TC21 and Ras share indistinguishable transforming and differentiating activities

Constitutively activated mutants of the Ras-related protein TC21/R-Ras2 cau se tumorigenic transformation of NIH3T3 cells. However, unlike Ras, TC21 fa ils to bind to and activate the Raf-1 serine-threonine kinase, Thus, wherea s Ras transformation is critically dependent on Raf-1 TC21 activity is prom oted by activation of Raf-independent signaling pathways. In the present st udy, we have further compared the functions of Ras and TC21, First we deter mined the basis for the inability of TC21 to activate Raf-1, Whereas Ras ca n interact with the two distinct Ras-binding sequences in NH2-terminus of R af-2, designated RBS1 and Raf-Cys, TC21 could only bind Raf-Cys, Thus, the inability of TC21 to bind to RBS1 may prevent it from promoting the translo cation of Raf-1 to the plasma membrane. Second, we found that TC21 is an ac tivator of the JNK and p38, but not ERK, mitogen-activated protein kinase c ascades and that TC21 transforming activity was dependent on Rac function. Thus, like Ras, TC21 may activate a Rac/JNK pathway, Third, we determined i f TC21 could cause the same biological consequences as Ras in three distinc t cell types. Like Ras, activated TC21 caused transformation of RIE-1 rat i ntestinal epithelial cells and terminal differentiation of PC12 pheochromoc ytoma cells. Finally, activated TC21 blocked serum starvation-induced diffe rentiation of C2 myoblasts, whereas dominant negative TC21 greatly accelera ted this differentiation process. Therefore, TC21 and Ras share indistingui shable biological activities in all cell types that we have evaluated. Thes e results support the importance of Raf-independent pathways in mediating t he actions of Ras and TC21.