Charged amine acids at the carboxyl-terminal portions determine the intracellular locations of two isoforms of cytochrome b₅

Outer mitochondrial membrane cytochrome b₅ (OMb), which is an isoform of cytochrome b₅ (cyt b₅) in the endoplasmic reticulum, is a typical tail- anchored protein of the outer mitochondrial membrane. We cloned cDNA containing the complete amino acid sequence of OMb and found that the protein has no typical structural feature common to the mitochondrial targeting signal at the amino terminus. To identify the region responsible for the mitochondrial targeting of OMb, various mutated proteins were expressed in cultured mammalian cells, and the subcellular localization of the expressed proteins was analyzed. The deletion of more than 11 amino acid residues from the carboxyl-terminal end of OMb abolished the targeting of the protein to the mitochondria. When the carboxyl-terminal 10 amino acids of OMb were fused to the cyt b₅ that was previously deleted in the corresponding 10 residues, the fused protein localized in the mitochondria, thereby indicating that the carboxyl-terminal 10 amino acid residues of OMb have sufficient information to transport OMb to the mitochondria. The replacement of either of the two positively charged residues within the carboxyl-terminal 10 amine acids by alanine resulted in the transport of the mutant proteins to the endoplasmic reticulum. The mutant cyt b₅, in which the acidic amine acid in its carboxyl-terminal end was replaced by basic amine acid, could be transported to the mitochondria. It would thus seem that charged amine acids in the carboxyl-terminal portion of these proteins determine their locations in the cell.