Domain Organization of Bile Salt Transporter (BSEP) BSEP is a member of the MDR/TAP subfamily. Transporters of this subfamily have the general form of (TMD-ABC)2 (TMD, transmembrane domain; ABC, ATP-binding cassette) although there are half transporters. The TMDs are composed of six helixes buried in the lipid bilayer of plasma membrane and the ATP binding ABCs are exposed in the cytosol. The TMD helixes are envisioned to form channels spanning the plasma membrane. There is experimental evidence for multiple substrate binding sites associated with the TMDs. Half transporters form tight functional coupling in multimolecular complexes.
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