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b'The cyclase-associated protein (CAP) from Dictyostelium discoideum was studied in detail regarding its structure and function relationships. The second part of the thesis describes the characterization of the novel actin-related protein filactin from D. discoideum. \\nCAP homologs are multifunctional proteins: they are involved in signal dependent changes in the actin cytoskeleton, in vesicle transport and cell development. The binding of monomeric actin through the C-terminal domain represents a common feature of all CAPs. From four highly conserved regions in this domain the verprolin homology region of the D. discoideum CAP was analyzed in this work. Loss of this region led to a clear decrease, but not suppression of the actin-sequestering activity. In agreement to these data, stable complexes of the modified CAP-C with G-actin could be identified in chemical crosslinking experiments, and it could be shown that CAP-C is able to dimerize.\\nConsidering the high conservation of cyclase-associated proteins and their importance for cell biological processes it is remarkable that no structure of this protein has been elucidated so far. Therefore, in the frame of this thesis the structure of the membrane-associated N-terminal domain of D. discoideum CAP was to be determined in cooperation with the group of T. Holak at the MPI f. Biochemistry (Martinsried). For this purpose numerous constructs from the N-terminal domain had to be cloned and expressed, to be purified and examined for their stability and threedimensional folding. It turned out that the stable core of the aminoterminal domain covers the amino acids 51-226. The structure was determined by nuclear magnetic resonance spectroscopy and X-ray crystallography. Six antiparallel a-helices are connected by loop elements and form a cylindrical core domain which can form a dimer in the crystal, dimerization occurs through Mg2+ ions. The clarification of the N-terminal structure of D. discoideum CAP will now simplify the research on its interaction with phospholipids and CAP binding proteins\\nThe second part of this work dealt with the characterization of filactin. The unique 105 kDa protein contains two filamin homologous regions in its N-terminal, and a clearly actin-related domain within the C-terminal part. While in resting cells the endogenous filactin shows cytoplasmic distribution and interaction with protein aggregates as well, the GFP construct of the actin-related domain displays an actin-like behavior during cell movement or phagocytosis. A stimulus-induced colocalization of actin and filactin was observed in experiments with chemotactically stimulated cells. The alignment of its C-terminal amino acid sequence with the structure of muscle actin predicts a globular, actin-related structure containing all residues that are important for ATP/ADP binding.'