Membrane targeting of the nucleotide exchange factor Sos is sufficient for activating the Ras signaling pathway

Functional roles for the pleckstrin and Dbl homology regions in the Ras exchange factor Son-of-sevenless

Activation of p21ras by receptor tyrosine kinases is thought to result from recruitment of guanine nucleotide exchange factors such as Son-of-sevenless (Sos) to plasma membrane receptor substrates via adaptor proteins such as Grb2. This hypothesis was tested in the present studies by evaluating the ability of truncation and deletion mutants of Drosophila (d)Sos to enhance [32P]GTP loading of p21ras when expressed in 32P-labeled COS or 293 cells. The dSos catalytic domain (residues 758-1125), expressed without the dSos NH2-terminal (residues 1-757) or adaptor-binding COOH-terminal (residues 1126-1596) regions, exhibits intrinsic exchange activity as evidenced by its rescue of mutant Saccharomyces cerevisiae deficient in endogenous GTP/GDP exchange activity. Here we show that this dSos catalytic domain fails to affect GTP p21ras levels when expressed in cultured mammalian cells unless the NH2-terminal domain is also present. Surprisingly, the COOH-terminal, adaptor binding domain of dSos was not sufficient to confer p21ras exchange activity to the Sos catalytic domain in these cells in the absence of the NH2-terminal domain. This function of promoting catalytic domain activity could be localized by mutational analysis to the pleckstrin and Dbl homology sequences located just NH2-terminal to the catalytic domain. The results demonstrate a functional role for these pleckstrin and Dbl domains within the dSos protein, and suggest the presence of unidentified cellular elements that interact with these domains and participate in the regulation of p21ras.

The Grb2 binding domain of mSos1 is not required for downstream signal transduction

Cellular Ras proteins are activated primarily by specific guanine-nucleotide releasing factors such as the Son of Sevenless (Sos) proteins. This activation event is thought to occur in response to plasma membrane localization of a complex containing Sos and a small adapter protein Grb2. We have isolated a dominant mutant allele of mSos1 which transforms Rat1 cells, yet is no longer able to bind Grb2. Biochemical experiments reveal that the subcellular distribution of this truncated Sos protein is not altered with respect to the wild type Sos protein. These data argue against a role for Grb2 in the direct recruitment of Sos proteins to the plasma membrane and suggest that Grb2 may function to overcome negative regulation of Sos by its C terminus.

Selective activation of the JNK signaling cascade and c-Jun transcriptional activity by the small GTPases Rac and Cdc42Hs

The Rho subfamily of GTPases is involved in control of cell morphology in mammals and yeast. The mammalian Rac and Cdc42 proteins control formation of lamellipodia and filopodia, respectively. These proteins also activate MAP kinase (MAPK) cascades that regulate gene expression. Constitutively activated forms of Rac and Cdc42Hs are efficient activators of a cascade leading to JNK and p38/Mpk2 activation. RhoA did not exhibit this activity, and none of the proteins activated the ERK subgroup of MAPKs. JNK, but not ERK, activation was also observed in response to Dbl, an oncoprotein that acts as a nucleotide exchange factor for Cdc42Hs. Results with dominant interfering alleles place Rac1 as an intermediate between Ha-Ras and MEKK in the signaling cascade leading from growth factor receptors and v-Src to JNK activation. JNK and p38 activation are likely to contribute to the biological effects of Rac, Cdc42Hs, and Dbl on cell growth and proliferation.

Altered expression of protein-tyrosine phosphatase 2C in 293 cells affects protein tyrosine phosphorylation and mitogen-activated protein kinase activation

PTP2C, an SH2 domain-containing protein-tyrosine phosphatase, is recruited to the growth factor receptors upon stimulation of cells. To investigate its role in growth factor signaling, we have overexpressed by approximately 6-fold the native PTP2C and a catalytically inactive mutant of the enzyme in 293 human embryonic kidney cells. The native PTP2C was located entirely in the cytosol, while the inactive mutant was nearly equally distributed in cytsolic and membrane fractions. Expression of the latter caused hyperphosphorylation on tyrosine of a 43-kDa protein, which was coimmunoprecipitated and co-partitioned in the plasma membrane fraction with the inactive PTP2C mutant. This protein may represent a physiological substrate of PTP2C. Overexpression of the native PTP2C enhanced epidermal growth factor (EGF)-stimulated mitogen-activated protein (MAP) kinase activity by 30%, whereas expression of the inactive mutant reduced the stimulated activity by 50%. Similar effects were observed for the activation of MAP kinase as determined by activity assay, gel mobility shift, and tyrosine phosphorylation. The data suggest that the phosphatase activity of PTP2C is partly required for MAP kinase activation by EGF and that PTP2C may function by dephosphorylating the 43-kDa membrane protein.

The dynamic role of palmitoylation in signal transduction

Guanine nucleotide binding protein (G protein)-linked receptors, the alpha-subunits of heterotrimeric G proteins and members of the Src family of nonreceptor tyrosine kinases are among many polypeptides that are posttranslationally modified by the addition of palmitate, a long-chain fatty acid. Attachment of palmitate to these proteins is dynamic and may be regulated by their activation. The presence of palmitate appears to play a key role in the membrane localization of either the entire polypeptide or parts of it, and may regulate the interactions of these polypeptides with other proteins.

The dishevelled protein is modified by wingless signaling in Drosophila

Wingless (Wg) is an important signaling molecule in the development of Drosophila, but little is known about its signal transduction pathway. Genetic evidence indicates that another segment polarity gene, dishevelled (dsh) is required for Wg signaling. We have recently developed a cell culture system for Wg protein activity, and using this in vitro system as well as intact Drosophila embryos, we have analyzed biochemical changes in the Dsh protein as a consequence of Wg signaling. We find that Dsh is a phosphoprotein, normally present in the cytoplasm. Wg signaling generates a hyperphosphorylated form of Dsh, which is associated with a membrane fraction. Overexpressed Dsh becomes hyperphosphorylated in the absence of extracellular Wg and increases levels of the Armadillo protein, thereby mimicking the Wg signal. A deletional analysis of Dsh identifies several conserved domains essential for activity, among which is a so-called GLGF/DHR motif. We conclude that dsh, a highly conserved gene, is not merely a permissive factor in Wg signaling but encodes a novel signal transduction molecule, which may function between the Wg receptor and more downstream signaling molecules.

Insulin-stimulated disassociation of the SOS-Grb2 complex

Insulin stimulation of differentiated 3T3-L1 adipocytes or Chinese hamster ovary cells expressing high levels of the insulin receptor resulted in a time-dependent decrease in the electrophoretic mobility of SOS on sodium dodecyl sulfate-polyacrylamide gels. The reduction in SOS mobility was completely reversed by alkaline phosphatase treatment, and the in vitro phosphorylation of SOS by mitogen-activated protein kinase resulted in a decrease of electrophoretic mobility identical to that following in vivo insulin stimulation. Immunoprecipitation of Grb2 followed by SOS immunoblotting demonstrated a disassociation of the SOS-Grb2 complex that paralleled the decrease in SOS electrophoretic mobility. Similarly, SOS immunoprecipitation followed by Grb2 immunoblotting also indicated an uncoupling of the SOS-Grb2 complex. Further, incubation of whole-cell extracts with glutathione-S-transferase-Grb2 fusion proteins demonstrated that insulin stimulation resulted in a decreased affinity of SOS for Grb2. In contrast, the dissociation of SOS from Grb2 did not affect the interactions between Grb2 and tyrosine-phosphorylated Shc. In addition to insulin, several other agents which activate the mitogen-activated protein kinase pathway (platelet-derived growth factor, serum, and phorbol ester) also resulted in the uncoupling of the SOS-Grb2 complex. Consistent with these results, expression of v-ras and v-raf resulted in a constitutive decrease in the association between SOS and Grb2. Together, these data suggest a molecular mechanism accounting for the transient activation of ras due to the uncoupling of the SOS-Grb2 complex following SOS phosphorylation.

In vivo functional analysis of the Ras exchange factor son of sevenless

The Son of sevenless (Sos) protein functions as a guanine nucleotide transfer factor for Ras and interacts with the receptor tyrosine kinase Sevenless through the protein Drk, a homolog of mammalian Grb2. In vivo structure-function analysis revealed that the amino terminus of Sos was essential for its function in flies. A molecule lacking the amino terminus was a potent dominant negative. In contrast, a Sos fragment lacking the Drk binding sites was functional and its activity was dependent on the presence of the Sevenless receptor. Furthermore, membrane localization of Sos was independent of Drk. A possible role for Drk as an activator of Sos is discussed and a Drk-independent interaction between Sos and Sevenless is proposed that is likely mediated by the pleckstrin homology domain within the amino terminus.

Suppression and promotion of tumor growth by monoclonal antibodies to ErbB-2 differentially correlate with cellular uptake

Amplification and overexpression of the erbB-2/neu protooncogene are frequently associated with aggressive clinical course of certain human adenocarcinomas, and therefore the encoded surface glycoprotein is considered a candidate target for immunotherapy. We previously generated a series of anti-ErbB-2 monoclonal antibodies (mAbs) that either accelerate or inhibit the tumorigenic growth of erbB-2-transformed murine fibroblasts. The present study extended this observation to a human tumor cell line grown as xenografts in athymic mice and addressed the biochemical differences between the two classes of mAbs. We show that the inhibitory effect is dominant in an antibody mixture, and it depends on antibody bivalency. By using radiolabeled mAbs we found that all of three tumor-inhibitory mAbs became rapidly inaccessible to acid treatment when incubated with tumor cells. However, a tumor-stimulatory mAb remained accessible to extracellular treatments, indicating that it did not undergo endocytosis. In addition, intracellular fragments of the inhibitory mAbs, but not of the stimulatory mAb, were observed. Electron microscopy of colloidal gold-antibody conjugates confirmed the absence of endocytosis of the stimulatory mAb but detected endocytic vesicles containing an inhibitory mAb. We conclude that acceleration of cell growth by ErbB-2 correlates with cell surface localization, whereas inhibition of tumor growth is associated with an intrinsic ability of anti-ErbB-2 mAbs to induce endocytosis. These conclusions are relevant to the selection of optimal mAbs for immunotherapy and may have implications for the mechanism of cellular transformation by an overexpressed erbB-2 gene.

The human leukemia oncogene bcr-abl abrogates the anchorage requirement but not the growth factor requirement for proliferation

Proliferation of normal cells in a multicellular organism requires not only growth factors but also the proper attachment to the extracellular matrix. A hallmark of neoplastic transformation is the loss of anchorage dependence which usually accompanies the loss of growth factor requirement. The Bcr-Abl tyrosine kinase of human leukemias is shown here to abrogate only the anchorage, not the growth factor, requirement. Bcr-Abl-transformed cells grow in soft agar but do not proliferate in serum-free media. Bcr-Abl does not activate the mitogenic pathway, as indicated by its inability to induce enhancers such as the serum response element or the tetradecanoyl phorbol acetate response element (TRE). However, Bcr-Abl can alleviate the anchorage requirement for the induction of the TRE enhancer; i.e., it allows serum to activate the TRE in detached cells. This activity is dependent on the association of an active Bcr-Abl tyrosine kinase with the actin filaments. Despite its association with the adapter protein Grb2, Bcr-Abl's effect on the TRE enhancer is not blocked by dominant negative Ras or Raf. The finding that Bcr-Abl tyrosine kinase abrogates only anchorage dependence may have important implications on the pathogenesis of chronic myelogenous leukemia.

Association between GRB2/Sos and insulin receptor substrate 1 is not sufficient for activation of extracellular signal-regulated kinases by interleukin-4: implications for Ras activation by insulin

Insulin receptor substrate 1 (IRS-1) mediates the activation of a variety of signaling pathways by the insulin and insulin-like growth factor 1 receptors by serving as a docking protein for signaling molecules with SH2 domains. We and others have shown that in response to insulin stimulation IRS-1 binds GRB2/Sos and have proposed that this interaction is important in mediating Ras activation by the insulin receptor. Recently, it has been shown that the interleukin (IL)-4 receptor also phosphorylates IRS-1 and an IRS-1-related molecule, 4PS. Unlike insulin, however, IL-4 fails to activate Ras, extracellular signal-regulated kinases (ERKs), or mitogen-activated protein kinases. We have reconstituted the IL-4 receptor into an insulin-responsive L6 myoblast cell line and have shown that IRS-1 is tyrosine phosphorylated to similar degrees in response to insulin and IL-4 stimulation in this cell line. In agreement with previous findings, IL-4 failed to activate the ERKs in this cell line or to stimulate DNA synthesis, whereas the same responses were activated by insulin. Surprisingly, IL-4's failure to activate ERKs was not due to a failure to stimulate the association of tyrosine-phosphorylated IRS-1 with GRB2/Sos; the amounts of GRB2/Sos associated with IRS-1 were similar in insulin- and IL-4-stimulated cells. Moreover, the amounts of phosphatidylinositol 3-kinase activity associated with IRS-1 were similar in insulin- and IL-4-stimulated cells. In contrast to insulin, however, IL-4 failed to induce tyrosine phosphorylation of Shc or association of Shc with GRB2. Thus, ERK activation correlates with Shc tyrosine phosphorylation and formation of an Shc/GRB2 complex. Thus, ERK activation correlates with Shc tyrosine phosphorylation and formation of an Shc/GRB2 complex. Previous studies have indicated that activation of ERks in this cell line is dependent upon Ras since a dominant-negative Ras (Asn-17) blocks ERK activation by insulin. Our findings, taken in the context of previous work, suggest that binding of GRB2/Sos to Shc may be the predominant mechanism whereby insulin as well as cytokine receptors activate Ras.

Protein modules and signalling networks

Communication between cells assumes particular importance in multicellular organisms. The growth, migration and differentiation of cells in the embryo, and their organization into specific tissues, depend on signals transmitted from one cell to another. In the adult, cell signalling orchestrates normal cellular behaviour and responses to wounding and infection. The consequences of breakdowns in this signalling underlie cancer, diabetes and disorders of the immune and cardiovascular systems. Conserved protein domains that act as key regulatory participants in many of these different signalling pathways are highlighted.

Specificity of receptor tyrosine kinase signaling: transient versus sustained extracellular signal-regulated kinase activation

A number of different intracellular signaling pathways have been shown to be activated by receptor tyrosine kinases. These activation events include the phosphoinositide 3-kinase, 70 kDa S6 kinase, mitogen-activated protein kinase (MAPK), phospholipase C-gamma, and the Jak/STAT pathways. The precise role of each of these pathways in cell signaling remains to be resolved, but studies on the differentiation of mammalian PC12 cells in tissue culture and the genetics of cell fate determination in Drosophila and Caenorhabditis suggest that the extracellular signal-regulated kinase (ERK-regulated) MAPK pathway may be sufficient for these cellular responses. Experiments with PC12 cells also suggest that the duration of ERK activation is critical for cell signaling decisions.

Polyoma virus middle T antigen: meddler or mimic?

Polyoma virus middle T antigen duplicates the actions of growth-factor receptors in binding the signalling molecules phosphatidylinositol 3'-OH kinase and Shc. These properties indicate that middle T is mitogenic and may be required to overcome inhibition of DNA replication during the lytic life cycle of the virus.

Proximity versus allostery: the role of regulated protein dimerization in biology

Regulated dimerization of proteins is increasingly understood to be important in many cellular processes, including signaling, transciption and protein degradation. Organic molecules that induce dimerization may offer as much potential to regulate biological processes as those that allosterically induce conformational change.