During the vegetative state, Dictyostelium are single-celled amoeba that feed on bacteria. The mechanism of chemotaxis is essentially identical in all eukaryotes, and because of its genetically tractability, the social amoebae Dictyostelium discoideum is often used as model organism to study chemotaxis ( Devreotes and Zigmond, 1988 Van Haastert and Devreotes, 2004). It plays a role in diverse functions such as the sourcing of nutrients by prokaryotes, the formation of multicellular structures in protozoa, the tracking of bacterial infections by neutrophils, and the organization of the embryo in metazoa ( Baggiolini, 1998 Campbell and Butcher, 2000 Crone and Lee, 2002 Iijima et al., 2002). As in Rap-regulated pseudopod formation in Dictyostelium, mammalian Rap and PI3K are essential for determining neuronal polarity, suggesting that the Rap/PI3K pathway is a conserved module regulating the establishment of cell polarity.Ĭhemotaxis or directional movement toward a chemical compound is an essential property of many cells ( Van Haastert and Devreotes, 2004). These results indicate that a GbpD/Rap/PI3K pathway helps control pseudopod formation and cell polarity. Consistently, upon overexpression of the PIP3-degradating enzyme PTEN in GbpD-overexpressing cells, the strong adhesion and cell morphology phenotype is largely lost. Furthermore we show that Rap1 directly binds to the Ras binding domain of PI3K, and overexpression of GbpD leads to strongly enhanced PIP3 levels. In this study we demonstrate that overexpression of GbpD in pi3k1/2-null cells does not induce the adhesion and cell morphology phenotype. Phg2, a serine/threonine-specific kinase, mediates Rap1-regulated cell-substrate adhesion, but not cell polarity or chemotaxis. Cells overexpressing GbpD are flat, exhibit strongly increased cell-substrate attachment, and extend many bifurcated and lateral pseudopodia. GbpD, a Dictyostelium discoideum guanine exchange factor specific for Rap1, has been implicated in adhesion, cell polarity, and chemotaxis.