Protons activate homomeric Kir6.2 channels by selective suppression of the long and intermediate closures

Abstract

The ATP-sensitive K+ channels (KATP) play an important role in regulating membrane excitability. These channels are regulated by H+ in addition to ATP, ADP, and phospholipids. To understand how protons affect the single-channel properties, Kir6.2DeltaC36 currents were studied in excised inside-out patches. We chose to study the homomeric Kir6.2 channel with 36 amino acids deleted at the C-terminal end, as there are ADP/ATP-binding sites in the SUR subunit, which may obscure the understanding of the channel-gating process. In the absence of ATP, moderate intracellular acidosis (pH 6.8) augmented P(open) with small suppression (by approximately 10%) of the single-channel conductance. The long and intermediate closures were selectively inhibited, leading to a shortening of the mean closed time without significant changes in the mean open time. Stronger acidification (<pH6.2) caused channel rundown. Although similar changes in the single-channel properties were observed in the presence of 1 mM ATP, the P(open)-pH relationship curve was shifted by 0.17 pH units toward lower pH levels. ATP had no effect on the inhibition of single-channel conductance during acidosis. Mutation of His175 eliminated the pH effect on the single-channel kinetics, while the single-channel response to acidic pH was retained in the K185E mutant that greatly reduces the ATP sensitivity. These results indicate that Kir6.2DeltaC36 channel gating by protons and ATP relies on two distinct mechanisms opposite to each other, although the pH sensitivity is modulated by ATP.