Signalling by Rho family proteins.

Introduction The Rho family is part of the Ras superfamily of small GTP-binding proteins, and consists of Rho (A, B and C), Rac (1 and 2), two Cdc42 isoforms, RhoD, RhoE, RhoG, TClO and TTF [l]. Different members of the large Ras superfamily regulate a diverse array of cellular processes, from vesicle trafficking to signal transduction. They all have in common the ability to bind and hydrolyse GTP, creating a switch between an active GTP-bound conformation and an inactive GDP-bound conformation (Figure 1) . Rho was the first member of the Rho family to be cloned, and because of its homology with the proto-oncogene Ras, it was initially tested for its ability to induce cellular transformation. Rho can induce a transformed phenotype in some cell lines but not in others [Z], and is certainly not as potent at transforming cells as Ras, nor has it been isolated from a human cancer or a retrovirus as an oncogene. Research into Rho function, however, was soon transformed after it was found to be the molecular target for C3 transferase, an exoenzyme produced by Clostridium botulinum. C3 transferase ADP-ribosylates and thereby inactivates RhoA, RhoB and RhoC with remarkable specificity, although it can also inefficiently ribosylate other members of the Rho family [3]. Many other bacterial toxins have now been shown to target Rho and in some cases other Rho family proteins, either inactivating or