Cell surface delivery and structural re-organization by pharmacological chaperones of an oligomerization-defective α1b-adrenoceptor mutant demonstrates membrane targeting of GPCR oligomers
Résumé
Many G protein-coupled receptors, including the α1b-adrenoceptor form homo-dimers or oligomers. Mutation of hydrophobic residues in transmembrane domains I and IV alters the organization of the α1b-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 α1b-adrenoceptor mutant cell surface delivery, receptor function and structural organization were recovered by treatment with a range of α1b-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 α1b-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 α1b-adrenoceptor now allowed co-recovery to the plasma membrane of the ligand binding deficient mutant. These results demonstrate that interactions between α1b-adrenoceptor monomers occur at an early stage in protein synthesis, that ligands of the α1b-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 α1b-adrenoceptor can be trafficked to the cell surface by binding of a ligand to only one component of the receptor oligomer.
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