The Dictyostelium Bcr/Abr-related protein DRG regulates both Rac- and Rab-dependent pathways

Dictyostelium discoideum DdRacGap1 (DRG) contains both Rho-GEF and Rho-GAP domains, a feature it shares with mammalian Ber and Abr. To elucidate the p hysiological role of this multifunctional protein, we characterized the enz ymatic activity of recombinant DRG fragments in vitro, created DRG-null cel ls, and studied the function of the protein in vivo by analysing the phenot ypic changes displayed by DRG-depleted cells and DRG-null cells complemente d with DRG or DRG fragments. Our results show that DRG-GEF modulates F-acti n dynamics and cAMP-induced F-actin formation via Rad-dependent signalling pathways. DRG's RacE-GAP activity is required for proper cytokinesis to occ ur. Additionally, we provide evidence that the specificity of DRG is not li mited to members of the Rho family of small GTPases. A recombinant DRG-GAP accelerates the GTP hydrolysis of RabD 30-fold in vitro and our complementa tion studies show that DRG-GAP activity is required for the RabD-dependent regulation of the contractile vacuole system in Dictyostelium.