The mitochondrial citrate transport protein: probing the secondary structure of transmembrane domain III, identification of residues that likely comprise a portion of the citrate transport pathway, and development of a model for the putative TMDIII-TMDIII' interface

The mitochondrial citrate transport protein (CTP) has been investigated by mutating 28 consecutive residues within transmembrane domain III (TMDIII), one at a time, to cysteine. A cysteine-less CTP that retains wild-type functional properties, served as the starting template. The single Cys CTP mutants were abundantly expressed in Escherichia coli, isolated, and functionally reconstituted in a liposomal system. The accessibility of each single Cys mutant to two methanethiosulfonate reagents was evaluated by determining the rate constants for inhibition of CTP function. These rate constants varied by over five orders of magnitude. With two independent data sets we observed peaks and troughs in the rate constant data at identical amino acid positions and a periodicity of 4 was observed from residues 123-137. Based on the pattern of accessibility we conclude that residues 123-137 exist as an alpha-helix. Although less certain, a combination of the rate constant data and the specific activity data with the single Cys mutants suggests that the alpha-helical secondary structure may extend to residue 113. Furthermore, the rate constant data define water-accessible and water-inaccessible faces of the helix. We infer that the water-accessible face comprises a portion of the substrate translocation pathway through the CTP, whereas the water-inaccessible surface faces the lipid bilayer. Finally, based on a combination of the CTP inhibition rate constant data and the existence of significant sequence identity with a transmembrane segment within glycophorin A that forms a portion of its dimer interface, a model for a putative CTP TMDIII-TMDIII' dimer interface has been developed.