Analysis of transmembrane segment 7 of the dipeptide transporter hPepT1 by cysteine-scanning mutagenesis

To investigate the involvement of transmembrane segment 7 (TMS7) of hPepT1 in forming the putative central aqueous channel through which the substrate traverses, we individually mutated each of the 21 amino acids in TMS7 to a cysteine and analyzed the mutated transporters using the scanning cysteine accessibility method. Y287C- and M292C-hPepT1 did not express at the plasma membrane. Out of the remaining 19 transporters, three (F293C-, L296C-, and F297C-hPepT1) showed negligible glycyl-sarcosine (gly-sar) uptake activity and may play an important role in defining the overall hPepT1 structure. K278C-hPepT1 showed similar to40% activity and the 15 other transporters exhibited more than 50% gly-sar uptake when compared with wild type (WT)-hPepT1. Gly-sar uptake for the 16 active transporters containing cysteine mutations was then measured in the presence of 2.5 mM 2-aminoethyl methanethiosulfonate hydrobromide (MTSEA) or 1 mM [2-(trimethylammonium) ethyl] methanethiosulfonate bromide (MTSET). Gly-sar uptake was significantly inhibited for each of the 16 single cysteine mutants in the presence of 2.5 mM MTSEA. In contrast, significant inhibition of uptake was only observed for K278C-, M279C-, V280C-, T281C-, M284C-, L286C-, P291C-, and D298C-hPepT1 in the presence of 1 mM MTSET. MTSET modification of R282C-hPepT1 resulted in a significant increase in gly-sar uptake. To investigate this further, we mutated WT-hPepT1 to R282A-, R282E-, and R282K-hPepT1. R282E-hPepT1 showed a 43% reduction in uptake activity, whereas R282A- and R282K-hPepT1 had activities comparable with WT-hPepT1, suggesting a role for the Arg-282 positive charge in substrate translocation. Most of the amino acids that were MTSET-sensitive upon cysteine mutation, including R282C, are located toward the intracellular end of TMS7. Hence, our results suggest that TMS7 of hPepT1 is relatively solvent-accessible along most of its length but that the intracellular end of the transmembrane domain is particularly so. From a structure-function perspective, we speculate that the extracellular end of TMS7 may shift following substrate binding, providing the basis for channel opening and substrate translocation.