An attenuating role of a WASP-related protein, WASP-B, in the regulation of F-actin polymerization and pseudopod formation via the regulation of RacC during Dictyostelium chemotaxis

•WASP-B (wasB) is a WASP-related protein found in Dictyostelium.•F-actin assembly response is increased and prolonged in wasB− cells.•wasB− cells exhibit frequent protrusion of pseudopods causing aberrant chemotaxis.•wasB− cells exhibit aberrant regulation of RacC activation and localization.•Chemotaxis defects of wasB− are likely to result from aberrant regulation of RacC. The WASP family of proteins has emerged as important regulators that connect multiple signaling pathways to regulate the actin cytoskeleton. Dictyostelium cells express WASP, as well as a WASP related protein, WASP-B, endoded by wasB gene. WASP-B contains many of the domains present in WASP. Analysis of wild type, wasB null cells revealed that WASP-B is required for proper control of F-actin polymerization in response to a cAMP gradient. Due to the lack of tight control on actin polymerization, wasB null cells exhibited higher level of F-actin polymerization. wasB− cells extend more de novo pseudopods laterally and their average life span is longer than those of wild type cells, causing more turns and inefficient chemotaxis. YFP-WASP-B appears to be uniformly distributed in the cytosol and shows no translocation to cortical membrane upon cAMP stimulation. Active RacC pull-down assay reveals that the level of active RacC in wasB− cells is significantly higher than wild type cells. Moreover, the distribution of active RacC is not localized in wasB− cells. We conclude that chemotaxis defects of wasB− cells are likely to result from the aberrant regulation of RacC activation and localization.