Coronin 1C harbours a second actin-binding site that confers co-operative binding to F-actin

Dynamic rearrangement of actin filament networks is critical for cell motility, phagocytosis, and endocytosis. Coronins facilitate these processes, in part, by their ability to bind F-actin. We previously identified a conserved surface-exposed arginine (R30) in the b-propeller of Coronin 1B required for F-actin binding in vitro and in vivo. However, whether this finding translates to other coronins has not been well defined. Using quantitative actin binding assays, we show that mutating the equivalent residue abolishes F-actin binding in Coronin 1A but not Coronin 1C. By mutagenesis and biochemical competition, we have identified a second actin binding site in the unique region of Coronin 1C. Interestingly, leading edge localization of Coronin 1C in fibroblasts requires the conserved site in the β-propeller but not the site in the unique region. Furthermore, in contrast to Coronin 1A and Coronin 1B, Coronin 1C displays highly cooperative binding to actin filaments. Here, we highlight a novel mode of coronin regulation, which has implications for how coronins orchestrate cytoskeletal dynamics.