The transmembrane domain region of the beta subunit of the organic solute transporter alpha‐beta is essential for heterodimerization with the alpha subunit

The organic solute transporter alpha‐beta (Ostα‐Ostβ) is a bile acid transporter in the small intestine that participates in their enterohepatic circulation. Because bile acids are required for intestinal lipid absorption, this transporter is thought to play a key role in lipid homeostasis. Ostα‐Ostβ consists of two subunits that heterodimerize to generate transport activity, and has a structure resembling GPCR‐RAMP complexes: namely, a predicted seven transmembrane (TM) domain protein Ostα, and a single TM domain protein Ostβ. In order to gather biomolecular information potentially useful for inhibiting this transporter, mutagenesis studies investigated the role of Ostβ in generating transport activity. Ostα and various Ostβ constructs were expressed in HEK293 cells, and the function and localization of the resultant complexes was assessed. When the Ostβ TM domain was deleted yielding Ostβ Δ34–53 , functional activity was lost. In turn, bimolecular fluorescence complementation (BiFC) analysis revealed that a peptide consisting of only the Ostβ TM domain region, Ostβ 29–53 , was sufficient for interaction with Ostα. Furthermore, mutagenesis of the highly conserved tryptophanasparagine (WN34‐35) sequence at the predicted start of Ostβ's TM helix abolished interaction with Ostα. Taken together, these results demonstrate that the TM domain region of Ostβ is essential for heterodimerization with Ostα.