The tolbutamide site of SUR1 and a mechanism for its functional coupling to K-ATP channel closure

Micromolar concentrations of tolbutamide will inhibit (SUR1/K(IR)6.2)(4) ch annels in pancreatic beta-cells, but not (SUR2A/K(IR)6.2)(4) channels in ca rdiomyocytes, Inhibition does not require Mg2+ or nucleotides and is enhanc ed by intracellular nucleotides. Using chimeras between SUR1 and SUR2A, we show that transmembrane domains 12-17 (TMD12-17) are required for high-affi nity tolbutamide inhibition of K-ATP channels. Deletions demonstrate involv ement of the cytoplasmic N-terminus of K(IR)6.2 in coupling sulfonylurea-bi nding with SUR1 to the stabilization of an interburst closed configuration of the channel, The increased efficacy of tolbutamide by nucleotides result s from an impairment of their stimulatory action on SUR1 which unmasks thei r inhibitory effects. The mechanism of inhibition of beta-cell KATP channel s by sulfonylureas during treatment of noninsulin-dependent diabetes mellit us thus involves two components, drug-binding and conformational changes wi thin SUR1 which are coupled to the pore subunit through its N-terminus and the disruption of nucleotide-dependent stimulatory effects of the regulator y subunit on the pore. These findings uncover a molecular basis for an inhi bitory influence of SUR1, an ATP-binding cassette (ABC) protein, on K(IR)6. 2, a ion channel subunit, (C) 1999 Federation of European Biochemical Socie ties.