Abstract Effective cell migration is dependent on dynamic remodeling of the actin-cytoskeleton, and plays an essential role in many physiological events, including tissue morphogenesis, wound healing and inflammatory responses. During these events, cells are typically directed to migrate towards targets by sensing chemical, physical, mechanical or electrical cues around them. To facilitate cell migration, actin must undergo rapid cycles of assembly and disassembly in a highly- coordinated manner. Type I coronins, a conserved class of actin binding proteins, have been shown to regulate the process of cell migration by modulating the turnover of the branched actin network at the leading edge. Recent work suggests that the type I coronin, Coronin 1C (Coro1C) mediates the formation of membrane protrusions during cell migration; while the structurally similar type I coronin Coronin 1B (Coro1B) regulates the disassembly of branched actin networks within these protrusions. However, despite these results our understanding of the function of Coro1B and Coro1C to actin dynamics and specifically in directed cell migration remains incomplete. Therefore, the overall objective of this proposal will be to elucidate the mechanistic contributions of Coro1B and Coro1C in branch stability and de-branching during directed migration. I hypothesize that Coro1B and Coro1C regulate branched actin dynamics by modulating the functions of various actin-regulators, including 1) the Arp2/3 complex, 2) cofilin and 3) Rac1 during directed cell migration. To test this hypothesis, I have established a fibroblast cell line derived from Coro1B knockout (KO), Coro1CFL/FL mice that is rescued with a flox-able Coro1B-GFP expression construct. Upon Cre-mediated recombination, both exogenous Coro1B- GFP and endogenous Coro1C are deleted to create a matched-pair null cell line to test the role of coronins in lamellipodial dynamics and cell motility. In Aim 1, I will utilize the null cells to delineate the mechanistic role of Coro1B and Coro1C on lamellipodia dynamics using optogenetically controlled Coro1B and Coro1C rescue constructs. In Aim 2, I will utilize the matched-pair and a microfabrication technique to assess the functional roles of Coro1B and Coro1C in haptotaxis and chemotaxis, two distinct and clinically relevant forms of directed migration. In all optogenetic and migration experiments proposed, cellular responses will be measured as changes to cell morphology, cell behavior, and cytoskeletal structure and organization. These studies will provide fundamental insights into the role of Coro1B and Coro1C in regulating the actin cytoskeleton during directed cell migration, as well as providing outstanding training opportunities to advance my career in biomedical science.