Novel inter- and intrasubunit contacts between transport-relevant residues of the homodimeric mitochondrial phosphate transport protein

Ser158 is located near the middle of the matrix loop connecting transmembrane helices C and D of the mitochondrial phosphate transport protein (PTP). The mutant Ser158Thr PTP is transport-inactive. His32 is located near the middle of transmembrane helix A and Thr79 is located 5 residues away from transmembrane helix B and its N-terminal (matrix end). Single site mutant PTPs that have either residue replaced with Ala are transport-inactive. Based on the high resolution structure of a subunit of the bovine ADP/ATP translocase, on sequence similarities between members of the mitochondrial transport protein family, and on the PTP subunit/subunit contact site between transmembrane A helices, it is now suggested that the Ser158 site is at the PTP subunit/subunit contact site. This contact site is essential for keeping the transport cycles catalyzed by the two PTP subunits 180° out of phase. The data also suggest that His32 and Thr79 of the same subunit interact and couple the phosphate and the proton transport paths.