The voltage dependence of large conductance channels in the intact chloroplast envelope of Nitellopsis obtusa was examined using the patch-clamp technique. The channel switched to the lower conducting substates with amplitudes of around 45 and 20% of that in the open state when potentials larger than 30 mV were applied. The steepness of the voltage dependence approximately corresponds to 4 elementary charges being transferred across the entire voltage drop to close the channel both at positive and negative potentials. The transition to the closed substates could also be induced by König's polyanion, a well known modulator of the mitochondrial outer membrane channel.
