Functional complementation of BLNK by SLP-76 and LAT linker proteins

Recent studies have demonstrated a requirement for the SLP-76 (SH2 domain-containing leukocyte protein of 76 kDa) and LAT (linker for activation of T cells) adaptor/linker proteins in T cell antigen receptor activation and T cell development as well as the BLNK (B cell linker) linker protein in B cell antigen receptor (BCR) signal transduction and B cell development. Whereas the SLP-76 and LAT adaptor proteins are expressed in T, natural killer, and myeloid cells and platelets, BLNK is preferentially expressed in B cells and monocytes. Although BLNK is structurally homologous to SLP-76, BLNK interacts with a variety of downstream signaling proteins that interact directly with both SLP-76 and LAT. Here, we demonstrate that neither SLP-76 nor LAT alone is sufficient to restore the signaling deficits observed in BLNK-deficient B cells. Conversely, the coexpression of SLP-76 and LAT together restored BCR-inducible calcium responses as well as activation of all three families of mitogen-activated protein kinases. Together, these data suggest functional complementation of SLP-76 and LAT in T cell antigen receptor function with BLNK in BCR function.     

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.     

HIV-1 Nef Limits Communication between Linker of Activated T Cells and SLP-76 To Reduce Formation of SLP-76-Signaling Microclusters following TCR Stimulation

Signal initiation by engagement of the TCR triggers actin rearrangements, receptor clustering, and dynamic organization of signaling complexes to elicit and sustain downstream signaling. Nef, a pathogenicity factor of HIV, disrupts early TCR signaling in target T cells. To define the mechanism underlying this Nef-mediated signal disruption, we employed quantitative single-cell microscopy following surface-mediated TCR stimulation that allows for dynamic visualization of distinct signaling complexes as microclusters (MCs). Despite marked inhibition of actin remodeling and cell spreading, the induction of MCs containing TCR-CD3 or ZAP70 was not affected significantly by Nef. However, Nef potently inhibited the subsequent formation of MCs positive for the signaling adaptor Src homology-2 domain-containing leukocyte protein of 76 kDa (SLP-76) to reduce MC density in Nef-expressing and HIV-1-infected T cells. Further analyses suggested that Nef prevents formation of SLP-76 MCs at the level of the upstream adaptor protein, linker of activated T cells (LAT), that couples ZAP70 to SLP-76. Nef did not disrupt pre-existing MCs positive for LAT. However, the presence of the viral protein prevented de novo recruitment of active LAT into MCs due to retargeting of LAT to an intracellular compartment. These modulations in MC formation and composition depended on Nef's ability to simultaneously disrupt both actin remodeling and subcellular localization of TCR-proximal machinery. Nef thus employs a dual mechanism to disturb early TCR signaling by limiting the communication between LAT and SLP-76 and preventing the dynamic formation of SLP-76-signaling MCs.     

Independent and Cooperative Roles of Adaptor Molecules in Proximal Signaling during Fc?RI-Mediated Mast Cell Activation

Activation through FcɛRI, a high-affinity IgE-binding receptor, is critical for mast cell function during allergy. The formation of a multimolecular proximal signaling complex nucleated by the adaptor molecules SLP-76 and LAT1 is required for activation through this receptor. Based on previous T-cell studies, current dogma dictates that LAT1 is required for plasma membrane recruitment and function of SLP-76. Unexpectedly, we found that the recruitment and phosphorylation of SLP-76 were preserved in LAT1^−/− mast cells and that SLP-76^−/− and LAT1^−/− mast cells harbored distinct functional and biochemical defects. The LAT1-like molecule LAT2 was responsible for the preserved membrane localization and phosphorylation of SLP-76 in LAT1^−/− mast cells. Although LAT2 supported SLP-76 phosphorylation and recruitment to the plasma membrane, LAT2 only partially compensated for LAT1-mediated cell signaling due to its decreased ability to stabilize interactions with phospholipase Cγ (PLCγ). Comparison of SLP-76^−/− LAT1^−/− and SLP-76^−/− mast cells revealed that some functions of LAT1 could occur independently of SLP-76. We propose that while SLP-76 and LAT1 depend on each other for many of their functions, LAT2/SLP-76 interactions and SLP-76-independent LAT1 functions also mediate a positive signaling pathway downstream of FcɛRI in mast cells.Mast cell activation during allergic inflammation is mediated by the high-affinity immunoglobulin E (IgE)-binding receptor FcɛRI. Cross-linking of FcɛRI on mast cells by IgE/cognate antigen complexes results in the rapid release of a wide array of inflammatory mediators, including vasoactive amines and cytokines/chemokines that give rise to allergic symptoms, ranging in severity from simple urticaria to anaphylactic shock and death (14). As allergy affects ∼30% of the population in developed countries (13), much attention has been placed on studying the signal transduction mechanisms involved in mast cell activation downstream of FcɛRI in hopes of finding novel targets for therapeutic intervention.Signal transduction downstream of FcɛRI is initiated by the phosphorylation of the immunoreceptor tyrosine-based activation motifs (ITAMs) contained in the signaling components (β and γ chains) of the FcɛRI complex (30, 37). Once phosphorylated, these chains serve as docking sites for several protein tyrosine kinases (PTKs), including Lyn and spleen tyrosine kinase (Syk) (9, 19, 34). Recruitment of Syk to the membrane by FcɛRI results in the phosphorylation of scaffold proteins known as adaptor molecules. Adaptor proteins lack enzymatic activity but instead contain protein-binding domains that are critical for the formation of a multimolecular complex, which orchestrates downstream signaling in a temporal and spatial manner. The adaptor molecules Src homology 2 (SH2) domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76) and linker of activated T cells 1 (LAT1) organize the assembly of a proximal signaling complex downstream of FcɛRI. Failure to form this complex is detrimental to FcɛRI-mediated mast cell function, as demonstrated by the finding that both SLP-76-deficient (22, 29, 41) and LAT1-deficient (25, 31, 32) mast cells display severely diminished degranulation and cytokine/chemokine production following FcɛRI ligation.Similar proximal signaling complexes are formed downstream of several different ITAM-containing receptors. Much of our understanding of the role of adaptor molecules in signal transduction has come from identification of phosphoproteins during T-cell receptor (TCR)-mediated activation of the human Jurkat T-cell line (1, 33). These studies eventually led to a paradigm describing the sequence of events in the formation of the SLP-76/LAT1 signaling complex. According to this model, SLP-76 is found constitutively bound to Grb2-related adaptor downstream of Shc (GADS) (24) and resides in the cytosol. Upon TCR activation, the tyrosines of membrane-resident LAT1 are phosphorylated and become attachment sites for proteins such as phospholipase Cγ (PLCγ) and GADS (43, 45). SLP-76 is drawn to the membrane through a GADS/LAT1 interaction, which then permits Syk family PTKs to maximally phosphorylate the N-terminal tyrosines of SLP-76 (5, 10). Several lines of evidence support this model whereby a LAT1/SLP-76 module organizes TCR signaling. First, both SLP-76- and LAT1-deficient Jurkat T cells display similar biochemical defects, such as diminished PLCγ and extracellular signal-regulated kinase (ERK) activation (10, 42). Second, T cells in SLP-76^−/− and LAT1^−/− mice are blocked at the same stage of development (7, 44). Third, SLP-76 can be coimmunoprecipitated with LAT1 but not with LAT1 harboring tyrosine-to-phenylalanine mutations (45). Finally, expression of a fusion protein comprised of the membrane-localizing domain of LAT1 and SLP-76 that forces localization of SLP-76 to the plasma membrane rescues the TCR-induced functional defects of both SLP-76- and LAT1-deficient Jurkat T cells (3). This model implies a mutually dependent relationship between SLP-76 and LAT1, where SLP-76 and LAT1 rely on each other to carry out their roles.One might suspect that this model for LAT1/SLP-76 function would operate in all other cells that utilize these adaptor molecules for ITAM-containing receptor-mediated signaling. However, the published defects of LAT1-deficient mast cells in FcɛRI-mediated signaling appeared milder than those of SLP-76-deficient mast cells, although a direct comparison has never been reported. In the present study, we show that LAT1-deficient mast cells display distinct functional and biochemical defects compared to SLP-76-deficient mast cells, implying that unlike in T cells, SLP-76 may not depend entirely on LAT1 for its function in mast cells. Surprisingly, the membrane recruitment and phosphorylation of SLP-76 were also preserved in LAT1^−/− mast cells. We show that LAT2 (also known as non-T-cell activation linker [NTAL] or linker for activation of B cells [LAB]), which is not expressed in naïve T cells but is expressed in mast cells (15), is responsible for phosphorylation and plasma membrane recruitment of SLP-76 in the absence of LAT1. However, LAT2 cannot support all LAT1/SLP-76-associated functions, such as sustained Ca²⁺ flux, likely due to decreased stability of the LAT2/SLP-76/PLCγ complex. Comparison of SLP-76^−/− LAT1^−/− and SLP-76^−/− mast cells also revealed that some functions of LAT1 could occur independently of SLP-76. We propose that although SLP-76 and LAT1 are interdependent for many of their functions, LAT2/SLP-76 interactions and SLP-76-independent LAT1 functions mediate positive signaling downstream of FcɛRI in mast cells.     

Differential SLP-76 expression and TCR-mediated signaling in effector and memory CD4 T cells

We present in this study novel findings on TCR-mediated signaling in naive, effector, and memory CD4 T cells that identify critical biochemical markers to distinguish these subsets. We demonstrate that relative to naive CD4 T cells, memory CD4 T cells exhibit a profound decrease in expression of the linker/adapter molecule SLP-76, while effector T cells express normal to elevated levels of SLP-76. The reduced level of SLP-76 is memory CD4 T cells is coincident with reduced phosphorylation overall, yet the residual SLP-76 couples to a subset of TCR-associated linker molecules, leading to downstream mitogen-activated protein (MAP) kinase activation. By contrast, effector CD4 T cells strongly phosphorylate SLP-76, linker for activation of T cells, and additional Grb2-coupled proteins, exhibit increased associations of SLP-76 to phosphorylated linkers, and hyperphosphorylate downstream Erk1/2 MAP kinases. Our results suggest distinct coupling of signaling intermediates to the TCR in naive, effector, and memory CD4 T cells. Whereas effector CD4 T cells amplify existing TCR signaling events accounting for rapid effector responses, memory T cells engage fewer signaling intermediates to efficiently link TCR triggering directly to downstream MAP kinase activation.     

The Adapter Protein SLP-76 Mediates “Outside-In” Integrin Signaling and Function in T Cells

The adapter protein SH2 domain-containing leukocyte protein of 76 kDa (SLP-76) is an essential mediator of signaling from the T-cell antigen receptor (TCR). We report here that SLP-76 also mediates signaling downstream of integrins in T cells and that SLP-76-deficient T cells fail to support adhesion to integrin ligands. In response to both TCR and integrin stimulation, SLP-76 relocalizes to surface microclusters that colocalize with phosphorylated signaling proteins. Disruption of SLP-76 recruitment to the protein named LAT (linker for activation of T cells) inhibits SLP-76 clustering downstream of the TCR but not downstream of integrins. Conversely, an SLP-76 mutant unable to bind ADAP (adhesion and degranulation-promoting adapter protein) forms clusters following TCR but not integrin engagement and fails to support T-cell adhesion to integrin ligands. These findings demonstrate that SLP-76 relocalizes to integrin-initiated signaling complexes by a mechanism different from that employed during TCR signaling and that SLP-76 relocalization corresponds to SLP-76-dependent integrin function in T cells.Coordinated signals from the extracellular environment direct T-cell functions, including cell trafficking and antigen-driven cell activation. One way that T cells make contact with their environment is through integrins expressed on the cell surface. Integrins are heterodimeric proteins that bind adhesion molecules in the extracellular matrix and on the surface of other cells (26). The predominant integrins expressed on T cells are leukocyte function-associated antigen 1 (LFA-1 [α_Lβ₂]) and very late antigen 4 (VLA-4 [α₄β₁]). By associating with both extracellular adhesion molecules and the cytoskeleton, integrins bridge the cell exterior and interior. Integrin function is critical for T-cell development, trafficking through the vasculature and tissues, the formation of conjugates with antigen-presenting cells, T-cell activation, and directed cytokine secretion (18, 38, 43, 47).Extensive studies of proximal signaling downstream of the engaged T-cell antigen receptor (TCR) have provided a model of how T cells respond to extracellular cues (44). TCR stimulation triggers the activation of the protein tyrosine kinases (PTKs) Lck and ζ-associated protein of 70 kDa (ZAP-70), resulting in the phosphorylation of the transmembrane adapter protein linker for activation of T cells (LAT). Another critical adapter protein, SH2 domain-containing leukocyte protein of 76 kDa (SLP-76), is recruited to phosphorylated LAT via its binding partner Grb2-related adapter downstream of Shc (Gads) (42). Together, SLP-76 and LAT form a multimolecular signaling complex at the cell membrane, resulting in activation of effector proteins and reorganization of the actin cytoskeleton. The dynamic relocalization of SLP-76 to TCR-initiated surface signaling complexes has been characterized using biochemical and live cell imaging approaches (7, 42, 51). In these studies, the TCR-induced recruitment of SLP-76 to microclusters coincident with the presence of PTKs and other signaling proteins was visualized in real time. As these reports show, SLP-76 microclusters arise at the periphery of the spreading T cell and then migrate inward (5) in a process shown recently to be modulated by TCR coligation with integrins (5, 35).Integrins in hematopoietic cells are maintained in an inactive conformation and possess low affinity for their ligands (40). “Inside-out” signals originating at either the TCR or chemokine receptors upregulate integrin affinity (47). Integrins in the active conformation then bind adhesion molecule ligands and initiate “outside-in” signals, resulting in altered cell morphology and enhanced adhesive and proliferative responses (reviewed in reference 3). Work to elucidate outside-in signaling pathways in various hematopoietic lineages has uncovered a role for PTKs as well as adapter proteins downstream of engaged integrins. Src and Syk family kinases have been shown to initiate signals from stimulated integrins, phosphorylating target adapter proteins (including SLP-76), and the adhesion and degranulation-promoting adapter protein (ADAP) (2, 15, 17, 49). SLP-76 and ADAP have also been found to organize integrin signals in myeloid lineages and platelets and to contribute to integrin-dependent cell functions (20, 22, 34, 46).While considerable progress has been made in defining how PTKs and adapters organize signaling from the TCR, much less is understood about how these molecules function downstream of integrins in T cells. Although a number of recent elegant papers have suggested that the immunoreceptor tyrosine-based activation motif (ITAM) and integrin second-messenger pathways intersect at a point very close to the membrane, how signals are then differentiated by the cell remains unclear (30, 52, 53). For this report, we made use of an inducible in vivo model to control SLP-76 expression and test the role of this adapter protein in integrin-initiated signaling in T cells. Our findings demonstrate an essential role for SLP-76 in T-cell integrin function that correlates with the relocalization of SLP-76 to signaling microclusters initiated by integrin ligation. Interestingly, the requirements for SLP-76 partner proteins and the domains of SLP-76 that they bind are distinct with respect to SLP-76 functions downstream of integrins versus the TCR, suggesting that adapter proteins mediate both overlapping and distinct signaling pathways from these receptors.     

Attenuation of T Cell Receptor Signaling by Serine Phosphorylation-mediated Lysine 30 Ubiquitination of SLP-76 Protein

SLP-76 (SH2 domain-containing leukocyte protein of 76 kDa) is an adaptor protein that is essential for T cell development and T cell receptor (TCR) signaling activation. Previous studies have identified an important negative feedback regulation of SLP-76 by HPK1 (hematopoietic progenitor kinase 1; MAP4K1)-induced Ser-376 phosphorylation. Ser-376 phosphorylation of SLP-76 mediates 14-3-3 binding, resulting in the attenuation of SLP-76 activation and downstream signaling; however, the underlying mechanism of this action remains unknown. Here, we report that phosphorylated SLP-76 is ubiquitinated and targeted for proteasomal degradation during TCR signaling. SLP-76 ubiquitination is mediated by Ser-376 phosphorylation. Furthermore, Lys-30 is identified as a ubiquitination site of SLP-76. Loss of Lys-30 ubiquitination of SLP-76 results in enhanced anti-CD3 antibody-induced ERK and JNK activation. These results reveal a novel regulation mechanism of SLP-76 by ubiquitination and proteasomal degradation of activated SLP-76, which is mediated by Ser-376 phosphorylation, leading to down-regulation of TCR signaling.     

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.     

The adaptor protein Gads (Grb2-related adaptor downstream of Shc) is implicated in coupling hemopoietic progenitor kinase-1 to the activated TCR

The hemopoietic-specific Gads (Grb2-related adaptor downstream of Shc) adaptor protein possesses amino- and carboxyl-terminal Src homology 3 (SH3) domains flanking a central SH2 domain and a unique region rich in glutamine and proline residues. Gads functions to couple the activated TCR to distal signaling events through its interactions with the leukocyte-specific signaling proteins SLP-76 (SH2 domain-containing leukocyte protein of 76 kDa) and LAT (linker for activated T cells). Expression library screening for additional Gads-interacting molecules identified the hemopoietic progenitor kinase-1 (HPK1), and we investigated the HPK1-Gads interaction within the DO11.10 murine T cell hybridoma system. Our results demonstrate that HPK1 inducibly associates with Gads and becomes tyrosine phosphorylated following TCR activation. HPK1 kinase activity is up-regulated in response to activation of the TCR and requires the presence of its proline-rich motifs. Mapping experiments have revealed that the carboxyl-terminal SH3 domain of Gads and the fourth proline-rich region of HPK1 are essential for their interaction. Deletion of the fourth proline-rich region of HPK1 or expression of a Gads SH2 mutant in T cells inhibits TCR-induced HPK1 tyrosine phosphorylation. Together, these data suggest that HPK1 is involved in signaling downstream from the TCR, and that SH2/SH3 domain-containing adaptor proteins, such as Gads, may function to recruit HPK1 to the activated TCR complex.     

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.     

The hematopoietic-specific adaptor protein gads functions in T-cell signaling via interactions with the SLP-76 and LAT adaptors

BACKGROUND: The adaptor protein Gads is a Grb2-related protein originally identified on the basis of its interaction with the tyrosine-phosphorylated form of the docking protein Shc. Gads protein expression is restricted to hematopoietic tissues and cell lines. Gads contains a Src homology 2 (SH2) domain, which has previously been shown to have a similar binding specificity to that of Grb2. Gads also possesses two SH3 domains, but these have a distinct binding specificity to those of Grb2, as Gads does not bind to known Grb2 SH3 domain targets. Here, we investigated whether Gads is involved in T-cell signaling. RESULTS: We found that Gads is highly expressed in T cells and that the SLP-76 adaptor protein is a major Gads-associated protein in vivo. The constitutive interaction between Gads and SLP-76 was mediated by the carboxy-terminal SH3 domain of Gads and a 20 amino-acid proline-rich region in SLP-76. Gads also coimmunoprecipitated the tyrosine-phosphorylated form of the linker for activated T cells (LAT) adaptor protein following cross-linking of the T-cell receptor; this interaction was mediated by the Gads SH2 domain. Overexpression of Gads and SLP-76 resulted in a synergistic augmentation of T-cell signaling, as measured by activation of nuclear factor of activated T cells (NFAT), and this cooperation required a functional Gads SH2 domain. CONCLUSIONS: These results demonstrate that Gads plays an important role in T-cell signaling via its association with SLP-76 and LAT. Gads may promote cross-talk between the LAT and SLP-76 signaling complexes, thereby coupling membrane-proximal events to downstream signaling pathways.     

A high-affinity Arg-X-X-Lys SH3 binding motif confers specificity for the interaction between Gads and SLP-76 in T cell signaling

A critical event in T cell receptor (TCR)-mediated signaling is the recruitment of hematopoietic-specific adaptor proteins that collect and transmit signals downstream of the TCR. Gads, a member of the Grb2 family of SH2 and SH3 domain-containing adaptors, mediates the formation of a complex between LAT and SLP-76 that is essential for signal propagation from the TCR. Here we examine the binding specificity of the Gads and Grb2 SH3 domains using peptide arrays and find that a nonproline-based R-X-X-K motif found in SLP-76 binds to the Gads carboxy-terminal SH3 domain with high affinity (K(D) = 240 +/- 45 nM). The Grb2 C-terminal SH3 domain also binds this motif, but with a 40-fold lower affinity than Gads. Single point mutations in either the relevant R (237) or K (240) completely abrogated SLP-76 association with Gads in vivo and impaired SLP-76 function. A chimeric Grb2 protein, possessing the C-terminal SH3 domain of Gads, was able to partially substitute for Gads in signaling downstream of the T cell receptor. These results provide a molecular explanation for the specific role of Gads in T cell receptor signaling, and identify a discrete subclass of SH3 domains whose binding is dependent on a core R-X-X-K motif.     

Quantitative Phosphoproteomics Reveals SLP-76 Dependent Regulation of PAG and Src Family Kinases in T Cells

The SH2-domain-containing leukocyte protein of 76 kDa (SLP-76) plays a critical scaffolding role in T cell receptor (TCR) signaling. As an adaptor protein that contains multiple protein-binding domains, SLP-76 interacts with many signaling molecules and links proximal receptor stimulation to downstream effectors. The function of SLP-76 in TCR signaling has been widely studied using the Jurkat human leukaemic T cell line through protein disruption or site-directed mutagenesis. However, a wide-scale characterization of SLP-76-dependant phosphorylation events is still lacking. Quantitative profiling of over a hundred tyrosine phosphorylation sites revealed new modes of regulation of phosphorylation of PAG, PI3K, and WASP while reconfirming previously established regulation of Itk, PLCγ, and Erk phosphorylation by SLP-76. The absence of SLP-76 also perturbed the phosphorylation of Src family kinases (SFKs) Lck and Fyn, and subsequently a large number of SFK-regulated signaling molecules. Altogether our data suggests unique modes of regulation of positive and negative feedback pathways in T cells by SLP-76, reconfirming its central role in the pathway.     

Evidence for glycosylphosphatidylinositol (GPI)-anchored eosinophil-derived neurotoxin (EDN) on human granulocytes

MIST (mast cell immunoreceptor signal transducer; also termed Clnk) is an adaptor protein structurally related to SLP-76-family hematopoietic cell-specific adaptor proteins. We demonstrate here that two major MIST-associated phosphoproteins expressed in mast cell lines are SLAP-130 and SKAP55, adaptors known to interact with the Src-homology (SH) 2 domain of Src-family protein tyrosine kinases (PTKs). MIST directly associated with SLAP-130 via its SH2 domain, and collaboration of SLAP-130 with SKAP55 was required for the recruitment of MIST to Lyn. Furthermore, MIST was preferentially recruited to Fyn rather than Lyn, which is regulated by higher affinity binding of SLAP-130 and SKAP55 with the Fyn-SH2 domain than the Lyn-SH2 domain. Our results suggest that the MIST–SLAP-130–SKAP55 adaptor complex functions downstream of high-affinity IgE receptor-associated Src-PTKs in mast cells.     

The missing link(er): a return to symmetry in antigen receptor signaling?

B lymphocytes and T lymphocytes utilize several proteins with common functions to transduce signals from their respective receptors. However, at the hierarchial signalling level of SLP-76 [Src homology 2(SH2) domain-containing leukocyte protein of 76-kDa] and LAT (linker for activation of T cells) in T cells, the only corresponding protein in B cells was known to be BLNK (B cell linker protein). It was thought that perhaps BLNK performed the cognate roles of SLP-76 and LAT in B cells; however, mounting evidence to the contrary revealed that this hypothesis was not robust. Two laboratories have recently described the characterization of a protein expressed in B cells and myeloid cells, alternatively termed NTAL (non-T cell activation linker) or LAB (linker for activation of B cells). NTAL/LAB and LAT may have arisen from a primordial gene-duplicating event, but genes that code for the two proteins do not share a very high degree of sequence identity. Wange discusses the results of the two reports, the evidence for functional homology between LAT and NTAL/LAB, and the possibility that the differences between them might lead to specific clinical therapeutics to manipulate immune cell responses.     

Role of two adaptor molecules SLP-76 and LAT in the PI3K signaling pathway in activated T cells

Previously, we identified p85, a subunit of PI3K, as one of the molecules that interacts with the N-terminal region of Src homology 2 domain-containing leukocyte protein of 76 kDa (SLP-76). We also demonstrated that tyrosine phosphorylation either at the 113 and/or 128 position is sufficient for the association of SLP-76 with the Src homology 2 domain near the N terminus of p85. The present study further examines the role of the association of these two molecules on the activation of PI3K signaling cascade. Experiments were done to determine the role of SLP-76, either wild-type, tyrosine mutants, or membrane-targeted forms of various SLP-76 constructs, on the membrane localization and phosphorylation of Akt, which is an event downstream of PI3K activation. Reconstitution studies with these various SLP-76 constructs in a Jurkat variant cell line that lacks SLP-76 or linker for activation of T cells (LAT) show that the activation of PI3K pathway following TCR ligation requires both SLP-76 and LAT adaptor proteins. The results suggest that SLP-76 associates with p85 after T cell activation and that LAT recruits this complex to the membrane, leading to Akt activation.     

Disruption of SLP-76 interaction with Gads inhibits dynamic clustering of SLP-76 and FcepsilonRI signaling in mast cells

We developed a confocal real-time imaging approach that allows direct observation of the subcellular localization pattern of proteins involved in proximal FcepsilonRI signaling in RBL cells and primary bone marrow-derived mast cells. The adaptor protein Src homology 2 (SH2) domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76) is critical for FcepsilonRI-induced calcium flux, degranulation, and cytokine secretion. In this study, we imaged SLP-76 and found it in the cytosol of unstimulated cells. Upon FcepsilonRI cross-linking, SLP-76 translocates to the cell membrane, forming clusters that colocalize with the FcepsilonRI, the tyrosine kinase Syk, the adaptor LAT, and phosphotyrosine. The disruption of the SLP-76 interaction with its constitutive binding partner, Gads, through the mutation of SLP-76 or the expression of the Gads-binding region of SLP-76, inhibits the translocation and clustering of SLP-76, suggesting that the interaction of SLP-76 with Gads is critical for appropriate subcellular localization of SLP-76. We further demonstrated that the expression of the Gads-binding region of SLP-76 in bone marrow-derived mast cells inhibits FcepsilonRI-induced calcium flux, degranulation, and cytokine secretion. These studies revealed, for the first time, that SLP-76 forms signaling clusters following FcepsilonRI stimulation and demonstrated that the Gads-binding region of SLP-76 regulates clustering of SLP-76 and FcepsilonRI-induced mast cell responses.     

Structural requirements of SLP-76 in signaling via the high-affinity immunoglobulin E receptor (Fc epsilon RI) in mast cells

The adapter SLP-76 plays an essential role in Fc epsilon RI signaling, since SLP-76(-/-) bone marrow-derived mast cells (BMMC) fail to degranulate and release interleukin-6 (IL-6) following Fc epsilon RI ligation. To define the role of SLP-76 domains and motifs in Fc epsilon RI signaling, SLP-76(-/-) BMMC were retrovirally transduced with SLP-76 and SLP-76 mutants. The SLP-76 N-terminal and Gads binding domains, but not the SH2 domain, were critical for Fc epsilon RI-mediated degranulation and IL-6 secretion, whereas all three domains are essential for T-cell proliferation following T-cell receptor (TCR) ligation. Unexpectedly, the three tyrosine residues in SLP-76 critical for TCR signaling, Y112, Y128, and Y145, were not essential for IL-6 secretion, but were required for degranulation and mitogen-activated protein kinase activation. Furthermore, a Y112/128F SLP-76 mutant, but not a Y145F mutant, strongly reconstituted mast cell degranulation, suggesting a critical role for Y145 in Fc epsilon RI-mediated exocytosis. These results point to important differences in the function of SLP-76 between T cells and mast cells.