Many putative downstream effectors of the small GTPases Cdc42 and Rac contain a GTPase binding domain (GBD), also called p21 binding domain (PBD), which has been shown to specifically bind the GTP bound form of Cdc42 or Rac, with a preference for Cdc42 [1,2]. The most conserved region of GBD/PBD domains is the N-terminal Cdc42/Rac interactive binding motif (CRIB), which consists of about 16 amino acids with the consensus sequence I-S-x-P-x(2,4)-F-x-H-x(2)-H-V-G [3]. Although the CRIB motif is necessary for the binding to Cdc42 and Rac, it is not sufficient to give high-affinity binding [4,5]. A less well conserved inhibitory switch (IS) domain responsible for maintaining the proteins in a basal (autoinhibited) state is located C-terminaly of the CRIB-motif [6,7,8].

GBD domains can adopt related but distinct folds depending on context. Although GBD domains are largely unstructured in the free state, the IS domain forms an N-terminal β hairpin that immediately follows the conserved CRIB motif and a central bundle of three α helices in the autoinhibited state. The interaction between GBD domains and their respective G proteins leads to the formation of a high-affinity complex in which unstructured regions of both the effector and the G protein become rigid. CRIB motifs from various GBD domains interact with Cdc42 in a similar manner, forming an intermolecular β-sheet with strand β-2 of Cdc42. Outside the CRIB motif, the C-termini of the various GBD domains are very divergent and show variation in their mode of binding to Cdc42, perhaps determining the specificity of the interaction. Binding of Cdc42 or Rac to the GBD domain causes a dramatic conformational change, refolding part of the IS domain and unfolding the rest [4,6,7,8,9,10].

Some proteins known to contain a CRIB domain are listed below:

• Mammalian activated Cdc42-associated kinases (ACKs), nonreceptor tyrosine kinases implicated in integrin-coupled pathways.
• Mammalian p21-activated kinases (PAK1 to PAK4), serine/threonine kinases that modulate cytoskeletal assembly and activate MAP-kinase pathways.
• Mammalian Wiskott-Aldrich Symdrom Proteins (WASPs), non-kinase proteins involved in the organization of the actin cytoskeleton.
• Yeast STE20 and CLA4, the homologues of mammalian PAKs. STE20 is involved in the mating/pheromone MAP kinase cascade.

The profile we developed covers the entire CRIB domain.