Signal anticipation in a GPCR pathway—Single cell measurements of scaffold recruitment

ALEJANDRO COLMAN LERNER

Montevideo, Uruguay

Conferencia; International Society of Computational Biology-Latin America; 2010

International Society of Computational Biology

Cells sense their chemical environment using receptor proteins that bind to extracellular molecules reversibly. However, the time it takes for binding and unbinding to equilibrate can be long and it might impose limits to the ability of cells to respond accurately to fast changing environments. In Saccharomyces cerevisiae an intracellular signal generated by a G-protein coupled receptor – MAP kinase signaling system precisely encodes receptor occupancy at equilibrium. However, the rates of α factor pheromone binding and unbinding to receptor are notably slow, potentially slowing cells response to fast changes in α factor. Here we show using quantitative confocal fluorescence microscopy that downstream events activated by the α factor-receptor complex, instead of reflecting the present level of receptor occupancy, “anticipate” equilibrium levels of receptor occupancy. For the dose that elicits semi-maximal activity, G protein-mediated recruitment of the MAP kinase cascade scaffold Ste5 to the plasma membrane has a response time (time to 50% recruitment) of ~60 seconds, ~20 fold faster than the α factor-receptor response time. We developed a simple mathematical model that captures this behavior. The model shows that anticipation may arise by a combination of an initial supersensitivity of the G protein module to its input (occupied receptor) followed by a gradual desensitization. Model suggested and our experiments support that negative feedback leading to slow (in the same order as receptor dynamics) activation of the Gα GAP protein Sst2 may underlie anticipation. Thus, our results suggest that G-protein-based signaling systems are capable to adjust to changes in ligand concentration much faster than the time it takes to the receptor-ligand complex to reach equilibrium. This hypothesis provides a plausible explanation for the fast response observed in several cell signaling systems and suggests that mutations that cause disease and drug treatments may act by affecting time-integration and response timing.