Regulation of Cytokinesis by Microtubules in Aspergillus Nidulans

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Division of one cell into two is one of the key critical functions of life. It is essential that cell division be precisely regulated, both spatially and temporally, in order to ensure appropriate distribution of subcellular components such as organelles and chromosomes to the daughter cells. Cell division is not always necessarily symmetrical; numerous examples of assymetrical division can be found among fungi, plants and animals, and these are often essential for differentiation and organismal morphogenesis. A great deal has been learned about fungal cell division (also termed cytokinesis or septation) from studies of various yeasts and filamentous fungi. In such fungi, septation is regulated by signal transduction pathways such as the septation initiation network (SIN) in fission yeast. SIN includes a MAP- kinase-like kinase cascade, and activation of this cascade initiates septation. The consequence of activation of the SIN network is to activate as-yet-unidentified components at the contractile medial ring, composed mainly of actin microfilaments and myosin plus various other proteins including actin-binding proteins. The ultimate function of this ring is to initiate the assembly of the septum, composed of long chain polysaccharides (beta-glucan and chitin), at the right place. While all SIN molecules localize to the spindle pole body in yeast, the terminal Sid2p kinase and its associated Mob1p partially translocate to the septation site to activate septation. This project addresses the question of how these SIN molecules (Sid2p and Mob1p) travel from the spindle pole body to the septation site. It is proposed that microtubules play essential roles in the localization of SIN molecules. This hypothesis will be tested in the filamentous fungus, Aspergillus nidulans. A. nidulans is a particularly good model organism for these studies because there exists a substantial armamentarium of experimental tools, and completion of the full sequence of its genome is anticipated shortly. In A. nidulans, the AnMOB1 protein localizes not only to the spindle pole body and the septation site, but also to the central spindle. The localization is dependent on microtubules. Cytoplamic dynein, a microtubule motor associated with the spindle during cell division, has been demonstrated to play a role in septum positioning. Dynein mutants will be used to test whether AnMOB1 localization is dependent on cytoplasmic dynein. Experiments will also be performed to test whether spindle pole body localization of AnMOB1 is dependent on minus end-directed kinesin KLPA. The roles of the plus end-directed kinesin BIMC in AnMOB1 localization to the central spindle will be explored. In addition, pharmacological and genetic approaches will be used to test whether microtubule dynamics plays a role in AnMOB1 localization. These studies will provide insights into the roles played by microtubule dynamics, microtubule motors, and the microtubule-organizing center in the intracellular movement of SIN molecules, and ultimately in cytokinesis.Broader Impacts: Aspergillus nidulans is not only a valuable research model, but it is also useful as teaching material for undergraduate and high school students. Dr. Liu uses living Aspergillus to demonstrate cell biological phenomena (cell division, organelle movement, and microtubule dynamics) to local Junior High School students and home-schooled students who visit his laboratory. He also uses Aspergillus as a teaching model in an undergraduate Introductory Biology course and in a graduate level course, Common Approaches in Plant Cell and Molecular Biology. Dr. Liu has also mentored several undergraduates who have performed research in his laboratory; these students have gone on to graduate school or to careers in biotechnology research.