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Nephrol Dial Transplant (2000) 15: 31-33
© 2000 European Renal Association-European Dialysis and Transplant Association

Tubular Transport of Inorganic Anions

Identification and functional analysis of three isoforms for the Na+-dependent phosphate co-transporter (NaPi-2) in rat kidney

Ken-ichi Miyamoto, Sawako Tatsumi, Hiroko Segawa, Ichiro Ohkido and Eiji Takeda

Department of Nutrition, School of Medicine, Tokushima University, Kuramoto-Cho 3, Tokushima City, Tokushima, Japan

Abstract

We have isolated three unique NaPi-2-related protein cDNAs (NaPi-2{alpha}, NaPi-2ß and NaPi-2{gamma}) from a rat kidney library. NaPi-2{alpha} cDNA encodes 337 amino acids which have high homology to the N-terminal half of NaPi-2 containing three transmembrane domains. NaPi-2ß encodes 327 amino acids which are identical to the N-terminal region of NaPi-2 containing four transmembrane domains; whereas the 146 amino acids in the C-terminal region are completely different. In contrast, NaPi-2{gamma} encodes 268 amino acids which are identical to the C-terminal half of NaPi-2. An analysis of phage and cosmid clones indicated that the three related proteins were produced by alternative splicing in the NaPi-2 gene. In a rabbit reticulocyte lysate system, NaPi-2{alpha}, ß, {gamma} were found to be 36, 36 and 29 kDa polypeptides, respectively. NaPi-2{alpha} and NaPi-2{gamma} were glycosylated and revealed to be 45 and 35 kDa proteins, respectively. A functional analysis demonstrated taht NaPi-2{gamma} and {alpha} markedly inhibited NaPi-2 activity in Xenopus oocytes. The results suggest that these short isoforms may function as a dominant-negative inhibitor of the full-length transporter.


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