Regulation of the ATP-sensitive potassium channel subunit, Kir6.2, by a Ca2+-dependent protein kinase C

The activity of ATP-sensitive potassium (K(ATP)) channels is governed by the concentration of intracellular ATP and ADP and is thus responsive to the metabolic status of the cell. Phosphorylation of K(ATP) channels by protein kinase A (PKA) or protein kinase C (PKC) results in the modulation of channel activity and is particularly important in regulating smooth muscle tone. At the molecular level the smooth muscle channel is composed of a sulfonylurea subunit (SUR2B) and a pore-forming subunit Kir6.1 and/or Kir6.2. Previously, Kir6.1/SUR2B channels have been shown to be inhibited by PKC, and Kir6.2/SUR2B channels have been shown to be activated or have no response to PKC. In this study we have examined the modulation of channel complexes formed of the inward rectifier subunit, Kir6.2, and the sulfonylurea subunit, SUR2B. Using a combination of biochemical and electrophysiological techniques we show that this complex can be inhibited by protein kinase C in a Ca(2+)-dependent manner and that this inhibition is likely to be as a result of internalization. We identify a residue in the distal C terminus of Kir6.2 (Ser-372) whose phosphorylation leads to down-regulation of the channel complex. This inhibitory effect is distinct from activation which is seen with low levels of channel activity.     

Arginine vasopressin inhibits Kir6.1/SUR2B channel and constricts the mesenteric artery via V1a receptor and protein kinase C

Kir6.1/SUR2B channel is the major isoform of K(ATP) channels in the vascular smooth muscle. Genetic disruption of either subunit leads to dysregulation of vascular tone and regional blood flows. To test the hypothesis that the Kir6.1/SUR2B channel is a target molecule of arginine vasopressin (AVP), we performed studies on the cloned Kir6.1/SUR2B channel and cell-endogenous K(ATP) channel in rat mesenteric arteries. The Kir6.1/SUR2B channel was expressed together with V1a receptor in the HEK-293 cell line. Whole cell currents of the transfected HEK cells were activated by K(ATP) channel opener pinacidil and inhibited by K(ATP) channel inhibitor glibenclamide. AVP produced a concentration-dependent inhibition of the pinacidil-activated currents with IC(50) 2.0 nM. The current inhibition was mediated by a suppression of the open-state probability without effect on single-channel conductance. An exposure to 100 nM PMA, a potent PKC activator, inhibited the pinacidil-activated currents, and abolished the channel inhibition by AVP. Such an effect was not seen with inactive phorbol ester. A pretreatment of the cells with selective PKC blocker significantly diminished the inhibitory effect of AVP. In acutely dissociated vascular smooth myocytes, AVP strongly inhibited the cell-endogenous K(ATP) channel. In isolated mesenteric artery rings, AVP produced concentration-dependent vasoconstrictions with EC(50) 6.5 nM. At the maximum effect, pinacidil completely relaxed vasoconstriction in the continuing exposure to AVP. The magnitude of the AVP-induced vasoconstriction was significantly reduced by calphostin-C. These results therefore indicate that the Kir6.1/SUR2B channel is a target molecule of AVP, and the channel inhibition involves G(q)-coupled V1a receptor and PKC.     

Evidence for presence of ATP-sensitive K+ channels in rat colonic smooth muscle cells.

Coexpression of sulfonylurea receptor (SUR) and inward-rectifying K+ channel (Kir6.1 or 6.2) subunit yields ATP-sensitive K+ (K(ATP)) channels. Three subtypes of SUR have been cloned: pancreatic (SUR1), cardiac (SUR2A), and vascular smooth muscle (SUR2B). The distinct responses to K+ channel openers (KCOs) produced in different tissues may depend on the SUR isoform of K(ATP) channel. Therefore, we investigated the effects of pinacidil and diazoxide, two KCOs, on K(ATP) currents in intestinal smooth muscle cells of the rat colon (circular layer) using whole-cell voltage clamp. Pinacidil stimulated a time-independent K+ current evoked by various test potentials from a holding potential of -70 mV. The reversal potential of the stimulated current was about -75 mV, which is close to the equilibrium potential for K+ (E(K)). Both pinacidil and diazoxide dose-dependently stimulated K+ currents (evoked by ramp pulses), with EC50 values of 1.3 and 34.2 microM, respectively. The stimulated current was completely reversed by glybenclamide (3 microM). Since the EC50 values are close to those reported for vascular smooth muscle (VSM) cells, the SUR subtype may be similar to that in VSM cells, and could form the functional K(ATP) channel in rat colonic smooth muscle cells.     

Physiological and pathophysiological roles of ATP-sensitive K+ channels

ATP-sensitive potassium (K(ATP)) channels are present in many tissues, including pancreatic islet cells, heart, skeletal muscle, vascular smooth muscle, and brain, in which they couple the cell metabolic state to its membrane potential, playing a crucial role in various cellular functions. The K(ATP) channel is a hetero-octamer comprising two subunits: the pore-forming subunit Kir6.x (Kir6.1 or Kir6.2) and the regulatory subunit sulfonylurea receptor SUR (SUR1 or SUR2). Kir6.x belongs to the inward rectifier K(+) channel family; SUR belongs to the ATP-binding cassette protein superfamily. Heterologous expression of differing combinations of Kir6.1 or Kir6.2 and SUR1 or SUR2 variant (SUR2A or SUR2B) reconstitute different types of K(ATP) channels with distinct electrophysiological properties and nucleotide and pharmacological sensitivities corresponding to the various K(ATP) channels in native tissues. The physiological and pathophysiological roles of K(ATP) channels have been studied primarily using K(ATP) channel blockers and K(+) channel openers, but there is no direct evidence on the role of the K(ATP) channels in many important cellular responses. In addition to the analyses of naturally occurring mutations of the genes in humans, determination of the phenotypes of mice generated by genetic manipulation has been successful in clarifying the function of various gene products. Recently, various genetically engineered mice, including mice lacking K(ATP) channels (knockout mice) and mice expressing various mutant K(ATP) channels (transgenic mice), have been generated. In this review, we focus on the physiological and pathophysiological roles of K(ATP) channels learned from genetic manipulation of mice and naturally occurring mutations in humans.     

新型钾通道开放剂对心血管ATP-敏感性钾通道基因表达的调节作用

目的：研究脂肪胺类的新型钾通道开放剂（KCO）埃他卡林（Ipt）和氰胍类的KCO吡那地尔（Pin）对大鼠心血管ATP-敏感性钾通道（KATP）的亚基SUR1、SUR2、Kir6．1和Kir6．2等在mRNA水平的调节作用。方法：SD大鼠给药1周后处死并取组织,提取总RNA,利用反转录-聚合酶链式反应（RT-PCR）研究以上基因在mRNA水平的改变。结果：与正常对照相比,心脏组织中,Ipt和Pin对KATP的4个亚基在mRNA水平均无显著影响;主动脉平滑肌上,Ipt对4个亚基的mRNA表达无显著影响,但Pin可显著上调SUR2的mRNA表达;尾动脉平滑肌上,Ipt对Kit6．1／Kit6．2、Pin对SUR2／Kir6．1均有显著下调的作用。结论：心肌、大动脉平滑肌和小动脉平滑肌KATP基因表达的调控不同,Ipt选择性调节小动脉平滑肌Kit6．1／Kit6．2;Ipt对心血管KATP基因表达的调节作用不同于Pin。     

Expression of ATP sensitive K+ channel subunit Kir6.1 in rat kidney

ATP-sensitive K+ (K(ATP)) channels in kidney are considered to play roles in regulating membrane potential during the change in intracellular ATP concentration. They are composed of channel subunits (Kir6.1, Kir6.2), which are members of the inwardly rectifying K+ channel family, and sulphonylurea receptors (SUR1, SUR2A and SUR2B), which belong to the ATP-binding cassette superfamily. In the present study, we have investigated the expression and localization of Kir6.1 in rat kidney with Western blot analysis, immunohistochemistry, in situ hybridization histochemistry, and immunoelectron microscopy. Western blot analysis showed that Kir6.1 was expressed in the mitochondria and microsome fractions of rat kidney and very weakly in the membrane fractions. Immunohistochemistry revealed that Kir6.1 was widely distributed in renal tubular epithelial cells, glomerular mesangial cells, and smooth muscles of blood vessels. In immunoelectron microscopy, Kir6.1 is mainly localized in the mitochondria, endoplasmic reticulum (ER), and very weakly in cell membranes. Thus, Kir6.1 is contained in the kidney and may be a candidate of mitochondrial K(ATP) channels.     

Interaction of vanadate with the cloned beta cell K(ATP) channel.

Vanadate is used as a tool to trap magnesium nucleotides in the catalytic site of ATPases. However, it has also been reported to activate ATP-sensitive potassium (K(ATP)) channels in the absence of nucleotides. K(ATP) channels comprise Kir6.2 and sulfonylurea receptor subunits (SUR1 in pancreatic beta cells, SUR2A in cardiac and skeletal muscle, and SUR2B in smooth muscle). We explored the effect of vanadate (2 mM), in the absence and presence of magnesium nucleotides, on different types of cloned K(ATP) channels expressed in Xenopus oocytes. Currents were recorded from inside-out patches. Vanadate inhibited Kir6.2/SUR1 currents by approximately 50% but rapidly activated Kir6.2/SUR2A ( approximately 4-fold) and Kir6. 2/SUR2B ( approximately 2-fold) currents. Mutations in SUR that abolish channel activation by magnesium nucleotides did not prevent the effects of vanadate. Studies with chimeric SUR indicate that the first six transmembrane domains account for the difference in both the kinetics and the vanadate response of Kir6.2/SUR1 and Kir6. 2/SUR2A. Boiling the vanadate solution, which removes the decavanadate polymers, largely abolished both stimulatory and inhibitory actions of vanadate. Our results demonstrate that decavanadate modulates K(ATP) channel activity via the SUR subunit, that this modulation varies with the type of SUR, that it differs from that produced by magnesium nucleotides, and that it involves transmembrane domains 1-6 of SUR.     

Evidence against functional heteromultimerization of the KATP channel subunits Kir6.1 and Kir6.2

K(ATP) channels consist of pore-forming potassium inward rectifier (Kir6.x) subunits and sulfonylurea receptors (SURs). Although Kir6.1 or Kir6.2 coassemble with different SUR isoforms to form heteromultimeric functional K(ATP) channels, it is not known whether Kir6.1 and Kir6.2 coassemble with each other. To define the molecular identity of K(ATP) channels, we used adenoviral gene transfer to express wild-type and dominant-negative constructs of Kir6.1 and Kir6.2 in a heterologous expression system (A549 cells) and in native cells (rabbit ventricular myocytes). Dominant-negative (DN) Kir6.2 gene transfer suppressed current through heterologously expressed SUR2A + Kir6.2 channels. Conversely, DN Kir6.1 suppressed SUR2B + Kir6.1 current but had no effect on coexpressed SUR2A + Kir6. 2. We next probed the ability of Kir6.1 and Kir6.2 to affect endogenous K(ATP) channels in adult rabbit ventricular myocytes, using adenoviral vectors to achieve efficient gene transfer. Infection with the DN Kir6.2 virus for 72 h suppressed pinacidil-inducible K(ATP) current density measured by whole-cell patch clamp. However, there was no effect of infection with the DN Kir6.1 on the K(ATP) current. Based on these functional assays, we conclude that Kir6.1 and Kir6.2 do not heteromultimerize with each other and that Kir6.2 is the sole K(ATP) pore-forming subunit in the surface membrane of heart cells.     

A short motif in Kir6.1 consisting of four phosphorylation repeats underlies the vascular KATP channel inhibition by protein kinase C

Vascular ATP-sensitive K(+) channels are inhibited by multiple vasoconstricting hormones via the protein kinase C (PKC) pathway. However, the molecular substrates for PKC phosphorylation remain unknown. To identify the PKC sites, Kir6.1/SUR2B and Kir6.2/SUR2B were expressed in HEK293 cells. Following channel activation by pinacidil, the catalytic fragment of PKC inhibited the Kir6.1/SUR2B currents but not the Kir6.2/SUR2B currents. Phorbol 12-myristate 13-acetate (a PKC activator) had similar effects. Using Kir6.1-Kir6.2 chimeras, two critical protein domains for the PKC-dependent channel inhibition were identified. The proximal N terminus of Kir6.1 was necessary for channel inhibition. Because there was no PKC phosphorylation site in the N-terminal region, our results suggest its potential involvement in channel gating. The distal C terminus of Kir6.1 was crucial where there are several consensus PKC sites. Mutation of Ser-354, Ser-379, Ser-385, Ser-391, or Ser-397 to nonphosphorylatable alanine reduced PKC inhibition moderately but significantly. Combined mutations of these residues had greater effects. The channel inhibition was almost completely abolished when 5 of them were jointly mutated. In vitro phosphorylation assay showed that 4 of the serine residues were necessary for the PKC-dependent (32)P incorporation into the distal C-terminal peptides. Thus, a motif containing four phosphorylation repeats is identified in the Kir6.1 subunit underlying the PKC-dependent inhibition of the Kir6.1/SUR2B channel. The presence of the phosphorylation motif in Kir6.1, but not in its close relative Kir6.2, suggests that the vascular K(ATP) channel may have undergone evolutionary optimization, allowing it to be regulated by a variety of vasoconstricting hormones and neurotransmitters.     

KATP通道基因在大鼠组织中的表达

为检测大鼠肺动脉平滑肌及支气管平滑肌中KATP．通道亚基的表达情况,应用RT-PCR技术．从原代培养的Wistar大鼠肺动脉平滑肌及支气管平滑肌细胞第3—5代提取总RNA,逆转录．并进行PCR扩增鉴定．发现肺动脉平滑肌细胞有Kir6.1、SUR1和SUR2B的表达,其中SUR1表达量较弱,支气管平滑肌细胞有Kir6.1和SUR2B的表达。     

Mutations in the linker domain of NBD2 of SUR inhibit transduction but not nucleotide binding

ATP-sensitive potassium (K(ATP)) channels are composed of an ATP-binding cassette (ABC) protein (SUR1, SUR2A or SUR2B) and an inwardly rectifying K(+) channel (Kir6.1 or Kir6.2). Like other ABC proteins, the nucleotide binding domains (NBDs) of SUR contain a highly conserved "signature sequence" (the linker, LSGGQ) whose function is unclear. Mutation of the conserved serine to arginine in the linker of NBD1 (S1R) or NBD2 (S2R) did not alter the ability of ATP or ADP (100 microM) to displace 8-azido-[(32)P]ATP binding to SUR1, or abolish ATP hydrolysis at NBD2. We co-expressed Kir6.2 with wild-type or mutant SUR in Xenopus oocytes and recorded the resulting currents in inside-out macropatches. The S1R mutation in SUR1, SUR2A or SUR2B reduced K(ATP) current activation by 100 microM MgADP, whereas the S2R mutation in SUR1 or SUR2B (but not SUR2A) abolished MgADP activation completely. The linker mutations also reduced (S1R) or abolished (S2R) MgATP-dependent activation of Kir6.2-R50G co-expressed with SUR1 or SUR2B. These results suggest that the linker serines are not required for nucleotide binding but may be involved in transducing nucleotide binding into channel activation.     

Molecular basis and structural insight of vascular K(ATP) channel gating by S-glutathionylation

The vascular ATP-sensitive K(+) (K(ATP)) channel is targeted by a variety of vasoactive substances, playing an important role in vascular tone regulation. Our recent studies indicate that the vascular K(ATP) channel is inhibited in oxidative stress via S-glutathionylation. Here we show evidence for the molecular basis of the S-glutathionylation and its structural impact on channel gating. By comparing the oxidant responses of the Kir6.1/SUR2B channel with the Kir6.2/SUR2B channel, we found that the Kir6.1 subunit was responsible for oxidant sensitivity. Oxidant screening of Kir6.1-Kir6.2 chimeras demonstrated that the N terminus and transmembrane domains of Kir6.1 were crucial. Systematic mutational analysis revealed three cysteine residues in these domains: Cys(43), Cys(120), and Cys(176). Among them, Cys(176) was prominent, contributing to >80% of the oxidant sensitivity. The Kir6.1-C176A/SUR2B mutant channel, however, remained sensitive to both channel opener and inhibitor, which indicated that Cys(176) is not a general gating site in Kir6.1, in contrast to its counterpart (Cys(166)) in Kir6.2. A protein pull-down assay with biotinylated glutathione ethyl ester showed that mutation of Cys(176) impaired oxidant-induced incorporation of glutathione (GSH) into the Kir6.1 subunit. In contrast to Cys(176), Cys(43) had only a modest contribution to S-glutathionylation, and Cys(120) was modulated by extracellular oxidants but not intracellular GSSG. Simulation modeling of Kir6.1 S-glutathionylation suggested that after incorporation to residue 176, the GSH moiety occupied a space between the slide helix and two transmembrane helices. This prevented the inner transmembrane helix from undergoing conformational changes necessary for channel gating, retaining the channel in its closed state.     

Regulation of ATP-sensitive potassium channel function by protein kinase A-mediated phosphorylation in transfected HEK293 cells

ATP-sensitive potassium (K(ATP)) channels regulate insulin secretion, vascular tone, heart rate and neuronal excitability by responding to transmitters as well as the internal metabolic state. K(ATP) channels are composed of four pore-forming alpha-subunits (Kir6.2) and four regulatory beta-subunits, the sulfonylurea receptor (SUR1, SUR2A or SUR2B). Whereas protein kinase A (PKA) phosphorylation of serine 372 of Kir6.2 has been shown biochemically by others, we found that the phosphorylation of T224 rather than S372 of Kir6.2 underlies the catalytic subunits of PKA (c-PKA)- and the D1 dopamine receptor-mediated stimulation of K(ATP) channels expressed in HEK293 cells. Specific changes in the kinetic properties of channels treated with c-PKA, as revealed by single-channel analysis, were mimicked by aspartate substitution of T224. The T224D mutation also reduced the sensitivity to ATP inhibition. Alteration of channel gating and a decrease in the apparent affinity for ATP inhibition thus underlie the positive regulation of K(ATP) channels by PKA phosphorylation of T224 in Kir6.2, which may represent a general mechanism for K(ATP) channel regulation in different tissues.     

PKA phosphorylation of SUR2B subunit underscores vascular KATP channel activation by beta-adrenergic receptors

ATP-sensitive K(+) (K(ATP)) channels are activated by several vasodilating hormones and neurotransmitters through the PKA pathway. Here, we show that phosphorylation at Ser1387 of the SUR2B subunit is critical for the channel activation. Experiments were performed in human embryonic kidney (HEK) 293 cells expressing the cloned Kir6.1/SUR2B channel. In whole cell patch, the Kir6.1/SUR2B channel activity was stimulated by isoproterenol via activation of beta(2) receptors. This effect was blocked in the presence of inhibitors for adenylyl cyclase or PKA. Similar channel activation was seen by exposing inside-out patches to the catalytic subunit of PKA. Because none of the previously suggested PKA phosphorylation sites accounted for the channel activation, we performed systematic mutational analysis on Kir6.1 and SUR2B. Two serine residues (Ser1351, Ser1387) located in the NBD2 of SUR2B were critical for the channel activation. In vitro phosphorylation experiments showed that Ser1387 but not Ser1351 was phosphorylated by PKA. The PKA-dependent activation of cell-endogenous K(ATP) channels was observed in acutely dissociated mesenteric smooth myocytes and isolated mesenteric artery rings, where activation of these channels contributed significantly to the isoproterenol-induced vasodilation. Taken together, these results indicate that the Kir6.1/SUR2B channel is a target of beta(2) receptors and that the channel activation relies on PKA phosphorylation of SUR2B at Ser1387.     

ATP modulation of ATP-sensitive potassium channel ATP sensitivity varies with the type of SUR subunit

ATP-sensitive potassium (K(ATP)) channels comprise Kir and SUR subunits. Using recombinant K(ATP) channels expressed in Xenopus oocytes, we observed that MgATP (100 microm) block of Kir6.2/SUR2A currents gradually declined with time, whereas inhibition of Kir6.2/SUR1 or Kir6.2DeltaC36 currents did not change. The decline in Kir6.2/SUR2A ATP sensitivity was not observed in Mg(2+) free solution and was blocked by the phosphatidylinositol (PI) 3-kinase inhibitors LY 294002 (10 microm) and wortmannin (100 microm), and by neomycin (100 microm). These results suggest that a MgATP-dependent synthesis of membrane phospholipids produces a secondary decrease in the ATP sensitivity of Kir6.2/SUR2A. Direct application of the phospholipids PI 4,5-bisphosphate and PI 3,4,5-trisphosphate in the presence of 100 microm MgATP activated all three types of channel, but the response was faster for Kir6.2/SUR2A. Chimeric studies indicate that the different responses of Kir6.2/SUR2A and Kir6.2/SUR1 are mediated by the first six transmembrane domains of SUR. The MgATP-dependent loss of ATP sensitivity of Kir6.2/SUR2A was enhanced by the actin filament disrupter cytochalasin and blocked by phalloidin (which stabilizes the cytoskeleton). Phalloidin did not block the effect of PI 3,4,5-trisphosphate. This suggests that MgATP may cause disruption of the cytoskeleton, leading to enhanced membrane phospholipid levels (or better targeting to the K(ATP) channel) and thus to decreased channel ATP sensitivity.     

ATP-sensitive K+-channel subunits on the mitochondria and endoplasmic reticulum of rat cardiomyocytes.

ATP-sensitive K(+) (K(ATP)) channel subunits on the subcellular structures of rat cardiomyocytes were studied with antibodies against Kir6.1 and Kir6.2. According to the results of Western blot analysis, Kir6.1 was strongly expressed in mitochondrial and microsome fractions, and faintly expressed in cell membrane fraction, whereas Kir6.2 was mainly expressed in the microsome fraction and weakly in cell membrane and mitochondrial fractions. Immunohistochemistry showed that Kir6.1 and Kir6.2 were expressed in the endocardium, atrial and ventricular myocardium, and in vascular smooth muscles. Immunoelectron microscopy revealed that Kir6.1 immunoreactivity was mainly localized in the mitochondria, whereas Kir6.2 immunoreactivity was mainly localized in the endoplasmic reticulum and a few in the mitochondria. Both Kir6.1 and Kir6.2 are candidates of mitochondrial K(ATP) channel subunits. The data obtained in this study will be useful for analyzing the composition of K(ATP) channels of cardiomyocytes and help to understanding the cardioprotective role of K(ATP) channels during heart ischemia.     

Assembly limits the pharmacological complexity of ATP-sensitive potassium channels

ATP-sensitive potassium channels (K(ATP) channels) are formed from an octameric complex of an inwardly rectifying K(+) channel (Kir6.1, Kir6.2) and a sulfonylurea receptor (SUR1, SUR2A, and SUR2B). In this study we have attempted to address the question of whether SUR heteromultimers can form using a combination of biochemical and electrophysiological approaches. We have constructed monoclonal stable lines in HEK293 cells co-expressing Kir6.2 with SUR1 and SUR2A. Using coimmunoprecipitation analysis with SUR isotype-specific antibodies two biochemical populations are distinguished, one containing SUR1 and the other SUR2A. It is not possible to detect immune complexes containing both SUR1 and SUR2A. Functional studies were undertaken and whole cell membrane currents were studied using the patch clamp. Concentrations of sulfonylureas and potassium channel openers were determined that selectively inhibited or activated SUR1/Kir6.2 and SUR2A/Kir6.2. In the cell line expressing SUR1/SUR2AKir6.2 we were unable to demonstrate a population of channels with unique pharmacological properties. Thus we conclude from these studies that heteromultimeric channel complexes containing both SUR1 and SUR2A are not formed, suggesting an incompatibility between different SUR subtypes. This incompatibility limits the pharmacological complexity of K(ATP) channels that may be observed in native tissues.     

On potential interactions between non-selective cation channel TRPM4 and sulfonylurea receptor SUR1

The sulfonylurea receptor SUR1 associates with Kir6.2 or Kir6.1 to form K(ATP) channels, which link metabolism to excitability in multiple cell types. The strong physical coupling of SUR1 with Kir6 subunits appears exclusive, but recent studies argue that SUR1 also modulates TRPM4, a member of the transient receptor potential family of non-selective cation channels. It has been reported that, following stroke, brain, or spinal cord injury, SUR1 is increased in neurovascular cells at the site of injury. This is accompanied by up-regulation of a non-selective cation conductance with TRPM4-like properties and apparently sensitive to sulfonylureas, leading to the postulation that post-traumatic non-selective cation currents are determined by TRPM4/SUR1 channels. To investigate the mechanistic hypothesis for the coupling between TRPM4 and SUR1, we performed electrophysiological and FRET studies in COSm6 cells expressing TRPM4 channels with or without SUR1. TRPM4-mediated currents were Ca(2+)-activated, voltage-dependent, underwent desensitization, and were inhibited by ATP but were insensitive to glibenclamide and tolbutamide. These properties were not affected by cotransfection with SUR1. When the same SUR1 was cotransfected with Kir6.2, functional K(ATP) channels were formed. In cells cotransfected with Kir6.2, SUR1, and TRPM4, we measured K(ATP)-mediated K(+) currents and Ca(2+)-activated, sulfonylurea-insensitive Na(+) currents in the same patch, further showing that SUR1 controls K(ATP) channel activity but not TRPM4 channels. FRET signal between fluorophore-tagged TRPM4 subunits was similar to that between Kir6.2 and SUR1, whereas there was no detectable FRET efficiency between TRPM4 and SUR1. Our data suggest that functional or structural association of TRPM4 and SUR1 is unlikely.