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Direct photoaffinity labeling of liver plasma membrane subfractions enriched in sinusoidal and canalicular membranes using 35Sadenosine 5'-O-(thiotriphosphate) (35SATP gamma S) allows the identification of ATP-binding proteins in these domains. Comparative photoaffinity labeling with 35SATP gamma S and with the photolabile bile salt derivative (7,7-azo-3 alpha, 12 alpha-dihydroxy-5 beta-3 beta-3H-cholan-24-oyl-2'- aminoethanesulfonate followed by immunoprecipitation with a monoclonal antibody (Be 9.2) revealed the identity of the ATP-binding and the bile salt-binding canalicular membrane glycoprotein with the apparent Mr of 110,000 (gp110). The isoelectric point of this glycoprotein was 3.7. Transport of bile salt was studied in vesicles enriched in canalicular and sinusoidal liver membranes. Incubation of canalicular membrane vesicles with 3H taurocholate in the presence of ATP resulted in an uptake of the bile salt into the vesicles which was sensitive to vanadate. ATP-dependent taurocholate transport was also observed in membrane vesicles from mutant rats deficient in the ATP-dependent transport of cysteinyl leukotrienes and related amphiphilic anions. Substrates of the P-glycoprotein (gp170), such as verapamil and doxorubicin, did not interfere with the ATP-dependent transport of taurocholate. Reconstitution of purified gp110 into liposomes resulted in an ATP-dependent uptake of 3Htaurocholate. These results demonstrate that gp110 functions as carrier in the ATP-dependent transport of bile salts from the hepatocyte into bile. This export carrier is distinct from hitherto characterized ATP-dependent transport systems.
Müller et al. (Tue,) studied this question.