Romeo Elisa, Dave Mital H, Bacic Desa, Ristic Zorica, Camargo Simone M R, Loffing Johannes, Wagner Carsten A, Verrey François
Institute of Physiology, Univ. of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland.
Am J Physiol Renal Physiol. 2006 Feb;290(2):F376-83. doi: 10.1152/ajprenal.00286.2005. Epub 2005 Sep 20.
The B degrees 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 degrees 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 Imino(B). Besides these two transporters with known functions, there are two other gene products belonging to the same phylogenetic B degrees AT-cluster, XT2 (SLC6A18) and rodent XT3 that are still "orphans." Quantitative real-time RT-PCR showed that the mRNAs of the four B degrees AT-cluster members are abundant in kidney, whereas only those of B degrees AT1 and XT3s1/SIT1 are elevated in small intestine. In brain, the XT3s1/SIT1 mRNA is more abundant than the other B degrees AT-cluster mRNAs. We show here by immunofluorescence that all four mouse B degrees 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 degrees AT1, XT2, and XT3. In summary, this study shows the luminal brush-border localization of the Na(+)-dependent amino and imino acid transporters B degrees 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°转运系统介导多种中性氨基酸在钠驱动下进入小肠和肾近端小管的上皮细胞。2004年在SLC6家族中鉴定出一种相应的转运体（A. Broer、K. Klingel、S. Kowalczuk、J. E. Rasko、J. Cavanaugh和S. Broer。《生物化学杂志》279: 24467 - 24476, 2004），并命名为B°AT1（SLC6A19）。一种在人类中称为XT3（SLC6A20）、在小鼠中称为XT3s1的系统发育相关转运体被证明作为亚氨基酸转运体发挥作用，也定位于肾脏和小肠，并重新命名为SIT1或亚氨基酸（B）。除了这两种具有已知功能的转运体外，还有另外两种属于同一系统发育B°AT簇的基因产物，XT2（SLC6A18）和啮齿动物的XT3，它们仍然是“孤儿”。定量实时RT - PCR显示，四个B°AT簇成员的mRNA在肾脏中丰富，而在小肠中只有B°AT1和XT3s1/SIT1的mRNA升高。在大脑中，XT3s1/SIT1的mRNA比其他B°AT簇的mRNA更丰富。我们在此通过免疫荧光显示，所有四种小鼠B°AT簇转运体以不同的轴向梯度定位于近端肾小管的刷状缘膜，除XT3可能例外，小肠的刷状缘膜也有定位。对刷状缘蛋白进行去糖基化和蛋白质印迹分析证实了B°AT1、XT2和XT3的糖基化以及腔面定位。总之，本研究显示了钠依赖性氨基酸和亚氨基酸转运体B°AT1和XT3s1/SIT1在肾脏和小肠中的腔面刷状缘定位。还表明，结构高度相似的孤儿转运体XT2和XT3具有相同的腔面定位，但沿近端肾小管的轴向定位略有不同。
