EFFECTOR DOMAIN MUTANTS OF RHO-DISSOCIATE CYTOSKELETAL CHANGES FROM NUCLEAR SIGNALING AND CELLULAR-TRANSFORMATION

The small GTP-binding Rho proteins control a variety of biological act ivities, including organization of the actin cytoskeleton, regulation of gene expression and cellular transformation. In contrast, Ras prote ins do not induce actin stress fibers, but potently transform cells wh ich exhibit a morphology clearly distinct from that caused by activate d forms of Rho. To investigate whether nuclear signaling and oncogenic potential of Rho are a consequence of its profound effect on cytoskel etal organization, we replaced each amino acid in the Rho effector loo p with those of Ras, or replaced conserved residues with others known to result in differential signaling capability when introduced into Ra s and Rad. These Rho mutants did not gain the ability to induce the MA PK, JNK or p38 pathways but, surprisingly, all Rho effector loop mutan ts still continued to induce actin stress fiber formation. However, th ree of these Rho mutants, with substitutions of leucine-39, glutamic a cid-39, or cysteine-42, lost the ability to stimulate gene transcripti on via the serum response factor (SRF) and failed to induce neoplastic transformation. Thus, these results indicate that cytoskeletal change s are not sufficient to induce the transformed phenotype, and that Rho -effector molecules regulating the actin cytostructure are distinct fr om those signaling to the nucleus and subverting normal growth control .