TCR/CD3 down-modulation and zeta degradation are regulated by ZAP-70

TCR down-modulation following binding to MHC/peptide complexes is considered to be instrumental for T cell activation because it allows serial triggering of receptors and the desensitization of stimulated cells. We studied CD3/TCR down-modulation and zeta degradation in T cells from two ZAP-70-immunodeficient patients. We show that, at high occupancy of the TCR, down-modulation of the CD3/TCR is comparable whether T cells express or do not express ZAP-70. However, if TCR occupancy was low, we found that CD3/TCR was down-regulated to a lesser extent in ZAP-70-negative than in ZAP-70-positive T cells. We studied CD3/TCR down-modulation in P116 (a ZAP-70-negative Jurkat cell-derived clone) and in P116 transfected with genes encoding the wild-type or a kinase-dead form of ZAP-70. Down-modulation of the TCR at high occupancy did not require ZAP-70, whereas at low TCR occupancy down-modulation was markedly reduced in the absence of ZAP-70 and in cells expressing a dead kinase mutant of ZAP-70. Thus, the presence of ZAP-70 alone is not sufficient for down-modulation; the kinase activity of this molecule is also required. The degradation of zeta induced by TCR triggering is also severely impaired in T cells from ZAP-70-deficient patients, P116 cells, and P116 cells expressing a kinase-dead form of ZAP-70. This defect in TCR-induced zeta degradation is observed at low and high levels of TCR occupancy. Our results identify ZAP-70, a tyrosine kinase known to be crucial for T cell activation, as a key player in TCR down-modulation and zeta degradation.     

Zap-70-independent Ca(2+) mobilization and Erk activation in Jurkat T cells in response to T-cell antigen receptor ligation

The tyrosine kinase ZAP-70 has been implicated as a critical intermediary between T-cell antigen receptor (TCR) stimulation and Erk activation on the basis of the ability of dominant negative ZAP-70 to inhibit TCR-stimulated Erk activation, and the reported inability of anti-CD3 antibodies to activate Erk in ZAP-70-negative Jurkat cells. However, Erk is activated in T cells receiving a partial agonist signal, despite failing to activate ZAP-70. This discrepancy led us to reanalyze the ZAP-70-negative Jurkat T-cell line P116 for its ability to support Erk activation in response to TCR/CD3 stimulation. Erk was activated by CD3 cross-linking in P116 cells. However, this response required a higher concentration of anti-CD3 antibody and was delayed and transient compared to that in Jurkat T cells. Activation of Raf-1 and MEK-1 was coincident with Erk activation. Remarkably, the time course of Ras activation was comparable in the two cell lines, despite proceeding in the absence of LAT tyrosine phosphorylation in the P116 cells. CD3 stimulation of P116 cells also induced tyrosine phosphorylation of phospholipase C-gamma1 (PLCgamma1) and increased the intracellular Ca(2+) concentration. Protein kinase C (PKC) inhibitors blocked CD3-stimulated Erk activation in P116 cells, while parental Jurkat cells were refractory to PKC inhibition. The physiologic relevance of these signaling events is further supported by the finding of PLCgamma1 tyrosine phosphorylation, Erk activation, and CD69 upregulation in P116 cells on stimulation with superantigen and antigen-presenting cells. These results demonstrate the existence of two pathways leading to TCR-stimulated Erk activation in Jurkat T cells: a ZAP-70-independent pathway requiring PKC and a ZAP-70-dependent pathway that is PKC independent.     

Itk/Emt/Tsk activation in response to CD3 cross-linking in Jurkat T cells requires ZAP-70 and Lat and is independent of membrane recruitment.

The Tec family tyrosine kinase, Itk has been implicated in T cell antigen receptor (TCR) signaling, yet little is known about Itk regulation. Here, we investigate the role of the tyrosine kinase ZAP-70 in regulating Itk. Whereas Itk was activated in Jurkat T cells in response to CD3 cross-linking, Itk activation was defective in the ZAP-70-deficient P116 Jurkat T cell line. Itk responsiveness to TCR engagement was restored in P116 cells stably transfected with ZAP-70 cDNA. ZAP-70 itself could not directly phosphorylate the Itk kinase domain, indicating an indirect regulation of Itk activity. No role was found for ZAP-70 in regulating Itk recruitment to the plasma membrane, an event that has been suggested to be rate-limiting for the activation of Tec family kinases. Indeed, Itk was found to be constitutively targeted to the membrane fraction in both Jurkat and P116 cells. Lat, a prominent in vivo substrate of ZAP-70 that mediates assembly of multimolecular signaling complexes at the plasma membrane of T cells was also found to be required for TCR-stimulated Itk activation. Itk could not be activated by CD3 cross-linking in a Lat-negative cell line, unless Lat expression was restored. Lat and Itk were observed to co-associate in response to CD3 cross-linking in Jurkat T cells, but not in P116 T cells. The Lat-Itk association correlated with Lat tyrosine phosphorylation, which was deficient in the P116 T cells. These data suggest that ZAP-70 and Lat play important, probably sequential, roles in regulating the activation of Itk following TCR engagement.     

A role of kinase inactive ZAP-70 in altered peptide ligand stimulated T cell activation

T cell activation signals induced by altered peptide ligands (APLs) are different from those induced by the original agonistic peptide. The characteristics of the former are partial phosphorylation of TCR-zeta and no tyrosine-phosphorylation of zeta-associated protein-70 (ZAP-70). To analyze further those signaling pathways, we introduced a dominant negative (DN) form of ZAP-70 into a human CD4(+) T cell clone in which fully and partially agonistic peptide ligands have been well characterized. We found that some over-expressed partially agonistic ligands (OPALs) induced T cell responses without tyrosine-phosphorylation and kinase activation of ZAP-70. However, those responses were inhibited in T cells expressing DN ZAP-70, which could associate with partially phosphorylated TCR-zeta. In OPAL-stimulated T cells, PLC-gamma1 was phosphorylated and it was suppressed by DN ZAP-70 expression, suggesting that the ZAP-70-TCR-zeta association mediates the activation of PLC-gamma1 leading to T cell responses even in the absence of kinase activation of ZAP-70.     

Identification of a novel isoform of ZAP-70, truncated ZAP kinase

We identified a novel cDNA encoding truncated ZAP-70, which lacked the SH2 domain and a part of interdomain B, and named it truncated ZAP kinase (TZK). TZK was expressed in the thymus, spleen, and lymph nodes with ZAP-70. TZK was expressed in CD44⁺CD25⁻ thymocytes up to mature T cells, but ZAP-70 was not expressed in CD44⁺CD25⁻ or CD44⁺CD25⁺ thymocytes. ZAP-70 or TZK was transfected into P116 cells derived from a Jurkat T-cell line deficient in ZAP-70. The P116 cells with ZAP-70 induced the T-cell receptor-mediated signal transduction, but the cells expressing TZK did not. While ZAP-70 was accumulated at the immune synapse, TZK was not. Meanwhile, impaired phosphorylation of SLP-76, one of the substrates of ZAP-70, in P116 cells upon pervanadate stimulation was rescued in the cells expressing TZK. These findings show that TZK is a novel isoform of ZAP-70, which is expressed in pre-T-cell receptor-minus thymocytes and functions as a kinase not associated with T-cell receptor.     

Differential requirement of ZAP-70 for CD2-mediated activation pathways of mature human T cells

This study addresses the role of the tyrosine kinase ZAP-70 in CD2-mediated T cell activation. Patients lacking ZAP-70 have few mature CD8+ T cells and high numbers of CD4+ T cells that are nonfunctional upon TCR triggering. Such a patient with a homozygous deletion in the zap-70 gene that resulted in the complete absence of ZAP-70 protein expression has been identified. Expression of the tyrosine kinases Lck, Fyn, and Syk was normal. The patient's T cells were activated with two different pairs of mitogenic mAbs. CD2-induced phosphorylation of the zeta-chain and influx of Ca2+ was defective in the ZAP-70-deficient T cells, whereas CD2-induced phosphorylation of several other proteins, including Syk, was not affected. CD2-induced proliferation as well as production of TNF-alpha and IFN-gamma was abrogated in ZAP-70-deficient T cells, whereas PMA plus ionomycin induced normal activation of these cells. Together, this study shows that CD2-activation triggers ZAP-70-dependent and -independent pathways. Deletion of ZAP-70 affected CD2- and CD3-mediated proliferation and cytokine production in a similar way, suggesting that one of the different CD2 pathways converges with a CD3 pathway at or upstream of the activation of ZAP-70.     

Herpesvirus saimiri replaces ZAP-70 for CD3- and CD2-mediated T cell activation.

The protein tyrosine kinase ZAP-70 plays a pivotal role involved in signal transduction through the T cell receptor and CD2. Defects in ZAP-70 result in severe combined immunodeficiency. We report that Herpesvirus saimiri, which does not code for a ZAP-70 homologue, can replace this tyrosine kinase. H. saimiri is an oncogenic virus that transforms human T cells to stable growth based on mutual CD2-mediated activation. Although CD2-mediated proliferation of ZAP-70-deficient uninfected T cells was absent, we could establish H. saimiri-transformed T cell lines from two unrelated patients presenting with ZAP-70 deficiencies. In these cell lines, CD2 and CD3 activation were restored in terms of [Ca(2+)](i), MAPK activation, cytokine production, and proliferation. Activation-induced tyrosine phosphorylation of zeta remained defective. The transformed cells expressed very high levels of the ZAP-70-related kinase Syk. This increased expression was not observed in the primary T cells from the patients and was not due to the transformation by the virus because transformed cell lines established from control T cells did not present this particularity. In conclusion, wild type H. saimiri can restore CD2- and CD3-mediated activation in signaling-deficient human T cells. It extends our understanding of interactions between the oncogenic H. saimiri and the infected host cells.     

Control of TCR-mediated activation of beta 1 integrins by the ZAP-70 tyrosine kinase interdomain B region and the linker for activation of T cells adapter protein

One of the earliest functional responses of T lymphocytes to extracellular signals that activate the Ag-specific CD3/TCR complex is a rapid, but reversible, increase in the functional activity of integrin adhesion receptors. Previous studies have implicated the tyrosine kinase zeta-associated protein of 70 kDa (ZAP-70) and the lipid kinase phosphatidylinositol 3-kinase, in the activation of beta(1) integrins by the CD3/TCR complex. In this report, we use human ZAP-70-deficient Jurkat T cells to demonstrate that the kinase activity of ZAP-70 is required for CD3/TCR-mediated increases in beta(1) integrin-mediated adhesion and activation of phosphatidylinositol 3-kinase. A tyrosine to phenylalanine substitution at position 315 in the interdomain B of ZAP-70 inhibits these responses, whereas a similar substitution at position 292 enhances these downstream signals. These mutations in the ZAP-70 interdomain B region also specifically affect CD3/TCR-mediated tyrosine phosphorylation of residues 171 and 191 in the cytoplasmic domain of the linker for activation of T cells (LAT) adapter protein. CD3/TCR signaling to beta(1) integrins is defective in LAT-deficient Jurkat T cells, and can be restored with expression of wild-type LAT. Mutant LAT constructs with tyrosine to phenylalanine substitutions at position 171 and/or position 191 do not restore CD3/TCR-mediated activation of beta(1) integrins in LAT-deficient T cells. Thus, these studies demonstrate that the interdomain B region of ZAP-70 regulates beta(1) integrin activation by the CD3/TCR via control of tyrosine phosphorylation of tyrosine residues 171 and 191 in the LAT cytoplasmic domain.     

Modification of immunocytochemical ZAP-70 assay for potential clinical application in B-cell chronic lymphocytic leukemia

The ZAP-70 protein is a member of the Syk/ZAP protein tyrosine kinase family, normally expressed in T cells and NK cells but not found in normal, mature B cells. The protein plays a critical role in the initiation of T-cell signaling. Leukemic cells from patients with B-cell chronic lymphocytic leukemia (B-CLL) that expressed nonmutated immunoglobulin V genes were found to express levels of ZAP-70 protein that were comparable to those detected in T cells of healthy adults. The ZAP-70 protein expression can be evaluated by flow cytometry and may be used as a prognostic marker in B-CLL patients. We modified the method of immunocytochemical assessment of ZAP-70 expression. The traditional two-step method with monoclonal anti-ZAP-70 antibody in the first step followed by FITC-conjugated goat anti-mouse IgG was changed for one-step method with monoclonal anti-ZAP-70 antibody labeled by Zenon Alexa Fluor 488. The method is simple and fast. The major advantage of Zenon labeling technique is its compatibility with simultaneous staining of surface antigens. The cells may be earlier immunostained for CD3, CD19 and/or CD5 to compare of the ZAP-70 kinase expression in B and T cells.     

CTLA-4 suppresses proximal TCR signaling in resting human CD4(+) T cells by inhibiting ZAP-70 Tyr(319) phosphorylation: a potential role for tyrosine phosphatases

The balance between positive and negative signals plays a key role in determining T cell function. CTL-associated Ag-4 is a surface receptor that can inhibit T cell responses induced upon stimulation of the TCR and its CD28 coreceptor. Little is known regarding the signaling mechanisms elicited by CTLA-4. In this study we analyzed CTLA-4-mediated inhibition of TCR signaling in primary resting human CD4(+) T cells displaying low, but detectable, CTLA-4 cell surface expression. CTLA-4 coligation with the TCR resulted in reduced downstream protein tyrosine phosphorylation of signaling effectors and a striking inhibition of extracellular signal-regulated kinase 1/2 activation. Analysis of proximal TCR signaling revealed that TCR zeta-chain phosphorylation and subsequent zeta-associated protein of 70 kDa (ZAP-70) tyrosine kinase recruitment were not significantly affected by CTLA-4 engagement. However, the association of p56(lck) with ZAP-70 was inhibited following CTLA-4 ligation, correlating with reduced actions of p56(lck) in the ZAP-70 immunocomplex. Moreover, CTLA-4 ligation caused the selective inhibition of CD3-mediated phosphorylation of the positive regulatory ZAP-70 Y319 site. In addition, we demonstrate protein tyrosine phosphatase activity associated with the phosphorylated CTLA-4 cytoplasmic tail. The major phosphatase activity was attributed to Src homology protein 2 domain-containing tyrosine phosphatase 1, a protein tyrosine phosphatase that has been shown to be a negative regulator of multiple signaling pathways in hemopoietic cells. Collectively, our findings suggest that CTLA-4 can act early during the immune response to regulate the threshold of T cell activation.     

Alternative antigen receptor (TCR) signaling in T cells derived from ZAP-70-deficient patients expressing high levels of Syk

ZAP-70-deficient patients present with nonfunctional CD4+ T cells in the periphery. We find that a subset of primary ZAP-70-deficient T cells, expressing high levels of the related protein-tyrosine kinase Syk, can proliferate in vitro. These cells (denoted herein as Syk(hi)/ZAP-70(-) T cells) provide a unique model in which the contribution of Syk to TCR-mediated responses can be explored in a nontransformed background. Importantly, CD3-induced responses, such as tyrosine phosphorylation of cellular substrates (LAT, SLP76, and PLC-gamma1), as well as calcium mobilization, which are defective in T cells expressing neither ZAP-70 nor Syk, are observed in Syk(hi)/ZAP-70(-) T cells. However, Syk(hi)/ZAP-70(-) T cells differ from control T cells with respect to the T cell antigen receptor (TCR)-mediated activation of the MAPK cascades: extracellular signal-regulated kinase activity and recruitment of the JNK and p38 stress-related MAPK pathways are diminished. This distinct phenotype of Syk(hi)/ZAP-70(-) T cells is associated with a profound decrease in CD3-mediated interleukin 2 secretion and proliferation relative to control T cells. Thus, ZAP-70 and Syk appear to play distinct roles in transducing a TCR-mediated signal.     

Carcinoembryonic antigen-related cell adhesion molecule 1 inhibits proximal TCR signaling by targeting ZAP-70

The long cytoplasmic tail (CT) isoforms of carcinoembryonic Ag-related cell adhesion molecule 1 (CEACAM1) are expressed on activated human T cells and possess two ITIM motifs in the CT. These isoforms of CEACAM1 are inhibitory for T cell responses initiated by the TCR/CD3 complex with the inhibition dependent upon the ITIMs of CEACAM1 and Src homology 2 domain-containing phosphatase 1 (SHP-1). However, the mechanism by which this inhibition occurs in T cells is unknown. We demonstrate here that the Src family kinase, Lck, and the ability of CEACAM1 to bind homophilically are required for the ITIM phosphorylation of CEACAM1 that is a prerequisite for CEACAM1 association with SHP-1. We further show that CEACAM1 associates with and recruits SHP-1 to the TCR/CD3 complex leading to decreased phosphorylation of CD3-zeta and ZAP-70 and consequently decreased activation of the elements downstream of ZAP-70. This is physiologically relevant because extinction of SHP-1 expression or blockade of homophilic binding by CEACAM1 using a Fab that specifically recognizes the homophilic binding region of human CEACAM1 increases the cytolytic function initiated by the TCR/CD3 complex. These studies show that long CT isoforms of CEACAM1 orchestrate an inhibitory program that abrogates extremely proximal events downstream of the TCR/CD3 complex by focusing on the activation of ZAP-70.     

Distinct T cell developmental consequences in humans and mice expressing identical mutations in the DLAARN motif of ZAP-70

The protein tyrosine kinase, ZAP-70, is pivotally involved in transduction of Ag-binding signals from the TCR required for T cell activation and development. Defects in ZAP-70 result in SCID in humans and mice. We describe an infant with SCID due to a novel ZAP-70 mutation, comparable with that which arose spontaneously in an inbred mouse colony. The patient inherited a homozygous missense mutation within the highly conserved DLAARN motif in the ZAP-70 kinase domain. Although the mutation only modestly affected protein stability, catalytic function was absent. Despite identical changes in the amino acid sequence of ZAP-70, the peripheral T cell phenotypes of our patient and affected mice are distinct. ZAP-70 deficiency in this patient, as in other humans, is characterized by abundant nonfunctional CD4(+) T cells and absent CD8(+) T cells. In contrast, ZAP-70-deficient mice lack both major T cell subsets. Although levels of the ZAP-70-related protein tyrosine kinase, Syk, may be sufficiently increased in human thymocytes to rescue CD4 development, survival of ZAP-70-deficient T cells in the periphery does not appear to be dependent on persistent up-regulation of Syk expression.     

Inhibition of ZAP-70 kinase activity via an analog-sensitive allele blocks T cell receptor and CD28 superagonist signaling

ZAP-70 is a cytoplasmic protein tyrosine kinase that is required for T cell antigen receptor (TCR) signaling. Both mice and humans deficient in ZAP-70 fail to develop functional T cells, thus demonstrating its necessity for T cell development and function. There is currently no highly specific, cell-permeable, small molecule inhibitor for ZAP-70; therefore, we generated a mutant ZAP-70 allele that retains kinase activity but is sensitive to inhibition by a mutant-specific inhibitor. We validated the chemical genetic inhibitor system in Jurkat T cell lines, where the inhibitor blocked ZAP-70-dependent TCR signaling in cells expressing the analog-sensitive allele. Interestingly, the inhibitor also ablated CD28 superagonist signaling, thereby demonstrating the utility of this system in dissecting the requirement for ZAP-70 in alternative mechanisms of T cell activation. Thus, we have developed the first specific chemical means of inhibiting ZAP-70 in cells, which serves as a valuable tool for studying the function of ZAP-70 in T cells.     

Dexamethasone induces rapid tyrosine-phosphorylation of ZAP-70 in Jurkat cells

Steroid hormones are known to mediate rapid non-genomic effects occurring within minutes, besides the classical genomic actions mediated by the nuclear translocation of the cytoplasmic glucocorticoid receptor (GR). The glucocorticoid hormone (GC) has significant role in the regulation of T-cell activation; however, the cross-talk between the GC and T-cell receptor (TcR) signal transducing pathways are still to be elucidated. We examined the rapid effects of GC exposure on in vitro cultured human T-cells. Our results showed that Dexamethasone (DX), a GC analogue, when applied at high dose (10 microM), induced rapid (within 5 min) tyrosine-phosphorylation events in Jurkat cells. Short DX pre-treatment strongly inhibited the tyrosine-phosphorylation stimulated by CD3 cross-linking. Furthermore, we also investigated the phosphorylation status of ZAP-70, an important member of tyrosine kinase mediated signalling pathway of TcR-elicited T-cell activation. Here, we demonstrate that high dose DX induced a rapid ZAP-70 tyrosine-phosphorylation in Jurkat T-cells. DX-induced ZAP-70 phosphorylation could be inhibited by RU486 (GR antagonist), suggesting that this process was GR mediated. DX-induced ZAP-70 phosphorylation did not occur in the absence of active p56-lck as examined in the p56-lck kinase-deficient Jurkat cell line JCaM1.6. Our results show that DX, at a high dose, can rapidly influence the initial tyrosine-phosphorylation events of the CD3 signalling pathway in Jurkat cells, thereby modifying TcR-derived signals. Lck and ZAP-70 represent an important molecular link between the TcR and GC signalling pathways.     

The linker phosphorylation site Tyr292 mediates the negative regulatory effect of Cbl on ZAP-70 in T cells

The protooncogene product Cbl has emerged as a negative regulator of tyrosine kinases. We have shown previously that Cbl binds to ZAP-70 through its N-terminal tyrosine kinase binding (TKB) domain. In this study, we demonstrate that overexpression of Cbl in Jurkat T cells decreases the TCR-induced phosphorylation of ZAP-70 and other cellular phosphoproteins. Coexpression of Cbl with ZAP-70 in COS cells reproduced the Cbl-induced reduction in the level of phosphorylated ZAP-70. The effect of Cbl was eliminated by the TKB-inactivating G306E mutation in Cbl as well as by a phenylalanine mutation of Tyr292 within the TKB domain binding site on ZAP-70. Notably, the oncogenic Cbl-70Z/3 mutant associated with ZAP-70, but did not reduce the levels of phosphorylated ZAP-70. Overexpression of Cbl, but not Cbl-G306E, in Jurkat T cells led to a decrease in the TCR-induced NF-AT luciferase reporter activity. Overexpression of the TKB domain itself, but not its G306E mutant, functioned in a dominant-negative manner and led to an increase in NF-AT reporter activity. Cbl-70Z/3-overexpressing cells exhibited an increase in both basal and TCR-induced NF-AT luciferase reporter activity, and this trend was reversed by the G306E mutation. Finally, by reconstituting a ZAP-70-deficient Jurkat T cell line, p116, we demonstrate that wild-type ZAP-70 is susceptible to the negative regulatory effect of Cbl, whereas the ZAP-70-Y292F mutant is resistant. Together, our results establish that the linker phosphorylation site Tyr292 mediates the negative regulatory effect of Cbl on ZAP-70 in T cells.     

CXCL12 is a costimulator for CD4+ T cell activation and proliferation in chronic lymphocytic leukemia patients

Activated T cells from patients with chronic lymphocytic leukemia (CLL) provide survival and proliferative signals to the leukemic clone within lymphoid tissues. Recruitment of both, CLL cells and T lymphocytes, to this supportive microenvironment greatly depends on CXCL12 production by stromal and myeloid cells. CXCL12 also supplies survival stimuli to leukemic B cells, but whether it exerts stimulatory effects on T lymphocytes from CLL patients is unknown. In order to evaluate the capacity of CXCL12 to increase CD4⁺ T cell activation and proliferation in CLL patients, peripheral blood mononuclear cells were cultured with or without recombinant human CXCL12 or autologous nurse-like cells, and then T cell activation was induced by anti-CD3 mAb. CXCL12 increases the proliferation and the expression of CD25, CD69, CD154, and IFNγ on CD3-stimulated CD4⁺ T cells from CLL patients, similarly in T cells from ZAP-70⁺ to ZAP-70^? patients. Autologous nurse-like cells establish a close contact with CD4⁺ T cells and increase their activation and proliferation partially through a CXCR4-dependent mechanism. In addition, we found that activated T cells in the presence of CXCL12 enhance the activation and proliferation of the leukemic clone. In conclusion, CXCL12 production by lymphoid tissue microenvironment in CLL patients might play a key dual role on T cell physiology, functioning not only as a chemoattractant but also as a costimulatory factor for activated T cells.     

Interaction of human MUC1 and beta-catenin is regulated by Lck and ZAP-70 in activated Jurkat T cells

The MUC1 transmembrane glycoprotein is aberrantly expressed by diverse hematologic malignancies, including those of the T cell lineage. The MUC1 cytoplasmic domain (CD) interacts with beta-catenin; however, the role of MUC1 in T cells is not known. In the present work, MUC1 was studied as a potential downstream effector of the Lck and ZAP-70 tyrosine kinases that are essential for T cell activation. The results demonstrate that anti-CD3-induced or PMA+ionomycin-induced activation of Jurkat T cells is associated with increased binding of MUC1 and Lck. Lck phosphorylates MUC1-CD on Y-46 and, in turn, stimulates the binding of MUC1 to beta-catenin. The results further demonstrate that MUC1 interacts with ZAP-70. In contrast to Lck, ZAP-70 phosphorylates MUC1-CD predominantly on Y-20. However, like Lck, ZAP-70-mediated phosphorylation of MUC1 Y-20 stimulates binding of MUC1 and beta-catenin. These findings indicate that MUC1 functions as a substrate for Lck and ZAP-70 in activated Jurkat T cells and that MUC1 integrates T cell receptor signaling with the beta-catenin pathway.