MOLECULAR-IDENTIFICATION OF A ROLE FOR TYROSINE-167 IN THE FUNCTION OF THE HUMAN INTESTINAL PROTON-COUPLED DIPEPTIDE TRANSPORTER (HPEPT1)

hPepT1 is a proton-coupled peptide transporter that mediates the absor ption of di- and tripeptides. Here we show that tyrosine 167 (Y167) in transmembrane domain 5 (TMD5) of this 12-transmembrane spanning prote in contributes to its transport function. We identified this particula r amino acid by a computer model of the arrangement of the TMDs of hPe pT1 and investigated its role by site-directed mutagenesis and dipepti de uptake studies. [H-3]Gly-sar uptake in cells transiently transfecte d with Y167A-hPepT1 was abolished completely, even though the level of Y167A-hPepT1 expression by Western blot analysis and cell surface exp ression by immunofluorescence microscopy was similar to those of the w ild type. Therefore, mutation affected transport function, but apparen tly not the steady-state protein level or trafficking of the transport er to the plasma membrane. Moreover, mutation of Y167 into phenylalani ne, serine, or histidine all abolished gly-sar uptake in transfected H EK 293 cells. Taken together, these findings suggest that Y167 plays a n essential role in hPepT1 function, perhaps due to the unique chemist ry of its phenolic side chain. (C) 1998 Academic Press.