Structural basis for the interaction of Ras with RaIGDS

The Ras protein signals to a number of distinct pathways by interacting with diverse downstream effecters. Among the effecters of Ras are the Raf kinase and RalGDS, a guanine nucleotide dissociation stimulator specific for Ral. Despite the absence of significant sequence similarities, both effecters bind directly to Ras, but with different specificities. We report here the 2.1 Angstrom crystal structure of the complex between Ras and the Ras-interacting domain (RID) of RalGDS. This structure reveals that the beta-sheet of the RID joins the switch I region of Ras to form an extended beta-sheet with a topology similar to that found in the Rap-Raf complex. However, the side chain interactions at the joining junctions of the two interacting systems and the relative orientation of the two binding domains are distinctly different. Furthermore, in the case of the Ras-RID complex a second RID molecule also interacts with a different part of the Ras molecule, the switch II region. These findings account for the cross-talk between the Ras and Ral pathways and the specificity with which Ras distinguishes the two effecters.