Cell division outside the cell; in vitro study of fission yeast cytokinesis

 Cytokinesis is the final stage of cell division, through which the cellular constituents of mother cells are partitioned into two daughter cells resulting in an increase in cell number. Failures in cytokinesis lead to aneuploidy associated with infertility, developmental defects and genetic instability that often lead to cancer. Cytokinesis in many eukaryotes involves the closure of an actomyosin based contractile ring. The relative contributions of actin polymerization, disassembly and myosin II function in ring closure are not fully understood. I will discuss the establishment of an in vitro system for the study of actomyosin ring assembly and closure in fission yeast, Schizaosaccharomyces pombe. We have partially purified and isolated actomyosin rings from fission yeast. These rings retain essential components of the contractile ring. Importantly, fully assembled rings contract to completion upon addition of ATP, in a myosin II dependent manner in an order of magnitude faster than that observed in intact cells, in the complete absence of cytoplasm. Surprisingly, unlike in intact cells, neither actin polymerization nor disassembly is required for closure of actomyosin rings.  Addition of both native and non-native actin crosslinking proteins block ring contraction. These studies establish that actin disassembly and myosin II function are sufficient for the mechanics of actomyosin ring closure. This opens up a new approach for studying cytokinesis and would compliment genetic and in vivo studies. I will discuss how this novel approach in combination with in vivo studies would help us understand important outstanding questions regarding the mechanisms and regulation of cytokinesis.