G3BP, a phosphorylation-dependent endoribonuclease that interacts with RasGAP, is recruited to stress granules (SGs) in cells exposed to arsenite. This study shows that G3BP may determine the fate of mRNAs during cellular stress. SG assembly is influenced by G3BP overexpression or inhibition, with the central domain of G3BP binding RasGAP and containing serine 149, which is dephosphorylated by arsenite treatment. A phosphomimetic mutant (S149E) fails to oligomerize and assemble SGs, whereas a nonphosphorylatable G3BP mutant (S149A) does both. These results suggest that G3BP is an effector of SG assembly, and that Ras signaling contributes to this process by regulating G3BP dephosphorylation. G3BP is an evolutionarily conserved RNA-binding protein that associates with RasGAP. Its RNase activity is positively regulated by phosphorylation at several sites, including serine 149, which is dependent on RasGAP and negatively regulated by Ras signaling. G3BP is recruited to SGs in response to stress, and its recruitment is influenced by p21ras. The NTF2-like and RNA-binding domains of G3BP mediate its recruitment to SGs. The acidic RasGAP binding domain of G3BP harbors an arsenite-regulated phosphorylation site and dominantly inhibits SG formation. Phosphorylation of G3BP at Ser 149 prevents its oligomerization, which is critical for SG assembly. G3BP is a regulated effector of SG assembly, and its phosphorylation state determines its ability to assemble SGs. The study provides the first evidence that ribonucleases are recruited to SGs, and that G3BP, along with other factors, plays a role in mRNA regulation during stress.G3BP, a phosphorylation-dependent endoribonuclease that interacts with RasGAP, is recruited to stress granules (SGs) in cells exposed to arsenite. This study shows that G3BP may determine the fate of mRNAs during cellular stress. SG assembly is influenced by G3BP overexpression or inhibition, with the central domain of G3BP binding RasGAP and containing serine 149, which is dephosphorylated by arsenite treatment. A phosphomimetic mutant (S149E) fails to oligomerize and assemble SGs, whereas a nonphosphorylatable G3BP mutant (S149A) does both. These results suggest that G3BP is an effector of SG assembly, and that Ras signaling contributes to this process by regulating G3BP dephosphorylation. G3BP is an evolutionarily conserved RNA-binding protein that associates with RasGAP. Its RNase activity is positively regulated by phosphorylation at several sites, including serine 149, which is dependent on RasGAP and negatively regulated by Ras signaling. G3BP is recruited to SGs in response to stress, and its recruitment is influenced by p21ras. The NTF2-like and RNA-binding domains of G3BP mediate its recruitment to SGs. The acidic RasGAP binding domain of G3BP harbors an arsenite-regulated phosphorylation site and dominantly inhibits SG formation. Phosphorylation of G3BP at Ser 149 prevents its oligomerization, which is critical for SG assembly. G3BP is a regulated effector of SG assembly, and its phosphorylation state determines its ability to assemble SGs. The study provides the first evidence that ribonucleases are recruited to SGs, and that G3BP, along with other factors, plays a role in mRNA regulation during stress.