The apical sodium-dependent bile acid transporter (ASBT) utilizes the Na+ concentration gradient to transport bile acids into cells. ASBT is essential in cholesterol homeostasis and blocking of ASBT reduces cholesterol concentration in plasma. A recent structure of a bacterial homolog of ASBT in an inward-open state advanced our understanding of the transporter . However, this single conformation does not resolve fundamental questions about the bile acid transport mechanism. We have solved crystal structures of an ASBT homolog from Yersinia Frederiksenii, ASBTYf, in a lipid enviroment in an inward-oopen state at 2.0 Angstrom and an outward-open state at 2.5 Angstrom. The structures reveal how bile acids could be transported across the membrane by a large-scale rigid-body rotation of a sokudm ion-binding domain, suggest the existence of a substrate binding site critical for translocation, and show a possible pathway for sodium ion release intot he cytosol.