Cloning of a novel T-cell protein FYB that binds FYN and SH2-domain-containing leukocyte protein 76 and modulates interleukin 2 production

Hematopoietic progenitor kinase 1 associates physically and functionally with the adaptor proteins B cell linker protein and SLP-76 in lymphocytes

B cell linker protein (BLNK) is a SLP-76-related adaptor protein essential for signal transduction from the BCR. To identify components of BLNK-associated signaling pathways, we performed a phosphorylation-dependent yeast two-hybrid analysis using BLNK probes. Here we report that the serine/threonine kinase hematopoietic progenitor kinase 1 (HPK1), which is activated upon antigen-receptor stimulation and which has been implicated in the regulation of MAP kinase pathways, interacts physically and functionally with BLNK in B cells and with SLP-76 in T cells. This interaction requires Tyr(379) of HPK1 and the Src homology 2 (SH2) domain of BLNK/SLP-76. Via homology modeling, we defined a consensus binding site within ligands for SLP family SH2 domains. We further demonstrate that the SH2 domain of SLP-76 participates in the regulation of AP-1 and NFAT activation in response to T cell receptor (TCR) stimulation and that HPK1 inhibits AP-1 activation in a manner partially dependent on its interaction with SLP-76. Our data are consistent with a model in which full activation of HPK1 requires its own phosphorylation on tyrosine and subsequent interaction with adaptors of the SLP family, providing a mechanistic basis for the integration of this kinase into antigen receptor signaling cascades.

Positive and negative regulation of T-cell activation by adaptor proteins

Adaptor proteins, molecules that mediate intermolecular interactions, are now known to be as crucial for lymphocyte activation as are receptors and effectors. Extensive work from numerous laboratories has identified and characterized many of these adaptors, demonstrating their roles as both positive and negative regulators. Studies into the molecular basis for the actions of these molecules shows that they function in various ways, including: recruitment of positive or negative regulators into signalling networks, modulation of effector function by allosteric regulation of enzymatic activity, and by targeting other proteins for degradation. This review will focus on a number of adaptors that are important for lymphocyte function and emphasize the various ways in which these proteins carry out their essential roles.

The role of the adapter molecule SLP-76 in platelet function

Following vascular injury, one of the most critical initial events is activation of platelets followed by formation of a hemostatic plug. Platelets are capable of responding to a diverse array of agonists resulting in adhesion and granule release. The biochemical events underlying platelet activation are just beginning to be understood. One class of molecules shown to play important roles in this process is adapters. Adapter molecules contain distinct modular domains which mediate protein-protein or protein-lipid interactions giving these proteins the ability to nucleate signal transduction complexes. In this review we will discuss the function of the hematopoietic cell specific adapter molecule, SLP-76 in both platelet activation and hemostasis. Because many parallels exist between signal transduction pathways in platelets and lymphocytes, we will also review the function of SLP-76 in coordinating signal transduction pathways following antigen bind to the T cell receptor.

Adaptor FYB (Fyn-binding protein) regulates integrin-mediated adhesion and mediator release: differential involvement of the FYB SH3 domain

Aggregation of the high-affinity IgE receptor (FcepsilonRI) on mast cells activates a tyrosine phosphorylation cascade that is required for adhesion and degranulation events leading to the release of histamine and other inflammatory mediators. The full range of intracellular mediators that regulate this process is unknown. Recent studies have identified a group of immune cell-specific adaptor proteins that include linker for activation of T-cell (LAT), SH2-domain-containing leukocyte protein (SLP-76), and Fyn-T-binding protein (FYB)/SLP-76-associated protein (SLAP). In this study, we demonstrate that FYB can up-regulate integrin-mediated adhesion to fibronectin and mediator release in RBL-2H3 mast cells. The regulation of these two events could be distinguished from each other by the requirement of the FYB SH3 domain in beta-hexosaminidase release, but not adhesion, and the up-regulation of mediator release by FYB in nonadherent cells. FcepsilonRI aggregation increased FYB tyrosine phosphorylation, whereas confocal immunofluorescence microscopy showed that FYB colocalizes with F-actin in membrane ruffles and plaques. Our findings identify FYB as a regulator of integrin-mediated adhesion and degranulation events, which, in the case of mast cells, has potential applications to inflammatory and allergic responses.

MIST functions through distinct domains in immunoreceptor signaling in the presence and absence of LAT

MIST (also termed Clnk) is an adaptor protein structurally related to SLP-76 and BLNK/BASH/SLP-65 hematopoietic cell-specific adaptor proteins. By using the BLNK-deficient DT40 chicken B cell system, we demonstrated MIST functions through distinct intramolecular domains in immunoreceptor signaling depending on the availability of linker for activation of T cells (LAT). MIST can partially restore the B cell antigen receptor (BCR) signaling in the BLNK-deficient cells, which requires phosphorylation of the two N-terminal tyrosine residues. Co-expression of LAT with MIST fully restored the BCR signaling and dispenses with the requirement of the two tyrosines in MIST for BCR signaling. However, some other tyrosine(s), as well as the Src homology (SH) 2 domain and the two proline-rich regions in MIST, is still required for full reconstitution of the BCR signaling, in cooperation with LAT. The C-terminal proline-rich region of MIST is dispensable for the LAT-aided full restoration of MAP kinase activation, although it is responsible for the interaction with LAT and for the localization in glycolipid-enriched microdomains. On the other hand, the N-terminal proline-rich region, which is a binding site of the SH3 domain of phospholipase Cgamma, is essential for BCR signaling. These results revealed a marked plasticity of MIST function as an adaptor in the cell contexts with or without LAT.

Positive regulation of T cell activation and integrin adhesion by the adapter Fyb/Slap

The molecular adapter Fyb/Slap regulates signaling downstream of the T cell receptor (TCR), but whether it plays a positive or negative role is controversial. We demonstrate that Fyb/Slap-deficient T cells exhibit defective proliferation and cytokine production in response to TCR stimulation. Fyb/Slap is also required in vivo for T cell-dependent immune responses. Functionally, Fyb/Slap has no apparent role in the activation of known TCR signaling pathways, F-actin polymerization, or TCR clustering. Rather, Fyb/Slap regulates TCR-induced integrin clustering and adhesion. Thus, Fyb/Slap is the first molecular adapter to be identified that couples TCR stimulation to the avidity modulation of integrins governing T cell adhesion.

Coupling of the TCR to integrin activation by Slap-130/Fyb

SLAP-130/Fyb (SLP-76-associated phosphoprotein or Fyn-binding protein; also known as Fyb/Slap) is a hematopoietic-specific adapter, which associates with and modulates function of SH2-containing leukocyte phosphoprotein of 76 kilodaltons (SLP-76). T cells from mice lacking SLAP-130/Fyb show markedly impaired proliferation following CD3 engagement. In addition, the T cell receptor (TCR) in SLAP-130/Fyb mutant cells fails to enhance integrin-dependent adhesion. Although TCR-induced actin polymerization is normal, TCR-stimulated clustering of the integrin LFA-1 is defective in SLAP-130/Fyb-deficient cells. These data indicate that SLAP-130/Fyb is important for coupling TCR-mediated actin cytoskeletal rearrangement with activation of integrin function, and for T cells to respond fully to activating signals.

Synergistic regulation of immunoreceptor signaling by SLP-76-related adaptor Clnk and serine/threonine protein kinase HPK-1

Recently, the identification of Clnk, a third member of the SLP-76 family of adaptors expressed exclusively in cytokine-stimulated hemopoietic cells, has been reported by us and by others. Like SLP-76 and Blnk, Clnk was shown to act as a positive regulator of immunoreceptor signaling. Interestingly, however, it did not detectably associate with known binding partners of SLP-76, including Vav, Nck, and GADS. In contrast, it became complexed in activated T cells and myeloid cells with an as yet unknown tyrosine-phosphorylated polypeptide of approximately 92 kDa (p92). In order to understand better the function of Clnk, we sought to identify the Clnk-associated p92. Using a yeast two-hybrid screen and cotransfection experiments with Cos-1 cells, evidence was adduced that p92 is HPK-1, a serine/threonine-specific protein kinase expressed in hemopoietic cells. Further studies showed that Clnk and HPK-1 were also associated in hemopoietic cells and that their interaction was augmented by immunoreceptor stimulation. A much weaker association was detected between HPK-1 and SLP-76. Transient transfections in Jurkat T cells revealed that Clnk and HPK-1 cooperated to increase immunoreceptor-mediated activation of the interleukin 2 (IL-2) promoter. Moreover, the ability of Clnk to stimulate IL-2 promoter activity could be blocked by expression of a kinase-defective version of HPK-1. Lastly we found that in spite of the differential ability of Clnk and SLP-76 to bind cellular proteins, Clnk was apt at rescuing immunoreceptor signaling in a Jurkat T-cell variant lacking SLP-76. Taken together, these results show that Clnk physically and functionally interacts with HPK-1 in hemopoietic cells. Moreover, they suggest that Clnk is capable of functionally substituting for SLP-76 in immunoreceptor signaling, albeit by using a distinct set of intracellular effectors.

Identification of a phospholipase C-gamma1 (PLC-gamma1) SH3 domain-binding site in SLP-76 required for T-cell receptor-mediated activation of PLC-gamma1 and NFAT

SLP-76 is an adapter protein required for T-cell receptor (TCR) signaling. In particular, TCR-induced tyrosine phosphorylation and activation of phospholipase C-gamma1 (PLC-gamma1), and the resultant TCR-inducible gene expression, depend on SLP-76. Nonetheless, the mechanisms by which SLP-76 mediates PLC-gamma1 activation are not well understood. We now demonstrate that SLP-76 directly interacts with the Src homology 3 (SH3) domain of PLC-gamma1. Structure-function analysis of SLP-76 revealed that each of the previously defined protein-protein interaction domains can be individually deleted without completely disrupting SLP-76 function. Additional deletion mutations revealed a new, 67-amino-acid functional domain within the proline-rich region of SLP-76, which we have termed the P-1 domain. The P-1 domain mediates a constitutive interaction of SLP-76 with the SH3 domain of PLC-gamma1 and is required for TCR-mediated activation of Erk, PLC-gamma1, and NFAT (nuclear factor of activated T cells). The adjacent Gads-binding domain of SLP-76, also within the proline-rich region, mediates inducible recruitment of SLP-76 to a PLC-gamma1-containing complex via the recruitment of both PLC-gamma1 and Gads to another cell-type-specific adapter, LAT. Thus, TCR-induced activation of PLC-gamma1 entails the binding of PLC-gamma1 to both LAT and SLP-76, a finding that may underlie the requirement for both LAT and SLP-76 to mediate the optimal activation of PLC-gamma1.

PRAM-1 is a novel adaptor protein regulated by retinoic acid (RA) and promyelocytic leukemia (PML)-RA receptor alpha in acute promyelocytic leukemia cells

The t(15;17) translocation, found in 95% of acute promyelocytic leukemia, encodes a promyelocytic leukemia (PML)-retinoic acid receptor alpha (RARalpha) fusion protein. Complete remission of acute promyelocytic leukemia can be obtained by treating patients with all-trans retinoic acid, and PML-RARalpha plays a major role in mediating retinoic acid effects in leukemia cells. A main model proposed for acute promyelocytic leukemia is that PML-RARalpha exerts its oncogenic effects by repressing the expression of retinoic acid-inducible genes critical to myeloid differentiation. By applying subtraction cloning to acute promyelocytic leukemia cells, we identified a retinoic acid-induced gene, PRAM-1 (PML-RARalpha target gene encoding an Adaptor Molecule-1), which encodes a novel adaptor protein sharing structural homologies with the SLAP-130/fyb adaptor. PRAM-1 is expressed and regulated during normal human myelopoiesis. In U937 myeloid precursor cells, PRAM-1 expression is inhibited by expression of PML-RARalpha in the absence of ligand and de novo superinduced by retinoic acid. PRAM-1 associates with other adaptors, SLP-76 and SKAP-55HOM, in myeloid cell lines and with protein tyrosine kinase lyn. By providing the first evidence that PML-RARalpha dysregulates expression of an adaptor protein, our data open new insights into signaling events that are disrupted during transformation by PML-RARalpha and induced by retinoic acid during de novo differentiation of acute promyelocytic leukemia cells.

Identification of potential diagnostic markers of prostate cancer and prostatic intraepithelial neoplasia using cDNA microarray

The identification of novel genes or groups of genes expressed in prostate cancer may allow earlier diagnosis or more accurate staging of the disease. We describe the assembly and use of a 1877-member microarray representing cDNA clones from a range of prostate cancer stages and grades, precursor lesions and normal tissue. Using labelled cDNA from tumour samples obtained from TURP or radical prostatectomy, analysis of expression patterns identified many up-regulated transcripts. Cell lines were found to over-express fewer genes than diseased tissue samples. 17 known genes were found to over-express more than 4-fold in 4 or more cancers out of 15 cancers. Only 2 genes were over-expressed in 6 out of 15 cancers or more, whilst no genes were consistently found to be over-expressed in all cancer samples. Novel prostate cancer associations for several well characterized genes or full length cDNAs were identified, including PLRP1, JM27, human UbcM2, dynein light intermediate chain 2 and human homologue of rat sec61. Novel associations with high-grade PIN include: breast carcinoma fatty acid synthase and cDNA DKFZp434B0335. We shortlist and discuss the most significant over-expressed genes in prostate cancer and PIN, and highlight expression differences between malignant and benign samples.

Protein tyrosine phosphatase CD148-mediated inhibition of T-cell receptor signal transduction is associated with reduced LAT and phospholipase Cgamma1 phosphorylation

In this study, we investigate the role of the receptor-like protein tyrosine phosphatase CD148 in T-cell activation. Overexpression of CD148 in the Jurkat T-cell line inhibited activation of the transcription factor nuclear factor of activated T cells following T-cell receptor (TCR) stimulation but not following stimulation through a heterologously expressed G protein-coupled receptor, the human muscarinic receptor subtype 1. Using a tetracycline-inducible expression system, we show that the TCR-mediated activation of both the Ras and calcium pathways was inhibited by expression of CD148 at levels that approximate those found in activated primary T cells. These effects were dependent on the phosphatase activity of CD148. Analysis of TCR-induced protein tyrosine phosphorylation demonstrated that most phosphoproteins were unaffected by CD148 expression. However, phospholipase Cgamma1 (PLCgamma1) and LAT were strikingly hypophosphorylated in CD148-expressing cells following TCR stimulation, whereas the phosphorylation levels of Slp-76 and Itk were modestly reduced. Based on these results, we propose that CD148 negatively regulates TCR signaling by interfering with the phosphorylation and function of PLCgamma1 and LAT.

Leukocyte-specific adaptor protein Grap2 interacts with hematopoietic progenitor kinase 1 (HPK1) to activate JNK signaling pathway in T lymphocytes

Immune cell-specific adaptor proteins create various combinations of multiprotein complexes and integrate signals from cell surface receptors to the nucleus, modulating the specificity and selectivity of intracellular signal transduction. Grap2 is a newly identified adaptor protein specifically expressed in lymphoid tissues. This protein shares 40--50% sequence homology in the SH3 and the SH2 domain with Grb2 and Grap. However, the Grap2 protein has a unique 120-amino acid glutamine- and proline-rich domain between the SH2 and C-terminal SH3 domains. The expression of Grap2 is highly restricted to lymphoid organs and T lymphocytes. In order to understand the role of this specific adaptor protein in immune cell signaling and activation, we searched for the Grap2 interacting protein in T lymphocytes. We found that Grap2 interacted with the hematopoietic progenitor kinase 1 (HPK1) in vitro and in Jurkat T cells. The interaction was mediated by the carboxyl-terminal SH3 domain of Grap2 with the second proline-rich motif of HPK1. Coexpression of Grap2 with HPK1 not only increased the kinase activity of HPK1 in the cell, but also had an additive effect on HPK1 mediated JNK activation. Furthermore, over expression of Grap2 and HPK1 induced significant transcriptional activation of c-Jun in the JNK signaling pathway and IL-2 gene reporter activity in stimulated Jurkat T cells. Therefore, our data suggest that the hematopoietic specific proteins Grap2 and HPK1 form a signaling complex to mediate the c-Jun NH(2)-terminal kinase (JNK) signaling pathway in T cells.

Dynamic actin polymerization drives T cell receptor-induced spreading: a role for the signal transduction adaptor LAT

T cell activation induces functional changes in cell shape and cytoskeletal architecture. To facilitate the collection of dynamic, high-resolution images of activated T cells, we plated T cells on coverslips coated with antibodies to the T cell receptor (TCR). Using these images, we were able to quantitate the morphological responses of individual cells over time. Here, we show that TCR engagement triggers the formation and expansion of contacts bounded by continuously remodeled actin-rich rings. These processes are associated with the extension of lamellipodia and require actin polymerization, tyrosine kinase activation, cytoplasmic calcium increases, and LAT, an important hematopoietic adaptor. In addition, the maintenance of the resulting contact requires sustained calcium influxes, an intact microtubule cytoskeleton, and functional LAT.

The molecular adapter SLP-76 relays signals from platelet integrin alphaIIbbeta3 to the actin cytoskeleton

Platelet adhesion to fibrinogen through integrin alpha(IIb)beta(3) triggers actin rearrangements and cell spreading. Mice deficient in the SLP-76 adapter molecule bleed excessively, and their platelets spread poorly on fibrinogen. Here we used human platelets and a Chinese hamster ovary (CHO) cell expression system to better define the role of SLP-76 in alpha(IIb)beta(3) signaling. CHO cell adhesion to fibrinogen required alpha(IIb)beta(3) and stimulated tyrosine phosphorylation of SLP-76. SLP-76 phosphorylation required coexpression of Syk tyrosine kinase and stimulated association of SLP-76 with the adapter, Nck, and with the Rac exchange factor, Vav1. SLP-76 expression increased lamellipodia formation induced by Syk and Vav1 in adherent CHO cells (p < 0.001). Although lamellipodia formation requires Rac, SLP-76 functioned downstream of Rac by potentiating adhesion-dependent activation of PAK kinase (p < 0.001), a Rac effector that associates with Nck. In platelets, adhesion to fibrinogen stimulated the association of SLP-76 with the SLAP-130 adapter and with VASP, a SLAP-130 binding partner implicated in actin reorganization. Furthermore, SLAP-130 colocalized with VASP at the periphery of spread platelets. Thus, SLP-76 functions to relay signals from alpha(IIb)beta(3) to effectors of cytoskeletal reorganization. Therefore, deficient recruitment of specific adapters and effectors to sites of adhesion may explain the integrin phenotype of SLP-76(-/-) platelets.

Defective signaling to Fyn by a T cell antigen receptor lacking the alpha -chain connecting peptide motif

A key role in the communication between the alphabetaTCR and the CD3/zeta complex is played by a specific motif within the connecting peptide domain of the TCR alpha chain (alpha-CPM). T cell hybridomas expressing an alpha-CPM-mutated TCR show a dramatic impairment in antigen-driven interleukin-2 production. This defect can be complemented by a calcium ionophore, indicating that activation of the calcium pathway is impaired. Several lines of evidence implicate Fyn in the regulation of calcium mobilization, at least in part through the activation of phospholipase Cgamma. Here we have investigated the potential involvement of Fyn in the TCR alpha-CPM signaling defect. Using T cell hybridomas expressing either a wild-type TCR or an alpha-CPM mutant, we show that Fyn fails to be activated by the mutant receptor following SEB binding and fails to generate tyrosine-phosphorylated Pyk2, a member of the focal adhesion kinase family. This defect correlated with an impairment in phospholipase Cgamma phosphorylation. Production of interlukin-2 and activation of the transcription factor NF-AT in response to triggering of the TCR alpha-CPM mutant with SEB were fully restored in the presence of constitutively active Fyn. Hence the signaling defect generated by the TCR alpha-CPM mutation results at least in part from an impaired coupling of the TCR.CD3 complex to Fyn activation.

Roles of SLP-76, phosphoinositide 3-kinase, and gelsolin in the platelet shape changes initiated by the collagen receptor GPVI/FcRγ-chain complex

How platelet shape change initiated by a collagen-related peptide (CRP) specific for the GPVI/FcRγ-chain complex (GPVI/FcRγ-chain) is coupled to SLP-76, phosphoinositide (PI) 3-kinase, and gelsolin is reported. As shown by video microscopy, platelets rapidly round and grow dynamic filopodial projections that rotate around the periphery of the cell after they contact a CRP-coated surface. Lamellae subsequently spread between the projections. All the actin-driven shape changes require SLP-76 expression. SLP-76 is essential for the Ca++mobilization induced by CRP, whereas PI 3-kinase only modulates it. The extension of lamellae requires net actin assembly and an exposure of actin filament barbed ends downstream of PI 3-kinase. Gelsolin expression is also required for the extension of lamellae, but not for the formation of filopodia. Altogether, the data describe the role of SLP-76 in the platelet activation initiated by GPVI/FcRγ-chain and the roles of PI 3-kinase and gelsolin in lamellae spreading.

Roles of SLP-76, phosphoinositide 3-kinase, and gelsolin in the platelet shape changes initiated by the collagen receptor GPVI/FcRγ-chain complex

How platelet shape change initiated by a collagen-related peptide (CRP) specific for the GPVI/FcRγ-chain complex (GPVI/FcRγ-chain) is coupled to SLP-76, phosphoinositide (PI) 3-kinase, and gelsolin is reported. As shown by video microscopy, platelets rapidly round and grow dynamic filopodial projections that rotate around the periphery of the cell after they contact a CRP-coated surface. Lamellae subsequently spread between the projections. All the actin-driven shape changes require SLP-76 expression. SLP-76 is essential for the Ca++mobilization induced by CRP, whereas PI 3-kinase only modulates it. The extension of lamellae requires net actin assembly and an exposure of actin filament barbed ends downstream of PI 3-kinase. Gelsolin expression is also required for the extension of lamellae, but not for the formation of filopodia. Altogether, the data describe the role of SLP-76 in the platelet activation initiated by GPVI/FcRγ-chain and the roles of PI 3-kinase and gelsolin in lamellae spreading.

Recruitment of SLP-76 to the membrane and glycolipid-enriched membrane microdomains replaces the requirement for linker for activation of T cells in T cell receptor signaling

Two hematopoietic-specific adapters, src homology 2 domain-containing leukocyte phosphoprotein of 76 kD (SLP-76) and linker for activation of T cells (LAT), are critical for T cell development and T cell receptor (TCR) signaling. Several studies have suggested that SLP-76 and LAT function coordinately to promote downstream signaling. In support of this hypothesis, we find that a fraction of SLP-76 localizes to glycolipid-enriched membrane microdomains (GEMs) after TCR stimulation. This recruitment of SLP-76 requires amino acids 224-244. The functional consequences of targeting SLP-76 to GEMs for TCR signaling are demonstrated using a LAT/SLP-76 chimeric protein. Expression of this construct reconstitutes TCR-inducted phospholipase Cgamma1 phosphorylation, extracellular signal-regulated kinase activation, and nuclear factor of activated T cells (NFAT) promoter activity in LAT-deficient Jurkat T cells (J.CaM2). Mutation of the chimeric construct precluding its recruitment to GEMs diminishes but does not eliminate its ability to support TCR signaling. Expression of a chimera that lacks SLP-76 amino acids 224-244 restores NFAT promoter activity, suggesting that if localized, SLP-76 does not require an association with Gads to promote T cell activation. In contrast, mutation of the protein tyrosine kinase phosphorylation sites of SLP-76 in the context of the LAT/SLP-76 chimera abolishes reconstitution of TCR function. Collectively, these experiments show that optimal TCR signaling relies on the compartmentalization of SLP-76 and that one critical function of LAT is to bring SLP-76 and its associated proteins to the membrane.

T-cell receptor antagonists induce Vav phosphorylation by selective activation of Fyn kinase

T cell receptor (TCR) antagonists inhibit antigen-induced T cell activation and by themselves fail to induce phenotypic changes associated with T cell activation. However, we have recently shown that TCR antagonists are inducers of antigen-presenting cell (APC)-T cell conjugates. The signaling pathway associated with this cytoskeleton-dependent event appears to involve tyrosine phosphorylation and activation of Vav. In this study, we investigated the role played by the protein tyrosine kinases Fyn, Lck, and ZAP-70 in antagonist-induced signaling pathway. Antagonist stimulation increased tyrosine phosphorylation and kinase activity of Fyn severalfold, whereas little or no increase in Lck and ZAP-70 activity was observed. Second, TCR stimulation of Lck(-), Fyn(hi) Jurkat cells induced strong tyrosine phosphorylation of Vav. In contrast, minimal increase in tyrosine phosphorylation of Vav was observed in Lck(hi), Fyn(lo) Jurkat cells. Finally, study of T cells from a Fyn-deficient TCR transgenic mouse also showed that Fyn was required for tyrosine phosphorylation and activation of Vav induced by both antagonist and agonist peptides. The deficiency in Vav phosphorylation in Fyn-deficient T cells was associated with a defect in the formation of APC-T cell conjugates when T cells were stimulated with either agonist or antagonist peptide. We conclude from these results that Vav is a selective substrate for Fyn, especially under conditions of low-affinity TCR-mediated signaling, and that this signaling pathway involving Fyn, Vav, and Rac-1 is required for the cytoskeletal reorganization that leads to T cell-APC conjugates and the formation of the immunologic synapse.

Roles of the SHP-1 tyrosine phosphatase in the negative regulation of cell signalling

The critical role for the SH2 domain-containing SHP-1 tyrosine phosphatase in regulating haemopoietic cell behaviour was initially revealed by data linking SHP-1 deficiency to the systemic autoimmunity and severe inflammation exhibited by motheaten mice. This discovery laid the groundwork for the identification of SHP-1 as an inhibitor of activation-promoting signalling cascades and for the coincident demonstration that protein tyrosine phosphatases (PTPs) such as SHP-1 show considerable specificity with respect to the mechanisms whereby they modulate the biochemical and biological sequelae of extracellular simulation. As outlined in this review, SHP-1 has now been implicated in the regulation of a myriad of signalling cascades and cell functions. As a result, the cumulative data generated from studies of this PTP have elucidated not only the functional relevance of SHP-1, but also a number of novel paradigms as to the molecular mechanisms whereby signalling cascades are regulated so as to either augment or abrogate specific cell behaviours.

SH3 domain recognition of a proline-independent tyrosine-based RKxxYxxY motif in immune cell adaptor SKAP55

Src-homology 3 (SH3) domains recognize PXXP core motif preceded or followed by positively charged residue(s). Whether SH3 domains recognize motifs other than proline-based sequences is unclear. In this study, we report SH3 domain binding to a novel proline-independent motif in immune cell adaptor SKAP55, which is comprised of two N-terminal lysine and arginine residues followed by two tyrosines (i.e. RKxxYxxY). Domains capable of binding to class I proline motifs bound to the motif, while the class II domains failed to bind. Peptide precipitation, alanine scanning and in vivo co-expression studies demonstrated a requirement for the arginine, lysine and tandem tyrosines of the motif. Two-dimensional NMR analysis of the peptide bound FYN-SH3 domain showed overlap with the binding site of a proline-rich peptide on the charged surface of the SH3 domain, while resonance signals for other residues (W119, W120, Y137) were not perturbed by the RKGDYASY based peptide. Expression of the RKGDYASY peptide potently inhibited TcRzeta/CD3-mediated NF-AT transcription in T cells. Our findings extend the repertoire of SH3 domain binding motifs to include a tyrosine-based motif and demonstrate a regulatory role for this motif in receptor signaling.

Adapter proteins in lymphocyte antigen-receptor signaling

Adapter molecules contain discrete modular domains that direct specific intermolecular interactions to orchestrate assembly of signaling complexes. A number of adapter proteins play critical roles in both positive and negative regulation of antigen-receptor signaling, influencing lymphocyte development and function.

In vitro and in vivo macrophage function can occur independently of SLP-76

Expression of SH2 domain-containing leukocyte-specific phosphoprotein of 76 kDa (SLP-76), a hematopoietic cell-specific adapter protein, is required to couple Syk family tyrosine kinase activation to downstream mediators such as phospholipase C (PLC)-gamma following TCR, platelet collagen receptor and mast cell Fc epsilon R stimulation. In addition to T cells, mast cells and platelets, SLP-76 is expressed in monocytes and macrophages. To determine the role of SLP-76 in Fc gamma R-stimulated signaling pathways in macrophages, we examined cultured bone marrow-derived macrophages (BMM) from SLP-76(-/-) and wild-type mice. In this study, we show that Fc gamma R cross-linking rapidly induces tyrosine phosphorylation of SLP-76 in wild-type BMM. Surprisingly, however, BMM from SLP-76(-/-) mice activate ERK2 and phosphorylate PLC-gamma 2 following Fc gamma R ligation. Furthermore, SLP-76(-/-) BMM display normal Fc gamma R-dependent phagocytic function and reactive oxygen intermediate production. SLP-76(-/-) and SLP-76(+/+) BMM secrete comparable levels of IL-12 in response to lipopolysaccharide and IFN-gamma. To examine macrophage function in vivo, SLP-76(-/-) mice were challenged i.v. with Listeria monocytogenes. SLP-76(-/-) mice survive and efficiently contain the acute phase of infection similar to wild-type mice but exhibit a stable chronic infection attributed to the lack of mature T cells. These data show that, although SLP-76 is required to couple Syk family PTK activity to downstream mediators and effector functions in Fc gamma R-induced pathways in some cell types, activation of Fc gamma R-dependent pathways occurs independently of SLP-76 in BM

Fyn-binding protein (Fyb)/SLP-76-associated protein (SLAP), Ena/vasodilator-stimulated phosphoprotein (VASP) proteins and the Arp2/3 complex link T cell receptor (TCR) signaling to the actin cytoskeleton

T cell receptor (TCR)-driven activation of helper T cells induces a rapid polarization of their cytoskeleton towards bound antigen presenting cells (APCs). We have identified the Fyn- and SLP-76-associated protein Fyb/SLAP as a new ligand for Ena/ vasodilator-stimulated phosphoprotein (VASP) homology 1 (EVH1) domains. Upon TCR engagement, Fyb/SLAP localizes at the interface between T cells and anti-CD3-coated beads, where Evl, a member of the Ena/VASP family, Wiskott-Aldrich syndrome protein (WASP) and the Arp2/3 complex are also found. In addition, Fyb/SLAP is restricted to lamellipodia of spreading platelets. In activated T cells, Fyb/SLAP associates with Ena/VASP family proteins and is present within biochemical complexes containing WASP, Nck, and SLP-76. Inhibition of binding between Fyb/SLAP and Ena/VASP proteins or WASP and the Arp2/3 complex impairs TCR-dependent actin rearrangement, suggesting that these interactions play a key role in linking T cell signaling to remodeling of the actin cytoskeleton.

The role of SLP-76 and LAT in lymphocyte development

SLP-76 and LAT are two recently identified adapter proteins that are involved in the signal transduction cascade initiated by engagement of the TCR. The role of these two molecules in thymocyte development has become clearer following studies of gene targeted mice. The data indicate that SLP-76 and LAT are each critical for the expansion and differentiation of double-negative thymocytes and that SLP-76 is essential for allelic exclusion at the TCRbeta locus.

Signaling pathways engaged by NK cell receptors: double concerto for activating receptors, inhibitory receptors and NK cells

Despite the absence of antigen-specific receptors at their surface, NK cells can selectively eliminate virus-infected cells, tumor cells and allogenic cells. A dynamic and precisely coordinated balance between activating and inhibitory receptors governs NK cell activation programs. Multiple activating and inhibitory NK cell surface molecules have been described, a group of them acting as receptors for MHC class I molecules. In spite of their heterogeneity, activating NK cell receptors present remarkable structural and functional homologies with T cell- and B cell-antigen receptors. Inhibitory NK cell receptors operate at early stages of activating cascades by recruiting protein tyrosine phosphatases via intra- cytoplasmic motifs (ITIM), a strategy which is widely conserved in hematopoietic and non-hematopoietic cells.

Functional association between SLAP-130 and SLP-76 in Jurkat T cells

T cell antigen receptor (TCR) engagement results in protein-tyrosine kinase activation which initiates signaling cascades leading to induction of the interleukin-2 gene. Previous studies identified two substrates of the TCR-induced protein-tyrosine kinases, SH2 domain-containing leukocyte specific protein of 76 kDa (SLP-76) and SLP-76-associated phosphoprotein of 130 kDa (SLAP-130). While SLP-76 appears to couple the TCR with downstream signals, SLAP-130 may play a negative regulatory role in T cell activation. In this study, we demonstrate that consistent with its ability to abrogate the SLP-76 augmentation of TCR-induced activation of the NFAT/AP1 region of the interleukin-2 promoter, overexpression of SLAP-130 also interferes with the rescue of signaling in SLP-76-deficient Jurkat cells in co-transfection experiments. The effect of SLAP-130 on SLP-76 function is specific for regulating TCR-induced ERK activation, but not phospholipase Cgamma 1 phosphorylation. By generating both deletion and point mutants of SLAP-130, we identified tyrosine 559 as critical for the interaction between SLP-76 and SLAP-130. We show that mutation of this residue in context of full-length SLAP-130 diminishes the ability of SLAP-130 to abrogate SLP-76 function. These data suggest that the SLAP-130/SLP-76 association is important for the negative regulatory role that SLAP-130 appears to play in T cell signaling.

Adapter proteins SLP-76 and BLNK both are expressed by murine macrophages and are linked to signaling via Fcgamma receptors I and II/III

The SLP-76 (Src homology 2 domain-containing leukocyte protein of 76 kDa) adapter protein is expressed in T cells and myeloid cells, whereas its homologue BLNK (B cell linker protein) is expressed in B cells. SLP-76 and BLNK link immunoreceptor tyrosine-based activation motif-containing receptors to signaling molecules that include phospholipase C-gamma, mitogen-activated protein kinases, and the GTPases Ras and Rho. SLP-76 plays a critical role in T cell receptor, FcvarepsilonRI and gpVI collagen receptor signaling, and participates in signaling via FcgammaR and killer cell inhibitory receptors. BLNK plays a critical role in B cell receptor signaling. We show that murine bone marrow-derived macrophages express both SLP-76 and BLNK. Selective ligation of FcgammaRI and FcgammaRII/III resulted in tyrosine phosphorylation of both SLP-76 and BLNK. SLP-76(-/-) bone marrow-derived macrophages display FcgammaR-mediated tyrosine phosphorylation of Syk, phospholipase C-gamma2, and extracellular signal regulated kinases 1 and 2, and normal FcgammaR-dependent phagocytosis. These data suggest that both SLP-76 and BLNK are coupled to FcgammaR signaling in murine macrophages.

Intracellular adapter molecules

Lymphocyte antigen receptor engagement leads to the initiation of numerous signal transduction pathways that direct ultimate cellular responses. In recent years, it has become apparent that adapter molecules regulate the coupling of receptor-proximal events, such as protein tyrosine kinase activation, with end results such as inducible gene expression and cytoskeletal rearrangements. While adapter molecules possess no intrinsic enzymatic activity, their ability to mediate protein-protein interactions is vital for the integration and propagation of signal transduction cascades in lymphocytes. Recent studies demonstrate that intracellular adapter molecules function as both positive and negative regulators of lymphocyte activation.

Cutting edge: a novel function for the SLAP-130/FYB adapter protein in beta 1 integrin signaling and T lymphocyte migration

The role of integrin-mediated signaling events in T cell function remains incompletely characterized. We report here that alpha4beta1 integrin stimulation of H9 T cells and normal human T cell blasts results in rapid and transient tyrosine phosphorylation of the adapter protein, SH2 domain-containing 76-kDa protein (SLP-76)-associated phosphoprotein of 130 kDa (SLAP-130)/FYB at levels comparable to those observed following TCR stimulation. Stimulation of T cells via the alpha4beta1 integrin enhances the association of tyrosine phosphorylated SLAP-130/FYB with the SH2 domain of the src tyrosine kinase p59fyn. Activation of normal T cells, but not H9 T cells, via alpha4beta1 leads to tyrosine phosphorylation of SLP-76 as well as SLAP-130/FYB. Overexpression of SLAP-130/FYB in normal T cells enhances T cell migration through fibronectin-coated filters in response to the chemokine stromal cell-derived factor (SDF)-1alpha. These results identify SLAP-130/FYB as a new tyrosine phosphorylated substrate in beta1 integrin signaling and suggest a novel function for SLAP-130/FYB in regulating T lymphocyte motility.

Genomic and expression analyses of alternatively spliced transcripts of the MLL septin-like fusion gene (MSF) that map to a 17q25 region of loss in breast and ovarian tumors

We previously defined a common region of 17q25 loss in breast and ovarian tumors, suggesting localization of at least one putative tumor suppressor gene. Genomic clones from the interval were used to isolate candidate transcripts. One novel transcript had strong homology to a septin family of GTPase genes involved in cytokinesis. This gene was recently identified as a myeloid/lymphoid leukemia (MLL) fusion protein partner in acute myeloid leukemia and was named MSF (MLL septin-like fusion). As this gene may play roles in both leukemogenesis and tumorigenesis, it is essential to understand its structure and normal expression. We cloned two human alternative transcripts and identified a third database variant of MSF. RNA expression studies with a probe common to the three novel sequences showed differential expression of 4.0- and 3.0-kb transcripts in all adult and fetal tissues tested. A probe spanning sequence unique to one MSF variant detected specific expression of the 4.0-kb transcript in all tissues. Another probe unique to a different MSF variant detected a 4.0-kb transcript only in skeletal muscle. Proteins of 422 and 586 amino acids were predicted from the novel alternate transcripts and included both a xylose isomerase 1 domain and a GTPase domain. Nine common exons, three alternatively spliced exons, and six polymorphisms were identified.

An essential role for BLNK in human B cell development

The signal transduction events that control the progenitor B cell (pro-B cell) to precursor B cell (pre-B cell) transition have not been well delineated. In evaluating patients with absent B cells, a male with a homozygous splice defect in the cytoplasmic adapter protein BLNK (B cell linker protein) was identified. Although this patient had normal numbers of pro-B cells, he had no pre-B cells or mature B cells, indicating that BLNK plays a critical role in orchestrating the pro-B cell to pre-B cell transition. The immune system and overall growth and development were otherwise normal in this patient, suggesting that BLNK function is highly specific.

Cutting Edge: SLP-76 Cooperativity with FYB/FYN-T in the Up-Regulation of TCR-Driven IL-2 Transcription Requires SLP-76 Binding to FYB at Tyr595 and Tyr651

SLP-76 (Src homology (SH) 2-domain-containing leukocyte protein of 76 kDa) and FYB/SLAP (FYN-T-binding protein/SLP-76-associated protein) are two hemopoietic cell-specific adaptor proteins downstream of TCR-activated protein tyrosine kinases. SLP-76 has been implicated as an essential component in T cell signaling. FYB is selectively phosphorylated by FYN-T, providing a template for the recruitment of FYN-T and SLP-76 SH2 domains. Coexpression of FYN-T, FYB, and SLP-76 can synergistically up-regulate IL-2 production in T cells upon TCR ligation. In this report, we show that two tyrosines, Tyr595 and Tyr651, of FYB are major sites of phosphorylation by FYN-T and mediate binding to SLP-76 in Jurkat T cells. Furthermore, the synergistic up-regulation of IL-2 promoter activity in the FYN-T-FYB-SLP-76 pathway is contingent upon the interaction between FYB and SLP-76, but not the interaction between FYB and FYN-T. These observations define a pathway by which SLP-76 interacts with downstream components in the up-regulation of T cell cytokine production.

Clnk, a novel SLP-76-related adaptor molecule expressed in cytokine-stimulated hemopoietic cells

We have identified a novel Src homology 2 domain-containing leukocyte protein of 76 kD (SLP-76)-related molecule which we have termed Clnk (for cytokine-dependent hemopoietic cell linker). Unlike its relatives SLP-76 and B cell linker protein (Blnk), Clnk is not expressed uniformly within a given hemopoietic cell lineage. Even though it can be detected in several cell types, including T cells, natural killer cells, and mast cells, its expression seems to be strictly dependent on sustained exposure to cytokines such as interleukin (IL)-2 and IL-3. Strong support for the notion that Clnk is involved in immunoreceptor signaling was provided by the observation that it inducibly associated with at least one tyrosine-phosphorylated polypeptide (p92) in response to immunoreceptor stimulation. Moreover, transient expression of Clnk caused an increase in immunoreceptor-mediated signaling events in a T cell line. Taken together, these results show that Clnk is a novel member of the SLP-76 family selectively expressed in cytokine-stimulated hemopoietic cells. Furthermore, they suggest that Clnk may be involved in a cross-talk mechanism between cytokine receptor and immunoreceptor signaling.

Novel isoform of lymphoid adaptor FYN-T-binding protein (FYB-130) interacts with SLP-76 and up-regulates interleukin 2 production

T-cell activation involves the participation of protein-tyrosine kinases p56(lck) and ZAP-70/SYK as well as lymphoid proteins such as SLP-76 and FYB/SLAP. FYB/SLAP has the hallmarks of an adaptor protein that binds to the SH2 domains of the Src kinase FYN-T and SLP-76. Whereas two forms of FYB at 120 and 130 kDa have been identified biochemically, a cDNA encoding only the lower molecular weight isoform has been cloned (termed FYB-120 or SLAP-130). In this study, we report the isolation of an alternative isoform of FYB with a molecular mass of 130 kDa (FYB-130) that has the same structure as FYB-120 except for an insertion of 46 amino acids toward the carboxyl-terminal region of the protein. FYB-120 and FYB-130 share an ability to bind to the SH2 domains of FYN-T and SLP-76, to act as substrates for p59(FYN-T), and to be expressed in the cytoplasm and nucleus of T-cells. Differences were noted between the isoforms in the efficiency of binding to SLP-76 and in the preferential expression of FYB-130 in mature T-cells. When co-expressed together with FYN-T and SLP-76, FYB-130 caused a significant increase in anti-CD3-driven NF-AT transcription. Finally, fluorescence in situ hybridization analysis localized the FYB gene to human chromosome 5 at position p13.1. FYB-130 therefore represents a novel variant of FYB protein that can up-regulate T-cell receptor-driven interleukin 2 production in mature T-cells.

Signaling scaffolds in immune cells

Of the past several years progress in understanding TCR signal transduction has led to the discovery of new kinases, adapter molecules and multiple signaling pathways. The study of molecules such as LAT, SLP-76, FYB, SKAP-55 and VAV have revealed multiple mechanisms with which to control the activation of downstream signaling pathways through RAS, PLC gamma-1 and ERK/MAPK. Signaling through SLP-76 can play a role in TCR-induced cytoskeleton changes through activation of effector molecules in the RAC/RHO-family of GTPases. In addition, SLP-76 through its association with FYB/FYN-T appears to play a role in IL-2 gene transcription following TCR activation. Finally, these newly identified adaptor molecules, such as LAT, may be crucial in T-cell activation by enhancing the recruitment of critical kinases to glycolipid-enriched microdomains of the activated T-cell receptor complex.

Targeted disruption of the murine fps/fes proto-oncogene reveals that Fps/Fes kinase activity is dispensable for hematopoiesis

The fps/fes proto-oncogene encodes a cytoplasmic protein-tyrosine kinase that is functionally implicated in the survival and terminal differentiation of myeloid progenitors and in signaling from several members of the cytokine receptor superfamily. To gain further insight into the physiological function of fps/fes, we targeted the mouse locus with a kinase-inactivating missense mutation. Mutant Fps/Fes protein was expressed at normal levels in these mice, but it lacked detectable kinase activity. Homozygous mutant animals were viable and fertile, and they showed no obvious defects. Flow cytometry analysis of bone marrow showed no statistically significant differences in the levels of myeloid, erythroid, or B-cell precursors. Subtle abnormalities observed in mutant mice included slightly elevated total leukocyte counts and splenomegaly. In bone marrow hematopoietic progenitor cell colony-forming assays, mutant mice gave slightly elevated numbers and variable sizes of CFU-granulocyte macrophage in response to interleukin-3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF). Tyrosine phosphorylation of Stat3 and Stat5A in bone marrow-derived macrophages was dramatically reduced in response to GM-CSF but not to IL-3 or IL-6. This suggests a distinct nonredundant role for Fps/Fes in signaling from the GM-CSF receptor that does not extend to the closely related IL-3 receptor. Lipopolysaccharide-induced Erk1/2 activation was also reduced in mutant macrophages. These subtle molecular phenotypes suggest a possible nonredundant role for Fps/Fes in myelopoiesis and immune responses.

Interaction of SLP adaptors with the SH2 domain of Tec family kinases

Activation of lymphocytes through their antigen receptors leads to mobilization of intracellular Ca(2+) ions. This process requires expression of SLP adaptors and involves phosphorylation of phospholipase C-gamma isoforms by the Tec-related protein tyrosine kinase Btk in B cells and Itk in T cells. The SH2 domain of Btk and Itk is essential for phospholipase C-gamma phosphorylation and mutations in this domain lead to the X-linked agammaglobulinemia immuno deficiency in humans. Here we show that, in contrast to SH2 domains from other signaling proteins, the Btk and Itk SH2 domains exhibit a restricted binding specificity. They bind selectively to tyrosine-phosphorylated SLP-65 and SLP-76 in activated B and T cells, respectively. Our findings suggest that Btk/Itk and phospholipase C-gamma both bind via their SH2 domain to phosphorylated SLP adaptors, and that this association is required for the activation of phospholipase C-gamma.

Coupling the TCR to downstream signalling pathways: the role of cytoplasmic and transmembrane adaptor proteins

Engagement of the T-cell receptor (TCR) complex initiates a cascade of intracellular events ultimately leading to T-cell proliferation and differentiation. One of the first detectable consequences of TCR triggering is the activation of cytoplasmic protein kinases which, through phosphorylation of specific substrates, couple the TCR to downstream signalling cascades. Although it is well established that activation of the Ras- and the calcium-dependent calcineurin pathway is required for the achievement of T-cell activation, the precise mechanism as to how the TCR is connected to these intracellular effector molecules is still unclear. Major progress has been made in this regard with the molecular characterization of novel cytoplasmic and transmembrane molecules called adaptor proteins which integrate TCR-mediated signals at the intracellular level thus allowing fine tuning of T-cell responses.

YopH dephosphorylates Cas and Fyn-binding protein in macrophages

The tyrosine phosphatase YopH is an essential virulence effector of pathogenic Yersinia spp. YopH, which is translocated from extracellularly located bacteria into interacting target cells, blocks phagocytosis by professional phagocytes. We show here that immunoprecipitation of YopH from lysates of J774 cells infected with Y. pseudotuberculosis expressing an inactive form of YopH resulted in co-precipitation of certain phosphotyrosine proteins. The association between the inactive YopH and phosphotyrosine proteins in the 120 kDa range was rapid and could be detected after 2 min of infection. The proteins were identified as the docking proteins Cas and Fyn-binding protein (FYB). Upon infection of J774 cells with Y. pseudotuberculosis lacking YopH expression both of these proteins became tyrosine phosphorylated. Moreover, this infection caused recruitment of Cas to peripheral focal complexes, and FYB was relocalized to areas surrounding these structures. Both Cas and FYB became dephosphorylated upon infection with Y. pseudotuberculosis expressing active YopH, and this was associated with disruption of focal complexes. With regard to the previous identification of Cas and focal complexes as targets of YopH in HeLa cells, the present study supports an important role for these targets in a general mechanism of bacterial uptake.

SLP-76 Binding to p56lck: A Role for SLP-76 in CD4-Induced Desensitization of the TCR/CD3 Signaling Complex

Nonreceptor protein tyrosine kinases and associated substrates play a pivotal role in Ag receptor stimulation of resting cells and in the initiation of activation-induced cell death (AICD) of preactivated T cells. CD4-associated p56lck has been implicated not only in the activation of primary T cells, but also in the inhibition of T cell responses. We have previously shown that CD4+ T cell clones can be rescued from AICD when surface CD4 is engaged before the TCR stimulus. In this study, we show that prevention of AICD is associated with a CD4-dependent inhibition of TCR-triggered tyrosine phosphorylation of the Src homology 2 domain-containing leukocyte protein of 76 kDa (SLP-76) and Vav. We provide evidence for a SLP-76 interaction with Src homology 3 domains of p56lck and identify amino acids 185–194 of SLP-76 as relevant docking site. In view of the multiple functions of p56lck and SLP-76/Vav in the initiation of TCR/CD3/CD4 signaling, we propose a model for the CD4-dependent inhibition of TCR signaling and AICD of preactivated T cells. Our data suggest that preformed activation complexes of adapter proteins and enzymes in the vicinity of the CD4/p56lck complex are no longer available for the TCR signal when CD4 receptors are engaged before TCR stimulation.

SHPS-1 is a scaffold for assembling distinct adhesion-regulated multi-protein complexes in macrophages

Inhibitory immunoreceptors downregulate signaling by recruiting Src homology 2 (SH2) domain-containing tyrosine and/or lipid phosphatases to activating receptor complexes [1]. There are indications that some inhibitory receptors might also perform other functions [2] [3]. In adherent macrophages, two inhibitory receptors, SHPS-1 and PIR-B, are the major proteins binding to the tyrosine phosphatase SHP-1. SHPS-1 also associates with two tyrosine-phosphorylated proteins (pp55 and pp130) and a protein tyrosine kinase [4]. Here, we have identified pp55 and pp130 as the adaptor molecules SKAP55hom/R (Src-kinase-associated protein of 55 kDa homologue) and FYB/SLAP-130 (Fyn-binding protein/SLP-76-associated protein of 130 kDa), respectively, and the tyrosine kinase activity as PYK2. Two distinct SHPS-1 complexes were formed, one containing SKAP55hom/R and FYB/SLAP-130, and the other containing PYK2. Recruitment of FYB/SLAP-130 to SHPS-1 required SKAP55hom/R, whereas PYK2 associated with SHPS-1 independently. Formation of both complexes was independent of SHP-1 and tyrosine phosphorylation of SHPS-1. Finally, tyrosine phosphorylation of members of the SHPS-1 complexes was regulated by integrin-mediated adhesion. Thus, SHPS-1 provides a scaffold for the assembly of multi-protein complexes that might both transmit adhesion-regulated signals and help terminate such signals through SHP-1-directed dephosphorylation. Other inhibitory immunoreceptors might have similar scaffold-like functions.

Antibody-induced CD3-CD4 coligation inhibits TCR/CD3 activation in the absence of costimulatory signals in normal mouse CD4(+) T lymphocytes

The effect of CD3-CD4 coligation on CD3-mediated activation of normal mouse CD4(+) T lymphocytes has been analyzed in the absence of exogenous lymphokines. If anti-CD3 and anti-CD4 antibodies are adsorbed to culture wells by means of previously adsorbed anti-Ig antibodies (indirect binding), CD3-CD4 coligation inhibits activation measured as cell proliferation or as secretion of IL-2, IL-4, and IFN-gamma. Addition of IL-2, anti-CD28 antibodies, or phorbol esters, but not IL-1, IL-4, or ionomycin, blocked CD4-mediated inhibition and restored the response to levels equal or higher than those of cultures activated by anti-CD3 alone. In contrast, CD3-CD4 coligation by antibodies directly adsorbed to culture wells potentiated anti-CD3-induced activation, either in the absence or in the presence of exogenous costimuli. Similar results were observed when CD4(+) T cells of naive phenotype (CD44(low), CD45RB(high)) were used in the experiments. The analysis of early tyrosine phosphorylation in CD4(+) T cells shows that phosphorylation of many cell substrates is clearly enhanced upon CD3-CD4 coligation using indirectly or directly bound antibodies, yet certain substrates are mainly phosphorylated under inhibitory conditions. Although CD28 ligation does not produce any clear change in the tyrosine phosphorylation pattern in lysates from cells activated by indirectly bound anti-CD3 plus anti-CD4 antibodies, the analysis of active forms of the MAP kinase ERK suggests that downstream signaling pathways involved in IL-2 gene activation can be differentially activated depending on the direct or indirect CD3-CD4 adsorption and CD28 ligation.

T cell receptor-mediated signs and signals governing T cell development

The developmental fate of T cells is largely controlled by the nature and success of signals mediated by the pre-T cell receptor (TCR) and TCR complexes. These intracellular signals are regulated by cascades of protein tyrosine phosphorylations initiated following ligand binding to the pre-TCR or TCR complexes. The phosphorylation cascades are primarily orchestrated by two distinct families of protein tyrosine kinases (PTKs), the Src- and the Syk/ZAP-70-families. Germline gene targeting experiments, several human immunodeficiencies, and somatic cell mutants have all contributed to our understanding of how these families of kinases coordinate their actions to promote signaling. Upon activation, the PTKs transmit their signals to a number of newly described adaptor proteins including LAT, SLP-76, and vav, among others. The following review combines results derived from different experimental strategies to examine the contributions of the PTKs and the adaptor molecules to pre-TCR and TCR signaling processes.

FYN-T-FYB-SLP-76 interactions define a T-cell receptor zeta/CD3-mediated tyrosine phosphorylation pathway that up-regulates interleukin 2 transcription in T-cells

Protein-tyrosine kinases p56(Lck), SYK, and ZAP-70 and downstream adaptors LAT and SLP-76 have been implicated as essential components in T-cell activation. Another lymphoid-specific adaptor FYB/SLAP has also been identified as a predominant binding partner of SLP-76 and the Src kinase FYN-T, although its role in the activation process has been unclear. In this study, we demonstrate that FYN-T selectively phosphorylates FYB providing a template for the recruitment of FYN-T and SLP-76 SH2 domain binding. This interaction is unusual in its distinct cytoplasmic localization and its long term stable kinetics of phosphorylation. Furthermore, we demonstrate for the first time that the co-expression of all three components of the FYN-T-FYB-SLP-76 matrix can synergistically up-regulate T-cell receptor-driven interleukin 2 transcription activity. These findings document the existence of a T-cell receptor-regulated FYN-T-FYB pathway that interfaces with the adaptor SLP-76 and up-regulates lymphokine production in T-cells.