The lck tyrosine protein kinase interacts with the cytoplasmic tail of the CD4 glycoprotein through its unique amino-terminal domain

Foreign glycoproteins expressed from recombinant vesicular stomatitis viruses are incorporated efficiently into virus particles

In a previous study we demonstrated that vesicular stomatitis virus (VSV) can be used as a vector to express a soluble protein in mammalian cells. Here we have generated VSV recombinants that express four different membrane proteins: the cellular CD4 protein, a CD4-G hybrid protein containing the ectodomain of CD4 and the transmembrane and cytoplasmic tail of the VSV glycoprotein (G), the measles virus hemagglutinin, or the measles virus fusion protein. The proteins were expressed at levels ranging from 23-62% that of VSV G protein and all were transported to the cell surface. In addition we found that all four proteins were incorporated into the membrane envelope of VSV along with the VSV G protein. The levels of incorporation of these proteins varied from 6-31% of that observed for VSV G. These results suggest that many different membrane proteins may be co-incorporated quite efficiently with VSV G protein into budding VSV virus particles and that specific signals are not required for this co-incorporation process. In fact, the CD4-G protein was incorporated with the same efficiency as wild type CD4. Electron microscopy of virions containing CD4 revealed that the CD4 molecules were dispersed throughout the virion envelope among the trimeric viral spike glycoproteins. The recombinant VSV-CD4 virus particles were about 18% longer than wild type virions, reflecting the additional length of the helical nucleocapsid containing the extra gene. Recombinant VSVs carrying foreign antigens on the surface of the virus particle may be useful for viral targeting, membrane protein purification, and for generation of immune responses.

Human immunodeficiency virus type 1 Nef binds directly to Lck and mitogen-activated protein kinase, inhibiting kinase activity

It is now well established that human immunodeficiency virus type I (HIV-1) Nef contributes substantially to disease pathogenesis by augmenting virus replication and markedly perturbing T-cell function. The effect of Nef on host cell activation could be explained in part by its interaction with specific cellular proteins involved in signal transduction, including at least a member of the src family kinase, Lck, and the serine/threonine kinase, mitogen-activated protein kinase (MAPK). Recombinant Nef directly interacted with purified Lck and MAPK in coprecipitation experiments and binding assays. A proline-rich repeat sequence [(Pxx)4] in Nef occurring between amino acid residues 69 to 78 is highly conserved and bears strong resemblance to a defined consensus sequence identified as an SH3 binding domain present in several proteins which can interact with the SH3 domain of various signalling and cytoskeletal proteins. Binding and coprecipitation assays with short synthetic peptides corresponding to the proline-rich repeat sequence [(Pxx)4] of Nef and the SH2, SH3, or SH2 and SH3 domains of Lck revealed that the interaction between these two proteins is at least in part mediated by the proline repeat sequence of Nef and the SH3 domain of Lck. In addition to direct binding to full-length Nef, MAPK was also shown to bind the same proline repeat motif. Nef protein significantly decreased the in vitro kinase activity of Lck and MAPK. Inhibition of key members of signalling cascades, including those emanating from the T-cell receptor, by the HIV-1 Nef protein undoubtedly alters the ability of the infected T cell to respond to antigens or cytokines, facilitating HIV-1 replication and contributing to HIV-1-induced disease pathogenesis.

HIV type 1 glycoprotein 120 amplifies tumor necrosis factor-induced NF-kappa B activation in Jurkat cells

This article demonstrates that human immunodeficiency virus type 1 (HIV-1) gp120 amplifies the activity of tumor necrosis factor alpha (TNF-alpha), a cytokine that stimulates HIV-1 replication through activation of NF-kappa B. In CD4-positive Jurkat cells, gp120 potentiates TNF-induced NF-kappa B activation. TNF-mediated activation of NF-kappa B is known to involve the intracellular formation of reactive oxygen intermediates (ROIs). Accordingly, we examined the influence of gp120 on the cellular redox state. We found that gp 120-modulated TNF-induced NK-kappa B activation was inhibited by the antioxidant butylated hydroxyanisole, indicating the involvement of redox-dependent mechanisms. In addition, we showed that gp120 induces intracellular formation of hydrogen peroxide, which is accompanied by a decrease in the ratio of glutathione to glutathione disulfide. In contrast, in the p56lck-deficient J.CaM1.6 T cell line, a derivative of the Jurkat cell line, gp120 was unable to stimulate hydrogen peroxide, to decrease the ratio of GSH to GSSG, and has no effect on TNF-induced NF-kappa B activation. This demonstrated that p56lck protein tyrosine kinase plays an active role in transmitting a signal that increases the oxidative state of the cell and as a consequence amplifies TNF-mediated NF-kappa B DNA binding. We have demonstrated that Tat protein decreased both the Mn-dependent superoxide dismutase (MnSOD) and the cellular glutathione content (GSH). Here we show that, in contrast to Tat, gp120 is unable to inhibit activity and expression of MnSOD and to decrease GSH content. Taken together, our data suggest that gp120 potentiates TNF-induced NF-kappa B activation by stimulating a signal pathway that involves p56lck and the increased formation of reactive oxygen intermediates such as H2O2. These findings may be relevant for the regulation of HIV-1 replication in T cells.

Requirement for p56lck tyrosine kinase activation in T cell receptor-mediated thymic selection

The nonreceptor protein tyrosine kinase p56lck (Lck) serves as a fundamental regulator of thymocyte development by delivering signals from the pre-T cell receptor (pre-TCR) that permit subsequent maturation. However, considerable evidence supports the view that Lck also participates in signal transduction from the mature TCR. We have tested this conjecture by expressing a dominant-negative form of Lck under the control of a promoter element (the distal lck promoter) that directs high expression in CD4+CD8+ thymocytes, mature thymocytes, and peripheral T cells, thereby avoiding, complications that result from the well-documented ability of dominant-negative Lck to block very early events in thymocyte maturation. Here we report that expression of the catalytically inactive Lck protein at twice normal concentrations inhibits thymocyte positive selection by as much as 80%, while leaving other aspects of cell maturation intact. This effect was studied in more detail in mice simultaneously bearing the male-specific H-Y alpha/beta TCR transgene and ovalbumin-specific DO10 alpha/beta TCR transgene, where even equimolar expression of the dominant-negative Lck protein substantially vitiated the positive selection process. Although deletion of H-Y alpha/beta thymocytes proceeded normally in male mice despite the presence of catalytically inactive Lck, modest inhibition of superantigen-mediated deletion was in some cases observed. These data further implicate Lck in the propagation of all TCR-derived signals, and indicate that even very modest deficiencies in the representation of functional Lck molecules could in humans, profoundly alter the character of the peripheral TCR repertoire.

Generation of lymphocyte cell lines coexpressing CD4 and wild-type or mutant HIV type 1 glycoproteins: implications for HIV type 1 Env-induced cell lysis

To gain more insight into the processes leading to HIV-1 Env-induced cell death, we aim to coexpress stably wild-type and relevant mutant variants of both HIV-1 Env and human CD4 in lymphocyte cell lines. Here we report on the generation and characterization of several cell lines inducibly or constitutively expressing wild-type or cleavage-defective HIV-1 glycoproteins and human CD4 either singly or in combination. Coexpression of CD4 and wild-type Env led to the formation of multinucleated syncytia, to growth arrest and cell death, effects that all could be prevented by cultivation in the presence of monoclonal antibodies that inhibit cell surface membrane fusion. Cell lines coexpressing CD4 and mutated, noncleavable Env, detectable at the cell surface and still retaining CD4-binding capacity, were not retarded in their growth and cytolysis did not occur. These results indicate that cell lysis requires cell surface interaction of CD4 and gp120/41 and cleavage of gp160 to gp120 and gp41.

Regulation, substrates and functions of src

Src is the best understood member of a family of 9 tyrosine kinases that regulates cellular responses to extracellular stimuli. Activated mutants of Src are oncogenic. Using Src as an example, and referring to other Src family members where appropriate, this review describes the structure of Src, the functions of the individual domains, the regulation of Src kinase activity in the cell, the selection of substrates, and the biological functions of Src. The review concentrates on developments in the last 6-7 years, and cites data resulting from the isolation and characterization of Src mutants, crystallographic studies of the structures of SH2, SH3 and tyrosine kinase domains, biochemical studies of Src kinase activity and binding properties, and the biology of transgenic and knockout mouse strains.

Multiple features of the p59fyn src homology 4 domain define a motif for immune-receptor tyrosine-based activation motif (ITAM) binding and for plasma membrane localization

The src family tyrosine kinase p59fyn binds to a signaling motif contained in subunits of the TCR known as the immune-receptor tyrosine-based activation motif (ITAM). This is a specific property of p59fyn because two related src family kinases, p60src and p56lck, do not bind to ITAMs. In this study, we identify the residues of p59fyn that are required for binding to ITAMs. We previously demonstrated that the first 10 residues of p59fyn direct its association with the ITAM. Because this region of src family kinases also directs their fatty acylation and membrane association (Resh, M.D. 1993, Biochim. Biophys. Acta 1155:307-322; Resh, M.D. 1994. Cell. 76:411-413), we determined whether fatty acylation and membrane association of p59fyn correlates with its ability to bind ITAMs. Four residues (Gly2, Cys3, Lys7, and Lys9) were required for efficient binding of p59fyn to the TCR. Interestingly, the same four residues are present in p56lyn, the other src family tyrosine kinase known to bind to the ITAM, suggesting that this set of residues constitutes an ITAM recognition motif. These residues were also required for efficient fatty acylation (myristoylation at Gly2 and palmitoylation at Cys3), and plasma membrane targeting of p59fyn. Thus, the signals that direct p59fyn fatty acylation and plasma membrane targeting also direct its specific ability to bind to TCR proteins.

Repression of human immunodeficiency virus type 1 long terminal repeat-driven gene expression by binding of the virus to its primary cellular receptor, the CD4 molecule

We have previously postulated that the binding of the human immunodeficiency virus type 1 (HIV-1) to cell surface CD4 induces signal transduction pathways that down-modulate production of progeny virions in acutely infected T cells (M. Tremblay, S. Meloche, S. Gratton, M. A. Wainberg, and R.-P. Sékaly, EMBO J. 13:774-783, 1994). To evaluate the possibility that CD4 cross-linking might indeed affect viral gene expression, we have introduced a molecular construct made of the luciferase reporter gene placed under the control of the regulatory elements of HIV-1 in several CD4-positive T-cell lines. We found that cross-linking of CD4 with defective HIV-1 particles and heat-inactivated viruses inhibits long terminal repeat-dependent luciferase expression. Experiments revealed that the gp120-CD4 interaction was necessary to repress HIV-1 long terminal repeat-dependent luciferase activity. The cytoplasmic domain of CD4 was also found to be required for this effect to occur. The virus-mediated signal transduction was shown to be mediated via p56lck-dependent and -independent pathways. These results indicate that the earliest event in the HIV-1 replicative cycle, namely, the binding of the virus to its cellular receptor, can lead to signal transduction culminating in down-modulation of viral gene expression. Thus we propose that defective viruses could regulate the pathogenesis of HIV disease as they constitute the vast majority of circulating HIV-1 particles.

TCR activation of ZAP70 is impaired in CD4+CD8+ thymocytes as a consequence of intrathymic interactions that diminish available p56lck

The fate of developing CD4+CD8+ thymocytes is determined by signals transduced through surface TCR complexes. Here, we report that cross-linking of TCR on CD4+ CD8+ thymocytes fails to activate ZAP70 protein tyrosine kinase and fails to initiate downstream signaling events, unless the TCR are coaggregated with surface coreceptor molecules. TCR signaling in CD4+CD8+ thymocytes is impaired because the number of available p56lck molecules is diminished by intrathymic CD4-Ia interactions that initially activate p56lck molecules, which are subsequently degraded. As a consequence of intrathymic CD4-Ia interactions, TCR zeta chains are initially phosphorylated to recruit ZAP70 molecules, but the recruited ZAP70 molecules are not subsequently phosphorylated, resulting in TCR complexes that are stably associated with inactive ZAP70 molecules. Thus, intrathymic interactions that diminish p56lck regulate TCR signaling thresholds and affect TCR structure in developing CD4+CD8+ thymocytes.

Discovery of a novel, potent, and Src family-selective tyrosine kinase inhibitor. Study of Lck- and FynT-dependent T cell activation

Here, we have studied the activity of a novel protein-tyrosine kinase inhibitor that is selective for the Src family of tyrosine kinases. We have focused our study on the effects of this compound on T cell receptor-induced T cell activation, a process dependent on the activity of the Src kinases Lck and FynT. This compound is a nanomolar inhibitor of Lck and FynT, inhibits anti-CD3-induced protein-tyrosine kinase activity in T cells, demonstrates selectivity for Lck and FynT over ZAP-70, and preferentially inhibits T cell receptor-dependent anti-CD3-induced T cell proliferation over non-T cell receptor-dependent phorbol 12-myristate 13-acetate/interleukin-2 (IL-2)-induced T cell proliferation. Interestingly, this compound selectively inhibits the induction of the IL-2 gene, but not the granulocyte-macrophage colony-stimulating factor or IL-2 receptor genes. This compound offers a useful new tool for examining the role of the Lck and FynT tyrosine kinases versus ZAP-70 in T cell activation as well as the role of other Src family kinases in receptor function.

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.

HIV-induced apoptosis requires the CD4 receptor cytoplasmic tail and is accelerated by interaction of CD4 with p56lck

The roles of the CD4 receptor and the src kinase p56lck were examined in the process of HIV-induced apoptosis of CD4+ T lymphocytes. The presence of the CD4 cytoplasmic tail was found to be essential in delivering an apoptotic signal, and interaction of CD4 with p56lck potentiated HIV-induced apoptosis. Apoptosis, but not HIV replication, was abrogated by deleting the NH2-terminal intracytoplasmic tail of CD4, or by mutating the two critical cysteines in this tail that are responsible for CD4-p56lck interaction. Introduction of p56lck in C8166-45 or MT-2 cells, CD4+ T cell lines deficient for this protein, greatly increased HIV-induced apoptosis and syncytium formation. The ability of p56lck to deliver an apoptotic signal did not depend on its kinase function, since a kinase-deficient mutant was as effective as its normal counterpart in inducing apoptosis, suggesting that p56lck may act as an adapter to anchor other proteins to transduce the death signal.

Downregulation of Lck-mediated signal transduction by tip of herpesvirus saimiri

A protein, called tip, of herpesvirus saimiri associates with Lck in transformed T cells. To investigate the effects of complex formation on cellular signal transduction, we constructed human Jurkat-T-cell lines expressing tip. The expression of tip in Jurkat-T cells dramatically suppressed cellular tyrosine phosphorylation and surface expression of lymphocyte antigens. The expression of tip also blocked the induction of tyrosine phosphorylation by anti-CD3 stimulation. The expression of tip in fibroblast cells suppressed the transforming activity of oncogenic F505 Lck. Binding assays showed that the SH3 domain of Lck is sufficient to form a stable complex with tip in vitro. These results demonstrate that tip acts at an early stage of the T-cell signal transduction cascade by associating with Lck and downregulating Lck-mediated activation. Inhibition of Lck-mediated signal transduction by tip in T cells appears to be analogous to the inhibition of Lyn/Syk-mediated signal transduction in B cells by LMP2A of the B-cell-tropic Epstein-Barr virus.

Identification of a novel repressive element in the proximal lck promoter

The T-cell-specific protooncogene lck, a src-related tyrosine kinase, is under the control of two promoters that give rise to transcripts differing only in their 5'-untranslated regions. The distal promoter is transcriptionally active in both peripheral and thymic T-cells, whereas expression of the proximal promoter is highest in developing thymocytes. The proximal promoter has also been shown to be selectively activated in a number of colon carcinoma cell lines. Approximately 570 base pairs of proximal promoter sequence is required for expression in both T-cells and colon carcinoma cell lines. Protein binding studies were initiated with an oligonucleotide homologous to a region that, when deleted, causes an increase in promoter activity in transgenic animals. Two proteins with approximate molecular masses of 35 and 75 kDa were found to bind to this region as determined by UV cross-linking studies. Absence of specific protein binding is correlated with a high level of proximal promoter expression. Competitive gel retardation analysis identified a 9-base pair binding site within the proximal lck promoter that is necessary for repression of transcription in cells that contain specific binding activity. Mutants of this binding site do not repress transcription. Repression does not occur in a cell line that expresses lck and lacks this activity. These data support the hypothesis that activation of lck transcription in colon carcinoma is due, at least in part, to the loss of a transcriptional repressor.

The kinase-dependent function of Lck in T-cell activation requires an intact site for tyrosine autophosphorylation

The cytoplasmic protein tyrosine kinase p56lck (Lck) has important signaling roles in T-cell development and activation. We have mutated the two known regulatory tyrosine residues of CD4-associated Lck and examined the effects on its kinase-dependent function in an antigen-specific CD4-dependent T-cell hybridoma. Substitution of phenylalanine for the negative regulatory tyrosine-505 within a CD4/Lck chimera resulted in a slightly increased response to antigen, whereas mutation of the major in vitro autophosphorylation site (tyrosine-394) completely abolished the kinase-dependent function of Lck. Even though its kinase activity was only slightly affected, the F394 mutant behaved similarly to a catalytically inactive chimeric protein. Cross-linking of the F505 mutant, but not of wild-type Lck or F394 mutants, resulted in tyrosine phosphorylation of multiple cellular proteins. Although the pattern of tyrosine phosphorylation resembled that observed upon T-cell receptor cross-linking, there was no induction of interleukin-2 synthesis upon cross-linking of the chimeric protein. These results suggest that the activity of the Lck kinase domain in vivo is controlled by dephosphorylation at the negative regulatory site and phosphorylation at the positive regulatory (autophosphorylation) site. Additionally, our data show that the specific kinase activity of Lck towards an artificial substrate need not correlate with its ability to phosphorylate cellular proteins or its biological function.

Role of tyrosine kinases in lymphocyte activation: targets for drug intervention

Recent developments in our understanding of lymphocyte receptor-associated signalling events have offered many new potential targets for modifying antigen and cytokine receptor signalling events in immune-related diseases such as allergy, autoimmunity and transplant rejection. As discussed below, these targets are largely tissue-restricted and are functionally confined to a limited set of receptors. Therefore, it is anticipated that selective inhibitors of these signalling events would offer safe and effective therapies for immunologically-based diseases. First, we review T and B cell antigen receptor signalling as targets for inhibiting lymphocyte responses. Second, targets in lymphocyte cytokine receptor signalling pathways are discussed. Finally, we review strategies for inhibition of receptor signalling.

Identification of Lck-binding elements in tip of herpesvirus saimiri

A protein called Tip (tyrosine kinase interacting protein) of herpesvirus saimiri associates with Lck in virus-transformed human T cells and is an in vitro substrate for Lck kinase. Mutational analyses of a GST-Tip fusion protein revealed that binding to Lck requires putative SH3 binding sequences and a sequence homologous to the carboxyl terminus of Src-related kinases. These sequences are referred to as SH3-Binding (SH3B) and C-terminal Src-related Kinase Homology (CSKH) elements. Peptide fragments as short as 37 amino acids containing both SH3B and CSKH elements were sufficient to form a stable complex with Lck in vitro. Furthermore, these same sequences of Tip were necessary for in vivo association with Lck when Tip and Lck were expressed transiently in COS-1 cells or stably in Rat-1 cell lines. These results demonstrate that the CSKH element of Tip participates in the binding of sequences within Lck. Tip of herpesvirus saimiri has apparently acquired such CSKH and SH3B elements for the purpose of targeting cellular protein kinases. The interaction of Tip with Lck may influence Lck kinase activity or its binding to other cellular proteins and thereby alter Lck function in T cells infected by h. saimiri.

CD4/major histocompatibility complex class II interaction analyzed with CD4- and lymphocyte activation gene-3 (LAG-3)-Ig fusion proteins

We analyzed CD4 major histocompatibility complex (MHC) class II interactions with CD4 and lymphocyte activation gene (LAG)-3 recombinant fusion proteins termed CD4Ig and LAG-3Ig. CD4Ig bound MHC class II molecules expressed on the cell surface only when used in the micromolar range. This weak CD4Ig binding was specific, since it was inhibited by anti-CD4 and anti-MHC class II mAb. LAG-3Ig bound MHC class II molecules with intermediate avidity (Kd = 60 nM at 37 degrees C). Using LAG-3Ig as a competitor in a CD4/MHC class II-dependent cellular adhesion assay, we showed that this recombinant molecule was able to block CD4/MHC class II interaction. In contrast, no inhibition was observed in a CD4/MHC class II-dependent T cell cytotoxicity assay. Together, these results suggest that co-engagement of the TcR with CD4 alters the CD4/MHC class II molecular interaction to become insensitive to LAG-3Ig competition.

Interaction between lck and syk family tyrosine kinases in Fc gamma receptor-initiated activation of natural killer cells

Ligation of the Fc gamma R on natural killer (NK) cells results in the tyrosine phosphorylation of multiple substrates critical for intracellular signaling and activation of NK cell effector functions. However, it remains unclear which nonreceptor protein-tyrosine kinases (PTK) participate in this process. In this report we demonstrate that Fc gamma R ligation induced the tyrosine phosphorylation and increased the catalytic activities of both syk family PTKs, ZAP-70, and syk. The phosphorylation of ZAP-70 and syk was enhanced markedly by overexpression of wild-type lck but not by a kinase-inactive mutant, suggesting that early Fc gamma R-initiated activation of lck results in the subsequent regulation of syk family PTKs. The regulatory interplay between src and syk family PTKs was emphasized further by the observation that lck overexpression enhanced the association of ZAP-70 with the zeta chain of the Fc gamma R complex. Additional analyses indicated that lck induced the subsequent tyrosine phosphorylation of phospholipase C (PLC)-gamma 2. Interestingly, the regulatory effects of lck on ZAP-70, syk, and PLC-gamma 2 could not be replaced by overexpression of either fyn or src, demonstrating a selective role for lck in effectively coupling Fc gamma R stimulation to critical downstream signaling events. Taken together, our results suggest not only that Fc gamma R stimulation on NK cells is coupled to the intracellular activation of both ZAP-70 and syk, but that the src family member, lck, can selectively regulate this tyrosine kinase cascade.

Detection of CD4+ T cells harboring human immunodeficiency virus type 1 DNA by flow cytometry using simultaneous immunophenotyping and PCR-driven in situ hybridization: evidence of epitope masking of the CD4 cell surface molecule in vivo

Human immunodeficiency virus type 1 (HIV-1) infection of T cells and cells of the monocyte/macrophage lineage requires a specific interaction between the CD4 antigen expressed on the cell surface and the HIV-1 external envelope glycoprotein (gp120). To study the association between HIV-1 infection and modulation of cell surface expression of the CD4 molecule in vivo, we examined the CD4+ T cells harboring proviral DNA obtained from HIV-1-infected individuals who had received no antiretroviral therapy for at least 90 days. Simultaneous immunophenotyping of CD4 cell surface expression and PCR-driven in situ hybridization for HIV-1 DNA were used to resolve the CD4+ T cells into distinct populations predicted upon the presence or absence of proviral DNA. Among the HIV-1-infected study subjects, the percentage of CD4+ T cells harboring proviral DNA ranged from 17.3 to 55.5%, with a mean of 40.5%. Cell surface fluorescent staining with anti-CD4 antibody directed against a non-gp120 binding site-related epitope (L120) or a conformation-dependent epitope of the gp120 binding site (Leu 3A) demonstrated either an equivalent or a 1.5- to 3-fold-lower cell surface staining intensity for the HIV-1 DNA-positive subpopulation relative to the HIV-1 DNA-negative subpopulation, respectively. These data suggest that masking or alteration of specific epitopes on the CD4 molecule occurs after viral infection.

Binding of ZAP-70 to phosphorylated T-cell receptor zeta and eta enhances its autophosphorylation and generates specific binding sites for SH2 domain-containing proteins

ZAP-70 is a protein tyrosine kinase thought to play a critical role in T-cell receptor (TCR) signal transduction. During T-cell activation, ZAP-70 binds to a conserved signalling motif known as the immune receptor tyrosine activating motif (ITAM) and becomes tyrosine phosphorylated. To determine whether binding of ZAP-70 to the phosphorylated ITAM was able to activate its kinase activity, we measured the kinase activity of ZAP-70 both when it was bound and when it was unbound to phosphorylated TCR subunits. The ability of ZAP-70 to phosphorylate itself, but not exogenous substrates, was enhanced when it was bound to the tyrosine-phosphorylated TCR zeta and eta chains or to a construct that contained duplicated epsilon ITAMs. No enhanced ZAP-70 autophosphorylation was noted when it was bound to tyrosine-phosphorylated CD3 gamma or epsilon. In addition, autophosphorylation of ZAP-70 when bound to zeta or eta resulted in the generation of multiple distinct ZAP-70 phosphorylated tyrosine residues which had the capacity to bind the SH2 domains of fyn, lck, GAP, and abl. As the effect was noted only when ZAP-70 was bound to TCR subunits containing multiple ITAMs, we propose that one of the roles of the tandem ITAMs is to facilitate the autophosphorylation of ZAP-70. Tyrosine-phosphorylated ZAP-70 then mediates downstream signalling by recruiting SH2 domain-containing signalling proteins.

The unique amino-terminal domain of p56lck regulates interactions with tyrosine protein phosphatases in T lymphocytes

The catalytic activity of p56lck is repressed by phosphorylation of a conserved carboxy-terminal tyrosine residue (tyrosine 505). Accumulating data show that this phosphorylation is mediated by the tyrosine protein kinase p50csk and that it is reversed by the transmembrane tyrosine protein phosphatase CD45. Recent studies have indicated that dephosphorylation of tyrosine 505 in resting T cells is necessary for the initiation of antigen-induced T-cell activation. To better understand this phenomenon, we have characterized the factors regulating tyrosine 505 phosphorylation in an antigen-specific T-cell line (BI-141). As is the case for other T-cell lines, Lck molecules from unstimulated BI-141 cells exhibited a pronounced dephosphorylation of the inhibitory carboxyl-terminal tyrosine. This state could be corrected by incubation of cells with the tyrosine protein phosphatase inhibitor pervanadate, suggesting that it reflected the unrestricted action of tyrosine protein phosphatases. In structure-function analyses, mutation of the site of Lck myristylation (glycine 2) partially restored phosphorylation at tyrosine 505 in BI-141 cells. Since the myristylation-defective mutant also failed to stably associate with cellular membranes, this effect was most probably the consequence of removal of p56lck from the vicinity of membrane phosphatases like CD45. Deletion of the unique domain of Lck, or its replacement by the equivalent sequence from p59fyn, also increased the extent of tyrosine 505 phosphorylation in vivo. This effect was unrelated to changes in Lck membrane association and therefore was potentially related to defects in crucial protein-protein interactions at the membrane. In contrast, deletion of the SH3 or SH2 domain, or mutation of the phosphotransfer motif (lysine 273) or the site of autophosphorylation (tyrosine 394), had no impact on phosphate occupancy at tyrosine 505. In combination, these results indicated that the hypophosphorylation of the inhibitory tyrosine of p56(lck) in T lymphocytes is likely the result of the predominant action of tyrosine protein phosphatases. Moreover, they showed that both the amino-terminal myristylation signal and the unique domain of p56(lck) play critical roles in this process.

The conserved box 1 motif of cytokine receptors is required for association with JAK kinases

The JAK2 tyrosine kinase is known to associate with the receptors for growth hormone (GH) and erythropoietin (EPO) and with the interleukin-6 receptor signal transducing protein, gp130. Here we demonstrate that chimeric cytokine receptors which contain the cytoplasmic domain of the receptors for GH and EPO or for gp130 can form complexes with JAK2 when transiently co-expressed in HeLa cells. Mutational analyses of chimeras for the the GH and EPO receptors and gp130 demonstrated that box 1, a motif critical for cytokine receptor signal transduction, was required for the association of JAK2. Although JAK2 was capable of associating with all three of the chimeras, JAK1 co-precipitated only with the gp130 chimera. Association of JAK1 and JAK2 with cytokine receptor proteins, therefore, requires the highly conserved box 1 domain, but other sequences within the receptor proteins may influence the specificity of JAK binding. Mutational analysis of JAK2 revealed that multiple or complex protein sequences within JAK2 are required for association with cytokine receptors.

The role of p56lck in CD4-mediated suppression of CD3-induced early signaling events in T lymphocytes

Crosslinking of the CD4 coreceptor can inhibit subsequent T-cell activation via the T-cell antigen receptor (TCR)/CD3 complex. The ability of the human immunodeficiency virus (HIV) envelope protein, gp 120, to cause similar inhibition has implicated this inhibitory signal in the induction of T-cell anergy and apoptosis observed in the acquired immunodeficiency syndrome (AIDS). In order to clarify the biochemical basis of this inhibition, we analyzed the effect of CD4 ligation on early signaling events induced by subsequent CD3xCD4 co-crosslinking. By comparison with CD3 crosslinking alone, CD3xCD4 co-crosslinking of a CD3+CD4+ human T-cell leukemia line (SupT1) resulted in an enhanced increase in free intracellular calcium concentration and tyrosine phosphorylation of several cellular substrates, including the prominent phosphorylation of an unidentified 120-kDa protein (p120). Prior CD4 ligation inhibited these responses. Similar results were obtained with A3.01, another CD3+CD4+ T leukemic line. However, P120 was only minor phosphorylated on tyrosine upon receptor crosslinking in A2.01/CD4(-cyt401), a derivative line expressing a truncated CD4 coreceptor lacking its cytoplasmic domain which binds the p56lck protein tyrosine kinase (PTK). Furthermore, prior CD4 ligation failed to inhibit in this line the increased tyrosine phosphorylation induced by subsequent CD3xCD4 co-crosslinking. Thus, prior CD4 crosslinking, or expression of truncated CD4, are both associated with reduced p120 phosphorylation. These results suggest that p120 is a p56lck substrate playing an important role during T-cell activation.

The human immunodeficiency virus type 1 (HIV-1) CD4 receptor and its central role in promotion of HIV-1 infection

Interactions between the viral envelope glycoprotein gp120 and the cell surface receptor CD4 are responsible for the entry of human immunodeficiency virus type 1 (HIV-1) into host cells in the vast majority of cases. HIV-1 replication is commonly followed by the disappearance or receptor downmodulation of cell surface CD4. This potentially renders cells nonsusceptible to subsequent infection by HIV-1, as well as by other viruses that use CD4 as a portal of entry. Disappearance of CD4 from the cell surface is mediated by several different viral proteins that act at various stages through the course of the viral life cycle, and it occurs in T-cell lines, peripheral blood CD4+ lymphocytes, and monocytes of both primary and cell line origin. At the cell surface, gp120 itself and in the form of antigen-antibody complexes can trigger cellular pathways leading to CD4 internalization. Intracellularly, the mechanisms leading to CD4 downmodulation by HIV-1 are multiple and complex; these include degradation of CD4 by Vpu, formation of intracellular complexes between CD4 and the envelope precursor gp160, and internalization by the Nef protein. Each of the above doubtless contributes to the ultimate depletion of cell surface CD4, although the relative contribution of each mechanism and the manner in which they interact remain to be definitively established.

Lck unique domain influences Lck specificity and biological function

Src-family tyrosine kinases share structural and amino acid sequence homology, particularly in the catalytic domain as well as in the SH2 and SH3 domains of the regulatory region. However, each src-family member also contains a unique domain which is specific to and characteristic of each individual tyrosine kinase. These unique or specific domains may contribute to the functional specificity of each src-family kinase. To address this possibility, we analyzed the kinase activities and substrate specificities of the lymphoid src-kinase, pp56lck, and a mutant of pp56lck lacking its specific domain. Our data show that both the wild type enzyme and the specific domain-deleted mutant displayed similar affinities for ATP and the non-physiological substrate denatured enolase. However, the specific domain-deleted mutant failed to phosphorylate a number of physiological substrates of pp56lck. In addition, the ability of pp56lck to mediate induction of the interleukin-2 promoter was strongly impaired upon deletion of its specific domain. Thus, the unique domain is not required for the intrinsic kinase activity of pp56lck, however, it influences substrate preference and contributes to the unique physiological function of this src-family tyrosine kinase.

Initiation of signal transduction by receptor aggregation: role of nonreceptor tyrosine kinases

Receptors which induce immune system effector function bear similar intracellular sequences and respond to aggregation through a nonreceptor tyrosine kinase-dependent pathway. The mechanism by which receptor aggregation leads to cell activation is poorly understood, but recent experiments with chimeric receptors and kinases have begun to simplify the analysis.

The function of the CD4 coreceptor in the development of T cells

T cells with helper activity can be found in mice that lack expression of the CD4 glycoprotein. The CD4 promoter is active in these cells; they respond to antigens presented by MHC class II molecules; they do not express CD8 and they do not depend on MHC class I for their development. By such criteria, these CD8- T cells resemble normal CD4+ helper T cells. The development of the helper lineage in CD4-null mice can be potentiated by expression of transgenes that encode either wild type CD4, or a deletion mutant of CD4 that lacks the cytoplasmic tail and therefore cannot interact with the tyrosine kinase p56lck. These observations suggest that CD4 is not absolutely required for the specification of the helper cell lineage. The role of the CD4 molecule in the development of T cells and possible mechanisms by which it achieves its functions are discussed.

Association of p62, a multifunctional SH2- and SH3-domain-binding protein, with src family tyrosine kinases, Grb2, and phospholipase C gamma-1

src family tyrosine kinases contain two noncatalytic domains termed src homology 3 (SH3) and SH2 domains. Although several other signal transduction molecules also contain tandemly occurring SH3 and SH2 domains, the function of these closely spaced domains is not well understood. To identify the role of the SH3 domains of src family tyrosine kinases, we sought to identify proteins that interacted with this domain. By using the yeast two-hybrid system, we identified p62, a tyrosine-phosphorylated protein that associates with p21ras GTPase-activating protein, as a src family kinase SH3-domain-binding protein. Reconstitution of complexes containing p62 and the src family kinase p59fyn in HeLa cells demonstrated that complex formation resulted in tyrosine phosphorylation of p62 and was mediated by both the SH3 and SH2 domains of p59fyn. The phosphorylation of p62 by p59fyn required an intact SH3 domain, demonstrating that one function of the src family kinase SH3 domains is to bind and present certain substrates to the kinase. As p62 contains at least five SH3-domain-binding motifs and multiple tyrosine phosphorylation sites, p62 may interact with other signalling molecules via SH3 and SH2 domain interactions. Here we show that the SH3 and/or SH2 domains of the signalling proteins Grb2 and phospholipase C gamma-1 can interact with p62 both in vitro and in vivo. Thus, we propose that one function of the tandemly occurring SH3 and SH2 domains of src family kinases is to bind p62, a multifunctional SH3 and SH2 domain adapter protein, linking src family kinases to downstream effector and regulatory molecules.

Inhibition of the T-cell receptor-mediated signal transduction by microinjection of anti-Lck monoclonal antibody into T-cells

Engagement of T-cell receptor (TcR)/CD3 complexes on T-cells rapidly provokes tyrosine phosphorylation of cellular proteins, which is thought to be an essential step to the following events of T-cell activation. p56lck, a member of src-related, non-receptor type protein tyrosine kinases, is expressed predominantly in lymphocytes. Accumulating data suggest that p56lck is one of the kinases responsible for TcR-mediated protein tyrosine phosphorylation. To investigate the role of p56lck in TcR-signaling in detail, we injected anti-Lck monoclonal antibody (mAb), MOL171 or MOL294, both specifically suppress Lck kinase activity in vitro, into Jurkat T-cells by the erythrocyte-ghost procedure in order to block the activity of p56lck. In Jurkat cells injected with anti-Lck mAb, intracellular Ca2+ mobilization induced by TcR-stimulation was markedly reduced in comparison with control mouse IgG-injected samples. This block of Ca2+ influx seems to be specific for TcR-signaling because anti-Lck mAb-injection did not cause significant suppression of phytohaemagglutinin-induced Ca2+ increase. Furthermore, injection of anti-Lck mAb inhibited TcR-mediated protein tyrosine phosphorylation of 100 kDa protein and phospholipase C gamma 1. These results confirm that p56lck is an indispensable element of TcR-signaling and p100 and phospholipase C gamma 1 are strongly presumed to be candidates for substrates for p56lck.

The protein tyrosine kinase p56lck regulates cell adhesion mediated by CD4 and major histocompatibility complex class II proteins

The CD4 protein is expressed on a subset of human T lymphocytes that recognize antigen in the context of major histocompatibility complex (MHC) class II molecules. Using Chinese hamster ovary (CHO) cells expressing human CD4, we have previously demonstrated that the CD4 protein can mediate cell adhesion by direct interaction with MHC class II molecules. In T lymphocytes, CD4 can also function as a signaling molecule, presumably through its intracellular association with p56lck, a member of the src family of protein tyrosine kinases. In the present report, we show that p56lck can affect cell adhesion mediated by CD4 and MHC class II molecules. The expression of wild-type p56lck in CHO-CD4 cells augments the binding of MHC class II+ B cells, whereas the expression of a mutant p56lck protein with elevated tyrosine kinase activity results in decreased binding of MHC class II+ B cells. Using site-specific mutants of p56lck, we demonstrate that the both the enzymatic activity of p56lck and its association with CD4 are required for this effect on CD4/MHC class II adhesion. Further, the binding of MHC class II+ B cells induces CD4 at the cell surface to become organized into structures resembling adhesions-type junctions. Both wild-type and mutant forms of p56lck influence CD4-mediated adhesion by regulating the formation of these structures. The wild-type lck protein enhances CD4/MHC class II adhesion by augmenting the formation of CD4-associated adherens junctions whereas the elevated tyrosine kinase activity of the mutant p56lck decreases CD4-mediated cell adhesion by preventing the formation of these structures.

Physical and functional association of the high affinity immunoglobulin G receptor (Fc gamma RI) with the kinases Hck and Lyn

The high affinity immunoglobulin G (IgG) receptor Fc gamma RI (CD64) is expressed constitutively on monocytes and macrophages, and is inducible on neutrophils. Fc gamma RI has recently been shown to be associated with the signal transducing gamma subunit of the high-affinity IgE receptor (Fc epsilon RI gamma). Induction of cytoplasmic protein tyrosine phosphorylation by Fc gamma RI cross-linking is known to be important in mediating Fc gamma RI-coupled effector functions. Recently, syk has been implicated in this role. We now report that the src-type kinases hck and lyn are physically and functionally associated with Fc gamma RI. Hck and lyn coimmunoprecipitated with Fc gamma RI from detergent lysates of normal human monocytes and of the monocytic line THP-1. Hck and lyn showed rapidly increased phosphorylation and increased exogenous substrate kinase activity after cross-linking of Fc gamma RI. These results demonstrate both physical and functional association of the Fc gamma RI/Fc epsilon RI gamma receptor complex with hck and lyn, and suggest a potential signal transducing role for these kinases in monocyte/macrophage activation.

Signals determining protein tyrosine kinase and glycosyl-phosphatidylinositol-anchored protein targeting to a glycolipid-enriched membrane fraction

Glycosyl-phosphatidylinositol (GPI)-anchored membrane proteins and certain protein tyrosine kinases associate with a Triton X-100-insoluble, glycolipid-enriched membrane fraction in MDCK cells. Also, certain protein tyrosine kinases have been shown to associate with GPI-anchored proteins in other cell types. To characterize the interaction between GPI-anchored proteins and protein tyrosine kinases, GPI-anchored proteins were coexpressed with p56lck in HeLa cells. Both proteins were shown to target independently to the glycolipid-enriched membranes. Coimmunoprecipitation of GPI-anchored proteins and p56lck occurred only when both proteins were located in the glycolipid-enriched membranes, and gentle disruption of these membranes abolished the interaction. The GPI anchor was found to be the targeting signal for this membrane fraction in GPI-anchored proteins. Analysis of mutants indicated that p56lck was nearly quantitatively palmitoylated at Cys-5 but not palmitoylated at Cys-3. The nonpalmitoylated cysteine at position 3 was very important for association of p56lck with the membrane fraction, while palmitoylation at Cys-5 promoted only a low level of interaction. Because other src family protein tyrosine kinases that are associated with GPI-anchored proteins always contain a Cys-3, we propose that this residue, in addition to the N-terminal myristate, is part of a common signal targeting these proteins to a membrane domain that has been linked to transmembrane signaling.

Signals determining protein tyrosine kinase and glycosyl-phosphatidylinositol-anchored protein targeting to a glycolipid-enriched membrane fraction

Glycosyl-phosphatidylinositol (GPI)-anchored membrane proteins and certain protein tyrosine kinases associate with a Triton X-100-insoluble, glycolipid-enriched membrane fraction in MDCK cells. Also, certain protein tyrosine kinases have been shown to associate with GPI-anchored proteins in other cell types. To characterize the interaction between GPI-anchored proteins and protein tyrosine kinases, GPI-anchored proteins were coexpressed with p56lck in HeLa cells. Both proteins were shown to target independently to the glycolipid-enriched membranes. Coimmunoprecipitation of GPI-anchored proteins and p56lck occurred only when both proteins were located in the glycolipid-enriched membranes, and gentle disruption of these membranes abolished the interaction. The GPI anchor was found to be the targeting signal for this membrane fraction in GPI-anchored proteins. Analysis of mutants indicated that p56lck was nearly quantitatively palmitoylated at Cys-5 but not palmitoylated at Cys-3. The nonpalmitoylated cysteine at position 3 was very important for association of p56lck with the membrane fraction, while palmitoylation at Cys-5 promoted only a low level of interaction. Because other src family protein tyrosine kinases that are associated with GPI-anchored proteins always contain a Cys-3, we propose that this residue, in addition to the N-terminal myristate, is part of a common signal targeting these proteins to a membrane domain that has been linked to transmembrane signaling.

Human immunodeficiency virus type 1 Nef-induced down-modulation of CD4 is due to rapid internalization and degradation of surface CD4

Human immunodeficiency virus type 1 (HIV-1) Nef is a myristylated protein with a relative molecular mass of 27 kDa, is localized to the cytoplasmic surfaces of cellular membranes, and has been reported to down-modulate CD4 in human T cells. To understand the mechanism of HIV-1 Nef-mediated down-modulation of cell surface CD4, we expressed Nef protein in human T-cell line VB. Expression of HIV-1 Nef protein down-modulated surface CD4 molecules. In pulse-chase experiments, CD4 molecules in Nef-expressing cells were synthesized at normal levels. However, the bulk of newly synthesized CD4 protein was degraded with a half-life of approximately 6 h, compared with the 24-h half-life in control cells. This Nef-induced acceleration of CD4 turnover was inhibited by lysosomotropic agents NH4Cl and chloroquine as well as by the protease inhibitor leupeptin. Surface CD4 biotinylation experiments demonstrated that CD4 molecules in Nef-expressing T cells are transported to the plasma membrane with normal kinetics but are then rapidly internalized. Therefore, HIV-1 Nef-induced down-modulation of CD4 is due to rapid internalization of surface CD4 and subsequent degradation by an acid-dependent process, potentially lysosomal. Additionally, in a Nef-expressing cell, we find accelerated dissociation of the T-cell tyrosine kinase p56lck and CD4 but only after the complex reaches the plasma membrane. This implies that HIV-1 Nef protein might play a role in triggering a series of T-cell activation-like events, which contribute to p56lck dissociation and internalization of surface CD4 molecules.

Mapping of a region of the paramyxovirus L protein required for the formation of a stable complex with the viral phosphoprotein P

The paramyxovirus large protein (L) and phosphoprotein (P) are both required for viral RNA-dependent RNA polymerase activity. Previous biochemical experiments have shown that L and P can form a complex when expressed from cDNA plasmids in vivo. In this report, L and P proteins of the paramyxovirus simian virus 5 (SV5) were coexpressed in HeLa T4 cells from cDNA plasmids, and L-P complexes were examined. To identify regions of the SV5 L protein that are required for L-P complex formation, 16 deletion mutants were constructed by mutagenesis of an SV5 L cDNA. Following coexpression of these L mutants with cDNA-derived P and radiolabeling with 35S-amino acids, cell lysates were analyzed for stable L-P complexes by a coimmunoprecipitation assay and by sedimentation on 5 to 20% glycerol gradients. Mutant forms of L containing deletions that removed as much as 1,008 residues from the C-terminal half of the full-length 2,255-residue L protein were detected in complexes with P by these two assays. In contrast, large deletions in the N-terminal half of L resulted in proteins that were defective in the formation of stable L-P complexes. Likewise, L mutants containing smaller deletions that individually removed N-terminal regions which are conserved among paramyxovirus and rhabdovirus L proteins (domain I, II, or III) were also defective in stable interactions with P. These results suggest that the N-terminal half of the L protein contains sequences important for stable L-P complex formation and that the C-terminal half of L is not directly involved in these interactions. SV5-infected HeLa T4 cells were pulse-labeled with 35S-amino acids, and cell extracts were examined by gradient sedimentation. Solubilized L protein was detected as an approximately 8 to 10S species, while the P protein was found as both a approximately 4S form (approximately 85%) and a species that cosedimented with L (approximately 15%). These data provide the first biochemical evidence in support of a simple domain structure for an L protein of the nonsegmented negative-sense RNA viruses. The results are discussed in terms of a structural model for the L protein and the interactions of L with the second viral polymerase subunit P.

Purification and characterization of an activated form of the protein tyrosine kinase Lck from an Escherichia coli expression system

The lymphocyte-specific, nonreceptor protein tyrosine kinase Lck has been purified from an Escherichia coli expression system using a monoclonal antibody column followed by dye-affinity chromatography. Polyacrylamide gel electrophoretic analysis of purified protein revealed a single 56 kDa band, indicating that recombinant Lck was purified to near-homogeneity. The purified enzyme displayed tyrosine kinase activity as measured by both autophosphorylation and phosphorylation of exogenous substrates. Biochemical properties including protein phosphorylation and kinetic characteristics of the enzyme have been assessed. Peptide map analysis revealed that bacterially expressed Lck is phosphorylated predominantly on the autophosphorylation site (tyrosine-394), which is characteristic for activated protein tyrosine kinases. Indeed, we found that the recombinant enzyme is approximately fivefold more active than Lck from resting T cells, which is extensively phosphorylated at the regulatory carboxy-terminal tyrosine residue (tyrosine-505). Thus, we have overproduced recombinant human Lck in E. coli and developed a simple two-step purification procedure which yields highly active enzyme. This will enable the identification and characterization of potential regulators and targets of Lck and thereby greatly facilitate studies which will clarify its role in T cell signal transduction.

CD4-mediated enhancement or inhibition of T cell activation does not require the CD4:p56lck association

CD4 is the coreceptor molecule expressed on the surface of T cells specific for or restricted by class II molecules of the major histocompatibility complex (MHC). Its expression on T cells is required for an optimal response to antigen (Ag). Three mechanisms have been invoked for the involvement of CD4 in T cell activation. First, it was shown that CD4 binds to MHC class II molecules on antigen presenting cells (APCs) thereby favoring an adhesion between effector cells and APCs. Association of CD4 to the T cell receptor and to the tyrosine kinase p56lck have also been shown to be critically involved in the positive function of CD4. Here, we demonstrate that the interaction of CD4 with p56lck is not required to enhance the response of two CD4-dependent, Ag-specific T cell hybridomas. Mutant forms of CD4 (TCD4), which lose association to p56lck, were expressed in these T cells and were shown to enhance the Ag-specific response as efficiently as the wild-type CD4. Moreover both CD4-dependent and independent T cell responses were inhibited by CD4-specific mAbs even when CD4 was not associated with p56lck. These results indicate that mechanisms distinct from sequestration of p56lck and/or negative signaling operate in these inhibitions. Results demonstrating enhancement of TCR-mediated signaling by the coaggregation of TCD4 mutant to the TCR further confirm that the association of p56lck to CD4 is not absolutely required for the regulatory functions of CD4. Our results suggest that the mechanisms implicated in the enhancement of T cell stimulation via CD4 depend solely on the extracellular and transmembrane domains of CD4.

The Src-family kinase, Fyn, regulates the activation of phosphatidylinositol 3-kinase in an interleukin 2-responsive T cell line

The proliferation of antigen-activated T cells is mediated by the T cell-derived growth factor, interleukin 2 (IL-2). The biochemical signaling cascades initiating IL-2-induced growth are dependent upon protein tyrosine kinase (PTK) activity. One IL-2-regulated PTK implicated in this cascade is the Src-family kinase, Fyn. Previous studies have described a physical association between Fyn and a potential downstream substrate, phosphatidylinositol 3-kinase (PI3-kinase) as well as the IL-2-dependent activation of PI3-kinase in T cells; however, the role of Fyn in IL-2-induced PI3-kinase activation remains unclear. In this report, we demonstrate that IL-2 stimulation triggers tyrosine phosphorylation of the p85 subunit of PI3-kinase in the murine T cell line, CTLL-2. Lysates prepared from growth factor-deprived and IL-2-stimulated T cells reconstituted both the binding of CTLL-2 cell-derived Fyn to and the IL-2-inducible tyrosine phosphorylation of exogenously added recombinant p85. Furthermore, overexpression of wild-type Fyn in these cells enhanced both the basal and IL-2-mediated activation of PI3-kinase. Additional studies of the Fyn-PI3-kinase interaction demonstrated that the Src homology 3 (SH3) domain of Fyn constitutes a direct binding site for the p85 subunit of PI3-kinase. These results support the notion that Fyn may be directly involved in the activation of the downstream signaling enzyme, PI3-kinase, in IL-2-stimulated T cells.

The T-cell antigen CD5 acts as a receptor and substrate for the protein-tyrosine kinase p56lck

CD5 is a T-cell-specific antigen which binds to the B-cell antigen CD72 and acts as a coreceptor in the stimulation of T-cell growth. CD5 associates with the T-cell receptor zeta chain (TcR zeta)/CD3 complex and is rapidly phosphosphorylated on tyrosine residues as a result of TcR zeta/CD3 ligation. However, despite this, the mechanism by which CD5 generates intracellular signals is unclear. In this study, we demonstrate that CD5 is coupled to the protein-tyrosine kinase p56lck and can act as a substrate for p56lck. Coexpression of CD5 with p56lck in the baculovirus expression system resulted in the phosphorylation of CD5 on tyrosine residues. Further, anti-CD5 and anti-p56lck coprecipitated each other in a variety of detergents, including Nonidet P-40 and Triton X-100. Anti-CD5 also precipitated the kinase from various T cells irrespective of the expression of TcR zeta/CD3 or CD4. No binding between p59fyn(T) and CD5 was detected in T cells. The binding of p56lck to CD5 induced a 10- to 15-fold increase in p56lck catalytic activity, as measured by in vitro kinase analysis. In vivo labelling with 32P(i) also showed a four- to fivefold increase in Y-394 occupancy in p56lck when associated with CD5. The use of glutathione S-transferase-Lck fusion proteins in precipitation analysis showed that the SH2 domain of p56lck could recognize CD5 as expressed in the baculovirus expression system. CD5 interaction with p56lck represents a novel variant of a receptor-kinase complex in which receptor can also serve as substrate. The CD5-p56lck interaction is likely to play roles in T-cell signalling and T-B collaboration.

Distinct p53/56lyn and p59fyn domains associate with nonphosphorylated and phosphorylated Ig-alpha

Among the earliest detectable events in B-cell antigen receptor-mediated signal transduction are the activation of receptor-associated Src-family tyrosine kinases and the tyrosine phosphorylation of Ig-alpha and Ig-beta receptor subunits. These kinases appear to interact with resting B-cell antigen receptor complexes primarily through the Ig-alpha chain antigen receptor homology 1 (ARH1) motif. Recent studies showed a dramatic increase in the amount of Src-family kinase p59fyn bound to Ig-alpha when ARH1 motif tyrosines were phosphorylated. To explore the submolecular basis of these interactions, we conducted mutational analysis to localize sites in p53/56lyn and p59fyn that bind nonphosphorylated and phosphorylated Ig-alpha. Here we report that distinct regions within these kinases bind nonphosphorylated and phosphorylated Ig-alpha ARH1 motifs. The N-terminal 10 residues mediate binding to the nonphosphorylated Ig-alpha ARH1 motif. Association with the phosphorylated Ig-alpha ARH1 motif is mediated by Src homology 2 domains. These findings suggest a mechanism whereby ligand-induced Ig-alpha tyrosine phosphorylation initiates a change in the orientation of an associated kinase that may alter its activity and/or access to substrates and other effectors.

Association of the transmembrane TGF-alpha precursor with a protein kinase complex

A variety of growth factors including transforming growth factor-alpha (TGF-alpha) are synthesized as transmembrane precursors. The short cytoplasmic domain of the transmembrane TGF-alpha precursor lacks any apparent motif associated with signal transduction. However, the sequence conservation of this cytoplasmic domain and its abundance of cysteine residues, reminiscent of the cytoplasmic domains of CD4 and CD8, suggest a biological function. In this study, we showed that transmembrane TGF-alpha was rapidly internalized after interaction with a specific antibody and that this internalization was greatly decreased when the COOH-terminal 31 amino acids were removed. Chemical cross-linking experiments revealed two associated proteins of 86 and 106 kD which coimmunoprecipitated with the TGF-alpha precursor. The association of p86 was dependent on the presence of the COOH-terminal cytoplasmic 31 amino acids of the TGF-alpha precursor, whereas p106 still remained associated when this segment was deleted. In addition, p106 was tyrosine-phosphorylated and exposed on the cell surface. The protein complex associated with transmembrane TGF-alpha displayed kinase activities towards tyrosine, serine, and threonine residues. These activities were not associated with transmembrane TGF-alpha when the COOH-terminal segment was truncated. The association of a protein kinase complex with transmembrane TGF-alpha may provide the basic elements for a "reverse" mode of signaling through the cytoplasmic domain of this growth factor, which may lead to two-directional communication during ligand-receptor interaction.

Two-step TCR zeta/CD3-CD4 and CD28 signaling in T cells: SH2/SH3 domains, protein-tyrosine and lipid kinases

A central question in T-cell immunity concerns the nature of intracellular signaling from the antigen receptor, the CD4/CD8 co-receptors and the CD28 antigen. Since the original discovery that T-cell receptors such as CD4 can interact with intracellular protein-tyrosine kinases such as p56lck, remarkable progress has been made in deciphering the signaling pathways that control T-cell growth and immune function. Here, Christopher Rudd and colleagues examine the role of protein-tyrosine kinases, SH2/SH3 domains and lipid kinases in the generation of signals from the TCR zeta/CD3 complex and the CD28 antigen.