Mitochondrial ATP-Mg/phosphate Carriers Transport Divalent Inorganic Cations in Complex with ATP

The ATP-Mg/phosphate carriers (APCs) modulate the intramitochondrial adenine nucleotide pool size. In this study the concentration-dependent effects of Mg and other divalent cations (Me) on the transport of [H]ATP in liposomes reconstituted with purified human and Arabidopsis APCs (hAPCs and AtAPCs, respectively, including some lacking their N-terminal domains) have been investigated. The transport of Me mediated by these proteins was also measured. In the presence of a low external concentration of [H]ATP (12 μM) and increasing concentrations of Me, Mg stimulated the activity (measured as initial transport rate of [H]ATP) of hAPCs and decreased that of AtAPCs; Fe and Zn stimulated markedly hAPCs and moderately AtAPCs; Ca and Mn markedly AtAPCs and moderately hAPCs; and Cu decreased the activity of both hAPCs and AtAPCs. All the Me-dependent effects correlated well with the amount of ATP-Me complex present. The transport of [C]AMP, which has a much lower ability of complexation than ATP, was not affected by the presence of the Me tested, except Cu. Furthermore, the transport of [H]ATP catalyzed by the ATP/ADP carrier, which is known to transport only free ATP and ADP, was inhibited by all the Me tested in an inverse relationship with the formation of the ATP-Me complex. Finally, direct measurements of Mg, Mn, Fe, Zn and Cu showed that they are cotransported with ATP by both hAPCs and AtAPCs. It is likely that in vivo APCs transport free ATP and ATP-Mg complex to different degrees, and probably trace amounts of other Me in complex with ATP.