The QCR8 genes encoding subunit VIII of the bc1 complex from Kluyveromyces lactis and Schizosaccharomyces pombe partially complement the respiratory-deficient phenotype of a S. cerevisiae QCR8-null mutant. This implies that the heterologous Qcr8 subunits can be imported by S. cerevisiae mitochondria and that they assemble to form a hybrid bc1 complex that is sufficiently active to support growth. In contrast, the QCR8 gene from bovine heart, encoding the 9.5-kDa subunit, is not able to restore respiratory function to the S. cerevisiae null mutant. This lack of functional complementation is directly attributable to the inability of S. cerevisiae mitochondria to import this protein as shown by in vitro assays. However, a hybrid gene encoding the N-terminal 26 residues of S. cerevisiae subunit VIII and the rest of the 9.5-kDa bovine heart homologue, was able to functionally complement the QCR8-null mutant, albeit to a very low extent. Successful import into S. cerevisiae mitochondria was confirmed by in vitro import experiments. Surprisingly, although assembly of these hybrid complexes is reduced to an extent that is proportional to the evolutionary distance of the homologue to S. cerevisiae, the specific activities of the assembled complexes are the same as for the wild-type bc1 complex. After solubilisation of the mitochondrial membranes with the mild detergent dodecyl maltoside, the wild-type enzyme can be inactivated by incubation at increased temperature, independent of protease activity. The rate of inactivation can be significantly increased by the addition of o-phenanthroline [Boumans, H., Grivell, L. A. & Berden, J. A. (1997) J. Biol. Chem. 272, 16753-16760]. The hybrid complexes are much more sensitive to both types of treatment. We conclude that substitution of subunit VIII by a homologous counterpart results in a loosening of the structure of the bc1 complex on the intermembrane space side, resulting in a less stable insertion of the Rieske Fe-S protein in vivo and therefore a lower stability of the assembled enzyme under certain in vitro conditions, but without an effect on catalytic activity.