Identification of a new pore in the mitochondrial outer membrane of a porin-deficient yeast mutant

Reconstitution experiments were performed on lipid bilayer membranes in the presence of detergent‐solubilized mitochondrial outer membranes of a porin‐free yeast mutant and of its parent strain. The addition of the detergent‐solubilized material resulted in a strong increase in the membrane conductance which was not observed if only the detergent was added to the aqueous phase. Surprisingly, the membrane conductance induced by the detergent extracts of the mutant membrane was only a factor of 20 less than that caused by the outer membrane of the parent strain under otherwise identical conditions. Single‐channel recordings of lipid bilayer membranes in the presence of mitochondrial outer membranes of the yeast mutant suggested the presence of a transient pore. The reconstituted pores had a single‐channel conductance of 0.21 nS in 0.1 M KCl and the characteristics of general diffusion pores with an estimated effective diameter of 1.2 nm. The pores present in the mitochondrial outer membranes of the yeast mutant shared some similarities with the pores formed by mitochondrial and bacterial porins although their effective diameter is much smaller than those of the ‘normal’ mitochondrial porins which have a single‐channel conductance of about 0.4 nS in 0.1 M KCl, corresponding to an effective diameter of 1.7 nm. Zero‐current membrane‐potential measurements suggested that the second mitochondrial porin is slightly cation‐selective. Its possible role in the metabolism of mitochondria is discussed.