Abstract

Cell division is a necessary process for growth and development of all organisms. Because fission yeast (Schizosaccharomyces pombe) cells grow in a bipolar fashion and divide symmetrically through contraction of an acto-myosin ring similar to the method of human cells, fission yeast supply a good model system to study polarity and cytokinetic mechanisms. Mid1 is a founding protein of the acto-myosin ring that helps recruit other ring proteins and define the position of division. In cells without functional Mid1, there is incomplete, asymmetrical cell division. One class of S. pombe mutants are classified by the apparent loss of cell polarity and round cellular shape due to problems in one of 12 orb genes. To further study whether these polarity defects are related to cytokinetic defects, we have investigated interactions between Mid1 and two orb mutants, Orb5 and Orb6. Analysis of orb5 and orb6 mutants reveal differing localizations of Mid1. Specifically, Mid1 localization appears more concentrated at specific subcellular sites in the orb6 mutants, suggesting the overall protein levels may be increased. Both orb5 and orb6 mutants have also shown a higher prevalence of binucleate cells and most of these binucleate cells have an internal septum. The majority of the orb mutants imaged also show a paired conformation, in which two cells are not completely separated from one another at one end. These unique phenotypes point out a relationship between cytokinetic defects and the polarity genes orb5 and orb6. The cytokinetic defects also present a link between Mid1 and the Orb proteins.