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Toward Linking Structure With Function in ATP-Sensitive K+ Channels

¹ Department of Molecular and Cellular Biology, Baylor College of Medicine, Houston, Texas
² Department of Molecular Physiology and Biophysics, Baylor College of Medicine, Houston, Texas
³ Department of Medicine, Baylor College of Medicine, Houston, Texas

	ABSTRACT

TOP ABSTRACT KATP CHANNELS ARE COMPOSED... SUR1 TMD0 "ACTIVATES" KIR6.2... THE L0 LINKER BIDIRECTIONALLY... TOWARD UNDERSTANDING THE ATP... SUR1 HAS A MULTIFACETED... LOCATION OF THE POTASSIUM... SUMMARY REFERENCES

 
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 (K_ATP channels) (K_IR6.0/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 -helices found in SURs, binds to and activates K_IR6.0. The adjacent cytoplasmic L0 linker serves a dual function, acting as a tether to link the MDR-like core to the K_IR6.2/TMD0 complex and exerting bidirectional control over channel gating via interactions with the NH₂-terminus of the K_IR. 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 K_IR NH₂-terminus.

The physiologic importance of the ionic mechanism is well illustrated by the dramatic changes in glucose homeostasis that can result from genetic alterations, which affect the P_O—the probability that K_ATP channels are in the open state. This is shown conceptually in Fig. 1, which relates the P_O to ß-cell membrane potential and insulin secretion. Depolarization activates voltage-gated Ca²⁺ channels, induces oscillation of cytosolic [Ca²⁺]_i, and thus pulsatile release of insulin (5,6). In humans, the loss of SUR1/K_IR6.2 K_ATP channel activity is the most common cause of persistent hyperinsulinemic hypoglycemia when ß-cells are persistently depolarized and oversecrete insulin (rev. in 7,8). A mild dominant form of hypoglycemia due to a mutation in SUR1, Glu1507Lys, produces hyperinsulinism at an early age and then progresses to decreased insulin secretory capacity in early adulthood and diabetes in middle age (9). At the other extreme, overexpression of a mutant pore subunit, engineered to activate channels by decreasing their sensitivity to inhibitory ATP, resulted in transgenic mice with neonatal diabetes (10). These results implied that "gain-of-activity" mutations were candidates for producing neonatal diabetes, and a genetic screen of patients identified mutations at two NH₂-terminal and two COOH-terminal positions (Q52, V59, R201, and I296) in K_IR6.2 associated with permanent neonatal diabetes (11). Interestingly, R201 had earlier been reported to be critical for ATP-inhibitory gating (12), while V59 and Q52 are in, or near, a helix argued to be critical for SUR/K_IR stimulatory coupling (13 and below). In addition, the E23K polymorphism, reported to increase K_ATP channel activity at physiologic nucleotide levels (14–16) and increase the channel sensitivity to stimulation by acyl CoAs (17), appears to be a predisposing factor for type 2 diabetes (18–23; see also 24,25).

View larger version (27K): [in this window] [in a new window]   FIG. 1. A conceptual view of the relationship between the PO of KATP channels, ß-cell membrane potential, and insulin secretion. A spectrum of mutations that affect channel open probability producing altered glucose homeostasis is illustrated. E23K, Glutamate23Lysine; GFGAAA, GlycinePhenylalanineGlycine in the selectivity filter substituted with AlanineAlanineAlanine; HI, hyperinsulinism.

	KATP CHANNELS ARE COMPOSED OF A WEAK INWARDLY RECTIFYING K+ PORE AND A QUARTET OF ATP-BINDING CASSETTE REGULATORY PROTEINS

TOP ABSTRACT KATP CHANNELS ARE COMPOSED... SUR1 TMD0 "ACTIVATES" KIR6.2... THE L0 LINKER BIDIRECTIONALLY... TOWARD UNDERSTANDING THE ATP... SUR1 HAS A MULTIFACETED... LOCATION OF THE POTASSIUM... SUMMARY REFERENCES

 
Beta-Cell K_ATP channels are hetero-octamers composed of two disparate subunits (Fig. 2): K_IR6.2, a two transmembrane helix pore forming subunit, and its regulatory subunit, the ATP-binding cassette (ABC) protein SUR1 (rev. in 40). K_IR6.2 is a member of the potassium inward rectifier family, channels that in physiological solutions conduct K⁺ better in the inward than outward direction (rev. in 41). Work on the structure(s) of K_IR6.0 pores necessary to understand their weak inward rectification and ATP-inhibitory gating has been limited. Eukaryotic ion channels are difficult to express and purify in the quantities needed for structural work. Thus, insight has come mainly from X-ray crystallographic studies on the cytoplasmic pore of mouse Kir3.1 (42), on the truncated bacterial channels KcsA (43) and KirBac1.1 (44), and on the use of these templates to generate homology models for the K_ATP channel (13) or various domains (45,46). KcsA provided the archetypal tetrameric K⁺ pore assembled from subunits with three helical elements: the outer M1, pore, and inner M2 helices. The selectivity filter, at the external face, is formed primarily from the main chain carbonyls of the GYG (GlyTyrGly) sequence between the M1 and pore helixes (47), whereas the bundle of four M2 helices at the internal face is thought to form the ligand-sensitive gate. An additional amphipathic helix, termed the slide helix, precedes M1 and appears to be lipid-anchored in KcsA (48). The slide helix is positioned perpendicular to the pore axis in the KirBac1.1 structure (44). The K_IR3.1 cytoplasmic pore and KirBac1.1 pore have provided insight into the organization of the long Mg²⁺ and polyamine binding permeation path that confers inward rectification.

View larger version (61K): [in this window] [in a new window]   FIG. 2. Summary of structures and mini-KATP channel data. The top panel illustrates the topology of SUR1 and KIR6.2. The SUR NH2-terminal domains that interact with and stabilize or destabilize the KIR open state are in red and blue, respectively. Glycosylation sites (trees), "+" exit, and RKR (Arg Lys Arg) ER retention signals are important for surface expression. The positions where SUR1 was cut to generate Nt and complementary Ct fragments are shown. The right middle panel summarizes the biphasic effect of progressively longer NH2-terminal segments on ligand-independent channel activity. Macrocurrents and single-channel recordings are shown. The red segments, single-channel recordings on an expanded time scale, illustrate differences in the open-state stability. See ref. 13 for details of the analysis. The left middle panel illustrates the interactions critical for assembly and gating of (SUR/KIR6.0)4 channels. TMD1-NBD1 and TMD2-NBD2 of one SUR1 (gray parts on the left) were modeled using the VC-MsbA structure (52) (see structural alignments in ref. 13). The KIR6.2 pore was modeled using the KirBac1.1 structure (44) (adjacent subunits are colored using differential tones). Loops that had to be built de novo are not shown; these include the linker between the inner helix and the COOH-terminal domain, which is longer in eukaryotic KIRs. Ile182, Lys185, and Gly334 are colocalized with Arg50 from the adjacent subunit (shown in yellow and light yellow residues). Phe133 of the GlyPheGly-based selectivity filter is in green. The TMS important for KIR/SUR recognition points outward from the M1 helix (red). The horizontal slide helix (red or light red), preceding M1, is accessible for interaction with the submembrane L0 domain (red cylinder). The first 44 residues of the KIR (the blue NH2 terminus, missing in the template, is shown for one KIR subunit) are not required for KATP assembly (65) and are at the periphery of the cytoplasmic pore accessible to parts of L0 (in blue). Residue 23 in KIR6.2 (*) and 116, an HI-SUR1 mutation in TMD0 () are indicated. The lower left panel provides an enlarged view, from the top, of one of the predicted inhibitory ATP-binding pockets shown in the figure above. ATP, in an extended conformation, is shown for illustrative purposes only. The exact coordination of ATP is unknown. The lower right panel shows a truncated view of the SUR1 core at the level of the ICDs. The NBDs have been cut off. The cyan residue in the lower left is S1237 and is pointed to by the cyan arrow. The russet colored residues are T1286 and M1290 on TM17, and are pointed to by the russet arrow. For illustrative purposes, the potassium channel opener diazoxide is shown to the left; glibenclamide in extended and compact low-energy configurations is at the right.

	SUR1 TMD0 "ACTIVATES" KIR6.2 AND ANCHORS IT TO THE ABCC8 CORE

TOP ABSTRACT KATP CHANNELS ARE COMPOSED... SUR1 TMD0 "ACTIVATES" KIR6.2... THE L0 LINKER BIDIRECTIONALLY... TOWARD UNDERSTANDING THE ATP... SUR1 HAS A MULTIFACETED... LOCATION OF THE POTASSIUM... SUMMARY REFERENCES

 
While the basic octameric stoichiometry of K_ATP channels is well established (59–61), a basic structural problem has been understanding what parts of the SUR and K_IR subunits interact to activate a fully functional channel. In the absence of inhibitory ligands, K_ATP channels exhibit spontaneous activity characterized by bursts of openings and brief closings. Bursts are separated by interburst intervals, so that channels conduct 65–70% of the time (P_Omax up to 0.7). Expressed in the absence of SUR, homomeric (K_IR6.2C)₄ pores, lacking their ER retention signals, display profoundly destabilized burst states—an 10-fold reduction in their P_Omax and a >30-fold reduction in their sensitivity to ATP. Therefore, we asked which domain(s) of SUR1 could convert the spiky activity of the pore to normal bursting inhibited with an IC_50(ATP) <10 µmol/l. Both the structure of the conserved K⁺ pore (43,62) and work using chimeras of K_IR6.2 and K_IR2.1, a subunit that does not associate with SUR1, implied the outer M1 helix was needed for coassembly (28). While there have been conflicting models of SUR/K_IR6.0 coupling (28,63,64), two recent studies have shown that TMD0 is the principal SUR coassembly domain (13,58). SUR1 TMD0 assembled stable complexes, or "mini-K_ATP channels," with the (K_IR6.2C)₄ pores resulting in restoration of full K_ATP-like bursting activity with about a fivefold increase in the P_Omax in ligand-free solutions (13,58) (Fig. 2).

The sensitivity to inhibitory ATP was not increased by TMD0 or TMD0-L0, implying that interaction with another part of SUR will be required to account for the increased affinity observed in the full channel. As expected, in the absence of the drug-binding SUR core, mini-K_ATP channels are not stimulated by ADP and their activity is not modulated by sulfonylureas or K_ATP openers. The approach demonstrated that TMD0 is a major SUR domain that interacts with K_IR6.2. The interaction activates the pore and restores bursting, presumably via contacts of transmembrane helices. One expects that other parts of the receptor might interact with cytoplasmic domains of K_IR6.2, but we have been unable to identify stable complexes between K_IR6.2 and the SUR1 core or to demonstrate the core has a functional effect on the K_IR. The results imply that these additional interactions, presumably regulatory in nature, are transient and that L0 serves as a de facto "tether" linking the ABC core to the TMD0/K_IR complex. Additional work is necessary to define how the core is "docked" to the K_IR6.2/TMD0-L0 complex.

	THE L0 LINKER BIDIRECTIONALLY CONTROLS PO

TOP ABSTRACT KATP CHANNELS ARE COMPOSED... SUR1 TMD0 "ACTIVATES" KIR6.2... THE L0 LINKER BIDIRECTIONALLY... TOWARD UNDERSTANDING THE ATP... SUR1 HAS A MULTIFACETED... LOCATION OF THE POTASSIUM... SUMMARY REFERENCES

 
Analysis of channels assembled from K_IR6.2 and chimeric receptors SUR1SUR2A demonstrated that TMD0-L0 was a determinant of the isoform differences in slow gating (38). The importance of L0 in the control of the slow component of gating was strongly reinforced by the observation that including parts of L0 in mini-K_ATP channels dramatically altered their interburst kinetics. Inclusion of the initial third of L0 dramatically reduced the rate of the burst termination, resulting in "hyperactivation" seen in Nt232/K_IR6.2C35 channels (Fig. 2). At a mechanistic level, this submembrane part of L0, predicted to contain a helix, appears to interact with a submembrane part of the pore to lock mini-K_ATP channels in the burst state. Inclusion of additional sequence attenuated this activation, implying L0 has a bidirectional effect and that the two actions, saturating versus attenuating the P_Omax, require structurally distinct segments of the linker. Existing data provide insight into which parts of K_IR6.2 are involved; deletion of the distal K_IR6.2 NH₂-terminus similarly locks NK_IR6.2/SUR1 channels in the burst state (65–67). The results imply that an interaction of the peripheral K_IR NH₂-terminus with a cytoplasmic segment of L0 attenuates the P_Omax. This hypothesis is supported by the observation that a synthetic, hydrophilic peptide, Ntp, based on the first 33 K_IR6.2 NH₂-terminal residues, can compete with the endogenous NH₂-terminus for a putative inhibitory linker in SUR1 and increase the P_Omax by reducing transitions from the burst state (64). The proposed interactions between TMD0-L0 and the pore are color-coded in Fig. 2: inhibitory contacts (in blue) between the cytoplasmic domains of the two subunits counterbalance the stimulatory interactions (in red) between the submembrane L0 helix and the K_IR "slide" helix (44), which saturate the P_Omax.

	TOWARD UNDERSTANDING THE ATP-INHIBITORY MACHINERY OF KATP CHANNELS

TOP ABSTRACT KATP CHANNELS ARE COMPOSED... SUR1 TMD0 "ACTIVATES" KIR6.2... THE L0 LINKER BIDIRECTIONALLY... TOWARD UNDERSTANDING THE ATP... SUR1 HAS A MULTIFACETED... LOCATION OF THE POTASSIUM... SUMMARY REFERENCES

 
Site-directed mutagenesis reduced the candidate residues involved in ATP binding, or its coupling to the ATP inhibition gate, to Lys185 and Gly334 in the COOH-domain and Arg50 in the NH₂-terminus of K_IR6.2. Substitutions at these positions decreased the IC_50(ATP) without affecting the P_Omax of homomeric K_IR pores and K_ATP channels in ligand-free solutions (65,68,69). Substitutions at other residues that affect ATP-binding, e.g., I182 (69,70), can alter the P_Omax. The first double mutant, K_IR6.2 Arg50Gln/Lys185Gln (65), demonstrated the additive contribution of NH₂- and COOH-terminal residues to ATP-induced stabilization of the closed state, consistent with the possibility that these two residues cooperate in the coordination of separate negatively charged phosphate groups of ATP in SUR1-containing channels. The GIRK1-based homology models of the K_IR6.0 C-domain (13,71) support the idea that Ile182, Lys185, and Gly334 are colocalized and that Lys185 and Gly334 probably contribute to the surface of the adenine- and ß-phosphate-selective ATP-binding pocket that we have argued is conserved in both K_IR6.0 subunits (39,72). Our modeling of the complete pore indicated an additional key structural aspect of the ATP-inhibitory machinery. Consistent with the data on the R50Q/K185Q double-mutant channels (65), we proposed that the COOH-terminal binding pocket on one K_IR6.0 subunit is complemented by Arg50 from the neighboring K_IR6.0 subunit, producing four intersubunit ATP-binding pockets (inset, Fig. 2). The key feature of this structural model is that Arg50 immediately precedes (by three residues) the slide helix that we hypothesize contacts the submembrane helix of L0 and moves with it during interburst transitions. Repositioning of these elements thus has a dual effect: Movement of the activating (red) domains away from the pore axis opens the gate and will also move Arg50 away from the nucleotide-binding cavities, effectively opening the intersubunit pockets and thus increasing the rate of dissociation (K_d) for inhibitory ATP. This mechanism can explain the marked increase in the IC_50(ATP) by Nt232, which effectively mimics the action of full-length SUR saturated with stimulatory ligands. Conversely, movement of the inhibitory region (blue) of L0 is hypothesized to push the K_IR NH₂-terminus, thus repositioning Arg50 and increasing the affinity for nucleotides, while stabilizing a straight, closed KcsA- and KirBac1.1-like conformation of the M2 helical bundle, thereby reducing the mean P_O. This attenuation of the P_Omax by the inhibitory segments of L0 (see Nt288 in Fig. 2) mimics the effect of full-length SUR1 saturated with a hypoglycemic compound when tested in nucleotide-free solutions (73).

	SUR1 HAS A MULTIFACETED BINDING POCKET

TOP ABSTRACT KATP CHANNELS ARE COMPOSED... SUR1 TMD0 "ACTIVATES" KIR6.2... THE L0 LINKER BIDIRECTIONALLY... TOWARD UNDERSTANDING THE ATP... SUR1 HAS A MULTIFACETED... LOCATION OF THE POTASSIUM... SUMMARY REFERENCES

 
The main action of oral hypoglycemic agents, both sulfonylurea and nonsulfonylurea compounds, is to stimulate insulin secretion by inhibiting K_ATP channels via their binding to SUR1. Early studies on structure-activity relationships indicated, over 30 years ago, that an effective pharmacophore consisted of three lipophilic or hydrophobic centers separated by a -CONH- group and a negative charge, respectively (Fig. 3). Recognition was posited to occur through interaction with a multifaceted bipartite pocket with distinct "A" and "B" binding sites within a "sulfonylurea receptor" (75–77,80; rev. in 81). More recent work has made it clear that the sulfonylurea group is not essential for hypoglycemic activity, but rather provides a properly positioned anionic group (78,79). The importance of this work is being rediscovered, and structure-function studies on SUR1 and SUR2 have begun to provide structural information about the nature of the "A" and "B" sites (rev. in 81). Chimeric receptors SUR1SUR2A were used to exploit the pharmacological differences between ß-cell and cardiomyocyte channels and identify the importance of the TMD2 domain for high-affinity recognition of tolbutamide (73,82). A detailed consideration of the available structure-activity data suggests that TMD2 forms part of the "A" site and that SUR1 is able to accommodate the lipophilic center adjacent to the anionic group on the sulfonylurea moiety (i.e., the butyl side chain in tolbutamide, cyclohexane substituted with a propyl group in nateglinide, or the cyclohexyl ring in glibenclamide) while SUR2A is not. The implication is that SUR1 has a hydrophobic cavity able to accommodate these side chains and that this cavity is missing or occluded in SUR2. Substitution of Ser1237 with Tyr, the analogous residue in SUR2, reduced the apparent affinity of SUR1 for tolbutamide and glibenclamide (82). One possibility is that Ser1237 is in close proximity to the lipophilic center and that Tyr1206 occludes binding in SUR2A; the reverse substitution, Tyr1206Ser in SUR2B, does increase the affinity for glibenclamide several fold but not to the level of SUR1 (83). The interpretational difficulty lies in not being able to distinguish experimentally between a direct interaction of Ser1237 with the drug versus an indirect effect of the substitution at a more distant drug binding site. In the homology model of the SUR1 core, Ser1237 is at the end of one of the -helices, which form the ICD (see Fig. 2). The ICDs are proposed to play a critical role in coupling the binding of stimulatory nucleotides to the NBD dimer with conformational changes in the substrate binding cavity of MsbA (52) or the maltose transporter (84). It is tempting to speculate that the A site lies at the level of the ICDs and that tolbutamide, or other insulin secretagogues, can "insert" between the halves of the core and restrict movement. This tentative structural picture provides a framework with which to interpret the abolishment of the "basal," Mg-nucleotide-dependent stimulation of SUR1 by oral hypoglycemic agents (73,85,86), which is their principal mechanism of action in vivo.

View larger version (36K): [in this window] [in a new window]   FIG. 3. Pharmacophores and affinity labels. A: The simple pharmacophore model in the upper panel is modified from Grell et al. (74) and is based on early structure-activity relation studies (75–79). The dashed line symbolizes the surrounding protein pocket. The structures of glibenclamide and tolbutamide are shown; the position of the azido group in azidoglibenclamide is given in parentheses. B: Alanine substitution of residues in L0 was used to identify residues important for affinity labeling. myc-tagged receptors were labeled with 125I-azidoglibenclamide, immunoprecipitated with anti-myc antibodies, and visualized by autoradiography. Results for the wild-type (WT) control, the Tyr230Ala (Y230A), and Trp231Ala (W232A) substitutions are shown. The results are summarized on a helical net for illustrative purposes. C: A myc-tagged NH2-terminal fragment of KIR6.2 containing M1 immunoprecipitates SUR1 and is affinity labeled. Progressive deletion of its NH2-terminus diminishes affinity labeling of the KIR fragment without significantly affecting labeling or coimmunoprecipitation of SUR1. D: The equivalent deletions in full-length NKIR6.2/SUR1 channels compromise coupling between binding of the inhibitory sulfonylurea tolbutamide (Tlb) and attenuation of the PO in the nucleotide-free solutions (73). The lower dashed line gives the NPO value for the full-length SUR1/KIR6.2 channel; the upper dashed line gives the mean NPO (channel number times open probability) for tolbutamide-insensitive cardiac SUR2A/KIR6.2 channels. The dotted lines are the standard deviations.

Deletion of the NH₂-terminus of K_IR6.2 reduced its affinity labeling with ¹²⁵I-azidoglibenclamide (64). Data in Fig. 3 show that a fragment of K_IR6.2 containing M1 but lacking M2 and the COOH-terminal cytoplasmic domain makes a stable complex with SUR1 and is affinity labeled. Progressive deletion of residues from the NH₂-terminus of this M1 fragment abolished affinity labeling without affecting its association with SUR1. Short NH₂-terminal deletions of K_IR6.2 also impaired the coupling of tolbutamide binding to SUR1 and consequent attenuation of the P_O in the absence of nucleotides (65). The results support the idea that the outer helix of K_IR6.0 subunits specifically interact with SUR (28), as well as the conservative interpretation that the principal affinity-labeling site is either in the first half of the K_IR6.2 NH₂-terminus or that the peripheral NH₂-terminus is essential for positioning more proximal residues near the nitrene (64).

	LOCATION OF THE POTASSIUM CHANNEL OPENER SITE

TOP ABSTRACT KATP CHANNELS ARE COMPOSED... SUR1 TMD0 "ACTIVATES" KIR6.2... THE L0 LINKER BIDIRECTIONALLY... TOWARD UNDERSTANDING THE ATP... SUR1 HAS A MULTIFACETED... LOCATION OF THE POTASSIUM... SUMMARY REFERENCES

 
Chimeric receptors were also used to localize regions important for recognition of several channel openers, including diazoxide, cromakalim, and pinacidil. The action of diazoxide required TMD1 and part of the first nucleotide binding domain, NBD1 (91), a localization supported by a study on the novel opener NNC 55-9216 (92). A region of TMD2 was shown to be critical for the action of cromakalim and pinacidil (91,93,94), and Moreau et al. (94) identified two residues in the last transmembrane helix of SUR—Leu1249 and Thr1253 in SUR2 and Thr1286 and Met1290 in SUR1. Swapping these two residues conferred sensitivity to cromakalim and pinacidil on SUR1 and abolished sensitivity in SUR2A. In the homology model of the SUR1 core (13,81), Met1290 is near the top of TMD2 facing the putative drug-binding cavity (see Fig. 2). The results are consistent with the conclusion that both halves of the SUR core are required for specific interaction with K_ATP channel openers (91). The positioning of Met1290 in central cavity, as well as earlier data indicating that the action of the potassium channel openers depends on the presence of hydrolysable MgATP (95–97; however, see 98), suggests a parallel with transport mechanisms in which site accessibility is dependent on nucleotide hydrolysis and implies that binding of an opener in the central cavity can trap a specific conformation of the core and thus potentiate channel activity.

	SUMMARY

TOP ABSTRACT KATP CHANNELS ARE COMPOSED... SUR1 TMD0 "ACTIVATES" KIR6.2... THE L0 LINKER BIDIRECTIONALLY... TOWARD UNDERSTANDING THE ATP... SUR1 HAS A MULTIFACETED... LOCATION OF THE POTASSIUM... SUMMARY REFERENCES

 
Current research is providing an increasingly detailed picture of ß-cell K_ATP channel structure, including homology models of the ion-conducting pore and the nucleotide- and drug-binding SUR1 core. A role for TMD0-L0 in the coupling of SUR1 to K_IR6.2 has been demonstrated, and the importance of these interactions for control of gating has been emphasized. This structural picture provides an increasingly sophisticated framework with which to interpret mutations and polymorphisms that affect glucose homeostasis by altering the normal coupling mechanism to produce both inactive and hyperactive channels. The spectrum of diseases linked with K_ATP channels, from neonatal to adult diabetes through congenital hyperinsulinism, is broad and of considerable social, medical, and economic importance. Increased understanding of the structure and coupling mechanism(s) in K_ATP channels will lead to improved pharmaceuticals directed at these disorders.

	ACKNOWLEDGMENTS

 
This work was funded in part by grants from the National Institutes of Health-National Institute of Diabetes and Digestive and Kidney Diseases (J.B. and L.A.-B.) and the American Heart Association (A.P.B.).

	FOOTNOTES

 
This article is based on a presentation at a symposium. The symposium and the publication of this article were made possible by an unrestricted educational grant from Servier.

Address correspondence and reprint requests to Joseph Bryan, PhD, Department of Molecular and Cellular Biology, Baylor College of Medicine, Houston, TX 77030. E-mail: jbryan{at}bcm.tmc.edu

Received for publication March 13, 2004 and accepted in revised form May 12, 2004

Abbreviations: ABC, ATP-binding cassette; [Ca²⁺]_i, intracellular Ca²⁺ concentration; ER, endoplasmic reticulum; ICD, intracellular coupling domain; K_ATP channel, ATP-sensitive K⁺ channel; K_IR channel, inwardly rectifying K⁺ channel; MDR, multidrug resistance protein; NBD, nucleotide-binding fold; P_O, open probability; SUR, sulfonylurea receptor; TMD, transmembrane domain

	REFERENCES

TOP ABSTRACT KATP CHANNELS ARE COMPOSED... SUR1 TMD0 "ACTIVATES" KIR6.2... THE L0 LINKER BIDIRECTIONALLY... TOWARD UNDERSTANDING THE ATP... SUR1 HAS A MULTIFACETED... LOCATION OF THE POTASSIUM... SUMMARY REFERENCES

 

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