The Specificity and Molecular Basis of α1-Adrenoceptor and CXCR Chemokine Receptor Dimerization

It is now well established that rhodopsin-like, family-A G protein-coupled receptors (GPCRs) can exist within homo- and heterodimeric/oligomeric complexes. However, limited information is currently available on the molecular basis of these interactions or their selectivity. Using the alpha1-adrenoceptor family as a model, this has been examined using assays including coimmunoprecipitation, saturation bioluminescence resonance energy transfer (BRET), time-resolved fluorescence resonance energy transfer (FRET), and bimolecular fluorescence complementation. We demonstrate key roles for transmembrane helices I and IV in homodimeric/oligomeric interactions of the alpha1b-adrenoceptor and suggest that other interactions indicate that this GPCR can exist as a higher-order oligomeric complex. Literature reports on heterodimerization between chemokine receptor family members and the effects or otherwise of agonist ligands are complex. It was recently indicated that although the CXCR2 receptor is able to homodimerize, this is not the case for the closely related CXCR1 receptor and that these two GPCRs do not heterodimerize. We have reinvestigated these issues using combinations of coimmunoprecipitation, saturation BRET, and a novel endoplasmic reticulum-trapping strategy. Unlike the previous report, we demonstrate that CXCR1 is able to both homodimerize and heterodimerize with the CXCR2 receptor and that the relative affinity of these interactions suggests that with coexpression of these two GPCRs a random mixture of homo- and heterodimers will be present.

Domain swapping in the human histamine H1 receptor

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 H(1) receptor. We also show the presence of domain-swapped H(1) 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 aspartate(107) in transmembrane (TM) 3 or phenylalanine(432) in TM6 to alanine results in two radioligand-binding-deficient mutant H(1) receptors. Coexpression of H(1)D(107) A and H(1)F(432)A, however, results in a reconstituted radioligand binding site that exhibits a pharmacological profile that corresponds to the wild-type H(1) receptor. Interestingly, the H(1) receptor radioligands [(3)H]mepyramine and [(3)H]-(-)-trans-1-phenyl-3-N,N-dimethylamino-1,2,3,4-tetrahydronaphthalene show differential saturation binding values (B(max)) for wild-type H(1) receptors but not for the radioligand binding site that is formed upon coexpression of H(1) D(107)A and H(1) F(432)A receptors, suggesting the presence of different H(1) receptor populations.