Abstract
Potassium channels permeate K+ ions at rates approaching 108 ions per second with strict selectivity. This process is crucial for producing proper electrical signals in biological systems. However, the mechanism of ion conduction through the selectivity filter remains unsolved. For the bacterial K+ channel, KcsA, high resolution crystal structures in the closed state have revealed the single-file distribution of ions and water molecules in the pore. Many simulation studies have been performed based on these static pictures but the results provided only a part of ion permeation process. Hence, the experimental data is essential to understand how water and ion cooperate to permeate through the open channel. Here we examined the streaming potential (Vstream) of K+ selective permeation in the KcsA channel using liposome patch-clamping technique. Vstream provides a quantitative measure of the number of water molecules accompanying a conducting ion (the water-ion coupling ratio), the knowledge of which is a prerequisite for elucidating the coordination of water and ion during permeation. Current-voltage relationship of single KcsA channel was recorded under varying osmotic pressure and the value was evaluated as a shift of the reversal potentials. Effects of concentration and species of permeating ions on the Vstream were investigated because those factors have been reported to perturb the ion coordination in the selectivity filter of crystal structure. Results will be discussed in relation to the ion conducting mechanism of K+ selectivity filter. [J Physiol Sci. 2008;58 Suppl:S77]
