Agonist occupancy of a single monomeric element is sufficient to cause internalization of the dimeric β2-adrenoceptor

A range of studies have indicated that many rhodopsin-like, family A G protein-coupled receptors, including the β2-adrenoceptor, exist and probably function as dimers. It is less clear if receptors internalize as dimers and if agonist occupancy of only one element of a dimer is sufficient to cause internalization of a receptor dimer into the cell. We have used a chemogenomic approach to demonstrate that this is the case. Following expression of the wild type β2-adrenoceptor, isoprenaline but not 1-(3′'4′-dihydroxyphenyl)-3-methyl-1-butanone, which does not have significant affinity for the wild type receptor, caused receptor internalization. By contrast, 1-(3′4′-dihydroxyphenyl)-3-methyl-1-butanone, but not isoprenaline that does not have high affinity for the mutated receptor, caused internalization of Asp113Serβ2-adrenoceptor. Following co-expression of wild type and Asp113Serβ2-adrenoceptors each of isoprenaline and 1-(3′4′-dihydroxyphenyl)-3-methyl-1-butanone caused the co-internalization of both of these two forms of the receptor. Co-expressed wild type and Asp113Serβ2-adrenoceptors were able to be co-immunoprecipitated and 1-(3′4′-dihydroxyphenyl)-3-methyl-1-butanone produced internalization of the wild type receptor that was not prevented by the β-adrenoceptor antagonist propranolol that binds with high affinity only to the wild type receptor. These results demonstrate that agonist occupancy of either single binding site of the β2-adrenoceptor dimer is sufficient to cause internalization of the dimer and that antagonist occupation of one of the two ligand binding sites is unable to prevent agonist-mediated internalization.