Bile Acid Transport by the Symporter ASBTNM: Substrate Binding and Conformational Change

Abstract

The apical sodium-dependent bile acid transporter (ASBT) allows reabsorption of bile acids from the intestine by coupling bile acid movement to the sodium gradient. Such transporters are attractive targets for drug delivery and in the treatment for hypercholesterolaemia. Several structures of bacterial homologues in both inward and outward facing conformations have been obtained experimentally, including a substrate-bound inward facing structure of the homologue from Neisseria meningitidis (ASBTNM). However, many details surrounding the binding of substrates and the conformational transition remain unclear. We have used experimental and computational methods to explore these details at an atomistic scale using the homologue ASBTNM and the bile acid taurocholate. Models of apo and substrate-bound ASBTNM in the outward-facing conformation were generated. Biased molecular dynamics simulations and free energy calculations were used to explore conformational changes and possible movement of taurocholate during the transition by an elevator mechanism. Probability distributions and the residues important for binding of sodium and taurocholate in wild type and mutant ASBTNM were investigated with unbiased simulations, revealing alternative sodium binding sites. These results further our understanding of the important molecular details of ASBT function.