Abstract Dictyostelium discoideum has proven indispensable to elucidate cytoskeletal dynamics. The cytoskeleton plays a key role in almost all cellular processes, including motility, cytokinesis, cell-to-cell and cell-substrate adhesions and intracellular transport. Several actin binding proteins are also involved in these processes, among them are actin crosslinking proteins (for example, filamin and a-actinin). Filamin (also known as ddfilamin or gelation factor or ABP 120) consists of an actin binding domain and six rod repeats of 100 amino acids, its last repeat being responsible for the formation of the homodimer. Both the domains are necessary for the actin crosslinking activity of filamin. Dictyostelium mutants lacking filamin have severe defects in multicellular slug migration towards light, phototaxis, and preferable temperature, thermotaxis. To study the phototaxis defect in filamin- mutants at the molecular level we expressed various domains in the mutant and tested their rescue potential. Expression of C terminally truncated and point mutated (at a putative phosphorylation site) filamin rescued the phototaxis defect partially. Full-length filamin when expressed under the control of the ecmA promoter in the anterior tip of the slug rescues the phototaxis, but not when expressed under the control of the cotB promoter which allows expression in the posterior ¾th of the slug. Phototaxis is a complex phenomenon, which includes more than 55 genes. To identify genes involved in this process we carried out a microarray analysis. Amoung 65 genes we selected in microarray analysis, 40 genes were up regulated and 25 genes were down regulated. From the functions of most of theses genes, we conclude that the phototactic behaviour of slugs is controlled by extracellular cAMP, Ca2+ ions and cell adhesion. To further focus on filamin's function in phototaxis we searched for proteins interacting with filamin by a yeast two hybrid screen and by immunoprecipitation. TipA, GAPA and SapA proteins were pulled down in the immunoprecipitation approach while the FIP, filamin interacting protein, was found earlier in a yeast two hybrid screen. Biochemical studies suggest that FIP is associated with F-actin and may function in vesicle trafficking. Detailed analysis of the mutants of LIM proteins, villidin and filamin for chemotactic migration towards cAMP, led us to conclude that alteration in chemotactic motility of individual cells may not affect the phototactic migration of the slug
