Different structural organization of Ras and Rho effector domains.

Ras regulates proliferation and differentiation signals in cells, and activation of the protein can lead to malignant transformation. Activation of the related protein, Rho, affects cell morphology, and it has been suggested that it may also have some oncogenic potential. We show here that Rho does not induce a malignant phenotype in NIH3T3 cells but instead is a potent activator of actin stress fibre formation. The limited homology between Ras and Rho has enabled us to determine the amino acids specifying their different biological activities and GTPase-activating protein (GAP) protein sensitivities using chimeras. Rho substituted with amino acids 23-46 of Ras induces transformed foci in NIH3T3 monolayers, and we conclude that Ras has a single effector domain required for downstream signalling. Although mutational analysis of Rho has revealed that residues 32-42 are also essential for its biological activity, Ras substituted with amino acids 25-48 of Rho does not induce actin stress fibre formation. On the basis of these experiments, we propose that Rho may have two effector domains: one at amino acids 32-42 and corresponding to the effector region of Ras and the second located elsewhere in the carboxy-terminal two-thirds of the molecule.