Interaction of Shc with Grb2 regulates association of Grb2 with mSOS

Functional importance of Shc tyrosine 317 on insulin signaling in Rat1 fibroblasts expressing insulin receptors

Shc is phosphorylated on Tyr-317, which serves as a docking site for Grb2. To investigate the specific role of Shc phosphorylation and Shc.Grb2 coupling on insulin signaling, we generated expression vectors for wild-type (WT-Shc) and a mutant Shc with a Tyr-317 --> Phe substitution (317Y/F-Shc) and stably transfected them into Rat1 fibroblasts expressing insulin receptors (HIRc). From different clonal cell lines, cells expressing 10 times greater amounts of WT-Shc or 317Y/F-Shc compared with endogenous Shc were chosen for analysis of insulin signaling. Insulin-induced Shc phosphorylation and subsequent association with Grb2 was enhanced in WT-Shc cells. Because of competition between Shc and IRS-1 for interaction with the insulin receptor, insulin-stimulated tyrosine phosphorylation of IRS-1 was decreased in WT-Shc cells compared with that in HIRc cells. Likewise, reduction of endogenous Shc expression by antisense Shc mRNA resulted in increased insulin stimulation of IRS-1 phosphorylation. Although 317Y/F-Shc was also able to interact with insulin receptor, decreased amounts of Shc phosphorylation and Shc association with Grb2 were observed in 317Y/F-Shc cells, indicating that 317Y/F-Shc functions as a dominant-negative mutant. The kinetics of mitogen-activated protein (MAP) kinase activation closely paralleled the kinetics of Shc phosphorylation. Thus, insulin stimulation of MAP kinase activation occurred more rapidly and was prolonged in WT-Shc cells, while the activation was delayed and transient in 317Y/F-Shc cells compared with that in HIRc cells. Importantly, WT-Shc cells displayed enhanced sensitivity to insulin stimulation of thymidine and bromodeoxyuridine incorporation, whereas the sensitivity was decreased in 317Y/F-Shc cells. These results indicate that Shc Tyr-317 phosphorylation plays an important role, via coupling with Grb2 and competition with IRS-1, in signal transduction to MAP kinase by insulin, ultimately leading to mitogenesis in Rat1 fibroblasts.

Identification of a novel pathway important for proliferation and differentiation of primary erythroid progenitors

Homodimerization of the erythropoietin (EPO) receptor (EPO-R) in response to EPO binding transiently activates the receptor-associated protein tyrosine kinase JAK2. Tyrosine phosphorylation of the EPO-R creates "docking sites" for SH2 domain(s) in signaling molecules such as the protein tyrosine phosphatases SH-PTP1 and SH-PTP2, phosphoinositide 3-kinase (PI3 kinase), and STAT5. However, little is known about the specific intracellular signals essential for proliferation and differentiation of erythroid progenitors. Here we show that an EPO-R containing only one cytosolic (phospho)tyrosine residue, Y479, induces a signal transduction pathway sufficient for proliferation and differentiation of fetal liver progenitors of erythroid colony-forming units from EPO-R(-/-) mice as well as for proliferation of cultured hematopoietic cells. This cascade involves sequential EPO-induced recruitment of PI3 kinase to the EPO-R and activation of mitogen-activated protein kinase activity, independent of the Shc/Grb2-adapter pathway and of STAT5. Protein kinase C epsilon may be one of the mediators connecting PI3 kinase with the mitogen-activated protein kinase signaling cascade. Our results identify a signaling cascade important in vivo for erythroid cell proliferation and differentiation.

A Tyrosine-Phosphorylated Protein of 140 kD Is Constitutively Associated With the Phosphotyrosine Binding Domain of Shc and the SH3 Domains of Grb2 in Acute Myeloid Leukemia Cells

The Shc gene encodes three proteins that have been implicated as mediators of signal transduction from growth factor receptors and nonreceptor tyrosine kinases to Ras. Overexpression of Shc in established murine fibroblasts results in oncogenic transformation, indicating that Shc has oncogenic potential. Shc proteins contain a carboxy terminal SH2 domain and a novel non-SH2 phosphotyrosine-binding (PTB) domain that specifically recognizes a phosphorylated NPXpY motif in target proteins such as the epidermal growth factor receptor. We show here that Shc is constitutively tyrosine-phosphorylated in all primary acute myeloid leukemias analyzed and that, in some of these leukemias, Shc is associated through its PTB domain with a tyrosinephosphorylated protein of 140 kD (p140) in vivo. In factor-dependent cells, this 140-kD protein can be tyrosine-phosphorylated in vitro in response to cytokines involved in myeloid proliferation and differentiation, ie, granulocyte-macrophage colony-stimulating factor and colony-stimulating factor-1. A similar or identical protein of 140 kD is constitutively bound to the C-terminal SH3 domain of Grb2 in the same acute myeloid leukemias. In addition to p140, other tyrosine-phosphorylated proteins of 61 and 200 kD are constitutively associated with Shc in some of the leukemias analyzed. Our results implicate Shc, Grb2, p140, and additional tyrosine-phosphorylated proteins of 61 and 200 kD in signalling of acute myeloid leukemia cells.

Cripto enhances the tyrosine phosphorylation of Shc and activates mitogen-activated protein kinase (MAPK) in mammary epithelial cells

Cripto-1 (CR-1), a recently discovered protein of the epidermal growth factor (EGF) family, was found to interact with a high affinity, saturable binding site(s) on HC-11 mouse mammary epithelial cells and on several different human breast cancer cell lines. This receptor exhibits specificity for CR-1, since other EGF-related peptides including EGF, transforming growth factor alpha, heparin-binding EGF-like growth factor, amphiregulin, epiregulin, betacellulin, or heregulin beta1 that bind to either the EGF receptor or to other type 1 receptor tyrosine kinases such as erb B-3 or erb B-4 fail to compete for binding. Conversely, CR-1 was found not to directly bind to or to activate the tyrosine kinases associated with the EGFR, erb B-2, erb B-3, or erb B-4 either alone or in various pairwise combinations which have been ectopically expressed in Ba/F3 mouse pro-B lymphocyte cells. However, exogenous CR-1 could induce an increase in the tyrosine phosphorylation of 185- and 120-kDa proteins and a rapid (within 3-5 min) increase in the tyrosine phosphorylation of the SH2-containing adaptor proteins p66, p52, and p46 Shc in mouse mammary HC-11 epithelial cells and in human MDA-MB-453 and SKBr-3 breast cancer cells. CR-1 was also found to promote an increase in the association of the adaptor Grb2-guanine nucleotide exchange factor-mouse son of sevenless (mSOS) signaling complex with tyrosine-phosphorylated Shc in HC-11 cells. Finally, CR-1 was able to increase p42(erk-2) mitogen-activated protein kinase (MAPK) activity in HC-11 cells within 5-10 min of treatment. These data demonstrate that CR-1 can function through a receptor which activates intracellular components in the ras/raf/MEK/MAPK pathway.

Tandem SH2 binding sites mediate the RasGAP-RhoGAP interaction: a conformational mechanism for SH3 domain regulation

Many cellular signaling proteins contain SH3 (Src homology 3) domains that mediate protein interactions via specific proline-containing peptides. Unlike SH2 domains, whose interactions with tyrosine-containing peptides are promoted by phosphorylation of the SH2 binding site, the regulatory mechanism for SH3 interactions is unclear. p120 RasGAP (GTPase-activating protein), which contains an SH3 domain flanked by two SH2 domains, forms an abundant SH2-mediated complex with p190 RhoGAP in cells expressing activated tyrosine kinases. We have identified two closely linked tyrosine-containing peptides in p190 that bind simultaneously to the RasGAP SH2 domains upon p190 phosphorylation. This interaction is expected to bring the two SH2 domains into close proximity. Consequently, RasGAP undergoes a conformational change that results in a 100-fold increase in the accessibility of the target binding surface of its SH3 domain. These results indicate that the tandem arrangement of SH2 and SH3 domains found in a variety of cellular signaling proteins can provide a conformational mechanism for regulating SH3-dependent interactions through tyrosine phosphorylation. In addition, it appears that the role of p190 in the RasGAP signaling complex is to promote additional protein interactions with RasGAP via its SH3 domain.

The role of a lymphoid-restricted, Grb2-like SH3-SH2-SH3 protein in T cell receptor signaling

We have characterized an SH3-SH2-SH3 linker protein that is prominently expressed in lymphoid tissues. This protein has 58% sequence identity to Grb2. An identical protein called Grap has been found in hematopoietic cells. In Jurkat cells, T cell receptor activation leads to the association of Grap with phosphoproteins p36/38 and, to a lesser degree, Shc. This interaction is mediated by the Grap SH2 domain, which has similar binding specificity to the Grb2 SH2 domain. Grap also associates via its SH3 domains with Sos, the Ras guanine nucleotide exchange factor; with dynamin, a GTPase involved in membrane protein trafficking; and with Sam68, a nuclear RNA-binding protein that serves as a substrate of Src kinases during mitosis. T cell activation effects an increase in Grap association with p36/38, Shc, Sos, and dynamin. Sam68 binding is constitutive. Phospholipase C-gamma1 and Fyn are also found in activated Grap signaling complexes, although these interactions may not be direct. We conclude that Grap is a prominent component of lymphocyte receptor signaling. Based on the known functions of bound effector molecules, Grap-mediated responses to antigen challenge may include endocytosis of the T cell receptor, cellular proliferation, and regulated entry into the cell cycle.

Interferon-β Interrupts Interleukin-6–Dependent Signaling Events in Myeloma Cells

Type I interferons (IFNs-α and IFN-β) bind to a common receptor to exert strong antiproliferative activity on a broad range of cell types, including interleukin-6 (IL-6)–dependent myeloma cells. In this study, we investigated the effect of IFN-β pretreatment on IL-6–stimulated mitogenic signaling in the human myeloma cell line U266. IL-6 induced transient tyrosine phosphorylation of the IL-6 receptor signal-transducing subunit gp130, the gp130-associated protein tyrosine kinases Jak1, Jak2, and Tyk2, the phosphotyrosine phosphatase PTP1D/Syp, the adaptor protein Shc and the mitogen-activated protein kinase Erk2, and accumulation of GTP-bound p21ras. Prior treatment of U266 cells with IFN-β downregulated IL-6–induced tyrosine phosphorylation of gp130, Jak2, PTP1D/Syp, Shc, and Erk2, and GTP-loading of p21ras. Further analysis indicated that treatment with IFN-β disrupted IL-6–induced binding of PTP1D/Syp to gp130 and the adaptor protein Grb2; IFN-β pretreatment also interfered with IL-6–induced interaction of Shc with Grb2 and a 145-kD tyrosine-phosphorylated protein. These results suggest a novel mechanism whereby type I IFNs interrupt IL-6–promoted mitogenesis of myeloma cells in part by preventing the formation of essential signaling complexes leading to p21ras activation.

Erythropoietin Activates Raf1 by an Shc-Independent Pathway in CTLL-EPO-R Cells

Stimulation of the erythropoietin receptor (EPO-R) or the interleukin-2 receptor (IL-2-R) by their respective ligands has been reported to activate tyrosine phosphorylation of the cytoplasmic protein, Shc. We have recently characterized a cell line, CTLL-EPO-R, that contains functional cell-surface receptors for both EPO and IL-2. Although stimulation with IL-2 or IL-15 resulted in the rapid, dose-dependent tyrosine phosphorylation of Shc, stimulation with EPO failed to activate Shc. EPO, IL-2, and IL-15 activated the tyrosine phosphorylation of the adaptor protein, Shp2, and the association of Shp2/Grb2/cytokine receptor complexes. In addition, EPO, IL-2, and IL-15 activated Raf1 and ERK2, demonstrating that the Raf1/MEK/MAP kinase pathway was activated. These results indicate that multiple biochemical pathways are capable of conferring a mitogenic signal in CTLL-EPO-R. EPO can activate the Raf1/MEK/ MAP kinase pathway via Shc-dependent or Shc-independent pathways, and Shc activation is not required for EPO-dependent cell growth in CTLL-EPO-R.

Interferon-β Interrupts Interleukin-6–Dependent Signaling Events in Myeloma Cells

Type I interferons (IFNs-α and IFN-β) bind to a common receptor to exert strong antiproliferative activity on a broad range of cell types, including interleukin-6 (IL-6)–dependent myeloma cells. In this study, we investigated the effect of IFN-β pretreatment on IL-6–stimulated mitogenic signaling in the human myeloma cell line U266. IL-6 induced transient tyrosine phosphorylation of the IL-6 receptor signal-transducing subunit gp130, the gp130-associated protein tyrosine kinases Jak1, Jak2, and Tyk2, the phosphotyrosine phosphatase PTP1D/Syp, the adaptor protein Shc and the mitogen-activated protein kinase Erk2, and accumulation of GTP-bound p21ras. Prior treatment of U266 cells with IFN-β downregulated IL-6–induced tyrosine phosphorylation of gp130, Jak2, PTP1D/Syp, Shc, and Erk2, and GTP-loading of p21ras. Further analysis indicated that treatment with IFN-β disrupted IL-6–induced binding of PTP1D/Syp to gp130 and the adaptor protein Grb2; IFN-β pretreatment also interfered with IL-6–induced interaction of Shc with Grb2 and a 145-kD tyrosine-phosphorylated protein. These results suggest a novel mechanism whereby type I IFNs interrupt IL-6–promoted mitogenesis of myeloma cells in part by preventing the formation of essential signaling complexes leading to p21ras activation.

Erythropoietin Activates Raf1 by an Shc-Independent Pathway in CTLL-EPO-R Cells

Stimulation of the erythropoietin receptor (EPO-R) or the interleukin-2 receptor (IL-2-R) by their respective ligands has been reported to activate tyrosine phosphorylation of the cytoplasmic protein, Shc. We have recently characterized a cell line, CTLL-EPO-R, that contains functional cell-surface receptors for both EPO and IL-2. Although stimulation with IL-2 or IL-15 resulted in the rapid, dose-dependent tyrosine phosphorylation of Shc, stimulation with EPO failed to activate Shc. EPO, IL-2, and IL-15 activated the tyrosine phosphorylation of the adaptor protein, Shp2, and the association of Shp2/Grb2/cytokine receptor complexes. In addition, EPO, IL-2, and IL-15 activated Raf1 and ERK2, demonstrating that the Raf1/MEK/MAP kinase pathway was activated. These results indicate that multiple biochemical pathways are capable of conferring a mitogenic signal in CTLL-EPO-R. EPO can activate the Raf1/MEK/ MAP kinase pathway via Shc-dependent or Shc-independent pathways, and Shc activation is not required for EPO-dependent cell growth in CTLL-EPO-R.

The Grb2-mSos1 complex binds phosphopeptides with higher affinity than Grb2

Epidermal growth factor (EGF) stimulation leads to autophosphorylation of the epidermal growth factor receptor (EGFR) and tyrosine phosphorylation of Shc. The Grb2 SH2 domain binds to Tyr1068 of EGFR and Tyr317 of Shc while its SH3 domains bind to mSos1. Therefore, EGF treatment potentially results in the formation of several multimeric signaling complexes, including EGFR-Grb2-mSos1, EGFR-Shc-Grb2-mSos1, and Shc-Grb2-mSos1, linking the receptor to activation of the Ras GTPase. We have purified Grb2, mSos1, and the Grb2-mSos1 complex to high homogeneity, and used these isolated proteins to obtain binding affinities of mSos1 for Grb2 and of either Grb2 or Grb2-mSos1 for phosphotyrosine-containing peptides. mSos1 bound Grb2 with a KD of 0.4 microM; the stoichiometry of the Grb2-mSos1 complex was 1:1. An EGFR-derived phosphopeptide bound Grb2 with a KD of 0.7 microM, whereas the Shc-derived phosphopeptide bound Grb2 with a KD of 0.2 microM. Since Grb2 exists in a stable complex with mSos1, and both proteins can exist in a constitutive complex in unstimulated cells, we performed phosphopeptide binding studies on the Grb2-mSos1 complex to gain a better understanding of binding events in the intact cell. Grb2-mSos1 bound to both EGFR- and Shc-derived phosphopeptides with higher affinities (KD of 0.3 microM and 31 nM, respectively) than Grb2 alone. These findings suggest that the proximity of mSos1 to Grb2 in the complex can influence the interactions of the Grb2 SH2 domain with phosphopeptides and raise the possibility that in the Grb2-mSos1 complex the SH2 and SH3 domains of Grb2 are not independent of each other but may be indirectly linked by mSos1.

Association between receptor protein-tyrosine phosphatase RPTPalpha and the Grb2 adaptor. Dual Src homology (SH) 2/SH3 domain requirement and functional consequences

Receptor protein-tyrosine phosphatase RPTPalpha is found associated in vivo with the adaptor protein Grb2. Formation of this complex, which contains no detectable levels of Sos, is known to depend on a C-terminal phosphorylated tyrosine residue (Tyr798) in RPTPalpha and on the Src homology (SH) 2 domain in Grb2 (, ). We show here that association of Grb2 with RPTPalpha also involves a critical function for the C-terminal SH3 domain of Grb2. Furthermore, Grb2 SH3 binding peptides interfere with RPTPalpha-Grb2 association in vitro, and the RPTPalpha protein can dissociate the Grb2-Sos complex in vivo. These observations constitute a novel mode of Grb2 association and suggest a model in which association with a tyrosine-phosphorylated protein restricts the repertoire of SH3 binding proteins with which Grb2 can simultaneously interact. The function of the Tyr798 tyrosine phosphorylation/Grb2 binding site in RPTPalpha was studied further by expression of wild type or mutant RPTPalpha proteins in PC12 cells. In these cells, wild type RPTPalpha interferes with acidic fibroblast growth factor-induced neurite outgrowth; this effect requires both the catalytic activity and the Grb2 binding Tyr798 residue in RPTPalpha. In contrast, expression of catalytically active RPTPalpha containing a mutated tyrosine phosphorylation/Grb2 association site enhances neurite outgrowth. Our observations associate a functional effect with tyrosine phosphorylation of, and ensuing association of signaling proteins with, a receptor protein-tyrosine phosphatase and raise the possibility that RPTPalpha association may modulate Grb2 function and vice versa.

Enhanced tumorigenic behavior of glioblastoma cells expressing a truncated epidermal growth factor receptor is mediated through the Ras-Shc-Grb2 pathway

A mutant epidermal growth factor receptor (DeltaEGFR) containing a deletion of 267 amino acids from the extracellular domain is common in human glioblastomas. We have previously shown that the mutant receptor fails to bind EGF, is constitutively phosphorylated, and confers upon U87MG glioblastoma cells expressing it (U87MG. DeltaEGFR), an increased ability to form tumors in mice. Here we demonstrate that the constitutively phosphorylated DeltaEGFR enhances growth of glioblastoma cells through increased activity of Ras: 1) there was an increase in the proportion of Ras present in the GTP-bound form, and 2) introduction of neutralizing anti-Ras 259 antibodies into U87MG and U87MG.DeltaEGFR cells by microinjection inhibited DNA synthesis to the same low level in both cell populations. We also show that the truncated EGF receptor constitutively associates with the adapter proteins Shc and Grb2 which are involved in the recruitment of Ras to activated receptors. Several derivatives of DeltaEGFR containing single, or multiple mutations at critical autophosphorylation sites were constructed and used to demonstrate that the major Shc binding site is Tyr-1148, and that Grb2 association occurs primarily through Tyr-1068. We conclude that the increased tumorigenic potential of glioblastoma cells expressing the truncated EGF receptor is due at least in part to Ras activation presumably involving the Shc and Grb2 adapter proteins.

Role of the Lck Src homology 2 and 3 domains in protein tyrosine phosphorylation

Many protein tyrosine phosphorylation events that occur as a result of T cell receptor (TCR) stimulation are enhanced when CD4 is co-cross-linked with the TCR, and this increased phosphorylation is thought to be a mechanism by which T cell functions are augmented by CD4. Such enhanced tyrosine phosphorylation was originally attributed to the kinase activity of the CD4-associated tyrosine kinase Lck. However, it has been shown that CD4-associated Lck lacking the catalytic domain can enhance T cell functions, suggesting that the noncatalytic domains of Lck are also important in CD4 signaling. Using T cells expressing various CD4-Lck chimeric molecules, we assessed the role of different Lck domains in early T cell signaling. Following TCR-CD4 co-cross-linking, cells expressing a CD4-Lck full-length chimera showed enhanced tyrosine phosphorylation of many cellular proteins in a CD4-dependent manner. Surprisingly, cells expressing a CD4-Lck chimera lacking the catalytic domain (termed CD4-N32) also showed enhanced phosphorylation. This enhancement of phosphorylation required both the Src homology 2 (SH2) and SH3 domains of Lck. Lck has been postulated to dimerize through the SH2 and SH3 domains. In this way CD4-N32 may interact with endogenous Lck, and although it lacks intrinsic kinase activity, it may be capable of enhancing phosphorylation through the associated full-length Lck. Consistent with this model, when CD4-Lck chimeric molecules were expressed in J. CaM1.6 cells lacking endogenous Lck, CD4-N32 failed to enhance tyrosine phosphorylation. Moreover, a Lck SH2 and SH3 domain fragment expressed as a glutathione S-transferase fusion protein associated with Lck when incubated with activated Jurkat T cell lysates, suggesting that the SH2 and SH3 domains of Lck can associate with endogenous full-length Lck upon activation. Thus, our data suggest that dimerization is an important mechanism of Lck function in T cell activation.

Functional importance of amino-terminal domain of Shc for interaction with insulin and epidermal growth factor receptors in phosphorylation-independent manner

Shc has two distinct domains, amino-terminal and SH2 domain, which can interact with activated growth factor receptors. Shc interacts with insulin receptor via Shc-amino-terminal (N) domain, whereas Shc associates with epidermal growth factor (EGF) receptor through both Shc-N and -SH2 domains. In accordance with the different functional roles between insulin and EGF receptors, EGF stimulated tyrosine phosphorylation of Shc faster than insulin. To clarify the functional importance of three distinct Shc domains on insulin and EGF signaling, we microinjected glutathione S-transferase (GST) fusion proteins containing the amino terminus plus collagen homology domain (NCH), collagen homology domain (CH), and Src homology 2 domain (SH2) into Rat1 fibroblasts expressing insulin receptors (HIRc). Bromodeoxyuridine (BrdUrd) incorporation into newly synthesized DNA was subsequently studied to assess the importance of the three distinct domains of Shc. Microinjection of the NCH-GST fusion protein inhibited BrdUrd incorporation induced by both EGF and insulin, whereas microinjection of the SH2-GST fusion protein inhibited EGF, but not insulin stimulation of DNA synthesis. Neither EGF- nor insulin-induced BrdUrd incorporation was inhibited by the CH-GST fusion protein. Following EGF or insulin stimulation, Shc is phosphorylated on single Tyr-317 residue serving as a docking site for Grb2. Microinjection of Shc-N+CH GST fusion protein with Tyr-317 --> Phe replacement (Y317F) also inhibited insulin stimulation of DNA synthesis. Next, we stably overexpressed wild-type Shc or Y317F mutant Shc into HIRc cells. Insulin-induced tyrosine phosphorylation of IRS-1 was compared among the transfected cell lines, since IRS-1 and Shc could competitively interact with insulin receptor. Insulin-stimulated tyrosine phosphorylation of IRS-1 was decreased in both WT-Shc and Y317F-Shc cells compared with that in HIRc cells. Furthermore, overexpression of the Shc-SH2 domain or Shc-N+CH domain with Y317F mutation interfered with EGF-stimulated endogenous Shc phosphorylation. These results suggest that the amino terminus domain of Shc is functionally important in insulin- and EGF-induced cell cycle progression and that the phosphorylation of Shc Tyr-317 residue is independent of Shc interaction with these receptors.

Insulin and epidermal growth factor receptors regulate distinct pools of Grb2-SOS in the control of Ras activation

Insulin and epidermal growth factor (EGF) stimulate a rapid but transient increase in the amount of GTP bound to Ras that returns to the basal GDP-bound state within 10-30 min. Although insulin stimulation resulted in a dissociation of the Grb2.SOS complex, EGF did not affect the Grb2.SOS complex but instead induced dissociation of Grb2-SOS from tyrosine-phosphorylated Shc. The dissociation of Grb2-SOS from Shc was not due to dephosphorylation as Shc remained persistently tyrosine-phosphorylated during this time. Furthermore, there was no decrease in the extent of insulin receptor substrate 1, insulin receptor, or EGF receptor tyrosine phosphorylation. Surprisingly, however, despite the EGF-induced decrease in the amount of Grb2-SOS bound to Shc, the extent of Grb2 associated with Shc remained constant, and there was a concomitant increase in the amount of SOS associated with Grb2. In addition, after the insulin-stimulated dissociation of Grb2 from SOS, EGF treatment induced the reassociation of the Grb2.SOS complex. Quantitative immunoprecipitation demonstrated that only a small fraction of the total cellular pool of Grb2 was associated with SOS. Similarly, only a small fraction of SOS and Grb2 were co-immunoprecipitated with Shc. Together, these data suggest the presence of distinct Grb2-SOS pools that are independently utilized by insulin and EGF in their recruitment to tyrosine-phosphorylated Shc.

Tight association of GRB2 with receptor protein-tyrosine phosphatase alpha is mediated by the SH2 and C-terminal SH3 domains

Receptor protein-tyrosine phosphatase alpha (RPTPalpha), a transmembrane member of the extensive family of protein-tyrosine phosphatases (PTPs), is constitutively phosphorylated on Tyr789, a consensus binding site for the SH2 domain of the SH3-SH2-SH3 adaptor protein GRB2. We have previously shown that GRB2 binds to P.Tyr789 in vivo and in vitro via its SH2 domain. Here, we report that not only the GRB2 SH2 domain, but also the C-terminal SH3 domain is involved in binding to RPTPalpha in vitro and in vivo. Although the N-terminal SH3 domain of GRB2 is essential for binding to the Ras guanine nucleotide exchange factor Son of Sevenless (Sos), an RPTPalpha-GRB2-Sos complex could not be detected. The inclusion of peptides encompassing an hSos1 proline-rich motif in cell lysates resulted in enhanced binding of RPTPalpha to GRB2 in vitro, suggesting that steric hindrance prohibits formation of the RPTPalpha-GRB2-Sos complex. In vitro binding experiments indicated that the binding of GRB2 to Sos/dynamin and RPTPalpha was mutually exclusive. Analysis of in vitro binding kinetics coupled with results from transient co-transfections demonstrated that RPTPalpha is tightly bound to GRB2. The site of interaction of the C-terminal SH3 domain of GRB2 with RPTPalpha was mapped using deletion mutants to an 18-residue region in the N-terminal PTP domain. Arg469, within this region, was identified as one of the residues that is involved in the interaction with the C-terminal SH3 domain of GRB2. RPTPalpha residues 469-486 are localized close to the catalytic site cleft in the structure of the N-terminal PTP-domain, suggesting that interaction with the C-terminal SH3 domain may block access to the catalytic site, thus inhibiting RPTPalpha activity.

Evidence for a role for the phosphotyrosine-binding domain of Shc in interleukin 2 signaling

Stimulation via the T-cell growth factor interleukin 2 (IL-2) leads to tyrosine phosphorylation of Shc, the interaction of Shc with Grb2, and the Ras GTP/GDP exchange factor, mSOS. Shc also coprecipitates with the IL-2 receptor (IL-2R), and therefore, may link IL-2R to Ras activation. We have further characterized the Shc-IL-2R interaction and have made the following observations. (i) Among the two phosphotyrosine-interaction domains present in Shc, the phosphotyrosine-binding (PTB) domain, rather than its SH2 domain, interacts with the tyrosine-phosphorylated IL-2R beta chain. Moreover, the Shc-PTB domain binds a phosphopeptide derived from the IL-2R beta chain (corresponding to residues surrounding Y338, SCFTNQGpYFF) with high affinity. (ii) In vivo, mutant IL-2R beta chains lacking the acidic region of IL-2Rbeta (which contains Y338) fail to phosphorylate Shc. Furthermore, when wild type or mutant Shc proteins that lack the PTB domain were expressed in the IL-2-dependent CTLL-20 cell line, an intact Shc-PTB domain was required for Shc phosphorylation by the IL-2R, which provides further support for a Shc-PTB-IL-2R interaction in vivo. (iii) PTB and SH2 domains of Shc associate with different proteins in IL-2- and T-cell-receptor-stimulated lysates, suggesting that Shc, through the concurrent use of its two different phosphotyrosine-binding domains, could assemble multiple protein complexes. Taken together, our in vivo and in vitro observations suggest that the PTB domain of Shc interacts with Y338 of the IL-2R and provide evidence for a functional role for the Shc-PTB domain in IL-2 signaling.

Phosphatidylinositol 3-kinase is an early intermediate in the G beta gamma-mediated mitogen-activated protein kinase signaling pathway

The beta gamma-subunit of Gi mediates mitogen-activated protein (MAP) kinase activation through a signaling pathway involving Shc tyrosine phosphorylation, subsequent formation of a multiprotein complex including Shc, Grb2, and Sos, and sequential activation of Ras, Raf, and MEK. The mechanism by which G beta gamma mediates tyrosine phosphorylation of Shc, however, is unclear. This study assesses the role of phosphatidylinositol 3-kinase (PI-3K) in G beta gamma-mediated MAP kinase activation. We show that Gi-coupled receptor- and G beta gamma-stimulated MAP kinase activation is attenuated by the PI-3K inhibitors wortmannin and LY294002 or by over expression of a dominant negative mutant of the p85 subunit of PI-3K. Wortmannin and LY294002 also inhibit Gi-coupled receptor-stimulated Ras activation. The PI-3K inhibitors do not affect MAP kinase activation stimulated by over-expression of Sos, a constitutively active mutant of Ras, or a constitutively active mutant of MEK. These results demonstrate that PI-3K activity is required in the G beta gamma-mediated MAP kinase signaling pathway at a point upstream of Sos and Ras activation.

Evidence for a physical association between the Shc-PTB domain and the beta c chain of the granulocyte-macrophage colony-stimulating factor receptor

Granulocyte-macrophage colony-stimulating factor (GM-CSF) regulates the growth and function of several myeloid cell types at different stages of maturation. The effects of GM-CSF are mediated through a high affinity receptor that is composed of two chains: a unique, ligand-specific alpha chain and a beta common chain (beta c) that is also a component of the receptors for interleukin 3 (IL-3) and IL-5. Beta c plays an essential role in the transduction of extra cellular signals to the nucleus through its recruitment of secondary messengers. Several downstream signaling events induced by GM-CSF stimulation have been described, including activation of tyrosine kinases and tyrosine phosphorylation of cellular proteins (including beta c) and activation of the Ras/mitogen-activated protein kinase and the JAK/STAT pathways. A region within the beta c cytoplasmic tail (amino acids 517-763) has been reported to be necessary for tyrosine phosphorylation of the adapter protein, Shc, and for the subsequent GM-CSF-induced activation of Ras. In this paper, we describe a physical association between the tyrosine phosphorylated GM-CSF receptor (GMR)-beta c chain and Shc in vivo. Using a series of cytoplasmic truncation mutants of beta c and various mutant Shc proteins, we demonstrate that the N-terminal phosphotyrosine-binding (PTB) domain of Shc binds to a short region of beta c (amino acids 549-656) that contains Tyr577. Addition of a specific phosphopeptide encoding amino acids surrounding this tyrosine inhibited the interaction between beta c and shc. Moreover, mutation of a key residue within the phosphotyrosine binding pocket of the Shc-PTB domain abrogated its association with beta c. These observations provide an explanation for the previously described requirement for Tyr577 of beta c for GM-CSF-induced tyrosine phosphorylation of Shc and have implications for Ras activation through the GM-CSF, IL-3, and IL-5 receptors.

Tyrosine phosphorylation of ACK in response to temperature shift-down, hyperosmotic shock, and epidermal growth factor stimulation

The mammalian Cdc42 protein regulates various kinds of cellular responses, including formation of filopodia, polarization of T cells, and cell cycle progression. A non-receptor tyrosine kinase ACK, which specifically binds to the GTP-bound form of Cdc42, was isolated as a putative target of Cdc42. Here we show the induction of tyrosine phosphorylation of ACK in response to temperature shift-down to 25 degrees C, and hypertonic shock, as well as stimulation with epidermal growth factor (EGF) in human embryonic kidney (HEK) 293 cells. The increased tyrosine phosphorylation level upon temperature shift-down was sustained for at least 60 min, whereas reversion of the temperature to 37 degrees C caused rapid tyrosine dephosphorylation to the initial level. The responses to EGF and the high osmolarity were transient. Furthermore, we observed association of ACK with an adaptor protein Grb2, which may suggest the involvement of Grb2 in EGF receptor-mediated tyrosine phosphorylation of ACK.

Co-purification and direct interaction of Ras with caveolin, an integral membrane protein of caveolae microdomains. Detergent-free purification of caveolae microdomains

Caveolae are plasma membrane specializations that have been implicated in signal transduction. Caveolin, a 21-24-kDa integral membrane protein, is a principal structural component of caveolae membranes in vivo. G protein alpha subunits are concentrated in purified preparations of caveolae membranes, and caveolin interacts directly with multiple G protein alpha subunits, including G(s), G(o), and G(i2). Mutational or pharmacologic activation of G alpha subunits prevents the interaction of caveolin with G proteins, indicating that inactive G alpha subunits preferentially interact with caveolin. Here, we show that caveolin interacts with another well characterized signal transducer, Ras. Using a detergent-free procedure for purification of caveolin-rich membrane domains and a polyhistidine tagged form of caveolin, we find that Ras and other classes of lipid-modified signaling molecules co-fractionate and co-elute with caveolin. The association of Ras with caveolin was further evaluated using two distinct in vitro binding assays. Wild-type H-Ras interacted with glutathione S-transferase (GST)-caveolin fusion proteins but not with GST alone. Using a battery of GST fusion proteins encoding distinct regions of caveolin, Ras binding activity was localized to a 41-amino acid membrane proximal region of the cytosolic N-terminal domain of caveolin. In addition, reconstituted caveolin-rich membranes (prepared with purified recombinant caveolin and purified lipids) interacted with a soluble form of wild-type H-Ras but failed to interact with mutationally activated soluble H-Ras (G12V). Thus, a single amino acid change (G12V) that constitutively activates Ras prevents or destabilizes this interaction. These results clearly indicate that (i) caveolin is sufficient to recruit soluble Ras onto lipid membranes and (ii) membrane-bound caveolin preferentially interacts with inactive Ras proteins. In direct support of these in vitro studies, we also show that recombinant overexpression of caveolin in intact cells is sufficient to functionally recruit a nonfarnesylated mutant of Ras (C186S) onto membranes, overcoming the normal requirement for lipid modification of Ras. Taken together, these observations suggest that caveolin may function as a scaffolding protein to localize or sequester certain caveolin-interacting proteins, such as wild-type Ras, within caveolin-rich microdomains of the plasma membrane.

Interleukin-4 induces activation of mitogen-activated protein kinase and phosphorylation of shc in human keratinocytes

Most cytokines stimulate the p21ras pathway, leading to MAP kinase activation. One exception is interleukin-4 (IL-4), which has been shown not to activate this pathway in hematopoietic cells. However, IL-4 acts on a broad range of cells, including keratinocytes, in which it induces IL-6 production. We report here that IL-4 stimulation of human keratinocytic cell lines or primary cultures activates MAP kinase. In these cells, IL-4 stimulation induces the tyrosine phosphorylation of p42/44 MAP kinase as well as its catalytic activity. We also observed an increased phosphorylation of p46shc, an SH2-containing protein involved in the Ras pathway, as a result of IL-4 stimulation in human keratinocytic cell lines but not in T lymphocytes.

Stimulation through the T cell receptor induces Cbl association with Crk proteins and the guanine nucleotide exchange protein C3G

We and others have recently identified Cbl, the protein product of the c-cbl protooncogene, as an early tyrosine kinase substrate upon T cell activation and have shown that Cbl forms in vivo complexes with Src family tyrosine kinases, Grb2 adaptor protein, and the p85 subunit of PI-3 kinase. Here we show that Cbl associates with all three forms of the human Crk protein, predominantly CrkL, following T cell receptor activation of Jurkat T cells. Association between Cbl and Crk proteins was confirmed in normal human peripheral blood-derived T cells. In vitro, Cbl was able to interact with the Crk SH2 domain but not the SH3 domain. A phosphopeptide corresponding to a potential Crk SH2 domain-binding motif in Cbl (pYDVP) specifically inhibited binding between Cbl and Crk SH2 domain. Anti-Cbl antibody completely immunodepleted the CrkL-associated 120kDa phosphotyrosyl polypeptide, suggesting that the recently described p130cas-related Crk-associated p116 of T cells may be Cbl. Consistent with this possibility, the 4F4 antibody used to characterize the p116 polypeptide cross-reacted with Cbl protein when it was resolved on one- or two-dimensional gels. CrkL was constitutively associated with a substantial amount of the guanine nucleotide exchange protein C3G, and a fraction of the C3G protein was coimmunoprecipitated with Cbl in activated Jurkat T cells. These results suggest the possibility that Cbl may participate in a signaling pathway that regulates guanine nucleotide exchange on small G-proteins in T cells.

Regulation of epidermal growth factor receptor signaling by phosphorylation of the ras exchange factor hSOS1

In response to stimulation with epidermal growth factor (EGF), the guanine nucleotide exchange factor human SOS1 (hSOS1) promotes the activation of Ras by forming a complex with Grb2 and the human EGF receptor (hEGFR). hSOS1 was phosphorylated in cells stimulated with EGF or phorbol 12-myristate 13-acetate or following co-transfection with activated Ras or Raf. Co-transfection with dominant negative Ras resulted in a decrease of EGF-induced hSOS1 phosphorylation. The mitogen-activated protein kinase (MAPK) phosphorylated hSOS1 in vitro within the carboxyl-terminal proline-rich domain. The same region of hSOS1 was phosphorylated in vivo, in cells stimulated with EGF. Tryptic phosphopeptide mapping showed that MAPK phosphorylated hSOS1 in vitro on sites which were also phosphorylated in vivo. Phosphorylation by MAPK did not affect hSOS1 binding to Grb2 in vitro. However, reconstitution of the hSOS1-Grb2-hEGFR complex showed that phosphorylation by MAPK markedly reduced the ability of hSOS1 to associate with the hEGFR through Grb2. Similarly, phosphorylated hSOS1 was unable to form a complex with Shc through Grb2. Thus phosphorylation of hSOS1, by affecting its interaction with the hEGFR or Shc, down-regulates signal transduction from the hEGFR to the Ras pathway.

The 145-kDa protein induced to associate with Shc by multiple cytokines is an inositol tetraphosphate and phosphatidylinositol 3,4,5-triphosphate 5-phosphatase

A 145-kDa tyrosine-phosphorylated protein that becomes associated with Shc in response to multiple cytokines has been purified from the murine hemopoietic cell line B6SUtA1. Amino acid sequence data were used to clone the cDNA encoding this protein from a B6SUtA1 library. The predicted amino acid sequence encodes a unique protein containing an N-terminal src homology 2 domain, two consensus sequences that are targets for phosphotyrosine binding domains, a proline-rich region, and two motifs highly conserved among inositol polyphosphate 5-phosphatases. Cell lysates immunoprecipitated with antiserum to this protein exhibited both phosphatidylinositol 3,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate polyphosphate 5-phosphatase activity. This novel signal transduction intermediate may serve to modulate both Ras and inositol signaling pathways. Based on its properties, we suggest the 145-kDa protein be called SHIP for SH2-containing inositol phosphatase.

The heterotrimeric G q protein-coupled angiotensin II receptor activates p21 ras via the tyrosine kinase-Shc-Grb2-Sos pathway in cardiac myocytes

p21 ras plays as important role in cell proliferation, transformation and differentiation. Recently, the requirement of p21 ras has been suggested for cellular responses induced by stimulation of heterotrimeric G protein-coupled receptors. However, it remains to be determined how agonists for G protein-coupled receptors activate p21 ras in metazoans. We show here that stimulation of the G q protein-coupled angiotensin II (Ang II) receptor causes activation of p21 ras in cardiac myocytes. The p21 ras activation by Ang II is mediated by an increase in the guanine nucleotide exchange activity, but not by an inhibition of the GTPase-activating protein. Ang II causes rapid tyrosine phosphorylation of Shc and its association with Grb2 and mSos-1, a guanine nucleotide exchange factor of p21 ras. This leads to translocation of mSos-1 to the membrane fraction. Shc associates with the SH3 domain of Fyn whose tyrosine kinase activity is activated by Ang II with a similar time course as that of tyrosine phosphorylation of Shc. Ang II-induced increase in the guanine nucleotide exchange activity was inhibited by a peptide ligand specific to the SH3 domain of the Src family tyrosine kinases. These results suggest that an agonist for a pertussis toxin-insensitive G protein-coupled receptor may initiate the cross-talk with non-receptor-type tyrosine kinases, thereby activating p21 ras using a similar mechanism as receptor tyrosine kinase-induced p21 ras activation.

Gastrin induces tyrosine phosphorylation of Shc proteins and their association with the Grb2/Sos complex

Gastrin/CCKB G protein-coupled receptors have been shown to mediate proliferative effects of their endogenous ligands. In the present study, we examined the signal transduction mechanisms linked to the G/CCKB receptor occupancy. We report here that gastrin stimulates MAP kinase activation in a dose- and time-dependent manner, a pathway known to play a key role in cell proliferation. We also characterized the molecular events, upstream of p21-Ras, that may link the MAP kinase pathway to G/CCKB receptors. Gastrin induced a rapid and transient increase in tyrosine phosphorylation of several proteins including the 2 isoforms (46 and 52 kDa) of the adaptor protein Shc. Phosphorylated Shc subsequently associated with a complex that includes Grb2 and the p21-Ras activator, Sos. Our results also indicate that Sos becomes phosphorylated in response to gastrin as shown by a reduction in electrophoretic mobility of the protein. Tyrosine phosphorylation of Shc and subsequent complex formation with Grb2 and Sos appear to be a common mechanism by which tyrosine kinase receptors and the G/CCKB G protein-coupled receptor stimulate the Ras-dependent MAP kinase pathway.

CD16-mediated p21ras activation is associated with Shc and p36 tyrosine phosphorylation and their binding with Grb2 in human natural killer cells

The Src homology (SH) 2/SH3 domain-containing protein Grb2 and the oncoprotein Shc have been implicated in a highly conserved mechanism that regulates p21ras activation. We investigated the involvement of these adaptor proteins in the signaling pathway induced by CD16 or interleukin (IL) 2R triggering in human natural killer (NK) cells. Both p46 and p52 forms of Shc were rapidly and transiently tyrosine phosphorylated upon CD16 or IL-2 stimulation with different kinetics. Shc immunoprecipitates from lysates of CD16- or IL-2-stimulated NK cells contained Grb2 and an unidentified 145-kD tyrosine phosphoprotein. Grb2 immunoprecipitates from anti-CD16-stimulated NK cells contained not only Shc, but also a 36-kD tyrosine phosphoprotein (p36). The interaction between Grb2 and Shc or p36 occurred via the Grb2SH2 domain as indicated by in vitro binding assays using a bacteriologically synthesized glutathione S-transferase-Grb2SH2 fusion protein. We also present evidence that p21ras is activated by CD16 and IL-2R cross-linking. Accumulation of guanosine triphosphate-bound Ras was detected within 1 minute and occurred with kinetics similar to inductive protein tyrosine phosphorylation and Grb2 association of Shc and p36 adaptor proteins.

CD4 and signal transduction

The CD4 molecule plays an important role in the development of CD4+T lymphocytes and it also acts as a coreceptor to enhance responses mediated via the TCR. It is now established that CD4 functions both as an adhesion molecule favoring the T cell: APC interaction and as a signaling molecule. The coreceptor function mediated via CD4 depends on its association with Lck, a src-family tyrosine kinase. Lck, while interacting via its unique NH2-terminal domain with CD4, also interacts via its SH2 and SH3 domains with other intracellular signaling proteins. Although the Lck association with CD4 is essential for CD4 coreceptor activity, the tyrosine kinase activity of CD4-associated Lck appears to be dispensable for CD4 function. Given the necessity of Lck kinase activity for T lymphocyte development and for mature T cell functions, perhaps Lck may function at different stages during T cell activation and at some stages the kinase activity of Lck may not be necessary. This raises an intriguing possibility that CD4-associated Lck may function more as an adapter protein than a kinase and may help to recruit other signaling proteins into the TCR/CD3 complex. However, determination of the precise role of Lck in CD4 coreceptor activity and the domains of Lck that are necessary for CD4-dependent and CD4-independent functions awaits further experiments.

Differential activation of the Ras/extracellular-signal-regulated protein kinase pathway is responsible for the biological consequences induced by the Axl receptor tyrosine kinase

To understand the mechanism of Axl signaling, we have initiated studies to delineate downstream components in interleukin-3-dependent 32D cells by using a chimeric receptor containing the recombinant epidermal growth factor (EGF) receptor extracellular and transmembrane domains and the Axl kinase domain (EAK [for EGF receptor-Axl kinase]). We have previously shown that upon exogenous EGF stimulation, 32D-EAK cells are capable of proliferation in the absence of interleukin-3. With this system, we determined that EAK-induced cell survival and mitogenesis are dependent upon the Ras/extracellular-signal-regulated protein kinase (ERK) cascade. Although the phosphatidylinositol-3 kinase pathway is activated upon EAK signaling, it appears to be dispensable for the biological actions of the Axl kinase. Furthermore, we demonstrated that different threshold levels of Ras/ERK activation are needed to induce a block to apoptosis or proliferation in 32D cells. Recently, we have identified an Axl ligand, GAS6. Surprisingly, GAS6-stimulated 32D-Axl cells exhibited no blockage to apoptosis or mitogenic response which is correlated with the absence of Ras/ERK activation. Taken together, these data suggest that different extracellular domains dramatically alter the intracellular response of the Axl kinase. Furthermore, our data suggest that the GAS6-Axl interaction does not induce mitogenesis and that its exact role remains to be determined.

Binding affinities of tyrosine-phosphorylated peptides to the COOH-terminal SH2 and NH2-terminal phosphotyrosine binding domains of Shc

The adaptor protein Shc has been implicated in Ras signaling via association with many tyrosine-phosphorylated receptors, including growth factor receptors, antigen receptors on T and B cells, and cytokine receptors. Shc could interact with the activated receptors through the carboxyl-terminal Src homology 2 (SH2) domain or the structurally unrelated amino-terminal phosphotyrosine binding (PTB) domain. Using NMR and surface plasmon resonance techniques, we have measured the binding affinities of the SH2 and the PTB domains of Shc to a series of phosphotyrosine-containing peptides derived from known Shc binding sites. Tyrosine-phosphorylated peptides derived from Trk (pY490), polyoma virus middle T-antigen (pY250), ErbB3 (pY1309), and epidermal growth factor receptor (pY1086, pY1148, and pY1114) that contain NPXpY sequences bind preferentially to the PTB domain of Shc with Kd values of 0.02-5.3 microM. The binding affinities of these peptides to the Shc SH2 domain were in the range of 220-1290 microM. In contrast, tyrosine-phosphorylated peptides from epidermal growth factor receptor (pY992, pY1173) and the zeta chain of the T-cell receptor bind preferentially to the SH2 domain (Kd = 50-130 microM) versus the PTB domain (Kd > 680 microM). From these studies, the relative contribution of the individual domains of Shc for binding to the phosphotyrosine-containing portions of these proteins was determined. In addition, our data indicate that the high affinity binding of the PTB domain to the NPXpY-containing peptides results from a very high association rate and a rapid dissociation rate, which is similar to previous results observed for the SH2-phosphopeptide complexes.

Shc is a substrate of the rat intestinal epidermal growth factor receptor tyrosine kinase

BACKGROUND & AIMS

Epidermal growth factor (EGF) has been shown to induce intestinal proliferation and maturation; however, little information is available regarding substrates of the intestinal EGF receptor tyrosine kinase. The purpose of this study was to determine if src homologous collagen-like protein (Shc) was an in vivo substrate of the intestinal EGF receptor.

METHODS

Ten-day-old rats were treated with EGF or were breast-fed. In some experiments, IEC-6 cells were treated with EGF. Intestinal tissue and cell fractions were studied by immunodetection to compare the tyrosine phosphorylation state and the subcellular localization of intestinal proteins.

RESULTS

The total tyrosine phosphorylation state of intestinal proteins was increased threefold by EGF. Tyrosine phosphorylation of the EGF receptor and Shc were rapidly increased by EGF. The association of Grb2 with Shc increased fourfold and fivefold. Plasma membrane translocation of Shc and associated phosphotyrosyl proteins was increased within 30 seconds of EGF treatment.

CONCLUSIONS

Shc is a substrate of the intestinal EGF receptor in vivo. EGF-induced association of Shc with the adapter protein Grb2 may have implications for activation of the p21ras signaling pathway in the intestine. The EGF-induced membrane association of Shc with two other phosphotyrosyl proteins suggests involvement of Shc in additional aspects of EGF-receptor signaling in the intestine.

Signal transduction in T lymphocytes using a conditional allele of Sos

While Ras activation has been shown to play an important role in signal transduction by the T-lymphocyte antigen receptor, the mechanism of its activation in T cells is unclear. Membrane localization of the guanine nucleotide exchange factor Sos, but not Vav or Dbl, was sufficient for Ras-mediated signaling in T lymphocytes. Activation of Sos appears to involve membrane recruitment and not allosteric changes, because interaction of Sos with the linking molecule Grb-2 was not required for Ras activation. To extend this analysis, we constructed a modified Sos that could be localized to the membrane inducibly by using a rationally designed chemical inducer of dimerization, FK1012. The role of Grb-2 in signaling was mimicked with this technique, which induced the association of a modified Sos with the membrane, resulting in rapid activation of Ras-induced signaling. In contrast, inducible localization of Grb-2 to the membrane did not activate signaling and suggests that the interaction of Grb-2 with Sos in T cells is subject to regulation. This conditional allele of Sos demonstrates that membrane localization of Sos is sufficient for Ras activation in T cells and indicates that the role of Grb-2 is to realize the biologic advantages of linker-mediated dimerization: enhanced specificity and favorable kinetics for signaling. This method of generating conditional alleles may also be useful in dissecting other signal transduction pathways regulated by protein localization or protein-protein interactions.

Binding of Shc to the NPXY motif is mediated by its N-terminal domain

Shc is an SH2-containing adapter protein that binds to and is phosphorylated by a large number of growth factor receptors. Phosphorylated Shc is able to interact with the Grb2-Sos complex which is responsible for mediating nucleotide exchange on Ras. We have shown previously that binding of Shc to the epidermal growth factor (EGF)-like receptor, c-ErbB-3, is through an NPXY motif (Prigent, S. A., and Gullick, W. J. (1994) EMBO J. 13, 2831-2841) shared by middle T antigen, TrkA, and EGF receptor. It has recently been reported that a region distinct from the SH2 domain is able to bind to tyrosine-phosphorylated proteins. In this paper we have used fusion proteins of various Shc domains to show that it is the N-terminal domain of Shc that is primarily responsible for binding EGF receptor and c-ErbB-3. Furthermore, by competition studies with synthetic phosphopeptides we have shown that this N-terminal domain binds to the previously identified NPXY motif.

Alternative signals to RAS for hematopoietic transformation by the BCR-ABL oncogene

Biological function of the BCR-ABL oncogene is dependent on its activated tyrosine kinase. Mutations that inactivate the SRC homology 2 (SH2) domain, the GRB2-binding site in BCR, or the major autophosphorylation site of the kinase domain selectively disrupt downstream signaling but not tyrosine kinase activity. Despite a loss of fibroblast transformation activity, all three mutants retain the ability to render hematopoietic cell lines growth factor independent and transform primary bone marrow cells in vitro. In vivo tests of malignant potential reveal a most critical role for signals dependent on the BCR-ABL SH2 domain. The efficiency of both fibroblast and hematopoietic transformation by BCR-ABL is strongly affected by increased dosage of the SHC adapter protein, which can connect tyrosine kinase signals to RAS. The BCR-ABL oncogene activates multiple alternative pathways to RAS for hematopoietic transformation.

The Grb2 adaptor

Grb2 is an 'adaptor' protein made of one SH2 and two SH3 domains. The SH3 domains bind to prolinerich motifs in the C-terminal part of the ras exchange factor Sos. Binding of the Grb2 SH2 domain to phosphotyrosine motifs on receptors, or other adaptor proteins such as Shc, recruits this Grb2/Sos complex at the plasma membrane where Sos stimulates nucleotide exchange on ras, then ras activates raf and leads to MAP kinase activation. The structure of Grb2, the precise motifs recognised by its SH2 and SH3 domains, the way Grb2 performs its function, a possible regulation of its association with Sos, and its ability to complex with other proteins in vivo, are discussed.

A functional T-cell receptor signaling pathway is required for p95vav activity

Stimulation of the T-cell antigen receptor (TCR) induces activation of multiple tyrosine kinases, resulting in phosphorylation of numerous intracellular substrates. One substrate is p95vav, which is expressed exclusively in hematopoietic and trophoblast cells. It contains a number of structural motifs, including Src homology 2, Src homology 3, and pleckstrin homology domains and a putative guanine nucleotide exchange domain. The role of p95vav in TCR-mediated signaling processes is unclear. Here, we show that overexpression of p95vav alone in Jurkat T cells leads to activation of the nuclear factors, including NFAT, involved in interleukin-2 expression. Furthermore, p95vav synergizes with TCR stimulation in inducing NFAT- and interleukin-2-dependent transcription. In contrast, NFAT activation by a G-protein-coupled receptor is not modulated by p95vav overexpression, suggesting that the effect is specific to the TCR signaling pathways. Although removal of the first 67 amino acids of p95vav activates its transforming potential in NIH 3T3 cells, this region appears to be required for its function in T cells. We further demonstrate that the p95vav-induced NFAT activation is not mimicked by Ras activation, though its function is dependent upon Ras and Raf. Furthermore, the activating function of p95vav is blocked by FK506, suggesting that its activity also depends on calcineurin. To further dissect p95vav involvement in TCR signaling, we analyzed various Jurkat mutants deficient in TCR signaling function or TCR expression and showed that an intact TCR signaling pathway is required for p95vav to function. However, overexpression of p95vav does not appear to influence TCR-induced protein tyrosine phosphorylation or increases in cytoplasmic free calcium. Taken together, our data suggest that p95vav plays an important role at an yet unidentified proximal position in the TCR signaling cascade.

Distinct pathways of Gi- and Gq-mediated mitogen-activated protein kinase activation

Receptors that couple to the heterotrimeric G proteins, Gi or Gq, can stimulate phosphoinositide (PI) hydrolysis and mitogen-activated protein kinase (MAPK) activation. PI hydrolysis produces inositol 1,4,5-trisphosphate and diacylglycerol, leading to activation of protein kinase C (PKC), which can stimulate increased MAPK activity. However, the relationship between PI hydrolysis and MAPK activation in Gi and Gq signaling has not been clearly defined and is the subject of this study. The effects of several signaling inhibitors are assessed including expression of a peptide derived from the carboxyl terminus of the beta adrenergic receptor kinase 1 (beta ARKct), which specifically blocks signaling mediated by the beta gamma subunits of G proteins (G beta gamma), expression of dominant negative mutants of p21ras (RasN17) and p74raf-1 (N delta Raf), protein-tyrosine kinase (PTK) inhibitors and cellular depletion of PKC. The Gi-coupled alpha 2A adrenergic receptor (AR) stimulates MAPK activation which is blocked by expression of beta ARKct, RasN17, or N delta Raf, or by PTK inhibitors, but unaffected by cellular depletion of PKC. In contrast, MAPK activation stimulated by the Gq-coupled alpha 1B AR or M1 muscarinic cholinergic receptor is unaffected by expression of beta ARKct or RasN17 expression or by PTK inhibitors, but is blocked by expression of N delta Raf or by PKC depletion. These data demonstrate that Gi- and Gq-coupled receptors stimulate MAPK activation via distinct signaling pathways. G beta gamma is responsible for mediating Gi-coupled receptor-stimulated MAPK activation through a mechanism utilizing p21ras and p74raf independent of PKC. In contrast, G alpha mediates Gq-coupled receptor-stimulated MAPK activation using a p21ras-independent mechanism employing PKC and p74raf. To define the role of G beta gamma in Gi-coupled receptor-mediated PI hydrolysis and MAPK activation, direct stimulation with G beta gamma was used. Expression of G beta gamma resulted in MAPK activation that was sensitive to inhibition by expression of beta ARKct, RasN17, or N delta Raf or by PTK inhibitors, but insensitive to PKC depletion. By comparison, G beta gamma-mediated PI hydrolysis was not affected by beta ARKct, RasN17, or N delta Raf expression or by PTK inhibitors. Together, these results demonstrate that G beta gamma mediates MAPK activation and PI hydrolysis via independent signaling pathways.