Structural insights into the HBV receptor and bile acid transporter NTCP

Around 250 million people are infected with hepatitis B virus (HBV) worldwide(1), and 15 million may also carry the satellite virus hepatitis D virus (HDV), which confers even greater risk of severe liver disease(2). The HBV receptor has been identified as sodium taurocholate co-transporting polypeptide (NTCP), which interacts directly with the first 48 amino acid residues ofthe N-myristoylated N-terminal preS1 domain ofthe viral large protein'. Despite the pressing need for therapeutic agentsto counter HBV, the structure of NTCP remains unsolved. This 349-residue protein is closely related to human apical sodium-dependent bile acid transporter (ASBT), another member of the solute carrier family SLC10. Crystal structures have been reported of similar bile acid transporters from bacteria(4,5), and these models are believed to resemble closely both NTCP and ASBT. Here we have used cryo-electron microscopy to solve the structure of NTCP bound to an antibody, clearly showing that the transporter has no equivalent ofthe first transmembrane helix found in other SLC10 proteins, and that the N terminus is exposed on the extracellular face. Comparison of our structure with those of related proteins indicates a common mechanism of bile acid transport, but the NTCP structure displays an additional pocket formed by residuesthat are known to interact with preS1, presenting new opportunities for structure-based drug design.

Automated summary

Around 250 million people worldwide are infected with hepatitis B virus (HBV), and 15 million of them may also carry hepatitis D virus (HDV), which increases the risk of severe liver disease. The structure of the HBV receptor NTCP has not been solved, but using cryo-electron microscopy, the structure of NTCP bound to an antibody has been determined. Comparison with other related proteins reveals a common mechanism of bile acid transport, and the NTCP structure presents a new pocket for potential drug design.