Stress granules (SGs) are formed in the cytoplasm in response to various toxic agents, and are believed to play a critical role in the regulation of mRNA metabolism during stress. In SGs, mRNAs are stored in an abortive translation initiation complex that can be routed to either translation initiation or degradation. Here, we show that G3BP, a phosphorylation-dependent endoribonuclease that interacts with RasGAP, is recruited to SGs in cells exposed to arsenite. G3BP may thus determine the fate of mRNAs during cellular stress. Remarkably, SG assembly can be either dominantly induced by G3BP overexpression, or on the contrary, inhibited by expressing a central domain of G3BP. This region binds RasGAP and contains serine 149, whose dephosphorylation is induced by arsenite treatment. Critically, a phosphomimetic mutant (S149E) fails to oligomerize and to 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.

A potential p120 GTPase-activating protein (RasGAP) effector, G3BP (RasGAP Src homology 3 [SH3] binding protein), was previously identified based on its ability to bind the SH3 domain of RasGAP. Here we show that G3BP colocalizes and physically interacts with RasGAP at the plasma membrane of serumstimulated but not quiescent Chinese hamster lung fibroblasts. In quiescent cells, G3BP was hyperphosphorylated on serine residues, and this modification was essential for its activity. Indeed, G3BP harbors a phosphorylation-dependent RNase activity which specifically cleaves the 3-untranslated region of human c-myc mRNA. The endoribonuclease activity of G3BP can initiate mRNA degradation and therefore represents a link between a RasGAP-mediated signaling pathway and RNA turnover.The Ras protein belongs to a family of low-molecular-weight GTPases which are essential components of multiple receptormediated signal transduction pathways controlling cell proliferation, differentiation, and cytoskeletal organization (23). Activated Ras is bound to GTP, while the GDP-bound form of Ras is inactive (27). Extracellular stimuli induce the exchange of GDP for GTP on Ras through a series of protein-protein interactions involving activated receptors, adaptor proteins (such as Grb2 or Shc), and Ras guanine nucleotide exchange factors (5,9,33,38). Mutations in the Ras gene which lock Ras in the GTP-bound form lead to cell growth in the absence of mitogenic signals and are associated with an oncogenic phenotype (17). Physiological inactivation of Ras involves interaction with GTPase-activating proteins (GAPs) (40), such as p120 (RasGAP) (41,43) or the product of the NF1 gene (neurofibromin) (26,44), which accelerate the hydrolysis of Ras-associated GTP, thereby converting Ras from an active to an inactive form. Disruption of either the RasGAP or the NF1 gene in mice results in an embryonic lethal phenotype (3, 14), indicating that Ras inactivation is a key process in normal cell signaling and development.In addition to being a negative regulator of Ras, RasGAP may also represent a downstream target of Ras (35). RasGAP is a widely expressed modular protein which comprises several structural features that likely enable it to function in the transduction cascade (29). While the carboxyl-terminal domain of RasGAP constitutes a catalytic domain (25), the N-terminal region is believed to mediate interactions with other signaling proteins (20). The N-terminal region is characterized by a Src homology 3 (SH3) domain flanked by two SH2 domains, as well as pleckstrin homology (PH) and calcium-dependent lipid binding domains (4, 34). Upon activation of many growth factor receptors, RasGAP becomes phosphorylated and associates with cytosolic proteins as well as with the autophosphorylated tyrosine kinase receptors (19). RasGAP has been shown to form a complex with G3BP (RasGAP SH3 binding protein) in a Ras-GTP-dependent manner (32). G3BP is composed of 466 amino acid and has a predicted molecular mass of 52 kDa; the carboxyl-terminal region contai...

Mitogen activation of mRNA decay pathways likely involves specific endoribonucleases, such as G3BP, a phosphorylation-dependent endoribonuclease that associates with RasGAP in dividing but not quiescent cells. G3BP exclusively cleaves between cytosine and adenine (CA) after a specific interaction with RNA through the carboxyl-terminal RRM-type RNA binding motif. Accordingly, G3BP is tightly associated with a subset of poly(A)؉ mRNAs containing its high-affinity binding sequence, such as the c-myc mRNA in mouse embryonic fibroblasts. Interestingly, c-myc mRNA decay is delayed in RasGAP-deficient fibroblasts, which contain a defective isoform of G3BP that is not phosphorylated at serine 149. A G3BP mutant in which this serine is changed to alanine remains exclusively cytoplasmic, whereas a glutamate for serine substitution that mimics the charge of a phosphorylated serine is translocated to the nucleus. Thus, a growth factor-induced change in mRNA decay may be modulated by the nuclear localization of a site-specific endoribonuclease such as G3BP.

The chemotropic guidance cue netrin-1 promotes neurite outgrowth through its receptor Deleted in Colorectal Cancer (DCC) via activation of Rac1. The guanine nucleotide exchange factor (GEF) linking netrin-1/ DCC to Rac1 activation has not yet been identified. Here, we show that the RhoGEF Trio mediates Rac1 activation in netrin-1 signaling. We found that Trio interacts with the netrin-1 receptor DCC in mouse embryonic brains and that netrin-1-induced Rac1 activation in brain is impaired in the absence of Trio. Trio ؊/؊ cortical neurons fail to extend neurites in response to netrin-1, while they are able to respond to glutamate. Accordingly, netrin-1-induced commissural axon outgrowth is reduced in Trio ؊/؊ spinal cord explants, and the guidance of commissural axons toward the floor plate is affected by the absence of Trio. The anterior commissure is absent in Trio-null embryos, and netrin-1/DCC-dependent axonal projections that form the internal capsule and the corpus callosum are defective in the mutants. Taken together, these findings establish Trio as a GEF that mediates netrin-1 signaling in axon outgrowth and guidance through its ability to activate Rac1.During the development of the central nervous system, axons are guided to their targets in response to molecular cues that can be either membrane-bound factors or secreted molecules, acting over short or long distances. The neuronal growth cone is a specialized structure found at the tip of the axon that integrates attractive and repulsive signals elicited by these extracellular cues and responds to them by triggering signaling pathways that regulate growth cone motility (16,18). Netrins are a family of secreted proteins that control axon outgrowth and guidance in multiple vertebrate and invertebrate species (3). Netrin-1 is a bifunctional molecule that attracts and repels different classes of axons. In vertebrates, netrin-1 was first shown to attract commissural axons of the developing spinal cord toward the ventral midline (21, 44). Since then, netrin-1 has been shown to promote outgrowth of a wide variety of axons, including growing cortical axons (30, 41). Two families of netrin-1 receptors in mammals have been identified: the Deleted in Colorectal Cancer (DCC) family, comprising DCC and neogenin, and the UNC-5 family of proteins (1,20,25). DCC mediates growth cone attraction induced by netrin-1 (1, 20, 25, 43) whereas the repulsive effect of netrin-1 is mediated by the UNC-5 family of netrin receptors, alone or in combination with DCC (17,22,35).DCC is a transmembrane protein without any obvious catalytic activity in its intracellular domain, and for this reason, it was unclear until recently how the intracellular signaling machinery leading to axon outgrowth was initiated. This process has begun to be elucidated with the identification in cortical and commissural neurons of different DCC-binding proteins, including the protein tyrosine kinases focal adhesion kinase, Src, and Fyn; the Nck adaptor protein; and phosphatidylinositol transfer protein ␣ (26...

Whereas DNA methylation is essential for genomic imprinting, the importance of histone methylation in the allelic expression of imprinted genes is unclear. Imprinting control regions (ICRs), however, are marked by histone H3-K9 methylation on their DNA-methylated allele. In the placenta, the paternal silencing along the Kcnq1 domain on distal chromosome 7 also correlates with the presence of H3-K9 methylation, but imprinted repression at these genes is maintained independently of DNA methylation. To explore which histone methyltransferase (HMT) could mediate the allelic H3-K9 methylation on distal chromosome 7, and at ICRs, we generated mouse conceptuses deficient for the SET domain protein G9a. We found that in the embryo and placenta, the differential DNA methylation at ICRs and imprinted genes is maintained in the absence of G9a. Accordingly, in embryos, imprinted gene expression was unchanged at the domains analyzed, in spite of a global loss of H3-K9 dimethylation (H3K9me2). In contrast, the placenta-specific imprinting of genes on distal chromosome 7 is impaired in the absence of G9a, and this correlates with reduced levels of H3K9me2 and H3K9me3. These findings provide the first evidence for the involvement of an HMT and suggest that histone methylation contributes to imprinted gene repression in the trophoblast.

The rabies virus phosphoprotein (P) and nucleoprotein (N) are involved in transcription and replication of the viral genome. Interaction between N and P was studied in vivo in transfected cells expressing both proteins. Coimmunoprecipitation assays revealed that the N-P complex is present in cells expressing both proteins as well as in infected cells. Furthermore, immunostaining showed that coexpression of N and P was sufficient to induce the formation of cytoplasmic inclusions similar to those found in infected cells. In addition, deletion mutant analysis of P was performed to identify the regions of P interacting with N. The results indicate that at least two independent N-binding sites exist on P protein: one is located in the carboxy-terminal part of the protein and another between amino acids 69 and 177. The formation of cytoplasmic inclusions seems to require the presence of both N-binding sites on P protein.

The transcriptional mechanisms underlying lineage specification and differentiation of embryonic stem (ES) cells remain elusive. Oct-3/4 (POU5f1) is one of the earliest transcription factors expressed in the embryo. Both the pluripotency and the fate of ES cells depend upon a tight control of Oct-3/4 expression. We report that transgene- or TGFbeta-induced increase in Oct-3/4 mRNA and protein levels in undifferentiated ES cells and at early stages of differentiation triggers expression of mesodermal and cardiac specific genes through Smad2/4. cDNA antisense- and siRNA-mediated inhibition of upregulation of Oct-3/4 in ES cells prevent their specification toward the mesoderm and their differentiation into cardiomyocytes. Similarly, Oct-3/4 siRNA injected in the inner cell mass of blastocysts impairs cardiogenesis in early embryos. Thus, quantitative Oct-3/4 expression is regulated by a morphogen, pointing to a pivotal and physiological function of the POU factor in mesodermal and cardiac commitments of ES cells and of the epiblast.

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