Priming GPCR signaling through the synergistic effect of two G proteins

Although individual G-protein-coupled receptors (GPCRs) are known to activate one or more G proteins, the GPCR-G-protein interaction is viewed as a bimolecular event involving the formation of a ternary ligand-GPCR-G-protein complex. Here, we present evidence that individual GPCR-G-protein interactions can reinforce each other to enhance signaling through canonical downstream second messengers, a phenomenon we term GPCR priming. Specifically, we find that the presence of noncognate Gq protein enhances cAMP stimulated by two Gs-coupled receptors, beta 2-adrenergic receptor (beta 2-AR) and D-1 dopamine receptor (D-1-R). Reciprocally, Gs enhances IP1 through vasopressin receptor (V-1A-R) but not alpha 1 adrenergic receptor (alpha 1-AR), suggesting that GPCR priming is a receptor-specific phenomenon. The C terminus of either the Gas or Gaq subunit is sufficient to enhance Ga subunit activation and cAMP levels. Interaction of Gas or Gaq C termini with the GPCR increases signaling potency, suggesting an altered GPCR conformation as the underlying basis for GPCR priming. We propose three parallel mechanisms involving (i) sequential G-protein interactions at the cognate site, (ii) G-protein interactions at distinct allosteric and cognate sites on the GPCR, and (iii) asymmetric GPCR dimers. GPCR priming suggests another layer of regulation in the classic GPCR ternary-complex model, with broad implications for the multiplicity inherent in signaling networks.