FUNCTIONAL-ANALYSIS OF A CHIMERIC MAMMALIAN PEPTIDE TRANSPORTER DERIVED FROM THE INTESTINAL AND RENAL ISOFORMS

1. Recently two genes have been identified by expression cloning that encode mammalian epithelial peptide transporters capable of translocat ing di- and tripeptides and selected peptidomimetics by stereoselectiv e and rheogenic substrate-H+ cotransport. PepT1 from rabbit or human s mall intestine induces a transport activity with high transport capaci ty but rather low substrate affinity when expressed in Xenopus oocyts. In contrast, the renal carrier PepT2 is a high affinity-type transpor ter with a lower maximal transport capacity. In addition, both transpo rters show differences in pH dependence and substrate specificity. 2. As a first approach to identify structural components of the transport proteins that determine their phenotypical characteristics, we constr ucted a recombinant chimeric peptide transporter (CH1Pep) in which the aminoterminal region (residues 1-401) is derived from PepT2 whereas t he carboxyterminal region (residues 402-707) starting at the end of tr ansmembrane domain 9 is derived from PepT1. Expression of PepT1, PepT2 and CH1Pep in Xenopus oocytes allowed the characteristics of the tran sporters to be determined by flux studies employing a radiolabelled di peptide and by the two-electrode voltage clamp technique. 3. Our studi es indicate that CH1Pep conserves the characteristics of PepT2 includi ng the high affinity for dipeptides and peptidomimetics, the substrate specificity, the pH dependence of transport activation and the electr ophysiological parameters. We conclude that the phenotypical character istics of the renal peptide transporter are determined by its aminoter minal region.