pH-dependent gating in the Streptomyces lividans K+ channel

Because of its size, high levels of expression, and unusual detergent stability, the small K+ channel from Streptomyces lividans (SKC1) is considered to be an ideal candidate for detailed structural analysis. In this paper, we have used planar lipid bilayers and radiotracer uptake experiments to study purified and reconstituted SKC1, in an attempt to develop a bulk assay for its functional characterization. In channels reconstituted into liposomes with external pH 3.5 and intravesicular pH 7.5, a time-dependent SKC1-catalyzed 86Rb+ uptake was observed. This cationic influx was blocked by Ba2+ ions with a Ki (external) of 0.4 mM and was shown to have the following selectivity sequence: K+ > Rb+ > NH4+ >> Na+ > Li+. In experiments with external pH 7.5 or in liposomes containing no channels, no 86Rb+ uptake was detected. When SKC1 was incorporated into planar lipid bilayers, we failed to observe significant single-channel activity at neutral pH but detected frequent multiple-channel openings a pH < 5.0. These results indicate that under these experimental conditions SKC1 behaves as a pH-gated K+ channel in which protonation of one or more residues promotes channel opening. At acidic pH and symmetrical 200 mM KCl solutions, SKC1 showed numerous brief openings with a main single-channel conductance of 135 pS and a subconductance state of 70 pS. Channel open probability showed a slight voltage dependence, with higher activities observed at negative potentials, a fact which may suggest that the protonation site lies within the transmembrane electrical field. Attempts to determine the pKa of channel activation were obscured by intrinsic limitations of the 86Rb+ flux assay. However, it appears to be lower than pH 4.0. Limited proteolysis experiments demonstrated that SKC1 reconstitutes vectorially, almost exclusively in the right-side-out configuration, indicating that the protonation site responsible for channel opening is located at the extracellular face of the channel. These results point toward a potentially novel gating mechanism for SKC1 and open the possibility of using transmembrane-driven radiotracer influx experiments as a reliable bulk functional assay for reconstituted SKC1.