Yeast ADP/ATP carrier isoform 2: conformational dynamics and role of the RRRMMM signature sequence methionines.

The mitochondrial ADP/ATP carrier, or Ancp, is a member of the mitochondrial carrier family responsible for exchanging ADP and ATP across the mitochondrial inner membrane. ADP/ATP transport involves Ancp switching between two conformational states. These can be analyzed using specific inhibitors, carboxyatractyloside (CATR) and bongkrekic acid (BA). The high resolution three-dimensional structure of bovine Anc1p (bAnc1p), as a CATR-carrier complex, has been solved. However, because the structure of the BA-carrier complex has not yet been determined, the detailed mechanism of transport remains unknown. Recently, sample processing for hydrogen/deuterium exchange experiments coupled to mass spectrometry was improved, providing novel insights into bAnc1p conformational transitions due to inhibitor binding. In this work we performed both hydrogen/deuterium exchange-mass spectrometry experiments and genetic manipulations. Because these are very difficult to apply with bovine Anc1p, we used Saccharomyces cerevisiae Anc isoform 2 (ScAnc2p). Significant differences in solvent accessibility were observed throughout the amino acid sequence for ScAnc2p complexed to either CATR or BA. Interestingly, in detergent solution, the conformational dynamics of ScAnc2p were dissimilar to those of bAnc1p, in particular for the upper half of the cavity, toward the intermembrane space, and the m2 loop, which is thought to be easily accessible to the solvent from the matrix in bAnc1p. Our study then focused on the methionyl residues of the Ancp signature sequence, RRRMMM. All our results indicate that the methionine cluster is involved in the ADP/ATP transport mechanism and confirm that the Ancp cavity is a highly dynamic structure.