Protein-protein interactions and membrane localization of the human organic solute transporter

Two proteins that mediate bile acid export from the ileal enterocyte, organic solute transporter (OST)-α and -β, have recently been identified. It is unclear whether these two proteins associate directly and how they interact to mediate transport function and membrane localization. In this study, the protein-protein interactions, transport functions, and membrane localization of human (h)OST-α and -β proteins were examined. The results demonstrated that coexpression of hOST-α and -β in transfected cells resulted in a three- to fivefold increase of the initial rate of taurocholate influx or efflux compared with cells expressing each protein individually and nontransfected cells. Confocal microscopy demonstrated plasma membrane colocalization of hOST-α and -β proteins in cells cotransfected with hOST-α and -β cDNAs. Protein-protein interactions between hOST-α and -β were demonstrated by mammalian two-hybrid and coimmunoprecipitation analyses. Truncation of the amino-terminal 50 amino acid extracellular residues of hOST-α abolished its interaction with hOST-β and led to an intracellular accumulation of the two proteins and to only background levels of taurocholate transport. In contrast, carboxyl-terminal 28 amino acid truncated hOST-α still interacted with hOST-β, and majority of this cytoplasmic tail-truncated protein was expressed on the basolateral membrane when it was stably cotransfected with hOST-β protein in Madin-Darby canine kidney cells. In summary, hOST-α and -β proteins are physically associated. The intracellular carboxyl-terminal domain of hOST-α is not essential for this interaction with hOST-β. The extracellular amino-terminal fragment of hOST-α may contain important information for the assembly of the heterodimer and trafficking to the plasma membrane.