Studies of cytokinesis in fission yeast protoplast

Abstract

In many organisms, cytokinesis is facilitated by an actomyosin-based contractile ring. Positioning of the ring requires close coordination of positive and negative-signaling cues, coupled with cell geometry and nuclear position. The fission yeast S. pombe relies on these spatial cues to accomplish stable ring assembly. After proper positioning of the actomyosin ring, the ring contracts to drive membrane invagination, cell wall assembly, and to complete cell separation into two daughter cells. Although this model organism has its specificities, knowledge of the basic mechanisms and roles of actomyosin ring could be useful to understand similar mechanisms in other organisms. We were curious about how a fission yeast maintain a robust cell division machinery, if majority of cell wall is absent. In cylindrical fission yeast cells, coordination of actomyosin ring positioning is facilitated by a positive spatial cue, mid1 and a negative spatial cue, the tip-complex. In spherical protoplast of fission yeast, we observed mislocalized spatial cues, while the actomyosin ring consistently assemble at the equatorial region. Although removal of mid1 and the tip-complex in cylindrical cells caused the actomyosin ring to assemble along the long axis, it did not hinder the equatorial assembly of actomyosin ring in the spherical protoplasts. We found that actin filaments played a role as major determinant of positioning actomyosin ring in the absence of spatial cues. Given the spheroplasts are capable of forming an equatorial ring, we then asked whether the actomyosin ring is capable of contracting and bring about cytofission. The cps1 mutant, a cell wall mutant lacking b-glucan synthase function is unable to overcome its turgor pressure and undergo actomyosin ring contraction. We found that by generating protoplasts, cytofission (which we define as a process that separates the cytosol into two membrane-bound entities) took place and the protoplast was divided into two entities. We report that this event does not require a-glucan (another component of cell wall), exoglucanases to facilitate breakdown residual septum, and ESCRT proteins. The actomyosin ring is however essential for cytofission