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

The outer membranes of plant cells contain channels which are highly selective for K⁺. In the giant-celled green alga Chara 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 of Chara are multi-ion pores.