CLONING AND FUNCTIONAL EXPRESSION OF A NA+-DEPENDENT PHOSPHATE COTRANSPORTER FROM HUMAN KIDNEY - CDNA CLONING AND FUNCTIONAL EXPRESSION

A cDNA clone encoding a protein 69% identical in amino acid sequence with that of the Na/P-i co-transporter NaPi-1 was isolated from a human kidney cDNA library. The DNA sequence was identical with that of NPT-1 cDNA published by Chong, Kristjansson, Zoghbi and Hughe (1993) (Genomics, 18, 355-359). In the present study, we have characterized the function of the encoded protein and the tissue distribution of its mRNA. Injection of RNA transcribed from NPT-1 into Xenopus oocytes resulted in expression of Na/P-i co-transport activity showing a high affinity for P-i transport (K-m 0.29 mM). Kinetic character ization ([P-i], [Na+]) demonstrated that the expressed transport activity has properties similar to those displayed by oocytes injected with human kidney poly(A)(+) RNA. Northern blotting demonstrated that NPT-1 mRNA is expressed in renal cortex, liver and brain but not in other tissues. Hybrid depletion with antisense oligonucleotides to NaPi-3 and NPT-1 completely inhibited poly(A)(+) RNA-induced Na+-dependent P-i uptake in oocytes. These findings indicate that two high-affinity Na/P-i cotransporters (NaPi-3 and NPT-1) are present in human kidney cortex.