Myo2 Motor Function in the Contractile Ring and the Regulation of Fission Yeast Cytokinesis

Animals, fungi, and amoebas require an actomyosin contractile ring at the division site to perform cytokinesis. The contractile ring initiates and guides the invagination of the plasma membrane as it forms new barriers between the nuclei at the cell equator. Defects in the contractile ring can result in misdirected, delayed, or premature cytokinesis, which leads to abnormal chromosome numbers. Aneuploidies resulting from failed cytokinesis sometimes lead to aggressive forms of cancer. This dissertation was motivated by the goal of better understanding the properties of the contractile ring and how it drives cytokinesis. Actomyosin is initially recruited to the cell equator through the coordination of scaffolding factors, actin-binding proteins, and signaling cascades. Subsequently, the sliding of actin filaments by myosin reshapes the resulting meshwork into a compact ring. Once fully assembled, the contractile ring establishes tension, which leads the plasma membrane inward. The primary motor proteins in the contractile ring of animal cells are class-II nonmuscle myosins, which typically function as bipolar filaments. Filament assembly is activated by phosphorylation and plays a central role in myosin function during cytokinesis. However, many underlying processes that regulate contractile ring function are poorly understood. Current models of cytokinesis have been based on mechanistic insights provided by two decades of work in the fission yeast system Schizosaccharomyces pombe. In fission yeast, the class-II myosin Myo2 provides the major source of motor activity in the contractile ring. Myo2 is two-headed and has a rod-like tail, which is consistent with other class-II myosins. Yet, it was unknown whether Myo2 assembles into filaments, or how phosphorylation affects its activity. To investigate these features, recombinant Myo2 was purified from the baculovirus/Sf9 insect cell expression system. Hydrodynamic measurements were used to examine whether Myo2 forms filaments. These sedimentation velocity data gave no indication that Myo2 self-assembles under the typical physiological salt concentrations, which suggests that Myo2 is unlike any class-II myosin known to date. Myo2 was also treated in vitro with its native kinase Pak1. Phosphorylation of Myo2 molecules had no effect on self-assembly, however it reduced actin-binding in motility assays and increased steady-state ATPase rates by two fold. Our results imply that the function and regulation of fission yeast Myo2 during cytokinesis depends on a specific scaffolding scheme at the plasma membrane, which has not been observed in other eukaryotes. Another interest of this dissertation was how the contractile ring is regulated during cytokinesis. We examined one cytokinesis protein, Cyk3, believed to mediate between the ring and extracellular processes. Genetics and live cell imaging analyses indicated that Cyk3 functions through a catalytically-inactive enzyme domain, which implicated Cyk3’s involvement in one of the primary cytokinesis signaling pathways. This dissertation sheds new light on core aspects of how fission yeast undergo cytokinesis, especially with respect to the mechanism of Myo2 activity in the contractile ring. Characterizing the physical and enzymatic properties of an essential myosin in a simple organism should provide insights into cytokinesis in higher organisms.