A Predictive Model for Yeast Cell Polarization in Pheromone Gradients
Fig 4
Response to pheromone gradients and model prediction.
A) Producing pheromone gradients in a laminar flow chamber. Pheromone mixed with a fluorescent dextran and a dilution buffer enters through two ports and diffusion between the two fluid streams creates a temporally stable, gradient (see Experimental Procedures in S1 Text, and Fig C in S1 Text for more details). The left hand view is from the top of the apparatus and right hand views are two different magnifications of a cross-section, showing the cells attached to the coverslip that forms the roof of the chamber. B) The transition between budding and shmooing, quantified as in Fig 2B. The lightly shaded, thick curves show the data from spatially uniform pheromone concentrations (Fig 2B) for comparison. For every cell, the difference in concentration between the two edges of the cell was > 5% (expressed relative to the mean concentration that the cell experienced). The inset shows the standard deviation. C) The accuracy of gradient detection as a function of pheromone concentration. Accuracy is defined as the mean cosine of the angle between the gradient and the line that connects the Spa2 polar cap to the center of the cell. D) The accuracy of gradient detection as a function of pheromone concentration for solutions to our model. For different damping coefficients λ, the gradient detection and the standard deviation were computed. We observe that the value of λ which fits the data the best is 101.25 which means 1/λopt ≈ 3 mn. E) We show the timing of polarization in representative numerical simulations of cells exposed to a uniform field and gradient field of α-factor (images are shown every 20 minutes). F) Comparing the timing of polarization in individual cells exposed to a uniform field (top) and a gradient (bottom) of α-factor. Images were taken every 20 minutes and pseudocolored to indicate the intensity of Spa2-YFP fluorescence. Note the small unstable Spa2 spots (arrowheads) that appeared in the homogenously stimulated cell long before a stable polar cap, which took four hours to develop. In contrast, in the gradient, a small Spa2 spot first appeared in the direction of the gradient and then gradually grew stronger allowing the cell polarize much faster.