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

Selective hydrolysis of a mitochondrial pool of sphingomyelin induces apoptosis

Our previous results have indicated that the major cellular pool of sphingomyelin present on the outer leaflet of the plasma membrane is not involved in the ceramide pathway of apoptosis. Thus, in this study we aimed at defining which intracellular pools of sphingomyelin and ceramide are involved in cell death. The bacterial sphingomyelinase (SMase) gene fused with green fluorescent protein was subcloned into mammalian vectors containing sequences that target the fusion proteins to cytoplasm, plasma membrane, mitochondria, Golgi apparatus, endoplasmic reticulum, or nucleus. Transfection of MCF7 breast cancer cells showed for all constructs an increase in SMase activity ranging from 2- to 60-fold, concomitant with an increase in total cellular ceramide levels (10-100%) as compared with vector-transfected cells. Next, the effect of overexpression of the SMase on cell death was examined. Results demonstrate that only when bacterial SMase was targeted to mitochondria did cells undergo apoptosis; its targeting to the other intracellular compartments was ineffective. Further, the results show that apoptosis induced by mitochondrial targeting of bacterial SMase requires SMase catalytic activity, is prevented by the overexpression of Bcl-2, and is mediated by inducing cytochrome c release. These results demonstrate that ceramide induces cell death specifically when generated in mitochondria. The results highlight the significance of compartment-specific lipid-mediated cell regulation, and they offer a novel general approach for these studies.

Cell transformation by the middle T-antigen of polyoma virus

The polyoma virus region expressed early in the lytic cycle encodes three proteins, or T-antigens, that together cause the infected cell to enter the cell cycle and so provide a suitable cellular environment for replication of the viral genome. Under some circumstances infection does not kill the cell, but the T-antigens are still produced, resulting in the cell becoming transformed and tumorigenic. Most of this transforming action is exerted by the middle T-antigen, which has the ability to convert established cell lines to an oncogenic state. Middle T is a membrane bound polypeptide that interacts with a number of the proteins used by tyrosine kinase associated receptors to stimulate mitogenesis, so MT can be considered as a permanently active analogue of a receptor. Through a defined series of interactions, MT assembles a large multi-protein complex at the cell membrane, consisting of MT, the core dimer of protein phosphatase 2A, an src-family tyrosine kinase, and via phosphotyrosines, ShcA, phosphatidylinositol (3') kinase, and phospholipase Cgamma-1. Tyrosine phosphorylation stimulates PI3K and PLCgamma-1 enzymatic activity, and on ShcA creates binding sites for Grb2 with its associated Sos1 and Gab1. This activates p21(ras), and hence, the MAP kinase cascade. Consequently, MT can be used as a model for studying cell transformation and growth factor receptor signalling pathways.

ShcA tyrosine phosphorylation sites can replace ShcA binding in signalling by middle T-antigen

ShcA and Grb2 are crucial components in signalling by most tyrosine kinase-associated receptors. How ever, it is not clear whether Grb2 bound directly to the receptor is equivalent to Grb2 associated via ShcA. We have used signalling stimulated by the middle T-antigen (MT) of polyoma virus to address this question. The two known Grb2-binding sites from murine ShcA, 313Y and 239/240YY, could functionally replace the MT ShcA-interacting region in transformation assays using Rat2 fibroblasts. This demonstrates that signal output from membrane-bound ShcA requires only these two sequences and the ShcA-binding site in MT does not recruit other signalling molecules. Two standard Grb2-interacting sequences, either from the EGF receptor or the ShcA 313Y region, could not replace the requirement for ShcA binding to MT, indicating an enhanced role for the ShcA 239/240YY motif. Sos1 and the docking protein Gab1 are brought into the MT complex through Grb2 association and this may be more effective using the 239/240YY sequence.

Early steps of insulin action in the skin of intact rats

Insulin is an important regulator of growth and initiates its action by binding to its receptor, which undergoes tyrosyl autophosphorylation and further enhances its tyrosine kinase activity towards other intermediate molecules, including insulin receptor substrate 1, insulin receptor substrate 2, and Shc. Insulin receptor substrate proteins can dock various src-homology-2-domain-containing signaling proteins, such as the 85 kDa subunit of phosphatidylinositol 3 kinase and growth-factor-receptor-bound protein 2. The serine-threonine kinase is activated downstream to phosphatidylinositol 3 kinase. Shc protein has been shown to directly induce the association with growth-factor-receptor-bound protein 2 and downstream the activation of the mitogen-activated protein kinase. In this study we investigated insulin signal transduction pathways in skin of intact rats by immunoprecipitation and immunoblotting with specific antibodies, and also by immunohistochemistry with anti-insulin-receptor antibody. Our results showed that skin fragments clearly demonstrated the presence of insulin receptor in cell bodies of the epidermis and hair follicles and some faint staining was also detected in fibroblasts of the dermis. It was also observed that acute stimulation with insulin can induce tyrosyl phosphorylation of insulin receptor, that the insulin receptor substrates and Shc proteins serve as signaling molecules for insulin in skin of rats, and that insulin is able to induce association of insulin receptor substrate 1/phosphatidylinositol 3 kinase and Shc/growth-factor-receptor-bound protein 2 in this tissue, as well as phosphorylation of mitogen-activated protein kinase and serine-threonine kinase, demonstrating that proteins involved in early steps of insulin action are expressed in skin of intact rats and are quickly activated after insulin stimulation.

Glucose-dependent and -independent signalling functions of the yeast glucose sensor Snf3

The yeast Snf3 protein has been described to function as a sensor for low concentrations of extracellular glucose. We have found that Snf3 is able to transduce a signal in the complete absence of extracellular glucose. High basal activity of the HXT7 promoter during growth on ethanol required Snf3 as well as other components of the signalling pathway activated by Snf3. Moreover, the C-terminal domain of Snf3 was sufficient to complement the role of Snf3 in this regulation. As the C-terminal tail of Snf3 interacted with other components at the plasma membrane independent of the carbon source, our data suggest that Snf3 is involved in signalling complexes which can be activated by other signals than extracellular glucose.

High mutation frequency at Ha-ras exons 1-4 in squamous cell carcinomas from PUVA-treated psoriasis patients

Clinical follow-up studies have revealed that PUVA-treated patients are at increased risk of skin cancer, particularly squamous cell carcinoma (SCC). However, since psoralen and UVA (PUVA) is not only a potent mutagen and carcinogen but also an immunosuppressor, and since other (co)carcinogenic factors often exist in psoriasis patients, the exact causes and mechanisms of PUVA-associated SCC are still not completely understood. In order to fill this gap the tools of molecular epidemiology are being used to study the SCC mutational spectra of p53 and Ha-ras, two of the most commonly mutated genes in human cancers. A previous mutation analysis revealed that SCC in PUVA-treated patients often carried mutated p53 genes and that many of the mutations had the UV fingerprint (i.e. C-->T or CC-->TT transitions at dipyrimidine sites). In the present study DNA-sequencing analysis revealed a total of 18 Ha-ras missense or nonsense mutations at exons 1-4 in 13 of 17 SCC (76%) from 8 of 11 (73%) PUVA-treated psoriasis patients. Six of the 18 mutations (33%) were of UV-fingerprint type (C-->T transitions), five (28%) were at 5'-TpG sites (i.e. potential psoralen-binding sites and thus potentially caused by PUVA) and seven were of other type (39%), including six G:C-->T:A transversions at hotspot codon 12. In addition, in the case of 6 of the 11 subjects (55%) both tumor and normal skin samples contained a T:A-->C:G base change at codon 27 (a 5'-ATT site), a change previously hypothesized to be a possible silent Ha-ras polymorphism at one allele. When we compared the present Ha-ras mutation spectrum with the p53 mutation spectrum from a previous study of the samples, we found that approximately half of the tumors harbored mutations in both Ha-ras and p53. Together, our results indicate that Ha-ras mutations are present in a large proportion of PUVA-associated SCC and that UVB, PUVA and other agents may induce Ha-ras mutations and act together with p53 in the formation of SCC in psoriasis patients.

Rac1 mediates STAT3 activation by autocrine IL-6

The activity of the small GTPase, Rac1, plays a role in various cellular processes including cytoskeletal rearrangement, gene transcription, and malignant transformation. In this report constitutively active Rac1 (Rac V12) is shown to stimulate the activation of STAT3, a member of the family of signal transducers and activators of transcription (STATs). The activity of Rac1 leads to STAT3 translocation to the nucleus coincident with STAT3-dependent gene expression. The expression of Vav (Delta1-187), a constitutively active guanine nucleotide exchange factor for the Rho GTPases, or activated forms of Ras or Rho family members, leads to STAT3-specific activation. The activation of STAT3 requires tyrosine phosphorylation at residue 705, but is not dependent on phosphorylation of Ser-727. Our studies indicate that Rac1 induces STAT3 activation through an indirect mechanism that involves the autocrine production and action of IL-6, a known mediator of STAT3 response. Rac V12 expression results in the induction of the IL-6 and IL-6 receptor genes and neutralizing antibodies directed against the IL-6 receptor block Rac1-induced STAT3 activation. Furthermore, inhibition of the nuclear factor-kappaB activation or disruption of IL-6-mediated signaling through the expression of IkappaBalpha S32AS36A and suppressor of cytokine signaling 3, respectively, blocks Rac1-induced STAT3 activation. These findings elucidate a mechanism dependent on the induction of an autocrine IL-6 activation loop through which Rac1 mediates STAT3 activation establishing a link between oncogenic GTPase activity and Janus kinase/STAT signaling.

Structure-based mutagenesis reveals distinct functions for Ras switch 1 and switch 2 in Sos-catalyzed guanine nucleotide exchange

Ras GTPases function as binary switches in signaling pathways controlling cell growth and differentiation. The guanine nucleotide exchange factor Sos mediates the activation of Ras in response to extracellular signals. We have previously solved the crystal structure of nucleotide-free Ras in complex with the catalytic domain of Sos (Boriack-Sjodin, P. A., Margarit, S. M., Bar-Sagi, D., and Kuriyan, J. (1998) Nature 394, 337-343). The structure demonstrates that Sos induces conformational changes in two loop regions of Ras known as switch 1 and switch 2. In this study, we have employed site-directed mutagenesis to investigate the functional significance of the conformational changes for the catalytic function of Sos. Switch 2 of Ras is held in a very tight embrace by Sos, with almost every external side chain coordinated by Sos. Mutagenesis of contact residues at the switch 2-Sos interface shows that only a small set of side chains affect binding, with the most important contact being mediated by tyrosine 64, which is buried in a hydrophobic pocket of Sos in the Ras.Sos complex. Substitutions of Ras and Sos side chains that are inserted into the Mg(2+)- and nucleotide phosphate-binding site of switch 2 (Ras Ala(59) and Sos Leu(938) and Glu(942)) have no effect on the catalytic function of Sos. These results indicate that the interaction of Sos with switch 2 is necessary for tight binding, but is not the critical driving force for GDP displacement. The structural distortion of switch 1 induced by Sos is mediated by a small number of specific contacts between highly conserved residues on both Ras and Sos. Mutations of a subset of these residues (Ras Tyr(32) and Tyr(40)) result in an increase in the intrinsic rate of nucleotide dissociation from Ras and impair the binding of Ras to Sos. Based on this analysis, we propose that the interactions of Sos with the switch 1 and switch 2 regions of Ras have distinct functional consequences: the interaction with switch 2 mediates the anchoring of Ras to Sos, whereas the interaction with switch 1 leads to disruption of the nucleotide-binding site and GDP dissociation.

The epithelium-specific ETS protein EHF/ESE-3 is a context-dependent transcriptional repressor downstream of MAPK signaling cascades

Exon trapping and cDNA selection procedures were used to search for novel genes at human chromosome 11p13, a region previously associated with loss of heterozygosity in epithelial carcinomas. Using these approaches, we found the ESE-2 and ESE-3 genes, coding for ETS domain-containing transcription factors. These genes lie in close proximity to the catalase gene within a approximately 200-kilobase genomic interval. ESE-3 mRNA is widely expressed in human tissues with high epithelial content, and immunohistochemical analysis with a newly generated monoclonal antibody revealed that ESE-3 is a nuclear protein expressed exclusively in differentiated epithelial cells and that it is absent in the epithelial carcinomas tested. In transient transfections, ESE-3 behaves as a repressor of the Ras- or phorbol ester-induced transcriptional activation of a subset of promoters that contain ETS and AP-1 binding sites. ESE-3-mediated repression is sequence- and context-dependent and depends both on the presence of high affinity ESE-3 binding sites in combination with AP-1 cis-elements and the arrangement of these sites within a given promoter. We propose that ESE-3 might be an important determinant in the control of epithelial differentiation, as a modulator of the nuclear response to mitogen-activated protein kinase signaling cascades.

Tyrosine-phosphorylated low density lipoprotein receptor-related protein 1 (Lrp1) associates with the adaptor protein SHC in SRC-transformed cells

v-Src transforms fibroblasts in vitro and causes tumor formation in the animal by tyrosine phosphorylation of critical cellular substrates. Exactly how v-Src interacts with these substrates remains unknown. One of its substrates, the adaptor protein Shc, is thought to play a crucial role during cellular transformation by v-Src by linking v-Src to Ras. We used Shc proteins with mutations in either the phosphotyrosine binding (PTB) or Src homology 2 domain to determine that phosphorylation of Shc in v-Src-expressing cells depends on the presence of a functional PTB domain. We purified a 100-kDa Shc PTB-binding protein from Src-transformed cells that was identified as the beta chain of the low density lipoprotein receptor-related protein LRP1. LRP1 acts as an import receptor for a variety of proteins and is involved in clearance of the beta-amyloid precursor protein. This study shows that LRP1 is tyrosine-phosphorylated in v-Src-transformed cells and that tyrosine-phosphorylated LRP1 binds in vivo and in vitro to Shc. The association between Shc and LRP1 may provide a mechanism for recruitment of Shc to the plasma membrane where it is phosphorylated by v-Src. It is at the membrane that Shc is thought to be involved in Ras activation. These observations further suggest that LRP1 could function as a signaling receptor and may provide new avenues to investigate its possible role during embryonal development and the onset of Alzheimer's disease.

Protein recruitment systems for the analysis of protein +/- protein interactions

The yeast Saccharomyces cerevisiae serves as an excellent genetic tool for the analysis of protein +/- protein interactions. The most common system, used to date, is the two-hybrid system. Although proven very powerful, the two-hybrid system exhibits several inherent problems and limitations. Recently, two alternative systems have been described that take advantage of the fact that localization of signal transduction effectors to the inner leaflet of the plasma membrane is absolutely necessary for yeast viability. These effectors can either be the Ras guanyl nucleotide exchange factor or Ras itself. The yeast strain used in both systems is a temperature-sensitive mutant in the yeast Ras guanyl nucleotide exchange factor, CDC25. Membrane localization of these effectors is achieved via protein +/- protein interaction. Each system can be used to test interaction between known protein pairs, as well as for isolation of novel protein interactions. Described here are the scientific and technical steps to be considered for both protein recruitment systems.

HBV X protein targets HIV Tat-binding protein 1

The HBV X protein (HBx) is implicated in infection and development of hepatocellular carcinoma. HBx has a pleiotropic effect on cells, suggesting multiple targets in the virus-host cell interaction. We employed the cytoplasmic-based two-hybrid screen and identified the HIV Tat-binding protein 1 (Tbp1) as a novel HBx interacting protein. Tbp1 interacts in vivo with HBx both in yeast and in animal cells. This interaction maps to the functionally important ATP-binding motif of Tbp1. Furthermore, HBx and Tbp1 interaction is functionally significant and regulates HBV transcription. Tbp1 homologues, such as Sug1, are known members of the proteasome 19S regulatory cap particle and have also been implicated in transcription coactivation. Remarkably, Tbp1 and Sug1 interact with multiple viral effector proteins including HIV Tat, SV40 large T antigen, and adenovirus E1A, establishing these proteins as important targets of the viral oncogenes.

L-selectin tyrosine phosphorylates cbl and induces association of tyrosine-phosphorylated cbl with crkl and grb2

L-Selectin-mediated rolling of leukocytes on endothelial cells is an important step for lymphocyte homing and an early event in the immune response to pathogens or inflammatory stimuli. We have previously elucidated intracellular signaling cascades upon L-selectin engagement resulting in activation of Ras, Rac and JNK as well as cytoskeletal changes, oxygen release, ceramide synthesis and receptor capping. Activation of the src-tyrosine kinase p56lck is followed by phosphorylation of the L-selectin molecule and MAP-K. Here we show a tyrosine kinase dependent phosphorylation of the Cbl adapter protein after L-selectin engagement in lymphocytes. Phosphorylation of Cbl was absent in Jurkat cells that are pharmacologically treated with tyrosine kinase inhibitors and in lck-deficient JCaM cells. There is an activation induced association of tyrosine phosphorylated Cbl with Grb2 and CrkL, respectively, but not CrkII. Therefore, the adapter protein Cbl plays a role in L-selectin signaling and might modulate immune function by the specific recruitment of signaling molecules to multiprotein complexes.

Dictyostelium: an ideal organism for genetic dissection of Ras signalling networks

Signalling pathways based on the small GTPase Ras regulate a multitude of cellular events in eukaryotic cells. Dictyostelium expresses a large and varied family of Ras proteins. It also uses a range of known Ras regulators, in particular RasGEFs, and effectors. The genetic tractability of Dictyostelium, together with the wide range of Ras proteins and regulators, make it an ideal model for the genetic dissection of Ras pathways. This review highlights the recent advances in our understanding of Ras function in Dictyostelium, and considers the implications of these findings for our understanding of eukaryotic signal transduction.

Analysis of function and regulation of proteins that mediate signal transduction by use of lipid-modified plasma membrane-targeting sequences

It is now established that the function of many signaling molecules is controlled, in part, by regulation of subcellular localization. For example, the dynamic recruitment of normally cytosolic proteins to the plasma membrane, by activated Ras or activated receptor tyrosine kinases, facilitates their interaction with other membrane-associated components that participate in their full activation (e.g., Raf-1). Therefore, the creation of chimeric proteins that contain lipid-modified signaling sequences that direct membrane localization allows the generation of constitutively activated variants of such proteins. The amino-terminal myristoylation signal sequence of Src family proteins and the carboxy-terminal prenylation signal sequence of Ras proteins have been widely used to achieve this goal. Such membrane-targeted variants have proved to be valuable reagents in the study of the biochemical and biological properties of many signaling molecules.

A computational study of feedback effects on signal dynamics in a mitogen-activated protein kinase (MAPK) pathway model

Exploiting signaling pathways for the purpose of controlling cell function entails identifying and manipulating the information content of intracellular signals. As in the case of the ubiquitously expressed, eukaryotic mitogen-activated protein kinase (MAPK) signaling pathway, this information content partly resides in the signals' dynamical properties. Here, we utilize a mathematical model to examine mechanisms that govern MAPK pathway dynamics, particularly the role of putative negative feedback mechanisms in generating complete signal adaptation, a term referring to the reset of a signal to prestimulation levels. In addition to yielding adaptation of its direct target, feedback mechanisms implemented in our model also indirectly assist in the adaptation of signaling components downstream of the target under certain conditions. In fact, model predictions identify conditions yielding ultra-desensitization of signals in which complete adaptation of target and downstream signals culminates even while stimulus recognition (i.e., receptor-ligand binding) continues to increase. Moreover, the rate at which signal decays can follow first-order kinetics with respect to signal intensity, so that signal adaptation is achieved in the same amount of time regardless of signal intensity or ligand dose. All of these features are consistent with experimental findings recently obtained for the Chinese hamster ovary (CHO) cell lines (Asthagiri et al., J. Biol. Chem. 1999, 274, 27119-27127). Our model further predicts that although downstream effects are independent of whether an enzyme or adaptor protein is targeted by negative feedback, adaptor-targeted feedback can "back-propagate" effects upstream of the target, specifically resulting in increased steady-state upstream signal. Consequently, where these upstream components serve as nodes within a signaling network, feedback can transfer signaling through these nodes into alternate pathways, thereby promoting the sort of signaling cross-talk that is becoming more widely appreciated.

Genetic and biochemical analysis of the yeast plasma membrane Ssy1p-Ptr3p-Ssy5p sensor of extracellular amino acids

Ssy1p and Ptr3p are known components of a yeast plasma membrane system that functions to sense the presence of amino acids in the extracellular environment. In response to amino acids, this sensing system initiates metabolic signals that ultimately regulate the functional expression of several amino acid-metabolizing enzymes and transport proteins, including multiple, genetically distinct amino acid permeases. We have found that SSY5 encodes a third component of this amino acid sensing system. Mutations in SSY5 manifest phenotypes that are indistinguishable from those resulting from either single ssy1 and ptr3 mutations or ssy5 ssy1 and ssy5 ptr3 double mutations. Although Ssy5p is predicted to be a soluble protein, it exhibits properties indicating that it is a peripherally associated plasma membrane protein. Each of the three sensor components, Ssy1p, Ptr3p, and Ssy5p, adopts conformations and modifications that are dependent upon the availability of amino acids and on the presence of the other two components. These results suggest that these components function as part of a sensor complex localized to the plasma membrane. Consistent with a sensor complex, the overexpression of SSY1 or the unique N-terminal extension of this amino acid permease homologue inactivates the amino acid sensor in a dominant-negative manner. Each of the components of the Ssy1p-Ptr3p-Ssy5p (SPS) signaling system undergoes rapid physical changes, reflected in altered electrophoretic mobility, when leucine is added to cells grown in media lacking amino acids. Furthermore, the levels of each SPS sensor component present in whole-cell extracts diminish upon leucine addition. The rapid physical alterations and reduced levels of sensor components are consistent with their being downregulated in response to amino acid availability. These results reveal the dynamic nature of the amino acid-initiated signals transduced by the SPS sensor.

The isoform-specific stretch of hSos1 defines a new Grb2-binding domain

hSos1 isoform II, defined by the presence of a 15 amino acid stretch in its carboxy-terminal region, exhibits higher Grb2 affinity than hSos1 isoform I. In this study, we investigated the cause for this difference and observed that, in addition to the four currently accepted Grb2-binding motifs, a number of additional, putative SH3-minimal binding sites (SH3-MBS) could be identified. The isoform II-specific 15 amino acid stretch contained one of them. Indeed, we demonstrated by site-directed mutagenesis that these SH3-MBS were responsible for the Grb2 interaction, and we found that C-terminal fragments of the two hSos1 isoforms (lacking the four cannonical Grb2-binding motifs, but containing the SH3-minimal binding sites) were able to bind Grb2, with the isoform II fragment showing higher Grb2 affinity than the corresponding isoform I fragment. Furthermore, we provide evidence that C-terminal truncated mutants of either hSos1 isoform, containing only the SH3-minimal binding sites, were able to originate in vivo stable complexes with Grb2. Although, Grb2-binding remains higher in both full-length isoforms, compared to the C-terminal truncated mutants, these mutants were also able to activate Ras, supporting a potential role of this C-terminal region as negative modulator of Sos1 activity. These findings document the existence of a new, functional, SH3-minimal binding site located in the specific stretch of hSos1 isoform II which may be responsible for the increased Grb2 affinity of this isoform in comparison to isoform I, and for the physiological properties differences between both isoforms. Moreover, these SH3-minimal binding sites may be sufficient to attain stable and functional hSosl-Grb2 complexes.

Interaction between two isoforms of the NF2 tumor suppressor protein, merlin, and between merlin and ezrin, suggests modulation of ERM proteins by merlin

The product of the neurofibromatosis type II (NF2) tumor suppressor gene, merlin, is closely related to the ezrin-radixin-moesin (ERM) family, a group of proteins believed to link the cytoskeleton to the plasma membrane. Mutation in the NF2 locus is associated with Schwann cell tumors (schwannomas). The two predominant merlin isoforms, I and II, differ only in the carboxy-terminal 16 residues and only isoform I is anti-proliferative. Merlin lacks an actin-binding domain conserved among ezrin, radixin and moesin. Because merlin, ezrin and moesin are co-expressed in Schwann cells, and all homodimerize, we have examined whether merlin and ezrin dimerize with one another. We found by immunoprecipitation and yeast two-hybrid assays that both merlin isoforms interact with ezrin. The interaction occurs in a head-to-tail orientation, with the amino-terminal half of one protein interacting with the carboxy-terminal half of the other. The two merlin isoforms behave differently in their interaction with ezrin. Isoform I binds only ezrin whose carboxy-terminus is exposed, whereas isoform II binds ezrin regardless of whether ezrin is in the open or closed conformation. The heterodimerization of merlin is a much stronger interaction than the interaction between either merlin isoform and ezrin, and can inhibit merlin-ezrin binding. This suggests that, in vivo, merlin dimerization could regulate merlin-ERM protein interaction, and could thus indirectly regulate other interactions involving ERM proteins.

Requirements of multiple domains of SLI-1, a Caenorhabditis elegans homologue of c-Cbl, and an inhibitory tyrosine in LET-23 in regulating vulval differentiation

SLI-1, a Caenorhabditis elegans homologue of the proto-oncogene product c-Cbl, is a negative regulator of LET-23-mediated vulval differentiation. Lack of SLI-1 activity can compensate for decreased function of the LET-23 epidermal growth factor receptor, the SEM-5 adaptor, but not the LET-60 RAS, suggesting that SLI-1 acts before RAS activation. SLI-1 and c-Cbl comprise an N-terminal region (termed SLI-1:N/Cbl-N, containing a four-helix bundle, an EF hand calcium-binding domain, and a divergent SH2 domain) followed by a RING finger domain and a proline-rich C-terminus. In a transgenic functional assay, the proline-rich C-terminal domain is not essential for sli-1(+) function. A protein lacking the SH2 and RING finger domains has no activity, but a chimeric protein with the SH2 and RING finger domains of SLI-1 replaced by the equivalent domains of c-Cbl has activity. The RING finger domain of c-Cbl has been shown recently to enhance ubiquitination of active RTKs by acting as an E3 ubiquitin-protein ligase. We find that the RING finger domain of SLI-1 is partially dispensable. Further, we identify an inhibitory tyrosine of LET-23 requiring sli-1(+) for its effects: removal of this tyrosine closely mimics the loss of sli-1 but not of another negative regulator, ark-1. Thus, we suggest that this inhibitory tyrosine mediates its effects through SLI-1, which in turn inhibits signaling upstream of LET-60 RAS in a manner not wholly dependent on the ubiquitin-ligase domain.

Protein recruitment systems for the analysis of protein-protein interactions

Following the completion of genome projects in a number of organisms, it is becoming evident that a relatively large proportion of the genes identified encode for proteins that have no sequence homology with known proteins. One possible approach towards understanding protein function is to identify the proteins with which a particular protein associates. Although very powerful, the most commonly used genetic method, the two-hybrid system, is limited in its ability to detect all possible protein-protein interactions. The development of novel approaches, such as the protein recruitment systems, provides attractive alternatives towards identification of protein-protein interactions where other methods have failed to function.

Characterization of RasGRP2, a plasma membrane-targeted, dual specificity Ras/Rap exchange factor

Ras proteins operate as molecular switches in signal transduction pathways downstream of tyrosine kinases and G-protein-coupled receptors. Ras is switched from the inactive GDP-bound state to the active GTP-bound state by guanine nucleotide exchange factors (GEFs). We report here the cloning and characterization of RasGRP2, a longer alternatively spliced form of the recently cloned RapGEF, CalDAG-GEFI. A unique feature of RasGRP2 is that it is targeted to the plasma membrane by a combination of N-terminal myristoylation and palmitoylation. In vivo, RasGRP2 selectively catalyzes nucleotide exchange on N- and Ki-Ras, but not Ha-Ras. RasGRP2 also catalyzes nucleotide exchange on Rap1, but this RapGEF activity is less potent than that associated with CalDAG-GEFI. The nucleotide exchange activity of RasGRP2 toward N-Ras is stimulated by diacylglycerol and inhibited by calcium. The effects of diacylglycerol and calcium are additive but are not accompanied by any detectable change in the subcellular localization of RasGRP2. In contrast, CalDAG-GEFI is localized predominantly to the cytosol and lacks Ras exchange activity in vivo. However, prolonged exposure to phorbol esters, or growth in serum, results in localization of CalDAG-GEFI to the cell membrane and restoration of Ras exchange activity. Expression of RasGRP2 or CalDAG-GEFI in NIH3T3 cells transfected with wild type N-Ras results in an accelerated growth rate but not morphologic transformation. Thus, under appropriate growth conditions, CalDAG-GEFI and RasGRP2 are dual specificity Ras and Rap exchange factors.

Abl interactor 1 binds to sos and inhibits epidermal growth factor- and v-Abl-induced activation of extracellular signal-regulated kinases

Recent studies have suggested that members of the Abl interactor (Abi) protein family negatively regulate cell growth and transformation. To date, however, no specific role in these cellular processes has been identified for the Abi family. Here we describe the inhibition by overexpressed Abi-1 of a mitogenic pathway activated by both growth factors and v-Abl. We have identified the guanine nucleotide exchange factors Sos1 and Sos2 as novel binding partners of Abi-1. A domain that is required for interaction with Sos in vivo has been mapped to the amino terminus of Abi-1. Overexpression of Abi-1 inhibits epidermal growth factor (EGF)-induced activation of extracellular signal-regulated kinases (Erks) but does not affect EGF-induced activation of c-Jun N-terminal kinase or Akt. In addition, overexpression of Abi-1 blocks Erk activation induced by v-Abl. In both cases, the maximal inhibitory effect requires an intact amino-terminal Sos-binding domain in Abi-1. Finally, we demonstrate that tyrosine phosphorylation of endogenous Abi-1 in fibroblasts is induced by both v-Abl and serum stimulation, further suggesting a role for Abi-1 in signal transduction initiated by v-Abl and growth factors. Taken together, these findings suggest that overexpressed Abi proteins negatively regulate cell growth and transformation by specifically targeting the Erk pathway.

Stimulation of Ras guanine nucleotide exchange activity of Ras-GRF1/CDC25(Mm) upon tyrosine phosphorylation by the Cdc42-regulated kinase ACK1

Ras-GRF1 is a brain-specific guanine nucleotide exchange factor (GEF) for Ras, whose activity is regulated in response to Ca(2+) influx and G protein-coupled receptor signals. In addition, Ras-GRF1 acts as a GEF for Rac when tyrosine-phosphorylated following G protein-coupled receptor stimulation. However, the mechanisms underlying the regulation of Ras-GRF1 functions remain incompletely understood. We show here that activated ACK1, a nonreceptor tyrosine kinase that belongs to the focal adhesion kinase family, causes tyrosine phosphorylation of Ras-GRF1. On the other hand, kinase-deficient ACK1 exerted no effect. GEF activity of Ras-GRF1 toward Ha-Ras, as defined by in vitro GDP binding and release assays, was augmented after tyrosine phosphorylation by ACK1. In contrast, GEF activity toward Rac1 remained latent, implying that ACK1 does not represent a tyrosine kinase that acts downstream of G protein-coupled receptors. Consistent with enhanced Ras-GEF activity, accumulation of the GTP-bound form of Ras within the cell was shown through the use of Ras-binding domain pull-down assays. Furthermore, Ras-dependent activation of ERK2 by Ras-GRF1 was enhanced following co-expression of activated ACK1. These results implicate ACK1 as an upstream modulator of Ras-GRF1 and suggest a signaling cascade consisting of Cdc42, ACK1, Ras-GRF1, and Ras in neuronal cells.

Intersectin can regulate the Ras/MAP kinase pathway independent of its role in endocytosis

We previously identified intersectin, a multiple EH and SH3 domain-containing protein, as a component of the endocytic machinery. Overexpression of the SH3 domains of intersectin blocks transferrin receptor endocytosis, possibly by disrupting targeting of accessory proteins of clathrin-coated pit formation. More recently, we identified mammalian Sos, a guanine-nucleotide exchange factor for Ras, as an intersectin SH3 domain-binding partner. We now demonstrate that overexpression of intersectin's SH3 domains blocks activation of Ras and MAP kinase in various cell lines. Several studies suggest that activation of MAP kinase downstream of multiple receptor types is dependent on endocytosis. Thus, the dominant-negative effect of the SH3 domains on Ras/MAP kinase activation may be indirectly mediated through a block in endocytosis. Consistent with this idea, incubating cells at 4 degrees C or with phenylarsine oxide, treatments previously established to inhibit EGF receptor endocytosis, blocks EGF-dependent activation of MAP kinase. However, under these conditions, Ras activity is unaffected and overexpression of the SH3 domains of intersectin is still able to block Ras activation. Thus, intersectin SH3 domain overexpression can effect EGF-mediated MAP kinase activation directly through a block in Ras, consistent with a functional role for intersectin in Ras activation.

Activation (tyrosine phosphorylation) of ErbB-2 (HER-2/neu): a study of incidence and correlation with outcome in breast cancer

PURPOSE

We hypothesize that phosphorylated ErbB-2 (P-ErbB-2, identified by a novel antibody PN2A) may provide either more significant or additional prognostic marker data for breast cancer patients. This study was designed to compare the incidence and prognostic value of ErbB-2 (HER-2/neu) and P-ErbB-2 immunoexpression in archival breast cancer samples.

MATERIALS AND METHODS

Eight hundred sixteen invasive breast cancers with a median of 16.3 years of follow-up were immunostained for ErbB-2 (using antibody CB11) and P-ErbB-2 (using antibody PN2A). ErbB-2 and P-ErbB-2 data were compared with clinical, histologic, immunohistochemical, and outcome variables.

RESULTS

Of 816 primary breast cancers, 307 (38%) were positive for ErbB-2 and 37 (12% of ErbB-2 positive and 5% of the study population) expressed P-ErbB-2. P-ErbB-2 was not detected in ErbB-2-negative cases (n = 509). ErbB-2 immunohistochemical data were bimodal; patients with > or = 80% cellular expression had the shortest disease-free and disease-specific survival. P-ErbB-2 was associated with a higher percentage of ErbB-2-positive cells, a higher number of positive lymph nodes, and cellular proliferation. ErbB-2 and P-ErbB-2 were indicators of poor prognosis in node-positive patients in both univariate and multivariate analyses. We found that either P-ErbB-2 expression or high (> or = 80%) ErbB-2 expression provided the most significant prognostic value in node-positive cases by multivariate analyses. There were too few P-ErbB-2-positive cases and events in the node-negative patient group to allow statistical analysis of P-ErbB-2 in that subgroup.

CONCLUSION

PN2A immunostaining identified a subset (approximately 12% of ErbB-2-positive breast cancers) with activation (phosphorylation) of the receptor ErbB-2. P-ErbB-2 expression was strongly associated with higher levels of ErbB-2 expression (> or = 80%), although it was not a surrogate. Identification of cases with a high percentage of invasive breast cancer cells expressing ErbB-2 or determination of receptor activation via P-ErbB-2 may provide additional prognostic value in node-positive breast cancers.

The adaptor protein Gads (Grb2-related adaptor downstream of Shc) is implicated in coupling hemopoietic progenitor kinase-1 to the activated TCR

The hemopoietic-specific Gads (Grb2-related adaptor downstream of Shc) adaptor protein possesses amino- and carboxyl-terminal Src homology 3 (SH3) domains flanking a central SH2 domain and a unique region rich in glutamine and proline residues. Gads functions to couple the activated TCR to distal signaling events through its interactions with the leukocyte-specific signaling proteins SLP-76 (SH2 domain-containing leukocyte protein of 76 kDa) and LAT (linker for activated T cells). Expression library screening for additional Gads-interacting molecules identified the hemopoietic progenitor kinase-1 (HPK1), and we investigated the HPK1-Gads interaction within the DO11.10 murine T cell hybridoma system. Our results demonstrate that HPK1 inducibly associates with Gads and becomes tyrosine phosphorylated following TCR activation. HPK1 kinase activity is up-regulated in response to activation of the TCR and requires the presence of its proline-rich motifs. Mapping experiments have revealed that the carboxyl-terminal SH3 domain of Gads and the fourth proline-rich region of HPK1 are essential for their interaction. Deletion of the fourth proline-rich region of HPK1 or expression of a Gads SH2 mutant in T cells inhibits TCR-induced HPK1 tyrosine phosphorylation. Together, these data suggest that HPK1 is involved in signaling downstream from the TCR, and that SH2/SH3 domain-containing adaptor proteins, such as Gads, may function to recruit HPK1 to the activated TCR complex.

Direct interaction of SOS1 Ras exchange protein with the SH3 domain of phospholipase C-gamma1

A recent report that microinjection of the SH3 domain of PLC-gamma1 could induce DNA synthesis raised the functional importance of the SH3 domain of PLC-gamma1 in mitogenic signaling. In this report, we provide evidence that SOS1, a p21Ras-specific guanine nucleotide exchange factor, directly binds to the SH3 domain of PLC-gamma1, and that the SH3 domain of PLC-gamma1 is involved in SOS1-mediated p21Ras activation. SOS1 was coprecipitated with the GST-fused SH3 domain of PLC-gamma1 in vitro. The interaction between SOS1 and the PLC-gamma1 SH3 domain is mediated by direct physical interaction. The carboxyl-terminal proline-rich domain of SOS1 is involved in the interaction with the PLC-gamma1 SH3 domain. Moreover, PLC-gamma1 could be co-immunoprecipitated with SOS1 antibody in cell lysates. From transient expression studies, we could demonstrate that the SH3 domain of PLC-gamma1 is necessary for the association with SOS1 in vivo. Intriguingly, overexpression of the SH3 domain of PLC-gamma1, lipase-inactive PLC-gamma1, or wild-type PLC-gamma1 elevated p21Ras activity and ERK activity when compared with vector transfected cells. The PLC-gamma1 mutant lacking the SH3 domain could not activate p21Ras. p21Ras activities in cell lines overexpressing either PLC-gamma1 or the SH2-SH2-SH3 domain of PLC-gamma1 were elevated about 2-fold compared to vector transfected cells. This study is the first to demonstrate that the PLC-gamma1 SH3 domain enhances p21Ras activity, and that the SH3 domain of PLC-gamma1 may be involved in the SOS1-mediated signaling pathway.

The EGF receptor provides an essential survival signal for SOS-dependent skin tumor development

The EGF receptor (EGFR) is required for skin development and is implicated in epithelial tumor formation. Transgenic mice expressing a dominant form of Son of Sevenless (SOS-F) in basal keratinocytes develop skin papillomas with 100% penetrance. However, tumor formation is inhibited in a hypomorphic (wa2) and null EGFR background. Similarly, EGFR-deficient fibroblasts are resistant to transformation by SOS-F and rasV12, however, tumorigenicity is restored by expression of the anti-apoptotic bcl-2 gene. The K5-SOS-F papillomas and primary keratinocytesfrom wa2 mice display increased apoptosis, reduced Akt phosphorylation and grafting experiments imply a cell-autonomous requirement for EGFR in keratinocytes. Therefore, EGFR functions as a survival factor in oncogenic transformation and provides a valuable target for therapeutic intervention in a broader range of tumors than anticipated.

Transformation and Stat activation by derivatives of FGFR1, FGFR3, and FGFR4

The fibroblast growth factor receptor (FGFR) family members mediate a number of important cellular processes, and are mutated or overexpressed in several forms of human cancer. Mutation of Lys650-->Glu in the activation loop of the FGFR3 kinase domain causes the lethal human skeletal disorder thanatophoric dysplasia type II (TDII) and is also found in patients with multiple myeloma, bladder and cervical carcinomas. This mutation leads to constitutive activation of FGFR3. To compare the signaling activity of FGFR family members, this activating mutation was generated in FGFR1, FGFR3, and FGFR4. We show that the kinase domains of FGFR1, FGFR3, and FGFR4 containing the activation loop mutation, when targeted to the plasma membrane by a myristylation signal, can transform NIH3T3 cells and induce neurite outgrowth in PC12 cells. Phosphorylation of Shp2, PLC-gamma, and MAPK was also stimulated by all three 'TDII-like' FGFR derivatives. Additionally, activation of Stat1 and Stat3 was observed in cells expressing the activated FGFR derivatives. Finally, we demonstrate that FGFR1, FGFR3, and FGFR4 derivatives can stimulate PI-3 kinase activity. Our comparison of these activated receptor derivatives reveals a significant overlap in the panel of effector proteins used to mediate downstream signals. This also represents the first demonstration that activation of FGFR4, in addition to FGFR1 and FGFR3, can induce cellular transformation. Moreover, our results suggest that Stat activation by FGFRs is important in their ability to act as oncogenes.

Natural and synthetic analogues of actinomycin D as Grb2-SH2 domain blockers

Natural analogues (D, C2, and VII) of actinomycin inhibit Grb2 SH2 domain binding with phosphopeptide-derived from Shc in vitro and in intracellular system. To study structure-activity relationships, 13 actinomycin analogues were synthesized and we found that the inhibition activity depended on the substituents of cyclic peptide groups in actinomycin and two analogues with Tyr residue are the most potent inhibitors with IC50 value of 0.5 and 0.8 microM, respectively.

The Ste20 kinase misshapen regulates both photoreceptor axon targeting and dorsal closure, acting downstream of distinct signals

We have previously shown that the Ste20 kinase encoded by misshapen (msn) functions upstream of the c-Jun N-terminal kinase (JNK) mitogen-activated protein kinase module in Drosophila. msn is required to activate the Drosophila JNK, Basket (Bsk), to promote dorsal closure of the embryo. A mammalian homolog of Msn, Nck interacting kinase, interacts with the SH3 domains of the SH2-SH3 adapter protein Nck. We now show that Msn likewise interacts with Dreadlocks (Dock), the Drosophila homolog of Nck. dock is required for the correct targeting of photoreceptor axons. We have performed a structure-function analysis of Msn in vivo in Drosophila in order to elucidate the mechanism whereby Msn regulates JNK and to determine whether msn, like dock, is required for the correct targeting of photoreceptor axons. We show that Msn requires both a functional kinase and a C-terminal regulatory domain to activate JNK in vivo in Drosophila. A mutation in a PXXP motif on Msn that prevents it from binding to the SH3 domains of Dock does not affect its ability to rescue the dorsal closure defect in msn embryos, suggesting that Dock is not an upstream regulator of msn in dorsal closure. Larvae with only this mutated form of Msn show a marked disruption in photoreceptor axon targeting, implicating an SH3 domain protein in this process; however, an activated form of Msn is not sufficient to rescue the dock mutant phenotype. Mosaic analysis reveals that msn expression is required in photoreceptors in order for their axons to project correctly. The data presented here genetically link msn to two distinct biological events, dorsal closure and photoreceptor axon pathfinding, and thus provide the first evidence that Ste20 kinases of the germinal center kinase family play a role in axonal pathfinding. The ability of Msn to interact with distinct classes of adapter molecules in dorsal closure and photoreceptor axon pathfinding may provide the flexibility that allows it to link to distinct upstream signaling systems.

Retinoic acid affects the EGF-R signaling pathway during differentiation induction of human endometrial adenocarcinoma cells

We have shown that moderately differentiated endometrial adenocarcinoma (RL95-2) cells differentiate in response to retinoic acid treatment, illustrated by their reorganization of actin filaments and cell enlargement (Carter et al., Anticancer Res. 16, 17-24, 1996). Tyrphostin, an inhibitor of epidermal growth factor receptor (EGF-R)-associated protein tyrosine kinases, caused a dramatic reorganization of actin filaments in RL95-2 cells, similar to retinoic-acid-treated cells (Carter and Bellido, J. Cell. Physiol. 178, 320-332, 1999). We evaluated the possibility that the differentiating effects of retinoids are due to retinoic-acid-induced decreases in phosphorylation of EGF-R and changes in downstream effector proteins. Retinoic acid caused a decrease in tyrosine phosphorylation of EGF-R. Retinoic acid treatment induced a dramatic actin filament reorganization and cell enlargement. Treatment with EGF reversed this effect, because cells treated with retinoic acid followed by EGF only possessed disrupted actin aggregates and appeared small, thus resembling medium controls. Retinoic acid induced a relocalization and decrease in the amount of Shc protein, another actin-binding protein which is an adaptor protein for EGF-R signaling. In addition, retinoic acid induced a relocalization of gelsolin from the plasma membrane to the cytoplasm. Retinoic acid decreased cell detachment in detachment assays; one-half as many retinoic-acid-treated cells detached as in controls. These results are consistent with the idea that retinoic acid induces differentiation of RL95-2 cells by interfering with the EGF-R signaling pathway.

The Rgr oncogene (homologous to RalGDS) induces transformation and gene expression by activating Ras, Ral and Rho mediated pathways

The effects of the 5'-truncated Rgr oncogene, a previously shown specific guanine exchange factor for Ral in vitro, in stimulating proliferation, cell transformation and gene expression were investigated. We have established TetRgr cell lines in which expression of Rgr can be inhibited by the presence of tetracycline in the medium. Using this system, we show that Rgr overexpressing cells are morphologically transformed and grow in a disorganized manner. At the transcriptional level, Rgr enhances the activity of the serum response element and c-Jun. Rgr induces phosphorylation of ERKs, p38 and JNK kinases, and increases the levels of the GTP-bound forms of Ral and Ras. Ras activation could account for the broad spectra of effects displayed by Rgr. The important role of these pathways is confirmed by experiments in which the transcriptional activation events can be blocked by dominant negative versions of Ras, Ral and Rho. Among all the Rgr-induced pathways, the Ras-Raf-MEK-ERK cascade is essential for the transforming properties of Rgr. Additional analysis has shown that the activation of this pathway by Rgr is not due to a feed back mechanism mediated by the Grb2 adaptor protein. Oncogene (2000).

Oncogenic transformation by ras and fos is mediated by c-Jun N-terminal phosphorylation

The nuclear phosphoprotein c-Jun is a major component of the AP-1 transcription factor, whose activity is augmented by many oncogenes. An important mechanism to stimulate AP-1 function is N-terminal phosphorylation of c-Jun at the serine residues 63 and 73 by the c-JunN-terminal kinases (JNKs). Mice and cells harboring a mutant allele of c-jun, which has the JNK phosphoacceptor serines changed to alanines (junAA), were used to determine the function of c-Jun N-terminal phosphorylation (JNP) during oncogenic transformation in vitro and in vivo. JunAA immortalized fibroblasts expressing v-ras and v-fos showed reduced tumorigenicity in nude mice, but the efficiency of v-src transformation was unaffected by the lack of JNP. To assess the significance of JNP in tumour development in vivo, two transgenic mouse tumour models were employed. Skin tumour development caused by constitutive activation of the ras pathway by K5-SOS-F expression and c-fos-induced osteosarcoma formation were impaired in mice lacking JNP. Inhibition of JNP may, therefore, be a novel therapeutic strategy to inhibit tumour growth in vivo. Oncogene (2000).

Identification and characterization of a new family of guanine nucleotide exchange factors for the ras-related GTPase Ral

Guanine nucleotide exchange factors (GEFs) are responsible for coupling cell surface receptors to Ras protein activation. Here we describe the characterization of a novel family of differentially expressed GEFs, identified by database sequence homology searching. These molecules share the core catalytic domain of other Ras family GEFs but lack the catalytic non-conserved (conserved non-catalytic/Ras exchange motif/structurally conserved region 0) domain that is believed to contribute to Sos1 integrity. In vitro binding and in vivo nucleotide exchange assays indicate that these GEFs specifically catalyze the GTP loading of the Ral GTPase when overexpressed in 293T cells. A central proline-rich motif associated with the Src homology (SH)2/SH3-containing adapter proteins Grb2 and Nck in vivo, whereas a pleckstrin homology (PH) domain was located at the GEF C terminus. We refer to these GEFs as RalGPS 1A, 1B, and 2 (Ral GEFs with PH domain and SH3 binding motif). The PH domain was required for in vivo GEF activity and could be functionally replaced by the Ki-Ras C terminus, suggesting a role in membrane targeting. In the absence of the PH domain RalGPS 1B cooperated with Grb2 to promote Ral activation, indicating that SH3 domain interaction also contributes to RalGPS regulation. In contrast to the Ral guanine nucleotide dissociation stimulator family of Ral GEFs, the RalGPS proteins do not possess a Ras-GTP-binding domain, suggesting that they are activated in a Ras-independent manner.

Why cytoplasmic signalling proteins should be recruited to cell membranes

It has been suggested that localization of signal-transduction proteins close to the cell membrane causes an increase in their rate of encounter after activation. We maintain that such an increase in the first-encounter rate is too small to be responsible for truly enhanced signal transduction. Instead, the function of membrane localization is to increase the number (or average lifetime) of complexes between cognate signal transduction proteins and hence increase the extent of activation of downstream processes. This is achieved by concentrating the proteins in the small volume of the area just below the plasma membrane. The signal-transduction chain is viewed simply as operating at low default intensity because one of its components is present at a low concentration. The steady signalling level of the chain is enhanced 1000-fold by increasing the concentration of that component. This occurs upon 'piggyback' binding to a membrane protein, such as the activated receptor, initiating the signal-transduction chain. For the effect to occur, the protein translocated to the membrane cannot be free but has to remain organized by being piggyback bound to a receptor, membrane lipid(s) or scaffold. We discuss an important structural constraint imposed by this mechanism on signal transduction proteins that might also account for the presence of adaptor proteins.

Differential interaction of CrkII adaptor protein with platelet-derived growth factor alpha- and beta-receptors is determined by its internal tyrosine phosphorylation

CrkII is an intracellular adaptor protein involved in signal transduction by various growth factors. Activation of PDGF alpha-receptor resulted in its association with CrkII in vivo. In contrast, binding of CrkII to the PDGF beta-receptor was negligible, despite its becoming prominently phosphorylated. Bacterially expressed GST-CrkII SH2 domain specifically bound to Tyr-762 and Tyr-771 in the activated PDGF alpha- and beta- receptors, respectively. GST fusion protein of full-length CrkII also bound to the activated PDGF beta-receptor. However, tyrosine phosphorylation of GST-CrkII diminished its binding to the beta-receptor. CrkI, a truncated version of CrkII lacking the phosphorylatable tyrosine residue, could bind to both PDGF alpha- and beta-receptors in vivo. In conclusion, tyrosine phosphorylation of CrkII negatively affects its binding to the PDGF receptors. The differential binding of CrkII to the PDGF alpha- and beta- receptors may be a rationale for functional diversity between the two receptors.

H-ras but not K-ras traffics to the plasma membrane through the exocytic pathway

Ras proteins must be localized to the inner surface of the plasma membrane to be biologically active. The motifs that effect Ras plasma membrane targeting consist of a C-terminal CAAX motif plus a second signal comprising palmitoylation of adjacent cysteine residues or the presence of a polybasic domain. In this study, we examined how Ras proteins access the cell surface after processing of the CAAX motif is completed in the endoplasmic reticulum (ER). We show that palmitoylated CAAX proteins, in addition to being localized at the plasma membrane, are found throughout the exocytic pathway and accumulate in the Golgi region when cells are incubated at 15 degrees C. In contrast, polybasic CAAX proteins are found only at the cell surface and not in the exocytic pathway. CAAX proteins which lack a second signal for plasma membrane targeting accumulate in the ER and Golgi. Brefeldin A (BFA) significantly inhibits the plasma membrane accumulation of newly synthesized, palmitoylated CAAX proteins without inhibiting their palmitoylation. BFA has no effect on the trafficking of polybasic CAAX proteins. We conclude that H-ras and K-ras traffic to the cell surface through different routes and that the polybasic domain is a sorting signal diverting K-Ras out of the classical exocytic pathway proximal to the Golgi. Farnesylated Ras proteins that lack a polybasic domain reach the Golgi but require palmitoylation in order to traffic further to the cell surface. These data also indicate that a Ras palmitoyltransferase is present in an early compartment of the exocytic pathway.

Constitutive activation of the Ras/MAP kinase pathway and enhanced TCR signaling by targeting the Shc adaptor to membrane rafts

The Shc adaptor is responsible for coupling receptor tyrosine kinases and tyrosine kinase-associated receptors to the Ras/MAP kinase pathway. Shc is believed to be regulated by a change in subcellular localization from the cytosol to the plasma membrane, where it recruits Grb-2/Sos complexes and hence permits juxtaposition of the guanine nucleotide exchange factor Sos to Ras, resulting in GDP/GTP exchange and Ras activation. Shc has been recently shown to inducibly colocalize in detergent-resistant membrane rafts together with the activated TCR and associated signaling molecules. To understand whether Shc localization in membrane rafts is sufficient to regulate Shc function, we constructed a Shc chimera containing the Ras membrane localization motif at the C-terminus. We show that membrane targeted Shc was constitutively localized in the plasma membrane of T-cells, and was mostly compartmentalized in lipid rafts. Membrane targeted Shc was phosphorylated on tyrosine residues and bound Grb-2/Sos in the absence of TCR engagement. Furthermore, expression of membrane targeted Shc resulted in constitutive downstream signaling, including Erk2 activation and enhancement of TCR dependent activation of the TCR responsive transcription factor NF-AT. Hence localization of Shc in membrane rafts is sufficient for Shc to acquire a signaling competent state. Interestingly, a membrane targeted Shc mutant lacking both Grb-2 binding sites was not only incapable of signaling in the absence of TCR triggering, but transdominantly inhibited endogenous Shc, supporting a non redundant role for Shc in the activation of the Ras/MAP kinase pathway in T-cells.

The endocytic protein intersectin is a major binding partner for the Ras exchange factor mSos1 in rat brain

We recently identified intersectin, a protein containing two EH and five SH3 domains, as a component of the endocytic machinery. The N-terminal SH3 domain (SH3A), unlike other SH3 domains from intersectin or various endocytic proteins, specifically inhibits intermediate events leading to the formation of clathrin-coated pits. We have now identified a brain-enriched, 170 kDa protein (p170) that interacts specifically with SH3A. Screening of combinatorial peptides reveals the optimal ligand for SH3A as Pp(V/I)PPR, and the 170 kDa mammalian son-of-sevenless (mSos1) protein, a guanine-nucleotide exchange factor for Ras, con- tains two copies of the matching sequence, PPVPPR. Immunodepletion studies confirm that p170 is mSos1. Intersectin and mSos1 are co-enriched in nerve terminals and are co-immunoprecipitated from brain extracts. SH3A competes with the SH3 domains of Grb2 in binding to mSos1, and the intersectin-mSos1 complex can be separated from Grb2 by sucrose gradient centrifugation. Overexpression of the SH3 domains of intersectin blocks epidermal growth factor-mediated Ras activation. These results suggest that intersectin functions in cell signaling in addition to its role in endocytosis and may link these cellular processes.

Induction of rac-guanine nucleotide exchange activity of Ras-GRF1/CDC25(Mm) following phosphorylation by the nonreceptor tyrosine kinase Src

Ras-GRF1/CDC25(Mm) has been implicated as a Ras-guanine nucleotide exchange factor (GEF) expressed in brain. Ras-GEF activity of Ras-GRF1 is augmented in response to Ca(2+) influx and G protein betagamma subunit (Gbetagamma) stimulation. Ras-GRF1 also acts as a GEF toward Rac, but not Rho and Cdc42, when activated by Gbetagamma-mediated signals. Tyrosine phosphorylation of Ras-GRF1 is critical for the induction of Rac-GEF activity as evidenced by inhibition by tyrosine kinase inhibitors. Herein, we show that the nonreceptor tyrosine kinase Src phosphorylates Ras-GRF1, thereby inducing Rac-GEF activity. Ras-GRF1 transiently expressed with v-Src was tyrosine-phosphorylated and showed significant GEF activity toward Rac, but not Rho and Cdc42, which was comparable with that induced by Gbetagamma. In contrast, Ras-GEF activity remained unchanged. The recombinant c-Src protein phosphorylated affinity-purified glutathione S-transferase-tagged Ras-GRF1 in vitro and thereby elicited Rac-GEF activity. Taken together, tyrosine phosphorylation by Src is sufficient for the induction of Rac-GEF activity of Ras-GRF1, which may imply the involvement of Src downstream of Gbetagamma to regulate Ras-GRF1.

ZAP-70 is essential for the T cell antigen receptor-induced plasma membrane targeting of SOS and Vav in T cells

Translocation of the SOS and Vav GDP/GTP exchange factors proximal to Ras and Rac GTPases localized in the plasma membrane glycolipid-enriched microdomains is a pivotal step required for T cell antigen receptor-induced T cell activation. Here we demonstrate that the T cell antigen receptor zeta-chain-associated ZAP-70 kinase and T cell antigen receptor zeta-chain immunoreceptor tyrosine-based activation motifs are essential for the membrane recruitment of SOS and Vav. Plasma membrane targeting of SOS or Vav begins with the assembly of ZAP-70 with Grb-2 and SOS. The subsequent tyrosine phosphorylation of LAT (linker for activation of T cell) by ZAP-70 leads to a shift in equilibrium from the ZAP-70.Grb-2.SOS(Vav) complex to the (Vav)SOS.Grb-2.LAT complex. This shift results in the targeting of SOS and Vav into glycolipid-enriched microdomains and initiation of the Ras and Rac signaling cascades involved in T cell activation, proliferation, and cytokine production.

Requirement for protein-tyrosine phosphatase SHP-2 in insulin-induced activation of c-Jun NH(2)-terminal kinase

Mitogen-activated protein kinases, including extracellular signal-regulated kinases and c-Jun NH(2)-terminal kinases (JNKs), are activated by insulin. Although the mechanism by which the insulin receptor activates extracellular signal-regulated kinases is relatively well defined, the pathway that leads to JNK activation is poorly understood. Overexpression of a catalytically inactive mutant (SHP-2C/S) of the protein-tyrosine phosphatase SHP-2 in Rat-1 fibroblasts that also express human insulin receptors has now revealed that activation of JNKs by insulin and epidermal growth factor, but not that by anisomycin or sorbitol, requires SHP-2. A dominant negative mutant (RasN17) of Ha-Ras blocked insulin-induced JNK activation, whereas a dominant negative mutant (RacN17) of Rac1 or a specific inhibitor (LY294002) of phosphoinositide 3-kinase did not, indicating a role for Ras, but not for Rac or phosphoinositide 3-kinase, in this effect. SHP-2C/S markedly inhibited Ras activation in response to insulin without affecting insulin-induced tyrosine phosphorylation of cellular substrates or the dissociation of the Crk-p130(Cas) complex. In contrast, SHP-2C/S did not inhibit activation of JNKs induced by a constitutively active mutant (RasV12) of Ha-Ras. Furthermore, expression of myristoylated SOS, which functions as a potent activator of Ras, induced JNK activation even when SHP-2 was inactivated. These results suggest that SHP-2 contributes to JNK activation in response to insulin by positively regulating the Ras signaling pathway at the same level as, or upstream from, SOS.