Membrane topology of system xc- light subunit reveals a re-entrant loop with substrate-restricted accessibility

Heteromeric amino acid transporters are composed of a heavy and a light subunit linked by a disulfide bridge. 4F2hc/xCT elicits sodium-independent exchange of anionic L-cysteine and L-glutamate (system x(c)(-)). Based on the accessibility of single cysteines to 3-(N-maleimidylpropionyl) biocytin, we propose a topological model for xCT of 12 transmembrane domains with the N and C termini located inside the cell. This location of N and C termini was confirmed by immunofluorescence. Studies of biotinylation and accessibility to sulfhydryl reagents revealed a re-entrant loop within intracellular loops 2 and 3. Residues His(110) and Thr(112), facing outside, are located at the apex of the re-entrant loop. Biotinylation of H110C was blocked by xCT substrates, by the nontransportable inhibitor (S)-4-carboxyphenylglycine, and by the impermeable reagent (2-sulfonatoethyl) methanethiosulfonate, which produced an inactivation of H110C that was protected by L-glutamate and L-cysteine with an IC50 similar to the K-m. Protection was temperature-independent. The data indicate that His110 may lie close to the substrate binding/permeation pathway of xCT. The membrane topology of xCT could serve as a model for other light subunits of heteromeric amino acid transporters.