All mitochondrial carriers are encoded by nuclear genes. Most contain a primary structure exhibiting regions of 100 homologous amino acid repeats, the N and C termini face the intermembrane space and there are six definable transmembrane segments in each carrier. All carriers also contain a common sequence, referred to as the MCF motif, in each repeated region, with some variation in one or two signature sequences.
Amongst the members of the mitochondrial carrier family that have been identified, it is the ADP/ATP carrier (AAC) that is responsible for importing ADP into the mitochondria and exporting ATP out of the mitochondria and into the cytosol following synthesis. The AAC is an integral membrane protein that is synthesised lacking a cleavable presequence, but instead contains internal targeting information. It forms a dimer of two identical subunits and consists of a basket shaped structure with six transmembrane helices that are tilted with respect to the membrane, 3 of them kinked at the level of proline residues.
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