Abstract

G-protein-coupled receptors (GPCRs) represent the largest family of receptors involved in transmembrane signaling. Although these receptors were generally believed to be monomeric entities, accumulating evidence supports the presence of GPCRs in multimeric forms. Here, using immunoprecipitation as well as time-resolved fluorescence resonance energy transfer to assess protein-protein interactions in living cells, we unambiguously demonstrate the occurrence of dimerization of the human histamine H₁ receptor. We also show the presence of domain-swapped H₁ receptor dimers in which there is the reciprocal exchange of transmembrane domain TM domains 6 and 7 between the receptors present in the dimer. Mutation of aspartate¹⁰⁷ in transmembrane (TM) 3 or phenylalanine⁴³² in TM6 to alanine results in two radioligand-binding-deficient mutant H₁ receptors. Coexpression of H₁D¹⁰⁷ A and H₁F⁴³²A, however, results in a reconstituted radioligand binding site that exhibits a pharmacological profile that corresponds to the wild-type H₁ receptor. Interestingly, the H₁ receptor radioligands [³H]mepyramine and [³H]-(–)-trans-1-phenyl-3-N,N-dimethylamino-1,2,3,4-tetrahydronaphthalene show differential saturation binding values (B_max) for wild-type H₁ receptors but not for the radioligand binding site that is formed upon coexpression of H₁ D¹⁰⁷A and H₁ F⁴³²A receptors, suggesting the presence of different H₁ receptor populations.