Simulating polarized growth factor delivery in ssion yeast

How di erent cell types can produce di erent shapes is a fundamental question in cell biology. As a relatively simple eukaryotic single-cell organism that is easy to control and manipulate, ssion yeast (Schizosaccharomyces pombe) became one of the most intensely studied model systems of single-cell morphogenesis. Fission yeast cells have an elongated, rod-like shape with a highly reproducible diameter of ca. 3 μm and a cell-cycle-dependent length on the order of 8− 14 μm [90]. The rod-like shape results from restriction of cell growth to two opposite regions on the cell cortex that are marked by accumulations of growth factor proteins. Knock-out experiments have identi ed many key components of ssion yeast polarization. Intriguingly, they suggest that the localization of growth factors to the growing poles of the cell is established via a symmetry-breaking mechanism that combines active transport on the cytoskeleton, di usion on the membrane and shuttling of proteins between cytoplasm, cytoskeleton and membrane. To elucidate this interplay we developed a spatially-resolved model of growth factor polarization that features microtubules and a fully membrane-enclosed cell volume, and conducted stochastic simulations of the model with eGFRD. Our model is representative of the Tea1/Mod5-system, which is described in detail in section 4.2. It contains two species: a permanently membrane-bound species M (Mod5) and species T (Tea1), which can di use in the cytoplasm, bind microtubules on which it can drift towards the cell poles, and form TM-complexes with M on the membrane. The TM-complexes have a nite lifetime; after TM-complex dissociation T moves back into the cytoplasm. This sets up a cycle in which T is transported actively from the cytoplasm to the membrane at the poles and passively returns from the membrane to the cytoplasm via di usion on the membrane. In order to identify the components of the considered system that are critical to establish proper growth factor polarization we compared models with di erent ways