Functionally characterized members of the BASS family catalyze Na:bile acid symport. These systems have been identified in intestinal, liver and kidney tissues of animals, and at least three isoforms are present in a single species such as humans. The BASS family is also called the Solute Carrier Family 10 (Claro da Silva et al. 2013).This family has been reported to have the NhaA fold (Ferrada and Superti-Furga 2022).

A BASS in the apical membrane of the human ileal intestine catalyzes the electrogenic uptake of bile acids with a stoichiometry of bile acid:Na of 1:2. This protein is associated with the 16 kDa subunit c of the vacuolar proton pump (Sun et al., 2004). This may account for its apical location. Thus, the vacuolar proton pump associated apical sorting machinery may be used to sort the apical Na:bile symporter.

Proteins of the BASS family vary in size from about 340 to 480 amino acyl residues and possess 7 to 10 putative transmembrane spanners (TMSs). The bile acid binding site appears to be localized to the last TMS (last 60 residues) (Kramer et al., 2001). The BASS family belongs to the BART superfamily (Mansour et al., 2007)

These symporters exhibit broad specificity, taking up a variety of non bile organic compounds as well as taurocholate and other bile salts. Homologues are found in plants, yeast, archaea and bacteria. For example, functionally uncharacterized homologues are present in Synechocystis (292 aas; gbD90911) and Bacillus subtilis (283 aas; spP55190; Z99104). The bacterial homologues exhibit 6-10 putative TMSs. Because the family is represented in widely divergent organisms, it is probably ubiquitous.

The rat liver Na/taurocholate cotransporter is subject to elaborate regulation in response to cyclic AMP and cell swelling (McConkey et al., 2004; Webster et al., 2000). It has two N-terminal, N-linked carbohydrate sites and two Tyr-based basolateral sorting motifs at its carboxyl terminus (YEKI and YKAA). The former targets the protein to the apical membrane in the absence of the latter, but the latter overrides the former, targeting the protein to the basolateral membrane (Sun et al., 2001). The ileal homologue has a 14-residue cytoplasmic tail with a β-turn structure that targets the protein to the apical membrane (Sun et al., 2003).

The human orthologue of the rat Na taurocholate symporter (TC #2.A.28.1.1) (NTCP; SLC10A1) exhibits multiple single nucleotide polymorphisms in populations of European, African, Chinese and Hispanic people (Ho et al., 2004). Four nonsynonymous single nucleotide polymorphisms are associated with significant loss of transport function or change in substrate specificity. One form, found in Chinese Americans does not catalyze bile acid uptake but is normal for estrone sulfate uptake. This transporter is responsible for maintenance of enterohepatic recirculation of bile acids (Ho et al., 2004).

High cholesterol levels greatly increase the risk of cardiovascular disease. About 50 per cent of cholesterol is eliminated from the body by its conversion into bile acids. However, bile acids released from the bile duct are constantly recycled, being reabsorbed in the intestine by the apical sodium-dependent bile acid transporter (ASBT, also known as SLC10A2). It has been shown that plasma cholesterol levels are considerably lowered by specific inhibitors of ASBT. Hu et al. (2011) reported the crystal structure of a bacterial homologue of ASBT from Neisseria meningitidis (ASBT(NM)) at 2.2 Å (3ZUY). ASBT(NM) contains two inverted structural repeats of five transmembrane helices. A core domain of six helices harbours two sodium ions, and the remaining four helices pack in a row to form a flat, 'panel'-like domain. Overall, the architecture of the protein is similar to that of the sodium/proton antiporter NhaA. The ASBT(NM) structure was captured with the substrate taurocholate present, bound between the core and panel domains in a large, inward-facing, hydrophobic cavity. Residues near this cavity have been shown to affect the binding of specific inhibitors of human ASBT.

The SLC10 family includes seven genes containing 1-12 exons that encode proteins in humans with sequence lengths of 348-477 amino acids (Döring et al. 2012). Only three out of seven (i.e. SLC10A1, SLC10A2, and SLC10A6) show sodium-dependent uptake of organic substrates across the cell membrane. These include the uptake of bile salts, sulfated steroids, sulfated thyroidal hormones, and certain statin drugs by SLC10A1 (Na⁺-taurocholate cotransporting polypeptide (NTCP; TC# 2..A.28.1.9)), the uptake of bile salts by SLC10A2 (apical sodium-dependent bile acid transporter (ASBT; TC#2.A.28.1.2)), and uptake of sulfated steroids and sulfated taurolithocholate by SLC10A6 (sodium-dependent organic anion transporter (SOAT; TC# 2.A.28.1.4)). Other members of the family are orphan carriers not all localized in the cell membrane. NTCP and ASBT are carriers for bile salts that establish their enterohepatic circulation. Information is available concerning their 2D and 3D membrane topologies, structure-transport relationships, and ligand and sodium-binding sites. The putative 3D structures have been deduced from the crystal structure of a bacterial homolog, ASBT(NM) (Döring et al. 2012). Knowledge about bile acid synthesis, bile acid hormonal functions, and individual members of the family in terms of expression, localization, substrate pattern, and protein topologies with emphasis on the three functional SLC10 carrier members is presented by (Döring et al. 2012).

Liraglutide, a glucagon-like peptide 1 analog used to treat type 2 diabetes and obesity, provides a potential new treatment modality for bile acid (BA) diarrhea. Nonogaki and Kaji 2024 showed that administration of liraglutide significantly decreased total BAs, especially the primary BAs, including cholic acid, chenodeoxycholic acid, taurocholic acid, taurochenodeoxycholic acid, glycocholic acid, and β-muricholic acid, in the liver and feces. In addition, liraglutide significantly decreased tryptophan metabolites, including L-tryptophan, serotonin, 5-hydroxy indole-3-acetic acid, L-kynurenine, and xanthurenic acid, in the colon, whereas it significantly increased indole-3-propionic acid. Moreover, the administration of liraglutide decreased the expression of the apical sodium-dependent bile acid transporter, which mediates BA uptake across the apical brush border member in the ileum, ileal BA binding protein, and fibroblast growth factor 15 in association with decreased expression of the BA-activated nuclear receptor farnesoid X receptor and the heteromeric organic solute transporter Ostα/β, which induces BA excretion, in the ileum. Liraglutide acutely decreased body weight and blood glucose levels in association with decreases in plasma insulin and serotonin levels in food-deprived mice. These findings suggest the potential of liraglutide as a novel inhibitor of primary BAs and serotonin in the colon (Nonogaki and Kaji 2024).

2.A.28 The Bile Acid:Na⁺ Symporter (BASS) Family Functionally characterized members of the BASS family catalyze Na:bile acid symport. These systems have been identified in intestinal, liver and kidney tissues of animals, and at least three isoforms are present in a single species such as humans. The BASS family is also called the Solute Carrier Family 10 (Claro da Silva et al. 2013).This family has been reported to have the NhaA fold (Ferrada and Superti-Furga 2022). A BASS in the apical membrane of the human ileal intestine catalyzes the electrogenic uptake of bile acids with a stoichiometry of bile acid:Na of 1:2. This protein is associated with the 16 kDa subunit c of the vacuolar proton pump (Sun et al., 2004). This may account for its apical location. Thus, the vacuolar proton pump associated apical sorting machinery may be used to sort the apical Na:bile symporter. Proteins of the BASS family vary in size from about 340 to 480 amino acyl residues and possess 7 to 10 putative transmembrane spanners (TMSs). The bile acid binding site appears to be localized to the last TMS (last 60 residues) (Kramer et al., 2001). The BASS family belongs to the BART superfamily (Mansour et al., 2007) These symporters exhibit broad specificity, taking up a variety of non bile organic compounds as well as taurocholate and other bile salts. Homologues are found in plants, yeast, archaea and bacteria. For example, functionally uncharacterized homologues are present in Synechocystis (292 aas; gbD90911) and Bacillus subtilis (283 aas; spP55190; Z99104). The bacterial homologues exhibit 6-10 putative TMSs. Because the family is represented in widely divergent organisms, it is probably ubiquitous. The rat liver Na/taurocholate cotransporter is subject to elaborate regulation in response to cyclic AMP and cell swelling (McConkey et al., 2004; Webster et al., 2000). It has two N-terminal, N-linked carbohydrate sites and two Tyr-based basolateral sorting motifs at its carboxyl terminus (YEKI and YKAA). The former targets the protein to the apical membrane in the absence of the latter, but the latter overrides the former, targeting the protein to the basolateral membrane (Sun et al., 2001). The ileal homologue has a 14-residue cytoplasmic tail with a β-turn structure that targets the protein to the apical membrane (Sun et al., 2003). The human orthologue of the rat Na taurocholate symporter (TC #2.A.28.1.1) (NTCP; SLC10A1) exhibits multiple single nucleotide polymorphisms in populations of European, African, Chinese and Hispanic people (Ho et al., 2004). Four nonsynonymous single nucleotide polymorphisms are associated with significant loss of transport function or change in substrate specificity. One form, found in Chinese Americans does not catalyze bile acid uptake but is normal for estrone sulfate uptake. This transporter is responsible for maintenance of enterohepatic recirculation of bile acids (Ho et al., 2004). High cholesterol levels greatly increase the risk of cardiovascular disease. About 50 per cent of cholesterol is eliminated from the body by its conversion into bile acids. However, bile acids released from the bile duct are constantly recycled, being reabsorbed in the intestine by the apical sodium-dependent bile acid transporter (ASBT, also known as SLC10A2). It has been shown that plasma cholesterol levels are considerably lowered by specific inhibitors of ASBT. Hu et al. (2011) reported the crystal structure of a bacterial homologue of ASBT from Neisseria meningitidis (ASBT(NM)) at 2.2 Å (3ZUY). ASBT(NM) contains two inverted structural repeats of five transmembrane helices. A core domain of six helices harbours two sodium ions, and the remaining four helices pack in a row to form a flat, 'panel'-like domain. Overall, the architecture of the protein is similar to that of the sodium/proton antiporter NhaA. The ASBT(NM) structure was captured with the substrate taurocholate present, bound between the core and panel domains in a large, inward-facing, hydrophobic cavity. Residues near this cavity have been shown to affect the binding of specific inhibitors of human ASBT. The SLC10 family includes seven genes containing 1-12 exons that encode proteins in humans with sequence lengths of 348-477 amino acids (Döring et al. 2012). Only three out of seven (i.e. SLC10A1, SLC10A2, and SLC10A6) show sodium-dependent uptake of organic substrates across the cell membrane. These include the uptake of bile salts, sulfated steroids, sulfated thyroidal hormones, and certain statin drugs by SLC10A1 (Na⁺-taurocholate cotransporting polypeptide (NTCP; TC# 2..A.28.1.9)), the uptake of bile salts by SLC10A2 (apical sodium-dependent bile acid transporter (ASBT; TC#2.A.28.1.2)), and uptake of sulfated steroids and sulfated taurolithocholate by SLC10A6 (sodium-dependent organic anion transporter (SOAT; TC# 2.A.28.1.4)). Other members of the family are orphan carriers not all localized in the cell membrane. NTCP and ASBT are carriers for bile salts that establish their enterohepatic circulation. Information is available concerning their 2D and 3D membrane topologies, structure-transport relationships, and ligand and sodium-binding sites. The putative 3D structures have been deduced from the crystal structure of a bacterial homolog, ASBT(NM) (Döring et al. 2012). Knowledge about bile acid synthesis, bile acid hormonal functions, and individual members of the family in terms of expression, localization, substrate pattern, and protein topologies with emphasis on the three functional SLC10 carrier members is presented by (Döring et al. 2012). Liraglutide, a glucagon-like peptide 1 analog used to treat type 2 diabetes and obesity, provides a potential new treatment modality for bile acid (BA) diarrhea. Nonogaki and Kaji 2024 showed that administration of liraglutide significantly decreased total BAs, especially the primary BAs, including cholic acid, chenodeoxycholic acid, taurocholic acid, taurochenodeoxycholic acid, glycocholic acid, and β-muricholic acid, in the liver and feces. In addition, liraglutide significantly decreased tryptophan metabolites, including L-tryptophan, serotonin, 5-hydroxy indole-3-acetic acid, L-kynurenine, and xanthurenic acid, in the colon, whereas it significantly increased indole-3-propionic acid. Moreover, the administration of liraglutide decreased the expression of the apical sodium-dependent bile acid transporter, which mediates BA uptake across the apical brush border member in the ileum, ileal BA binding protein, and fibroblast growth factor 15 in association with decreased expression of the BA-activated nuclear receptor farnesoid X receptor and the heteromeric organic solute transporter Ostα/β, which induces BA excretion, in the ileum. Liraglutide acutely decreased body weight and blood glucose levels in association with decreases in plasma insulin and serotonin levels in food-deprived mice. These findings suggest the potential of liraglutide as a novel inhibitor of primary BAs and serotonin in the colon (Nonogaki and Kaji 2024). The generalized transport reaction catalyzed by members of the BASS family is: organic acid (out) + Na⁺ (out) → organic acid (in) + Na⁺ (in)
This family belongs to the IT Superfamily.
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Takemori, T., A. Sugimoto-Ishige, H. Nishitsuji, Y. Futamura, M. Harada, T. Kimura-Someya, T. Matsumoto, T. Honma, M. Tanaka, M. Yaguchi, K. Isono, H. Koseki, H. Osada, D. Miki, T. Saito, T. Tanaka, T. Fukami, T. Goto, M. Shirouzu, K. Shimotohno, and K. Chayama. (2022). Establishment of a Monoclonal Antibody against Human NTCP That Blocks Hepatitis B Virus Infection. J. Virol. 96: e0168621.
van der Mark, V.A., D.R. de Waart, K.S. Ho-Mok, M.M. Tabbers, H.W. Voogt, R.P. Oude Elferink, A.S. Knisely, and C.C. Paulusma. (2014). The lipid flippase heterodimer ATP8B1-CDC50A is essential for surface expression of the apical sodium-dependent bile acid transporter (SLC10A2/ASBT) in intestinal Caco-2 cells. Biochim. Biophys. Acta. 1842: 2378-2386.
Wang, L., Q. Liu, X. Hu, C. Zhou, Y. Ma, X. Wang, Y. Tang, K. Chen, X. Wang, and Y. Liu. (2022). Enhanced Oral Absorption and Liver Distribution of Polymeric Nanoparticles through Traveling the Enterohepatic Circulation Pathways of Bile Acid. ACS Appl Mater Interfaces 14: 41712-41725.
Wawrzycka, D., K. Markowska, E. Maciaszczyk-Dziubinska, M. Migocka, and R. Wysocki. (2016). Transmembrane topology of the arsenite permease Acr3 from Saccharomyces cerevisiae. Biochim. Biophys. Acta. 1859: 117-125. [Epub: Ahead of Print]
Webster, C.R., C.J. Blanch, J. Phillips, and M.S. Anwer. (2000). Cell swelling-induced translocation of rat liver Na⁺/taurocholate cotransport polypeptide is mediated via the phosphoinositide 3-kinase signaling pathway. J. Biol. Chem. 275: 29754-29760.
Weinman, S.A., M.W. Carruth, and P.A. Dawson. (1998). Bile acid uptake via the human apical sodium-bile acid cotransporter is electrogenic. J. Biol. Chem. 273: 34691-34695.
Wu, M.R., Y.Y. Huang, and J.K. Hsiao. (2020). Role of Sodium Taurocholate Cotransporting Polypeptide as a New Reporter and Drug-Screening Platform: Implications for Preventing Hepatitis B Virus Infections. Mol Imaging Biol 22: 313-323.
Yan, H., G. Zhong, G. Xu, W. He, Z. Jing, Z. Gao, Y. Huang, Y. Qi, B. Peng, H. Wang, L. Fu, M. Song, P. Chen, W. Gao, B. Ren, Y. Sun, T. Cai, X. Feng, J. Sui, and W. Li. (2012). Sodium taurocholate cotransporting polypeptide is a functional receptor for human hepatitis B and D virus. elife 1: e00049.
Yu, L., H. Chang, W. Xie, Y. Zheng, L. Yang, Q. Wu, F. Bu, Y. Zhu, Y. Xie, G. Pan, K. Lan, and Q. Deng. (2025). Manganese is a potent inducer of lysosomal activity that inhibits de novo HBV infection. PLoS Pathog 21: e1012800.
Zakrzewicz, D. and J. Geyer. (2023). Interactions of Na/taurocholate cotransporting polypeptide with host cellular proteins upon hepatitis B and D virus infection: novel potential targets for antiviral therapy. Biol Chem. [Epub: Ahead of Print]
Zhang J., Fu LL., Tian M., Liu HQ., Li JJ., Li Y., He J., Huang J., Ouyang L., Gao HY. and Wang JH. (2015). Design and synthesis of a novel candidate compound NTI-007 targeting sodium taurocholate cotransporting polypeptide [NTCP]-APOA1-HBx-Beclin1-mediated autophagic pathway in HBV therapy. Bioorg Med Chem. 23(5):976-84.
Zhang, E.Y., M.A. Phelps, A. Banerjee, C.M. Khantwal, C. Chang, F. Helsper, and P.W. Swaan. (2004). Topology scanning and putative three-dimensional structure of the extracellular binding domains of the apical sodium-dependent bile acid transporter (SLC10A2). Biochemistry 43: 11380-11392.
Zhao, Y., X. Ai, M. Wang, L. Xiao, and G. Xia. (2016). A putative pyruvate transporter TaBASS2 positively regulates salinity tolerance in wheat via modulation of ABI4 expression. BMC Plant Biol 16: 109.
Zhou, X., E.J. Levin, Y. Pan, J.G. McCoy, R. Sharma, B. Kloss, R. Bruni, M. Quick, and M. Zhou. (2014). Structural basis of the alternating-access mechanism in a bile acid transporter. Nature 505: 569-573.
Examples:
TC#	Name	Organismal Type	Example
2.A.28.1.1	Organic acid/(conjugated) bile acid (taurocholate):Na⁺ symporter. Taurine conjugates > glycine conjugates > unconjugated bile salts. The initial effect on hepatic bile flow of cholestatic agents such as thorazine and estradiol 17beta-glucuronide are on water flow and not bile salt export pump-mediated bile acid transport (Javitt 2020).	Animals	Liver bile acid uptake system of Rattus norvegicus

2.A.28.1.10	BASS family homologue	Proteobacteria	BASS family homologue of Myxococcus xanthus

2.A.28.1.11	*Apical sodium-dependent bile acid transporter, SBAT of 546 aas and 9 - 12 TMSs. It is essential for survival of a carcinogenic liver fluke Clonorchis sinensis* in the bile (Dai et al. 2020).**		SBAT of Clonorchis sinensis

2.A.28.1.2	Liver/ileal bile acid:Na⁺ symporter, ASBT, ISBT or NTCP2 (SLC10A2) of 348 aas and 7 TMSs (Mareninova et al. 2005) (essential for liver or intestinal bile acid transport and homeostasis (Rao et al., 2008). This BART superfamily protein has been modeled in 3-dimensions using the 3-D structure of bacteriorhodopsin (a TOG superfamily member) (Zhang et al. 2004). TMS4 forms part of the substrate translocation pathway (Khantwal and Swaan, 2008); TMS7 plays a role in substrate binding and translocation (González et al., 2012); TMS1 contributes to substrate translocation and protein stability (da Silva et al., 2011), and TMS2 coordinates Na⁺ translocation (Sabit et al. 2013). NTCPserves asthe Hepatitis B Virus (HBV) receptor, and drugs developed to target NTCP induce autophagy and may provide therapy for HBV (Zhang et al. 2015). Decreased activity leads to luminal bile salt concentrations and either increased eletrolyte secretion or decreased reabsolption (van der Mark et al., 2014). Function and stability depend on N-glycosylation (Muthusamy et al. 2015). Specific inhibitors are known (Slijepcevic and van de Graaf 2017). It has a 7 TMS topology (Banerjee and Swaan 2006). Chronic hepatitis B, C and D viruses (HBV, HCV and HDV) infect the liver and cause cancer. The three viruses are exclusively hepatotropic, and NTCP mediates the transport of bile acids and plays a key role in HBV HCV and HDV entry into hepatocytes. It modulates HCV infection by regulating innate antiviral immune responses in the liver (Eller et al. 2018). The S-acylation status of hASBT regulates its function, metabolic stability, membrane expression, and phosphorylation state (Ayewoh et al. 2020). The GXXXG/A motifs in TMS2 and TMS7 are important for proper folding and sorting of NTCP, and they indirectly affect glycosylation, homodimerization, and bile acid transport, as well as its HBV/HDV receptor function (Palatini et al. 2021). Structural plasticity is a feature of rheostat positions in the human Na⁺/taurocholate cotransporting polypeptide (NTCP) (Ruggiero et al. 2022). A monoclonal antibody against human NTCP blocks Hepatitis B virus infection (Takemori et al. 2022). NTCP interacts directly with the first 48 residues of the N-myristoylated N-terminal preS1 domain of the hepatitis viral large protein. 3-d structural analyses suggest that members of the SLC10 family share a common mechanism of bile acid transport, but the NTCP structure displays an additional pocket formed by residues that are known to interact with preS1 (Park et al. 2022). Genetic variants of NTCP gene influence hepatitis B vaccine failure (Chen et al. 2022). Enhanced oral absorption and liver distribution of polymeric nanoparticles can be achieved through traveling the enterohepatic circulation pathways of bile acids using ASBT (Wang et al. 2022). Novel inhibitors of NTCP have been identified (Song et al. 2024).	Animals	NTCP of Homo sapiens

2.A.28.1.3	The organic anion:Na⁺ symporter, SOAT (transports estrone-3-sulfate (Km= 31 μM) and dehydropiandrosterone sulfate (Km = 30 μM) but not taurocholate, estradiol-17β-glucuronide or ouabain) (Geyer et al., 2004)	Animals	SOAT of Rattus norvegicus (Q70EX6)

2.A.28.1.4	The organic anion:Na⁺ symporter, SOAT (probable paralogue of 2.A.28.1.3); a 7 TMS protein with the N-terminus out and the C-terminus in. Transports dehydroepiandrosterone sulfate, estrone-3-sulfate, and pregnenolone sulfate with Km values of 30, 12 and 11 μM, respectively. Sulfoconjugated taurolithocholate is also a substrate. Cholate, taurocholate and chenodeoxycholate are not substrates. (Geyer et al., 2007). It is expressed in the CNS (Sreedharan et al. 2011).	Animals	SLC10A6 of Homo sapiens

2.A.28.1.5	solute carrier family 10 (sodium/bile acid cotransporter family), member 3	Animals	*SLC10-3 of Bos taurus* (Q0V8N6)**

2.A.28.1.6	solute carrier family 10 (sodium/bile acid cotransporter family), member 5	Animals	SLC10A5 of Homo sapiens

2.A.28.1.7	Solute carrier family 10 (sodium/bile acid cotransporter family), member 4. The rat orthologue is found in cholinergic neurons of the brain together with the vesicular acetyl choline transporter, VAChT (TC# 2.A.1.2.28), and the high affinity choline transporter, CHT1 (TC#s 2.A.21.8.1 & 2) (Geyer et al. 2008). It is a protease-activated bile acid transporter (Abe et al. 2013). It has also been reported to be a vesicular monoaminergic and cholinergic associated transporter that is important for dopamine homeostasis and neuromodulation in vivo, and it may play a role in neurotransmitter release at the neuromuscular junction (Larhammar et al. 2015; Patra et al. 2015). It's loss in mice results in cognitive impairment (Melief et al. 2016).	Animals	SLC10A4 of Homo sapiens

2.A.28.1.8	P3 protein (Solute carrier family 10 member 3)	Animals	SLC10A3 of Homo sapiens

2.A.28.1.9	Sodium/bile acid cotransporter (Cell growth-inhibiting gene 29 protein; Na^+/bile acid cotransporter; Na⁺/taurocholate transport protein; NTCP; Solute carrier family 10 member 1). Transports steroids and xenobiotics, including HMG-CoA reductase inhibiitors (statins). This protein is the hepatitis B and D virus receptor (Yan et al. 2012). Specific inhibitors are known and include cyclosporin A (Wu et al. 2020, Slijepcevic and van de Graaf 2017). hNTCP-membrane vesicles effectively prevent viral infection, spreading, and replication in a human-liver-chimeric mouse model of HBV infection (Liu et al. 2018). The dye, indocyaine green (ICG), is a substrate, and NTCP and ICG form a reporter system with applications in cancer biology, robust drug-drug interactions, and drug screening in HBV/HDV infections (Wu et al. 2020). Structural plasticity is a feature of rheostat positions in the Human Na⁺/taurocholate cotransporting polypeptide (NTCP) (Ruggiero et al. 2022). Findings on protein-protein interactions (PPIs) between NTCP and cofactors relevant for entry of the virus/NTCP receptor complex have been summarized (Zakrzewicz and Geyer 2023). Hepatitis B virus (HBV) is a globally prevalent human DNA virus responsible for over 250 million cases of chronic liver infections, leading to conditions such as liver inflammation, cirrhosis and hepatocellular carcinoma (HCC). Sodium taurocholate co-transporting polypeptide (NTCP) is highly expressed in human hepatocytes and functions as a bile acid (BA) transporter. NTCP is the receptor that HBV and its satellite virus, hepatitis delta virus (HDV), use to enter hepatocytes. HBV entry into hepatocytes is tightly regulated by various signaling pathways, and NTCP plays an important role in the initial stage of HBV infection. NTCP acts as an initiation signal, causing metabolic changes in hepatocytes, facilitating the entry of HBV into hepatocytes. Li et al. 2024 examined the regulatory mechanisms governing HBV pre-S1 binding to liver membrane NTCP, the role of NTCP in HBV internalization, and the transcriptional and translational regulation of NTCP expression. Additionally, they discussed clinical drugs targeting NTCP, including combination therapies involving NTCP inhibitors. Sodium taurocholate co-transporting polypeptide (NTCP) has been identified as an entry receptor for hepatitis B virus (HBV). Manganese is a potent inducer of lysosomal activity that inhibits de novo hepatitis B virus (HBV) infection (Yu et al. 2025).	Animals	SLC10A1 of Homo sapiens

Examples:
TC#	Name	Organismal Type	Example
2.A.28.2.1	The chloroplastic glucosinolate uptake porter, BAT5 (Gigolashvili et al., 2009) [glucosinolates are thioglucosides of amino acid derivatives. These bitter natural pesticides are present in most plants of the order Brassicales among others].	Plants	*BAT5 of Arabidopsis thaliana* (Q3EA49)**

2.A.28.2.2	*Chloroplast envelope membrane pyruvate:Na⁺ symporter, called Bile acid:sodium symporter, protein 2, BASS2 (widely distributed in all land plants tested) (Furumoto et al., 2011).*	Plants	*BASS2 of Flaveria trinervia* (E0D3H5)**

2.A.28.2.3	Chloroplast envelope membrane pyruvate:Na⁺ symporter, BASS2. (Orthologous to 2.A.28.2.2) (Furumoto et al., 2011; Furumoto 2016). The wheat ortholog functions in salt tolerance (Zhao et al. 2016).	Plants	*BASS2 pf Arabidopsis thaliana* (Q1EBV7)**

2.A.28.2.4	Na⁺:bile acid symporter (AstB). The 3-d structure is available (3ZUX).	Bacteria	*AstB of Neisseria meningitidis* (Q9K0A9)**

2.A.28.2.5	Putative Na⁺ symporter	Bacteria	*YqcL of Paenibacillus* sp. JDR-2 (C6CWW0)**

2.A.28.2.6	*Probable macrolide resistance porter (very similar to the orthologue in B. brevis) (Margolles et al.* 2005).**	Actinobacteria	Macrolide resistance protein of Bifidobacterium longum

2.A.28.2.7	Putative Na⁺ symporter (10 TMSs)	Archaea	*Putative Na⁺ symporter of Halomicrobium mukohataei* (C7NY93)**

2.A.28.2.8	Putative integral membrane protein	Actinobacteria	Putative integral membrane protein of Streptomyces coelicolor

2.A.28.2.9	Sodium bile acid symporter family protein, ASBT, of 307 aas and 10 TMSs. The 3-d structure has been solved (4N7W and 4N7X) (Zhou et al. 2014). This structure has been used to model the yeast Acr3 protein (TC# 2.A.59.1.1) which is in a distinct family of the BART superfamily (Wawrzycka et al. 2016).		ASBT of Yersinia frederiksenii

Examples:
TC#	Name	Organismal Type	Example
2.A.28.3.1	Solute carrier family 10, member 7 protein (358 aas. Present in the plasma membrane with 10 established TMSs with the N- and C-termini in the cytoplasm (Godoy et al. 2007)) (Zou et al., 2005). Slc10a7 KO mice exhibit moderate skeletal dysplasia (osteochondrodyplasia), characterized by markedly shortened and mildly bowed limbs (Brommage et al. 2014). SLC10A7 plays roles in glycosaminoglycan synthesis and skeletal development, and mutants in its gene can cause skeletal dysplasia in mice and humans (Dubail et al. 2018). It plays a role in bone mineralization and transport of glycoproteins to the extracellular matrix (Ashikov et al. 2018). However it has been reported to not show transport activity towards bile acids and steroid sulfates (including taurocholate, cholate, chenodeoxycholate, estrone-3-sulfate, dehydroepiandrosterone sulfate (DHEAS) and pregnenolone sulfate) (Godoy et al. 2007). SLC10A7 regulates O-GalNAc glycosylation and Ca²⁺ homeostasis in the secretory pathway (Durin et al. 2025).	Animals	SLC10A7 of Homo sapiens

2.A.28.3.2	Putative Na⁺-dependent transporter	Plants	*Putative transporter of Arabidopsis thaliana* (Q9LYM5)**

2.A.28.3.3	Putative Na⁺-dependent transporter	Bacteria	Putative transporter of Paracoccus denitrificans

2.A.28.3.4	Putative Na⁺-dependent transporter, YfeH (332 aas; 7-10 TMSs)	Bacteria	*YfeH of E. coli* (P39836)**

2.A.28.3.5	Putative Na⁺-dependent transporter (322 aas; 8-10 TMSs)	Bacteria	*Transporter of Lentisphaera araneosa* (A6DUG7)**

2.A.28.3.6	Fusion Protein of 928aas: N-terminal Cysteine proteinase/Cathepsin F (residues 1-578/Peptidase CIA family) C-terminal BART sugar family domain (579-928).	Plants	*Protease/transporter fusion protein of Ostreococcus tauri* (Q01E11)**

2.A.28.3.7	RCh1p transporter (SLC10 family). Regulates cytosolic Ca²⁺ homeostasis (Jiang et al., 2012). Rch1p is part of the low-affinity calcium uptake system (LACS) system and does not functionally interact with Cch1p (Alber et al. 2013). It regulates O-GalNAc glycosylation as well as Ca²⁺ homeostasis in the secretory pathway: insights into SLC10A7-CDG (congenital disorders of glycosylation) (Durin et al. 2025).	Yeast	*RCh1p of Candida albicans* (Q59UQ7)**

Examples:
TC#	Name	Organismal Type	Example
2.A.28.4.1	Uncharacterized protein of 360 aas with an N-terminal hydrophobic domain of 4 - 5 TMSs homologous to members of this family. The C-terminal hydrophilic domain may be related to TC# 1.C.96, and 1.C.96 may be related to 1.C.5 in the aerolysin superfamily.		UP of Crassostrea gigas (Pacific oyster) (Crassostrea angulata)