Direct interaction of Syk and Lyn protein tyrosine kinases in rat basophilic leukemia cells activated via type I Fcε receptors

Activation of rat mast cells through the receptor with high affinity for IgE (FcεRI) requires a complex set of interactions involving transmembrane subunits of the FcεRI and two classes of nonreceptor protein tyrosine kinase (PTK), the Src family PTK p53/p56^lyn (Lyn) and the Syk/ZAP-family PTK p72^syk (Syk). Early activation events involve increased activity of Lyn and Syk kinases and their translocation into membrane domains containing aggregated FcεRI, but the molecular mechanisms responsible for these changes have remained largely unclear. To determine the role of FcεRI subunits in this process, we have analyzed Syk- and Lyn-associated proteins in activated rat basophilic leukemia (RBL) cells and their variants deficient in the expression of FcεRI β or γ subunits. Sepharose 4B gel chromatography of postnuclear supernatants from Nonidet-P40-solubilized antigen (Ag)- or pervanadate-activated RBL cells revealed extensive changes in the size of complexes formed by Lyn and Syk kinases and other cellular components. A fusion protein containing Src homology 2 (SH2) and SH3 domains of Lyn bound Syk from lysates of nonactivated RBL cells; an increased binding was observed when lysates from Ag- or pervanadate-activated cells were used. A similar amount of Syk was bound when lysates from pervanadate-activated variant cells deficient in the expression of FcεRI β or γ subunits were used, suggesting that FcεRI does not function as the only intermediate in the formation of the Syk-Lyn complexes. Further experiments have indicated that Syk-Lyn interactions occur in Ag-activated RBL cells under in vivo conditions and that these interactions could involve direct binding of the Lyn SH2 domain with phosphorylated tyrosine of Syk. The physical association of Lyn and Syk during mast-like cell activation supports the recently proposed functional cooperation of these two tyrosine kinases in FcεRI signaling.     

The role of tyrosine phosphorylation in the interaction of cellular tyrosine kinases with the T cell receptor ζ chain tyrosine-based activation motif

Immunoglobulin receptor family tyrosine-based activation motifs (ITAM) define a conserved signaling sequence, EX₂ YX₂L/IX₇YX₂L/I, that mediates coupling of the T cell antigen receptor (TCR) to protein tyrosine kinases (PTK). In the present study, we explored the role of phosphorylation of the two ITAM tyrosine residues in the interactions of the motif with the PTK ZAP-70 and p59fyn. The data show that the phosphorylation of a single tyrosine within the motif enables binding of p59fyn, whereas phosphorylation of both tyrosines within the motif is required for maximal binding of the PTK ZAP-70. Quantitative binding experiments show that nanomolar concentrations of the doubly phosphorylated ζ1-ITAM are sufficient for ZAP-70 recruitment, whereas micromolar levels of singly phosphorylated ITAM are necessary for p59fyn binding. ZAP-70 binds with low efficiency to a singly phosphorylated ITAM, but shows preferential binding to the C-terminal phosphotyrosine in the ITAM, whereas p59fyn binds selectively to the N-terminal phosphotyrosine. The present data thus show that there is the potential for a singly phosphorylated ITAM to couple to cellular PTK. Moreover, the data suggest a mechanism for heterogeneity in signal transduction responses by the TCR, since ITAM could differentially couple the TCR to downstream signaling events depending on their phosphorylation state.     

CD2 singnaling in T cells involves tyrosine phosphorylation and activation of the Tec family kinase, EMT/ITK/TSK

Ligation of the CD2 cell surface glycoprotein expressed on Tlymphocytes and NK cells induces protein tyrosine phosphorylationand activation of the Src kinases, LCK and FYN. We show herethat in Jurkat T leukemia cells and in peripheral blood T cells,CD2 stimulation also leads to tyrosine phosphorylation and activationof the Tec family kinase, EMThTKITSK. Activation of EMT by CD2was induced by mitogenic pairs of CD2 mAb, certain single CD2mAb followed by secondary antibody cross-linking, and CD58-bearingsheep red blood cells. With the use of different Jurkat cellmutants it was demonstrated that CD2-mediated activation ofEMT required expression of LCK, but did not require surfaceexpression of the CD3 chain. Receptor-mediated activation ofLCK does not in itself lead to activation of this Tec kinasesince induction of LCK by ligation of CD4 or CD5 did not resultin activation of EMT. The activation of EMT during CD2 signalingsuggests an important role for this kinase in CD2 co-stimulationof T cell responses.     

Positive and negative regulation of Fc epsilon receptor I-mediated signaling events by Lyn kinase C-terminal tyrosine phosphorylation

Although it is known that the Src family tyrosine kinase Lyn initiates Fc epsilon receptor I (Fc epsilon RI) signaling by phosphorylation of the receptor subunits, regulation of Lyn kinase activity and its consequences for receptor signaling are incompletely understood. Using a phospho-Lyn-specific antiserum, we show an increased phosphorylation of the Lyn C-terminal regulatory tyrosine and decreased Lyn kinase activity during Fc epsilon RI-mediated mast cell activation. Mutant Lyn, defective in the C-terminal tyrosine, constitutively phosphorylated several substrates in resting cells, but did not cause Fc epsilon RI internalization or spontaneous degranulation. Fc epsilon RI-induced signaling in the presence of constitutively active Lyn exhibited enhanced phosphorylation of the receptor subunits, Syk, LAT, Gab2, phospholipase C (PLC)gamma 1 and PLC gamma 2, and production of phosphatidylinositol 3,4,5-trisphosphate. Although enzymatic activities of PLC gamma 1 and PLC gamma 2 were also up-regulated, amounts of inositol 1,4,5-trisphosphate, mobilization of intracellular calcium and degranulation were suppressed. Additionally, constitutively active Lyn was strikingly less efficient than wild-type Lyn in restoring the receptor-mediated calcium responses in bone marrow mast cells derived from Lyn(-/-) mice. These findings pinpoint the tight regulation of Lyn kinase activity as a critical event in mast cell degranulation.     

Induction of the increased Fyn kinase activity in anergic T helper type 1 clones requires calcium and protein synthesis and is sensitive to cyclosporin A

Several alterations in T cell receptor-associated signal transduction have been observed following induction of anergy of T helper type 1 (Th1) clones, including a modified intracellular free calcium ([Ca²⁺]_i) response and increased kinase activity associated with the protein tyrosine kinase p59^fyn. In the current study, we demonstrate that, although the kinetics of acquisition of both of these signaling alterations correlated with the generation of anergy, a normal calcium response returned within 48 h after removal from the anergizing stimulus, whereas the increased p59^fyn activity persisted and the cells remained hyporesponsive. Generation of both the anergic state and the increased p59^fyn activity was prevented in the presence of calcium-free medium, cycloheximide (CHX), or cyclosporin A (CsA), and could be mimicked by the calcium ionophore ionomycin. In contrast, the altered calcium response was inhibited by stimulation in the presence of calcium-free medium or CsA, but not CHX. Thus, surprisingly, these data suggest that a chronic elevation of [Ca²⁺]_i is proximal to and necessary for the increase in p59^fyn-associated kinase activity observed in anergic Th1 clones. Increased p59^fyn activity, but not the altered calcium response, correlates with maintenance of the anergic state.     

T cell stimulation in the absence of exogenous antigen: a T cell signal is induced by both MHC-dependent and -independent mechanisms

Mature T cells residing in peripheral lymphoid organs have frequent contact with antigen presenting cells (APC). Such contact may be required for T cell survival, but the degree to which signals in mature T cells are induced by TCR recognition of self peptide/MHC complexes is unclear. We have used induction of the early growth response gene 1 (Egr1) as an indicator of signal transduction in 3.L2 (I-Ek-restricted) T cells interacting with APC in the absence of exogenous antigen. The data show that Egr1 can be induced in 3.L2 T cells by TCR recognition of self peptides presented by I-Ek. However, a more transient induction of Egr1 can be induced in 3.L2 T cells interacting with dendritic cells derived from class II/beta2m double-deficient mice. Egr1 induction after T cell-APC contact was also observed in a freshly isolated polyclonal CD4 T cell population. The data suggest that self peptide/MHC recognition by the TCR induces a signal in T cells and that dendritic cells can also induce a more transient T cell signal by an MHC-independent mechanism.     

酪氨酸磷酸化抑制剂AG490抑制Jurkat T细胞增殖

目的:通过AG490对Jurkat T细胞活化、增殖、周期、凋亡及ICBP90蛋白表达的影响,探讨阻断JAK/STAT信号通路以抑制Jurkat T细胞生长的可能性及其初步机制。方法:以Jurkat T细胞为模型,应用双荧光抗体标记结合流式细胞仪检测AG490对Jurkat T细胞表面分子CD69和CD25表达的影响;利用噻唑蓝(MTT)比色法观察AG490对Jurkat T细胞增殖的影响;采用碘化丙锭(PI)染色检测AG490对Jur-kat T细胞周期的影响;应用Annexin V-FITC和PI双染色检测AG490对Jurkat T细胞凋亡的影响;Western blot检测AG490对Jurkat T细胞中ICBP90蛋白表达的影响以确定其与AG490抑制Jurkat T细胞增殖的关系。结果:随着AG490浓度从1 mmol/L增至30 mmol/L,细胞停滞于G0/G1期,阻止其进入S期和G2/M期,导致Jurkat T细胞ICBP90蛋白的表达显著降低;AG490对细胞的抑制作用于24 h最为明显,抑制率可达27.37%,呈剂量依赖关系;AG490不能明显抑制Jurkat T细胞的活化或促进其凋亡。结论:AG490能明显抑制Jurkat T细胞的生长,其抑制作用可能通过下调Jurkat T细胞ICBP90蛋白的表达与细胞周期阻滞有关,而不是通过促进细胞凋亡而实现。     

Induction of interleukin-2 unresponsiveness and down-regulation of the JAK-STAT system upon activation through the T cell receptor

Full activation of T cells with antigen (Ag) and antigen-presenting cells initiates effector functions and proliferation. When T cells are re-stimulated through the T cell receptor (TCR) after a primary stimulation with Ag, growth arrest and cell death are induced. Activation of a T cell clone by cross-linking of TCR induces interleukin (IL)-2 unresponsiveness and ultimately cell death. While the proliferative signal delivered by IL-2 induces c-myc, bcl-2 and cyclin D3 expression, the expression of bcl-2 and cyclin D3 is completely suppressed upon TCR stimulation. Furthermore, TCR stimulation induces a decrease in the protein levels of JAK3 and STAT5, suggesting that IL-2 unresponsiveness and growth arrest of T cells result from down-regulation of JAK3 and STAT5.     

A protein kinase C inhibitory activity is present in the human T lymphoblast cell line Jurkat

The human lymphoblast cell line Jurkat is widely used as a model system for studying signal transduction pathways during lymphocyte activation. We report the presence of a potent endogenous inhibitor of protein kinase C (PKC) in the cytosolic fraction of Jurkat cells. This inhibitor is not diffusible and is thermolabile; it is assumed to be a protein. It was separated from PKC by ion-exchange chromatography on DEAE-cellulose. The inhibitory activity was partially reversed by increasing the concentration of the PKC substrate; increasing that of PKC activators (calcium and phospholipids) was without effect. PKC activity was inhibited by more than 90% in the crude cytosolic fraction but the inhibition could be completely reversed by diluting the cell extract. This inhibitory activity could not be detected in the cytosol from normal lymphocytes or from lymphoblasts from leukemic patients.     

Cyclosporin A inhibits T cell receptor-induced interleukin-2 synthesis of human T lymphocytes by selectively preventing a transmembrane signal transduction pathway leading to sustained activation of a protein kinase C isoenzyme,protein kinase C-β

Stimulation of human peripheral blood lymphocytes via T cell receptor/CD3 complex resulted in a bimodal activation of protein kinase(s) C (PKC). Within 10 min of stimulation PKC-α was translocated to, and thus activated in, the plasma membranes of human lymphocytes, followed by a fast dissociation of this isotype from the plasma membrane. This short term activation and translocation PKC-α proved to be cyclosporin A (CsA) insensitive. After 90 min of stimulation PKC-β was translocated to and remained bound to the plasma membranes for up to 4 h. Preincubation of human lymphocytes with 200 ng/ml CsA specifically and completely abolished the sustained activation of PKC-β. Neither the phorbol ester-induced direct activation of PKC nor the specific activity of the plasma membrane-bound enzyme was influenced by CsA, suggesting that a signal transduction pathway leading to sustained activation of PKC-β was influenced by the immunosuppressive agent. In fact, CsA inhibited, in a concentration-dependent manner, the activation of lysophosphatid acyltransferase-catalyzed elevated incorporation of cis-polyunsaturated fatty acids into plasma membrane phospholipids. While interleukin-2 (IL-2) synthesis and cellular proliferation were completely inhibited by 200 ng/ml CsA in BMA 030- or BMA 031-stimulated cells, expression of high-affinity IL-2 receptors was not influenced by the immunosuppressive drug. These results suggest that synthesis and expression of high-affinity IL-2 receptors might be regulated by a signal-transducing pathway involving activation and translocation of PKC-α. Lysophosphatid acyltransferase-catalyzed incorporation of cis-polyunsaturated fatty acids might represent another mechanism of signal transduction implicated in the activation and translocation of PKC-β, which is specifically inhibited by CsA. Neutralization of PKC-β by introducing anti-PKC-β antibodies prevented IL-2 synthesis and proliferation in stimulated human lymphocytes. The results suggest a possible link between activation of PKC-β and regulation of IL-2 synthesis in activated human lymphocytes. Thus, inhibition of the activation and translocation of PKC-β by CsA may result in inhibition of IL-2 gene expression in human lymphocytes.     

Analysis of the tyrosine phosphorylation and calcium fluxing of human CD6 isoforms with different cytoplasmatic domains

CD6 is a cell surface glycoprotein that functions both as a co-stimulatory and adhesion receptor on T cells. Recently we have described CD6 isoforms (CD6a, b, c, d, e) that arise via alternative splicing of exons encoding the cytoplasmic region of the molecule. CD6 becomes phosphorylated on tyrosine (Tyr) residues following stimulation through the T cell receptor (TCR) complex. Since the phosphorylation of Tyr residues renders some cell surface receptors competent for interactions with proteins of intracellular signaling pathways, we wanted to determine which region(s) and residues in the cytoplasmic domain of CD6 were important for phosphorylation on Tyr residues. We engineered and stably expressed chimeric receptors that consisted of the extracellular region of mouse CD6 and the cytoplasmic regions of either naturally occurring isoforms of human CD6, truncated proteins, or point mutants. We were able to demonstrate that of the nine Tyr residues in the cytoplasmic domain of the largest isoform CD6a, the two C-terminal Tyr residues (Tyr 629/662) are critical for the phosphorylation of CD6 following TCR cross-linking. Isoform CD6e, which is missing a region that contains two proline-rich motifs, is not phosphorylated. We further analyzed the ability of the different CD6 isoforms and truncated receptors to mobilize intracellular calcium after CD6/TCR co-ligation. All CD6 isoforms, including CD6e, as well as the truncation mutant Δ 555, which is missing approximately the C-terminal half of the cytoplasmic domain, are able to increase Ca²⁺ influx. Taken together, these results suggest that the region of CD6 which is critical for Ca²⁺ mobilization is located N-terminal from amino acid 555 and is therefore different from the region located at the C terminus of CD6, which is necessary for tyrosine phosphorylation.     

Expression and function of the adaptor protein Gads in murine B cells

Nearly all hematopoietic receptors are dependent on adaptor proteins for the activation of downstream signaling pathways. The Gads adaptor protein is expressed in many hematopoietic tissues, including bone marrow, lymph node, and spleen. Using intracellular staining, we detected Gads protein in a number cells, including B cells, T cells, NK cells, monocytes, and plasmacytoid DC, but not in macrophages, neutrophils, or monocyte-derived DC. In the B cell compartment, Gads was first expressed after immature B cells leave the bone marrow and was down-regulated after B cell antigen receptor (BCR) ligation. Female Gads(-/-) mice had increased numbers of splenic B cells, as compared to female Gads(+/+) mice, suggesting a role for Gads in B cell homeostasis. Although B cell production and turnover of splenic B cell subsets appeared normal in Gads(-/-) mice, homeostatic proliferation was significantly impaired in Gads(-/-) B cells. Whereas BCR ligation can induce apoptosis in wild-type transitional stage 1 (T1) B cells, Gads(-/-) T1 B cells were resistant to BCR-induced apoptosis. Gads(-/-) B cells also showed increased BCR-mediated calcium mobilization. We conclude that Gads may have a negative regulatory role in signaling through survival pathways, and is necessary for normal homeostatic proliferation in B cells.     

A model for antigen-induced T cell unresponsiveness based on autophosphorylative protein tyrosine kinase activity

Helper T cell signaling is initiated by the aggregation of TCRwith the induction of tyrosine kinase activity as one of theearliest consequences. Here, a theoretical model for antigen-inducedunresponsiveness is presented that relies on a cascade of tyrosinephosphorylation- dephoshorylation cycles. A mechanism is describedfor both desensitization in the presence of antigen and persistentlowering of cell responsiveness after stimulus removal. An importantcomponent of the model, leading to bistability, is the presenceof autophosphorylating protein tyrosine kinases in the earlysteps of TCR signaling. One of its predictions is that, followingstimulation, the net phosphorylative activity of these receptor-associatedtyrosine kinases will remain above background level after removalof the antigen. It is proposed that this residual tyrosine kinaseactivity is linked to a deficient signal transduction capacityof the TCR system that leads to a state of prolonged unresponsiveness.In addition, the present analysis defines the notion of a signalingthreshold for hyporesponsiveness induction, associated witha durable switch and amplification of the net tyrosine kinaseactivity. This approach emphasizes the role of tyrosine kinasesin the down-regulation of cellular competence.     

Roles of Lck,Syk and ZAP-70 tyrosine kinases in TCR-mediated phosphorylation of the adapter protein Shc

The adapter protein Shc has been implicated in mitogenic signaling via growth factor receptors, antigen receptors and cytokine receptors. Recent studies have suggested that tyrosine phosphorylation of Shc may play a key role in T lymphocyte proliferation via interaction of phosphorylated Shc with downstream molecules involved in activation of Ras and Myc proteins. However, the sites on Shc that are tyrosine phosphorylated in response to TCR engagement and the ability of different T cell tyrosine kinases to phosphorylate Shc have not been defined. In this report, we show that during TCR signaling, the tyrosines Y239, Y240 and Y317 of Shc are the primary sites of tyrosine phosphorylation. Mutation of all three tyrosines completely abolished tyrosine phosphorylation of Shc following TCR stimulation. Our data also suggest that multiple T cell tyrosine kinases contribute to tyrosine phosphorylation on Shc. In T cells, CD4/Lck-dependent tyrosine phosphorylation on Shc was markedly diminished when Y317 was mutated, suggesting a preference of Lck for the Y317 site. The syk-family kinases (Syk and ZAP-70) were able to phosphorylate the Y239 and Y240 sites, and less efficiently the Y317 site. Moreover, co-expression of Syk or ZAP-70 with Lck resulted in enhanced phosphorylation of Shc on all three sites, suggesting a synergy between the syk -family and scr -family kinases. Of the two potential Grb2 binding sites (Y239 and Y317), Y239 appears to play a greater role in recruiting Sos through Grb2. These studies have implications for Ras activation and mitogenic signaling during T cell activation.     

T cell receptor α-chain tail is required for protein kinase C-mediated down-regulation,but not for signaling

Antigen stimulation through the T cell receptor (TCR) induces phosphorylation of the associated CD3 γδσ- and ζ-chain cytoplasmic tails. These events lead to the induction of the intracellular signaling pathways with concomitant receptor down-regulation. The TCR is down-regulated from the cell surface by the activation of protein kinase C (PKC) and subsequent serine phosphorylation of the CD3 γ-chain. We report here that the TCR α-chain cytoplasmic tail is also necessary for PKC-mediated internalization of the TCR complex. The requirement for the TCR α-chain cytoplasmic tail is specific for internalization of the TCR complex, since down-regulation of CD4 is still intact in hybridoma cells expressing a tailless TCR α-chain. The absence of TCR internalization directly correlates with defective PKC-mediated phosphorylation of the CD3 γ-chain. Despite deficient PKC-mediated TCR down-regulation, the tailless αβ TCR still transduces antigenic signals resulting in the production of interleukin-2. Although the TCR tails are not obviously required for signal transduction, the TCR α-tail may serve as a targeting domain for PKC-mediated down-regulation of the TCR complex.     

Signal extinction and T cell repolarization in T helper cell-antigen-presenting cell conjugates

We have previously demonstrated that in T cell-antigen-presenting cell (APC) conjugates many T cell receptors (TCR) are serially triggered by a few peptide-MHC complexes, resulting in sustained signaling. Here, we investigate the mechanisms that determine the duration and extent of signaling. We show that in the course of the T helper cell-APC interaction, down-regulation of triggered TCR leads to extinction of signaling. However, T cells that have been activated by a previous encounter with peptide-pulsed APC and have extinguished signaling can swiftly repolarize towards APC displaying higher antigen concentrations and dedicate their help to these cells. These results demonstrate that TCR down-regulation allows T cells to calibrate their response and dedicate their help to APC offering the highest stimulus.     

The internalization of the IgG2a antigen receptor does not require the association with Ig-α and Ig-β but the activation of protein tyrosine kinases does

The B cell antigen receptor of class IgM is a multimeric protein complex containing the membrane-bound immunoglobulin molecule and a heterodimer of the two B cell-specific transmembrane proteins Ig-α and Ig-β. The B cell antigen receptor fulfills a dual role on the surface of B cells. First, it is a signal transduction complex which can activate protein tyrosine kinases and induce the release of Ca²⁺ ions from intracellular stores. Second, its internalization mediates the specific uptake of bound antigens, which are processed intracellularly and presented as major histocompatibility complex-bound peptides on the cell surface. In case of the IgM antigen receptor, the association with the heterodimer is necessary for expression of large amounts of IgM on the surface. We show here that the IgG2a antigen receptor can be expressed on the surface of myeloma cells in two structurally different forms: either with or without the Ig-α/Ig-β heterodimer. A functional comparison of the two forms of antigen receptors demonstrates that the Ig-α and Ig-β molecules are required for the activation of protein tyrosine kinases after cross-linking of the B cell antigen receptor. In contrast, both forms of IgG2a are equally well internalized. This suggests that Ig-α and Ig-β are essential for signal transduction through the IgG2a antigen receptor, whereas internalization can occur independently of the heterodimer.     

Decreased CD4 expression by polarized T helper 2 cells contributes to suboptimal TCR-induced phosphorylation and reduced Ca2+ signaling

Polarized Th1 and Th2 cells expressing the same TCR produce distinct biochemical responses to ligand engagement. Compared to Th1 cells, Th2 cells show altered substrate tyrosine phosphorylation and a diminished or transient Ca2+ response. Here we demonstrate that agonist stimulation of Th1 cells leads to the predominant appearance of fully phosphorylated (p23) TCR zeta, substantial phosphorylation of zeta-associated protein 70 (ZAP-70), and strong elevation of intracellular Ca2+, whereas agonist stimulation of Th2 cells expressing an identical TCR results in an elevated p21:p23 TCR zeta ratio, little or no detectable ZAP-70 phosphorylation, and a more limited elevation in intracellular Ca2+. Th2 cells consistently had twofold lower surface CD4 expression as compared to Th1 cells with the same TCR. When CD4 levels in Th2 cells were raised to Th1 levels using retroviral gene transfer, the transduced cells showed greater generation of p23 phospho-zeta, measurable phosphorylation of ZAP-70, and increased Ca2+ responses. These findings suggest that the apparent qualitative differences in TCR signaling characterizing Th1 versus Th2 cells are largely the result of modest quantitative variation in CD4 expression, with decreased CD4 expression playing a significant role in attenuating the proximal signaling responsiveness of Th2 cells to TCR ligands.     

The T cell receptor associated CD3-ϵ protein is phosphorylated upon T cell activation in the two tyrosine residues of a conserved signal transduction motif

Signal transduction through the Tcell receptor for antigen, the TcR/CD3 complex, involves phosphorylation of tyrosine residues in the CD3-? chain. Since both CD3-?and the ζ, chain contain a tyrosine-based signaling motif, we examine phosphorylation of CD3-? in human T cells. Engagement of the TcR/CD3 complex induced tyrosine phosphorylation of CD3-? in vivo. Induction of CD3-? phosphorylation followed similar kinetics to that of the ζ, chain phosphorylation. In contrast to ζ, CD3-? phosphorylation was strictly dependent upon cell surface expression of this member of the TcR/CD3 complex. Chemical and proteolytic cleavage combined with peptide-specific Western blotting established that CD3-? phosphorylation occurred in the two tyrosine residues located in the signal transduction motif in the C-terminal portion of the molecule. Taken together, these data indicated that phosphorylation of CD3-? by tyrosine protein kinases may serve to couple the TcR/CD3 complex to other effector molecules in the signaling cascade.