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.

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).

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.

Proteins of the BASS family are homologous to members of the Acr3 family (2.A.59) of arsenite/antimonite efflux carriers. We have designated the superfamily comprising the BASS and Acr3 families the Anion Transporter (AT) Superfamily.

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).

The generalized transport reaction catalyzed by members of the BASS family is:

organic acid (out) + Na⁺ (out) → organic acid (in) + Na⁺ (in).