Regulators of G-protein signaling form a quaternary complex with the agonist, receptor, and G-protein - A novel explanation for the acceleration of signaling activation kinetics

Regulators of G-protein signaling (RGS) proteins modulate signaling through heterotrimeric G-proteins. They act to enhance the intrinsic GTPase activity of the G alpha subunit but paradoxically have also been shown to enhance receptor-stimulated activation. To study this paradox, we used a G-protein gated K+ channel to report the dynamics of the G-protein cycle and fluorescence resonance energy transfer techniques with cyan and yellow fluorescent protein-tagged proteins to report physical interaction. Our data show that the acceleration of the activation kinetics is dissociated from deactivation kinetics and dependent on receptor and RGS type, G-protein isoform, and RGS expression levels. By using fluorescently tagged proteins, fluorescence resonance energy transfer microscopy showed a stable physical interaction between the G-protein alpha subunit and RGS (RGS8 and RGS7) that is independent of the functional state of the G-protein. RGS8 does not directly interact with G-protein-coupled receptors. Our data show participation of the RGS in the ternary complex between agonist-receptor and G-protein to form a "quaternary complex." Thus we propose a novel model for the action of RGS proteins in the G-protein cycle in which the RGS protein appears to enhance the "kinetic efficacy" of the ternary complex, by direct association with the G-protein alpha subunit.