The heteromeric transporter OST Alpha/OST Beta facilitates the transport of bile and other steroid solutes across the basolateral epithelial cell membrane of intestine, liver, testis, kidney and adrenal gland. OST Alpha/OST Beta expression is induced by bile acids through ligand-dependent transactivation of their genes by FXR (Farnesoid X-activated receptor). This genetic regulation suggests that in response to changes in intracellular bile acid levels, bile acids adjust the rate of their own efflux from enterocytes. OST Beta is a 128 amino acid single-pass transmembrane protein that requires OST Alpha to localize to the plasma membrane. Coexpression of OST Alpha and OST Beta is also required to convert the OST Alpha subunit to a mature glycosylated endoglycosidase H-resistant form, suggesting that co-expression facilitates trafficking of OST Alpha through the golgi apparatus. Though widely expressed, OST Beta is present at highest levels in ileum.

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