='''Sodium Taurocholate Co-Transporting Polypeptide'''=

<StructureSection load='' size='350' side='right' scene='95/952721/Structure_overview/6'>

== Introduction ==
[[image:Taurocholate.png|thumb|250 px| "Fig. 1 Image of Taurocholic acid a crystalline bile acid"]]

Sodium Taurocholate Co-Transporting Polypeptide, or NTCP, is a [https://en.wikipedia.org/wiki/Membrane_transport_protein membrane transporter protein] that is found in the plasma membrane of liver cells, or [https://en.wikipedia.org/wiki/Hepatocyte hepatocytes]. NTCP's primary function is the transportation of taurocholates, or '''bile salts''', (Fig. 1) into the liver and out of the liver to the small intestine. <Ref> Stieger B. The role of the sodium-taurocholate cotransporting polypeptide (NTCP) and of the bile salt export pump (BSEP) in physiology and pathophysiology of bile formation. Handb Exp Pharmacol. 2011;(201):205-59. doi: 10.1007/978-3-642-14541-4_5. PMID: 21103971. [https://dx.doi.org/10.1007/978-3-642-14541-4_5 DOI: DOI: 10.1007/978-3-642-14541-4_5]. </Ref> Bile salts play various roles in metabolism and digestion, but their main function is the [https://en.wikipedia.org/wiki/Emulsion emulsification] of lipid droplets into smaller fragments. This enables lipases to break down the droplets into their monomers, or triglycerides which are then able to be digested. NTCP is part of the [https://en.wikipedia.org/wiki/Solute_carrier_family solute carrier superfamily], more specifically SLC10. NTCP is the founding member of the SLC10 family, first discovered in rat hepatocytes in 1978. <ref name = "SLC10"> Geyer, J., Wilke, T. & Petzinger, E. The solute carrier family SLC10: more than a family of bile acid transporters regarding function and phylogenetic relationships. Naunyn Schmied Arch Pharmacol 372, 413–431 (2006). https://doi.org/10.1007/s00210-006-0043-8 </ref> NTCP has a key role in [https://en.wikipedia.org/wiki/Enterohepatic_circulation Enterohepatic circulation] or '''bile salt recycling''', and its unique ability to transport other solutes lends it therapeutic potential for lowering cholesterol and treating [https://en.wikipedia.org/wiki/Liver_disease liver disease].

NTCP also serves as a binding site for [https://en.wikipedia.org/wiki/Hepatitis_B hepatitis B virus] and [https://en.wikipedia.org/wiki/Hepatitis_D hepatitis D virus]. <ref name = "Park"> Park, JH., Iwamoto, M., Yun, JH. et al. Structural insights into the HBV receptor and bile acid transporter NTCP. Nature 606, 1027–1031 (2022). https://doi.org/10.1038/s41586-022-04857-0. </ref> Future studies into HBV binding mechanism can help understand infection pathways and the development of viral inhibitors.

== Structure ==
[[Image:NTCP topology official.jpeg|300px|left|thumb| '''Figure 2: cartoon depiction of NTCP topology''']]

=== Overview ===
NTCP is one continuous polypeptide chain consisting of a total of <scene name='95/952722/Labeled_9_helices/1'>9 Transmembrane alpha helices</scene> (Fig. 2). The N-terminus of the polypeptide chain is found on the extracellular region of the plasma membrane while the C-terminus is located on the intracellular region. There are <scene name='95/952722/Ntcp_core_domain-_blue/7'>two distinct domains</scene> within the quaternary structure of NTCP: a core domain and a panel domain both being a part of the same polypeptide chain (Fig. 2). The <scene name='95/952722/Ntcp_core_domain-_blue/6'>core domain</scene> (blue) includes 6 transmembrane α helices (TM2-4 and TM7-9) and demonstrates [https://en.wikipedia.org/wiki/Protein_structure two-fold pseudosymmetry]. The <scene name='95/952722/Ntcp_panel_domain-_red/3'>panel domain</scene> (red) consists of 3 transmembrane α helices (TM1 and TM5-6) and does not display symmetry. Within the core domain, there is a unique crossover between TM-3 and TM-8 that is known as the <scene name='95/952722/Ntcp_x_motif/7'>X motif</scene>. This motif is important because this is where the transporter's substrate binding site is located, and within this motif lies essential residues that aid in the conformational change that NTCP undergoes. The core and panel domains are connected by both extracellular and intracellular <scene name='95/952722/Connector_helices/1'>connecter helices</scene> that are separate from the nine transmembrane α helices.

=== Binding Sites ===
==== Sodium ====
NTCP, among others in the SLC10 family, have <scene name='95/952721/Sodium_binding/5'>two sodium binding sites</scene>. 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. This suggests sodium binding impacts bile salt binding.
<Ref name = "Goutam"> Goutam, K., Ielasi, F.S., Pardon, E. et al. Structural basis of sodium-dependent bile salt uptake into the liver. Nature 606, 1015–1020 (2022). [https://doi.org/10.1038/s41586-022-04723-z DOI: 10.1038/s41586-022-04723-z]. </Ref> It is understood that sodium binding and release facilitates changes from open-pore to inward-facing states of NTCP. The inward-facing state is favored in the absence of sodium ions, while the open-pore state is favored in the presence of sodium ions. This also allows for sodium concentrations to regulate the uptake of taurocholates. When intracellular sodium levels are higher, the open-pore state is favored allowing for the diffusion of taurocholates. However, when extracellular sodium levels are high, the inward-facing state is favored preventing diffusion of taurocholates. <ref name="Goutam"/>

==== Bile Salts ====
The <scene name='95/952721/Amphipathic_patterns/2'>amphipathic pore</scene> is also characteristic of NTCP. The pore surface remains {{Template:ColorKey_Hydrophobic}}, while lining of the open pore state is largely {{Template:ColorKey_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 [https://en.wikipedia.org/wiki/Steroid steroids] and [https://en.wikipedia.org/wiki/Thyroid_hormones thyroid hormones]. <Ref name = Qi> Qi X. and Li W. (2022). Unlocking the secrets to human NTCP structure. The Innovation 3(5), 100294. https://doi.org/10.1016/j.xinn.2022.100294 </ref> Thus the channel provides specificity while preventing leakage of other substrates. When observing the relevant <scene name='95/952722/Bile_salts_res/1'>bile salt binding residues</scene> 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 ===
<table align='right' border='0' width='0' cellpadding='0' bgcolor='#d0d0d0' hspace='0'><tr><td rowspan='2'> </td><td bgcolor='#e8e8e8'>[[Image:Cartoon_NTCP_confchange.gif|100px‎]]</td></tr><tr><td bgcolor='#e8e8e8'>Cartoon representation of NTCP conformational change.</td></tr></table>

<table align='right' border='0' width='4' cellpadding='0' bgcolor='#d0d0d0' hspace='0'><tr><td rowspan='2'> </td><td bgcolor='#e8e8e8'>[[Image:Surface_NTCP_confchange.gif‎]]</td></tr><tr><td bgcolor='#e8e8e8'>Cartoon representation of NTCP conformational change.</td></tr></table>
NTCP exists in two different conformations; the <scene name='95/952722/Ntcp_open_pore/1'>open pore conformation</scene> and the <scene name='95/952722/Ntcp_inward_facing/1'>inward-facing conformation</scene>. In order to transport bile salts across the plasma membrane of hepocytes, NTCP must undergo the conformational change from inward facing to open pore. This movement consists of the core and panel domains both rotating 20° and the panel domain moving 5 Å away from the core domain, which remains relatively rigid. This conformational change reveals the two sodium ion binding sites as well as the amphipathic pore in the membrane that bile salts can pass through. This movement of the panel domain is facilitated by <scene name='95/952722/Pro_and_gly_residues/4'>proline and glycine residues</scene> located in the connector helices between the panel and core domains. These residues act as hinges that assist in the movement of the panel domain away from the core domain.

== Bile Salt Transport ==

[[Image:NTCP_mech.png|left|600 px|thumb| '''Figure 5: Diagram of Proposed Bile Salt Transport Process''']]
A proposed pathway for NTCP bile salt transport suggests that both sodium ions are translocated with the transport of one bile salt.<Ref name = "Liu"> Liu, H., Irobalieva, R.N., Bang-Sørensen, R. et al. Structure of human NTCP reveals the basis of recognition and sodium-driven transport of bile salts into the liver. Cell Res 32, 773–776 (2022). https://doi.org/10.1038/s41422-022-00680-4 </Ref> Initally all <scene name='95/952721/Mech_step_1/1'>ligands and sodium ions are bound</scene> then both sodium ions are released along with the inner bile salt into the cytoplasm (Fig. 5). The <scene name='95/952721/Mech_step_2/2'>outermost bile salt remains bound</scene> however in the pore, likely helping to prevent leakage. <Ref name = "Liu"/> The <scene name='95/952721/Mech_step_3/2'> outer bile salt is displaced </scene> into the inner bile salt placement by the movement of sodium ions that facilitates the conformational change to the inward-facing, pore inaccessible conformation (Fig. 5). <Ref name = "Liu"/> It utilizes an [https://www.sciencedirect.com/science/article/pii/S0092867417302891 elevator-alternating mechanism] where one domain (core) does most of the translocation, and the other domain (panel) remains stationary. <Ref name = "Asami"> Asami, J., Kimura, K.T., Fujita-Fujiharu, Y. et al. Structure of the bile acid transporter and HBV receptor NTCP. Nature 606, 1021–1026 (2022). https://doi.org/10.1038/s41586-022-04845-4 </ref> Sodium ions then bind to NTCP, favoring the open-pore state and also allowing for the binding of another outer bile salt (Fig 5). The <scene name='95/952721/Mech_step_1/1'>protein is then reset</scene> and the process can then start again releasing the next inner bile salt with the translocation of the sodium ions into the cytoplasm.

== HBV Binding and Infection==
NTCP is the only [https://rupress.org/jcb/article/195/7/1071/54877/The-cell-biology-of-receptor-mediated-virus entry receptor] into the liver for HBV. <Ref name = "Asami"/> The [https://en.wikipedia.org/wiki/Myristoylation myristolated] PreS1 domain of HBV binds to NTCP through a <scene name='95/952721/Hbv_patch/2'>hydrophobic patch</scene> containing residues 157-165 on the open pore surface. <Ref name = "Asami"/> These residues form part of the tunnel resulting in HBV binding and bile salt transport directly competing and interfering with one another. <Ref name = "Asami"/> Another hydrophobic patch consisting of residues 84-87 found on the N-terminus of NTCP does not overlap with bile salt binding and may be used for the development of [https://en.wikipedia.org/wiki/Antiviral_drug antivirals] that don't inhibit bile uptake <Ref name = "Park"/>. Other minor variations within NTCP provide species specificity for HBV or virus resistance, such as mutant S267F found in East Asia. <Ref name = "Park"/>

The exact mechanism by which NTCP mediates viral internalization is still yet to be determined; however, current studies speculate it works through [https://en.wikipedia.org/wiki/Viral_entry#Entry_via_endocytosis endocytosis.] <Ref name = "Herrscher"> Herrscher C, Roingeard P, Blanchard E. Hepatitis B Virus Entry into Cells. Cells. 2020 Jun 18;9(6):1486. doi: 10.3390/cells9061486. PMID: 32570893; PMCID: PMC7349259. </ref> Once HBV is bound the NTCP/HBV complex is taken into the cell where viral contents are dumped into the cytoplasm to then begin [https://en.wikipedia.org/wiki/Viral_replication viral replication]. It is currently unknown whether HBV also interacts with other receptors or host cell factors, but NTCP alone is not sufficient for infection. <Ref name = "Herrscher"/>

== Medical Relevancy ==

This is a sample scene created with SAT to <scene name="/12/3456/Sample/1">color</scene> by Group, and another to make <scene name="/12/3456/Sample/2">a transparent representation</scene> of the protein. You can make your own scenes on SAT starting from scratch or loading and editing one of these sample scenes.

</StructureSection>
== References ==
<references/>

== Student Contributors ==
*Isabelle White

*Lena Barko