Rac1 GTPases control filopodia formation, cell motility, endocytosis, cytokinesis and development in Dictyostelium

The function of the highly homologous Rac1A, Rac1B, and Rac1C GTPases of th e Dictyostelium Rad group was investigated. All three GTPases bound with an equal capacity to the IQGAP-related protein DGAP1, with a preference for t he activated GTP-bound form. Strong overexpression of wild-type Rad GTPases N-terminally tagged with green fluorescent protein (GFP), predominantly in duced the formation of numerous long filopodia, Remarkably, expression of t he constitutively-activated GTPases resulted in dominant-negative phenotype s: these Rac1-V12 mutants completely lacked filopodia but formed numerous c rown shaped structures resembling macropinosomes. Moreover, these mutants w ere severely impaired in cell motility, colony growth, phagocytosis, pinocy tosis, cytokinesis and development. Transformants expressing constitutively -inactivated Rac1-N17 proteins were similar to wild-type cells, but display ed abundant and short filopodia and exhibited a moderate defect in cytokine sis. Taken together, our results indicate that the three GTPases play an id entical role in signaling pathways and are key regulators of cellular activ ities that depend on the re-organization of the actin cytoskeleton in Dicty ostelium.