Calcium-independent ATP-Mg/Pi exchanger that catalyzes the electroneutral exchange of Mg-ATP or free ADP against an hydrogenphosphate and participates in the net transport of adenine nucleotides across the mitochondria inner membrane. ATP(out) + Mg(2+)(out) + phosphate(in) = ATP(in) + Mg(2+)(in) + phosphate(out) ADP(out) + phosphate(in) = ADP(in) + phosphate(out) May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Belongs to the mitochondrial carrier (TC 2.A.29) family. Wrong choice of CDS.