Structure of Cdc42 in complex with the GTPase-binding domain of the 'Wiskott-Aldrich syndrome' protein

The Rho-family GTP-hydrolysing proteins (GTPases), Cdc42, Rac and Rho, act as molecular switches in signalling pathways that regulate cytoskeletal arc hitecture, gene expression and progression of the cell cycle(1). Cdc42 and Rac transmit many signals through GTP-dependent binding to effector protein s containing a Cdc42/Rac-interactive-binding (CRIB) motif(2). One such effe ctor, the Wiskott-Aldrich syndrome protein (WASP), is postulated to link ac tivation of Cdc42 directly to the rearrangement of acting(3). Human mutatio ns in WASP cause severe defects in haematopoietic cell function, leading to clinical symptoms of thrombocytopenia, immunodeficiency and eczema. Here w e report the solution structure of a complex between activated Cdc42 and a minimal GTPase-binding domain (GBD) from WASP. An extended amino-terminal G BD peptide that includes the CRIB motif contacts the switch I, beta 2 and a lpha 5 regions of Cdc42. A carboxy-terminal beta-hairpin and alpha-helix pa ck against switch II. The Phe-X-His-X-2-His portion of the CRIB motif and t he alpha-helix appear to mediate sensitivity to the nucleotide switch throu gh contacts to residues 36-40 of Cdc42. Discrimination between the Rho-fami ly members is likely to be governed by GBD contacts to the switch I and alp ha 5 regions of the GTPases. Structural and biochemical data suggest that G BD-sequence divergence outside the CRIB motif may reflect additional regula tory interactions with functional domains that are specific to individual e ffecters.