Two GTPases, cdc42 and rac, bind directly to a protein implicated in the immunodeficiency disorder Wiskott-Aldrich syndrome

Background: Members of the Rho family of small GTPases play an essential role in controlling the motile behaviour of animal cells. Specifically, Cdc42 and Pac have been shown to induce the formation of filopodia and lamellipodia, respectively, at the cell periphery of Swiss 3T3 fibroblasts. In addition, both GTPases are required for progression through G(1) phase of the cell cycle, possibly by regulating the activity of the Jun N-terminal kinase (JNK) signalling pathway, In order to examine more closely the mechanisms underlying the diverse functions of Rho GTPases in mammalian cells, we searched for downstream targets of these proteins. Results: A yeast two-hybrid screen for proteins interacting with the human Cdc42 GTPase identified WASP, a protein implicated in the immunodeficiency disorder Wiskott-Aldrich syndrome (WAS). Recombinant WASP, expressed in Escherichia coli, also bound to Cdc42 and weakly to Pac, but not at all to Rho. The Cdc42/Rac-binding domain was identified in a region between amino acids 201-321 of WASP, and binding was dependent on Cdc42 and Pac being in the GTP-bound conformation. Furthermore, WASP did not catalyze GTPase activation or nucleotide exchange activity on Cdc42. Conclusions: Positional cloning has implicated WASP in causing WAS, and the protein is defective in patients suffering from the disease. WASP is expressed exclusively in cells of hematopoietic lineage, and lymphocytes from WAS patients have a distorted cell-surface and exhibit reduced proliferative capacity. WASP has recently been found to bind to the Src-homology 3 (SH3) domain of the adapter protein Nck. This observation, and the results presented here, suggest that WAS is the result of defects in signal transduction pathways regulated by Cdc4a/Rac and Nck.