Cell Size Control in Fission Yeast

Among all living organisms, there is almost much variety in cell size as there is for cell function and cell type. However, within each cell type, cells stay remarkably faithful to a defined size over generations. Many factors have been found to influence this ability to specify and maintain cell size, yet clear mechanisms have yet to be elucidated. The fission yeast Schizosaccharomyces pombe is an ideal model organism whose simple but conserved cell biology has led to the identification of many important cell size regulators common to all eukaryotes. In this thesis, I have quantitatively analyzed the dynamics and localization of several key players of cell size regulation, which lead to a new physical model on cell size regulation based on the localization and accumulation of a size sensing kinase cdr2p. In this model, cdr2p molecules accumulate in proportion to cell size into complexes called midsomes, which localize to the cortex at the central section of the cell. Upon reaching the desired cell size, cdr2p accumulation surpasses a concentration threshold and the cell will divide. This accumulation is partly facilitated by the key negative regulator pom1p, which prevents midsome formation at the cell tip. Evidence also suggests that the ER serves a role in confining midsome localization to the medial plasma membrane, perhaps by providing a physical link to the nucleus. Together, this work elucidates a mechanistic understanding of how cell size can be determined and controlled.