A novel Dictyostelium RasGEF is required for normal endocytosis, cell motility and multicellular development

Background: Dictyostelium possesses a surprisingly large number of Ras prot eins and little is known about their activators, the guanine nucleoticle ex change factors (GEFs). It is also unclear, in Dictyostelium or in higher eu karyotes, whether Ras pathways are linear, with each Ras controlled by its own GEF, or networked, with multiple GEFs acting on multiple Ras proteins. Results: We have identified the Dictyostelium gene that encodes RasGEFB, a protein with homology to known RasGEFs such as the Son-of-sevenless (Sos) p rotein. Dictyostelium cells in which the gene for RasGEFB was disrupted mov ed unusually rapidly, but lost the ability to perform macropinocytosis and therefore to grow in liquid medium. Crowns, the sites of macropinocytosis, were replaced by polarised lamellipodia. Mutant cells were also profoundly defective in early development, although they eventually formed tiny but no rmally proportioned fruiting bodies. This defect correlated with loss of di scoidin I gamma mRNA, a starvation-induced gene, although other genes requi red for development were expressed normally or even precociously. RasGEFB w as able to rescue a Saccharomyces CDC25 mutant, indicating that it is a gen uine GEF for Ras proteins. Conclusions: RasGEFB appears to be the principal activator of the RasS prot ein, which regulates macropinocytosis and cell speed, but it also appears t o regulate one or more other Ras proteins.