title: Receptor sites involved in posttranslational transport of apocytochrome c into mitochondria: specificity, affinity, and number of sites.
abstract: Assembly of cytochrome c involves a series of steps: synthesis of apocytochrome c on free ribosomes, specific binding of apocytochrome c to the mitochondrial surface, transfer across the outer membrane, covalent addition of protoheme, refolding of the polypeptide chain, and association of holocytochrome c with its functional sites at the inner membrane. The binding step of apocytochrome c to Neurospora crassa mitochondria was studied by inhibiting the subsequent transfer steps with the heme analogue deuterohemin. The binding sites are highly specific for mitochondrial apocytochromes c. Bound labeled Neurospora apocytochrome c was competitively displaced by unlabeled apocytochrome c from various species. These exhibited different abilities for displacement. Apocytochrome c from Paracoccus denitrificans, the amino-terminal (heme-binding) fragment of Neurospora apocytochrome c, and Neurospora holocytochrome c did not recognize the binding sites. Polylysine did not interfere with apocytochrome c binding. Apocytochrome c is reversibly bound. The binding sites are present in limited number. High-affinity binding sites were present at about 90 pmol/mg of mitochondrial protein. They displayed an association constant of 2.2 X 10(7) M-1. Apocytochrome c was imported into mitochondria and converted to holocytochrome c directly from the binding sites when inhibition by deuterohemin was relieved. We conclude that the apocytochrome c binding sites on mitochondria represent receptors that function in the recognition and import of this precursor by mitochondria.
mesh_terms: D000818:Animals; D001059:Apoproteins / Q000235:genetics*; D001665:Binding Sites; D003574:Cytochrome c Group / Q000235:genetics*; D045304:Cytochromes c; D007700:Kinetics; D008928:Mitochondria / Q000378:metabolism*; D009491:Neurospora / Q000235:genetics*; D009492:Neurospora crassa / Q000235:genetics*; D011485:Protein Binding; D011499:Protein Processing, Post-Translational; D013045:Species Specificity
chemical_list: D001059:Apoproteins; D003574:Cytochrome c Group; D045304:Cytochromes c