Functional and structural role of amino acid residues in the even-numbered transmembrane alpha-helices of the bovine mitochondrial oxoglutarate carrier

The mitochondrial oxoglutarate carrier exchanges cytosolic malate for 2-oxoglutarate from the mitochondrial matrix. Orthologs of the carrier havea high degree of amino acid sequence conservation, meaning that it isimpossible to identify residues important for function on the basis of thiscriterion alone. Therefore, each amino acid residue in the transmembraneα-helices H2 and H6 was replaced by a cysteine in a functionalmitochondrial oxoglutarate carrier that was otherwise devoid of cysteineresidues. The effects of the cysteine replacement and subsequentmodification by sulfhydryl reagents on the initial uptake rate of 2-oxoglutarate were determined. The results were evaluated using astructural model of the oxoglutarate carrier. Residues involved in interhelicaland lipid bilayer interactions tolerate cysteine replacements or theirmodifications with little effect on transport activity. In contrast, themajority of cysteine substitutions in the aqueous cavity had a severe effecton transport activity. Residues important for function of the carrier clusterin three regions of the transporter. The first consists of residues in the[YWLF]- [KR]-G-X-X-P sequence motif, which is highly conserved in allmembers of the mitochondrial carrier family. The residues may fulfill astructural role as a helix breaker or a dynamic role as a hinge region forconformational changes during translocation. The second cluster ofimportant residues can be found at the carboxy-terminal end of theeven-numbered transmembrane α-helices at the cytoplasmic side of thecarrier. Residues in H6 at the interface with H1 are the most sensitive tomutation and modification, and may be essential for folding of the carrierduring biogenesis. The third cluster is at the midpoint of the membraneand consists of residues that are proposed to be involved in substratebinding.