Identification of a Potential Substrate Binding Domain in the Mammalian Peptide Transporters PEPT1 and PEPT2 Using PEPT1-PEPT2 and PEPT2-PEPT1 Chimeras

The mammalian peptide transporters PEPT1 and PEPT2 are energized by a transmembrane electrochemical H+ gradient and exhibit similar broad substrate specificity. These transporters however differ in their affinity for substrates, PEPT1 being a low-affinity transporter and PEPT2 being a high-affinity transporter. To identify the substrate binding domain in PEPT1 and PEPT2 which is responsible for the differing affinities, we constructed a series of PEPT1-PEPT2 and PEPT2-PEPT1 chimeras using an in vivo restriction site-independent procedure and determined their substrate affinities. A comparison of these kinetic data for different chimeras with those of the wild-type PEPT1 and PEPT2 in conjunction with the specific structural PEPT1/PEPT2 crossover regions in these chimeras has led to the identification of a putative substrate binding site, which is comprised of the transmembrane domains 7, 8 and 9 of the transporters.