Accelerated publication -: Hetero-concatemeric KIR6.X4/SUR14 channels display distinct conductivities but uniform ATP inhibition

K(IR)6.1 and K(IR)6.2 are the pore-forming subunits of K-NDP, the nucleotide-diphosphate-activated K-ATP channels, and classical K-ATP channels, respectively. "Hybrid" channels, in which the structure is predetermined by concatemerizing K(IR)6.1 and K(IR)6.2, exhibit distinct conductivities specified by subunit number and position. Inclusion of one K(IR)6.2 is sufficient to open K(IR)6.X-X-X-X/SUR1(4) in the absence of nucleotide stimulation through sulfonylurea receptor-1 (SUR1). ATP inhibited the spontaneous bursting of hybrid channels with an IC50(ATP) similar to 10(-5) M, similar to that of K(IR)6.2(4)-containing channels. These findings and a transient increase in K-NDP channel activity following rapid wash-out of MgATP suggested that K(IR)6.1 is not ATP-insensitive as previously believed. We propose that SUR-dependent, inhibitory ATP-enhanced interactions of the cytoplasmic domains of both K(IR)6.1 and K(IR)6.2 stabilize a closed form of the M2 bundle in the gating apparatus.