Structural requirement of CRK SH2 region for binding to phosphotyrosine-containing proteins. Evidence from reactivity to monoclonal antibodies

c-Crk proto-oncogene contributes to transcriptional repression of p120-catenin in non-small cell lung cancer cells

As a member of adherens junction, p120-catenin (p120ctn) plays a major role in cell adhesions through stabilization of E-cadherin. p120ctn is transcriptionally down-regulated in non-small cell lung cancer (NSCLC), although the molecular mechanisms underlying p120ctn repression are incompletely defined. Here we further investigated transcriptional regulation of p120ctn in NSCLC. We prepared a promoter reporter plasmid construct that contained p120ctn promoter region from position -1082 to +320 relative to transcription start site. Through serial deletion mutation analysis of the p120ctn promoter, we pinpointed cis-acting elements involved in regulation of p120ctn. We identified transcription factor SP1 as a transcriptional repressor of p120ctn that directly binds to segment (-9 to +36) of the p120ctn promoter. SP1 can receive multiple signals from several intracellular signaling pathways. Through examination of SP1 binding partners, we identified proto-oncogene c-Crk to be involved in transcriptional down-regulation of p120ctn. RNAi mediated silencing of CRK in A549, H157 and H358 cells increased p120ctn protein levels. On the other hand, over-expression of CRK-I and CRK-II in NSCLC cells down-regulated p120ctn, an effect that was abrogated by simultaneous silencing of SP1. In summary, our data provide evidence for the role of c-Crk proto-oncogene in transcriptional repression of p120ctn that further clarifies the mechanism by which this biochemical signal promotes metastasis in NSCLC.

Brk activates rac1 and promotes cell migration and invasion by phosphorylating paxillin

Brk (for breast tumor kinase) is a nonreceptor tyrosine kinase containing SH3, SH2, and tyrosine kinase catalytic domains. Brk was originally identified from a human metastatic breast tumor, and its overexpression is frequently observed in breast cancer and several other cancer types. However, the molecular mechanism by which this kinase participates in tumorigenesis remains poorly characterized. In the present study, we not only identified paxillin as the binding partner and substrate of Brk but also discovered a novel signaling pathway by which Brk mediates epidermal growth factor (EGF)-induced paxillin phosphorylation. We show that EGF stimulation activates the catalytic activity of Brk, which in turn phosphorylates paxillin at Y31 and Y118. These phosphorylation events promote the activation of small GTPase Rac1 via the function of CrkII. Through this pathway, Brk is capable of promoting cell motility and invasion and functions as a mediator of EGF-induced migration and invasion. In accordance with these functional roles, Brk translocates to membrane ruffles, where it colocalizes with paxillin during cell migration. Together, our findings identify novel signaling and biological roles of Brk and indicate the first potential link between Brk and metastatic malignancy.

CrkII regulates focal adhesion kinase activation by making a complex with Crk-associated substrate, p130Cas

CrkII is an adaptor protein possessing oncogenic potential despite the lack of an enzymatic domain. We investigated here the physiological functions of CrkII by studying its ability to induce anchorage-independent cell growth. We found that inhibition or null mutation of focal adhesion kinase (FAK) blocked the anchorage-independent growth induced by CrkII overexpression, indicating that FAK is a critical determinant of the transforming activity of CrkII. CrkII overexpression enhanced the autophosphorylation of FAK at Tyr-397 and tyrosine phosphorylation of p130(Cas) (Crk-associated substrate, Cas) upon stimulation of integrin by fibronectin. Moreover, the constitutive phosphorylation of FAK and Cas was observed in CrkII-overexpressing cells, even when they were in the suspended condition, consistent with the ability of CrkII to induce anchorage-independent growth. Using Cas-deficient cells, we showed Cas function to be essential for both the CrkII-induced phosphorylation of FAK (Tyr-397) and anchorage-independent cell growth. The CrkII-induced FAK autophosphorylation depended upon CrkII-Cas complex formation. Furthermore, we showed that CrkII knockdown resulted in defects in integrin-mediated events, such as cell spreading, haptotactic migration, and FAK autophosphorylation. The integrin-mediated FAK autophosphorylation was also reduced in Cas-deficient cells. These results suggest that the CrkII-Cas complex functions in integrin-mediated FAK activation signaling. Our findings show the importance of CrkII in integrin-mediated events, acting upstream of FAK to affect the activation of this kinase, which appears to have a central role in this pathway.

Cytoplasmic c-Abl provides a molecular 'Rheostat' controlling carcinoma cell survival and invasion

Tumor cell metastasis involves the coordinated activation of migration and survival mechanisms necessary for cell invasion of foreign tissues. Here, we report that cytoplasmic c-Abl tyrosine kinase determines whether a cell invades the ECM or commits suicide. c-Abl phosphorylates the cytoskeleton-associated adaptor protein, Crk, at tyrosine 221, causing disassociation of Crk from the Crk-associated substrate (CAS) and disassembly of Crk/CAS complexes. c-Abl-induced disruption of Crk/CAS complexes inhibits cell migration and promotes apoptosis in normal cells, and is deregulated in highly invasive carcinoma cells. c-Abl-mediated disassembly of Crk/CAS complexes and induction of death occur via disruption of the cytoskeleton, which is distinct from nuclear c-Abl-induced apoptosis in response to DNA-damaging agents. Inhibition of c-Abl kinase activity or Crk binding to Abl's polyproline region prevents Crk phosphorylation and apoptosis, leading to increased cell survival and invasion of the extracellular matrix. Together, these data illustrate that c-Abl prevents aberrant motility and survival through Crk 221 phosphorylation and modulation of Crk/CAS complexes, and that deregulation of this pathway contributes to cell metastasis.

Novel function of Chat in controlling cell adhesion via Cas-Crk-C3G-pathway-mediated Rap1 activation

Chat (Cas/HEF1-associated signal transducer) is a novel signaling molecule with an N-terminal SH2 domain and C-terminal Cas/HEF1 association domain that is implicated in the regulation of cell adhesion. The Cas/HEF1 association domain also shows sequence similarity with guanine nucleotide exchange factors for Ras family small GTPases. In this study, we found significant activation of Rap1 in Chat-overexpressing cells. Myr-Chat, a membrane-targeted form of Chat, activated Rap1 more efficiently. Interestingly, Chat and Cas synergistically activated Rap1. Certain Cas, Crk or C3G mutants suppressed Rap1 activation by Chat. We also confirmed the ternary complex formation consisting of Chat, Cas and Crk. Thus, it is likely that Chat-induced Rap1 activation was mediated by upregulation of the Cas-Crk-C3G signaling pathway rather than direct guanine nucleotide exchange factor activity of Chat. We further demonstrated that Myr-Chat expression induced cell periphery spreading and cell shape branching and that this activity also depended on the Cas-Crk-C3G pathway and Rap1 activity. Moreover, expression of Myr-Chat enhanced integrin-mediated cell adhesion. Taken together we propose a novel role for the Chat-Cas complex in controlling cell adhesion via the activation of Rap1.

Rho GTPase signalling pathways in the morphological changes associated with apoptosis

The killing and removal of superfluous cells is an important step during embryonic development, tissue homeostasis, wound repair and the resolution of inflammation. A specific sequence of biochemical events leads to a form of cell death termed apoptosis, and ultimately to the disassembly of the dead cell for phagocytosis. Dynamic rearrangements of the actin cytoskeleton are central to the morphological changes observed both in apoptosis and phagocytosis. Recent research has highlighted the importance of Rho GTPase signalling pathways to these changes in cellular architecture. In this review, we will discuss how these signal transduction pathways affect the structure of the actin cytoskeleton and allow for the efficient clearance of apoptotic cells.

Purification of pseudopodia from polarized cells reveals redistribution and activation of Rac through assembly of a CAS/Crk scaffold

Initiation of cell migration requires morphological polarization with formation of a dominant leading pseudopodium and rear compartment. A molecular understanding of this process has been limited, due to the inability to biochemically separate the leading pseudopodium from the rear of the cell. Here we examine the spatio-temporal localization and activation of cytoskeletal-associated signals in purified pseudopodia directed to undergo growth or retraction. Pseudopodia growth requires assembly of a p130Crk-associated substrate (CAS)/c-CrkII (Crk) scaffold, which facilitates translocation and activation of Rac1. Interestingly, Rac1 activation then serves as a positive-feedback loop to maintain CAS/Crk coupling and pseudopodia extension. Conversely, disassembly of this molecular scaffold is critical for export and down regulation of Rac1 activity and induction of pseudopodia retraction. Surprisingly, the uncoupling of Crk from CAS during pseudopodium retraction is independent of changes in focal adhesion kinase activity and CAS tyrosine phosphorylation. These findings establish CAS/Crk as an essential scaffold for Rac1-mediated pseudopodia growth and retraction, and illustrate spatio-temporal segregation of cytoskeletal signals during cell polarization.

Inhibition of cell migration by Abl family tyrosine kinases through uncoupling of Crk-CAS complexes

c-Abl and the Abl-related gene product (Arg) are nonreceptor tyrosine kinases that regulate the actin cytoskeleton of cells by direct association with F-actin and localization to focal contacts. However, the biological significance of this interaction is not known. We show here that transfection of COS-7 cells with a kinase-inactive form of c-Abl (Abl) promotes c-Crk II/p130(CAS) (Crk-CAS) coupling, enhancing cell migration. Moreover, embryonic fibroblast cells isolated from mice devoid of endogenous Abl and Arg (abl-/- arg-/-) demonstrate increased Crk-CAS coupling and motility. Conversely, expression of a kinase-active form of Abl or reconstitution of abl-/- arg-/- cells with wild-type Abl prevents Crk-CAS coupling and inhibits cell migration. Thus, Abl and Arg kinases play a critical role in preventing cell migration through regulation of Crk and CAS adaptor protein complexes, which are necessary for cell movement.

Wee1-regulated apoptosis mediated by the crk adaptor protein in Xenopus egg extracts

Many of the biochemical reactions of apoptotic cell death, including mitochondrial cytochrome c release and caspase activation, can be reconstituted in cell-free extracts derived from Xenopus eggs. In addition, because caspase activation does not occur until the egg extract has been incubated for several hours on the bench, upstream signaling processes occurring before full apoptosis are rendered accessible to biochemical manipulation. We reported previously that the adaptor protein Crk is required for apoptotic signaling in egg extracts (Evans, E.K., W. Lu, S.L. Strum, B.J. Mayer, and S. Kornbluth. 1997. EMBO (Eur. Mol. Biol. Organ.) J. 16:230-241). Moreover, we demonstrated that removal of Crk Src homology (SH)2 or SH3 interactors from the extracts prevented apoptosis. We now report the finding that the relevant Crk SH2-interacting protein, important for apoptotic signaling in the extract, is the well-known cell cycle regulator, Wee1. We have demonstrated a specific interaction between tyrosine-phosphorylated Wee1 and the Crk SH2 domain and have shown that recombinant Wee1 can restore apoptosis to an extract depleted of SH2 interactors. Moreover, exogenous Wee1 accelerated apoptosis in egg extracts, and this acceleration was largely dependent on the presence of endogenous Crk protein. As other Cdk inhibitors, such as roscovitine and Myt1, did not act like Wee1 to accelerate apoptosis, we propose that Wee1-Crk complexes signal in a novel apoptotic pathway, which may be unrelated to Wee1's role as a cell cycle regulator.

NGF-Dependent neurite outgrowth in PC12 cells overexpressing the Src homology 2-domain protein shb requires activation of the Rap1 pathway

The Src homology 2 (SH2) domain adaptor protein Shb has been shown to transmit NGF- and FGF-2-dependent differentiation signals in PC12 cells. To study if this involves signaling through the small GTPase Rap1, Rap1 activity was assessed in Shb-overexpressing PC12 cells. We demonstrate that NGF and EGF induce Rap1 activation in PC12-Shb cells, while FGF-2 fails to do so. However, PC12 cells expressing Shb with an inactivated SH2 domain do not respond to NGF stimulation with Rap1 activation. The CrkII SH2 domain interacts with Shb and a 130- to 135-kDa phosphotyrosine protein present mainly in PC12-Shb cells and these interactions may thus relate to the effect of Shb on Rap1 activation. Transient expression of RalGDS-RBD or Rap1GAP to block the Rap1 pathway reduces the NGF-dependent neurite outgrowth in PC12-Shb cells. These results suggest a role of Shb in NGF-dependent Rap1 signaling and this pathway may be of significance for neurite outgrowth under certain conditions.

GTK, a Src-related tyrosine kinase, induces nerve growth factor-independent neurite outgrowth in PC12 cells through activation of the Rap1 pathway. Relationship to Shb tyrosine phosphorylation and elevated levels of focal adhesion kinase

The rat pheochromocytoma cell line PC12 is extensively used as a model for studies of neuronal cell differentiation. These cells develop a sympathetic neuron-like phenotype when cultured in the presence of nerve growth factor. The present study was performed in order to assess the role of mouse GTK (previously named BSK/IYK), a cytoplasmic tyrosine kinase belonging to the Src family, for neurite outgrowth in PC12 cells. We report that PC12 cells stably overexpressing GTK exhibit a larger fraction of cells with neurites as compared with control cells, and this response is not accompanied by an increased ERK activity. Treatment of the cells with the MEK inhibitor PD98059 did not reduce the GTK-dependent increased in neurite outgrowth. GTK expression induces a nerve growth factor-independent Rap1 activation, probably through altered CrkII signaling. We observe increased CrkII complex formation with p130(Cas), focal adhesion kinase (FAK), and Shb in PC12-GTK cells. The expression of GTK also correlates with a markedly increased content of FAK, phosphorylation of the adaptor protein Shb, and an association between these two proteins. Transient transfection of GTK-overexpressing cells with RalGDS-RBD or Rap1GAP, inhibitors of the Rap1 pathway, reduces the GTK-dependent neurite outgrowth. These data suggest that GTK participates in a signaling pathway, perhaps involving Shb, FAK and Rap1, that induces neurite outgrowth in PC12 cells.

Extracellular-regulated kinase activation and CAS/Crk coupling regulate cell migration and suppress apoptosis during invasion of the extracellular matrix

Regulation of cell migration/invasion is important for embryonic development, immune function, and angiogenesis. However, migratory cells must also coordinately activate survival mechanisms to invade the extracellular matrix and colonize foreign sites in the body. Although invasive cells activate protective programs to survive under diverse and sometimes hostile conditions, the molecular signals that regulate these processes are poorly understood. Evidence is provided that signals that induce cell invasion also promote cell survival by suppressing apoptosis of migratory cells. Extracellular-regulated kinase (ERK) activation and molecular coupling of the adaptor proteins p130 Crk-associated substrate (CAS) and c-CrkII (Crk) represent two distinct pathways that induce cell invasion and protect cells from apoptosis in a three-dimensional collagen matrix. CAS/Crk-mediated cell invasion and survival requires activation of the small GTPase Rac, whereas ERK-induced cell invasion, but not survival requires myosin light chain kinase activation and myosin light chain phosphorylation. Uncoupling CAS from Crk or inhibition of ERK activity prevents migration and induces apoptosis of invasive cells. These findings provide molecular evidence that during invasion of the extracellular matrix, cells coordinately regulate migration and survival mechanisms through ERK activation and CAS/Crk coupling.

Phosphorylation of tyrosine residues 31 and 118 on paxillin regulates cell migration through an association with CRK in NBT-II cells

Identification of signaling molecules that regulate cell migration is important for understanding fundamental processes in development and the origin of various pathological conditions. The migration of Nara Bladder Tumor II (NBT-II) cells was used to determine which signaling molecules are specifically involved in the collagen-mediated locomotion. We show here that paxillin is tyrosine phosphorylated after induction of motility on collagen. Overexpression of paxillin mutants in which tyrosine 31 and/or tyrosine 118 were replaced by phenylalanine effectively impaired cell motility. Moreover, stimulation of motility by collagen preferentially enhanced the association of paxillin with the SH2 domain of the adaptor protein CrkII. Mutations in both tyrosine 31 and 118 diminished the phosphotyrosine content of paxillin and prevented the formation of the paxillin-Crk complex, suggesting that this association is necessary for collagen-mediated NBT-II cell migration. Other responses to collagen, such as cell adhesion and spreading, were not affected by these mutations. Overexpression of wild-type paxillin or Crk could bypass the migration-deficient phenotype. Both the SH2 and the SH3 domains of CrkII are shown to play a critical role in this collagen-mediated migration. These results demonstrate the important role of the paxillin-Crk complex in the collagen-induced cell motility.

ErbB2 is necessary for induction of carcinoma cell invasion by ErbB family receptor tyrosine kinases

The epidermal growth factor (EGF) family of tyrosine kinase receptors (ErbB1, -2, -3, and -4) and their ligands are involved in cell differentiation, proliferation, migration, and carcinogenesis. However, it has proven difficult to link a given ErbB receptor to a specific biological process since most cells express multiple ErbB members that heterodimerize, leading to receptor cross-activation. In this study, we utilize carcinoma cells depleted of ErbB2, but not other ErbB receptor members, to specifically examine the role of ErbB2 in carcinoma cell migration and invasion. Cells stimulated with EGF-related peptides show increased invasion of the extracellular matrix, whereas cells devoid of functional ErbB2 receptors do not. ErbB2 facilitates cell invasion through extracellular regulated kinase (ERK) activation and coupling of the adaptor proteins, p130CAS and c-CrkII, which regulate the actin-myosin cytoskeleton of migratory cells. Overexpression of ErbB2 in cells devoid of other ErbB receptor members is sufficient to promote ERK activation and CAS/Crk coupling, leading to cell migration. Thus, ErbB2 serves as a critical component that couples ErbB receptor tyrosine kinases to the migration/invasion machinery of carcinoma cells.

Non-adherent cell-specific expression of DOCK2, a member of the human CDM-family proteins

Human DOCK180, which was originally identified as a major protein bound to the Crk oncogene product, is an archetype of the CDM family of proteins, including Ced-5 of Caenorhabditis elegans and Mbc of Drosophila melanogaster. After DOCK180, at least three putative human proteins that manifest high amino acid sequence similarity to DOCK180 have been registered in the GenBank/EMBL database. We have designated one of them, KIAA0209, as DOCK2 and characterize here. DOCK2 mRNA was expressed mostly in peripheral blood cells, followed by slight expression in the spleen and thymus, whereas DOCK180 was expressed in all tissues tested except in peripheral blood cells. Immunostaining of human cadaver tissues revealed that the expression of DOCK2 was limited to the lymphocytes and macrophages of various organs. DOCK2 bound to and activated Rac1, as did DOCK180; however, DOCK2 did not bind to CrkII, which transduces signals at focal adhesions. Thus, DOCK180 and DOCK2 are regulators of Rac and function in adherent and non-adherent cells, respectively.

Epidermal growth factor-dependent dissociation of CrkII proto-oncogene product from the epidermal growth factor receptor in human glioma cells

Human glioma cells frequently overexpress epidermal growth factor receptor (EGFR). We found that the CrkII proto-oncogene product was associated with the EGFR in human glioma cells in the absence of epidermal growth factor (EGF). EGF stimulation of glioma cells induced the phosphorylation of tyrosine 221 of the CrkII protein, which correlates with its dissociation from the EGFR. By contrast, Shc and Grb2 were inducibly associated with the EGFR in response to EGF stimulation of glioma cells. In A431 cells, epidermoid carcinoma cells which overexpress EGFR, CrkII was tyrosine-phosphorylated and associated with the EGFR in an EGF-dependent manner. Therefore, the dissociation of CrkII from the EGFR upon stimulation with EGF appears to be specific to glioma cells. The Cbl oncogene product was also tyrosine-phosphorylated in U87MG glioma cells upon EGF stimulation. However, unlike in other cell lines, CrkII was not inducibly bound to Cbl in U87MG glioma cells. Thus, EGF-dependent binding of CrkII to phosphotyrosine-containing proteins appears to be suppressed in glioma cells. To evaluate the physiological role of dissociation of CrkII from EGFR, we expressed the CrkII-23 mutant in glioma cells. CrkII-23 mutant, which was isolated as a suppressor gene of the EGF-dependent transformation of NRK cells, binds constitutively to EGFR. We found that expression of CrkII-23 inhibited the anchorage-independent growth of the glioma cells in the presence of EGF. Taken together, these data implicate EGF-dependent dissociation of CrkII from EGFR in the oncogenicity of human glioma cells.

Regulation of cell contraction and membrane ruffling by distinct signals in migratory cells

Cell migration and wound contraction requires assembly of actin into a functional myosin motor unit capable of generating force. However, cell migration also involves formation of actin-containing membrane ruffles. Evidence is provided that actin-myosin assembly and membrane ruffling are regulated by distinct signaling pathways in the migratory cell. Interaction of cells with extracellular matrix proteins or cytokines promote cell migration through activation of the MAP kinases ERK1 and ERK2 as well as the molecular coupling of the adaptor proteins p130CAS and c-CrkII. ERK signaling is independent of CAS/Crk coupling and regulates myosin light chain phosphorylation leading to actin-myosin assembly during cell migration and cell-mediated contraction of a collagen matrix. In contrast, membrane ruffling, but not cell contraction, requires Rac GTPase activity and the formation of a CAS/Crk complex that functions in the context of the Rac activating protein DOCK180. Thus, during cell migration ERK and CAS/Crk coupling operate as components of distinct signaling pathways that control actin assembly into myosin motors and membrane ruffles, respectively.

Association of CrkL with STAT5 in hematopoietic cells stimulated by granulocyte-macrophage colony-stimulating factor or erythropoietin

CrkL is an adapter protein comprising Src homology (SH) 2 and SH3 domains. We investigated the molecule(s) associated with CrkL in factor-dependent cell lines. In the granulocyte-macrophage colony-stimulating factor (GM-CSF)-dependent cell lines TF-1 and UT-7, an approximately 95-kDa tyrosine-phosphorylated protein was precipitated along with CrkL after GM-CSF stimulation. The same protein was also observed when we used the erythropoietin (EPO)-dependent cell line UT-7/EPO, in an EPO stimulation-dependent manner. We identified it as STAT5 (signal transducer and activator of transcription 5, 96 kDa) by STAT5-specific antibodies. The direct binding of the SH2 domain of CrkL to STAT5 was demonstrated in far Western blotting and pull-down experiments using the glutathione S-transferase (GST) fusion construct CrkL-SH2. The addition of the oligopeptide containing phosphotyrosine 694 in STAT5A impaired the association between GST-CrkL-SH2 and STAT5. Furthermore, in a gel shift assay using prolactin-inducible element (PIE) as the probe, the DNA binding activity of STAT5 was inhibited by the interaction with GST-CrkL-SH2 in vitro. Finally, we found that STAT5 associated with CrkL did not bind to PIE sequence. These results suggest that CrkL participates in the Janus kinase (JAK)-STAT pathway by direct association with STAT5.

Phosphorylation of CrkII adaptor protein at tyrosine 221 by epidermal growth factor receptor

CrkII adaptor protein becomes tyrosine-phosphorylated upon various types of stimulation. We examined whether tyrosine 221, which has been shown to be phosphorylated by c-Abl, was phosphorylated also by other tyrosine kinases, such as epidermal growth factor (EGF) receptor. For this purpose, we developed an antibody that specifically recognizes Tyr221-phosphorylated CrkII, and we demonstrated that CrkII was phosphorylated on Tyr221 upon EGF stimulation. When NRK cells were stimulated with EGF, the tyrosine-phosphorylated CrkII was detected at the periphery of the cells, where ruffling is prominent, suggesting that signaling to CrkII may be involved in EGF-dependent cytoskeletal reorganization. The EGF-dependent phosphorylation of CrkII was also detected in a c-Abl-deficient cell line. Moreover, recombinant CrkII protein was phosphorylated in vitro by EGF receptor. These results strongly suggest that EGF receptor directly phosphorylates CrkII. Mutational analysis revealed that the src homology 2 domain was essential for the phosphorylation of CrkII by EGF receptor but not by c-Abl, arguing that these kinases phosphorylate CrkII by different phosphorylation mechanisms. Finally, we found that the CrkII protein phosphorylated upon EGF stimulation did not bind to the phosphotyrosine-containing peptide and that CrkII initiated dissociation from EGF receptor within 3 min even with the sustained tyrosine phosphorylation of EGF receptor. This result implicated phosphorylation of Tyr221 in the negative regulation of the src homology 2-mediated binding of CrkII to EGF receptor.

Constitutive association of EGF receptor with the CrkII-23 mutant that inhibits transformation of NRK cells by EGF and TGF-beta

Crk belongs to the adapter proteins that participate in many signalling pathways from cell surface receptors. We have characterised the CrkII-23 mutant that inhibits the transformation of NRK cells induced by epidermal growth factor (EGF) and transforming growth factor (TGF)-beta. To study the biochemical difference, cDNAs of the wild-type CrkII and the CrkII-23 mutant were introduced stably into NIH 3T3 cells expressing EGF receptor (EGFR). Both CrkII and CrkII-23 were phosphorylated on tyrosine upon EGF simulation with similar time course and dose dependency. Whereas the wild-type CrkII bound to EGFR only after EGF stimulation, CrkII-23 bound to EGFR from before stimulation. Mutation in the Src homology (SH) 2 or amino-terminal SH3 domain did not abolish the binding of CrkII-23 to EGFR in the quiescent cells, suggesting that the binding is mediated by a novel mechanism. These CrkII-23-derived mutants, however, did not suppress transformation of NRK cells by EGF and TGF-beta. Hence, both the SH2 and amino-terminal SH3 domains are required to inhibit transformation of NRK cells. These results suggest that persistent signalling from CrkII-23 bound to EGFR suppresses transformation by EGF and TGF-beta in NRK23 cells.

CAS/Crk coupling serves as a "molecular switch" for induction of cell migration

Carcinoma cells selected for their ability to migrate in vitro showed enhanced invasive properties in vivo. Associated with this induction of migration was the anchorage-dependent phosphorylation of p130CAS (Crk-associated substrate), leading to its coupling to the adaptor protein c-CrkII (Crk). In fact, expression of CAS or its adaptor protein partner Crk was sufficient to promote cell migration, and this depended on CAS tyrosine phosphorylation facilitating an SH2-mediated complex with Crk. Cytokine-stimulated cell migration was blocked by CAS lacking the Crk binding site or Crk containing a mutant SH2 domain. This migration response was characterized by CAS/Crk localization to membrane ruffles and blocked by the dominant-negative GTPase, Rac, but not Ras. Thus, CAS/Crk assembly serves as a "molecular switch" for the induction of cell migration and appears to contribute to the invasive property of tumors.

B cell antigen receptor signaling induces the formation of complexes containing the Crk adapter proteins

Crk proteins are Src homology (SH) 2/SH3-containing adapter proteins that can mediate the formation of signaling complexes. We show that engaging the B cell antigen receptor (BCR) on the RAMOS B cell line caused both Crk-L and Crk II to associate with several tyrosine-phosphorylated proteins. We identified two of these phosphoproteins as Cas and Cbl and showed that both bound to the Crk SH2 domain after BCR engagement. BCR ligation also increased the amount of Crk proteins in the particulate fraction of the cells and induced the formation of Crk.Cas and Crk.Cbl complexes in the particulate fraction. We propose that tyrosine phosphorylation of membrane-associated Cas and Cbl creates binding sites for the Crk SH2 domain and recruits Crk complexes to cellular membranes. Thus, Crk proteins may participate in BCR signaling by using their SH2 domains to direct the interactions and subcellular localization of proteins that bind to their SH3 domains. In RAMOS cells, we found that the SH3 domains of Crk-L and Crk II bound C3G. Since C3G activates Rap, a negative regulator of the Ras pathway, Crk proteins may participate in regulation of Ras signaling by the BCR.