Coordinating Cytokinesis with Mitosis by a Conserved Signal Transduction Network in the Fission Yeast Schizosaccharomyces Pombe: a Dissertation

Abstract

Cytokinesis is the final event of the cell division cycle and results in physical and irreversible separation of a mother cell into two daughter cells. Cytokinesis must only occur after chromosomes have segregated during mitosis to ensure each daughter cell receives the proper complement of genetic material. Failure to execute normal cytokinesis can result in aneuploidy and/or polyploidy, a hallmark of many cancers. Cytokinesis occurs mechanically through constriction of an actin-myosin based contractile ring, while initiation of ring constriction is temporally and spatially mediated by complex signaling networks. It is absolutely crucial that cytokinesis is tightly coordinated with the cell cycle in order to preserve the fidelity of cell division. We hypothesized that to achieve such tight control of cytokinesis, cells may utilize both promotional and inhibitory signals, however how cells maintained this control was poorly understood. The goal of this thesis was to characterize how cells regulate signaling of cytokinesis, both positively and negatively, during cell division using the fission yeast Schizosaccharomyces pombe as a model organism. Two approaches were employed. (1) We first sought to characterize the positive timing mechanism that signals cytokinesis though a detailed investigation of Sid1p, a protein kinase essential for activation of ring constriction. (2) Secondly, we sought to define how cells negatively regulate cytokinesis through investigation of Dma1p, a spindle checkpoint protein implicated in inhibition of cytokinesis. Our results reveal a conserved signaling network, termed the Septation Initiation Network (SIN), of which Sid1p is an intermediate component, that controls temporal and spatial regulation of cytokinesis. We found Sid1p is additionally controlled by Cyclin Dependent Kinase activity, uncovering an important link between mitotic events and initiation of cytokinesis. Furthermore, we found that aberrant SIN activation can override a microtubule-damage-induced spindle checkpoint arrest. This effect is counteracted by Dma1p, which normally inhibits the SIN during checkpoint activation to preserve cell viability until damage is repaired. We conclude that signaling cytokinesis is tightly coordinated with mitosis in S. pombe by positive signals acting through Sid1p and the SIN, and under certain conditions, negative signals acting through Dma1p. Considering the conservation of cell cycle regulators in the eukaryotic kingdom, it is likely that similar mechanisms to control cytokinesis exist in humans.