Sandbox Reserved 1793

Introduction

Sodium Taurocholate Co-Transporting Polypeptide, or NTCP, is a membrane transporter protein that is found in the plasma membrane of liver cells, or hepatocytes. NTCP's primary function is the transportation of taurocholates, or bile salts, into the liver and out of the liver to the small intestine ^[1] NTCP is part of the solute carrier superfamily, more specifically SLC10. NTCP is the founding member of the SLC10 family, first discovered in rat hepatocytes in 1978 ^[2] NTCP has a key role in Enterohepatic circulation, and it's unique ability to transport other solutes lends it therapeutic potential for lowering cholesterol and liver disease.

NTCP also serves as a binding site for hepatitis B virus and hepatitis d virus ^[3] Future studies into HBV binding mechanism can help understand infection pathways and the development of viral inhibitors.

Structure

Binding Sites

Sodium

NTCP, among others in the SLC10 family, have . Many polar and negatively charged residues are characteristic of these active sites. The high level of conservation among sodium binding placement and interacting residues suggests sodium binding is coupled to bile salt transport. Additional mutations in the X-motif near sodium binding sites have shown that bile salt transport function is lost also suggesting that sodium allows bile salt binding. ^[4] It is understood that these sodium binding sites facilitate changes from open-pore to closed pore states of NTCP that allow for the binding or release of bile salts. Closed-pore state is favored in the absence of sodium ions, while open-pore state is favored in the presence of sodium ions. This also allows for sodium concentrations to regulate uptake of taurocholates. When intracellular sodium levels are higher, open-pore state is favored allowing for the diffusion of taurocholates. However, when extracellular sodium levels are high, closed-state is favored preventing diffusion of taurocholates. ^[4]

Bile Salt

The is also characteristic of NTCP. The pore surface remains Hydrophobic, while lining of the open pore state is largely Polar. However, in the "inward-facing conformation" the polar pore residues are inaccessible. When the pore is closed only the surface hydrophobic residues are observed. As the pore opens up inner polar residues become accessible allowing for the binding of substrates. The pattern of hydrophobic and polar residues within the pore is believed to follow similar amphipathic patterns within taurocholate and other NTCP substrates, such as steroids and thyroid hormones. ^[5] Thus the channel provides specificity while preventing leakage of other substrates. When observing the relevant it is shown that some residues form Van der Waals interactions while others will form dipole-dipole or ionic interactions with bile salt substrates. The core domain appears to contribute most of the polar domains, while the panel domain contributes more hydrophobic residues.

Conformational Change

	Cartoon representation of NTCP conformational change.

	Cartoon representation of NTCP conformational change.

Bile Salt Transport

A proposed pathway for NTCP bile salt transport suggests that both sodium ions are translocated with the transport of one bile salt.^[6] In this mechanism both sodium ions are released along with the inner bile salt into the cytoplasm (Fig. 5). The outermost bile salt remains bound however in the pore, likely helping to prevent leakage ^[6]