Prenylation of target GTPases contributes to signaling specificity of Ras-guanine nucleotide exchange factors

Ras-GRF1 and Ras-GRF2 constitute a family of calmodulin-regulated guanine-n ucleotide exchange factors (GEFs) that activate Ras proteins. Here we show that whereas Ras-GRF1 activated both Ha-Ras and R-Ras in cells, Ras-GRF2 ac tivated only Ha-Ras. The inability of Ras-GRF2 to activate R-Ras was the co nsequence of the GTPase being post-translationally modified, since Ras-GRF2 activated unprocessed R-Ras as effectively as unprocessed Ha-Ras when assa ys were performed either in vivo or in vitro. Moreover, Ras-GRF2 failed to activate fully processed R-Ras in vitro. The particular C-terminal lipid at tached to the GTPases played an important role in determining signaling spe cificity, since R-Ras became more responsive to Ras-GRF2 when it was farnes ylated instead of geranylgeranylated. Similarly, Ha-Ras became less respons ive to Ras-GRF2 when it was geranylgeranylated instead of farnesylated. Ana lysis of chimeras between Ras-GRF1 and Ras-GRF2 demonstrated that a 30-amin o acid segment embedded with their catalytic domains was responsible for re cognizing the presence of different lipids on Ras proteins. These results i ndicate that the specific lipid moiety attached to GTPases can contribute t o signaling specificity of Ras-GEFs.