Identification and Functional Reconstitution of Effector Proteins for the GTPases Rac and CDC42HS

Rho-related GTPase proteins, Rat 1,2, Rho A, B, G, and CDC42Hs, constitute a distinct subfamily in the Ras super family of GTPases ( 1 ) During the last few years a large body of evidence has been accumulated that suggests that Rho-like proteins play a critical role in the organization of the actin cytoskeleton and are tinpltcated in cell growth and transformation ( 2 ). Like other GTPases, Rho-family members cycle between the inactive guanosme drphosphate (GDP)-bound form and the active guanosine triphosphate (GTP) -bound form. By virtue of this molecular activity, these proteins can act like molecular switches. The cycling between the “on” and “off” states IS regulated by GTP-GDP exchange factors (GEFs) and GTPase-activating proteins (GAPS), respectively ( 3 ) (Fig. 1 ). Once a GTPase is activated, rt binds rapidly to an effector molecule that subsequently is activated to initiate a specific response. The diversity of cellular responses that are implicated by Rho GTPases suggests that each GTPase protein activates multiple-effector proteins. Thus, to delineate the signal transductions that are regulated by GTPases, it IS essential to identify the molecular targets for these GTPases. Thus far, several effecters were identified for each member of the family of Rho-like proteins. Rat effectors include the p21-activated kinases (PAK) ( 4 , 5 ), and the p67-phox of nicotrnamide adenine dinueleotide phosphate (reduced form) (NADPH) oxrdase ( 6 ). CDC42Hs effecters include PAK kinases and the Wiskott Aldrich Syndrome Protein (WASP) ( 7 , 8 ). Rho was recently shown to interact with several protein kinases related to protein kinase N ( 9 , 10 ).

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