Slc15a1 transporters in teleosts fish: PepT1a and PepT1b, comparative functional studies

Di/tripeptides are key nutrients in animal diets, and fundamental for growth. They are transported across the membrane of enterocytes via the Slc15a1/PepT1 peptide transporter, that uses an inwardly-directed proton electrochemical gradient to drive the uptake. Due to a genome duplication event, PepT1a and PepT1b paralogues are found in teleost fish. Two PepT1a transporters, respectively cloned from the Atlantic salmon (Salmo salar) and zebrafish (Danio rerio), namely asPepT1a and zfPepT1a, have been characterized. For both orthologs, function was verified by heterologous expression in Xenopus laevis oocytes, highlighting electrogenic, Na+-independent and pH-dependent transport similarly to the well-known PepT1b. The transient currents and the transport currents recorded from asPept1a and zfPepT1a indicate significant functional differences with respect to PepT1b. PepT1a can be described as a low‐affinity/high‐capacity system, but its substrates preference profile is peculiar in the species, particularly for charged dipeptides. Moreover, the pre-steady state (PSS) currents, that reflect the first steps of transport cycle display in the charge/voltage relationship differences with respect to Pept1b in the voltage dependence. Considering that the PSS are consequence of the rearrangement of the protein in the membrane electric field, PepT1a transporters interact with the substrate at physiological pH, differently from PepT1b. In addition, PepT1a and PepT1b have similar expression profile but different expression levels. These data together with a significant dissimilar substrate specificity, support the idea of distinct roles for these proteins in peptide recognition and transport.