THE AIMLESS RASGEF IS REQUIRED FOR PROCESSING OF CHEMOTACTIC SIGNALS THROUGH G-PROTEIN-COUPLED RECEPTORS IN DICTYOSTELIUM

Background: Ras proteins are small GTP-binding proteins that play an e ssential role in a wide range of processes, particularly in mammalian growth control, They act as molecular switches, being inactive when GD P is bound, and active when associated with GTP. Activation is accompl ished by guanine nucleotide exchange factors (RasGEFs); when RasGEFs i nteract with Ras proteins, GDP is allowed to escape, and is replaced b y GTP. Dictyostelium responds to chemoattractants through typical seve n transmembrane domain receptors and heterotrimeric G proteins. There are at least five different Dictyostelium Ras genes, whose functions a re not yet known. Results: We have isolated the aimless gene, which en codes the Dictyostelium homologue of RasGEFs, during a screen for inse rtional mutants that fail to aggregate. We found that aimless null mut ants grew at a normal rate, but were severely impaired in both chemota xis and activation of adenylyl cyclase, both of which are critical for the early stages of development. Although coupling between receptors and their G proteins is unaffected, and several cyclic AMP (cAMP)-medi ated responses appear normal, activation of adenylyl cyclase by recept ors and GTP gamma S (a non-hydrolyzable GTP analogue) is reduced by up to 95%. The motility of mutant cells appears normal, suggesting a tru e defect in gradient sensing. Conclusions: The discovery of the aimles s gene adds an interesting new member to the family. of RasGEFs. Our d ata suggest an unforeseen role for a RasGEF, and therefore presumably a complete Ras pathway, in the processing of chemotactic signals throu gh G-protein-coupled receptors.