ADP:ATP antiporter that catalyzes the exchange of ADP and ATP across the endoplasmic reticulum membrane. ADP(in) + ATP(out) = ADP(out) + ATP(in) The transmembrane helices are not perpendicular to the plane of the membrane, but cross the membrane at an angle. At least 2 of the odd-numbered transmembrane helices exhibit a sharp kink, due to the presence of a conserved proline residue. Dramatic growth retardation. Belongs to the mitochondrial carrier (TC 2.A.29) family.