Motors and Mitosis: Characterizing the Role of Dynein in Mammalian Epithelial Spindle Positioning Phenomena
Reproduction distinguishes living organisms from inanimate objects, as cells cannot be made except by division from preexisting cells. Despite the importance of proper cell division for growth, aging, healing, and the prevention of diseases such as cancer, many of the complicated protein interactions involved remain a mystery. Mitosis, the segregation of replicated chromosomes to two new daughter cells, is central to the process of cell division. The forces generated by motor proteins, a special class of proteins able to convert chemical energy from ATP into mechanical energy, are necessary for the movements of the cytoskeleton during mitosis. Dynein is a motor protein that walks along the microtubules of the mitotic spindle, performing several essential roles throughout the phases of mitosis. Research in budding yeast and other model systems has implicated dynein located at the cell cortex as a key player in spindle positioning (Kozlowski et al., 2007; Carminati and Stearns, 1997). The regulation and mechanics of spindle positioning are poorly understood yet critical for both asymmetric and symmetric cell divisions. The characterization of spindle positioning phenomena in a line of mammalian epithelial cells revealed a pattern of asymmetric spindle position at anaphase onset followed by unequal movement of each set of sister chromatids in a significant subpopulation of cells. In addition to phase contrast observation of spindle positioning, microscopic observation of mammalian epithelial cells expressing fluorescent gamma tubulin revealed asymmetric movement of each spindle pole during anaphase. Immunofluorescent fixing and staining of mammalian epithelial cells suggested that dynein localized to the cell cortex plays a role in spindle positioning phenomena.