Insulin and epidermal growth factor stimulate a conformational change in Rap1 and dissociation of the CrkII-C3G complex

Crk proteins activate the Rap1 guanine nucleotide exchange factor C3G by segregated adaptor-dependent and -independent mechanisms

BACKGROUND

C3G is a guanine nucleotide exchange factor (GEF) that activates Rap1 to promote cell adhesion. Resting C3G is autoinhibited and the GEF activity is released by stimuli that signal through tyrosine kinases. C3G is activated by tyrosine phosphorylation and interaction with Crk adaptor proteins, whose expression is elevated in multiple human cancers. However, the molecular details of C3G activation and the interplay between phosphorylation and Crk interaction are poorly understood.

METHODS

We combined biochemical, biophysical, and cell biology approaches to elucidate the mechanisms of C3G activation. Binding of Crk adaptor proteins to four proline-rich motifs (P1 to P4) in C3G was characterized in vitro using isothermal titration calorimetry and sedimentation velocity, and in Jurkat and HEK293T cells by affinity pull-down assays. The nucleotide exchange activity of C3G over Rap1 was measured using nucleotide-dissociation kinetic assays. Jurkat cells were also used to analyze C3G translocation to the plasma membrane and the C3G-dependent activation of Rap1 upon ligation of T cell receptors.

RESULTS

CrkL interacts through its SH3N domain with sites P1 and P2 of inactive C3G in vitro and in Jurkat and HEK293T cells, and these sites are necessary to recruit C3G to the plasma membrane. However, direct stimulation of the GEF activity requires binding of Crk proteins to the P3 and P4 sites. P3 is occluded in resting C3G and is essential for activation, while P4 contributes secondarily towards complete stimulation. Tyrosine phosphorylation of C3G alone causes marginal activation. Instead, phosphorylation primes C3G lowering the concentration of Crk proteins required for activation and increasing the maximum activity. Unexpectedly, optimal activation also requires the interaction of CrkL-SH2 domain with phosphorylated C3G.

CONCLUSION

Our study revealed that phosphorylation of C3G by Src and Crk-binding form a two-factor mechanism that ensures tight control of C3G activation. Additionally, the simultaneous SH2 and SH3N interaction of CrkL with C3G, required for the activation, reveals a novel adaptor-independent function of Crk proteins relevant to understanding their role in physiological signaling and their deregulation in diseases. Video abstract.

C3G downregulation induces the acquisition of a mesenchymal phenotype that enhances aggressiveness of glioblastoma cells

Glioblastoma (GBM) is the most aggressive tumor from the central nervous system (CNS). The current lack of efficient therapies makes essential to find new treatment strategies. C3G, a guanine nucleotide exchange factor for some Ras proteins, plays a dual role in cancer, but its function in GBM remains unknown. Database analyses revealed a reduced C3G mRNA expression in GBM patient samples. C3G protein levels were also decreased in a panel of human GBM cell lines as compared to astrocytes. Based on this, we characterized C3G function in GBM using in vitro and in vivo human GBM models. We report here that C3G downregulation promoted the acquisition of a more mesenchymal phenotype that enhanced the migratory and invasive capacity of GBM cells. This facilitates foci formation in anchorage-dependent and -independent growth assays and the generation of larger tumors in xenografts and chick chorioallantoic membrane (CAM) assays, but with a lower cell density, as proliferation was reduced. Mechanistically, C3G knock-down impairs EGFR signaling by reducing cell surface EGFR through recycling inhibition, while upregulating the activation of several other receptor tyrosine kinases (RTKs) that might promote invasion. In particular, FGF2, likely acting through FGFR1, promoted invasion of C3G-silenced GBM cells. Moreover, ERKs mediate this invasiveness, both in response to FGF2- and serum-induced chemoattraction. In conclusion, our data show the distinct dependency of GBM tumors on C3G for EGF/EGFR signaling versus other RTKs, suggesting that assessing C3G levels may discriminate GBM patient responders to different RTK inhibition protocols. Hence, patients with a low C3G expression might not respond to EGFR inhibitors.

Combination of Gentiana rhodantha and Gerbera anandria in the BL02 formula as therapeutics to non-small cell lung carcinoma acting via Rap1/cdc42 signaling: A transcriptomics/ bio-informatics biological validation approach

BACKGROUND

Non-small cell lung cancer (NSCLC) ranks the most commonly diagnosed and highest mortality-leading cancer worldwide despite a variety of treatment strategies are available. The highly heterogeneous and aggressive property of NSCLC as well as its poor prognosis indicates the need for novel therapeutic targets identification. The objective of this study is to identify potential targets from the adjuvant herbal formula BL02 using a combined approach of high throughput transcriptomics and network pharmacology.

METHODS

The quality and stability of BL02 were assessed by UHPLC analysis. The inhibitory effect of BL02 on NSCLC was measured by in vivo orthotopic intrathoracic mouse model and in vitro cellular models. EGFR-mutant HCC827 and wild type A549 cell lines were employed. Transcriptomics analysis was introduced to profile the gene expression of NSCLC cells treated with BL02; Network pharmacology and molecular docking analyses predicted the interaction of compounds and NSCLC targets. Immuno-blotting and pull-down assays verified the putative targets.

RESULTS

The UHPLC analysis revealed that BL02 was relatively stable between batches of production and for 24 months of storage. Orally administration of BL02 was safe and effective to inhibit pulmonary NSCLC growth in mice implanted with A549 and HCC827-generated tumors. BL02 exhibited relatively low cytotoxicity to NSCLC cells in vitro, but potently suppressed NSCLC cell motility. The transcriptomic analysis illustrated that EGFR and cellular adhesion-related signaling is involved in BL02 action. Further bioinformatics analysis validated BL02 activity is mediated by cdc42-regulated signaling. BL02 depolymerized the actin cytoskeleton through suppressing cdc42 and deactivating its upstream molecule Rap1. These effects may be primarily mediated by the direct binding of 5-methylcoumarin-4-cellobioside and mangiferin from BL02 to Rap1 protein.

CONCLUSION

Our study proposes an integration model of experimental, transcriptomic and bioinformatics analyses in the identification of novel therapeutic target of NSCLC from an adjuvant herbal formula BL02. Our findings revealed that inhibition of Rap1/cdc42 signaling by active compounds 5-methylcoumarin-4-cellobioside and mangiferin from BL02 might be potentially effective therapy for NSCLC.

Continuous or Transient High Level of Glucose Exposure Differentially Increases Coronary Artery Endothelial Cell Proliferation and Human Colon Cancer Cell Proliferation

We studied effect of high glucose levels on coronary artery endothelial cell proliferation and human colon cancer cell proliferation. To examine the long-term effect of glucose exposure on cell growth, cells were cultured for 14 days in the absence or presence of 183 mg/dL D-glucose addition in the culture medium. Short effect of elevated glucose levels was examined by addition of 183 mg/dL D-glucose addition in the culture medium for just one hour per day followed by changing the culture to standard medium (5.5 mM D-glucose) during the next 23-hours period. Cell proliferation was estimated by 2,3-Bis (2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carbox-anilide (XTT) assay and phosphor-Erk western blot analysis. We found that coronary artery endothelial cell proliferation was significantly increased in the culture medium with the acute one-hour addition of 183 mg/dL D-glucose compared to the absence or chronic presence of 183 mg/dL D-glucose addition in the culture medium. In contrast, colon cancer cell proliferation was significantly increased in the continuous presence of 183 mg/dL D-glucose addition in the culture medium compared to the acute one-hour addition of glucose. The extent of Erk2 phosphorylation paralleled with the relative changes in cellular proliferation in both cell types. Taken together, these results suggested that continuous or transient high level of glucose exposure differentially effects coronary artery endothelial and human colon cancer cell proliferation.

Identification of targets of c-Src tyrosine kinase by chemical complementation and phosphoproteomics

The cellular proto-oncogene c-Src is a nonreceptor tyrosine kinase involved in cell growth and cytoskeletal regulation. Despite being dysregulated in a variety of human cancers, its precise functions are not fully understood. Identification of the substrates of c-Src remains a major challenge, because there is no simple way to directly stimulate its activity. Here we combine the chemical rescue of mutant c-Src and global quantitative phosphoproteomics to obtain the first high resolution snapshot of the range of tyrosine phosphorylation events that occur in the cell immediately after specific c-Src stimulation. After enrichment by anti-phosphotyrosine antibodies, we identified 29 potential novel c-Src substrate proteins. Tyrosine phosphopeptide mapping allowed the identification of 382 nonredundant tyrosine phosphopeptides on 213 phosphoproteins. Stable isotope labeling of amino acids in cell culture-based quantitation allowed the detection of 97 nonredundant tyrosine phosphopeptides whose level of phosphorylation is increased by c-Src. A large number of previously uncharacterized c-Src putative protein targets and phosphorylation sites are presented here, a majority of which play key roles in signaling and cytoskeletal networks, particularly in cell adhesion. Integrin signaling and focal adhesion kinase signaling pathway are two of the most altered pathways upon c-Src activation through chemical rescue. In this context, our study revealed the temporal connection between c-Src activation and the GTPase Rap1, known to stimulate integrin-dependent adhesion. Chemical rescue of c-Src provided a tool to dissect the spatiotemporal mechanism of activation of the Rap1 guanine exchange factor, C3G, one of the identified potential c-Src substrates that plays a role in focal adhesion signaling. In addition to unveiling the role of c-Src in the cell and, specifically, in the Crk-C3G-Rap1 pathway, these results exemplify a strategy for obtaining a comprehensive understanding of the functions of nonreceptor tyrosine kinases with high specificity and kinetic resolution.

Signalling to actin: role of C3G, a multitasking guanine-nucleotide-exchange factor

C3G (Crk SH3-domain-binding guanine-nucleotide-releasing factor) is a ubiquitously expressed member of a class of molecules called GEFs (guanine-nucleotide-exchange factor) that activate small GTPases and is involved in pathways triggered by a variety of signals. It is essential for mammalian embryonic development and many cellular functions in adult tissues. C3G participates in regulating functions that require cytoskeletal remodelling such as adhesion, migration, maintenance of cell junctions, neurite growth and vesicle traffic. C3G is spatially and temporally regulated to act on Ras family GTPases Rap1, Rap2, R-Ras, TC21 and Rho family member TC10. Increased C3G protein levels are associated with differentiation of various cell types, indicating an important role for C3G in cellular differentiation. In signalling pathways, C3G serves functions dependent on catalytic activity as well as protein interaction and can therefore integrate signals necessary for the execution of more than one cellular function. This review summarizes our current knowledge of the biology of C3G with emphasis on its role as a transducer of signals to the actin cytoskeleton. Deregulated C3G may also contribute to pathogenesis of human disorders and therefore could be a potential therapeutic target.

Conformational dynamics of calmodulin in complex with the calmodulin-dependent kinase kinase alpha calmodulin-binding domain

As the primary intracellular calcium sensor, calcium-saturated calmodulin (CaM) regulates numerous and diverse proteins. Several mechanisms, including tissue-specific expression, localization, and sequestration, work in concert to limit the total number of available targets of calmodulin within a cell. While the free energies of binding of calmodulin-binding domains of regulated proteins by CaM have been shown to be highly similar, they result from vastly different enthalpic and entropic contributions. Here, we report the backbone and side-chain methyl dynamics of calcium-activated calmodulin in complex with a peptide corresponding to the CaM-binding domain of calmodulin kinase kinase, along with the thermodynamic underpinnings of complex formation. The results show a considerable reduction in side-chain mobility throughout CaM upon binding the CaMKKalpha peptide, which is consistent with the enthalpically driven nature of the binding. Site-specific comparison to another kinase-derived peptide complex with similar thermodynamic values reveals significant differences in dynamics largely localized to the hydrophobic binding sites.

Uncoupling Crk signal transduction by Pseudomonas exoenzyme T

Exoenzyme T (ExoT) is a bifunctional type III cytotoxin of Pseudomonas aeruginosa that possesses both Rho GTPase-activating protein and ADP-ribosyltransferase activities. The ADP-ribosyltransferase activity of ExoT stimulated depolymerization of the actin cytoskeleton independent of Rho GTPase-activating protein function, and ExoT was subsequently shown to ADP-ribosylate Crk (CT10 regulator of kinase)-I and Crk-II. Crk proteins are eukaryotic adaptor proteins comprising SH2 and SH3 domains that are components of the integrin signaling pathway leading to Rac1 and Rap1 functions. Mass spectroscopic analysis identified Arg20 as the site of ADP-ribosylation by ExoT. Arg20 is a conserved residue located within the SH2 domain that is required for interactions with upstream signaling molecules such as paxillin and p130cas. Glutathione S-transferase pull-down and far Western assays showed that ADP-ribosylated Crk-I or Crk-I(R20K) failed to bind p130cas or paxillin. This indicates that ADP-ribosylation inhibited the direct interaction of Crk with these focal adhesion proteins. Overexpression of wild-type Crk-I reduced cell rounding by ExoT, whereas expression of dominant-active Rac1 interfered with the ability of ExoT to round cells. Thus, the ADP-ribosylation of Crk uncouples integrin signaling by direct inhibition of the binding of Crk to focal adhesion proteins.

Mutational analysis of the interaction between insulin receptor and IGF-I receptor with c-Crk and Crk-L in a yeast two-hybrid system

The SH2/SH3 adapter proteins of the Crk family are potent signal transducers after receptor tyrosine kinase stimulation with insulin or IGF-1. We have employed a yeast two-hybrid approach and mutational analysis to dissect the capabilities of the insulin receptor and the IGF-I receptor to directly associate with Crk isoforms. Insulin receptor stably recruits full length Crk by association with its SH2 domain in an auto-phosphorylation dependent manner. In contrast, interaction of the IGF-I receptor with the Crk-IISH2 domain was only detectable when Crk-II was truncated in its C-terminal part, indicating the transient nature of this interaction. From these data it can be concluded that members of the insulin receptor family activate Crk proteins in a differential manner.

Cortactin and Crk cooperate to trigger actin polymerization during Shigella invasion of epithelial cells

Shigella, the causative agent of bacillary dysentery, invades epithelial cells in a process involving Src tyrosine kinase signaling. Cortactin, a ubiquitous actin-binding protein present in structures of dynamic actin assembly, is the major protein tyrosine phosphorylated during Shigella invasion. Here, we report that RNA interference silencing of cortactin expression, as does Src inhibition in cells expressing kinase-inactive Src, interferes with actin polymerization required for the formation of cellular extensions engulfing the bacteria. Shigella invasion induced the recruitment of cortactin at plasma membranes in a tyrosine phosphorylation–dependent manner. Overexpression of wild-type forms of cortactin or the adaptor protein Crk favored Shigella uptake, and Arp2/3 binding–deficient cortactin derivatives or an Src homology 2 domain Crk mutant interfered with bacterial-induced actin foci formation. Crk was shown to directly interact with tyrosine-phosphorylated cortactin and to condition cortactin-dependent actin polymerization required for Shigella uptake. These results point at a major role for a Crk–cortactin complex in actin polymerization downstream of tyrosine kinase signaling.

Trafficking the NGF signal: implications for normal and degenerating neurons

Nerve growth factor (NGF) activates TrkA to trigger signaling events that promote the survival, differentiation and maintenance of neurons. The mechanism(s) that controls the retrograde transport of the NGF signal from axon terminals to neuron cell bodies is not known. The 'signaling endosome' hypothesis stipulates that NGF, TrkA and signaling proteins are retrogradely transported on endocytic vesicles. Here, we provide evidence for the existence of signaling endosomes. Following NGF treatment, clathrin-coated vesicles (CCVs) contain NGF bound to TrkA together with activated signaling proteins of the Ras/pErk1/2 pathway. NGF signals from isolated CCVs through the Erk1/2 pathway. Early endosomes appear to represent a second type of signaling endosomes. We found that NGF induced a sustained activation of Rap1, a small monomeric GTP-binding protein of the Ras family, and that this activation occurred in early endosomes that contain key elements of Rap1/pErk1/2 pathway. We discuss the possibility that the failure of retrograde NGF signaling in a mouse model of Down syndrome (Ts65Dn) may be due to the failure to retrograde transport signaling endosomes. It is important to define further the significance of signaling endosomes in the biology of both normal and degenerating neurons.

The guanine nucleotide exchange factor C3G is necessary for the formation of focal adhesions and vascular maturation

The Ras signalling pathway has major roles in normal cell function and oncogenesis. C3G is a guanine nucleotide exchange factor for members of the Ras family of GTPases. We generated a mouse strain with a hypomorphic C3G allele. C3G(gt/gt) mutant embryos died of vascular defects around E11.5 due to haemorrhage and vascular integrity defects. Vascular supporting cells did not develop appropriately. C3G-deficient fibroblasts responded to PDGF-BB abnormally, exhibited cell adhesion defects and lacked paxillin and integrin-beta1-positive cell adhesions. In contrast, integrin-beta3-positive cell adhesions formed normally. These results show that C3G is required for (1) vascular myogenesis, (2) the formation of paxillin- and integrin beta1-positive, but not integrin beta3-positive, cell adhesions and (3) normal response to PDGF, necessary for vascular myogenesis.

Crk family adaptors-signalling complex formation and biological roles

Crk family adaptors are widely expressed and mediate the timely formation of signal transduction protein complexes upon a variety of extracellular stimuli, including various growth and differentiation factors. Selective formation of multi-protein complexes by the Crk and Crk-like (CRKL) proteins depends on specific motifs recognized by their SH2 and SH3 domains. In the case of the first SH3 domains [SH3(1)] a P-x-x-P-x-K motif is crucial for highly selective binding, while the SH2 domains prefer motifs which conform to the consensus pY-x-x-P. Crk family proteins are involved in the relocalization and activation of several different effector proteins which include guanine nucleotide releasing proteins like C3G, protein kinases of the Abl- and GCK-families and small GTPases like Rap1 and Rac. Crk-type proteins have been found not only in vertebrates but also in flies and nematodes. Major insight into the function of Crk within organisms came from the genetic model organism C. elegans, where the Crk-homologue CED-2 regulates cell engulfment and phagocytosis. Other biological outcomes of the Crk-activated signal transduction cascades include the modulation of cell adhesion, cell migration and immune cell responses. Crk family adaptors also appear to play a role in mediating the action of human oncogenes like the leukaemia-inducing Bcr-Abl protein. This review summarizes some key findings and highlights recent insights and open questions.

Nerve growth factor activates persistent Rap1 signaling in endosomes

We investigated a role for endogenous Rap1, a small monomeric GTP-binding protein of the Ras family, in nerve growth factor (NGF) signaling in PC12 cells. Although both epidermal growth factor (EGF) and NGF caused transient activation of Ras, only NGF induced the activation of Rap1. Moreover, Rap1 activation was sustained for hours, an effect that matched the sustained activation of the mitogen-activated protein kinase (MAPK) pathway. To investigate the molecular basis for Rap1 activation, we examined complexes containing C3G, a guanine nucleotide exchange factor for Rap1, and CrkL, an adapter protein known to influence Rap1 signaling. NGF induced the formation of a long-lived complex containing C3G/CrkL/Shp2/Gab2/TrkA. Linking the complex to Rap1 activation, we coprecipitated activated TrkA and activated MAPK with activated Rap1 in NGF-treated cells. Confocal microscopy and subcellular fractionation showed that activated Rap1 and the other proteins of the signaling complex were present in endosomes. Pretreatment of PC12 cells with brefeldin A (BFA), which disrupts the Golgi and endosomal compartments, had little effect on Ras activation but strongly inhibited NGF-induced Rap1 activation and continuing MAPK activation. We propose that endosomes are a site from which NGF induces the prolonged activation of Rap1 and MAPK.

Identification of the mechanisms regulating the differential activation of the mapk cascade by epidermal growth factor and nerve growth factor in PC12 cells

In PC12 cells, epidermal growth factor (EGF) transiently stimulates the mitogen-activated protein (MAP) kinases, ERK1 and ERK2, and provokes cellular proliferation. In contrast, nerve growth factor (NGF) stimulation leads to the sustained activation of the MAPKs and subsequently to neuronal differentiation. It has been shown that both the magnitude and longevity of MAPK activation governs the nature of the cellular response. The activations of MAPKs are dependent upon two distinct small G-proteins, Ras and Rap1, that link the growth factor receptors to the MAPK cascade by activating c-Raf and B-Raf, respectively. We found that Ras was transiently stimulated upon both EGF and NGF treatment of PC12 cells. However, EGF transiently activated Rap1, whereas NGF stimulated prolonged Rap1 activation. The activation of the ERKs was due almost exclusively (>90%) to the action of B-Raf. The transient activation of the MAPKs by EGF was a consequence of the formation of a short lived complex assembling on the EGF receptor itself, composed of Crk, C3G, Rap1, and B-Raf. In contrast, NGF stimulation of the cells resulted in the phosphorylation of FRS2. FRS2 scaffolded the assembly of a stable complex of Crk, C3G, Rap1, and B-Raf resulting in the prolonged activation of the MAPKs. Together, these data provide a signaling link between growth factor receptors and MAPK activation and a mechanistic explanation of the differential MAPK kinetics exhibited by these growth factors.

Rap1 signalling: adhering to new models

Ras-like GTPases are ubiquitously expressed, evolutionarily conserved molecular switches that couple extracellular signals to various cellular responses. Rap1, the closest relative of Ras, has attracted much attention because of the possibility that it regulates Ras-mediated signalling. Rap1 is activated by extracellular signals through several regulatory proteins, and it might function in diverse processes, ranging from modulation of growth and differentiation to secretion, integrin-mediated cell adhesion and morphogenesis.

The association of CRKII with C3G can be regulated by integrins and defines a novel means to regulate the mitogen-activated protein kinases

In studies to define mechanisms of ERK activation in Chinese hamster ovary cells, we have observed an inverse correlation between CRKII-C3G complex formation and ERK activity. That is, we were able to coprecipitate the guanine nucleotide exchange factor C3G with the adaptor protein CRKII in lysates from suspended cells that had low ERK activity, but we could not do so or could do so less efficiently in lysates of adherent cells with increased ERK activity. Consistent with the presence of a functional CRKII-C3G complex, we detected more GTP-loaded RAP1 in suspension than adherent lysates. Overexpression of cDNAs encoding B-RAF, CRKII W109L, and PTP1B C215S activated ERK in suspension cells, the latter two constructs also disrupting CRKII-C3G complex formation. Finally, we have also observed that certain integrin alpha subunit cytoplasmic splice variants differentially regulate ERK1/2 but also in a manner that correlated with levels of a CRKII-C3G complex. Thus, these data suggest the involvement of integrins in an ERK suppression pathway mediated by CRKII-C3G complex formation and downstream signaling from activated RAP1.

The role of G proteins in insulin signalling

Insulin modulates many intracellular processes including cellular metabolism, cell proliferation and cell differentiation. Some of these processes involve significant changes in the traffic of intracellular vesicles or in the structural organization of the cell. These phenomena have been linked to the activity of regulatory GTP-binding proteins. Most, if not all functions, of the insulin receptor are associated with its tyrosine kinase activity. Thus, over the past few years, a significant effort has been dedicated to elucidate the cross-talk between the tyrosine kinase activity of the receptor and the regulation of G protein-mediated pathways. Recent progress indicates that G proteins may mediate the control of several of insulin's intracellular functions. These include the regulation of the MAP kinase pathway, the activation of phospholipase D and the regulation of glucose uptake. This article discusses some recent advances in this area.

Innovative treatment programs against cancer. I. Ras oncoprotein as a molecular target

Modulation of Ras function may provide a novel means by which cancer cells with oncogenic mutations can be sensitized to chemotherapeutic or radiotherapeutic regimens. Moreover, cancer cells without ras oncogene mutations can also be eliminated by compounds that interfere with the mevalonate pathway, which is more fundamental to mitogenesis because it allows the synthesis of sterol and nonsterol lipids and without which many Ras-related proteins and nuclear lamins would not be prenylated and functional.

Constitutive association of JAK1 and STAT5 in pro-B cells is dissolved by interleukin-4-induced tyrosine phosphorylation of both proteins

The bipartite human interleukin-4 (IL-4) receptor was functionally expressed in murine pro-B cells and activated by human IL-4 to evoke intracellular signaling. Mutual association of signal transducing proteins within the receptor complex was then studied in dependence of ligand stimulation. Besides ligand-induced receptor heterodimerization and contacts of the two IL-4 receptor subunits alpha and gamma with Janus kinases JAK1 and JAK3 a prominent constitutive binding between JAK1 and signal transducer and activator of transcription STAT5 was detected. Since both these proteins become phosphorylated in response to IL-4 receptor stimulation, the influence of tyrosine phosphorylation on their mutual contact was analyzed. Association of JAK1 and STAT5 was found to occur exclusively between unphosphorylated proteins.

Direct binding of p130(Cas) to the guanine nucleotide exchange factor C3G

p130(Cas) (Cas; crk-associated substrate) belongs to a new family of docking molecules. It contains one Src homology (SH) 3 domain in its amino-terminal region followed by a region containing binding motifs for SH2 and SH3 domains. To gain further insight into Cas signaling we used the SH3 domain of Cas in a two-hybrid screen to search a human placenta library for binding partners. The screen confirmed a previous finding of its binding to the focal adhesion kinase (FAK) but also identified C3G, a guanine nucleotide exchange factor. We found direct interaction between Cas and C3G in vitro and in vivo. A series of analysis with C3G deletion mutants revealed a proline-rich Cas-binding site (Ala0-Pro1-Pro2-Lys3-Pro4-Pro5-Leu6-Pro7) located NH2-terminal to the previously characterized Crk binding motifs in C3G. Mutagenesis studies showed that Pro1, Lys3, and Pro4 within the ligand-binding site are critical for high affinity interaction. These results, combined with sequence alignments of proline-rich binding elements from proteins known for Cas binding, define the consensus sequence XXPXKPX which is recognized by the CasSH3 domain. Cas shows structural characteristics of a docking molecule and may serve to bring C3G to specific compartments within the cell.

Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) stimulation of GLUT4 translocation is tyrosine kinase-dependent

Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) treatment of permeabilized adipocytes results in GLUT4 translocation similar to that elicited by insulin treatment. However, although the selective phosphatidylinositol 3-kinase inhibitor, wortmannin, completely prevented insulin-stimulated GLUT4 translocation, it was without effect on GTPgammaS-stimulated GLUT4 translocation. In addition, insulin was an effective stimulant, whereas GTPgammaS was a very weak activator of the downstream Akt serine/threonine kinase. Consistent with an Akt-independent mechanism, guanosine 5'-O-2-(thio)diphosphate inhibited insulin-stimulated GLUT4 translocation without any effect on the Akt kinase. Surprisingly, two functionally distinct tyrosine kinase inhibitors, genistein and herbimycin A, as well as microinjection of a monoclonal phosphotyrosine specific antibody, inhibited both GTPgammaS- and insulin-stimulated GLUT4 translocation. Phosphotyrosine immunoblotting and specific immunoprecipitation demonstrated that GTPgammaS did not elicit tyrosine phosphorylation of insulin receptor or insulin receptor substrate-1. In contrast to insulin, proteins in the 120-130-kDa and 55-75-kDa range were tyrosine-phosphorylated following GTPgammaS stimulation. Several of these proteins were identified and include protein-tyrosine kinase 2 (also known as CAKbeta, RAFTK, and CADTK), pp125 focal adhesion tyrosine kinase, pp130 Crk-associated substrate, paxillin, and Cbl. These data demonstrate that the GTPgammaS-stimulated GLUT4 translocation utilizes a novel tyrosine kinase pathway that is independent of both the phosphatidylinositol 3-kinase and the Akt kinase.