ROBUSTNESS AND INFORMATION PROCESSING IN A CELL FATE DECISION SYSTEM

ALEJANDRO COLMAN LERNER

Salta

Congreso; Congreso conjunto SAIB-PABMB 2019; 2019

SAIB y PABMB

Cell behavior requires the ability to detect and respond to extracellular signals. We use the response to mating pheromone in the budding yeast Saccharomyces cerevisiaeas a model system. I will present recent work that addresses two general problems signaling pathways must solve: eliciting robust responses despite large variability in cellular components and appropriate integration of extracellular and intracellular signals. Robustness: The effect of ligands usually depends on the amount of the ligand?receptor complex. Thus, changes in receptor abundance should have quantitative effects. However, the response to pheromone in yeast is robust (unaltered) to variation in the abundance of the GPCR receptor, Ste2, responding instead to the fraction of occupied receptor. To learn how fractional occupancy is measured by the cell, we developed a mathematical model of GPCR activation, which suggested that the ability to compute fractional occupancy depended on the physical interaction between the inhibitory RGS, Sst2, and the receptor. I will present here the experimental evidence that supports this model and explain how it might apply to other GPCR systems and to signal transduction pathways in general. Signal integration: The pheromone pathway has an antagonistic relationship with the cell cycle. The pathway MAPK (Fus3) arrests the cell cycle by activating a CDK inhibitor. Conversely, when cells pass START, the CDK (Cln2-Cdc28) blocks pheromone response by phosphorylating the scaffold protein Ste5, which prevents its association with the plasma membrane. We found that efficient Cln2-Cdc28 inhibition of Ste5 requires the help of Fus3. This is surprising since previous literature suggested that CDK should be able to do the blocking by itself. However, our results thus show that Fus3, which usually serves to promote mating, also serves to block mating during the wrong part of the cell cycle. We made this discovery thanks to our ability to perform fast single-cell measurement of Ste5 membrane recruitment in a large number of mutant strains. This approach, in combination with analyses of Ste5 phosphorylation via gel mobility assays and quantitative mass-spectrometry, led us to propose a detailed molecular model that I will present here to explain this unexpected MAPK-CDK collaboration.