Chemoattractant-induced Ras activation during Dictyostelium aggregation

Ras proteins are highly conserved molecular switches that regulate cellular response to external stimuli. Dictyostelium discoideum contains an extensive family of Ras proteins that function in regulation of mitosis, cytoskeletal function and motility, and the onset of development. Little is known about the events that lead to the activation of Ras proteins in Dictyostelium, primarily owing to a lack of a biochemical assay to measure the levels of activated Ras. We have adapted an assay, used successfully to measure activated Ras in mammalian cells, to monitor activation of two Dictyostelium Ras proteins, RasC and RasG. We have found that the Ras-binding domain (RBD) of mammalian Raf1 was capable of binding to the activated form of RasG, but not to the activated form of RasC; however, the RBD of Schizosaccharomyces pombe Byr2 was capable of binding preferentially to the activated forms of both RasC and RasG. Using this assay, we discovered that RasC and RasG showed a rapid and transient activation when aggregation-competent cells were stimulated with the chemoattractant cAMP, and this activation did not occur in a number of cAMP signalling mutants. These data provide further evidence of a role for both RasC and RasG in the early development of Dictyostelium.