ADP:ATP antiporter that mediates import of ADP into the mitochondrial matrix for ATP synthesis, and export of ATP out to fuel the cell (PubMed:2167309). Cycles between the cytoplasmic-open state (c-state) and the matrix-open state (m-state): operates by the alternating access mechanism with a single substrate-binding site intermittently exposed to either the cytosolic (c-state) or matrix (m-state) side of the inner mitochondrial membrane (By similarity). ADP(in) + ATP(out) = ADP(out) + ATP(in) The matrix-open state (m-state) is inhibited by the membrane-permeable bongkrekic acid (BKA). The cytoplasmic-open state (c-state) is inhibited by the membrane-impermeable toxic inhibitor carboxyatractyloside (CATR). Monomer. The transmembrane helices are not perpendicular to the plane of the membrane, but cross the membrane at an angle. Odd-numbered transmembrane helices exhibit a sharp kink, due to the presence of a conserved proline residue. Belongs to the mitochondrial carrier (TC 2.A.29) family.