Potassium channels in the plasmalemma ofChara corallina are multi-ion pores: Voltage-dependent blockade by Cs+ and anomalous permeabilities

Abstract

The outer membranes of plant cells contain channels which are highly selective for K+. In the giant-celled green algaChara corallina, K+ currents in the plasmalemma were measured when the cell was depolarized to the K+ equilibrium potential in relatively high external K+ concentrations. K+ current was reduced by externally added Cs+. Cs+ mainly inhibited inward K+ current, in a strongly voltage-dependent manner; the effective valence of the blocking reaction was often greater than 1, increasing with higher external Cs+ concentrations and with lower K+ concentrations. This is consistent with the channels being single-file, multi-ion pores. Outward current could also be inhibited by Cs+, when external K+ concentrations were low relative to Cs+ concentrations. As the ratio of K+/Tl+ was changed (keeping the sum of the two ions equal), both the resting potential and plasmalemma conductance went through minimums; this is the so-called “anomalous mole fraction effect,” and is consistent with a channel whose pore can be multiply occupied. These effects together strongly suggest that the K+ channels found in the plasmalemma ofChara are multi-ion pores.