Abstract
ATP-sensitive potassium channels (K-ATP channels) play a key role in adjusting the membrane potential to the metabolic state of cells. They result from the unique combination of two proteins: the sulfonylurea receptor (SUR), an ATP-binding cassette (ABC) protein, and the inward rectifier K+ channel Kir6.2. Both subunits associate to form a heterooctamer (4 SUR/4 Kir6.2). SUR modulates channel gating in response to the binding of nucleotides or drugs and Kir6.2 conducts potassium ions. The activity of K-ATP channels varies with their localization. In pancreatic β-cells, SUR1/Kir6.2 channels are partly active at rest while in cardiomyocytes SUR2A/Kir6.2 channels are mostly closed. This divergence of function could be related to differences in the interaction of SUR1 and SUR2A with Kir6.2. Three residues (E1305, I1310, L1313) located in the linker region between transmembrane domain 2 and nucleotide-binding domain 2 of SUR2A were previously found to be involved in the activation pathway linking binding of openers onto SUR2A and channel opening. To determine the role of the equivalent residues in the SUR1 isoform, we designed chimeras between SUR1 and the ABC transporter multidrug resistance-associated protein 1 (MRP1), and used patch clamp recordings on Xenopus oocytes to assess the functionality of SUR1/MRP1 chimeric K-ATP channels. Our results reveal that the same residues in SUR1 and SUR2A are involved in the functional association with Kir6.2, but they display unexpected side-chain specificities which could account for the contrasted properties of pancreatic and cardiac K-ATP channels.
Highlights
ATP-sensitive potassium channels (K-ATP channels) allow potassium ions to cross-selectively cell membranes as a function of the internal ATP/ADP ratio (Noma 1983; Ashcroft et al 1988; Nichols et al 1996)
Our results reveal that the same residues in SUR1 and SUR2A are involved in the functional association with Kir6.2, but they display unexpected side-chain specificities which could account for the contrasted properties of pancreatic and cardiac K-ATP channels
Using inside-out patch clamp recordings, multidrug resistance-associated protein 1 (MRP1)/SUR1 chimeras coexpressed with Kir6.2 in Xenopus oocytes were first tested for their response to openers (Diazoxide, 300 lmol/L and MgADP, 100 lmol/L)
Summary
ATP-sensitive potassium channels (K-ATP channels) allow potassium ions to cross-selectively cell membranes as a function of the internal ATP/ADP ratio (Noma 1983; Ashcroft et al 1988; Nichols et al 1996). While ATP inhibits the channels, MgADP acts as a physiological opener. Because of this unique regulation by intracellular nucleotides, K-ATP channels are usually considered as direct sensors of the metabolic state of the cell. In pancreatic b-cells, where their physiological function is best understood, they couple the cytoplasmic concentration of nucleotides levels to the insulin secretion machinery via the modulation of the membrane voltage. K-ATP channels are the target of inhibitors such as sulfonylureas (Gribble and Ashcroft 1999), and potassium channels openers (KCO) such as Diazoxide (Moreau et al 2000, 2005a,b), which are commercialized as type 2 diabetes and antihypertensive vasodilator medications, respectively
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