Abstract

K_ATP channels are composed of a small inwardly rectifying K⁺ channel subunit, either K_IR6.1 or K_IR6.2, plus a sulfonylurea receptor, SUR1 or SUR2 (A or B), which belong to the ATP-binding cassette superfamily. SUR1/K_IR6.2 reconstitute the neuronal/pancreatic -cell channel, whereas SUR2A/K_IR6.2 and SUR2B/K_IR6.1 (or K_IR6.2) are proposed to reconstitute the cardiac and the vascular-smooth-muscle-type K_ATP channels, respectively. We report that potassium channel openers (KCOs) bind to and act through SURs and that binding to SUR1, SUR2A and SUR2B requires ATP. Non-hydrolysable ATP-analogues do not support binding, and Mg²⁺ or Mn²⁺ are required. Point mutations in the Walker A motifs or linker regions of both nucleotide-binding folds (NBFs) abolish or weaken [³H]P1075 binding to SUR2B, rendering reconstituted SUR2B/K_IR6.2 channels insensitive towards KCOs. The C-terminus of SUR affects KCO affinity with SUR2B SUR1 > SUR2A. KCOs belonging to different structural classes inhibited specific [³H]P1075 binding to SUR2B in a monophasic manner, with the exception of minoxidil sulfate, which induced a biphasic displacement. The affinities of KCO binding to SUR2B were 3.5–8-fold higher than their potencies for activation of SUR2B/K_IR6.2 channels. The results establish that SURs are the KCO receptors of K_ATP channels and suggest that KCO binding requires a conformational change induced by ATP hydrolysis in both NBFs.