Comparative Characterization of the Atlantic salmon, Salmo salar L., Di/Tripeptide Transporters PepT1a and PepT1b

Amino acids and small peptides in the form of di/tripeptides are key nutrients in animal diets and essential contributors to growth. In mammals, di/tripeptides represent a highly significant fraction of dietary protein after digestion and are transported across the brush‐border membrane of the intestinal epithelium via a single transport protein, the Peptide Transporter 1 (PepT1), a member of the Solute Carrier 15 (Slc15) family. PepT1 is a low‐affinity/high‐capacity transporter that uses an inwardly‐directed proton electrochemical gradient to drive the uptake of di/tripeptides, and peptidomimetics, across the cell membrane. However, in teleost fishes two pept1 genes, namely pept1b and pept1a, have been identified. PepT1b has been studied in many teleost species, including the Atlantic (A.) salmon (Salmo salar L.), whereas PepT1a remain unexplored. The aim of our study was to characterize, the A. salmon PepT1a vs. PepT1b in terms of tissue/organ distribution and function, and possibly extrapolate their physiological role(s). Despite the extra (4th) whole genome duplication (WGD) in the Actinopterygian lineage, A. salmon has only two pept1 genes (namely pept1a and pept1b), like in other telosts that didn't undergo the extra WGD, e.g., the zebrafish (Danio rerio). At the amino acid level, the two paralogues share 77% similarity, 64% identity. Tissue distribution and relative abundance of A. salmon pept1a and pept1b revealed that both transcripts have a similar expression profile. A. salmon pept1a is highly expressed in the proximal intestine (pyloric caeca ≈ anterior midgut > midgut >> posterior midgut), but ~5‐fold less than pept1b, which is expressed in the same gut regions. No expression signals were detected in the anterior and posterior stomach or the anterior and posterior hindgut. As assessed by heterologous expression in Xenopus laevis oocytes and Two‐Electrode Voltage Clamp measurements, A. salmon PepT1a is, like PepT1b, a low‐affinity/high‐capacity system, able to sustain an electrogenic, sodium‐independent and pH‐dependent transport of di/tripeptides. Transport associated currents show that the protein exhibits a K0.5 value for the reference dipeptide Gly‐Gln of 0.5 mM at pH 7.6 and of 0.07 mM at pH 6.5. The pre‐steady state and steady‐state transport currents analysis indicate important functional differences between A. salmon PepT1a and PepT1b transporters. The difference in expression levels and a significantly dissimilar substrate specificity for certain di/tripeptides, such as lysine‐containing peptides, supports the idea of distinct roles in peptide recognition and transport for PepT1a and PepT1b. Support or Funding Information Funded by RFFV; 247978 SalmoFeedPlus and Cargill