Identification and characterization of a mammalian mitochondrial ATP-binding cassette membrane protein

Membrane proteins of the ATP-binding cassette (ABC) superfamily are involved in the transport of diverse substrates across organellar and plasma membranes of the mammalian cell. Most human ABC proteins identified to date are associated with genetically linked diseases or clinically relevant phenotypes. We describe a new human half-molecule ABC protein, designated M-ABC1, that contains a predicted single membrane and ATP-binding cassette domain. M-ABC1 is localized to membranes of the mitochondria and its transcript is expressed in all tissues. The N-terminal region of the M-ABC1 protein was shown to function independently as a mitochondrial signal sequence by its ability to target the green fluorescent protein to the mitochondria. The monomeric 60 kDa M-ABC1 protein was chemically crosslinked in vivo into a major protein species of 120-130 kDa, thereby confirming that M-ABC1 exists within a higher ordered ABC protein complex. A dominant negative repression approach using M-ABC1 protein with site-directed mutations in its Walker A motif revealed that the mutant protein was rapidly degraded and indicated that the intact Walker A motif of M-ABC1 was required for its stability. The identification of M-ABC1 extends the known distribution of members of the ABC protein family into the mammalian mitochondrion.