Protein sorting between the outer and inner mitochondrial membranes: submitochondrial localization of cytochrome c1 whose presequence is replaced by the amino-terminal region of a 70 kDa outer membrane protein.

The amino-terminal region of a 70 kDa mitochondrial outer membrane protein of yeast and the presequence of cytochrome c1, an inner membrane protein exposed to the intermembrane space, are thought to be responsible for localizing the proteins in their final destinations after synthesis in the cytosol. Gene fusion experiments were used to identify signals that are responsible for protein sorting between the outer and inner mitochondrial membranes. The submitochondrial localization of cytochrome c1 whose presequence was replaced by the amino-terminal region of the 70 kDa mitochondrial outer membrane protein has been investigated. We have also used an in vivo complementation assay to determine whether or not a 70k-cyt c1 fusion protein is functional. Both the first half and all of the presequence of cytochrome c1 can be replaced by the amino-terminal 12 or 29 residues of the 70 kDa protein for transport to the inner membrane and functional assembly into succinate-cytochrome c reductase. However, replacements by the amino-terminal 61 residues of the 70 kDa protein result in exclusive localization of the fusion proteins to the outer membrane, and the fusions cannot be assembled into the enzyme complex. These data indicate that a mitochondrial targeting signal alone is sufficient to direct cytochrome c1 of mature size to the inner membrane.