than for Kir6.2/SUR2A or Kir6.2/SUR2B currents (IC50, 3 and 5 μM, respectively). Inhibition of Kir6.2/SUR1 currents was only slowly reversible on the time scale of electrophysiological experiments. Kir6.2/SUR1-S1237Y currents, which previously have been shown to lack high affinity tolbutamide inhibition, resembled Kir6.2/SUR2 currents in being unaffected by 100 nM but blocked by 10 μM mitiglinide. Our results show that mitiglinide is a high-affinity drug that shows a 1000 fold greater affinity for the β-cell type than the cardiac and smooth muscle types of KATP channel, when measured in excised patches. British Journal of Pharmacology (2001) 132, 1542–1548; doi:10.1038/sj.bjp.0703962
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Effects of mitiglinide (S 21403) on Kir6.2/SUR1, Kir6.2/SUR2A and Kir6.2/SUR2B types of ATP-sensitive potassium channel.
British journal of pharmacology, 2001Co-Authors: Frank Reimann, Peter Proks, Frances M AshcroftAbstract:1. We have investigated the mechanism of action of the novel anti-diabetic agent mitiglinide (S 21403) on Kir6.2/SUR1, Kir6.2/SUR2A and Kir6.2/SUR2B types of ATP-sensitive potassium (K(ATP)) channel. These possess a common pore-forming subunit, Kir6.2, and different regulatory sulphonylurea receptor (SUR) subunits. It is believed that they correspond to native K(ATP) channels in pancreatic beta-cells, heart and non-vascular smooth muscle, respectively. 2. Kir6.2 was coexpressed with SUR1, SUR2A or SUR2B in Xenopus oocytes and macroscopic currents were recorded in giant inside-out membrane patches. Mitiglinide was added to the intracellular membrane surface. 3. Mitiglinide inhibited Kir6.2/SUR currents at two sites: a low-affinity site on Kir6.2 and a high-affinity site on SUR. Low-affinity inhibition was similar for all three types of K(ATP) channel but high-affinity inhibition was greater for Kir6.2/SUR1 currents (IC(50), 4 nM) than for Kir6.2/SUR2A or Kir6.2/SUR2B currents (IC(50), 3 and 5 microM, respectively). 4. Inhibition of Kir6.2/SUR1 currents was only slowly reversible on the time scale of electrophysiological experiments. 5. Kir6.2/SUR1-S1237Y currents, which previously have been shown to lack high affinity tolbutamide inhibition, resembled Kir6.2/SUR2 currents in being unaffected by 100 nM but blocked by 10 microM mitiglinide. 6. Our results show that mitiglinide is a high-affinity drug that shows a 1000 fold greater affinity for the beta-cell type than the cardiac and smooth muscle types of K(ATP) channel, when measured in excised patches.
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Direct photoaffinity labeling of Kir6.2 by [gamma-(32)P]ATP-[gamma]4-azidoanilide.
Biochemical and biophysical research communications, 2000Co-Authors: Kouichi Tanabe, Stephen J Tucker, Frances M Ashcroft, Peter Proks, Teruo Amachi, Noriyuki Kioka, Kazumitsu UedaAbstract:ATP-sensitive potassium (K(ATP)) channels are under complex regulation by intracellular ATP and ADP. The potentiatory effect of MgADP is conferred by the sulfonylurea receptor subunit of the channel, SUR, whereas the inhibitory effect of ATP appears to be mediated via the pore-forming subunit, Kir6.2. We have previously reported that Kir6.2 can be directly labeled by 8-azido-[gamma-(32)P]ATP. However, the binding affinity of 8-azido-ATP to Kir6.2 was low probably due to modification at 8' position of adenine. Here we demonstrate that Kir6.2 can be directly photoaffinity labeled with higher affinity by [gamma-(32)P]ATP-[gamma]4-azidoanilide ([gamma-(32)P]ATP-AA), containing an unmodified adenine ring. Photoaffinity labeling of Kir6.2 by [gamma-(32)P]ATP-AA is not affected by the presence of Mg(2+), consistent with Mg(2+)-independent ATP inhibition of K(ATP) channels. Interestingly, SUR1, which can be strongly and specifically photoaffinity labeled by 8-azido-ATP, was not photoaffinity labeled by ATP-AA. These results identify key differences in the structure of the nucleotide binding sites on SUR1 and Kir6.2.
Andre Terzic - One of the best experts on this subject based on the ideXlab platform.
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protection conferred by myocardial atp sensitive k channels in pressure overload induced congestive heart failure revealed in kcnj11 Kir6.2 null mutant
The Journal of Physiology, 2006Co-Authors: Satsuki Yamada, Susumu Seino, Takashi Miki, Garvan C Kane, Atta Behfar, Xiao Ke Liu, Roy B Dyer, Randolph S Faustino, Andre TerzicAbstract:Ventricular load can precipitate development of the heart failure syndrome, yet the molecular components that control the cardiac adaptive response to imposed demand remain partly understood. Compromised ATP-sensitive K+ (KATP) channel function renders the heart vulnerable to stress, implicating this metabolic sensor in the homeostatic response that would normally prevent progression of cardiac disease. Here, pressure overload was imposed on the left ventricle by transverse aortic constriction in the wild-type and in mice lacking sarcolemmal KATP channels through Kir6.2 pore knockout (Kir6.2-KO). Despite equivalent haemodynamic loads, within 30 min of aortic constriction, Kir6.2-KO showed an aberrant prolongation of action potentials with intracellular calcium overload and ATP depletion, whereas wild-type maintained ionic and energetic handling. On catheterization, constricted Kir6.2-KO displayed compromised myocardial performance with elevated left ventricular end-diastolic pressure, not seen in the wild-type. Glyburide, a KATP channel inhibitor, reproduced the knockout phenotype in the wild-type, whereas the calcium channel antagonist, verapamil, prevented abnormal outcome in Kir6.2-KO. Within 48 h following aortic constriction, fulminant biventricular congestive heart failure, characterized by exercise intolerance, cardiac contractile dysfunction, hepatopulmonary congestion and ascites, halved the Kir6.2-KO cohort, while no signs of organ failure or mortality were seen in wild-type. Surviving Kir6.2-KO developed premature and exaggerated fibrotic myocardial hypertrophy associated with nuclear up-regulation of calcium-dependent pro-remodelling MEF2 and NF-AT pathways, precipitating chamber dilatation within 3 weeks. Thus, KATP channels appear mandatory in acute and chronic cardiac adaptation to imposed haemodynamic load, protecting against congestive heart failure and death.
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Knockout of Kir6.2 negates ischemic preconditioning-induced protection of myocardial energetics.
American journal of physiology. Heart and circulatory physiology, 2003Co-Authors: Richard J. Gumina, Susumu Seino, Takashi Miki, Darko Pucar, Peter Bast, Denice M. Hodgson, Christopher E. Kurtz, Petras P. Dzeja, Andre TerzicAbstract:Although ischemic preconditioning induces bioenergetic tolerance and thereby remodels energy metabolism that is crucial for postischemic recovery of the heart, the molecular components associated with preservation of cellular energy production, transfer, and utilization are not fully understood. Here myocardial bioenergetic dynamics were assessed by (18)O-assisted (31)P-NMR spectroscopy in control or preconditioned hearts from wild-type (WT) or Kir6.2-knockout (Kir6.2-KO) mice that lack metabolism-sensing sarcolemmal ATP-sensitive K(+) (K(ATP)) channels. In WT vs. Kir6.2-KO hearts, preconditioning induced a significantly higher total ATP turnover (232 +/- 20 vs. 155 +/- 15 nmol x mg protein(-1) x min(-1)), ATP synthesis rate (58 +/- 3 vs. 46 +/- 3% (18)O labeling of gamma-ATP), and ATP consumption rate (51 +/- 4 vs. 31 +/- 4% (18)O labeling of P(i)) after ischemia-reperfusion. Moreover, preconditioning preserved cardiac creatine kinase-catalyzed phosphotransfer in WT (234 +/- 26 nmol x mg protein(-1) x min(-1)) but not Kir6.2-KO (133 +/- 18 nmol x mg protein(-1) x min(-1)) hearts. In contrast with WT hearts, preconditioning failed to preserve contractile recovery in Kir6.2-KO hearts, as tight coupling between postischemic performance and high-energy phosphoryl transfer was compromised in the K(ATP)-channel-deficient myocardium. Thus intact K(ATP) channels are integral in ischemic preconditioning-induced protection of cellular energetic dynamics and associated cardiac performance.
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Physical association between recombinant cardiac ATP-sensitive K+ channel subunits Kir6.2 and SUR2A.
Journal of molecular and cellular cardiology, 1999Co-Authors: Eva Lorenz, Andre TerzicAbstract:Abstract The inwardly-rectifying K + channel Kir6.2 serves as a common pore-forming core in various ATP-sensitive K + (K ATP ) channels, and it is through assembly with sulfonylurea-receptor (SUR) isoforms, which are ATP-binding cassette (ABC) proteins, that tissue-specific channel phenotypes can be generated. In this regard, Kir6.2 has been shown to physically associate with SUR1 to form the pancreatic K ATP channel. While cardiac K ATP channel activity can be reconstituted by coexpression of Kir6.2 with a distinct SUR isoform, SUR2A, no direct proof has been provided for physical association between these two proteins. Therefore, we tested, by a coimmunoprecipitation procedure in conjunction with an amino-terminal Kir6.2-antibody, physical association between recombinant Kir6.2 and SUR2A. From a mixture of Kir6.2 and SUR2A in vitro -translated proteins, the Kir6.2-specific antibody coimmunoprecipitated 38-kDa and 140-kDa proteins corresponding to Kir6.2 and SUR2A, respectively. In the absence of Kir6.2, SUR2A was not precipitated by the anti-Kir6.2 antibody, indicating that the antibody recognized SUR2A only when SUR2A formed a complex with Kir6.2. A Kir6.2 deletion mutant lacking 37 amino acids from the carboxy-terminus still coimmunoprecipitated with SUR2A, indicating that the distal carboxy-terminus of Kir6.2 is unnecessary for subunit association. Kir6.2 mutants lacking more proximal carboxy-terminus regions, including the M2 transmembrane domain, failed to immunoprecipitate SUR2A, suggesting that the proximal carboxy-terminus together with the M2 domain are required for channel assembly. These deletion constructs supported cellular distribution of Kir6.2. Thus, the present study provides direct evidence for physical association between Kir6.2 and SUR2A, essentially reconstituting the cardiac K ATP channel in vitro . The demonstration of complex formation between Kir6.2 and SUR2A indicates that the structural basis for channel function may rely on direct physical interaction of the two subunits.
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evidence for direct physical association between a k channel Kir6.2 and an atp binding cassette protein sur1 which affects cellular distribution and kinetic behavior of an atp sensitive k channel
Molecular and Cellular Biology, 1998Co-Authors: Eva Lorenz, Alexey E. Alekseev, Grigory Krapivinsky, Antonio J Carrasco, David E Clapham, Andre TerzicAbstract:Potassium channels are the most diverse group of ion channels, with molecular cloning revealing a number of structurally distinct families, including the subfamily of inwardly rectifying K+ (Kir) channels (11, 27, 35). Channel diversity is increased by the ability of constitutive subunits to form not only homomeric but also heteromultimeric complexes with distinct functional and regulatory properties (8, 9, 15, 21, 27, 30, 39, 53). Present in most excitable tissues, ATP-sensitive K+ (KATP) channels belong to the Kir family and are involved in signaling networks that transduce cellular metabolic events into membrane potential changes (1, 9, 40). These channels are regulated by intracellular nucleotides and have been implicated in hormone secretion, cardioprotection, and neurotransmitter release, with their function best understood in the pancreatic β cell, where KATP channels are essential in glucose-mediated membrane depolarization and insulin secretion (7, 9, 14, 31, 34, 42, 44, 52). Structurally unique among K+ channels, KATP channel activity can be reconstituted by coexpressing two unrelated proteins: the Kir channel Kir6.2 and the ATP-binding cassette (ABC) protein SUR, specifically the SUR1 isoform for the pancreatic channel phenotype (2, 22, 38). Expression of Kir6.2 alone does not result in functional ion channels, suggesting an intimate and required interaction between Kir6.2 with SUR1 (1, 7, 40, 41). Actually, expression of Kir6.2-SUR1 fusion constructs indicates that a subunit stoichiometry of 1:1 is necessary for assembly of active KATP channels (10, 24). Furthermore, Kir6.2 and SUR1 genes are clustered on chromosome 11 (p15.1), separated by a short intergenic sequence of 4.3 kb, suggesting that these genes could be cotranscribed and cotranslated to form a functional heteromultimeric channel (1, 9, 22, 40). To date, evidence for physical association between Kir6.2 and SUR1 is based on photoaffinity labeling of both channel subunits by radioactive sulfonylurea (10). Labeling of Kir6.2 was dependent on coexpression of SUR1, suggesting close association between the two subunits (10). However, photoaffinity labeling is based primarily on proximity rather than physical interaction between proteins (18). Recent evidence indicates that K+ channels are tetramers of single subunits comprising the K+-selective pore (27). The measurement of KATP channel activity in cells expressing mutant carboxy-truncated Kir6.2 has been interpreted to mean that the presence of the carboxy terminus in Kir6.2 prevents functional expression of the channel in the absence of SUR (51). However, it is not known whether the distal carboxy terminus of Kir6.2 merely serves as a suppressor of channel activity or is also important in regulating physical interaction between Kir6.2 and SUR1. To determine whether Kir6.2 and SUR1 can physically associate with each other, and to investigate the role of the carboxy terminus of Kir6.2 in complex formation, we used a Kir6.2-specific antibody to coimmunoprecipitate and to immunostain channel subunits. We truncated the carboxy terminus of Kir6.2 polypeptide to yield functional channels in the absence of SUR1 (49, 51) and then used such mutants to measure single-channel properties when expressed alone or with SUR1. We demonstrate that Kir6.2 and SUR1 physically associate in functional complexes and that the carboxy terminus of Kir6.2 is not required for subunit association. Furthermore, we provide evidence that the intraburst behavior of KATP channels is defined by Kir6.2 alone, whereas burst channel behavior is modulated by association with SUR1.
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Burst Kinetics of Co-expressed Kir6.2/SUR1 Clones: Comparison of Recombinant with Native ATP-sensitive K+ Channel Behavior
The Journal of membrane biology, 1997Co-Authors: Alexey E. Alekseev, M. E. Kennedy, B. Navarro, Andre TerzicAbstract:Co-expression of clones encoding Kir6.2, a K+ inward rectifier, and SUR1, a sulfonylurea receptor, reconstitutes elementary features of ATP-sensitive K+ (KATP) channels. However, the precise kinetic properties of Kir6.2/SUR1 clones remain unknown. Herein, intraburst kinetics of Kir6.2/SUR1 channel activity, heterologously co-expressed in COS cells, displayed mean closed times from 0.7 ± 0.1 to 0.4 ± 0.03 msec, and from 0.4 ± 0.1 to 2.0 ± 0.2 msec, and mean open times from 1.9 ± 0.4 to 4.5 ± 0.8 msec, and from 12.1 ± 2.4 to 5.0 ± 0.2 msec between −100 and −20 mV, and +20 to +80 mV, respectively. Burst duration for Kir6.2/SUR1 activity was 17.9 ± 1.8 msec with 5.6 ± 1.5 closings per burst. Burst kinetics of the Kir6.2/SUR1 activity could be fitted by a four-state kinetic model defining transitions between one open and three closed states with forward and backward rate constants of 1905 ± 77 and 322 ± 27 sec−1 for intraburst, 61.8 ± 6.6 and 23.9 ± 5.8 sec−1 for interburst, 12.4 ± 6.0 and 13.6 ± 2.9 sec−1 for intercluster events, respectively. Intraburst kinetic properties of Kir6.2/SUR1 clones were essentially indistinguishable from pancreatic or cardiac KATP channel phenotypes, indicating that intraburst kinetics per se were insufficient to classify recombinant Kir6.2/SUR1 amongst native KATP channels. Yet, burst kinetic behavior of Kir6.2/SUR1 although similar to pancreatic, was different from that of cardiac KATP channels. Thus, expression of Kir6.2/SUR1 proteins away from the pancreatic micro-environment, confers the burst kinetic identity of pancreatic, but not cardiac KATP channels. This study reports the kinetic properties of Kir6.2/SUR1 clones which could serve in the further characterization of novel KATP channel clones.
Colin G Nichols - One of the best experts on this subject based on the ideXlab platform.
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conserved functional consequences of disease associated mutations in the slide helix of kir6 1 and Kir6.2 subunits of the atp sensitive potassium channel
Journal of Biological Chemistry, 2017Co-Authors: Paige E Cooper, Conor Mcclenaghan, Xingyu Chen, Anna Staryweinzinger, Colin G NicholsAbstract:Cantu syndrome (CS) is a condition characterized by a range of anatomical defects, including cardiomegaly, hyperflexibility of the joints, hypertrichosis, and craniofacial dysmorphology. CS is associated with multiple missense mutations in the genes encoding the regulatory sulfonylurea receptor 2 (SUR2) subunits of the ATP-sensitive K+ (KATP) channel as well as two mutations (V65M and C176S) in the Kir6.1 (KCNJ8) subunit. Previous analysis of leucine and alanine substitutions at the Val-65-equivalent site (Val-64) in Kir6.2 indicated no major effects on channel function. In this study, we characterized the effects of both valine-to-methionine and valine-to-leucine substitutions at this position in both Kir6.1 and Kir6.2 using ion flux and patch clamp techniques. We report that methionine substitution, but not leucine substitution, results in increased open state stability and hence significantly reduced ATP sensitivity and a marked increase of channel activity in the intact cell irrespective of the identity of the coassembled SUR subunit. Sulfonylurea inhibitors, such as glibenclamide, are potential therapies for CS. However, as a consequence of the increased open state stability, both Kir6.1(V65M) and Kir6.2(V64M) mutations essentially abolish high-affinity sensitivity to the KATP blocker glibenclamide in both intact cells and excised patches. This raises the possibility that, at least for some CS mutations, sulfonylurea therapy may not prove to be successful and highlights the need for detailed pharmacogenomic analyses of CS mutations.
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structural and functional determinants of conserved lipid interaction domains of inward rectifying Kir6.2 channels
The Journal of General Physiology, 2002Co-Authors: Catherine A. Cukras, Iana Jeliazkova, Colin G NicholsAbstract:All members of the inward rectifiier K+ (Kir) channel family are activated by phosphoinositides and other amphiphilic lipids. To further elucidate the mechanistic basis, we examined the membrane association of Kir6.2 fragments of KATP channels, and the effects of site-directed mutations of these fragments and full-length Kir6.2 on membrane association and KATP channel activity, respectively. GFP-tagged Kir6.2 COOH terminus and GFP-tagged pleckstrin homology domain from phospholipase C δ1 both associate with isolated membranes, and association of each is specifically reduced by muscarinic m1 receptor–mediated phospholipid depletion. Kir COOH termini are predicted to contain multiple β-strands and a conserved α-helix (residues ∼306–311 in Kir6.2). Systematic mutagenesis of D307-F315 reveals a critical role of E308, I309, W311 and F315, consistent with residues lying on one side of a α-helix. Together with systematic mutation of conserved charges, the results define critical determinants of a conserved domain that underlies phospholipid interaction in Kir channels.
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Structural and Functional Determinants of Conserved Lipid Interaction Domains of Inward Rectifying Kir6.2 Channels
The Journal of general physiology, 2002Co-Authors: Catherine A. Cukras, Iana Jeliazkova, Colin G NicholsAbstract:All members of the inward rectifiier K(+) (Kir) channel family are activated by phosphoinositides and other amphiphilic lipids. To further elucidate the mechanistic basis, we examined the membrane association of Kir6.2 fragments of K(ATP) channels, and the effects of site-directed mutations of these fragments and full-length Kir6.2 on membrane association and K(ATP) channel activity, respectively. GFP-tagged Kir6.2 COOH terminus and GFP-tagged pleckstrin homology domain from phospholipase C delta1 both associate with isolated membranes, and association of each is specifically reduced by muscarinic m1 receptor-mediated phospholipid depletion. Kir COOH termini are predicted to contain multiple beta-strands and a conserved alpha-helix (residues approximately 306-311 in Kir6.2). Systematic mutagenesis of D307-F315 reveals a critical role of E308, I309, W311 and F315, consistent with residues lying on one side of a alpha-helix. Together with systematic mutation of conserved charges, the results define critical determinants of a conserved domain that underlies phospholipid interaction in Kir channels.
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sulfonylurea and k channel opener sensitivity of katp channels functional coupling of Kir6.2 and sur1 subunits
The Journal of General Physiology, 1999Co-Authors: J C Koster, Qun Sha, Colin G NicholsAbstract:The sensitivity of KATP channels to high-affinity block by sulfonylureas and to stimulation by K+ channel openers and MgADP (PCOs) is conferred by the regulatory sulfonylurea receptor (SUR) subunit, whereas ATP inhibits the channel through interaction with the inward rectifier (Kir6.2) subunit. Phosphatidylinositol 4,5-bisphosphate (PIP2) profoundly antagonized ATP inhibition of KATP channels expressed from cloned Kir6.2+SUR1 subunits, but also abolished high affinity tolbutamide sensitivity. By stabilizing the open state of the channel, PIP2 drives the channel away from closed state(s) that are preferentially affected by high affinity tolbutamide binding, thereby producing an apparent loss of high affinity tolbutamide inhibition. Mutant KATP channels (Kir6.2[ΔN30] or Kir6.2[L164A], coexpressed with SUR1) also displayed an “uncoupled” phenotype with no high affinity tolbutamide block and with intrinsically higher open state stability. Conversely, Kir6.2[R176A]+SUR1 channels, which have an intrinsically lower open state stability, displayed a greater high affinity fraction of tolbutamide block. In addition to antagonizing high-affinity block by tolbutamide, PIP2 also altered the stimulatory action of the PCOs, diazoxide and MgADP. With time after PIP2 application, PCO stimulation first increased, and then subsequently decreased, probably reflecting a common pathway for activation of the channel by stimulatory PCOs and PIP2. The net effect of increasing open state stability, either by PIP2 or mutagenesis, is an apparent “uncoupling” of the Kir6.2 subunit from the regulatory input of SUR1, an action that can be partially reversed by screening negative charges on the membrane with poly-l-lysine.
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Control of Rectification and Gating of Cloned KATP Channels by the Kir6.2 Subunit
The Journal of general physiology, 1997Co-Authors: Show-ling Shyng, T. Ferrigni, Colin G NicholsAbstract:KATP channels are a functional complex of sulphonylurea receptor (SUR1, SUR2) and inward rectifier K+ (Kir6.1, Kir6.2) channel subunits. We have studied the role of the putative pore forming subunit (Kir6.2) in regulation of rectification and gating of KATP channels generated by transfection of SUR1 and Kir6.2 cDNAs in COSm6 cells. In the absence of internal polyvalent cations, the current-voltage relationship is sigmoidal. Mg2+ or spermine4+ (spm) each induces a mild inward rectification. Mutation of the asparagine at position 160 in Kir6.2 to aspartate (N160D) or glutamate (N160E) increases the degree of rectification induced by Mg2+ or spermine4+, whereas wild-type rectification is still observed after mutation to other neutral residues (alanine–N160A, glutamine–N160Q). These results are consistent with this residue lining the pore of the channel and contributing to the binding of these cations, as demonstrated for the equivalent site in homomeric ROMK1 (Kir1.1) channels. Since Kir6.2 contains no consensus ATP binding site, whereas SUR1 does, inhibition by ATP has been assumed to depend on interactions with SUR1. However, we found that the [ATP] causing half-maximal inhibition of current (Ki) was affected by mutation of N160. Channels formed from N160D or N160Q mutant subunits had lower apparent sensitivity to ATP (Ki,N160D = 46.1 μM; Ki,N160Q = 62.9 μM) than wild-type, N160E, or N160A channels (Ki = 10.4, 17.7, 6.4 μM, respectively). This might suggest that ATP binding to the channel complex was altered, although examination of channel open probabilities indicates instead that the residue at position 160 alters the ATP-independent open probability, i.e., it controls the free energy of the open state, thereby affecting the “coupling” of ATP binding to channel inhibition. The results can be interpreted in terms of a kinetic scheme whereby the residue at Kir6.2 position 160 controls the rate constants governing transitions to and from the open state, without directly affecting ATP binding or unbinding transitions.
