Role of Rac in controlling the actin cytoskeleton and chemotaxis in motilecells

We have used the chemotactic ability of Dictyostelium cells to examine the roles of Rho family members, known regulators of the assembly of F-actin, i n cell movement. Wild-type cells polarize with a leading edge enriched in F -actin toward a chemoattractant. Overexpression of constitutively active Di ctyostelium Rac1B(61L) Or disruption of DdRacGAP1, which encodes a Dictyost elium Rad GAP, induces membrane ruffles enriched with actin filaments aroun d the perimeter of the cell and increased levels of F-actin in resting cell s. Whereas wild-type cells move linearly toward the cAMP source, Rad1B(61L) and Ddracgap1 null cells make many wrong turns and chemotaxis is inefficie nt, which presumably results from the unregulated activation of F-actin ass embly and pseudopod extension. Cells expressing dominant-negative DdRac1B(1 7N) do not have a well-defined F-actin-rich leading edge and do not protrud e pseudopodia, resulting in very poor cell motility. From these studies and assays examining chemoattractant-mediated F-actin assembly, we suggest DdR ac1 regulates the basal levels of F-actin assembly, its dynamic reorganizat ion in response to chemoattractants. and cellular polarity during chemotaxi s.