Na-Pi cotransport in flounder: same transport system in kidney and intestine

The cloning of a renal Na-Pi contransport in system from winter flounder (P eudopleuronectes americanus) has recently been reported. We used this information to answer the questions 1) what is the distribution of the transport protein along the nephron? and 2) how are renal and intestinal transporters related? The distribution of the flounder NaPi-II protein was tested using two antisera raised against partial sequences (amino acids 1-14 and 388-441) of the transporter. Antibody-specific fluorescence was detected at the basolateral membrane of epithelial cells in the proximal tubular segment PII. Two clones corresponding to the renal Na-Pi cotransporter were isolated from a flounder intestinal cDNA library. Their functional properties were determined using Xenopus laevis oocytes. The apparent affinities for Pi [Michaelis constant (K(m)) = 0.063 mM] and Na (K(m) = 45.3 mM), as well as the pH dependency (increasing transport activity with increasing pH), showed the same characteristics in both intestinal and the renal systems. Sequence analysis revealed that the two intestinal clones were 100% homologous to the renal cDNA, Flounder NaPi-II-specific immunofluorescence was observed predominantly at the apical membrane on intestinal cross sections. We report the cloning and expression of the first intestinal Na-Pi cotransport system. This transporter belongs to the small group of proteins that exhibit the same function in the apical and the basolateral membranes of different cells.