Oligomerization of the α1a- and α1b-Adrenergic Receptor Subtypes

We combined biophysical, biochemical, and pharmacological approaches to investigate the ability of the α 1a - and α 1b -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 α 1a - and α 1b -AR can form homo-oligomers with similar transfer efficiency of ∼0.10. Hetero-oligomers could also be observed between the α 1b - and the α 1a -AR subtypes but not between the α 1b -AR and the β 2 -AR, the NK1 tachykinin, or the CCR5 chemokine receptors. Oligomerization of the α 1b -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 α 1b -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 α 1b -AR. Confocal imaging revealed that oligomerization of the α 1 -AR subtypes correlated with their ability to co-internalize upon exposure to the agonist. The α 1a -selective agonist oxymetazoline induced the co-internalization of the α 1a - and α 1b -AR, whereas the α 1b -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 α 1a - and α 1b -AR subtypes.