The Ras proteins are low molecular weight GTPases that play a critical role in the control of proliferation, differentiation, and cytoskeletal organization. RasGAP acts as a negative regulator of Ras, as well as a downstream target of Ras. It participates in the signal transduction cascade through interactions with other signaling proteins. Via its C-terminal domain, RasGAP interacts with Ras, but the N-terminus interacts with other signaling molecules such as p62 and p190. This function of RasGAP is mediated by the presence of SH2, SH3, and PH domains in the N-terminal region. However, some signaling proteins cannot bind to these domains. In an attempt to identify RasGAP-interacting molecules, a GAP SH3-Binding Protein (G3BP) was identified. This protein exhibits sequence homology to the hnRNP superfamily. It contains RNP1 and RNP2 motifs and is found primarily in the cytosol. G3BP interacts with GAP in proliferating cells and is dependent on the activation state of Ras. In addition, G3BP contains an intrinsic endonuclease activity that cleaves 3'-untranslated region (3'-UTR) mRNA. These data indicate that G3BP may link the Ras pathway to mRNA degradation.