Inactivation of the p53 tumor suppressor pathway is a critical step in human tumorigenesis. In addition to mutations, p53 can be functionally silenced through its increased degradation, inhibition of its transcriptional activity, and/or its inappropriate subcellular localization. In breast cancer for example, the TP53 gene is mutated in only 25% of tumors, suggesting that p53 functional inactivation is likely to play an essential role in the pathogenesis of this disease. Using a proteomic approach, we have found that members of the Ras network of proteins, Ras-GAP-SH3-domain-binding protein-1 and -2 (G3BP-1, 2), bind to p53 in vitro and in vivo. G3BP1 and G3BP2 are encoded by distinct genes, yet share a 59% amino acid identity. Although the precise role of G3BPs in Ras signaling and other cellular processes is still unclear, increasing data implicate G3BPs as having a role in tumor progression. Both isoforms have been shown to be upregulated in a number of human cancers including lung, prostate colon, and thyroid. Interestingly, G3BP-2 overexpression was recently detected in over 80% of human breast cancers. In this study, we investigated the functional consequences of the interaction between G3BPs and p53. Our data show that expression of G3BPs leads to the redistribution of p53 from the nucleus to the cytoplasm. Stable transduction of breast cancer cells with short-hairpin RNAi targeting G3BPs resulted in marked upregulation of p53 protein levels, induction of downstream p53 targets, like p21waf1, and an increased amount of apoptosis, indicating that G3BPs may act as negative regulators of p53. Since estrogen is known to play a critical role in mammary gland tumorigenesis and can cause cytoplasmic redistribution of p53, we investigated a possible connection between estrogen and G3BPs. Exposure of estrogen responsive breast cancer cells to estradiol caused a dose-dependent upregulation of G3BPs, which was accompanied by the downregulation of p53 responsive targets, such as the pro-apoptotic proteins PUMA and Bax. An estrogen inhibitor, IC 182,780, in the presence of estradiol, completely abrogated the induction of G3BPs and restored p53 transcriptional activity, confirming the effects were estrogen dependent. Together, our results show that estrogen mediated induction of G3BP expression leads to the functional inactivation of p53, and the partial relocalization of p53 to the cytoplasm might explain this effect. This data may also further define a molecular mechanism in which increased estrogen exposure leads to increased breast cancer risk. Since knock-down of G3BPs results in p53 activation, targeting G3BPs might prove to be an effective approach to increase the sensitivity of tumors to conventional chemotherapy or radiation.