Structural insight into sodium-dependent bile acid transport by members of the SLC10 family

Human apical sodium-dependent bile acid transporter (ASBT) and Na⁺-dependent co-transporting polypeptide (NTCP) are secondary transporters from the SLC10 family. These 9- transmembrane-helix proteins play critical roles in enterohepatic recycling of bile acids. Here we solve the crystal structure of a bacterial homolog, also with nine transmembrane helices and high sequence homology to human ASBT. We report two different structures of the bacterial homolog in the inward-facing state and a humanized version in the outward-facing state, with and without bile acid bound. Structures of NTCP show a pore through the protein in violation of the classical alternating access mechanism. In our structures, the flexible TM6 seals this pore, showing that an outward-closed structure is not necessarily precluded in this 9-transmembrane-helix protein family. Molecular dynamics simulations of bacterial and human proteins highlight that while the bile acid substrate is anchored to residues at the center of the transporter, lipids from the surrounding membrane interact with the hydrophobic sterol group.