Yeast cells show prominent polarization during the mating process, when two cells of opposite mating type extend cellular projections towards each other in response to pheromone signalling. Fission yeast cells exist as homothallic strains, in which cells can switch mating type after division, yielding sister-cells of opposite mating types. Each of these mating types secretes a pheromone which is recognized by a cognate receptor on the partner cell and activates a downstream signalling cascade. We are interested in exploring the mechanisms by which cells re-orient their polarization machinery towards a mating partner.
We have shown that, in the early stages of the mating process before cells have selected a mating partner, or upon exposure to low-level pheromone levels, fission yeast cells display a dynamic site of polarization. The small GTPase Cdc42, a central eukaryotic regulator of cell polarization, forms clusters that appear and disappear dynamically at the plasma membrane. The timelapse shows cells of distinct mating types, in which activators of Cdc42, which are co-dynamic with Cdc42, have been labelled green or red. The green-labeled cell appear to explore several potential partner before settling on one for fusion.
Direct effectors of Cdc42, such as formin and exocyst subunits, also colocalize to these dynamic clusters, but not cell wall synthases. Consistently, cells do not grow during this phase. When cells experiment high-level pheromone signalling, they bypass this dynamic polarization phase, fail to orient growth towards neighbouring mating partners and mate preferentially to sister-cells. We are interested in defining the molecular mechanisms behind this dynamic polarization behaviour and partner choice.
We also explore the mechanism of cell-cell fusion to produce a diploid progeny.