Bile salt export pump is highly conserved during vertebrate evolution and its expression is inhibited by PFIC type II mutations

Bile secretion is a fundamental function of the liver of all vertebrates an d is generated by ATP-dependent transport proteins at the canalicular membr ane of hepatocytes, particularly by the bile salt export pump BSEP. To dete rmine the evolutionary origin and structure-function relationship of this t ransport mechanism, a liver cDNA library from the marine skate Raja erinace a, a 200 million-year-old vertebrate, was screened for BSEP orthologues. A full-length clone was isolated that encodes for 1,348 amino acids and share s 68.5% identity to human BSEP. Northern blot analysis revealed a 5-kb tran script only in skate liver. Expression of skate Bsep in Sf9 cells demonstra ted a sixfold stimulation of ATP-dependent taurocholate transport compared with controls, with a Michaelis-Menten constant of 15 muM, which is compara ble to rat Bsep. Sequences at the site of published mutations in human BSEP are also conserved in skate Bsep. When two of these mutations were introdu ced into the skate Bsep cDNA, this resulted in defective expression of the mutant proteins in Sf9 cells. These studies demonstrate that Bsep is a live r-specific ATP-dependent export pump that is highly conserved throughout ev olution and provide insights into critical determinants for the function of this transporter in higher vertebrates.