Primary Structure and Functional Expression of a cDNA Encoding the Bile Canalicular, Purine-specific Na⁺-Nucleoside Cotransporter ∗

We previously characterized a purine-specific Na⁺-nucleoside cotransport system in bile canalicular membrane. The function of this transport system may be related to conserving nucleosides and preventing cholestasis. We report here the isolation of a cDNA encoding a Na⁺-dependent nucleoside transporter from rat liver using an expression cloning strategy. The substrate specificities and kinetic characteristics of the cloned cotransporter are consistent with the properties of the Na⁺-dependent, purine-selective nucleoside transporter in bile canalicular membranes. The nucleotide sequence predicts a protein of 659 amino acids (72 kDa) with 14 putative membrane-spanning domains. Northern blot analysis showed that the transcripts are present in liver and several other tissues. Data base searches indicate significant sequence similarity to the pyrimidine-selective nucleoside transporter (cNT1) of rat jejunum. Although these two subtypes of Na⁺-nucleoside cotransporter have different substrate specificities and tissue localizations, they are members of a single gene family.