Role of polo kinase and Mid1p in determining the site of cell division in fission yeast

The fission yeast Schizosaccharomyces pombe divides symmetrically using a m edial F-actin-based contractile ring to produce equal-sized daughter cells. Mutants defective in two previously described genes, mid1 and pom1, freque ntly divide asymmetrically. Here we present the identification of three new temperature-sensitive mutants defective in localization of the division pl ane. All three mutants have mutations in the polo kinase gene, plo1, and sh ow defects very similar to those of midi mutants in both the placement and organization of the medial ring. In both cases, ring formation is frequentl y initiated near the cell poles, indicating that Mid1p and Plo1p function i n recruiting medial ring components to the cell center. It has been reporte d previously that during mitosis Mid1p becomes hyperphosphorylated and relo cates from the nucleus to a medial ring. Here we show that Mid1p first form s a diffuse cortical band during spindle formation and then coalesces into a ring before anaphase. Plo1p is required for Mid1p to exit the nucleus and form a ring, and Pom1p is required for proper placement of the Mid1p ring. Upon overexpression of Plo1p, Mid1p exits the nucleus prematurely and disp lays a reduced mobility on gels similar to that of the hyperphosphorylated form observed previously in mitotic cells. Genetic and two-hybrid analyses suggest that Plo1p and Mid1p act in a common pathway distinct from that inv olving Pom1p. Plo1p localizes to the spindle pole bodies and spindles of mi totic cells and also to the medial ring at the time of its formation. Taken together, the data indicate that Plo1p plays a role in the positioning of division sites by regulating Mid1p. Given its previously known functions in mitosis and the timing of cytokinesis, Plo1p is thus implicated as a key m olecule in the spatial and temporal coordination of cytokinesis with mitosi s.