Requirement for association of p56lck with CD4 in antigen-specific signal transduction in T cells

Modification of Ser59 in the unique N-terminal region of tyrosine kinase p56lck regulates specificity of its Src homology 2 domain

During T-cell activation, Ser59 in the unique N-terminal region of p56lck is phosphorylated. Mutation of Ser59 to Glu59 mimics Ser59 phosphorylation, and upon CD4 crosslinking, this mutant p56lck induces tyrosine phosphorylation of intracellular proteins distinct from those induced by wild-type p56lck. Mutant and wild-type p56lck have similar affinities for CD4 and similar kinase activities. In glutathione S-transferase fusion proteins, the p56lck Src homology 2 (SH2) domain with the SH3 domain and the unique N-terminal region (including Ser59) has a different binding specificity for phosphotyrosyl proteins than the SH2 domain alone. Either deletion of the unique N-terminal region or mutation of Ser59 to Glu59 in the fusion protein reverts the phosphotyrosyl protein binding specificity back to that of the SH2 domain alone. These results suggest that phosphorylation of Ser59 regulates the function of p56lck by controlling binding specificity of its SH2 domain.

Alterations in the signal-transducing molecules of T cells and NK cells in colorectal tumor-infiltrating, gut mucosal and peripheral lymphocytes: correlation with the stage of the disease

T cells from mice bearing an experimental colon carcinoma, and from patients with colorectal and renal carcinomas, have atypical T-cell receptors (TCR). In the present study, further characterization of modulations in CD3- and CD16-associated zeta chain in peripheral blood lymphocytes (PBL) and tumor-infiltrating lymphocytes (TIL) from colorectal carcinomas was performed. Relative to PBL, the percentage of natural killer (NK) cells among fresh TIL was reduced, while a higher proportion of T cells expressing HLA-DR was found. As previously reported, we found significantly reduced levels of the CD3- and CD16-associated zeta chain in TIL and, to a lesser extent, also in patients' PBL. Levels of zeta chain in T and NK cells from non-cancerous colorectal tissue from patients were lower than in PBL but higher than in TIL, with a direct relationship between levels of this signal-transducing molecule and the distance from the tumor. In addition, zeta levels correlated with the Dukes' stage of the disease, since PBL from patients with lymph-node involvement or distant organ metastases (Dukes' stages C and D) had significantly less CD3 zeta than patients with localized disease (stages A and B). Patients' T cells also had decreased levels of cell-surface and cytoplasmic CD3 epsilon. We also observed reduced levels of the TCR accessory molecules CD4 and CD8, mainly on TIL but to a lesser extent also on patients' PBL. Biochemical analysis of anti-CD3 epsilon-immunoprecipitated TCR complexes demonstrated that the CD3 complex was not associated with the zeta chain, either on TIL or on PBL or on lymphocytes from non-cancerous colon tissue, suggesting a defect in the assembly of the TCR complex. Following several days of in vitro culture with recombinant interleukin-2 and phytohemagglutinin, anti-CD3 or anti-CD2 monoclonal antibodies (MAbs), levels of CD3 zeta chain as well as of cell surface CD3 epsilon were normalized. Our findings suggest an abnormal expression as well as assembly of several different signal-transducing molecules of T cells and NK cells, which correlate with the stage of the disease in patients with colorectal carcinomas.

Signalling by the p60c-src family of protein-tyrosine kinases

The c-src gene family has nine known members (blk, c-fgr, fyn, hck, lck, lyn, c-src, c-yes and yrk), each encoding a cytoplasmic protein-tyrosine kinase (PTK) believed to be involved in signal transduction. The c-src PTKs contain three domains (SH1, SH2 and SH3) that are found in many other signalling proteins. The SH1 domain has PTK activity, whilst the SH2 and SH3 domains are involved in mediating protein-protein interactions by binding to phosphotyrosine-containing and proline-rich motifs, respectively. The expression patterns of the c-src PTKs suggest that they function in a broad range of biological situations, in many cases regulating the behaviour of terminally-differentiated, post-mitotic cell types. Targeted disruption of members of the c-src family in transgenic mice has confirmed important roles for p56lck and p59fym(T) in T-lymphocyte maturation and activation, but has also revealed unexpected roles for p60c-src in bone maintenance and for p59fym(B) in learning and memory. There is increasingly detailed information about the biochemical nature of the signalling pathways in which the c-src PTKs operate and about the other signalling proteins with which they interact. The c-src PTKs can associate with activated receptor PTKs, including the receptors for platelet-derived growth factor and epidermal growth factor, by means of SH2-phosphotyrosine binding. The c-src PTKs also associated with transmembrane proteins that lack PTK activity, frequently by means of interactions involving their unique amino-terminal sequences.

T lymphocyte development in p56lck deficient mice: allelic exclusion of the TcR beta locus is incomplete but thymocyte development is not restored by TcR beta or TcR alpha beta transgenes

The protein tyrosine kinase, p56lck, is involved in signal transduction in mature T cells and in the molecular events controlling early thymocyte differentiation. Thymuses of mice deficient for p56lck expression (p56lck-/-) consist of immature CD4-CD8- double-negative (DN) and CD4+CD8+ double-positive (DP) thymocytes and are severely reduced in total cell number. In this report we have studied DN thymocytes from p56lck-/- mice and found an increase in the proportion of the CD44-CD25+ subset, suggesting that transit through this stage, which is known to require T cell receptor (TcR) beta expression, may be delayed in the absence of p56lck expression. In addition, the expression of a transgenic TcR beta chain or TcR alpha beta pair did not restore thymic development in p56lck-/- mice. However, in contrast to mice expressing a dominant negative isoform of p56lck in which DP thymocytes do not develop, DP thymocytes still develop in nontransgenic and TcR transgenic p56lck-/- mice. These results demonstrate that expansion of the DP subset is impaired in p56lck-/- mice. In contrast, allelic exclusion is not severely compromised. Although there was an increase in the number of peripheral T cells expressing more than one V beta chain in TcR transgenic p56lck-/- mice, we found that inhibition of endogenous TcR beta gene rearrangement was almost complete in thymocytes of V beta transgenic p56lck-/- mice and we could not detect any peripheral T cells that expressed more than one V beta chain in non-transgenic p56lck-/- mice.

HIV-1 glycoprotein gp120 disrupts CD4-p56lck/CD3-T cell receptor interactions and inhibits CD3 signaling

Using the CD4+ human T cell clone P28, we demonstrated that the HIV-1 glycoprotein gp120 inhibited CD3-induced inositol trisphosphate production, calcium influx and T cell proliferation. Additionally, gp120 was shown to dissociate the tyrosine kinase p56lck from CD4 in CEM cells, with a concommittant inhibition of CD4-linked kinase activity. We have addressed the question whether disruption of CD4/p56lck or CD4/CD3-T cell receptor interactions, or both, could account for the inhibitory effect of gp120 in P28 cells. By comparing the effects of various anti-CD4 monoclonal antibodies (mAb) with those of gp120, we show that gp120 and IOT4a modulate CD4 expression, and decrease CD4-associated p56lck and CD4-linked kinase activity at the plasma membrane. In contrast, OKT4A and OKT4 anti-CD4 mAb have no inhibitory effect. Interestingly, gp120 also inhibits CD3-induced Lck activation and cellular tyrosine phosphorylation, particularly of phosphoinositide-specific phospholipase C-gamma-1. Kinetic experiments reveal that the inhibitory effect of gp120 on CD3-induced tyrosine phosphorylation appears as early as 30 min, but culminate when CD4-p56lck complexes disappear from the cell surface after 4 h. These results suggest that a negative signal is triggered by gp120 that results, after a few hours, in down-modulation of CD4-p56lck complexes and the impairment of CD3 signaling. Supporting this hypothesis, gp120 inhibits CD3-linked kinase activity as shown by the inhibition of the phosphorylation of CD3 chains, leading to the inhibition of subsequent signal transduction.

NKR-P1A is a target-specific receptor that activates natural killer cell cytotoxicity

NKR-P1A is a lectinlike surface molecule expressed on rat natural killer (NK) cells. NKR-P1A has structural and functional features of an activating NK cell receptor, but a requirement for NKR-P1A in target cell lysis has not been determined. To define the role of NKR-P1A in natural killing, we have generated a mutant of the rat NK cell line, RNK-16, lacking expression of all members of the NKR-P1 receptor family. Although these NKR-P1-deficient NK cells were able to kill many standard tumor targets, including YAC-1, they were selectively deficient in the lysis of IC-21 macrophage, B-16 melanoma, and C1498 lymphoma targets. Reexpression of a single member of the NKR-P1 family, NKR-P1A, on mutant cells restored lysis of IC-21, and killing of IC-21 targets through rat NKR-P1A was completely blocked by F(ab')2 anti-NKR-P1A. Reexpression of NKR-P1A also restored transmembrane signaling to IC-21, as assessed by the generation of inositol-1,4,5-trisphosphate. The generation of inositol-1,4,5-trisphosphate was also restored in response to B-16 targets, but both B-16 and C1498 cells remained resistant to lysis, indicating that other NK cell molecules, perhaps within the NKR-P1 family, are required for the efficient killing of these tumors. These results are the first to demonstrate that NKR-P1A is a target-specific receptor that activates natural killing.

Type 2 phosphatidylinositol 4-kinase is recruited to CD4 in response to CD4 cross-linking

CD4 serves as a cell-cell adhesion molecule, with specific affinity for class II MHC molecules, and as a receptor for the human immunodeficiency virus type 1 (HIV-1) viral coat protein. Phosphoinositide (PI)-3-kinase and 1-phosphatidylinositol (PtdIns)-4-kinase activities were previously found to associate with the CD4:p56lck complex, but the protein responsible for PtdIns 4-kinase activity was not identified. Here we demonstrate that the 53 kDa type 2 PtdIns 4-kinase associates with CD4 using a monoclonal antibody specific for this enzyme. We also show that an increase in PtdIns 4-kinase activity is due to recruitment of the type 2 PtdIns 4-kinase protein to the CD4:p56lck complex after cross-linking with anti-CD4.

p56lck plays a key role in transducing apoptotic signals in T cells

The CD4 receptor synergizes with the T-cell antigen receptor (TCR) in helper T-cell activation. However CD4 cross-linking in the absence of simultaneous TCR engagement leaves the cells primed to activation dependent apoptosis. To assess the role of the CD4 associated protein tyrosine kinase p56lck in CD4 priming to apoptosis we have constructed Jurkat T-cell lines stably transfected with a constitutively active form of p56lck. These cells were constitutively primed to undergo apoptosis upon TCR crosslinking with specific antibodies. In addition the Jurkat JCaM1 line, which is defective for p56lck expression, was resistant to TCR induced apoptosis. These data indicate that p56lck is required for T-cell apoptosis and that CD4 priming of T-cells for antigen dependent apoptosis is due to inappropriate or partial activation of the p56lck signal transduction pathway.

Transmembrane signaling by antigen receptors of B and T lymphocytes

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

Interactions between the tyrosine kinases p56lck, p59fyn and p50csk in CD4 signaling in T cells

Interaction of the CD4 co-receptor with major histocompatibility complex (MHC) class II molecules during antigen presentation results in enhancement of antigen receptor signaling. The synergism between the two receptors is believed to result from the juxtaposition of the CD4-associated tyrosine kinase p56lck with the cytoplasmic domains of CD3 complex components. Here, we report that cross-linking of CD4 on the surface of Jurkat cells using monoclonal antibodies results in activation of the CD3-associated kinase p59fyn. Co-cross-linking of CD4 and CD3 results in synergistic activation of p59fyn. The p59fyn kinase is also hyperactive in a Jurkat cell line stably transfected with a constitutively active p56lck mutant, indicating that p56lck mediates CD4 activation of p59fyn. In support of this hypothesis, expression of a dominant inhibitory mutant of p59fyn blocks CD4 signals involved in gene activation. In addition, the p59fyn dominant inhibitor mutant blocks gene-activating signals induced by expression of a constitutively active mutant of p56lck. Overexpression of the regulatory kinase p50csk, which attenuates TcR signaling by inactivation of p59fyn, inhibits signaling from the constitutively active form of p56lck. Taken together, these data suggest that CD4/p56lck enhancement of TcR signaling is, at least in part, mediated by activation of p59fyn, and may be regulated by p50csk.

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.

Cross-linking of CD4 in a TCR/CD3-juxtaposed inhibitory state: a pFRET study

Instances when T cell activation via the T cell receptor/CD3 complex is suppressed by anti-CD4 Abs are generally attributed either to the topological separation of CD4-p56lck from CD3, or their improper apposition. Photobleaching fluorescence resonance energy transfer measurements permitted direct analysis of these alternatives on human peripheral blood lymphocytes. Distinction between changes of relative antigen densities or positioning was made possible by simultaneously recording donor and acceptor fluorescence in the energy transfer experiment performed on homogeneous populations of flow-sorted cells. We show here that CD4 stays in the molecular vicinity of CD3, while anti-CD3 stimulation is suppressed by anti-CD4 or cross-linked HIV gp120. Our data suggest that cross-linking of CD4 through particular epitopes is capable of inhibiting activation driven by Abs binding to specific sites on CD3 without major topological sequestration of the Ags, in such a way that additional positive signals will also be affected. Thus, these and other related cases of negative signaling via CD4 may be interpreted in terms of functional uncoupling rather than a wide physical separation of CD4 from the T cell receptor-CD3 complex.

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.

Staphylococcal enterotoxin A and toxic shock syndrome toxin compete with CD4 for human major histocompatibility complex class II binding

We have examined the role of the CD4 molecule in primary T-lymphocyte responses to the staphylococcal enterotoxins SEA, SEB, SEC1, and the toxic shock syndrome toxin TSST-1. Proliferating cells were predominantly CD4+; however, the responses to SEA and TSST-1 were most sensitive to inhibition by the anti-CD4 antibody Leu-3a. T-lymphocyte responses to the bacterial superantigens were inhibited by site-directed mutations of residues in the DR beta membrane-proximal domain (DR beta 2) that are also known to be important for interactions with CD4. SEA and TSST-1 binding to DR was reduced by the DR beta 2 mutations and by competition with soluble recombinant CD4. We propose that bacterial superantigens sequentially, or simultaneously with CD4, stabilize complexes of T-cell antigen receptors and major histocompatibility complex class II molecules. The superantigen qualities of these toxins may be due, in part, to a molecular mimicry of CD4 and other adhesion molecules.

Significance of T-cell stimulation by altered peptide ligands in T cell biology

Investigations of T-cell responses to altered peptide ligands have provided functional evidence that a T-cell receptor can interpret subtle structural changes in its ligand, highlighting the complexity of this antigen receptor signaling system. Over the past year, observations from many studies have suggested several roles for such analog peptides in various aspects of immune responses. Collectively, these data strongly suggest the existence of naturally occurring altered peptide ligands in the endogenous peptide repertoire, that can actively participate in the development and shaping of T-cell immunity.

Human immunodeficiency virus type 1 Nef and p56lck protein-tyrosine kinase interact with a common element in CD4 cytoplasmic tail

The human immunodeficiency virus type 1 nef gene induces endocytosis of CD4 antigen and disrupts the association between CD4 and p56lck protein-tyrosine kinase (EC 2.7.1.112). We demonstrate that in T cells these effects of the viral protein require a cluster of hydrophobic amino acids in a membrane-proximal region of the CD4 cytoplasmic tail; other amino acids in the C-terminal segment of CD4 cytoplasmic tail also contribute to the interaction. Mutations in CD4 that prevent down-modulation by Nef also decrease CD4 association with p56lck and prevent Nef-induced disruption of CD4-p56lck complexes. Together, the overlap in CD4 sequences required for interaction with Nef and p56lck and the tight correlation between Nef-induced CD4 down-modulation and disruption of CD4-p56lck association suggest that Nef, or cellular factors recruited by Nef, interact with this segment of CD4 to displace p56lck from the complex and induce CD4 endocytosis.

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.

Contribution of p56lck to the upregulation of cytokine production and T cell proliferation by IL-2 in human CD3-stimulated T cell clones

Interaction of the interleukin 2 receptor (IL-2R) beta chain with the lymphocyte-specific protein tyrosine kinase (PTK), p56lck, has led to the speculation that p56lck participates in growth signal transduction. Although activation of T cells with interleukin 2 (IL-2) results in the activation of p56lck, accumulating data support the notion that Lck does not play an essential role in mitogenic signal delivery from the IL-2R. Since this src-related PTK has been shown to enhance TCR/CD3-mediated T cell responsiveness, here we investigated whether activation of Lck by IL-2 could contribute to enhance TCR/CD3-mediated T cell functions. This was achieved by using human CD4(+)-cloned T cells and comparing the effects of IL-2 on p56lck kinase activation and cytokine production. Results show that p56lck kinase activity increased as early as 1 min, reached a maximum within 5 min and decreased within 60 min after IL-2 stimulation. Such treatment with IL-2 also resulted in enhancing T cell responsiveness to CD3+PMA stimulation, as assessed by IL-2 and IFN-gamma secretion and by T cell proliferation. This increase of T cell functions was correlated with IL-2-induced p56lck activation in both dose-response and time-course experiments. Taken together these results strongly suggest that activation of Lck by IL-2 may play a role in regulating CD3-mediated T cell functions.

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.

NPK15, a tobacco protein-serine/threonine kinase with a single hydrophobic region near the amino-terminus

A cDNA clone (cNPK15) was isolated from tobacco cells in suspension culture, which encodes a predicted protein kinase of 422 amino acids. The predicted NPK15 protein consists of a hydrophobic region near the amino-terminus, a linker domain and the catalytic domain of a protein-serine/threonine kinase in the carboxyl-half. NPK15 was not found to be closely related to any reported protein, but its putative catalytic domain shares some structural similarity with those of receptor-like protein kinases of plants, such as ZmPK1 from Zea mays and TMK1 from Arabidopsis, even though no receptor-like domain is found in NPK15. Recombinant NPK15 expressed in Escherichia coli as a fusion protein was found capable of autophosphorylation and of phosphorylation of the histone H1 protein on both serine and threonine residues. Upon overexpression of cNPK15 under control of the promoter of cauliflower mosaic virus 35S RNA in tobacco cells, into which it had been introduced by Agrobacterium-mediated transformation, the NPK15 gene acted as a "suicide" gene and blocked proliferation of the host cells. By contrast, such a suicide effect was not observed with the gene for a kinase-negative mutant protein in which the nucleotide sequence for the ATP-binding site had been mutated or with a mutant derivative encoding a protein in which the hydrophobic region had been deleted. Thus, the protein kinase activity of NPK15 and the hydrophobic region of the protein are responsible for the suicide effect. The NPK15 protein kinase seems to be associated with specific cellular functions. Southern blot analysis with cNPK15 as the probe detected several fragments in restriction digests of genomic DNAs from both tobacco and other members of the Solanaceae. This results suggests that NPK15-related genes constitute a small gene family in the genomes of Solanaceae.

Physical and functional interactions between SH2 and SH3 domains of the Src family protein tyrosine kinase p59fyn

The Src family protein tyrosine kinases participate in signalling through cell surface receptors that lack intrinsic tyrosine kinase domains. All nine members of this family possess adjacent Src homology (SH2 and SH3) domains, both of which are essential for repression of the enzymatic activity. The repression is mediated by binding between the SH2 domain and a C-terminal phosphotyrosine, and the SH3 domain is required for this interaction. However, the biochemical basis of functional SH2-SH3 interaction is unclear. Here, we demonstrate that when the SH2 and SH3 domains of p59fyn (Fyn) were present as adjacent domains in a single protein, binding of phosphotyrosyl peptides and proteins to the SH2 domain was enhanced, whereas binding of a subset of cellular polypeptide ligands to the SH3 domain was decreased. An interdomain communication was further revealed by occupancy with domain-specific peptide ligands: occupancy of the SH3 domain with a proline-rich peptide enhanced phosphotyrosine binding to the linked SH2 domain, and occupancy of the SH2 domain with phosphotyrosyl peptides enhanced binding of certain SH3-specific cellular polypeptides. Second, we demonstrate a direct binding between purified SH2 and SH3 domains of Fyn and Lck Src family kinases. Heterologous binding between SH2 and SH3 domains of closely related members of the Src family, namely, Fyn, Lck, and Src, was also observed. In contrast, Grb2, Crk, Abl, p85 phosphatidylinositol 3-kinase, and GTPase-activating protein SH2 domains showed lower or no binding to Fyn or Lck SH3 domains. SH2-SH3 binding did not require an intact phosphotyrosine binding pocket on the SH2 domain; however, perturbations of the SH2 domain induced by specific high-affinity phosphotyrosyl peptide binding abrogated binding of the SH3 domain. SH3-SH2 binding was observed in the presence of proline-rich peptides or when a point mutation (W119K) was introduced in the putative ligand-binding pouch of the Fyn SH3 domain, although these treatments completely abolished the binding to p85 phosphatidylinositol 3-kinase and other SH3-specific polypeptides. These biochemical SH2-SH3 interactions suggest novel mechanisms of regulating the enzymatic activity of Src kinases and their interactions with other proteins.

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.

Physical and functional interactions between SH2 and SH3 domains of the Src family protein tyrosine kinase p59fyn

The Src family protein tyrosine kinases participate in signalling through cell surface receptors that lack intrinsic tyrosine kinase domains. All nine members of this family possess adjacent Src homology (SH2 and SH3) domains, both of which are essential for repression of the enzymatic activity. The repression is mediated by binding between the SH2 domain and a C-terminal phosphotyrosine, and the SH3 domain is required for this interaction. However, the biochemical basis of functional SH2-SH3 interaction is unclear. Here, we demonstrate that when the SH2 and SH3 domains of p59fyn (Fyn) were present as adjacent domains in a single protein, binding of phosphotyrosyl peptides and proteins to the SH2 domain was enhanced, whereas binding of a subset of cellular polypeptide ligands to the SH3 domain was decreased. An interdomain communication was further revealed by occupancy with domain-specific peptide ligands: occupancy of the SH3 domain with a proline-rich peptide enhanced phosphotyrosine binding to the linked SH2 domain, and occupancy of the SH2 domain with phosphotyrosyl peptides enhanced binding of certain SH3-specific cellular polypeptides. Second, we demonstrate a direct binding between purified SH2 and SH3 domains of Fyn and Lck Src family kinases. Heterologous binding between SH2 and SH3 domains of closely related members of the Src family, namely, Fyn, Lck, and Src, was also observed. In contrast, Grb2, Crk, Abl, p85 phosphatidylinositol 3-kinase, and GTPase-activating protein SH2 domains showed lower or no binding to Fyn or Lck SH3 domains. SH2-SH3 binding did not require an intact phosphotyrosine binding pocket on the SH2 domain; however, perturbations of the SH2 domain induced by specific high-affinity phosphotyrosyl peptide binding abrogated binding of the SH3 domain. SH3-SH2 binding was observed in the presence of proline-rich peptides or when a point mutation (W119K) was introduced in the putative ligand-binding pouch of the Fyn SH3 domain, although these treatments completely abolished the binding to p85 phosphatidylinositol 3-kinase and other SH3-specific polypeptides. These biochemical SH2-SH3 interactions suggest novel mechanisms of regulating the enzymatic activity of Src kinases and their interactions with other proteins.

Protein tyrosine kinase p56-Lck regulates lymphocyte function-associated 1 adhesion molecule expression, granule exocytosis, and cytolytic effector function in a cloned T cell

Elevated levels of p56-Lck kinase activity were achieved in an interleukin 2 (IL-2)-dependent cloned cytolytic T cell CTLL-2 through gene transfer approaches. CTLL-2-Lck cells remained dependent on IL-2 for growth and survival in culture but exhibited markedly elevated, IL-2-independent cytolytic activity against a variety of tumor targets. This immune cell effector function was similar to the non-major histocompatibility complex-restricted cytolytic activity previously described for lymphokine activated killer (LAK) cells, and involved a cytolytic mechanism that was independent of protein synthesis in either the T cells or the tumor targets. Characterization of CTLL-2-Lck cells revealed markedly elevated levels of both the alpha (CD11a) and beta (CD18) chains of the cell adhesion molecule lymphocyte function-associated 1 (LFA-1) and increased binding of these T cells to a recombinant protein representing the extracellular domain of the LFA-ligand, intercellular adhesion molecule 1 (ICAM-1). Antibodies to CD11a partially abrogated cytolytic killing of tumor target cells by CTLL-2-Lck cells, suggesting that the upregulation in LFA protein levels potentially accounts at least in part for the phenotype of these T cells. Gene transfer-mediated elevations in p56-Lck kinase activity in an IL-3-dependent myeloid cell clone 32D.3 also resulted in increased LFA-1 expression, demonstrating that the findings are not unique to CTLL-2 cells. In addition to upregulation of LFA-1 expression, CTLL-Lck cells also exhibited more efficient exocytosis of cytotoxic granules upon activation with Ca(2+)-ionophore and phorbol ester, relative to control transfected and untransfected CTLL-2 cells. The findings functionally link the Lck kinase to a T cell effector pathway involved in cell-mediated cytotoxicity.

GRB2 and phospholipase C-gamma 1 associate with a 36- to 38-kilodalton phosphotyrosine protein after T-cell receptor stimulation

GRB2, a 25-kDa protein comprising a single SH2 domain flanked by two SH3 domains, has been implicated in linking receptor protein tyrosine kinases (PTKs) to the Ras pathway by interacting with the guanine nucleotide exchange protein SOS. Previous studies have demonstrated that GRB2 directly interacts with Shc, a proto-oncogene product that is tyrosine phosphorylated upon receptor and nonreceptor PTK activation. In this report, we detected low levels of tyrosine phosphorylation of Shc and induced association with GRB2 upon T-cell receptor (TCR) stimulation. Instead, a prominent 36- to 38-kDa tyrosine phosphoprotein (pp36-38) associated with the SH2 domain of GRB2 and formed a stable complex with GRB2/SOS upon TCR stimulation. Cellular fractionation studies showed that whereas both GRB2 and SOS partitioned to the soluble and particulate fractions, pp36-38 was present exclusively in the particulate fraction. This phosphoprotein had the same apparent mobility in sodium dodecyl sulfate-polyacrylamide gel electrophoresis as the phosphoprotein that associates with phospholipase C-gamma 1 (PLC-gamma 1). Furthermore, following partial immunodepletion of GRB2 and of the associated pp36-38, there was a significant reduction in the amount of the 36-kDa phosphoprotein associated with PLC-gamma 1, suggesting that a trimeric PLC-gamma 1/pp36-38/GRB2 complex could form. In support of this notion, we have also been able to detect low levels of PLC-gamma 1 in GRB2 immunoprecipitates. We suggest that pp36-38 may be a bridging protein, coupling different signalling molecules to cytoplasmic PTKs regulated by the TCR.

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.

GRB2 and phospholipase C-gamma 1 associate with a 36- to 38-kilodalton phosphotyrosine protein after T-cell receptor stimulation

GRB2, a 25-kDa protein comprising a single SH2 domain flanked by two SH3 domains, has been implicated in linking receptor protein tyrosine kinases (PTKs) to the Ras pathway by interacting with the guanine nucleotide exchange protein SOS. Previous studies have demonstrated that GRB2 directly interacts with Shc, a proto-oncogene product that is tyrosine phosphorylated upon receptor and nonreceptor PTK activation. In this report, we detected low levels of tyrosine phosphorylation of Shc and induced association with GRB2 upon T-cell receptor (TCR) stimulation. Instead, a prominent 36- to 38-kDa tyrosine phosphoprotein (pp36-38) associated with the SH2 domain of GRB2 and formed a stable complex with GRB2/SOS upon TCR stimulation. Cellular fractionation studies showed that whereas both GRB2 and SOS partitioned to the soluble and particulate fractions, pp36-38 was present exclusively in the particulate fraction. This phosphoprotein had the same apparent mobility in sodium dodecyl sulfate-polyacrylamide gel electrophoresis as the phosphoprotein that associates with phospholipase C-gamma 1 (PLC-gamma 1). Furthermore, following partial immunodepletion of GRB2 and of the associated pp36-38, there was a significant reduction in the amount of the 36-kDa phosphoprotein associated with PLC-gamma 1, suggesting that a trimeric PLC-gamma 1/pp36-38/GRB2 complex could form. In support of this notion, we have also been able to detect low levels of PLC-gamma 1 in GRB2 immunoprecipitates. We suggest that pp36-38 may be a bridging protein, coupling different signalling molecules to cytoplasmic PTKs regulated by the TCR.

Interactions of p59fyn and ZAP-70 with T-cell receptor activation motifs: defining the nature of a signalling motif

The tyrosine-based activation motif is a 20- to 25-amino-acid sequence contained in the cytoplasmic domains of many hematopoietic receptors which is sufficient by itself to reconstitute signalling. This motif is characterized by two YXXL/I sequences separated by approximately 10 residues. The molecular basis of signalling by this motif is unknown. Here we demonstrate that the tyrosine-based activation motif is required and sufficient for association with the tyrosine kinases p59fyn and ZAP-70, suggesting that association with these kinases is a general feature of this motif. Focusing on the single activation motif present in epsilon, we analyzed which residues of the motif were critical for binding of p59fyn and ZAP-70. Surprisingly, we found that no single mutation of any residue of epsilon resulted in the loss of p59fyn association. In contrast, single mutations at five residues of the epsilon activating motif abrogated ZAP-70 binding. Both of the tyrosines and the leucine or isoleucine residues that follow them were critical. The spacing between the tyrosines was also important, as deletion of two residues disrupted binding of ZAP-70, although p59fyn binding was not disrupted. Most of the defined features of the tyrosine activation motif are therefore requirements for ZAP-70 binding. Interestingly, the interaction of ZAP-70 with the motif was dependent on the presence of both ZAP-70 SH2 domains and both of the tyrosine residues in the motif, suggesting that ZAP-70 interacts with two phosphotyrosine residues and that the binding of the two SH2 domains is cooperative. In addition, we demonstrate that the interaction between the tyrosine activation motif is direct and requires prior tyrosine phosphorylation of the motif. We propose that the activation of cells by the tyrosine activating motif occurs in four discrete steps: binding of p59fyn, phosphorylation of the motif, binding of ZAP-70, and activation of ZAP-70 kinase activity.

Adhesion receptors in lymphocyte activation

The past several years have seen significant progress in understanding the role of T lymphocyte coreceptors in adhesion and activation. New insights have been gained in several areas: the avidity regulation of beta 1 and beta 2 integrins and their role in signal transduction; the regulation of CD8 avidity; the role of Lck in CD4 coreceptor activity; and the novel role for CD2 adhesion in the T cell antigen response.

Interactions of p59fyn and ZAP-70 with T-cell receptor activation motifs: defining the nature of a signalling motif

The tyrosine-based activation motif is a 20- to 25-amino-acid sequence contained in the cytoplasmic domains of many hematopoietic receptors which is sufficient by itself to reconstitute signalling. This motif is characterized by two YXXL/I sequences separated by approximately 10 residues. The molecular basis of signalling by this motif is unknown. Here we demonstrate that the tyrosine-based activation motif is required and sufficient for association with the tyrosine kinases p59fyn and ZAP-70, suggesting that association with these kinases is a general feature of this motif. Focusing on the single activation motif present in epsilon, we analyzed which residues of the motif were critical for binding of p59fyn and ZAP-70. Surprisingly, we found that no single mutation of any residue of epsilon resulted in the loss of p59fyn association. In contrast, single mutations at five residues of the epsilon activating motif abrogated ZAP-70 binding. Both of the tyrosines and the leucine or isoleucine residues that follow them were critical. The spacing between the tyrosines was also important, as deletion of two residues disrupted binding of ZAP-70, although p59fyn binding was not disrupted. Most of the defined features of the tyrosine activation motif are therefore requirements for ZAP-70 binding. Interestingly, the interaction of ZAP-70 with the motif was dependent on the presence of both ZAP-70 SH2 domains and both of the tyrosine residues in the motif, suggesting that ZAP-70 interacts with two phosphotyrosine residues and that the binding of the two SH2 domains is cooperative. In addition, we demonstrate that the interaction between the tyrosine activation motif is direct and requires prior tyrosine phosphorylation of the motif. We propose that the activation of cells by the tyrosine activating motif occurs in four discrete steps: binding of p59fyn, phosphorylation of the motif, binding of ZAP-70, and activation of ZAP-70 kinase activity.

Role of tyrosine kinases in lymphocyte activation

Interaction of T- and B-cell antigen receptors with cytoplasmic non-receptor tyrosine protein kinases is critical to the activation of lymphocytes by antigen. Both the src-family tyrosine protein kinases Lck, Fyn, Lyn and Blk and the syk-family tyrosine protein kinases Syk and ZAP-70 play a role in lymphocyte activation. The antigen receptors are coupled to this cluster of kinases by the cytoplasmic tails of the gamma, delta, epsilon, zeta, and eta subunits of the T-cell receptor, and the Ig-alpha and Ig-beta subunits of the B-cell receptor. Each of these proteins contains one or more 'tyrosine based activation motifs', with the amino acid sequence D/EX7D/EXXYXXL/IX7YXXL/I. This motif appears to allow binding of one or more src-like kinases, via their unique amino termini, before the onset of lymphocyte activation. Invariant tyrosines in the motif become phosphorylated following the triggering of lymphocyte activation, and this modification induces the binding of the src- and syk-family tyrosine protein kinases, and potentially other signalling molecules, through SH2 domains to the antigen receptors.

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.

Amino acid residues that flank core peptide epitopes and the extracellular domains of CD4 modulate differential signaling through the T cell receptor

Hen egg lysozyme 52-61-specific CD4+ T cells responded by interleukin 2 (IL-2) secretion to any peptide containing this epitope regardless of length of NH2- and COOH-terminal composition. However, CD4- variants could only respond to peptides containing the two COOH-terminal tryptophans at positions 62 and 63. Substitutions at these positions defined patterns of reactivity that were specific for individual T cells inferring a T cell receptor (TCR)-based phenomenon. Thus, the fine specificity of major histocompatibility complex (MHC)-peptide recognition by the TCR was dramatically affected by CD4 and the COOH-terminal peptide composition. Peptides that failed to induce IL-2 secretion in the CD4- variants nevertheless induced strong tyrosine phosphorylation of CD3 zeta. Thus, whereas the TCR still recognized and bound to the MHC class II-peptide complex resulting in protein phosphorylation, this interaction failed to induce effective signal transduction manifested by IL-2 secretion. This provides a clear example of differential signaling mediated by peptides known to be naturally processed. In addition, the external domains of CD4, rather than its cytoplasmic tail, were critical in aiding TCR recognition of all peptides derived from a single epitope. These data suggest that the nested flanking residues, which are present on MHC class II but not class I bound peptides, are functionally relevant.

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.

Rapid T-cell receptor-mediated tyrosine phosphorylation of p120, an Fyn/Lck Src homology 3 domain-binding protein

Tyrosine phosphorylation of cellular proteins is the earliest identifiable event following T-cell antigen receptor (TCR) stimulation and is essential for activating downstream signaling machinery. Two Src-family protein-tyrosine kinases, the TCR-associated p59fyn (Fyn) and the CD4/8-associated p56lck (Lck), have emerged as the likely mediators of early tyrosine phosphorylation in T cells. Here, we show direct binding of a 120-kDa TCR-induced phosphotyrosyl polypeptide, p120, to glutathione S-transferase fusion proteins of the Src homology 3 (SH3) domains of Fyn, Lck, and p60src (Src) but not other proteins. While binding of p120 to Fyn SH2 domain was phosphotyrosine-dependent as expected, its binding to the SH3 domain was independent of tyrosine phosphorylation, as shown by lack of competition with a phosphotyrosyl competitor peptide. In contrast, an SH3-specific proline-rich peptide completely abolished p120 binding to SH3. p120 was tyrosine-phosphorylated within 10 sec following stimulation of Jurkat cells with anti-CD3 monoclonal antibody, with maximal phosphorylation at 30 sec. Importantly, p120 was found associated with Fyn and Lck proteins in unstimulated Jurkat cells and served as an in vitro substrate for these kinases. These results provide evidence for a role of the SH3 domains of Fyn and Lck in the recruitment of early tyrosine-phosphorylation substrates to the TCR-associated tyrosine kinases.

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.

Isoforms of the transmembrane tyrosine phosphatase CD45 differentially affect T cell recognition

Activation of T cells has been shown to require CD45. CD45 is expressed on T cells as distinct isoforms and these isoforms are expressed differentially on subsets of CD4 T cells. We have generated T cell lines expressing a T cell receptor (TCR) of known specificity, with or without CD4, and examined the effect of different CD45 isoforms on stimulation through the antigen receptor. We find that isoforms differ in their ability to participate in antigen recognition, with the null isoform that is predominantly found on memory CD4 T cells being the most effective. The ability of the CD4 T cells being the most effective. The ability of the CD45 ectodomain to differentially affect sensitivity to specific ligands represents a novel way of regulating the efficacy of signaling through a receptor without altering its specificity. It may play a crucial role both in immunological memory and during intrathymic maturation of T cells.

Dissection of T cell antigen receptor signaling using protein tyrosine kinase inhibitors

T cell antigen receptor (TcR) recognition of appropriately presented antigen results in the rapid activation of protein tyrosine kinases. Subsequent events include activation of protein kinase C and increased intracellular free calcium which lead to the activation of transcription factors involved in regulating interleukin-2 gene expression. We have assayed the ability of a panel of protein tyrosine kinase (PTK) inhibitors to interfere with activation of the NF-AT transcription factor by TcR ligation or treatment with phorbol ester and calcium ionophore which bypass many of the early events of TcR signal transduction. The results indicate that PTK are involved in early and late stages of TcR signaling. Moreover, one inhibitor (genistein) revealed the existence of a PTK which down-regulates specifically calcium-mediated signaling at a point downstream of the PTK p56lck but upstream of calcium mobilization.

Distinct intracellular localization of Lck and Fyn protein tyrosine kinases in human T lymphocytes

Two src family kinases, lck and fyn, participate in the activation of T lymphocytes. Both of these protein tyrosine kinases are thought to function via their interaction with cell surface receptors. Thus, lck is associated with CD4, CD8, and Thy-1, whereas fyn is associated with the T cell antigen receptor and Thy-1. In this study, the intracellular localization of these two protein tyrosine kinases in T cells was analyzed by immunofluorescence and confocal microscopy. Lck was present at the plasma membrane, consistent with its proposed role in transmembrane signalling, and was also associated with pericentrosomal vesicles which co-localized with the cation-independent mannose 6-phosphate receptor. Surprisingly, fyn was not detected at the plasma membrane in either Jurkat T cells or T lymphoblasts but was closely associated with the centrosome and to microtubule bundles radiating from the centrosome. In mitotic cells, fyn co-localized with the mitotic spindle and poles. The essentially non-overlapping intracellular distributions of lck and fyn suggest that these kinases may be accessible to distinct regulatory proteins and substrates and, therefore, may regulate different aspects of T cell activation. Anti-phosphotyrosine antibody staining at the plasma membrane increases dramatically after CD3 cross-linking of Jurkat T cells. The localization of lck to the plasma membrane suggests that it may participate in mediating this increase in tyrosine phosphorylation, rather than fyn. Furthermore, the distribution of fyn in mitotic cells raises the possibility that it functions at the M phase of the cell cycle.