Coactivation of G Protein Signaling by Cell-Surface Receptors and an Intracellular Exchange Factor

G protein-coupled receptors (GPCRs) mediate responses to a broad range of chemical and environmental signals. In yeast a pheromone-binding GPCR triggers events leading to the fusion of haploid cells. In general, GPCRs function as guanine nucleotide exchange factors (GEFs); upon agonist binding the receptor induces a conformational change in the G protein α subunit, resulting in exchange of GDP for GTP and in signal initiation. Signaling is terminated when GTP is hydrolyzed to GDP [1]. This well-established paradigm has in recent years been revised to include new components that alter the rates of GDP release, GTP binding [2-8], and GTP hydrolysis [9, 10]. Here we report the discovery of a non-receptor GEF, Arr4. Like receptors, Arr4 binds directly to the G protein, accelerates guanine nucleotide exchange, and stabilizes the nucleotide-free state of the α subunit. Moreover, Arr4 promotes G protein-dependent cellular responses including mitogen-activated protein kinase (MAPK) phosphorylation, new gene transcription and mating. In contrast to known GPCRs, however, Arr4 is not a transmembrane receptor, but rather a soluble intracellular protein. Our data suggest that intracellular proteins function in cooperation with mating pheromones to amplify G protein signaling, thereby leading to full pathway activation