Single cell analyses of fate decisions in yeast

ALEJANDRO COLMAN LERNER

Homerton College, Cambridge, UK

Conferencia; ADVANCES IN STEM CELL RESEARCH: Stem cells, systems & synthetic biology; 2009

European Molecular Biology Organization (EMBO)

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 limits the ability of cells to respond accurately to fast changes in the signal. 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 (kon= 1.67 104 M-1 s-1, koff= 9 10-4 s-1), limiting the ability of cells to respond accurately and fast to changes in α factor. We show using 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 ~30 seconds, a ~13 fold acceleration with respect to the α factor-receptor response time. We developed a simple model that captures this behavior. Our results suggest that G-protein-based signaling systems are capable to adjust to changes in concentration of ligand 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.