Understanding Regulation of the Cytoskeleton during Cell Cycle Transitions through Examination of Crosstalk between Homologous Fission Yeast Pathways, Septation Initiation Network and Morphogenesis ORB6 Network: A Dissertation

The fission yeast Schizosaccharomyces pombe has become a powerful model system for studying cytokinesis, a process of cytoplasmic division by which one cell divides into two identical daughter cells. Like mammalian cells, S. pombe divides through the use of an actomyosin contractile ring, which is composed of a set of highly conserved cytoskeletal proteins. Cytokinesis in S. pombe is primarily regulated by the SIN pathway, which is activated in late mitosis and is required for actomyosin contractile ring and septum assembly, and also plays a role in spindle checkpoint inactivation, and telophase nuclear positioning. The various functions of the SIN are carried out by the terminal kinase in the pathway called Sid2. The lack of information in the downstream targets of Sid2 has limited our understanding of the different functions of the SIN. We recently showed that, in addition to its other functions, the SIN promotes cytokinesis through inhibition the MOR signaling pathway, which normally drives cell separation and initiation of polarized growth following completion of cytokinesis (Ray et al, 2010). The molecular details of this inhibition and the physiological significance of inhibiting MOR during cytokinesis was unclear. The results presented in Chapter II describe our approach to identify Sid2 substrates, particularly focusing on Nak1 and Sog2 that function in the MOR signaling cascade. We identified and characterized Sid2 phosphorylation sites on the Nak1 and Sog2 proteins. Chapter III explores how post translational modification of MOR proteins by Sid2 regulates polarized growth during cytokinesis. This includes delineating the effect of Sid2 mediated phosphorylation of Nak1 and Sog2 on protein-protein interactions in the MOR pathway as well as on the regulation of their localization during late mitosis. Finally, results in Chapter IV demonstrate that failure to inhibit MOR signaling is lethal because cells initiate septum degradation/cell separation before completing cytokinesis thereby emphasizing the importance of cross-regulation between the two pathways to prevent initiation of the interphase polarity program during cytokinesis.