Topological organization of subunits VII and VIII in the ubiquinol-cytochrome c oxidoreductase of Saccharomyces cerevisiae

To determine the topology of subunit Vm of the yeast ubiquinol-cytochrome c oxidoreductase in the mitochondrial inner membrane, an epitope has been introduced in the N-terminal half of this protein, Previous topology studies had shown that at least the C-terminus faces the intermembrane space [Hemrika and Berden (1990) fur. J, Biochem. 192, 761-765]., Based on sensitivity of the protein to proteinase K digestion we now suggest that the N-terminus of subunit VIII is similarly oriented, implying that this subunit does not span the membrane, Despite this, however, subunit VIII cannot be extracted from the membrane even after treatment with 0.1 M Na2CO3 at pH 11.5, showing that the protein is integrally embedded in the membrane, A similar behaviour was displayed by another low molecular weight protein of the complex, subunit VII, which faces the matrix side, A model for the topology of these subunits in the membrane is discussed with respect to the structure of the complex and their involvement in quinone binding.