Oligomerization of the α1a- and α1b-Adrenergic Receptor Subtypes

We combined biophysical, biochemical, and pharmacological approaches to investigate the ability of the α1a- and α1b-adrenergic receptor (AR) subtypes to form homo- and hetero-oligomers. Receptors tagged with different epitopes (hemagglutinin and Myc) or fluorescent proteins (cyan and green fluorescent proteins) were transiently expressed in HEK-293 cells either individually or in different combinations. Fluorescence resonance energy transfer measurements provided evidence that both the α1a- and α1b-AR can form homo-oligomers with similar transfer efficiency of ∼0.10. Hetero-oligomers could also be observed between the α1b- and the α1a-AR subtypes but not between the α1b-AR and the β2-AR, the NK1 tachykinin, or the CCR5 chemokine receptors. Oligomerization of the α1b-AR did not require the integrity of its C-tail, of two glycophorin motifs, or of the N-linked glycosylation sites at its N terminus. In contrast, helix I and, to a lesser extent, helix VII were found to play a role in the α1b-AR homo-oligomerization. Receptor oligomerization was not influenced by the agonist epinephrine or by the inverse agonist prazosin. A constitutively active (A293E) as well as a signaling-deficient (R143E) mutant displayed oligomerization features similar to those of the wild type α1b-AR. Confocal imaging revealed that oligomerization of the α1-AR subtypes correlated with their ability to co-internalize upon exposure to the agonist. The α1a-selective agonist oxymetazoline induced the co-internalization of the α1a- and α1b-AR, whereas the α1b-AR could not co-internalize with the NK1 tachykinin or CCR5 chemokine receptors. Oligomerization might therefore represent an additional mechanism regulating the physiological responses mediated by the α1a- and α1b-AR subtypes.