The Influence of Fic1 on Polarity and Cytokinesis in Fission Yeast

Abstract

Cytokinesis is the final process in the cell cycle which divides two daughter cells. Many eukaryotic organisms use an actin- myosin-based structure known as the cytokinetic ring (CR) to drive cell abscission. The rod-shaped fission yeast Schizosaccharomyces pombe is a well-established, genetically tractable system for studying cytokinesis. While cytokinesis is crucial for daughter cell abscission, cytokinesis also influences S. pombe growth polarity. S. pombe cells elongate at their two cell tips in a cell cycle-controlled manner, transitioning from monopolar to bipolar growth in interphase when new ends established by the most recent cell division begin to extend. It has been established that Fic1, a cytokinetic factor, is required for efficient cytokinesis and normal growth polarity establishment. In these studies, I report that Fic1 is phosphorylated on two C-terminal residues. Disrupting these phosphorylation sites prevents the establishment of bipolar growth which promotes the dimorphic switch into invasive pseudohyphal growth. I also determined that the fic1 phospho-ablating mutant, fic1-2A, is a gain-of-function allele that suppresses myo2-E1, the temperature-sensitive allele of the essential type-II myosin, myo2. This suppression highlighted Fic1’s role in promoting septum formation which requires its interactions with the F-BAR proteins Cdc15 and Imp2. Additionally, I found that Fic1 interacts with Cyk3 and that this interaction was likewise required for Fic1’s role in septum formation. Fic1, Cdc15, Imp2, and Cyk3 are the orthologs of the Saccharomyces cerevisiae ingression progression complex, which stimulates Chs2, a chitin synthase, to promote primary septum formation. However, my findings indicate that Fic1 promotes septum formation and cell abscission independently of the S. pombe Chs2 ortholog. Together these studies characterize the influence of Fic1’s phosphorylation state on polarity and reveal Fic1’s role in septum formation in S. pombe.