The X-ray crystal structure of the complex between the Ras-related protein Rap1A in the GTP-analogue (GppNHp) form and the pas-binding domain (RED) of the Ras effector molecule c-Raf1, a ser/Thr-specific protein kinase, has been solved to a resolution of 2.2 Angstrom. It shows that RED has the ubiquitin superfold and that the structure of Rap1A is very similar to that of Ras. The interaction between the two proteins is mediated by an apparent central antiparallel beta-sheet formed by strands B1-B2 from RED and strands beta 2-beta 3 from Rap1A. Complex formation is mediated by main-chain and side-chain interactions of the so-called effector residues in the switch I region of Rap1A.