Bile acid and steroid hormone homeostasis are critical for human health, with disruptions linked to metabolic and endocrine disorders1,2. The organic solute transporter Ostα/β, essential for bile acid efflux in enterohepatic circulation3, has long defied mechanistic elucidation. Here we present cryogenic electron microscopy structures of human Ostα/β in apo and substrate-bound states at 2.6–3.1 Å resolution, revealing a distinctive membrane protein architecture that defines a new transporter class. Ostα/β forms a symmetric tetramer of heterodimers, with each Ostα subunit showing a new seven-transmembrane fold, augmented by a single transmembrane helix of Ostβ. This architecture is stabilized by extensive lipid modifications, including a palmitoylated cysteine-rich motif that forms a lateral substrate-binding groove. The structures uncover a unique transport pathway featuring two substrate-binding sites connected by an amphipathic helix-gated conduit. This design, conserved in the evolutionarily related TMEM184 family, suggests an ancient mechanism for substrate translocation. Electrophysiological studies demonstrate voltage-sensitive, bidirectional transport driven by electrochemical gradients, elucidating the efflux role of Ostα/β in vivo. Lipid interactions, notably palmitoylation-dependent trafficking, emerge as critical for stability and function. These findings clarify the molecular mechanism of Ostα/β, provide a structural basis for disease-associated mutations4,5 and establish a paradigm for lipid-modified membrane transport.
