Human intestinal H⁺/peptide cotransporter. Cloning, functional expression, and chromosomal localization

In mammalian small intestine, a H⁺-coupled peptide transporter is responsible for the absorption of small peptides arising from digestion of dietary proteins. Recently a cDNA clone encoding a H⁺/peptide cotransporter has been isolated from a rabbit intestinal cDNA library (Fei, Y. J., Kanai, Y., Nussberger, S., Ganapathy, V., Leibach, F. H., Romero, M. F., Singh, S. K., Boron, W. F., and Hediger, M. A. (1994) Nature 368, 563-566). Screening of a human intestinal cDNA library with a probe derived from the rabbit H⁺/peptide cotransporter cDNA resulted in the identification of a cDNA which when expressed in HeLa cells or in Xenopus laevis oocytes induced H⁺- dependent peptide transport activity. The predicted protein consists of 708 amino acids with 12 membrane-spanning domains and two putative sites for protein kinase C-dependent phosphorylation. The cDNA-induced transport process accepts dipeptides, tripeptides, and amino β-lactam antibiotics but not free amino acids as substrates. The human H⁺/peptide cotransporter exhibits a high degree of homology (81% identity and 92% similarity) to the rabbit H⁺/peptide cotransporter. But surprisingly these transporters show only a weak homology to the H⁺-coupled peptide transport proteins present in bacteria and yeast. Chromosomal assignment studies with somatic cell hybrid analysis and in situ hybridization have located the gene encoding the cloned human H⁺/peptide cotransporter to chromosome 13 q33→q34.