Bile acids are essential for digestion, metabolism, and hormonal signaling. Their circulation between the liver and intestine, known as the enterohepatic circulation, depends on a coordinated network of membrane transporters. Most of these transporters were identified and mechanistically understood decades ago. However, one critical step remained unresolved for many years: the export of bile acids from enterocytes into the bloodstream. This missing link was often referred to as the “Northwest Passage” of bile acid transport because it proved unusually difficult to identify and explain.

In hepatocytes, bile acid transport follows a well-established paradigm. Sodium-coupled or facilitative transporters mediate bile acid uptake at the sinusoidal membrane, while ATP-binding cassette (ABC) transporters drive bile acid export at the canalicular membrane. For many years, it was assumed that enterocytes and other bile acid–transporting epithelia, including kidney, biliary tract, and gallbladder, would follow the same logic. Contrary to expectations, this paradigm was overturned in 2004 with the identification of a completely different transporter: the heterodimeric organic solute transporter Ostα/β. Rather than an ABC transporter, this unusual heterodimer was shown to be the major basolateral bile acid exporter.

In a study published in Nature, a collaborative research team led by Eric H. Xu (XU Huaqiang), from the Shanghai Institute of Materia Medica of the Chinese Academy of Sciences, and Xiong Ma from Renji Hospital, Shanghai Jiao Tong University School of Medicine, addressed this long-standing question using combined structural and functional approaches. By integrating state-of-the-art structural biology with electrophysiological analyses, the researchers determined how Ostα/β transports bile acids and why it differs fundamentally from previously characterized carriers.

The team expressed and purified the human Ostα/β complex in mammalian cells and solved its structure using single-particle cryo–electron microscopy at resolutions between 2.6 and 3.1 Å. The structures revealed that Ostα/β assembles as a symmetric tetramer composed of two heterodimers. Each Ostα subunit forms a unique seven-transmembrane fold, which is completed by a single transmembrane helix from Ostβ. This architecture explained why Ostα/β had long resisted classification within known transporter families.

Structural analysis uncovered a lateral substrate-binding groove embedded within the membrane. This groove is stabilized by a cysteine-rich loop that undergoes extensive palmitoylation. The lipid modifications create a hydrophobic environment that accommodates amphipathic substrates such as bile acids and steroid sulfates. Structures bound to taurolithocholic acid and dehydroepiandrosterone sulfate revealed how charged residues within the groove interact with negatively charged substrate groups, conferring specificity.

The researchers also identified a hydrophilic tunnel extending from the binding groove toward the extracellular side of the transporter. Molecular dynamics simulations and electrophysiological recordings showed that substrates move through this pathway in a voltage-dependent manner. Notably, the team leveraged the intrinsic charge of cholic acid to directly convert bile acid flux into an electrical signal: by recording transporter-associated currents, they provided a direct, quantitative readout of bile acid transport. This represents a conceptual advance for the field—using a charged bile acid as the transported species to measure bile acid transport by current recording—linking structure to transport in real time and with polarity control.

Together, the data support a model in which Ostα/β functions as a facilitative carrier whose transport direction is governed by the combined electrochemical gradient of its substrates. The transporter mediates both influx and efflux, with directionality determined by substrate concentration gradients, membrane potential, and electrostatic interactions within the binding pocket. In this framework, membrane voltage is not a passive background variable but an active determinant that biases bidirectional transport, effectively tuning whether Ostα/β operates in an export- or import-favored mode under physiological conditions.

The work also has implications beyond bile acid biology. Structural similarities between Ostα/β and members of the TMEM184 protein family suggest that these proteins are likely orphan transporters rather than receptors and may operate through related transport mechanisms. This insight opens new avenues for studying poorly characterized membrane proteins and for understanding how lipid environments shape transporter function.

DOI: 10.1038/s41586-025-10029-7

Article link: https://www.nature.com/articles/s41586-025-10029-7

Keywords:Bile acid transport; Cryo–electron microscopy; Voltage dependent

Contact:

DIAO Wentong

Shanghai Institute of Materia Medica

E-mail: diaowentong@simm.ac.cn