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Identification of a Functionally Important Negatively Charged Identification of a Functionally Important Negatively Charged Residue Within the Second Catalytic Site of the SUR1 Nucleotide-Binding Domains Jeff D. Campbell,1,2 Peter Proks,1 Jonathan D. Lippiat,1 Mark S.P. Sansom,2 and Frances M. Ashcroft1 ؉ Two very different protein subunits make up the K The ATP-sensitive K channel (KATP channel) couples ATP glucose metabolism to insulin secretion in pancreatic channel (3). The pore is composed of four inwardly recti- ϩ ␤-cells. It is comprised of sulfonylurea receptor (SUR)-1 fying K channel (Kir channel) subunits (Kir6.2), and each and Kir6.2 proteins. Binding of Mg nucleotides to the of these is associated with a sulfonylurea receptor (SUR) nucleotide-binding domains (NBDs) of SUR1 stimulates subunit (SUR1), which modulates the opening and closing channel opening and leads to membrane hyperpolariza- (gating) of the channel. Metabolically generated changes tion and inhibition of insulin secretion. To elucidate the in intracellular adenine nucleotides influence channel gat- structural basis of this regulation, we constructed a molecular model of the NBDs of SUR1, based on the ing via interaction with both types of subunit. Thus, ATP crystal structures of mammalian proteins that belong to (and ADP) closes the channel by binding to Kir6.2, where- the same family of ATP-binding cassette transporter as channel opening is mediated by interaction of Mg nu- proteins. This model is a dimer in which there are two cleotides (MgATP and MgADP) with SUR1 (4,5). nucleotide-binding sites, each of which contains resi- SUR belongs to the ATP-binding cassette (ABC) family dues from NBD1 as well as from NBD2. It makes the of transporter proteins (6). Like other ABC transporters, it novel prediction that residue D860 in NBD1 helps coor- has two sets of transmembrane domains (TMDs), each dinate Mg nucleotides at site 2. We tested this predic- containing six membrane spanning helices, and two nucle- tion experimentally and found that, unlike wild-type channels, channels containing the SUR1-D860A muta- otide binding domains (NBDs). It also possesses an addi- tion were not activated by MgADP in either the pres- tional NH2-terminal set of five transmembrane helices (Fig. ence or absence of MgATP. Our model should be useful 1A). The NBDs of all ABC proteins are highly conserved for designing experiments aimed at elucidating the and contain several motifs involved in ATP binding and relationship between the structure and function of the hydrolysis, including a Walker-A (WA) and Walker-B (WB) KATP channel. Diabetes 53 (Suppl. 3):S123–S127, 2004 motif linked by a “signature” sequence (LLSGGQ) and two shorter sequences containing conserved glutamine (Gln- loop) and histidine (His-loop) residues (Fig. 1A). The ϩ crystal structures of a number of isolated NBDs suggest he ATP-sensitive K channel (KATP channel) plays a crucial role in insulin secretion by cou- that their topology is also highly conserved across both pling ␤-cell metabolism to insulin secretion (1). prokaryotes and eukaryotes. Furthermore, the X-ray struc- tures show that the NBDs form a dimer containing two At low glucose levels, the KATP channel is open, T nucleotide-binding pockets. These do not correspond to keeping the resting membrane potential hyperpolarized and preventing electrical activity and insulin secretion (2). the NBDs encoded in the primary sequence, however, Increased glucose metabolism results in closure of the because the ATP molecules are sandwiched between the ␤ KATP channels, leading to a depolarization of the -cell WA of one NBD and the signature sequence of the opposite membrane, initiation of electrical activity, opening of NBD (Fig. 1B). For this reason, we henceforth refer to the voltage-gated calcium channels, and an influx of Ca2ϩ ions two nucleotide-binding pockets as site 1 (Walker motifs of that triggers the release of insulin granules. NBD1, linker NBD2) and site 2 (Walker motifs of NBD2, linker NBD1). Because the sequences of NBD1 and NBD2 of SUR1 are From the 1University Laboratory of Physiology, University of Oxford, Oxford, U.K.; and 2Laboratory of Molecular Biophysics, University of Oxford, Oxford, not identical, the structure of these two sites will also not U.K. be identical. This may provide a structural explanation for Address correspondence and reprint requests to Prof. Frances M. Ashcroft, University Laboratory of Physiology, University of Oxford, Parks Rd., Oxford, the observed differences in function between the two sites OX1 3PT, U.K. E-mail: [email protected]. of SUR1 (8). As described previously (7), it is possible to Received for publication 12 March 2004 and accepted in revised form equate the properties described for NBD1 in functional 25 May 2004. This article is based on a presentation at a symposium. The symposium and studies with those of site 1 and to equate those described the publication of this article were made possible by an unrestricted educa- for NBD2 with site 2 (8). Therefore, site 1 binds nucleo- tional grant from Servier. ABC, ATP-binding cassette; CFTR, cystic fibrosis transmembrane regulator; tides with higher affinity than site 2 (half-maximal inhibi- ϩ ϩ ␮ KATP channel, ATP-sensitive K channel; Kir channel, inwardly rectifying K tory concentration [IC50] of 4 and 60 mol/l for ATP and 26 channel; NBD, nucleotide-binding domain; SUR, sulfonylurea receptor; TAP, and 100 ␮mol/l for ADP at sites 1 and 2, respectively). In transporter associated with antigen processing; TMD, transmembrane do- addition, ATP is preferentially hydrolyzed at site 2. main; WA, Walker-A; WB, Walker-B. © 2004 by the American Diabetes Association. To examine these differences in nucleotide handling DIABETES, VOL. 53, SUPPLEMENT 3, DECEMBER 2004 S123 KATP CHANNEL REGULATION BY SUR1 FIG. 1. A: Left, the transmembrane topology of SUR1. A schematic diagram of SUR1 showing the TMDs and the NBDs. Right, schematic showing the structure of the NBD of an ABC transporter. B: Model of the NBDs of SUR1 as viewed from the extracellular surface. The model shows the nucleotide sandwich dimer and the conserved motifs. The WA motifs, WB motifs, and signa- ture sequence are yellow, orange, and green, respec- tively. The ATP molecule is shown in red, and Mg2؉ is shown in gray. NBD1 is blue and NBD1 is purple. Sites 1 and 2 are indicated. C: Expanded view of site 2 of the SUR1 model showing the proximity of D860 to the ATP ␥-phosphate in Site 2. further, we built a molecular model of the SUR1-NBD likely to be slight structural differences between the two heterodimer using a combination of mammalian and bac- NBDs. To take account of these differences, we used two terial crystal structures as templates. We then mutated different templates to construct a molecular model of the residues putatively identified in the model as being in- SUR1 heterodimer: NBD1 was modeled on the crystal volved in nucleotide handling or catalysis and tested their structure of NBD1 of mouse CFTR (15) and NBD2 was functional effects on KATP channel activity. modeled on the X-ray structure of the human TAP1 NBD (16). The two NBD models were then assembled in the RESEARCH DESIGN AND METHODS nucleotide sandwich dimer configuration by least-squares Modeling. The sequences of NBD1 and NBD2 of SUR1 were aligned with fitting to the bacterial NBD homodimer MJ0796 (11). The those of CFTR (cystic fibrosis transmembrane regulator) (protein database CFTR-NBD1 and TAP1-NBD sequences share 33 and 26% [PDB] code 1ROZ) and TAP1 (transporter associated with antigen processing) (PDB code 1JJ7), respectively, using ClustalW (9) and individual monomers sequence identity to the NBD1 and NBD2 of SUR1, respec- generated using MODELLER 6 (version 2) (10). The individual NBD models tively, which is the highest homology currently available were then least-squares fitted to the MJ0796 dimer crystal structure (PDB for NBDs of known structure. Additionally, the CFTR- code 1L2T) (11). Molecular graphics diagrams were generated using VMD NBD1 and TAP1-NBD2 structures were cocrystallized with (visual molecular dynamics) (12) and PovRay (http://www.povray.org). Molecu- lar dynamic simulations were performed as previously described (13). MgATP and MgADP, respectively. Thus, our model will Electrophysiology. Xenopus oocytes were defolliculated and coinjected represent a structure with ATP docked at NBD1 and ADP with ϳ5 ng each of mRNAs encoding Kir6.2 (GenBank accession no. D50581) docked at NBD2. It is thought that this corresponds to the and either wild-type or mutant SUR1 (GenBank accession no. L40624). The “active” conformation of SUR. final injection volume was ϳ50 nl per oocyte. Isolated oocytes were main- tained as previously described (14), and currents were studied 1–4 days after Predictions of the molecular model. Figure 1B shows injection. Macroscopic currents were recorded from giant excised inside-out the model of the NBD heterodimer of SUR1, with ATP patches at 20–24°C (14). The pipette solution contained (in mmol/l) 140 KCl, docked at site 1 and ADP docked at site 2. Given that site 1.2 MgCl , 2.6 CaCl , and 10 HEPES (pH 7.4 with KOH), and the internal (bath) 2 2 2 shows an enhanced ability to hydrolyze ATP (17), we solution contained 110 KCl, 1.44 MgCl2, 30 KOH, 10 EGTA, 10 HEPES (pH 7.2 with KOH), and nucleotides as indicated. Currents were recorded with an searched for negatively charged residues proximal to the Axopatch 200B amplifier (Axon Instruments), filtered at 0.2 kHz and sampled phosphate tail of ATP that could participate in coordina- at 0.5 kHz. tion and hydrolysis of the nucleotide. Two negatively charged residues, D1505 and E1506 in the WB motif of RESULTS AND DISCUSSION NBD2, were observed to line site 2. Mutation of either of Modeling the SUR1 NBD heterodimer.
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