title: Yeast GPCR signaling reflects the fraction of occupied receptors, not the number.
abstract: According to receptor theory, the effect of a ligand depends on the amount of agonist-receptor complex. Therefore, changes in receptor abundance should have quantitative effects. However, the response to pheromone in Saccharomyces cerevisiae is robust (unaltered) to increases or reductions in the abundance of the G-protein-coupled receptor (GPCR), Ste2, responding instead to the fraction of occupied receptor. We found experimentally that this robustness originates during G-protein activation. We developed a complete mathematical model of this step, which suggested the ability to compute fractional occupancy depends on the physical interaction between the inhibitory regulator of G-protein signaling (RGS), Sst2, and the receptor. Accordingly, replacing Sst2 by the heterologous hsRGS4, incapable of interacting with the receptor, abolished robustness. Conversely, forcing hsRGS4:Ste2 interaction restored robustness. Taken together with other results of our work, we conclude that this GPCR pathway computes fractional occupancy because ligand-bound GPCR-RGS complexes stimulate signaling while unoccupied complexes actively inhibit it. In eukaryotes, many RGSs bind to specific GPCRs, suggesting these complexes with opposing activities also detect fraction occupancy by a ratiometric measurement. Such complexes operate as push-pull devices, which we have recently described.
authors.affiliation: Department of Physiology, Molecular and Cellular Biology, University of Buenos Aires, Buenos Aires, Argentina.|Institute of Physiology, Molecular Biology and Neurosciences, National Research Council (CONICET), Buenos Aires, Argentina.
authors.forename: Alan
authors.identifier: http://orcid.org/0000-0002-0407-5750
authors.initials: A
authors.lastname: Bush
authors.affiliation: Department of Physiology, Molecular and Cellular Biology, University of Buenos Aires, Buenos Aires, Argentina.|Institute of Physiology, Molecular Biology and Neurosciences, National Research Council (CONICET), Buenos Aires, Argentina.
authors.forename: Gustavo
authors.initials: G
authors.lastname: Vasen
authors.affiliation: Department of Physiology, Molecular and Cellular Biology, University of Buenos Aires, Buenos Aires, Argentina.|Institute of Physiology, Molecular Biology and Neurosciences, National Research Council (CONICET), Buenos Aires, Argentina.
authors.forename: Andreas
authors.initials: A
authors.lastname: Constantinou
authors.affiliation: Department of Physiology, Molecular and Cellular Biology, University of Buenos Aires, Buenos Aires, Argentina.|Institute of Physiology, Molecular Biology and Neurosciences, National Research Council (CONICET), Buenos Aires, Argentina.
authors.forename: Paula
authors.initials: P
authors.lastname: Dunayevich
authors.affiliation: Department of Physiology, Molecular and Cellular Biology, University of Buenos Aires, Buenos Aires, Argentina.|Institute of Physiology, Molecular Biology and Neurosciences, National Research Council (CONICET), Buenos Aires, Argentina.
authors.forename: Inés Lucía
authors.initials: IL
authors.lastname: Patop
authors.affiliation: Department of Physiology, Molecular and Cellular Biology, University of Buenos Aires, Buenos Aires, Argentina.|Institute of Physiology, Molecular Biology and Neurosciences, National Research Council (CONICET), Buenos Aires, Argentina.
authors.forename: Matías
authors.initials: M
authors.lastname: Blaustein
authors.affiliation: Department of Physiology, Molecular and Cellular Biology, University of Buenos Aires, Buenos Aires, Argentina colman-lerner@fbmc.fcen.uba.ar.|Institute of Physiology, Molecular Biology and Neurosciences, National Research Council (CONICET), Buenos Aires, Argentina.
authors.forename: Alejandro
authors.identifier: http://orcid.org/0000-0002-2557-8883
authors.initials: A
authors.lastname: Colman-Lerner
mesh_terms: D020690:GTPase-Activating Proteins / Q000378:metabolism*; D006801:Humans; D008962:Models, Theoretical; D011485:Protein Binding; D020710:RGS Proteins / Q000378:metabolism; D051964:Receptors, Mating Factor / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism*
chemical_list: D020690:GTPase-Activating Proteins; D020710:RGS Proteins; D051964:Receptors, Mating Factor; C094301:SST2 protein, S cerevisiae; C498031:STE2 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; C098799:RGS4 protein
keywords: fraction measurement; paradoxical components; ratiometric signaling; robustness