INVESTIGADORES
COLMAN LERNER Alejandro Ariel
congresos y reuniones científicas
Título:
Robustness and information processing in a signal transduction pathway
Autor/es:
ALEJANDRO COLMAN LERNER; ALAN BUSH; GUSTAVO VASEN
Reunión:
Congreso; Winter Q-bio 2017; 2017
Resumen:
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 changes 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. A mathematical model of this step suggested that fractional occupancy depended on the physical interaction between the inhibitory Regulator of G-protein Signaling (RGS), Sst2, and the receptor. These GPCR-RGS complexes have opposing activities: they activate or inactivate G-proteins, depending on receptor occupancy. Therefore, the activation state of a given G protein (ie, GTP or GDP-bound) depends only on the ligand-occupancy state of the last GPCR-RGS complex it visited, independently on the absolute number of GPCRs. To test this core prediction of the model, we uncoupled the receptor from the RGS by replacing Sst2 by the heterologous hsRGS4, incapable of interacting with the receptor. The uncoupled system lacked robustness to changes in receptor abundance. Conversely, forcing hsRGS4:Ste2 interaction, restored robustness. In a second test of the model, we reduced the fractional occupancy while maintaining the absolute occupancy constant by co-expressing WT and mutant receptors locked in the unbound state. As predicted by our model, this system showed a correspondingly reduced signaling, but only if the mutant receptors could interact with the RGS. Taken together, our results show that the yeast GPCR pathway computes fractional occupancy because ligand-bound Ste2-Sst2 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 fractional occupancy by a ratiometric measurement. Such complexes operate as push-pull devices, which we have recently described (Andrews et al, 2016).