Different Binding Properties and Affinities for ATP and ADP among Sulfonylurea Receptor Subtypes, SUR1, SUR2A, and SUR2B

Abstract

ATP-sensitive potassium (K ATP )1 channels are inwardly rectifying potassium channels, which are inhibited by ATP and stimulated by MgADP (1-3). They play important roles by linking cellular metabolic level to membrane potential by sensing intracellular ATP and ADP levels in various tissues such as pancreatic ␤-cells, heart, brain, skeletal muscle, and vascular smooth muscle. The K ATP channel is a hetero-octamer composed of sulfonylurea receptor (SURx) and Kir6.x subunits in 4:4 stoichiometry (4 -7). SURx is a member of the ATP-binding cassette (ABC) superfamily including P-glycoprotein (MDR1), multidrug resistance-associated protein (MRP1), and the cystic fibrosis transmembrane conductance regulator (CFTR) (8, 9), all of which have two nucleotide-binding folds (NBFs) per molecule; Kir6.x is a member of the inwardly rectifying potassium channel family (10 -12). Both SURx and Kir6.x have a number of subtypes as follows: SUR1, SUR2A, and SUR2B and Kir6.1 and Kir6.2. SUR1 has been cloned as a high affinity binding protein for sulfonylurea (9), the most commonly used drug for treatment of patients with type 2 diabetes. SUR2A shares 68% amino acid identity with SUR1, and SUR2B is a splicing variant of SUR2A differing only in its C-terminal 42 amino acids (13,14). The Cterminal 42 amino acids of SUR2B are similar to those of SUR1. Kir6.1 and Kir6.2 share 71% amino acid identity with each other, both of which have two putative transmembrane domains and an ion pore-forming (H5) region.Pancreatic ␤-cell K ATP channels, composed of SUR1 and Kir6.2, regulate insulin secretion by altering the ␤-cell membrane potential (1,3,15,16). Coexpression of SUR2A/Kir6.2, SUR2B/Kir6.2, and SUR2B/Kir6.1 has been reported to reconstitute cardiac, smooth muscle, and vascular smooth muscle K ATP channels, respectively (2, 3). These channels have different sensitivities to ATP and show different responses to sulfonylurea drugs and potassium channel openers (10,11,14,17,18). The IC 50 (ATP) of SUR1/Kir6.2 K ATP channels is about 10 M, whereas those of SUR2A/Kir6.2 and SUR2B/Kir6.2 are about 100 and 300 M, respectively (10,11,14). SUR2B/Kir6.1 K ATP channels are not inhibited by ATP but are stimulated by ADP and ATP (17). SUR1/Kir6.2 K ATP channels are inhibited by glibenclamide at K i ϳ10 nM, and SUR2A/Kir6.2, SUR2B/ Kir6.2, and SUR2B/Kir6.1 K ATP channels are inhibited with K i values in the low micromolar range (10,11,14,17). Both SUR1/Kir6.2 and SUR2B/Kir6.2 K ATP channels are stimulated by diazoxide, but SUR2A/Kir6.2 K ATP channels are not (10,11,14). The differences between these channels may be caused, at least in part, by differences in SUR subtype. However, it is not clear how SUR subtypes cause the different properties of K ATP channel subtypes.We have already shown that NBF1 of SUR1 is a Mg 2ϩ -independent high affinity ATP-binding site, that NBF2 is a Mg 2ϩ -dependent low affinity ATP-binding site, and that MgADP binding at NBF2 stabilizes the 8-azido-ATP binding at

ATP-sensitive K channel channel/enzyme multimer: Metabolic gating in the heart

“…orthovanadate) which is usually applied in order to prolong the lifetime of nucleotide-NBD complexes securing the probability for covalent bonding of azidonucleotides in a binding pocket [42,43]. Thus in contrast to certain ABC proteins NBD1 of SUR apparently does not hydrolyze ATP but possesses relatively stable nucleotide binding.…”

“…A conformational/functional association of the two NBDs of SUR is supported by biochemical studies of cooperative nucleotide binding. Specifically, MgADP, either by direct binding or as a product of ATP hydrolysis at NBD2, facilitates ADP/ATP-binding to NBD1, an effect abolished by mutations in the Walker A and Walker B motifs of NBD2 [42,43,59]. Furthermore, mutation of the Walker A lysine of NBD1 is crucial for K ATP channel activation induced by stabilization of NBD2·ADP·V i or inhibition by NBD2·ADP·BeF x complexes, indicating a joint action of NBD1 and NBD2 on channel gating [69].…”

“…Indeed, the classical allosteric model was successfully applied to nucleotidedependent K ATP channel gating [15] which presumed that four identical binding sites for ATP and ADP co-exist within the octameric stoichiometry of the K ATP channel complex [22]; binding of ATP to the pore-forming Kir6.2 subunit inhibits channel opening [25,26], whereas binding of ADP to the corresponding regulatory SUR subunit antagonizes ATP-binding to Kir6.2 [43,81]. Although this classic allosteric model of channel regulation was in agreement with experimentally defined nucleotide-dependent gating (i.e.…”

Nucleotide-gated KATPchannels integrated with creatine and adenylate kinases: Amplification, tuning and sensing of energetic signals in the compartmentalized cellular environment

“…Nucleotide-dependent K ATP channel gating was simulated by an allosteric model where: (i) 4 identical binding sites for ATP and ADP co-exist within the octameric stoichiometry of the K ATP channel complex [15,19]; (ii) binding of ATP to the pore-forming Kir6.2 subunit inhibits channel opening [16,17]; (iii) binding of ADP to the regulatory SUR subunit antagonizes ATPbinding to Kir6.2 [18,20,21]. T i and D i (i = 0-4) are channel species with bound ATP and/or ADP.…”

“…1A). NBD1 binds nucleotides whereas NBD2 hydrolyzes ATP, with NBDs working in tandem to gate K ATP channels [21][22][23]. The ATP hydrolysis cycle at SUR2A drives conformational transitions with distinct outcomes on channel gating imparting low or high ATP-sensitivity to the channel [24].…”