Investigating the roles of turnover and mobility in the actomyosin ring of Schizosaccharomyces pombe

In many eukaryotes cytokinesis is facilitated by the contraction of an actomyosin ring. The exact mechanisms that lead to this contractility are unknown, although a number of models posit that actin turnover in the ring is essential. The effect of reduced actin dynamics during ring formation has been well studied in Schizosaccharomyces pombe; however the corresponding effects on ring contraction are not well understood. By using mutants of the fission yeast actin severing protein Adf1, we observed that contracting actomyosin rings display a ‘peeling’ phenotype. In these cells bundles of actin and myosin peel off from one side of the ring, and are then pulled across to the opposite side, which typically occurs 3 times during cytokinesis. We also found that this phenotype is dependant on the activity of the three myosin species present in the ring. We hypothesise that reduced actin turnover leads to a non-­‐ uniform distribution of tension around the ring, which causes regions of higher tension to peel off. This model predicts that adf1 mutant cells might be sensitive to additional mutations that compromise membrane anchoring of the ring, and that the tension imbalance could lead to non-­‐ uniform septum ingression during cytokinesis. Subsequent experiments confirmed these predictions, which further supported our model. We also attempted to recreate the phenotype in silico, by adapting a previously used mathematical model of the actomyosin ring. However, in doing so we encountered a number of problems, as we realised that certain parts of the model were unrealistic, and attempting to fix these aspects introduced new problems. As a result of this, we were unable to achieve this goal in the available timeframe. Finally, we also used mathematical modelling to analyse FRAP experiments, to more accurately extract information on the mobile fractions of ring proteins from FRAP recovery curves. Application of this method enabled the identification of interesting behaviour from the membrane anchoring protein Cdc15, potentially linking its mobility to its function in the ring.