Abstract
Cardiac ATP-sensitive K+ (KATP) channels are composed of a K+ channel subunit, Kir6.2 and sulfonylurea receptor (SUR) 2A, whereas vascular nucleotide-dependent K+ channels are formed from Kir6.1 and SUR2B. SUR is an ABC protein possessing two cytoplasmic nucleotide-binding domains (NBD1 and NBD2). Intracellular ATP and ADP inhibit KATP channels by binding to Kir6.2 while activating them through NBDs. It was reported that the E171Q mutant of MJ0796, a prokaryotic ABC protein which is entirely an NBD, forms an ATP sandwich dimer in the presence of Mg-free, Na-bound ATP. SUR2A and SUR2B bearing the corresponding mutation in both NBDs (SUR2A(D834N, E1471Q) and SUR2B(D834N, E1471Q)) were coexpressed with Kir6.2 in HEK293T cells. Both SUR2A(D834N, E1471Q)/Kir6.2 and SUR2B(D834N, E1471Q)/Kir6.2 channels were spontaneously activated by intracellular Na- but not K- or Mg-ATP as measured with the inside-out configuration of the patch-clamp method. This reaction was never observed with wild-type SUR2A/Kir6.2 and SUR2B/Kir6.2 channels. Na-ATP increased the mutant channel activity up to ~40% of the maximum in a concentration-dependent manner. Nicorandil, a KATP channel opener, increased the activity of the mutant than the wild-type KATPchannels with higher potency in the presence Na-ATP. Therefore, dimerization of NBD1 and NBD2 seems to be sufficient to induce opening of the SUR2A- or SUR2B-containing KATP channels. [Jpn J Physiol 54 Suppl:S75 (2004)]
