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-nucleotide 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 activated only Ha-Ras. The inability of Ras-GRF2 to activate R-Ras was the consequence of the GTPase being post-translationally modified, since Ras-GRF2 activated unprocessed R-Ras as effectively as unprocessed Ha-Ras when assays 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 attached to the GTPases played an important role in determining signaling specificity, since R-Ras became more responsive to Ras-GRF2 when it was farnesylated instead of geranylgeranylated. Similarly, Ha-Ras became less responsive to Ras-GRF2 when it was geranylgeranylated instead of farnesylated. Analysis of chimeras between Ras-GRF1 and Ras-GRF2 demonstrated that a 30-amino acid segment embedded with their catalytic domains was responsible for recognizing the presence of different lipids on Ras proteins. These results indicate that the specific lipid moiety attached to GTPases can contribute to signaling specificity of Ras-GEFs.