Fig. 2. Design of different fluorescent probes for detection of Rho family GTPase activity in living cells. (A) Bimolecular Rac FRET probe (Kraynov et al., 2000). Red pentagon represents Alexa-546 dye. (B) Unimolecular `GTPase-effector fusion' Rac and Cdc42 FRET probes (Itoh et al., 2002). Non-natural isoprenoid moiety due to K-Ras4B CAAX-box is depicted in red. (C) `Effector domain only' probes (Itoh et al., 2002; Seth et al., 2003). (D) Biosensor based on covalently labeling a domain with a solvatochromic dye. Red pentagon represents the dye. Black dashed arrows represent excitation light. Black solid arrows represent emission light. Red arrows represent direction of FRET.