Abstract
The bile salt export pump (BSEP/ABCB11) transports bile salts from hepatocytes into bile canaliculi. Its malfunction is associated with severe liver disease. One reason for functional impairment of BSEP is systemic administration of drugs, which as a side effect inhibit the transporter. Therefore, drug candidates are routinely screened for potential interaction with this transporter. Hence understanding the functional biology of BSEP is of key importance. In this study we engineered the transporter in order to dissect interdomain communication paths. We introduced mutations in non-canonical and in conserved residues of either of the two nucleotide binding domains and determined the effect on BSEP basal and substrate stimulated ATPase activity as well as on taurocholate transport. Replacement of the non-canonical methionine residue M584 (Walker B sequence of nucleotide binding site 1) by glutamate imparted hydrolysis competency to this site. Importantly this mutation was able to sustain 15% of wild-type transport activity, when the catalytic glutamate of the canonical nucleotide binding site 2 was mutated to glutamine. Kinetic modeling of experimental results for the ensuing M584E/E1244Q mutant suggests that a transfer of hydrolytic capacity from the canonical to the non-canonical nucleotide binding site results in loss of active and adoption of facilitative characteristics. This facilitative transport is ATP-gated. To the best of our knowledge this result is unprecedented in ABC proteins with one non-canonical NBS. Our study promotes an understanding of the domain interplay in BSEP as a basis for exploration of drug interactions with this transporter.
- The American Society for Pharmacology and Experimental Therapeutics