Elucidation of the Multi-Faceted Roles of the SIN (Septation Initiation Network); Understanding How the SIN Promotes Cytokinesis and Inhibits Interphase Growth in the Fission Yeast Schizosaccharomyces pombe: A Dissertation

Cytokinesis is the cytoplasmic division of one cell into two independent daughter cells. Precise regulation of cytokinesis during cell cycle is essential for healthy and rapid multiplication of any organism. Schizosaccharomyces pombe has emerged as an excellent model system to study eukaryotic cell division regulation. This rod shaped organism grows by bipolar elongation in interphase when its actin cytoskeleton is concentrated at the cell ends (poles). However, growth stops in mitosis and the actin cytoskeleton is rearranged to facilitate assembly of the contractile actomyosin ring at the cell middle. Although several studies have focused on the separate processes of growth and division, it was unclear how cells regulate the cytoskeletal remodeling during the transition between the different stages of the cell cycle. The Septation Initiation Network (SIN) is a signaling cascade essential for fission yeast cytokinesis (Balasubramanian et al., 1998; Mishra et al., 2004) and the MOR (morphogenesis) signaling pathway is essential for interphase bipolar growth (Kanai et al, 2005). Interestingly, inactivation of the SIN not only failed to maintain the cytokinetic apparatus at the cell middle but also caused the redistribution of the cytoskeletal elements like actin to the cell ends that led to bipolar cell elongation similar to cells in interphase (Mishra et al., 2004). These results suggested that SIN signaling inhibits interphase bipolar growth, but it was not clear if the SIN had a direct role in inhibition of interphase growth during mitosis and this question was the major focus of this thesis. The results presented in Chapter II show a novel cross-pathway interaction between the SIN and the MOR in the fission yeast. Our results in Chapter III suggest that some of the MOR pathway components might be important for coordination between nuclear and cytoplasmic divisions in mitosis, revealing novel roles of the pathway. In a separate study (Chapter IV) we sought to identify additional regulators of the SIN and cytokinesis through a suppressor screen and found that the nucleolar rDNA transcription machinery inhibits cytokinesis in fission yeast.