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Rearrangement of the actin cytoskeleton occurs in a variety of cellular processes and structures and involves a wide spectrum of proteins. Among these, the gelsolin superfamily proteins (GSPs) control actin organization by severing filaments, capping filament ends and bundling filaments. Structural changes within the GSPs are key in controling their functions. This thesis is aimed in understanding the activation mechanisms of the C-terminal halves of GSPs through investigating the atomic structures of gelsolin, adseverin and villin. X-ray crystallography was used to determine the structures of C-terminal fragments of these 3 proteins. The results demonstrate that: 1) The structure of the activated form of the C-terminal half of gelsolin displays an open conformation, with the actin-binding site on gelsolin domain 4 (G4) fully exposed and all three type-II calcium binding sites (CBS) occupied. Neither actin nor the type-I calcium, which is normally sandwiched between actin and G4, is required to achieve this conformation. 2) Calcium ions at both type-I and type-II CBSs of gelsolin were exchangable within the crystals. Extraction of calcium ions from the CBSs triggered local conformation changes which we speculate are the initial steps toward restoration of the arrangement of domains found in the calcium-free inactive form of gelsolin in solution. 3) The long helix of G6 in the calcium-bound structure is similar to the helix of calcium-free isolated villin domain 6 (V6). 4) The conformation of the C-terminal half of adseverin in the active state is similar to that of gelsolin. These results suggest that the C-terminal halves of GSPs are activated before forming a complex with actin. The activation involves straightening the helix of domain 6 which is a key component in the global conformation changes of C-terminal halves of these proteins. The results also suggest that a calcium ion may bind to the type-I CBS on domain 4 of the active conformation of GSPs concurrently with forming the complex with actin, hence, stabilizing the GSP:actin complex.