A voltage- and K+-dependent K+ channel from a membrane fraction enriched in contractile vacuole of Dictyostelium discoideum

摘要

We obtained a membrane fraction enriched in the contractile vacuole by aqueous-polymer two-phase partitioning and its channel activities were analysed by incorporating it into artificial planar lipid bilayers. In asymmetrical KCl solutions (cis, 300 mM/100 mM, trans), we observed single-channel currents of a highly K+-selective channel with slope conductance of 102 pS and reversal potential of ?20.4 mV, which corresponded to PK+/PCl?=7. They showed bursts separated by infrequent quiescent periods. At 0 mV the mean open time was 2.0 ms. Among monovalent cations, Na+ and Li+ were impermeable, whereas Rb+ showed permeability equivalent to that of K+, although the unitary conductance was apparently reduced when the current flowed from the Rb+ containing side, suggesting that Rb+ is a permeant blocking ion. The open probability within bursts remained constant at approx.0.6 as long as the holding potential was positive on the cis side with respect to the trans side, but it decreased to 0 at negative potential. This channel was blocked by submillimolar concentrations of quinine and 30 mM TEA+. The open probability-voltage relationship showed a striking dependency on the KCl concentration on either side. This channel may play a role in water transport in this organelle.