Cloning and functional characterization of a sodium-dependent phosphate transporter expressed in human lung and small intestine.

A cDNA clone with 53% amino acid identity to the human type II sodium-dependent phosphate transporter (NaPi-3) was isolated from human small intestine and lung. Functional characterization in Xenopus laevis oocytes showed this cDNA to encode a sodium-dependent phosphate transporter. The electrogenic response is similar to that found in other type II transporters but an inverse pH dependence was observed. By Northern blot, a 4.2-kb transcript was found to be abundantly expressed in lung and, to a lesser degree, in several other tissues of epithelial origin including small intestine, pancreas, prostate, and kidney. This transcript encompasses a 2.073-kb open reading frame which is most closely related (78% amino acid identity) to the mouse sodium-dependent phosphate transporter IIb isoform. This novel transporter, designated human NaPi-3b (Genbank AF111856), appears to be an isoform of the mammalian renal type II co-transporter family.