Fig. S5.

Proposed mechanisms of GPCR priming. Agonist-binding to a Gs-coupled GPCR canonically activates Gαs in a heterotrimeric G-protein complex (Top, “Canonical pathway”). GPCR priming (red conformational state) may be attained by three parallel mechanisms (blue box). First, a G protein undergoes association and dissociation from the “Cognate site” of the GPCR. Although the G protein itself may or may not be activated, the GPCR is primed for subsequent interactions. Second, two G-protein entities bind to distinct cognate and allosteric sites on the same GPCR. The G protein bound to the “Allosteric site” primes the GPCR to activate the cognate G protein. Interaction of the two G proteins may be simultaneous or sequential. Third, an “Asymmetric dimer” composed of two GPCR–G-protein pairs primes signaling through one of the G proteins. Population of the primed state (blue box) increases G-protein activation, and consequently augments downstream signaling.