Mitochondrial targeting signal of rat liver monoamine oxidase B is located at its carboxy terminus.

Monoamine oxidase B, a typical intrinsic protein of the outer mitochondrial membrane, has an uncleavable targeting signal and is inserted into the membrane without proteolytic maturation. To investigate the region responsible for targeting the enzyme to the outer mitochondrial membrane, various mutated proteins were expressed in cultured mammalian cells, and the distributions of the expressed proteins were analyzed by immunofluorescence microscopy and subcellular fractionation. Deletion of the carboxy-terminal 28 amino acids of monoamine oxidase B abolished the transfer of the enzyme to mitochondria, while the deletion of the amino-terminal 55 amino acids had no effect on the transfer to mitochondria. The existence of the targeting signal at the carboxy-terminal portion of the enzyme was confirmed by using hybrid proteins in which the amino- or carboxy-terminal portion of the enzyme was fused to the hydrophilic portion of cytochrome b5. The fused protein with the carboxy-terminal 29 amino acid residues of monoamine oxidase B was localized in mitochondria, whereas that with 10 amino acids remained in the cytoplasm. These results indicate that the targeting signal of monoamine oxidase B is present within its carboxy-terminal 29 amino acid residues.