Molecular/Functional Characterization of the zebrafish intestinal peptide transporter PEPT1

Cell uptake of small peptides is mediated by membrane transport proteins of the Peptide TRansporter (PTR) family. Members of this family have been described in bacteria, fungi, plants, fruitfly, nematodes, birds and mammals, but not in lower vertebrates. We hereby report the first molecular and functional characterization of a piscine peptide transporter, orthologue to mammalian and avian PEPT1, from the zebrafish Danio rerio. An EST cDNA clone was obtained from IMAGE and the nucleotide sequence of its 2746 pb insert determined. The open reading frame was 2157 bp and encoded for a protein of 718 amino acids with 12 potential transmembrane domains and highest identity (60-62%) to PEPT1 transporters within the PTR family. Electrophysiological analysis after cRNA injection in Xenopus laevis oocytes suggested that zebrafish PEPT1 is a low-affinity/high-capacity system, kinetically similar to the mammalian counterparts. However, in contrast to mammalian transporters which maximal transport activity is independent on extracellular pH, zebrafish PEPT1 maximal transport rates unexpectedly increased at alkaline extracellular pH. Zebrafish pept1 was highly expressed in the proximal intestine since day 4 post-fertilisation, thus preceding functional maturation of the gut, first feeding and complete yolk resorption. Zebrafish PEPT1 will help to understand the evolutionary/functional relations among vertebrate peptide transporters. Moreover, zebrafish pept1 can be a useful marker to screen mutations affecting gut regionalization, differentiation and morphogenesis.