Membrane topography of the subunits of ubiquinol-cytochrome-c oxidoreductase of Saccharomyces cerevisiae. The 14-kDa and the 11-kDa subunits face opposite sides of the mitochondrial inner membrane.

The topology of the subunits of ubiquinol-cytochrome-c oxidoreductase of the yeast Saccharomyces cerevisiae has been determined using a digitonin/proteinase K assay. With this assay we were able selectively to disrupt the mitochondrial membranes and to identify the subunits which became proteinase-K sensitive after disruption of either the outer or both outer and inner membranes. This approach confirmed previous indications for the localization of the core I protein, cytochrome c1, cytochrome b, the FeS protein and the 17-kDa subunit, while it also provided direct evidence for the site of accessibility to proteinase K of the 14-kDa and 11-kDa subunits. The 14-kDa subunit faces the mitochondrial matrix and the 11-kDa subunit faces the intermembrane space.