CONDUCTION PROPERTIES OF THE CLONED SHAKER K+ CHANNEL

The conduction properties of the cloned Shaker K+ channel were studied using electrophysiological techniques. Single channel conductance increases in a sublinear manner with symmetric increases in K+ activity, reaching saturation by 0.6 M K+. The Shaker K+ channel is highly selective among monovalent cations; under bi-ionic conditions, its selectivity sequence is K+ > Rb+ > NH4+ > Cs+ > Na+, whereas, by relative conductance in symmetric solutions, it is K+ > NH4+ > Rb+ > Cs+. In Cs+ solutions, single channel currents were too small to be measured directly, so nonstationary fluctuation analysis was used to determine the unitary Cs+ conductance. The single channel conductance displays an anomalous mole-fraction effect in symmetric mixtures of K+ and NH4+ suggesting that the conducting pore is occupied by multiple ions simultaneously.