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Abstract

Actin is involved in cell movement, maintaining cell shape and anchoring cytoskeletal proteins. These functions are regulated by many actin-binding proteins, including those of the gelsolin superfamily. Gelsolin superfamily members regulate actin organization by severing, capping F-actin, nucleating the formation of F-actin and/or bundling F-actin. Although abundant structures are available for gelsolin and gelsolin fragments in complexes with actin, the detailed mechanisms for gelsolin activation, and for gelsolin severing and capping of F-actin are still unknown. Structures for gelsolin family members and their complexes with actin help elucidate these mechanisms. In this thesis, I describe the purification, crystallization and solution of the structures of the following four proteins and protein complexes, including human G1-G3/actin complex, a novel equine G1-G3/actin complex, human villin domain V6 and actin monomer. The structure of human G1-G3/actin indicates cooperative binding of calcium ions in G2 and G6 is responsible for opening the G2/G6 latch to expose the F-actin binding site on G2. A new equine G1-G3/actin structure suggests G2-G3 can adopt a CapG-like conformation and reveals novel interactions between gelsolin and actin. The villin V6 structure implies a common spring-loaded activation mechanism in the gelsolin superfamily. Finally, a new actin monomer structure is the first reported for G-actin in an ATP state, without ABPs or modification. All these structures contribute to our understanding of actin's physiological roles and their regulation by the gelsolin superfamily.