BME Master's Defense Announcement for June Hyung Kim (T. Kim, advisor) Everyone is invited to attend the public presentation beginning at 12:30 PM. Title: Probing the roles of actin dynamics in the cytoskeleton of animal and plant cells Date: April 16, 2024 (Tuesday) Time: 12:30 PM Location: MJIS 2001 Committee: Taeyoon Kim, Chair; Chris J. Staiger; Daniel M. Suter Abstract: The actin cytoskeleton is a dynamic structure that regulates various important cellular processes, such as cell protrusion, migration, transport, and cell shape changes. Cells employ different actin architectures best suited for each of these functions. We have employed an agent-based model to illuminate how the actin cytoskeleton plays such functions in animal and plant cells, via dynamic interactions between molecular players. Lamellipodia found in animal cells are two-dimensional actin protrusion formed on the leading edge of cells, playing an important role in sensing surrounding mechanical environments via focal adhesions. Various molecular players, architecture, and dynamics of the lamellipodia have been investigated extensively during recent decades. Nevertheless, it still remains elusive how each component in the lamellipodia mechanically interacts with each other to attain a stable, dynamic steady state characterized by a retrograde flow emerging in the branched actin network. Using the agent-based model, we investigated how the balance between different subcellular processes is achieved for the dynamic steady state. We simulated a branched network found in the lamellipodia, consisting of actin filament (F-actin), myosin motor, Arp2/3 complex, and actin crosslinking protein. We found the importance of a balance between F-actin assembly at the leading edge of cells and F-actin disassembly at the rear end of the lamellipodia. We also found that F-actin severing is crucial to allow for the proper disassembly of an actin bundle formed via network contraction induced by motor activity. In addition, it was found that various dynamic steady states can exist. The actin cytoskeleton in plant cells plays a crucial role in cellular transport and cytoplasmic streaming, and its structure is very different from actin cytoskeleton in animal cells. The plant actin cytoskeleton is known to show distinct dynamic behaviors with homeostasis. We used the agent-based model to simulate the plant actin cytoskeleton with the consideration of the key governing mechanisms, including F-actin polymerization/depolymerization, different types of F-actin nucleation events, severing, and capping. We succeeded in reproducing experimental observations in terms of F-actin density, length, nucleation frequency, and rates of severing, polymerization, and depolymerization. We found that the removal of nucleators results in lower F-actin density in the network, which supports recent experimental findings.