Constitutive Formation of Oligomeric Complexes between Family B G Protein-Coupled Vasoactive Intestinal Polypeptide and Secretin Receptors

Formation of oligomeric complexes of family A G protein-coupled receptors has been shown to influence their function and regulation. However, little is known about the existence of such complexes for family B receptors in this superfamily. We previously used bioluminescence resonance energy transfer (BRET) to demonstrate that the prototypic family B secretin receptor forms ligand-independent oligomeric complexes. Here, we show that subtypes of human vasoactive intestinal polypeptide receptors (VPAC1 and VPAC2) that represent the closest structurally related receptors to the secretin receptor also form constitutive oligomers with themselves and with the secretin receptor. We prepared tagged constructs expressing Renilla reniformis luciferase, yellow fluorescent protein, or cyan fluorescent protein at the carboxyl terminus of VPAC1, VPAC2, and secretin receptors, and performed BRET and morphologic fluorescence resonance energy transfer (FRET) studies with all combinations. The specificity of the BRET and FRET signals was confirmed by control studies. These constructs bound their natural ligands specifically and saturably, with these agonists able to elicit full cAMP responses. BRET studies showed that, like the secretin receptor, both VPAC receptors exhibited constitutive homo-oligomerization in COS cells. Unlike secretin receptor oligomers that were unaffected by ligand binding, the VPAC receptor homo-oligomers were modulated by vasoactive intestinal polypeptide. In addition, each of these three receptors formed hetero-oligomers with each other. The VPAC1-VPAC2 hetero-oligomers were modulated by vasoactive intestinal polypeptide binding, whereas the secretin-VPAC1 and secretin-VPAC2 receptor hetero-oligomers were unaffected by ligand treatment. Morphologic FRET studies demonstrated that each of the homo-oligomers and the VPAC1-VPAC2 receptor hetero-oligomers reached the cell surface, where receptor interactions were clear. However, coexpression of secretin receptors with either type of VPAC receptor resulted in intracellular trapping of the hetero-oligomeric complexes within the biosynthetic pathway. These studies provide new insight into the ability of family B G protein-coupled receptors to associate with each other in cells.