Interactions of MAP17 with the NaPi-IIa/PDZK1 protein complex in renal proximal tubular cells.

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 tre...