AN ACTIVATED RAC MUTANT FUNCTIONS AS A DOMINANT-NEGATIVE FOR MEMBRANERUFFLING

Previous studies have shown that point mutations in the effector domai n of Rad block specific downstream pathways such as PAK, JNK/SAPK kina ses and membrane ruffling, Specifically, the F37A mutation, made in a constitutively activated Q61L background, activates PAK but fails to i nduce membrane ruffles. We now show that Q61L/F37A Rac not only fails to induce ruffling but potently blocks membrane ruffling induced by se rum or PDGF, In the presence of serum, cells do extend filopodia, sugg esting that this mutant only blocks a subset of the effecters that ind uce cytoskeletal reorganization. At later times, this rac mutant induc es membrane blebbing, but not apoptosis, These results show that Q61L/ F37A Rac, is constitutively activated with respect to PAK activation b ut functions as a dominant negative for another pathway, membrane ruff ling, That an effector domain point mutant can simultaneously function as a dominant negative and dominant positive for different pathways i mplies that effects of these variants on cell functions must be interp reted with caution.