Substrate-modulated ADP/ATP-transporter dynamics revealed by NMR relaxation dispersion

The ADP/ATP carrier (AAC) transports ADP and ATP across the inner mitochondrial membrane. Unlike most transporters, which have two-fold direct or inverted quasisymmetry, AAC has apparent three-fold rotational symmetry. Further, its transport rate is relatively fast for transporters that carry large solutes. Here, we study the yeast AAC carrier 3 by obtaining comprehensive NMR relaxation dispersion measurements, which provide residue-specific information on the protein's conformational exchange. Our data indicate that AAC is predominantly in the cytosol-facing open state and converts to a sparsely populated state in an asymmetric manner despite its three-fold structural symmetry. Binding of the substrate ADP substantially increases the rate of conformational exchange, whereas the inhibitor CATR slows the exchange. These results suggest that although the transporter catalyzes the translocation of substrate the substrate also facilitates interconversion between alternating states, and this interconversion may be relevant to the transport function.