Abstract

Using combinations of bioluminescence resonance energy transfer, time-resolved fluorescence resonance energy transfer and the functional complementation of pairs of inactive receptor-G protein fusion proteins, the human α_1A-1-adrenoceptor was shown to form homodimeric/oligomeric complexes when expressed in human embryonic kidney (HEK) 293 cells. Saturation bioluminescence resonance energy transfer studies indicated the α_1A-1-adrenoceptor homodimer interactions to be high affinity and some 75 times greater than interactions between the α_1A-1-adrenoceptor and the δ opioid peptide receptor. Only a fraction of the α_1A-1-adrenoceptors was at the plasma membrane of HEK293 cells at steady state. However, dimers of α_1A-1-adrenoceptors were also present in intracellular membranes, and the dimer status of those delivered to the cell surface was unaffected by the presence of agonist. Splice variation can generate at least three forms of the human α_1A-1-adrenoceptor with differences limited to the C-terminal tail. Each of the α_1A-1, α_1A-2a, and α_1A-3a-adrenoceptor splice variants formed homodimers/oligomers, and all combinations of these splice variants were able to generate heterodimeric/oligomeric interactions. Despite the coexpression of these splice variants in human tissues that possess the pharmacologically defined α_1L-adrenoceptor binding site, coexpression of any pair in HEK293 cells failed to generate ligand binding characteristic of the α_1L-adrenoceptor.