The basolateral efflux of amino acids from the epithelial cells into the extracellular space is mediated by facilitated diffusion pathway(s) and exchangers ( 36).ДЄ major player of the luminal amino acid uptake machinery was identified in 2004 in mice, namely an Na +-dependent neutral amino acid transporter corresponding to system BВ° ( 4). The first step of this transcellular transport is the influx across the luminal brush-border membrane that is mediated by symporters (cotransporters) and antiporters (exchangers see Refs. Similarly, small oligopeptides and single amino acids are reabsorbed across kidney proximal tubule epithelial cells to prevent their loss in the urine. Ingested dietary proteins are cleaved into small oligopeptides and single amino acids that are then absorbed across small intestine enterocytes.

It also shows that the structurally highly similar orphan transporters XT2 and XT3 have the same luminal but a slightly differing axial localization along the kidney proximal tubule. In summary, this study shows the luminal brush-border localization of the Na +-dependent amino and imino acid transporters BвЉT1 and XT3s1/SIT1 in kidney and intestine. Deglycosylation and Western blotting of brush-border proteins demonstrated the glycosylation and confirmed the luminal localization of BвЉT1, XT2, and XT3. We show here by immunofluorescence that all four mouse BвЉT-cluster transporters localize, with differential axial gradients, to the brush-border membrane of proximal kidney tubule and, with the possible exception of XT3, also of intestine. In brain, the XT3s1/SIT1 mRNA is more abundant than the other BвЉT-cluster mRNAs. Besides these two transporters with known functions, there are two other gene products belonging to the same phylogenetic BвЉT-cluster, XT2 (SLC6A18) and rodent XT3 that are still вЂњorphans.вЂќ Quantitative real-time RT-PCR showed that the mRNAs of the four BвЉT-cluster members are abundant in kidney, whereas only those of BвЉT1 and XT3s1/SIT1 are elevated in small intestine. A phylogenetically related transporter known as XT3 in human (SLC6A20) and XT3s1 in mouse was shown to function as an imino acid transporter, to localize also to kidney and small intestine and renamed SIT1 or Imino B. A corresponding transporter was identified in 2004 (A. The BВ° transport system mediates the Na +-driven uptake of a broad range of neutral amino acids into epithelial cells of small intestine and kidney proximal tubule.

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