The mitochondrial phosphate transport protein (PTP) has six (A-F) transmembrane (TM) helices per subunit of functional homodimer with all mutations referring to the subunit of the homodimer. In earlier studies, conservative replacements of several residues located either at the matrix end (Asp39/ helix A, Glu137/helix C, Asp236/helix E) or at the membrane center (His32/helix A, Glu136/helix C) of TM helices yielded inactive single mutation PTPs. Some of these residues were suggested to act as phosphate ligands or as part of the proton cotransport path. We now show that the mutation Ser158Thr, not part of a TM helix but located near the center of the matrix loop (Ile141-Ser171) between TM helices C and D, inactivates PTP and is thus also functionally relevant. On the other side of the membrane, the single mutation Glu192Asp at the intermembrane space end of TM helix D yields a PTP with 33% wild-type activity. We constructed double mutants by adding this mutation to the six transport-inactivating mutations. Transport was detected only in those with Asp39Asn, Glu137Gln, or Ser158Thr. We conclude that TM helix D can interact with TM helices A and C and matrix loop Ile141-Ser171 and that Asp39, Glu137, and Ser158 are not essential for phosphate transport. Since our results are consistent with residues present in all 12 functionally identified members of the mitochondrial transport protein (MTP) family, they lead to a general rule that specifies MTP residue types at 7 separate locations. The conformations of all the double mutation PTPs (except that with the matrix loop Ser158Thr) are significantly different from those of the single mutation PTPs, as indicated by their very low liposome incorporation efficiency and their requirement for less detergent (Triton X-100) to stay in solution. These dramatic conformational differences also suggest an interaction between TM helices D and E. The results are discussed in terms of TM helix movements and changes in the PTP monomer/dimer ratio.
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