Anillin-related protein Mid1 regulates timely formation of the contractile ring in the fission yeast Schizosaccharomyces japonicus.

In the fission yeast Schizosaccharomyces pombe (Sp), Mid1/Dmf1 plays an important role in positioning the division site by inducing formation of the contractile ring (CR). Mid1, emanating from the nucleus located in the cell center, forms a dozen of nodes in the middle cell cortex ahead of mitosis, and actin filaments and myosin II accumulated at each node interact and assemble the CR in metaphase. Curiously, in another fission yeast S. japonicus (Sj), CR formation begins after nuclear segregation in late anaphase. Here, we investigated the role of S. japonicus Mid1 during mitosis to compare the molecular mechanisms that determine the cell division site in Schizosaccharomyces. Similar to Sp Mid1, Sj Mid1 often accumulated in the nucleus of interphase cells. Moreover, Sj Mid1 localized to cortical dots with myosin II in the future division site and formed a medial ring in mitotic cells. However, S. japonicus cells without Mid1 function still carried out symmetrical binary division. Therefore, the Mid1 dependency for positional control of the cell division site is possibly different between the two species. Meanwhile, we found that Sj Mid1 enhanced CR formation, in a manner possibly similar to that by Sp Mid1.