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

We have isolated three unique NaPi-2-related protein cDNAs (NaPi-2 alpha, NaPi-2 beta 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-2 beta 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, beta and 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 that 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.