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 alpha subunit, resulting in exchange of guanine diphosphate (GDP) for guanine triphosphate (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 rates of GDP release, GTP binding [2-8], and GTP hydrolysis[9, 10]. Here we report the discovery of a nonreceptor GEF, Arr4. Like receptors, Arr4 binds directly to the G protein,accelerates guanine-nucleotide exchange, and stabilizes the nucleotide-free state of the a subunit. Moreover, Arr4 promotes G protein-dependent cellular responses, including mitogen-activated protein kinase (MAPK) phosphorylation,new-gene transcription, and mating. In contrast to knownGPCRs, 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.
Lee
Michael J
MJ
Department of Pharmacology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7365, USA.
Dohlman
Henrik G
HG
D000251:Adenosine Triphosphatases; D019204:GTP-Binding Proteins / Q000378:metabolism*; D015966:Gene Expression Regulation, Fungal / Q000502:physiology; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics* / Q000378:metabolism*; D010675:Pheromones / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics* / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology*; D013997:Time Factors
D020662:Guanine Nucleotide Exchange Factors; D010675:Pheromones; D011956:Receptors, Cell Surface; D029701:Saccharomyces cerevisiae Proteins; D000251:Adenosine Triphosphatases; D019204:GTP-Binding Proteins; C529048:Get3 protein, S cerevisiae