ATPase activity of the sulfonylurea receptor: a catalytic function for theK-ATP channel complex

ATP-sensitive K+ (K-ATP) channels are unique metabolic sensors formed by as sociation of Kir6.2, an inwardly rectifying K+ channel, and the sulfonylure a receptor SUR, an ATP binding cassette protein. We identified an ATPase ac tivity in immunoprecipitates of cardiac K-ATP channels and in purified fusi on proteins containing nucleotide binding domains NBD1 and NBD2 of the card iac SUR2A isoform. NBD2 hydrolyzed ATP with a twofold higher rate compared to NBD1, The ATPase required Mg2+ and was insensitive to ouabain, oligomyci n, thapsigargin, or levamisole, K1348A and D1469N mutations in NBD2 reduced ATPase activity and produced channels with increased sensitivity to ATP. K -ATP channel openers, which bind to SUR, promoted ATPase activity in purifi ed sarcolemma. At higher concentrations, openers reduced ATPase activity, p ossibly through stabilization of MgADP at the channel site, K1348A and D146 9N mutations attenuated the effect of openers on K-ATP channel activity. Op ener-induced channel activation was also inhibited by the creatine kinase/c reatine phosphate system that removes ADP from the channel complex, Thus, t he K-ATP channel complex functions not only as a K+ conductance, but also a s an enzyme regulating nucleotide-dependent channel gating through an intri nsic ATPase activity of the SUR subuniT. Modulation of the channel ATPase a ctivity and/or scavenging the product of the ATPase reaction provide novel means to regulate cellular functions associated with K-ATP channel opening.