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.     

MIST functions through distinct domains in immunoreceptor signaling in the presence and absence of LAT.

MIST (also termed Clnk) is an adaptor protein structurally related to SLP-76 and BLNK/BASH/SLP-65 hematopoietic cell-specific adaptor proteins. By using the BLNK-deficient DT40 chicken B cell system, we demonstrated MIST functions through distinct intramolecular domains in immunoreceptor signaling depending on the availability of linker for activation of T cells (LAT). MIST can partially restore the B cell antigen receptor (BCR) signaling in the BLNK-deficient cells, which requires phosphorylation of the two N-terminal tyrosine residues. Co-expression of LAT with MIST fully restored the BCR signaling and dispenses with the requirement of the two tyrosines in MIST for BCR signaling. However, some other tyrosine(s), as well as the Src homology (SH) 2 domain and the two proline-rich regions in MIST, is still required for full reconstitution of the BCR signaling, in cooperation with LAT. The C-terminal proline-rich region of MIST is dispensable for the LAT-aided full restoration of MAP kinase activation, although it is responsible for the interaction with LAT and for the localization in glycolipid-enriched microdomains. On the other hand, the N-terminal proline-rich region, which is a binding site of the SH3 domain of phospholipase Cgamma, is essential for BCR signaling. These results revealed a marked plasticity of MIST function as an adaptor in the cell contexts with or without LAT.     

Biochemical properties of the Cdc42-associated tyrosine kinase ACK1. Substrate specificity, authphosphorylation, and interaction with Hck

ACK1 (activated Cdc42-associated kinase 1) is a nonreceptor tyrosine kinase and the only tyrosine kinase known to interact with Cdc42. To characterize the enzymatic properties of ACK, we have expressed and purified active ACK using the baculovirus/Sf9 cell system. This ACK1 construct contains (from N to C terminus) the kinase catalytic domain, SH3 domain, and Cdc42-binding Cdc42/Rac interactive binding (CRIB) domain. We characterized the substrate specificity of ACK1 using synthetic peptides, and we show that the specificity of the ACK1 catalytic domain most closely resembles that of Abl. Purified ACK1 undergoes autophosphorylation, and autophosphorylation enhances kinase activity. We identified Tyr284 in the activation loop of ACK1 as the primary autophosphorylation site using mass spectrometry. When expressed in COS-7 cells, the Y284F mutant ACK1 showed dramatically reduced levels of tyrosine phosphorylation. Although the SH3 and CRIB domains of purified ACK1 are able to bind ligands (a polyproline peptide and Cdc42, respectively), the addition of ligands did not stimulate tyrosine kinase activity. To characterize potential interacting partners for ACK1, we screened several SH2 and SH3 domains for their ability to bind to full-length ACK1 or to the catalytic-SH3-CRIB construct. ACK1 interacts most strongly with the SH3 domains of Src family kinases (Src or Hck) via its C-terminal proline-rich domain. Co-expression of Hck with kinase-inactive ACK1(K158R) in mammalian cells resulted in tyrosine phosphorylation of ACK1, suggesting that ACK1 is a substrate for Hck. Our data suggest that Hck is a novel binding partner for ACK1 that can regulate ACK1 activity by phosphorylation.     

一种新的蛋白酪氨酸磷酸酶基因的克隆、定位和组织表达谱研究

从人胎肝cDNA库分离出一长度为5248bp的cDNA克隆,该基因包含26个外显子和25个内含子,染色体定位于在某些肿瘤细胞中易缺失的3p21.1-21.33。其可读框编码1636个氨基酸,该蛋白属于蛋白酪氨酸磷酸酶（PTP）家族,其C端有一个典型的PTP结构域,N端含有约800氨基酸残基的BRO1样结构域及随后2个可能的SH3结构域结合位点,在这两个结构域之间及C末端还各有一个脯氨酸富集区。Northern杂交和点杂交分析显示,该基因以大约5.4kb的单一转录物广泛表达于人体各种组织,而且在人部分肿瘤细胞中高表达。结果提示,人源PTP－TD14是一个新的蛋白酪氨酸磷酸酶。     

A novel membrane glycoprotein, SHPS-1, that binds the SH2-domain-containing protein tyrosine phosphatase SHP-2 in response to mitogens and cell adhesion.

Protein tyrosine phosphatases (PTPases), such as SHP-1 and SHP-2, that contain Src homology 2 (SH2) domains play important roles in growth factor and cytokine signal transduction pathways. A protein of approximately 115 to 120 kDa that interacts with SHP-1 and SHP-2 was purified from v-src-transformed rat fibroblasts (SR-3Y1 cells), and the corresponding cDNA was cloned. The predicted amino acid sequence of the encoded protein, termed SHPS-1 (SHP substrate 1), suggests that it is a glycosylated receptor-like protein with three immunoglobulin-like domains in its extracellular region and four YXX(L/V/I) motifs, potential tyrosine phosphorylation and SH2-domain binding sites, in its cytoplasmic region. Various mitogens, including serum, insulin, and lysophosphatidic acid, or cell adhesion induced tyrosine phosphorylation of SHPS-1 and its subsequent association with SHP-2 in cultured cells. Thus, SHPS-1 may be a direct substrate for both tyrosine kinases, such as the insulin receptor kinase or Src, and a specific docking protein for SH2-domain-containing PTPases. In addition, we suggest that SHPS-1 may be a potential substrate for SHP-2 and may function in both growth factor- and cell adhesion-induced cell signaling.     

SNX9 as an adaptor for linking synaptojanin-1 to the Cdc42 effector ACK1

Sorting nexin 9 (SNX9, also referred to as SH3PX1) is a binding partner for the non-receptor and Cdc42-associated kinase (ACK) in Drosophila and mammals. ACK1 is known to bind clathrin and influence EGF receptor endocytosis. SNX9 comprises an N-terminal Src homology domain 3 (SH3), a central PHOX homology (PX) domain, and a carboxyl-terminal coiled-coil region. In order to investigate SNX9 further we have made use of a novel in vivo biotinylation system to label various GST-SH3 domains and perform blot overlays, thereby identifying synaptojanin-1 as a partner for SNX9. Biotinylated SH3 domains were also used for specific identification of target proline-rich sequences in synaptojanin and ACK1 on synthetic peptides arrays. Direct assessment of SH3 binding efficiencies at different positions within the extensive proline-rich regions of these proteins were thus determined. While SNX9 targets a number of sequences within the proline-rich regions of synaptojanin, a single site was identified in human ACK1. By testing the association of various truncations of ACK1 with SNX9 we confirmed the dominant SNX9 binding domain in human ACK1 (residues 920-955). In the presence of SNX9 we find that synaptojanin is able to colocalize with distinct ACK1 containing vesicles, indicating that this tyrosine kinase is linked to many components involved in vesicle dynamics including clathrin, AP2 and synaptojanin-1.     

GRID: a novel Grb-2-related adapter protein that interacts with the activated T cell costimulatory receptor CD28

Adapter proteins such as Grb2 play a central role in the formation of signaling complexes through their association with multiple protein binding partners. These interactions are mediated by specialized domains such as the well-characterized Src homology SH2 and SH3 motifs. Using yeast three-hybrid technology, we have identified a novel adapter protein, expressed predominantly in T lymphocytes, that associates with the activated form of the costimulatory receptor, CD28. The protein is a member of the Grb2 family of adapter proteins and contains an SH3-SH2-SH3 domain structure. A unique glutamine/proline-rich domain (insert domain) of unknown function is situated between the SH2 and N-terminal SH3 domains. We term this protein GRID for Grb2-related protein with insert domain. GRID coimmunoprecipitates with CD28 from Jurkat cell lysates following activation of CD28. Using mutants of CD28 and GRID, we demonstrate that interaction between the proteins is dependent on phosphorylation of CD28 at tyrosine 173 and integrity of the GRID SH2 domain, although there are also subsidiary stabilizing contacts between the PXXP motifs of CD28 and the GRID C-terminal SH3 domain. In addition to CD28, GRID interacts with a number of other T cell signaling proteins, including SLP-76 (SH2 domain-containing leukocyte protein of 76 kDa), p62dok, and RACK-1 (receptor for activated protein kinase C-1). These findings suggest that GRID functions as an adapter protein in the CD28-mediated costimulatory pathway in T cells.     

Tyrosine phosphorylation of Grb2 by Bcr/Abl and epidermal growth factor receptor: a novel regulatory mechanism for tyrosine kinase signaling.

Growth factor receptor-binding protein-2 (Grb2) plays a key role in signal transduction initiated by Bcr/Abl oncoproteins and growth factors, functioning as an adaptor protein through its Src homology 2 and 3 (SH2 and SH3) domains. We found that Grb2 was tyrosine-phosphorylated in cells expressing BCR/ABL and in A431 cells stimulated with epidermal growth factor (EGF). Phosphorylation of Grb2 by Bcr/Abl or EGF receptor reduced its SH3-dependent binding to Sos in vivo, but not its SH2-dependent binding to Bcr/Abl. Tyr209 within the C-terminal SH3 domain of Grb2 was identified as one of the tyrosine phosphorylation sites, and phosphorylation of Tyr209 abolished the binding of the SH3 domain to a proline-rich Sos peptide in vitro. In vivo expression of a Grb2 mutant where Tyr209 was changed to phenylalanine enhanced BCR/ABL-induced ERK activation and fibroblast transformation, and potentiated and prolonged Grb2-mediated activation of Ras, mitogen-activated protein kinase and c-Jun N-terminal kinase in response to EGF stimulation. These results suggest that tyrosine phosphorylation of Grb2 is a novel mechanism of down-regulation of tyrosine kinase signaling.     

Interactions of Drosophila Cbl with epidermal growth factor receptors and role of Cbl in R7 photoreceptor cell development.

The human proto-oncogene product c-Cbl and a similar protein in Caenorhabditis elegans (Sli-1) contain a proline-rich COOH-terminal region that binds Src homology 3 (SH3) domains of proteins such as the adapter Grb2. Cb1-Grb2 complexes can be recruited to tyrosine-phosphorylated epidermal growth factor (EGF) receptors through the SH2 domain of Grb2. Here we identify by molecular cloning a Drosophila cDNA encoding a protein (Drosophila Cbl [D-Cbl]) that shows high sequence similarity to the N-terminal region of human c-Cbl but lacks proline-rich sequences and fails to bind Grb2. Nonetheless, in COS-1 cells, expression of hemagglutinin epitope-tagged D-Cbl results in its coimmunoprecipitation with EGF receptors in response to EGF. EGF also caused tyrosine phosphorylation of D-Cbl in such cells, but no association of phosphatidylinositol 3-kinase was detected in assays using anti-p85 antibody. A point mutation in D-Cbl (G305E) that suppresses the negative regulation of LET-23 by the Cbl homolog Sli-1 in C. elegans prevented tyrosine phosphorylation of D-Cbl as well as binding to the liganded EGF receptor in COS-1 cells. Colocalization of EGF receptors with both endogenous c-Cbl or expressed D-Cbl in endosomes of EGF-treated COS-1 cells is also demonstrated by immunofluorescence microscopy. In lysates of adult transgenic Drosophila melanogaster, GST-DCbl binds to the tyrosine-phosphorylated 150-kDa torso-DER chimeric receptor. Expression of D-Cbl directed by the sevenless enhancer in intact Drosophila compromises severely the development of the R7 photoreceptor neuron. These data suggest that despite the lack of Grb2 binding sites, D-Cbl functions as a negative regulator of receptor tyrosine kinase signaling in the Drosophila eye by a mechanism that involves its association with EGF receptors or other tyrosine kinases.     

The Cdc42 target ACK2 interacts with sorting nexin 9 (SH3PX1) to regulate epidermal growth factor receptor degradation.

Activated Cdc42-associated kinase-2 (ACK2) is a non-receptor tyrosine kinase that serves as a specific effector for Cdc42, a Rho family small G-protein. Recently, we have found that ACK2 directly interacts with clathrin heavy chain through a clathrin-binding motif that is conserved in all endocytic adaptor proteins and regulates clathrin assembly, suggesting that ACK2 plays a role in clathrin-coated vesicle endocytosis (Yang, W., Lo, C. G., Dispenza, T., and Cerione, R. A. (2001) J. Biol. Chem. 276, 17468-17473). Here we report the identification of another binding partner for ACK2 that has previously been implicated in endocytosis, namely the sorting nexin protein SH3PX1 (sorting nexin 9). The interaction occurs between a proline-rich domain of ACK2 and the Src homology 3 domain (SH3) of SH3PX1. Co-immunoprecipitation studies indicate that ACK2, clathrin, and SH3PX1 form a complex in cells. Epidermal growth factor (EGF) stimulated the tyrosine phosphorylation of SH3PX1, whereas co-transfection of ACK2 with SH3PX1 resulted in the constitutive phosphorylation of SH3PX1. However, co-transfection of the kinase-dead mutant ACK2(K158R) with SH3PX1 blocked EGF-induced tyrosine phosphorylation of SH3PX1, indicating that the EGF-stimulated phosphorylation of SH3PX1 is mediated by ACK2. EGF receptor levels were significantly decreased following EGF stimulation of cells co-expressing ACK2 and SH3PX1, thus highlighting a novel role for ACK2, working together with SH3PX1 to promote the degradation of the EGF receptor.     

Cytoplasmic ACK1 Interaction with Multiple Receptor Tyrosine Kinases Is Mediated by Grb2: AN ANALYSIS OF ACK1 EFFECTS ON Axl SIGNALING*

ACK1 (activated Cdc42-associated kinase 1), a cytoplsmic tyrosine kinase, is implicated in metastatic behavior, cell spreading and migration, and epidermal growth factor receptor (EGFR) signaling. The function of ACK1 in the regulation of receptor tyrosine kinases requires a C-terminal region that demonstrates a significant homology to the EGFR binding domain of MIG6. In this study, we have identified additional receptor tyrosine kinases, including Axl, leukocyte tyrosine kinase, and anaplastic lymphoma kinase, that can bind to the ACK1/MIG6 homology region. Unlike the interaction between MIG6 and EGFR, our data suggest that these receptor tyrosine kinases require the adaptor protein Grb2 for efficient binding, which interacts with highly conserved proline-rich regions that are conserved between ACK1 and MIG6. We have focused on Axl and compared how ACK1/Axl differs from the ACK1/EGFR axis by investigating effects of knockdown of endogenous ACK1. Although EGFR activation promotes ACK1 turnover, Axl activation by GAS6 does not; interestingly, the reciprocal down-regulation of GAS6-stimulated Axl is blocked by removing ACK1. Thus, ACK1 functions in part to control Axl receptor levels. Silencing of ACK1 also leads to diminished ruffling and migration in DU145 and COS7 cells upon GAS6-Axl signaling. The ability of ACK1 to modulate Axl and perhaps anaplastic lymphoma kinase (altered in anaplastic large cell lymphomas) might explain why ACK1 can promote metastatic and transformed behavior in a number of cancers.     

Dynamic interaction of CD2 with the GYF and the SH3 domain of compartmentalized effector molecules

Intracellular protein interaction domains are essential for eukaryotic signaling. In T cells, the CD2BP2 adaptor binds two membrane-proximal proline-rich motifs in the CD2 cytoplasmic tail via its GYF domain, thereby regulating interleukin-2 production. Here we present the structure of the GYF domain in complex with a CD2 tail peptide. Unlike SH3 domains, which use two surface pockets to accommodate proline residues of ligands, the GYF domain employs phylogenetically conserved hydrophobic residues to create a single interaction surface. NMR analysis shows that the Fyn but not the Lck tyrosine kinase SH3 domain competes with CD2BP2 GYF-domain binding to the same CD2 proline-rich sequence in vitro. To test the in vivo significance of this competition, we used co-immunoprecipitation experiments and found that CD2BP2 is the ligand of the membrane-proximal proline-rich tandem repeat of CD2 in detergent-soluble membrane compartments, but is replaced by Fyn SH3 after CD2 is translocated into lipid rafts upon CD2 ectodomain clustering. This unveils the mechanism of a switch of CD2 function due to an extracellular mitogenic signal.     

Growth transformation of human T cells by herpesvirus saimiri requires multiple Tip-Lck interaction motifs

Lymphoma induction and T-cell transformation by herpesvirus saimiri strain C488 depends on two viral oncoproteins, StpC and Tip. The major interaction partner of Tip is the protein tyrosine kinase Lck, a key regulator of T-cell activation. The Lck binding domain (LBD) of Tip comprises two interaction motifs, a proline-rich SH3 domain-binding sequence (SH3B) and a region with homology to the C terminus of Src family kinase domains (CSKH). In addition, biophysical binding analyses with purified Lck-SH2 domain suggest the phosphorylated tyrosine residue 127 of Tip (pY127) as a potential third Lck interaction site. Here, we addressed the relevance of the individual binding motifs, SH3B, CSKH, and pY127, for Tip-Lck interaction and for human T-cell transformation. Both motifs within the LBD displayed Lck binding activities and cooperated to achieve a highly efficient interaction, while pY127, the major tyrosine phosphorylation site of Tip, did not enhance Lck binding in T cells. Herpesvirus saimiri strain C488 recombinants lacking one or both LBD motifs of Tip lost their transforming potential on human cord blood lymphocytes. Recombinant virus expressing Tip with a mutation at position Y127 was still able to transform human T lymphocytes but, in contrast to wild-type virus, was strictly dependent on exogenous interleukin-2. Thus, the strong Lck binding mediated by cooperation of both LBD motifs was essential for the transformation of human T cells by herpesvirus saimiri C488. The major tyrosine phosphorylation site Y127 of Tip was particularly required for transformation in the absence of exogenous interleukin-2, suggesting its involvement in cytokine signaling pathways.     

Regulation of c-Fes tyrosine kinase activity by coiled-coil and SH2 domains: analysis with Saccharomyces cerevisiae

The c-Fes protein-tyrosine kinase regulates the growth and differentiation of diverse cell types, including myeloid hematopoietic cells, vascular endothelial cells, and neurons. Structurally, Fes is composed of a unique N-terminal region with coiled-coil oligomerization motifs, followed by SH2 and kinase domains. Although Fes kinase activity is tightly regulated in cells, the structural basis for its negative regulation is not clear. In this report, c-Fes was expressed in Saccharomyces cerevisiae to determine whether regulation is kinase-intrinsic or dependent upon protein factors found in mammalian cells. Wild-type Fes kinase activity was completely repressed in yeast and did not affect cell growth. Mutation or deletion of the more N-terminal c-Fes coiled-coil domain reversed negative regulation, leading to strong kinase activation and suppression of yeast cell growth. Similarly, replacement of the wild-type SH2 domain with that of v-Src induced strong kinase activation and the growth-inhibitory phenotype. Immunoblotting with phosphospecific antibodies shows that activation of Fes by either mechanism induced autophosphorylation of the activation loop tyrosine residue (Tyr 713). These data support the idea that Fes naturally adopts an inactive conformation in vivo, and that maintenance of the inactive structure requires the coiled-coil and SH2 domains.     

The N-terminal SH2 domains of Syk and ZAP-70 mediate phosphotyrosine-independent binding to integrin beta cytoplasmic domains

Syk and ZAP-70 form a subfamily of nonreceptor tyrosine kinases that contain tandem SH2 domains at their N termini. Engagement of these SH2 domains by tyrosine-phosphorylated immunoreceptor tyrosine-based activation motifs leads to kinase activation and downstream signaling. These kinases are also regulated by beta3 integrin-dependent cell adhesion via a phosphorylation-independent interaction with the beta3 integrin cytoplasmic domain. Here, we report that the interaction of integrins with Syk and ZAP-70 depends on the N-terminal SH2 domain and the interdomain A region of the kinase. The N-terminal SH2 domain alone is sufficient for weak binding, and this interaction is independent of tyrosine phosphorylation of the integrin tail. Indeed, phosphorylation of tyrosines within the two conserved NXXY motifs in the integrin beta3 cytoplasmic domain blocks Syk binding. The tandem SH2 domains of these kinases bind to multiple integrin beta cytoplasmic domains with varying affinities (beta3 (Kd = 24 nm) > beta2 (Kd = 38 nm) > beta1 (Kd = 71 nm)) as judged by both affinity chromatography and surface plasmon resonance. Thus, the binding of Syk and ZAP-70 to integrin beta cytoplasmic domains represents a novel phosphotyrosine-independent interaction mediated by their N-terminal SH2 domains.     

Characterization of the tyrosine kinase Tnk1 and its binding with phospholipase C-gamma1

Tnk1 is a nonreceptor tyrosine kinase cloned from CD34+/Lin-/CD38- hematopoietic stem/progenitor cells. The cDNA predicts a 72-kDa protein containing an NH(2)-terminal kinase, a Src Homology 3 (SH3) domain, and a proline-rich (PR) tail. We generated rabbit antiserum to a GST-Tnk1(SH3) fusion protein. Affinity-purified anti-Tnk1 antibodies specifically recognized a 72-kDa protein in Tnk1-transfected COS-1 cells and cells which express Tnk1 mRNA. Western blot analysis indicated that Tnk1 is expressed in fetal blood cells, but not in any other hematopoietic tissues examined. Tnk1 immunoprecipitated from cell lysates possessed kinase activity and was tyrosine phosphorylated. In binding experiments with a panel of GST-fusion constructs, only GST-PLC-gamma1(SH3) interacted with in vitro translated Tnk1. GST-protein precipitations from cell lysates confirmed that GST-PLC-gamma1(SH3) associated with endogenously expressed Tnk1. Conversely, GST-Tnk1(PR) protein constructs complexed with endogenously expressed PLC-gamma1. The association of Tnk1 with PLC-gamma1 suggests a role for Tnk1 in phospholipid signal transduction.     

Latent membrane protein 2A of Epstein-Barr virus binds WW domain E3 protein-ubiquitin ligases that ubiquitinate B-cell tyrosine kinases

The latent membrane protein (LMP) 2A of Epstein-Barr virus (EBV) is implicated in the maintenance of viral latency and appears to function in part by inhibiting B-cell receptor (BCR) signaling. The N-terminal cytoplasmic region of LMP2A has multiple tyrosine residues that upon phosphorylation bind the SH2 domains of the Syk tyrosine kinase and the Src family kinase Lyn. The LMP2A N-terminal region also has two conserved PPPPY motifs. Here we show that the PPPPY motifs of LMP2A bind multiple WW domains of E3 protein-ubiquitin ligases of the Nedd4 family, including AIP4 and KIAA0439, and demonstrate that AIP4 and KIAA0439 form physiological complexes with LMP2A in EBV-positive B cells. In addition to a C2 domain and four WW domains, these proteins have a C-terminal Hect catalytic domain implicated in the ubiquitination of target proteins. LMP2A enhances Lyn and Syk ubiquitination in vivo in a fashion that depends on the activity of Nedd4 family members and correlates with destabilization of the Lyn tyrosine kinase. These results suggest that LMP2A serves as a molecular scaffold to recruit both B-cell tyrosine kinases and C2/WW/Hect domain E3 protein-ubiquitin ligases. This may promote Lyn and Syk ubiquitination in a fashion that contributes to a block in B-cell signaling. LMP2A may potentiate a normal mechanism by which Nedd4 family E3 enzymes regulate B-cell signaling.     

Tyrosine phosphorylation of BCR by FPS/FES protein-tyrosine kinases induces association of BCR with GRB-2/SOS.

The human bcr gene encodes a protein with serine/threonine kinase activity, CDC24/dbl homology, a GAP domain, and an SH2-binding region. However, the precise physiological functions of BCR are unknown. Coexpression of BCR with the cytoplasmic protein-tyrosine kinase encoded by the c-fes proto-oncogene in Sf-9 cells resulted in stable BCR-FES protein complex formation and tyrosine phosphorylation of BCR. Association involves the SH2 domain of FES and a novel binding domain localized to the first 347 amino acids of the FES N-terminal region. Deletion of the homologous N-terminal BCR-binding domain from v-fps, a fes-related transforming oncogene, abolished transforming activity and tyrosine phosphorylation of BCR in vivo. Tyrosine phosphorylation of BCR in v-fps-transformed cells induced its association with GRB-2/SOS, the RAS guanine nucleotide exchange factor complex. These data provide evidence that BCR couples the cytoplasmic protein-tyrosine kinase and RAS signaling pathways.