Functional importance of a structurally distinct homodimeric complex of the family B G protein-coupled secretin receptor

Oligomerization of G protein-coupled receptors has been described, but its structural basis and functional importance have been inconsistent. Here, we demonstrate that the agonist occupied wild type secretin receptor is predominantly in a guanine nucleotide sensitive high affinity state and exhibits negative cooperativity, while the monomeric receptor is primarily in a guanine nucleotide insensitive lower affinity state. We previously demonstrated constitutive homo-dimerization of this receptor through the lipid-exposed face of TM IV. We now use cysteine-scanning mutagenesis of fourteen TM IV residues, bioluminescence resonance energy transfer (BRET), and functional analysis to map spatial approximations and functional importance of specific residues in this complex. All, except for three helix-facing mutants, trafficked to the cell surface, where secretin was shown to bind and elicit cAMP production. Cells expressing complementary-tagged receptors were treated with cuprous phenanthroline to establish disulfide bonds between spatially-approximated cysteines. BRET was measured as an indication of receptor oligomerization, and was repeated after competitive disruption of oligomers with TM IV peptide to distinguish covalent from non-covalent associations. While all constructs generated a significant BRET signal, this was disrupted by peptide in all except for single site mutants replacing five residues with cysteine. Of these, covalent stabilization of receptor homo-dimers through positions of Gly(243), Ile(247), and Ala(250) resulted in a GTP-sensitive high affinity state of the receptor, while the same procedure with Ala(246) and Phe(240) mutants resulted in a GTP-insensitive lower affinity state. We propose the existence of a functionally-important, structurally-specific high affinity dimeric state of the secretin receptor, which may be typical of Family B G protein-coupled receptors.