Redistribution of the electric field within the pore contributes to the voltage-dependence of mitochondrial porin channel.

The effects of pH on the integral conductance and on the properties of single channels induced by porin from rat liver mitochondria in a lipid bilayer have been studied. When the membrane potential increases, the conductance of the multi-channel membrane decreases more sharply at acidic pH than at neutral or basic pH. The channel is shown to have several states with different conductance and selectivity. The number of levels and their conductance do not depend on pH, while the selectivity as well as the dependence of steady-state probabilities of different levels on the membrane potential are substantially affected by a pH change. This dependence curve steepens in the pH region where charges of carboxyl groups of aspartic and glutamic amino acids are neutralized. It is concluded that at neutral pH the channel gate is controlled by a great number of the positively and negatively charged groups. The high steepness of the conductance-voltage curve in the acidic region suggests that at least 60 positive charges participate in controlling the channel gate. This number, compared with that of the positively charged side chain amino acids per channel, according to the amino acid analysis of the porin, led us to conclude that almost all amino groups of the channel former must pass through the entire membrane potential difference upon random motion of the channel among the states. The assumption that channel closing leads to redistribution of the electric field within the pore, changing the energy of the charges on the voltage sensor, may be the only explanation of this phenomenon.