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MINI-REVIEW Biogenesis of Mitochondrial c-Type Cytoehromes

Cytochromes e and c I are essential components of the mitochondrial respiratory chain. In both cytochromes the heme group is covalently linked to the polypeptide chain via thioether bridges. The location of the two cytochromes is in the intermembrane space; cytochrome c is loosely attached to the surface of the inner mitochondrial membrane, whereas cytochrome c~ is firmly anchored to the inner membrane. Both cytochrome c and c~ are encoded by nuclear genes, translated on cytoplasmic ribosomes, and are transported into the mitochondria where they become covalently modified and assembled. Despite the many similarities, the import pathways of cytochrome c and c 1 are drastically different. Cytochrome c I is made as a precursor with a complex bipartite presequence. In a first step the precursor is directed across outer and inner membranes to the matrix compartment of the mitochondria where cleavage of the first part of the presequence takes place. In a following step the intermediate-size form is redirected across the inner membrane; heme addition then occurs on the surface of the inner membrane followed by the second processing reaction. The import pathway of cytochrome c is exceptional in practically all aspects, in comparison with the general import pathway into mitochondria. Cytochrome c is synthesized as apocytochrome c without any additional sequence. It is translocated selectively across the outer membrane. Addition of the heine group, catalyzed by cytochrome c heme lyase, is a requirement for transport. In summary, cytochrome c I import appears to follow a "conservative pathway" reflecting features of cytochrome c~ sorting in prokaryotic cells. In contrast, cytochrome c has "invented" a rather unique pathway which is essentially "non-conservative."