Macropinocytosis-the large-scale, non-specific uptake of fluid by cells-is used 
by Dictyostelium discoideum amoebae to obtain nutrients. These cells form 
circular ruffles around regions of membrane defined by a patch of 
phosphatidylinositol (3,4,5)-trisphosphate (PIP3) and the activated forms of the 
small G-proteins Ras and Rac. When this ruffle closes, a vesicle of the medium 
is delivered to the cell interior for further processing. It is accepted that 
PIP3 is required for efficient macropinocytosis. Here, we assess the roles of 
Ras and Rac in Dictyostelium macropinocytosis. Gain-of-function experiments show 
that macropinocytosis is stimulated by persistent Ras activation and genetic 
analysis suggests that RasG and RasS are the key Ras proteins involved. Among 
the activating guanine exchange factors (GEFs), GefF is implicated in 
macropinocytosis by an insertional mutant. The individual roles of Rho family 
proteins are little understood but activation of at least some may be 
independent of PIP3. This article is part of the Theo Murphy meeting issue 
'Macropinocytosis'.