Identification of Key Amino Acids that Impact Organic Solute Transporter / (OSTα/β).

Organic solute transporter α/β (OSTα/β) is a bidirectional bile acid transporter localized on the basolateral membrane of hepatic, intestinal, and renal epithelial cells. OST/ plays a critical role in intestinal bile acid reabsorption and is upregulated in hepatic diseases characterized by elevated bile acids, whereas genetic variants in / have been associated with clinical cholestasis. OST/ also transports and is inhibited by commonly used medications. However, there is currently no high-resolution structure of OST/, and structure-function data for OST, the proposed substrate-binding subunit, are lacking. The present study addressed this knowledge gap and identified amino acids in OST that are important for bile acid transport. This was accomplished using computational modeling and site-directed mutagenesis of the OST subunit to generate OST/ mutant cell lines. Out of the 10 OST/ mutants investigated, four (S228K, T229S, Q269E, Q269K) exhibited decreased [H]-taurocholate (TCA) uptake (ratio of geometric means relative to OST/ wild type (WT) of 0.76, 0.75, 0.79, and 0.13, respectively). Three OST/ mutants (S228K, Q269K, E305A) had reduced [H]-TCA efflux % (ratio of geometric means relative to OST/ WT of 0.86, 0.65, and 0.79, respectively). Additionally, several OST/ mutants demonstrated altered expression and cellular localization when compared with OST/ WT. In summary, we identified OST residues (Ser228, Thr229, Gln269, Glu305) in predicted transmembrane domains that affect expression of OST/ and may influence OST/-mediated bile acid transport. These data advance our understanding of OST/ structure/function and can inform future studies designed to gain further insight into OST/ structure or to identify additional OST/ substrates and inhibitors. SIGNIFICANCE STATEMENT: OST/ is a clinically important transporter involved in enterohepatic bile acid recycling with currently no high-resolution protein structure and limited structure-function data. This study identified four OST amino acids (Ser228, Thr229, Gln269, Glu305) that affect expression of OST/ and may influence OSTα/β-mediated bile acid transport. These data can be utilized to inform future investigation of OST/ structure and refine molecular modeling approaches to facilitate the identification of substrates and/or inhibitors of OST/.