Engulfment of extracellular material by phagocytosis or macropinocytosis depends on the ability of cells to generate specialised cup shaped protrusions. To effectively capture and internalise their targets, these cups are organised into a ring or ruffle of actin-driven protrusion encircling a static interior domain. These functional domains depend on the combined activities of multiple Ras and Rho family small GTPases, but how their activities are integrated and differentially regulated over

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... ce and time is unknown. Here, we show that the amoeba Dictyostelium discoideum coordinates Ras and Rac activity using the multidomain protein RGBARG (RCC1, RhoGEF, BAR and RasGAP-containing protein). We find RGBARG uses a tripartite mechanism of Ras, Rac and phospholipid interactions to localise at the protruding edge and interface with the interior of both macropinocytic and phagocytic cups. There, RGBARG shapes the protrusion by driving Rac activation at the rim whilst suppressing expansion of the active Ras interior domain. Consequently, cells lacking RGBARG form enlarged, flat interior domains unable to form large macropinosomes. During phagocytosis, we find that disruption of RGBARG causes a geometry-specific defect in engulfing rod-shaped bacteria and ellipsoidal beads. This demonstrates the importance of co-ordinating small GTPase activities during engulfment of more complex shapes and thus the full physiological range of microbes, and how this is achieved in a model professional phagocyte.