Bile acids (BAs) are crucial amphipathic surfactants that function as multifaceted regulators in various physiological processes, including nutrient absorption and distribution, lipid metabolism and inflammation1,2. The human organic solute transporter αβ (OSTα–OSTβ; hereafter referred to as OSTα/β) is a BA transporter that has a key role in the secretion and distribution of BAs3–6. Pathogenic mutations in OSTα/β have been associated with cholestasis7,8. Despite the functional importance of OSTα/β in BA homeostasis, the stoichiometry and assembly of the complex and the molecular mechanism that underlies BA transport by OSTα/β remain unknown. Here we present cryo-electron microscopy structures of human OSTα/β in complex with cholesterols and an endogenous substrate, elucidating the structural basis for the function of OSTα/β. OSTα/β is assembled in a novel dimer-of-heterodimers manner: two OSTα units form the homodimeric core, with two OSTβ units bound to the periphery. OSTα 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 OSTα connects the BA site and facilitates the transmembrane translocation of BAs through OSTα/β. Together, this study reveals the architecture and transport mechanism of OSTα/β and provides insights into the structure–function relationships of this crucial transporter in BA homeostasis.
