The Dictyostelium RasS protein is required for macropinocytosis, phagocytosis and the control of cell movement

Endocytosis and cell migration both require transient localised remodelling of the cell cortex. Several lines of evidence suggest a key regulatory rol e in these activities for members of the Ras family of small GTPases. We ha ve generated Dictyostelium cells lacking one member of this family, RasS, a nd the mutant cells are perturbed in endocytosis and cell migration. Mutant amoebae are defective in phagocytosis and fluid-phase endocytosis and are impaired in growth. Conversely, the rasS- cells show an enhanced rate of ce ll migration, moving three times faster than wild-type controls. The mutant cells display an aberrant morphology, are highly polarised, carry many elo ngated actin protrusions and show a concomitant decrease in formation of pi nocytic crowns on the cell surface. These morphological aberrations are par alleled by changes in the actin cytoskeleton, with a significant proportion of the cortical F-actin relocalised to prominent pseudopodia. Rapid migrat ion and endocytosis appear to be mutually incompatible and it is likely tha t RasS protein is required to maintain the normal balance between these two actin-dependent processes.