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1Institute of Physiology and Center for Integrative Human Physiology and 2Institute of Anatomy, University of Zürich, Zürich, Switzerland
Submitted 12 July 2005 ; accepted in final form 12 September 2005
The B° 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. A corresponding transporter was identified in 2004 (A. Broer, K. Klingel, S. Kowalczuk, J. E. Rasko, J. Cavanaugh, and S. Broer. J Biol Chem 279: 24467–24476, 2004) within the SLC6 family and named B°AT1 (SLC6A19). 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 IminoB. Besides these two transporters with known functions, there are two other gene products belonging to the same phylogenetic B°AT-cluster, XT2 (SLC6A18) and rodent XT3 that are still "orphans." Quantitative real-time RT-PCR showed that the mRNAs of the four B°AT-cluster members are abundant in kidney, whereas only those of B°AT1 and XT3s1/SIT1 are elevated in small intestine. In brain, the XT3s1/SIT1 mRNA is more abundant than the other B°AT-cluster mRNAs. We show here by immunofluorescence that all four mouse B°AT-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. Deglycosylation and Western blotting of brush-border proteins demonstrated the glycosylation and confirmed the luminal localization of B°AT1, XT2, and XT3. In summary, this study shows the luminal brush-border localization of the Na+-dependent amino and imino acid transporters B°AT1 and XT3s1/SIT1 in kidney and intestine. 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.
B°AT1; XT2; SIT1; epithelial transporters; small intestine
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