The dynamic localization of Mob1 and its function in the regulation of the chromosomal passenger complex during mitosis and cytokinesis in mammalian cells

The science of cell biology had its simple but significant beginnings over 300 years ago and progress was slow. Over the last 40 years, advances in imaging biochemistry and genetics, have afforded a more detailed and dynamic picture of the internal environment of the cell has come to light, yet there is still so much to ascertain. Among these open questions is the manner by which a cell coordinates the division of genetic material (mitosis) with the division of the cytoplasm (cytokinesis). The proper temporal regulation of mitosis and cytokinesis is critical to ensure that each daughter cell receive the correct complement of chromosomes, otherwise ending in destabilizing chromosome segregation errors. In yeast, mitotic exit and cytokinesis are coordinated by a signaling cascade whose terminal components include the protein kinase Sid2/Dbf2 and a non-catalytic subunit, Mob1. There are five human Mob1 isoforms, all of which display redundant localization patterns at the centrosomes and kinetochores in early mitosis, and the spindle midzone during cytokinesis. Employing a combination of RNA interference-, imaging- and pharmacological approaches, we sought to determine how mammalian orthologs for the Mitotic Exit Network function to regulate the final events of cell division. Mob1 shares similar localizations patterns with two key mitotic regulators that are required for mitotic progression and cytokinesis. Mob1's recruitment to the centrosomes required Polo-like kinase (Plk1) which functions in several capacities including the formation of the mitotic spindle. Mob1's recruitment to the kinetochores requires the Chromosomal Passenger Complex (CPC) which regulates chromosomal dynamics during mitosis and for completion of cytokinesis. Depletion of Mob1 revealed that Mob1 was not required for completion of mitosis or cytokinesis in human cells, but did disrupt the temporal and spatial localization of the CPC during mitosis. These results suggests Mob1 and the CPC are mutually dependent on each other for localization during mitosis, and that Mob1 may be playing a novel role in regulating mitotic progression through the CPC.