Apocytochrome c is synthesized in the cytoplasm, transported to the mitochondrial intermembrane space, and subsequently covalently attached to heme in a reaction catalyzed by the enzyme cytochrome c heme lyase. We have investigated the amino acid sequences in cytochrome c which are required for mitochondrial import, using a systematic series of site-directed alterations of the CYC7-H3 gene which encodes iso-2-cytochrome c in the yeast Saccharomyces cerevisiae. Import of the altered apocytochromes c was assayed in yeast strains that overexpressed cytochrome c heme lyase. Under these conditions, there was efficient mitochondrial accumulation of forms of apocytochrome c which are incapable of having heme covalently attached. In fact, all apocytochromes c containing deletions located to the carboxyl-terminal side of His efficiently accumulated in the mitochondria of strains overexpressing heme lyase, even though all but one of these deletion-containing proteins were incapable of heme attachment. A minimum length of polypeptide chain at the extreme amino terminus of cytochrome c, rather than any specific sequence element in this region, appears to be required for efficient mitochondrial import. Certain amino acid substitutions in the region extending from Gly to Leu, at residue Phe and at residue His, lead to reduced mitochondrial import of apocytochrome c, resulting from stalling of the altered apocytochrome c in partially imported states.

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