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Physics Special Colloquium, January 31, 2005Long-range Cooperativity in Yeast Mating PolarizationChinlin GuoHarvard UniversityPolarity formation is important for eukaryote cell development and differentiation but the general mechanism is yet not well understood. Haploid budding yeast cells exist in two mating types. When they meet, pheromone-mediated communication induces mating projections (shmoo) that fuse with each other to form diploid. The ability to control the external pheromone profile and genetically manipulate the responding cells makes budding yeast an attractive model system to test different models. We marked several polarized proteins to follow cellular responses to uniform fields and defined gradients of pheromone. The dynamics are complex, resulting in cells eventually polarizing along a single axis. We observed large-scale symmetry breaking in which only one half of the cell is activated and small-scale focusing in which the activation is converged into a small region. Using cytoskeleton depolymerizing agent reveals that polarity formation depends on long range cooperativity. The experiments also exclude models based on lateral inhibition produced by short-range cooperativity and by reaction-diffusion schemes. We proposed a different model which predicts the key feature of pheromone response, the formation of a single, actin-dependent polarization axis at all levels of homogeneous stimulation. We also addressed how gradient detection could be affected by this model. 10:30 a.m., Smith Laboratory, Room 1094Refreshments served in Smith 1094 at 10:00 a.m. |
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