Overexpression of a novel rho family GTPase, RacC, induces unusual actin-based structures and positively affects phagocytosis in Dictyostelium discoideum.

Rho family proteins have been implicated in regulating various cellular processes, including actin cytoskeleton organization, endocytosis, cell cycle, and gene expression. In this study, we analyzed the function of a novel Dictyostelium discoideum Rho family protein (RacC). A cell line was generated that conditionally overexpressed wild-type RacC three- to fourfold relative to endogenous RacC. Light and scanning electron microscopy indicated that the morphology of the RacC-overexpressing cells [RacC WT(+) cells] was significantly altered compared with control cells. In contrast to the cortical F-actin distribution normally observed, RacC WT(+) cells displayed unusual dorsal and peripheral F-actin-rich surface blebs (petalopodia, for flower-like). Furthermore, phagocytosis in the RacC WT(+) cells was induced threefold relative to control Ax2 cells, whereas fluid-phase pinocytosis was reduced threefold, primarily as the result of an inhibition of macropinocytosis. Efflux of fluid-phase markers was also reduced in the RacC WT(+) cells, suggesting that RacC may regulate postinternalization steps along the endolysosomal pathway. Treatment of cells with Wortmannin and LY294002 (phosphatidylinositol 3-kinase inhibitors) prevented the RacC-induced morphological changes but did not affect phagocytosis, suggesting that petalopodia are probably not required for RacC-induced phagocytosis. In contrast, inactivating diacylglycerol-binding motif-containing proteins by treating cells with the drug calphostin C completely inhibited phagocytosis in control and RacC WT(+) cells. These results suggest that RacC plays a role in actin cytoskeleton organization and phagocytosis in Dictyostelium.