Abstract
Many G-protein-coupled receptors, including the alpha(1b)-adrenoceptor, form homo-dimers or oligomers. Mutation of hydrophobic residues in transmembrane domains I and IV alters the organization of the alpha(1b)-adrenoceptor oligomer, with transmembrane domain IV playing a critical role. These mutations also result in endoplasmic reticulum trapping of the receptor. Following stable expression of this alpha(1b)-adrenoceptor mutant, cell surface delivery, receptor function and structural organization were recovered by treatment with a range of alpha(1b)-adrenoceptor antagonists that acted at the level of the endoplasmic reticulum. This was accompanied by maturation of the mutant receptor to a terminally N-glycosylated form, and only this mature form was trafficked to the cell surface. Co-expression of the mutant receptor with an otherwise wild-type form of the alpha(1b)-adrenoceptor that is unable to bind ligands resulted in this wild-type variant also being retained in the endoplasmic reticulum. Ligand-induced cell surface delivery of the mutant alpha(1b)-adrenoceptor now allowed co-recovery to the plasma membrane of the ligand-binding-deficient mutant. These results demonstrate that interactions between alpha(1b)-adrenoceptor monomers occur at an early stage in protein synthesis, that ligands of the alpha(1b)-adrenoceptor can act as pharmacological chaperones to allow a structurally compromised form of the receptor to pass cellular quality control, that the mutated receptor is not inherently deficient in function and that an oligomeric assembly of ligand-binding-competent and -incompetent forms of the alpha(1b)-adrenoceptor can be trafficked to the cell surface by binding of a ligand to only one component of the receptor oligomer.