A peptide-sensitive channel of large conductance is localized on mitochondrial outer membrane

Applying the technique of 'tip-dip' to mitochondria, we have shown the existence in this organelle of a cationic channel of large conductance, which is blocked by a 13-residue peptide possessing the sequence of the N-terminal extremity of the cytochrome c oxidase subunit IV precursor. To study the submitochondrial localization of the channel, the effect of trypsin on isolated channels and on entire mitochondria were compared. One side of isolated channels is sensitive to trypsin, which eliminates the voltage dependence. Channels isolated from trypsinized mitochondria were devoid of voltage dependence and were blocked by the peptide. This suggests a localization of the channel on the outer membrane. Consistent with this hypothesis, the channel was observed with the highest frequency in outer membrane fractions purified by different procedures, either from bovine adrenal cortex or from rat liver mitochondria. Such a localization is also consistent with digitonin solubilization experiments. The channel was solubilized before the inner membrane marker, cytochrome c oxidase. The orientation of the channel was inferred from its trypsin sensitivity and its potential dependence: a transmembrane potential (inside negative) will close the channel.