Yeast GPCR signaling reflects the fraction of occupied receptors, not the number.

ANDREAS CONSTANTINOU; ALAN BUSH; MATÍAS BLAUSTEIN; PAULA DUNAYEVICH; ALEJANDRO COLMAN-LERNER; GUSTAVO VASEN; INÉS LUCÍA PATOP

Águas de Lindóia, São Paulo

Congreso; 46th Annual Meeting of the Brazilian Society for Biochemistry and Molecular Biology; 2017

SBBq

According to receptor theory, the effect of a ligand depends on the amount of agonist?receptor complex. Therefore, changes in recep- tor 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 occu- pancy because ligand-bound GPCR?RGS complexes stimulate signaling while unoccupied complexes actively inhibit it. In eukary- otes, 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.