A transmembrane domain of the sulfonylurea receptor mediates activation ofATP-sensitive K+ channels by K+ channel openers

ATP-sensitive K+ (K-ATP) channels are a complex of an ATP-binding cassette transporter, the sulfonylurea receptor (SUR), and an inward rectifier K+ ch annel subunit, Kir6.2, The diverse pharmacological responsiveness of K-ATP channels from various tissues are thought to arise from distinct SUR isofor ms. Thus, when assembled with Kir6.2, the pancreatic beta cell isoform SUR1 is activated by the hyperglycemic drug diazoxide but not by hypotensive dr ugs like cromakalim, whereas the cardiac muscle isoform SUR2A is activated by cromakalim and not by diazoxide. We exploited these differences between SUR1 and SUR2A to pursue a chimeric approach designed to identify the struc tural determinants of SUR involved in the pharmacological activation of K-A TP channels. Wild-type and chimeric SUR were coexpressed with Kir6.2 in Xen opus oocytes, and we studied the resulting channels with the patch-clamp te chnique in the excised inside-out configuration. The third transmembrane do main of SUR is found to be an important determinant of the response to crom akalim, which possibly harbors at least part of its binding site. Contrary to expectations, diazoxide sensitivity could not be linked specifically to the carboxyl-terminal end (nucleotide-binding domain 2) of SUR but appeared to involve complex allosteric interactions between transmembrane and nucle otide-binding domains. In addition to providing direct evidence for the str ucture-function relationship governing K-ATP channel activation by potassiu m channel-opening drugs, a family of drugs of the highest therapeutic inter est, these findings delineate the determinants of ligand specificity within the modular ATP-binding cassette-transporter architecture of SUR.