Functional characterization of Atlantic salmon (Salmo salar L.) PepT2 transporters

The high-affinity/low-capacity system Slc15a2 (PepT2) is responsible for the reuptake ofdi/tripeptides from the renal proximal tubule, but it also operates in many other tissues and organs.Information regarding PepT2 in teleost fish is limited and, to date, functional data are available fromthe zebrafish (Danio rerio) only. Here, we report the identification of twoslc15a2genes in the Atlanticsalmon (Salmo salar)genome,namelyslc15a2aandslc15a2b. The two encoded PepT2 proteins share87% identity and resemble both structurally and functionally the canonical vertebrate PepT2 system.The mRNA tissue distribution analyses reveal a widespread distribution ofslc15a2atranscripts,being more abundant in the brain and gills, whileslc15a2btranscripts are mainly expressed in thekidney and the distal part of the gastrointestinal tract. The function of the two transporters wasinvestigated by heterologous expression inXenopus laevisoocytes and two-electrode voltage-clamprecordings of transport and presteady-state currents. Both PepT2a and PepT2b in the presence ofGly-Gln elicit pH-dependent and Na+independent inward currents. The biophysical and kineticanalysis of the recorded currents defined the transport properties, confirming that the two Atlanticsalmon PepT2 proteins behave as high-affinity/low-capacity transporters. The recent structures andthe previous kinetic schemes of rat and human PepT2 qualitatively account for the characteristicsof the two Atlantic salmon proteins. This study is the first to report on the functional expressionof two PepT2-type transporters that operate in the same vertebrate organism as a result of (a) geneduplication process(es)