Cytochrome c reductase from potato has been extensively studied with respect to its catalytic activities, its subunit composition, and the biogenesis of individual subunits. Molecular characterization of all 10 subunits revealed that the high-molecular-weight subunits exhibit striking homologies with the components of the general mitochondrial processing peptidase (MPP) from fungi and mammals. Some of the other subunits show differences in the structure of their targeting signals or in their molecular composition when compared to their counterparts from heterotrophic organisms. The proteolytic activity of MPP was found in the cytochrome c reductase complexes from potato, spinach, and wheat, suggesting that the integration of the protease into this respiratory complex is a general feature of higher plants.