Abstact

Bile acids (BAs) are crucial amphipathic surfactants that function as multifaceted regulators in various physiological processes, including nutrient absorption and distribution, lipid metabolism and inflammation(1,2). The human organic solute transporter alphabeta (OSTalpha-OSTbeta; hereafter referred to as OSTalpha/beta) is a BA transporter that has a key role in the secretion and distribution of BAs(3-6). Pathogenic mutations in OSTalpha/beta have been associated with cholestasis(7,8). Despite the functional importance of OSTalpha/beta in BA homeostasis, the stoichiometry and assembly of the complex and the molecular mechanism that underlies BA transport by OSTalpha/beta remain unknown. Here we present cryo-electron microscopy structures of human OSTalpha/beta in complex with cholesterols and an endogenous substrate, elucidating the structural basis for the function of OSTalpha/beta. OSTalpha/beta is assembled in a novel dimer-of-heterodimers manner: two OSTalpha units form the homodimeric core, with two OSTbeta units bound to the periphery. OSTalpha adopts the G-protein-coupled-receptor (GPCR) fold and contains a unique cysteine-rich loop with seven palmitoylation sites; these cooperate with transmembrane helices 5 and 6, constituting a BA recognition site. A positive cavity in OSTalpha connects the BA site and facilitates the transmembrane translocation of BAs through OSTalpha/beta. Together, this study reveals the architecture and transport mechanism of OSTalpha/beta and provides insights into the structure-function relationships of this crucial transporter in BA homeostasis.