Pribanic Sandra, Gisler Serge Mike, Bacic Desa, Madjdpour Caveh, Hernando Nati, Sorribas Victor, Gantenbein Andrea, Biber Jürg, Murer Heini
Institute of Physiology, University of Zürich, 8057 Zürich, Switzerland.
Am J Physiol Renal Physiol. 2003 Oct;285(4):F784-91. doi: 10.1152/ajprenal.00109.2003. Epub 2003 Jul 1.
An essential role in phosphate homeostasis is played by Na/Pi cotransporter IIa that is localized in the brush borders of renal proximal tubular cells. Recent studies identified several PDZ proteins interacting with the COOH-terminal tail of NaPi-IIa, such as PDZK1 and NHERF-1. Here, by using yeast two-hybrid screen of mouse kidney cDNA library, we attempted to find proteins interacting with the NH2-terminal part of NaPi-IIa. We identified MAP17, a 17-kDa membrane protein that has been described to be associated with various human carcinomas, but it is also expressed in normal kidneys. Results obtained by various in vitro analyses suggested that MAP17 interacts with the fourth domain of PDZK1 but not with other PDZ proteins localized in proximal tubular brush borders. As revealed by immunofluorescence, MAP17 was abundant in S1 but almost absent in S3 segments. No alterations of the apical abundance of MAP17 were observed after maneuvers undertaken to change the content of NaPi-IIa (parathyroid hormone treatment, different phosphate diets). In agreement, no change in the amount of MAP17 mRNA was observed. Results obtained from transfection studies using opossum kidney cells indicated that the apical localization of MAP17 is independent of PDZK1 but that MAP17 is required for apical localization of PDZK1. In summary, we conclude that MAP17 1) interacts with PDZK1 only, 2) associates with the NH2 terminus of NaPi-IIa within the PDZK1/NaPi-IIa/MAP17 complex, and 3) acts as an apical anchoring site for PDZK1.
位于肾近端小管细胞刷状缘的钠/磷酸盐共转运蛋白IIa在磷酸盐稳态中发挥着重要作用。最近的研究发现了几种与钠-磷协同转运蛋白IIa的COOH末端相互作用的PDZ蛋白，如PDZK1和NHERF-1。在这里，我们通过对小鼠肾脏cDNA文库进行酵母双杂交筛选，试图找到与钠-磷协同转运蛋白IIa的NH2末端相互作用的蛋白质。我们鉴定出了MAP17，一种17 kDa的膜蛋白，该蛋白已被描述与多种人类癌症相关，但它也在正常肾脏中表达。各种体外分析结果表明，MAP17与PDZK1的第四个结构域相互作用，但不与位于近端小管刷状缘的其他PDZ蛋白相互作用。免疫荧光显示，MAP17在S1段丰富，但在S3段几乎不存在。在采取改变钠-磷协同转运蛋白IIa含量的措施（甲状旁腺激素治疗、不同的磷酸盐饮食）后，未观察到MAP17顶端丰度的改变。同样，未观察到MAP17 mRNA量的变化。使用负鼠肾细胞进行的转染研究结果表明，MAP17的顶端定位独立于PDZK1，但PDZK1的顶端定位需要MAP17。总之，我们得出结论：1）MAP17仅与PDZK1相互作用；2）在PDZK1/钠-磷协同转运蛋白IIa/MAP17复合物中，MAP17与钠-磷协同转运蛋白IIa的NH2末端相关联；3）MAP17作为PDZK1的顶端锚定位点。
