Promotes the exchange of Ras-bound GDP by GTP (By similarity). Involved in phagocytosis, fluid-phase endocytosis, regulation of macropinocytosis and control of cell movement. Expressed during development; especially between 8 and 14 hours of development. Expression reaches a maximum at around 16 hours, when the cell aggregation was near completion and phagocytosis started. Grows relatively normally on bacterial plates, but is unable to grow in axenic culture due to a loss of fluid-phase endocytosis. The morphology of growing cells appears flattened, elongate, polarized and dominated by a single large lamella. Able to aggregate but not to form slugs, and does make highly aberrant fruiting bodies. Cells move unusually rapidly and have lost the ability to perform macropinocytosis. Crowns, the sites of macropinocytosis, are replaced by polarized lamellipodia. The majority of F-actin is concentrated in a single large lamella, at the leading edge of the cell. Cells lacking rasS and gefB show that the cell speed of unstimulated, bacterially-grown double mutants is around three times the speed of wild type cells; comparable with both rasS- and gefB- parents and reveal a serious defect in phagocytosis.