Global functions of extracellular, transmembrane and cytoplasmic domains of organic solute transporter ß-subunit.

ABSTRACT

Transport of bile acids across the basolateral membrane of the intestinal enterocyte is carried out by the organic solute transporter (Ost) composed of a seven-transmembrane domain (TMD) subunit (Ostα) and an ancillary single TMD subunit (Ostß). Although previous investigations have demonstrated the importance of the TMD of Ostß for its activity, further studies were conducted to assess the contributions of other regions of the Ostß subunit. Transport activity was retained when Ostß was truncated to contain only the TMD with 15 additional residues on each side and co-expressed with Ostα, whereas shorter fragments were inactive. To probe the broader functions of Ostß segments, chimeric proteins were constructed in which N-terminal, TMD or C-terminal regions of Ostß were fused to corresponding regions of receptor activity-modifying protein (RAMP1), a single TMD protein required by several seven-TMD G-protein-coupled receptors including the calcitonin receptor-like receptor (CLR). Ostß/RAMP1 chimeras were expressed with Ostα and CLR. As expected, replacing the Ostß TMD abolished transport activity; however, replacing either the entire N-terminal or entire C-terminal domain of Ostß with RAMP1 sequences did not prevent plasma membrane localization or the ability to support [3H]taurocholate uptake. Co-immunoprecipitation experiments revealed that the C-terminus of Ostß is a previously unrecognized site of interaction with Ostα. All chimeras containing N-terminal RAMP1 segments allowed co-expressed CLR to respond to agonists with strong increases in cyclic AMP. These results provide new insights into the structure and function of the heteromeric Ost transporter complex.