An iso-1-cytochrome c-chloramphenicol acetyltransferase fusion protein (iso-1/CAT) was expressed in Saccharomyces cerevisiae and used to delineate two stages in the cytochrome c import pathway in vivo (S. H. Nye and R. C. Scarpulla, Mol. Cell. Biol. 10:5753-5762, 1990 [this issue]). Fusion proteins with the CAT reporter domain in its native conformation were arrested at the initial stage of mitochondrial membrane recognition and insertion. In contrast, those with a deletional disruption of the CAT moiety were relieved of this block and allowed to translocate to the intermembrane space, where they functioned in respiratory electron transfer. In the present study, iso-1/CAT was used to map structural determinants in apoiso-1-cytochrome c involved in the initial step of targeting to the mitochondrial membrane. Carboxy-terminal deletions revealed that one of these determinants consisted of the amino-terminal 68 residues. Deletion mutations either within or at the ends of this determinant destroyed mitochondrial targeting activity, suggesting that functionally important information spans the length of this fragment. Disruption of an alpha-helix near the amino terminus by a helix-breaking proline substitution for leucine 14 also eliminated the targeting activity of the 1 to 68 determinant, suggesting a contribution from this structure. A second, functionally independent targeting determinant was found in the carboxy half of the apoprotein between residues 68 and 85. This determinant coincided with a stretch of 11 residues that are invariant in nearly 100 eucaryotic cytochromes c. Therefore, in lieu of an amino-terminal presequence, apocytochrome c has redundant structural information located in both the amino and carboxy halves of the molecule that can function independently to specify mitochondrial targeting and membrane insertion in vivo.
Import of cytochrome c heme lyase into mitochondria: a novel pathway into the intermembrane space.