A complex of Grb2 adaptor protein, Sos exchange factor, and a 36-kDa membrane-bound tyrosine phosphoprotein is implicated in ras activation in T cells

Regulation of T cell receptor signaling by tyrosine phosphatase SYP association with CTLA-4

The absence of CTLA-4 results in uncontrolled T cell proliferation. The T cell receptor-specific kinases FYN, LCK, and ZAP-70 as well as the RAS pathway were found to be activated in T cells of Ctla-4-/- mutant mice. In addition, CTLA-4 specifically associated with the tyrosine phosphatase SYP, an interaction mediated by the SRC homology 2 (SH2) domains of SYP and the phosphotyrosine sequence Tyr-Val-Lys-Met within the CTLA-4 cytoplasmic tail. The CTLA-4-associated SYP had phosphatase activity toward the RAS regulator p52SHC. Thus, the RAS pathway and T cell activation through the T cell receptor are regulated by CTLA-4-associated SYP.

The protein interactions of the immunoglobulin receptor family tyrosine-based activation motifs present in the T cell receptor zeta subunits and the CD3 gamma, delta and epsilon chains

Immunoglobulin family tyrosine-based activation motifs (ITAM), which define the conserved signaling sequence EX2YX2L/IX7YX2L/I, couple the T cell antigen receptor (TCR) to cellular proteins including protein tyrosine kinases (PTK) and adapter molecules. The TCR is a multichain complex with four invariant chains CD3 gamma, delta and epsilon that each contain a single ITAM and the TCR zeta chain that contains three ITAM. The present study explores the protein interactions of the doubly phosphorylated CD3 gamma, delta, epsilon ITAM to determine whether they have common or unique biochemical properties. The data show that the doubly phosphorylated ITAM all bind the PTK ZAP-70, but the ITAM also variably bind the PTK p59fyn and the adapters Shc, Grb-2 and the p85 regulatory subunit of phosphoinositol 3' kinase. The CD3 and zeta ITAM display a hierarchy of ZAP-70 binding: zeta 1 = gamma = delta > zeta 3 > zeta 2 = epsilon. Shc, Grb-2 and p85 could bind the zeta ITAM and the CD3 gamma and delta ITAM, but not the CD3 epsilon ITAM. There were also subtle differences in the hierarchy of reactivity of these adapters for the CD3 gamma, delta and zeta ITAM that show that the zeta, CD3 gamma, delta and epsilon ITAM have different binding properties. The present study thus shows that the different ITAM of the TCR/CD3 complex can interact with different cytosolic effectors, indicating that differential ITAM phosphorylation during T cell activation could be a mechanism to generate signaling diversity by the TCR complex.

Two distinct intracytoplasmic regions of the T-cell adhesion molecule CD28 participate in phosphatidylinositol 3-kinase association

Through the interaction with its ligands, CD80/B7-1 and CD86/B7-2 or B70, the human CD28 molecule plays a major functional role as a costimulator of T cells along with the CD3-TcR complex. We and others have previously reported that phosphatidylinositol 3-kinase inducibly associates with CD28. This association is mediated by the SH2 domains of the p85 adaptor subunit interacting with a cytoplasmic YMNM consensus motif present in CD28 at position 173-176. Disruption of this binding site by site-directed mutagenesis abolishes CD28-induced activation events in a murine T-cell hybridoma transfected with human CD28 gene. Here we show that the last 10 residues of the intracytoplasmic domain of CD28 (residues 193-202) are required for its costimulatory function. These residues are involved in interleukin-2 secretion, p85 binding, and CD28-associated phosphatidylinositol 3-kinase activity. In contrast, the CD28/CD8O interaction is unaffected by this deletion, as is the induction of other second messengers such as the rise in intracellular calcium and tyrosine phosphorylation of CD28-specific substrates. Furthermore, we also demonstrate that, within these residues, the tyrosine at position 200 is involved in p85 binding, probably together with the short proline-rich motif present between residues 190 and 194 (PYAPP).

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

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

T cell antigen receptor signal transduction pathways

The T cell antigen receptor (TCR) regulates the activation and growth of T lymphocytes. The initial membrane proximal event triggered by the TCR is activation of protein tyrosine kinases with the resultant phosphorylation of cellular proteins. This biochemical response couples the TCR to a divergent array of signal transduction molecules including enzymes that regulate lipid metabolism, GTP binding proteins, serine/threonine kinases, and adapter molecules. The ultimate aim of studies of intracellular signaling mechanisms is to understand the functional consequences of a particular biochemical event for receptor function. The control of cytokine gene expression is one of the mechanism that allows the TCR to control immune responses. Accordingly, one object of the present review is to discuss the role of the different TCR signal transduction pathways in linking the TCR to nuclear targets: the transcription factors that control the expression of cytokine genes.

Interactions of Cbl with two adapter proteins, Grb2 and Crk, upon T cell activation

Several recent studies have demonstrated that Grb2, composed entirely of SH2 and SH3 domains, serves as an adaptor protein in tyrosine kinase signaling pathways. Cb1, the protein product of c-cbl proto-oncogene, has been reported to be phosphorylated on tyrosine residues upon T cell receptor (TCR) engagement. Here we show that in unstimulated Jurkat cells Cbl is co-immunoprecipitated with monoclonal antibody against Grb2. However, in lymphocytes activated through the TCR, Cbl loses its ability to bind to Grb2 precipitated either with anti-Grb2 antibody or with an immobilized tyrosine phosphopeptide, Y1068-P, derived from the epidermal growth factor receptor. In vitro studies confirm that the ability of Cb1 to bind to both SH3 domains of Grb2 is strongly reduced in activated T lymphocytes. Investigation of the time course of Cbl dissociation from Grb2 reveals that it is transient and correlates with the kinetics of tyrosine phosphorylation of Cbl. Moreover, Cb1 is co-immunoprecipitated with Crk, another SH2/SH3 domain-containing protein, upon TCR stimulation. Tyrosine-phosphorylated Cbl binds exclusively to the SH2 domain of Crk. These results suggest that different adaptor proteins may have different roles in the regulation of c-cbl proto-oncogene product.

Analysis of the T-cell activation signaling pathway mediated by tyrosine kinases, protein kinase C, and Ras protein, which is modulated by intracellular cyclic AMP

T-cell receptor (TCR) triggering by an anti-CD3 antibody or phytohemagglutinin (PHA) as well as the treatment with phorbol myristate acetate (PMA), a direct activator of protein kinase C (PKC), induces activation of Ras in T-lymphocytes (Downward, J. et al. (1990)) Nature 364, 719-723). In this paper, we studied the role of Ras in the process of TCR-mediated T-cell activation using a human lymphomic Jurkat cell line. The stimulatory effect of TCR cross-linking on Ras activation was inhibited by herbimycin A, a specific inhibitor of protein tyrosine kinases (PTKs), whereas PMA-induced Ras activation was not affected. On the other hand, calphostin C, a specific inhibitor of PKC, blocked not only PMA-induced, but also TCR-mediated formation of Ras.GTP. Furthermore, down-regulation of PMA-sensitive PKC severely impaired the activation of Ras in response to TCR-stimulation. Tyrosine-phosphorylation and translocation to the particulate fraction of phospholipase C-gamma 1 (PLC-gamma 1) were observed upon T-cell activation. Subcellular localization of PKC was also changed when the cells were stimulated with an anti-CD3 antibody or PMA. While TCR-stimulated translocation of PKC was observed only transiently, PMA-induced translocation of PKC was more sustained. These results suggest that the activation of PLC-gamma 1 by PTK and subsequent activation of PKC are important for TCR-mediated Ras activation in Jurkat cells. An activated form of Ras enhanced the activation of interleukin 2 (IL-2) promoter by TCR stimulation or PMA treatment, although the activated Ras by itself was insufficient for IL-2 promoter activation. On the other hand, a dominant-inhibitory Ras diminished almost completely the activation of IL-2 promoter induced by PMA plus calcium ionophore, indicating that Ras is essential for transduction of T-cell activation signals. Cholera toxin (CTX), which directly activates Gs alpha, is shown to inhibit the activation of IL-2 promoter. TCR-mediated Ras activation, tyrosine phosphorylation and translocation of cellular proteins including ZAP-70, PLC-gamma 1 , and PKC. An activated Gs alpha mutant as well as dibutylyl cAMP (dBcAMP) also showed similar inhibitory effects.

Interleukin-6 induces tyrosine phosphorylation of the Ras activating protein Shc, and its complex formation with Grb2 in the human multiple myeloma cell line LP-1

Like many other cytokines and growth factors, interleukin-6 (IL-6) activates p21ras. However, the precise biochemical mechanisms inducing this activation are unknown. Therefore, we investigated the effects of IL-6 on some recently identified signaling intermediates, Shc (Src homology and collagen) and Grb2 (growth factor receptor bound protein 2), known to activate p21ras. In the multiple myeloma cell line LP-1, IL-6 stimulated the tyrosine phosphorylation of Shc in a time- and concentration-dependent manner. This led to the complex formulation of Shc with Grb2, an adaptor protein known to relocate a p21ras-GDP exchange factor. Sos1 (Son-of-sevenless), to the cell membrane. Taken together, these findings suggest that IL-6 might activate the Ras signaling pathway via tyrosine phosphorylation of Shc and subsequent recruitment of Grb2. Further studies will elucidate which of the IL-6 receptor associated non-receptor tyrosine kinases of the Src kinase or Janus kinase family, mediate these effects.

Signal transduction by CD28 costimulatory receptor on T cells. B7-1 and B7-2 regulation of tyrosine kinase adaptor molecules

This study compares the biochemical responses in T cells activated with the CD28 ligands B7-1 and B7-2. The patterns of tyrosine phosphorylation induced in T cells by these two CD28 ligands are identical, but clearly different from the tyrosine phosphorylation induced by the T cell receptor (TCR). The TCR regulates protein complexes mediated by the adapter Grb2 both in vivo and in vitro. In contrast, there is no apparent regulation of in vivo Grb2 complexes in response to B7-1 or B7-2. Rather, B7-1 and B7-2 both induce tyrosine phosphorylation of a different adaptor protein, p62. The regulation of p62 is a unique CD28 response that is not shared with the TCR. These data indicate that B7-1 and B7-2 induce identical tyrosine kinase signal transduction pathways. The data show also that the TCR and CD28 couple to different adapter proteins, which could explain the divergence of TCR and CD28 signal transduction pathways during T cell activation.

CD4 and signal transduction

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

c-Cbl is inducibly tyrosine-phosphorylated by epidermal growth factor stimulation in fibroblasts, and constitutively tyrosine-phosphorylated and associated with v-Src in v-src-transformed fibroblasts

The c-cbl gene was cloned as the cellular homolog of the v-cbl oncogene that is the transforming component of a murine tumorigenic retrovirus, CAS NS-1, though the biological roles of c-Cbl remain to be elucidated. We have previously reported that c-Cbl is implicated in the signal transduction triggered by granulocyte-macrophage colony-stimulating factor or erythropoietin in hematopoietic cells. Here, we observed tyrosine phosphorylation of C-cbl in cells expressing epidermal growth factor receptor depending on EGF stimulation and in v-src transformed cells. Furthermore, c-Cbl was revealed to associate with v-Src in vivo. By means of binding experiments using glutathione S-transferase fusion proteins, we have found that the SH2 and SH3 domains of many proteins bind to c-Cbl. These findings strongly suggest that c-Cbl is implicated in a wide variety of signal transduction pathways, including those of EGF receptor and Src protein, as well as in the signaling pathways of hematopoietic cells.

Ras is not required for the interleukin 3-induced proliferation of a mouse pro-B cell line, BaF3

It has been demonstrated that Ras is involved in interleukin 3 (IL-3)-stimulated signal transduction in various hematopoietic cultured cells (Satoh, T., Nakafuku, M., Miyajima, A., and Kaziro, Y. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 3314-3318; Duronio, V., Welham, M. J., Abraham, S., Dryden, P., and Schrader, J. W. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1587-1591). However, it has not been fully understood which of IL-3-promoted cellular responses, i.e. proliferation, survival, and differentiation, requires Ras function. We employed a system of inducible expression of the dominant-negative (S17N) or dominant-active (G12V) mutant of Ras in BaF3 mouse pro-B cell line to analyze the role of Ras in IL-3-stimulated signal transduction. Induction of the dominant-negative Ras(S17N) effectively inhibited the IL-3-induced activation of c-Raf-1 and mitogen-activated protein kinase (MAPK). Furthermore, the activation of fos gene promoter following IL-3 stimulation was almost completely abolished when Ras(S17N) was induced. Under these conditions, Ras(S17N) exhibited no inhibitory effect on IL-3-dependent proliferation assessed by the increase of cell numbers and a mitochondrial enzyme activity. The results indicate that Ras-dependent pathways, including the Raf/MAPK/Fos pathway, are dispensable for IL-3-induced growth stimulation. When BaF3 cells were treated with a tyrosine kinase inhibitor, herbimycin A, IL-3-dependent proliferation of the cells was impaired, suggesting that tyrosine kinase-mediated pathways are critical for growth promotion. On the other hand, apoptotic cell death caused by deprivation of IL-3 was prevented by the induction of the activated mutant Ras(G12V), although the rate of cell number increase was markedly reduced. Thus, it is likely that Ras-independent pathways play important roles to facilitate the proliferation although they may not be essential for IL-3-stimulated antiapoptotic signal transduction.

T cell activation-dependent association between the p85 subunit of the phosphatidylinositol 3-kinase and Grb2/phospholipase C-gamma 1-binding phosphotyrosyl protein pp36/38

Tyrosine phosphorylation of cellular proteins is an early and an essential step in T cell receptor-mediated lymphocyte activation. Tyrosine phosphorylation of transmembrane receptor chains (such as zeta and CD3 chains) and membrane-associated proteins provides docking sites for SH2 domains of adaptor proteins and signaling enzymes, resulting in their recruitment in the vicinity of activated receptors. pp36/38 is a prominent substrate of early tyrosine phosphorylation upon stimulation through the T cell receptor. The tyrosine-phosphorylated form of pp36/38 is membrane-associated and directly interacts with phospholipase C-gamma 1 and Grb2, providing one mechanism to recruit downstream effectors to the cell membrane. Here, we demonstrate that in Jurkat T cells, pp36/38 associates with the p85 subunit of phosphatidylinositol 3-kinase (PI-3-K p85) in an activation-dependent manner. Association of pp36/38 with PI-3-K p85 was confirmed by transfection of a hemagglutinin-tagged p85 alpha cDNA into Jurkat cells followed by anti-hemagglutinin immunoprecipitation. In vitro binding experiments with glutathione S-transferase fusion proteins of PI-3-K p85 demonstrated that the SH2 domains, but not the SH3 domain, mediated binding to pp36/38. This binding was selectively abrogated by phosphopeptides that bind to p85 SH2 domains with high affinity. Filter binding assays demonstrated that association between pp36/38 and PI-3-K p85 SH2 domains was due to direct binding. These results strongly suggest the role of pp36/38 in recruiting PI-3-K to the cell membrane and further support the idea that pp36/38 is a multifunctional docking protein for SH2 domain-containing signaling proteins in T cells.

The SH3 domain-binding T cell tyrosyl phosphoprotein p120. Demonstration of its identity with the c-cbl protooncogene product and in vivo complexes with Fyn, Grb2, and phosphatidylinositol 3-kinase

Previously, we have identified p120 as a Fyn/Lck SH3 and SH2 domain-binding protein that is tyrosine phosphorylated rapidly after T cell receptor triggering. Here, we used direct protein purification, amino acid sequence analysis, reactivity with antibodies, and two-dimensional gel analyses to identify p120 as the human c-cbl protooncogene product. We demonstrate in vivo complexes of p120cbl with Fyn tyrosine kinase, the adaptor protein Grb2, and the p85 subunit of phosphatidylinositol (PI) 3-kinase. The association of p120cbl with Fyn and the p85 subunit of PI 3-kinase (together with PI 3-kinase activity) was markedly increased by T cell activation, consistent with in vitro binding of p120cbl to their SH2 as well as SH3 domains. In contrast, a large fraction of p120cbl was associated with Grb2 prior to activation, and this association did not change upon T cell activation. In vitro, p120cbl interacted with Grb2 exclusively through its SH3 domains. These results demonstrate a novel Grb2-p120cbl signaling complex in T cells, distinct from the previously analyzed Grb2-Sos complex. The association of p120cbl with ubiquitous signaling proteins strongly suggests a general signal transducing function for this enigmatic protooncogene with established leukemogenic potential but unknown physiological function.

Ligation of the T-cell antigen receptor (TCR) induces association of hSos1, ZAP-70, phospholipase C-gamma 1, and other phosphoproteins with Grb2 and the zeta-chain of the TCR

Signaling by the T-cell antigen receptor (TCR) involves both phospholipase C (PLC)-gamma 1 and p21ras activation. While failing to induce Shc/Grb2 association, ligation of the TCR/CD3 receptor in Jurkat T-cells induced hSos1-Grb2 complexes. In addition to hSos1, Grb2 participates in the formation of a tyrosine phosphoprotein complex that includes 145-, 95-, 70-, 54-, and 36-38-kDa proteins. p145 was identified as PLC-gamma 1 and p70 as the protein tyrosine kinase, ZAP-70. Although of the same molecular weight, p95 was not recognized by an anti-serum to p95 Vav. The SH2 domains of Grb2 and PLC-gamma 1 were required for the formation of this protein complex. In anti-CD3-treated cells, Grb2 redistributed from the cytosol to a particulate cell compartment along with p36/p38, ZAP-70, and PLC-gamma 1. Part of the Grb2 complex associated with the particulate compartment could be extracted with Nonidet P-40, while the rest was Nonidet P-40 insoluble. In both the detergent-soluble and -insoluble fractions, Grb2 coimmunoprecipitated with the zeta-chain of the TCR. Taken together, these results indicate that anti-CD3 induces Grb2-hSos1-PLC-gamma 1-p36/p38-ZAP70 complexes, which localize in the vicinity of TCR-zeta.

A comparison of the interaction of Shc and the tyrosine kinase ZAP-70 with the T cell antigen receptor zeta chain tyrosine-based activation motif

Tyrosine-based activation motifs (TAMs) define a conserved signaling sequence, EX2YX2L/IX7YX2L/I, that couples the T cell antigen receptor to protein tyrosine kinases and adapter molecules. The present study shows that phosphorylation of both tyrosines within the motif is required for high affinity binding of the tyrosine kinase ZAP-70 whereas phosphorylation of the single COOH-terminal tyrosine within the motif is optimal for the binding of the adapter Shc. There were also quantitative differences in the ZAP-70 and Shc association with the zeta 1-TAM since nM concentrations of the doubly phosphorylated zeta 1-TAM are sufficient for ZAP-70 recruitment whereas micromolar levels of singly phosphorylated TAMs are necessary for Shc binding. Shc is tyrosine phosphorylated in antigen receptor-activated T cells and can potentially form a complex with the adapter molecule Grb2 and could thus recruit the Ras guanine nucleotide exchange protein Sos into the antigen receptor complex. The present data show that Grb2 can bind to the phosphorylated TAM, but this binding is independent of Shc and there is no formation of zeta 1-TAM.Shc.Grb2.Sos complexes in antigen receptor-activated cells. Accordingly, Shc function should not be considered in the context of Grb2/Sos recruitment to the T cell antigen receptor complex.

The proto-oncogene product c-Cbl becomes tyrosine phosphorylated by stimulation with GM-CSF or Epo and constitutively binds to the SH3 domain of Grb2/Ash in human hematopoietic cells

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and erythropoietin (Epo) are hematopoietic growth factors that regulate proliferation and differentiation of hematopoietic cells. They elicit and control a cascade of biochemical events, the earliest of which is tyrosine phosphorylation of several cellular proteins. Grb2/Ash is composed of SH2 and SH3 domains. The SH2 domain binds to tyrosine-phosphorylated proteins, and the SH3 domains bind to proteins containing proline-rich regions. It is considered that Grb2/Ash functions as an adapter protein linking tyrosine kinases and Ras in downstream of receptors for growth factors in fibroblasts. However, the mechanisms of signal transduction through Grb2/Ash and the roles of proteins associated with Grb2/Ash remain to be determined in hematopoietic cells. By means of the binding experiments using the glutathione S-transferase fusion protein including the full-length Grb2/Ash, we have found that Shc and unidentified 130- and 135-kDa proteins are associated with Grb2/Ash and that they are tyrosine phosphorylated by treatment with GM-CSF or Epo in a human leukemia cell line, UT-7. We have purified the 130-kDa protein (pp130) using the glutathione S-transferase-Grb2/Ash affinity column. The amino acid sequence analysis of the three peptides derived from the in situ protease digestion of the purified pp130 showed that the pp130 was identical to the human c-cbl proto-oncogene product (c-Cbl). c-Cbl constitutively binds to the SH3 domain of Grb2/Ash both in vitro and in vivo but not to the SH2 domain of Grb2/Ash, and the binding of Grb2/Ash to c-Cbl or Sos was not altered by GM-CSF stimulation. Moreover, c-Cbl (pp130) becomes tyrosine phosphorylated rapidly and transiently depending on GM-CSF or Epo stimulation. These findings strongly suggest that c-Cbl is implicated in the signal transduction of GM-CSF or Epo in hematopoietic cells and that c-Cbl is involved in another signaling pathway different from the Ras signaling pathway.

Regulation of the adapter molecule Grb2 by the Fc epsilon R1 in the mast cell line RBL2H3

Antigenic cross-linking of the high affinity IgE receptor (Fc epsilon R1) on mast cells results in protein tyrosine kinase activation. The object of the present study was to explore the regulation of the SH2 and SH3 domain containing adapter molecule Grb2 by Fc epsilon R1-stimulated PTK signal transduction pathways. Affinity purification of in vivo Grb2 complexes together with in vitro experiments with Grb2 glutathione S-transferase fusion proteins were used to analyze Grb2 complexes in the mast cell line RBL2H3. The data show that in RBL2H3 cells several different proteins are complexed to the SH3 domains of Grb2. These include the p21ras guanine nucleotide exchange factor Sos, two basally tyrosine-phosphorylated 110- and 120-kDa molecules, and a 75-kDa protein that is a substrate for Fc epsilon R1-activated PTKs. By analogy with Sos, p75, p110 and p120 are candidates for Grb2 effector proteins which suggests that Grb2 may be a pleiotropic adapter. Two Grb2 SH2-binding proteins were also characterized in RBL2H3 cells; the adapter Shc and a 33-kDa molecule. Shc is constitutively tyrosine phosphorylated in unstimulated cells and Fc epsilon R1 ligation induces no changes in its phosphorylation or binding to Grb2. In contrast, p33 is a substrate for Fc epsilon R1-activated PTKs and binds to Grb2 SH2 domains in Fc epsilon R1 activated but not quiescent cells. The beta subunit of the Fc epsilon R1 is a 33-kDa tyrosine phosphoprotein, but the p33 Grb2-binding protein described in the present report is not the Fc epsilon R1 beta chain and its identity is unknown. The present report thus demonstrates that there are multiple Grb2 containing protein complexes in mast cells of which a subset are Fc epsilon R1-regulated. Two other of the Grb2-binding proteins described herein are tyrosine phosphorylated in response to Fc epsilon R1 ligation: the 75-kDa protein which binds to Grb2 SH3 domains and the 33-kDa protein that associates with the Grb2 SH2 domain. We propose that protein complex formation by Grb2 is an important consequence of Fc epsilon R1 cross-linking and that this may be a signal transduction pathway which acts synergistically with calcium/PKC signals to bring about optimal mast cell end function.

Transmembrane signaling by antigen receptors of B and T lymphocytes

The specificity of immune responses depends upon the activation of only those lymphocytes that recognize the introduced antigen. In recent years, a great deal has been learned about the structure of lymphocyte receptors for antigens and about their signal transduction mechanism. These receptors activate intracellular protein tyrosine kinases of at least two families, the Src family and the Syk/ZAP-70 family. Recent studies have given us considerable insight into the interactions of these two types of kinases and how they mediate antigen receptor signaling.

Molecular cloning of SLP-76, a 76-kDa tyrosine phosphoprotein associated with Grb2 in T cells

The activation of protein tyrosine kinases is a critical event in T cell antigen receptor (TCR)-mediated signaling. One substrate of the TCR-activated protein tyrosine kinase pathway is a 76-kDa protein (pp76) that associates with the adaptor protein Grb2. In this report we describe the purification of pp76 and the molecular cloning of its cDNA, which encodes a novel 533-amino acid protein with a single carboxyl-terminal Src homology 2 (SH2) domain. Although no recognizable motifs related to tyrosine, serine/threonine, or lipid kinase domains are present in the predicted amino acid sequence, it contains several potential motifs recognized by SH2 and SH3 domains. A cDNA encoding the murine homologue of pp76 was also isolated and predicts a protein with 84% amino acid identity to human pp76. Northern analysis demonstrates that pp76 mRNA is expressed solely in peripheral blood leukocytes, thymus, and spleen; and in human T cell, B cell and monocytic cell lines. In vitro translation of pp76 cDNA gives rise to a single product of 76 kDa that associates with a GST/Grb2 fusion protein, demonstrating a direct association between these two molecules. Additionally, a GST fusion protein consisting of the predicted SH2 domain of pp76 precipitates two tyrosine phosphoproteins from Jurkat cell lysates, and antiserum directed against phospholipase C-gamma 1 coprecipitates a tyrosine phosphoprotein with an electrophoretic mobility identical to that of pp76. These results demonstrate that this novel protein, which we term SLP-76 (SH2 domain-containing Leukocyte Protein of 76 kDa), is likely to play an important role in TCR-mediated intracellular signal transduction.

The regulation and function of p21ras during T-cell activation and growth

The delivery of signals that control the growth of T cells is a key event for effective co-ordination of T-cell-dependent immune responses. It is now recognized that guanine nucleotide binding proteins play an important role in signal transduction by the T-cell receptor (TCR) and cytokine receptors. Here, Manolo Izquierdo Pastor, Karin Reif and Doreen Cantrell review the numerous recent advances in understanding how the p21ras guanine nucleotide binding protein couples the TCR to the T-cell signalling cascade.

Crk interacts with tyrosine-phosphorylated p116 upon T cell activation

Products of the crk oncogene are expressed in all tissues. Crk proteins are composed exclusively of Src homology 2 (SH2) and Src homology 3 (SH3) domains, and they have been implicated in intracellular signaling. For example, they participate as mediators of Ras activation during nerve growth factor stimulation of PC12 pheochromocytoma cells. We examined the role of Crk proteins during T cell receptor-mediated signaling and observed that Crk proteins specifically interact, via their SH2 domains, with a tyrosine-phosphorylated 116-kDa protein upon T cell activation. p116 may be related to the recently cloned fibroblast p130cas and/or p120-Cbl. In addition, we observed that GST-Crk fusion proteins and Crk-L bind, most likely via their SH3 domain, to C3G, a Ras guanine nucleotide exchange factor. Thus, the interaction of Crk with p116 and C3G strongly implicates Crk as a mediator of T cell receptor signaling, possibly involved in Ras activation.

T-cell antigen receptor-induced signal-transduction pathways--activation and function of protein kinases C in T lymphocytes

CONTENTS. T-cell activation--Structure of the T-cell antigen receptor--Modular organisation of the T-cell antigen receptor--T-cell antigen receptor-coupled signaling pathways: Activation of protein-tyrosine kinase by the T-cell antigen receptor; Signal transduction in lymphoid cells involves several protein-tyrosine kinases in parallel; Regulation of T-cell antigen receptor signaling by the phosphoprotein phosphatase CD45--Consequences of T-cell antigen receptor-induced tyrosine phosphorylation: Activation of phosphoinositol-lipid-turnover pathways--Activation of phospholipase C-gamma-1: p59fyn or p56lck?--G-protein motif of CD3-gamma: relevance for signal transduction--Association of lipid kinase with the T-cell antigen receptor--Intracellular signaling by phospholipid metabolites and calcium: activation of protein kinase C--Protein kinase C isoenzymes--Heterogenity of protein kinase C and mode of activation--Phospholipid-derived mediators in activation of protein kinase C in T-cells--Role of phospholipase D metabolites in activation of protein kinase C--Polyunsaturated fatty acids and lysophosphatidylcholine as activators of protein kinase C--Potein kinase C and p21ras function in interdependent and distinct signaling pathways during T-cell activation--Raf-1 kinase: regulator or target of protein kinase C?--Summary and perspectives.

Signal transduction by the antigen receptors of B and T lymphocytes

B and T lymphocytes of the immune system recognize and destroy invading microorganisms but are tolerant to the cells and tissues of one's own body. The basis for this self/non-self-discrimination is the clonal nature of the B and T cell antigen receptors. Each lymphocyte has antigen receptors with a single unique antigen specificity. Multiple mechanisms ensure that self-reactive lymphocytes are eliminated or silenced whereas lymphocytes directed against foreign antigens are activated only when the appropriate antigen is present. The key element in these processes is the ability of the antigen receptors to transmit signals to the interior of the lymphocyte when they bind the antigen for which they are specific. Whether these signals lead to activation, tolerance, or cell death is dependent on the maturation state of the lymphocytes as well as on signals from other receptors. We review the role of antigen receptor signaling in the development and activation of B and T lymphocytes and also describe the biochemical signaling mechanisms employed by these receptors. In addition, we discuss how signal transduction pathways activated by the antigen receptors may alter gene expression, regulate the cell cycle, and induce or prevent programmed cell death.

B cell antigen receptor cross-linking induces tyrosine phosphorylation and membrane translocation of a multimeric Shc complex that is augmented by CD19 co-ligation

The SH2 domain-containing transforming Shc protein has been implicated in mitogenic signaling via several surface receptors through p21ras. Following tyrosine phosphorylation by either receptor or non-receptor tyrosine kinases, Shc may interact with the adaptor protein Grb2, which is linked to Sos1, a guanine nucleotide exchange factor for human ras. Ligation of the antigen receptor complex on B cells (BCR) is known to activate various intracellular signaling pathways, which may accumulate in mitogenic responses. With respect to the initial steps, the activation of BCR-associated non-receptor tyrosine kinases appears to be indispensible. In this report we show that Shc proteins become tyrosine phosphorylated after BCR ligation on both transformed and normal human B cells. This is accompanied by the association of Shc with Grb2 proteins and a yet unidentified 145-kDa tyrosine phosphorylated protein. Subcellular fractionation revealed that this activation-induced multimeric Shc complex rapidly translocates towards the plasma membrane. Co-ligation of the BCR with the CD19 molecule results in a marked increase of these events, whereas CD19 cross-linking alone does not induce Shc tyrosine phosphorylation or translocation. Thus, in B cells the Shc complex may represent a molecular junction between the BCR and the mitogenic p21ras cascade.

G proteins in lymphocyte signalling

The guanine nucleotide binding proteins p21ras are activated by the T-cell antigen receptor and play a critical role in transducing signals from the T-cell receptor to the cell nucleus. The mechanism that couples the T-cell receptor to p21ras is the focus of many current studies, but is as yet unidentified.