Requirement for the SLP-76 adaptor GADS in T cell development

The human and mouse complement of SH2 domain proteins-establishing the boundaries of phosphotyrosine signaling

SH2 domains are interaction modules uniquely dedicated to the recognition of phosphotyrosine sites and are embedded in proteins that couple protein-tyrosine kinases to intracellular signaling pathways. Here, we report a comprehensive bioinformatics, structural, and functional view of the human and mouse complement of SH2 domain proteins. This information delimits the set of SH2-containing effectors available for PTK signaling and will facilitate the systems-level analysis of pTyr-dependent protein-protein interactions and PTK-mediated signal transduction. The domain-based architecture of SH2-containing proteins is of more general relevance for understanding the large family of protein interaction domains and the modular organization of the majority of human proteins.

Grb2 and Gads exhibit different interactions with CD28 and play distinct roles in CD28-mediated costimulation

Although both CD28 and ICOS bind PI3K and provide stimulatory signal for T cell activation, unlike CD28, ICOS does not costimulate IL-2 secretion. CD28 binds both PI3K and Grb2, whereas ICOS binds only PI3K. We have generated an ICOS mutant, which can bind Grb2 by replacement of its PI3K binding motif YMFM with the CD28 YMNM motif, and shown that it induces significant activation of the IL-2 promoter. However, this mutant ICOS was insufficient to activate the NF-kappaB pathway. In this study, we show that Gads, but not Grb2, is essential for CD28-mediated NF-kappaB activation, and its binding to CD28 requires the whole CD28 cytoplasmic domain in addition to the YMNM motif. Mutagenesis experiments have indicated that mutations in the N-terminal and/or C-terminal PXXP motif(s) of CD28 significantly reduce their association with Gads, whereas their associations with Grb2 are maintained. They induced strong activity of the NFAT/AP-1 reporter comparable with the CD28 wild type, but weak activity of the NF-kappaB reporter. Grb2- and Gads-dominant-negative mutants had a strong effect on NFAT/AP-1 reporter, but only Gads-dominant-negative significantly inhibited NF-kappaB reporter. Our data suggest that, in addition to the PI3K binding motif, the PXXP motif in the CD28 cytoplasmic domain may also define a functional difference between the CD28- and ICOS-mediated costimulatory signals by binding to Gads.

Human SLP-65 isoforms contribute differently to activation and apoptosis of B lymphocytes

The SH2 domain-containing leukocyte adaptor protein of 65 kDa (SLP-65) is the key effector for signaling downstream of the B-cell antigen receptor (BCR). SLP-65 controls not only B lymphopoiesis and humoral immunity but also possesses a yet poorly defined tumor suppressor activity that is lost in many cases of acute lymphoblastic leukemia. We found that the 2 isoforms of human SLP-65 are differentially involved in positive and negative B-cell signaling. Reconstitution experiments revealed that an atypical SH3 domain-binding motif, which is present in the long but not in the short SLP-65 isoform, mediates association to Grb2 and suppresses activation of mitogen-activated protein kinases p38 and JNK as well as up-regulation of c-Fos expression. In turn, the short isoform activates not only AP1-driven but also NF-kappaB-driven gene transcription more potently than the long isoform. Conversely, the long rather than the short SLP-65 isoform promotes BCR-induced B-cell apoptosis. Our data further delineate the structural requirements of positive and negative SLP-65 signal transduction in normal and neoplastic cells.

Crystal structure of the C-terminal SH3 domain of the adaptor protein GADS in complex with SLP-76 motif peptide reveals a unique SH3-SH3 interaction

The Grb2-like adaptor protein GADS is essential for tyrosine kinase-dependent signaling in T lymphocytes. Following T cell receptor ligation, GADS interacts through its C-terminal SH3 domain with the adaptors SLP-76 and LAT, to form a multiprotein signaling complex that is crucial for T cell activation. To understand the structural basis for the selective recognition of GADS by SLP-76, herein is reported the crystal structure at 1.54 Angstrom of the C-terminal SH3 domain of GADS bound to the SLP-76 motif 233-PSIDRSTKP-241, which represents the minimal binding site. In addition to the unique structural features adopted by the bound SLP-76 peptide, the complex structure reveals a unique SH3-SH3 interaction. This homophilic interaction, which is observed in presence of the SLP-76 peptide and is present in solution, extends our understanding of the molecular mechanisms that could be employed by modular proteins to increase their signaling transduction specificity.

T-Cell Antigen Receptor-Induced Signaling Complexes: Internalization Via a Cholesterol-Dependent Endocytic Pathway

T-cell antigen receptor engagement causes the rapid assembly of signaling complexes. The adapter protein SLP-76, detected as SLP-yellow fluorescent protein, initially clustered with the TCR and other proteins, then translocated medially on microtubules. As shown by total internal reflection fluorescence microscopy and the inhibition of SLP-76 movement at 16 degrees C, this movement required endocytosis. Immunoelectron microscopy showed SLP-76 staining of smooth pits and tubules. Cholesterol depletion decreased the movement of SLP-76 clusters, as did coexpression of the ubiquitin-interacting motif domain from eps15. These data are consistent with the internalization of SLP-76 via a lipid raft-dependent pathway that requires interaction of the endocytic machinery with ubiquitinylated proteins. The endocytosed SLP-76 clusters contained phosphorylated SLP-76 and phosphorylated LAT. The raft-associated, transmembrane protein LAT likely targets SLP-76 to endocytic vesicles. The endocytosis of active SLP-76 and LAT complexes suggests a possible mechanism for downregulation of signaling complexes induced by TCR activation.

Adhesion- and degranulation-promoting adapter protein is required for efficient thymocyte development and selection

Adhesion- and degranulation-promoting adapter protein (ADAP) is required in TCR-induced activation and proliferation of peripheral T cells. Loss of ADAP also impairs TCR-initiated inside-out activation of the integrin LFA-1 (CD11a/CD18, alphaLbeta2). In this study, we demonstrate that ADAP-deficient CD4/CD8 double-positive (DP) cells have a diminished ability to proliferate, and that these DP thymocytes up-regulate CD69 poorly in vivo. Moreover, in both MHC class I- and class II-restricted TCR transgenic models, loss of ADAP interferes with both positive and negative selection. ADAP deficiency also impairs the ability of transgene-bearing DP thymocytes to form conjugates with Ag-loaded presenting cells. These findings suggest that ADAP is critical for thymocyte development and selection.

A role for MAPK in feedback inhibition of Tcrb recombination

The Tcrb locus is subject to a host of regulatory mechanisms that impart a strict cell and developmental stage-specific order to variable (V), diversity (D), and joining (J) gene segment recombination. The Tcrb locus is also regulated by allelic exclusion mechanisms, which restrict functional rearrangements to a single allele. The production of a functional rearrangement in CD4-CD8- double-negative (DN) thymocytes leads to the assembly of a pre-TCR and initiates signaling cascades that allow for DN to CD4+CD8+ double-positive (DP) differentiation, proliferation, and feedback inhibition of further Vbeta to DJbeta rearrangement. Feedback inhibition is believed to be controlled, in part, by the loss of Vbeta gene segment accessibility during the DN to DP transition. However, the pre-TCR signaling pathways that lead to the inactivation of Vbeta chromatin have not been determined. Because activation of the MAPK pathway is documented to promote DP differentiation in the absence of allelic exclusion, we characterized the properties of Vbeta chromatin within DP thymocytes generated by a constitutively active Raf1 (Raf-CAAX) transgene. Consistent with previous reports, we show that the Raf-CAAX transgene does not inhibit Tcrb recombination in DN thymocytes. Nevertheless, DP thymocytes generated by Raf-CAAX signals display normal down-regulation of Vbeta segment accessibility and normal feedback inhibition of the Vbeta to DJbeta rearrangement. Therefore, our results emphasize the distinct requirements for feedback inhibition in the DN and DP compartments. Although MAPK activation cannot impose feedback in DN thymocytes, it contributes to feedback inhibition through developmental changes that are tightly linked to DN to DP differentiation.

Gene repression by Pax5 in B cells is essential for blood cell homeostasis and is reversed in plasma cells

The transcription factor Pax5 represses lineage-inappropriate genes and activates B cell-specific genes in B lymphocytes. By identifying 110 Pax5-repressed genes, we now demonstrate that Pax5 downregulates diverse biological activities including receptor signaling, cell adhesion, migration, transcriptional control, and cellular metabolism at B cell commitment. The T lymphoid or myeloid expression of these genes demonstrates that Pax5(-/-) pro-B cells and common lymphoid progenitors display lymphoid and myeloid promiscuity of gene expression. These lineage-inappropriate genes require continuous Pax5 activity for their repression, as they are reactivated in committed pro-B cells and mature B cells following conditional Pax5 deletion. Pax5-repressed genes are also reexpressed in plasma cells, which depend for normal function on Cd28 and Ccr2 reactivation. The loss of Pax5 during terminal differentiation thus contributes to the plasma cell transcription program. Finally, ectopic expression of the Pax5-repressed chemokine gene Ccl3 in B cells results in increased osteoclast formation and bone loss, demonstrating that Pax5-mediated gene repression is essential for normal homeostasis of hematopoietic development.

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.

Expression, refolding and crystallizations of the Grb2-like (GADS) C-terminal SH3 domain complexed with a SLP-76 motif peptide

The Grb2-like adaptor protein GADS is composed of an N-terminal SH3 domain, an SH2 domain, a proline-rich region and a C-terminal SH3 domain. GADS interacts through its C-terminal SH3 domain with the adaptor protein SLP-76, thus recruiting this protein and other associated molecules to the linker for activation of T-cell (LAT) protein. The DNA encoding the C-terminal SH3 domain of GADS (GADS-cSH3) was assembled synthetically using a recursive PCR technique and the protein was overexpressed in Escherichia coli, refolded and purified. Several crystals of this domain in complex with the SLP-76 peptide were obtained and characterized.

Spi-1 and Spi-B control the expression of the Grap2 gene in B cells

The Ets family members Spi-1 and Spi-B have been implicated in the regulation of genes important for B cell antigen receptor (BCR) signaling. Mice deficient in Spi-B exhibit reduced B cell proliferation in response to BCR cross-linking and impaired T cell-dependent immune responses. This defect is exacerbated in the presence of Spi-1 haplo-insufficiency (Spi1+/- SpiB-/-). Tyrosine phosphorylation and calcium mobilization induced by BCR engagement is diminished in Spi1+/- SpiB-/- B lymphocytes, although many key BCR signaling proteins are expressed, suggesting that Spi-1 and Spi-B regulate expression of additional, unidentified signaling molecules. We now demonstrate that expression of the adaptor protein Grap2 is impaired in Spi1+/- SpiB+/- and Spi1+/- SpiB-/- B lymphocytes. Analysis of two alternate murine Grap2 promoters revealed a functionally important Spi-1 and Spi-B DNA binding element located in the downstream promoter. Ectopic expression of Grap2 in Grap2-deficient B cells reduced the recruitment of BLNK to Igalpha and the phosphorylation of specific substrates. Regulation of BLNK recruitment was dependent upon the Grap2 proline-rich domain, while modulation of phosphorylation was dependent upon both the proline-rich and SH2 domains. These data indicate that Spi-1 and Spi-B directly regulate the expression of Grap2 and that Grap2 functions to modulate BCR signaling, but that reduced Grap2 expression is unlikely to account for the BCR signaling defects observed in Spi1+/- SpiB-/- B cells.

Genes, pathways and checkpoints in lymphocyte development and homeostasis

A plethora of genes involved in murine B and T cell development have been identified, and developmental pathways within the primary lymphoid tissues have been well delineated. The generation of a functional, but non-self reacting lymphocyte repertoire results from the completion of several checkpoints during lymphocyte development and competition for survival factors in the periphery. Improved knowledge of these developmental checkpoints and homeostatic mechanisms is critical for understanding human immunodeficiency, leukaemia/lymphoma and autoimmunity, which are conditions where checkpoints and homeostasis are likely to be deregulated.

Expression and function of the adaptor protein Gads in murine B cells

Nearly all hematopoietic receptors are dependent on adaptor proteins for the activation of downstream signaling pathways. The Gads adaptor protein is expressed in many hematopoietic tissues, including bone marrow, lymph node, and spleen. Using intracellular staining, we detected Gads protein in a number cells, including B cells, T cells, NK cells, monocytes, and plasmacytoid DC, but not in macrophages, neutrophils, or monocyte-derived DC. In the B cell compartment, Gads was first expressed after immature B cells leave the bone marrow and was down-regulated after B cell antigen receptor (BCR) ligation. Female Gads(-/-) mice had increased numbers of splenic B cells, as compared to female Gads(+/+) mice, suggesting a role for Gads in B cell homeostasis. Although B cell production and turnover of splenic B cell subsets appeared normal in Gads(-/-) mice, homeostatic proliferation was significantly impaired in Gads(-/-) B cells. Whereas BCR ligation can induce apoptosis in wild-type transitional stage 1 (T1) B cells, Gads(-/-) T1 B cells were resistant to BCR-induced apoptosis. Gads(-/-) B cells also showed increased BCR-mediated calcium mobilization. We conclude that Gads may have a negative regulatory role in signaling through survival pathways, and is necessary for normal homeostatic proliferation in B cells.

The SLP-76 family of adapter proteins

Adapter molecules are multidomain proteins lacking intrinsic catalytic activity, functioning instead by nucleating molecular complexes during signal transduction. The SLP-76 family of adapters includes SH2 domain-containing leukocyte phosphoprotein of 76kDa (SLP-76), B cell linker protein (BLNK), and cytokine-dependent hematopoietic cell linker (Clnk). These proteins are critical for integration of numerous signaling cascades downstream of immunotyrosine-based activation motif (ITAM)-bearing receptors and integrins in diverse hematopoietic cell types. Mutations in genes encoding SLP-76 family adapters result in severe phenotypes, underscoring the critical role these proteins play in cellular development and function by directing formation of signaling complexes in a temporally- and spatially-specific manner.

Signaling control of memory T cell generation and function

Memory T cells exhibit low activation thresholds and mediate rapid effector responses when recalled by antigen; contrasting the higher activation threshold, slower responses and predominant IL-2 production by naive T cells. While the sequence of intracellular events coupling the T cell-receptor (TCR) to naive T cell activation is well characterized, biochemical control of memory T cell differentiation and function remains undefined. In this review, we will discuss recent developments in T cell-receptor signal transduction as they pertain to memory T cells, and will discuss how signal dampening may drive memory generation, and more efficient spatial organization of signaling molecules may promote rapid recall responses.

T cell receptor-induced activation of phospholipase C-gamma1 depends on a sequence-independent function of the P-I region of SLP-76

SLP-76 forms part of a hematopoietic-specific adaptor protein complex, and is absolutely required for T cell development and activation. T cell receptor (TCR)-induced activation of phospholipase C-gamma1 (PLC-gamma1) depends on three features of SLP-76: the N-terminal tyrosine phosphorylation sites, the Gads-binding site, and an intervening sequence, denoted the P-I region, which binds to the SH3 domain of PLC-gamma1 (SH3(PLC)) via a low affinity interaction. Despite extensive research, the mechanism whereby SLP-76 regulates PLC-gamma1 remains uncertain. In this study, we uncover and explore an apparent paradox: whereas the P-I region as a whole is essential for TCR-induced activation of PLC-gamma1 and nuclear factor of activated T cells (NFAT), no particular part of this region is absolutely required. To better understand the contribution of the P-I region to PLC-gamma1 activation, we mapped the PLC-gamma1-binding site within the region, and created a SLP-76 mutant that fails to bind SH3(PLC), but is fully functional, mediating TCR-induced phosphorylation of PLC-gamma1 at tyrosine 783, calcium flux, and nuclear factor of activated T cells activation. Unexpectedly, full functionality of this mutant was maintained even under less than optimal stimulation conditions, such as a low concentration of the anti-TCR antibody. Another SLP-76 mutant, in which the P-I region was scrambled to abolish any sequence-dependent protein-binding motifs, also retained significant functionality. Our results demonstrate that SLP-76 need not interact with SH3(PLC) to activate PLC-gamma1, and further suggest that the P-I region of SLP-76 serves a structural role that is sequence-independent and is not directly related to protein-protein interactions.

The Gads (GrpL) adaptor protein regulates T cell homeostasis

Little is known about the role of the Gads (GrpL) adaptor protein in mature T cell populations. In this study we show that the effects of Gads deficiency on murine CD4(+) and CD8(+) T cells are markedly different. Gads(-/-) CD4(+) T cells were markedly deficient in the spleen and had an activated phenotype and a rapid turnover rate. When transferred into a wild-type host, Gads(-/-) CD4(+) T cells continued to proliferate at a higher rate than wild-type CD4(+) T cells, demonstrating a defect in homeostatic proliferation. Gads(-/-) CD8(+) T cells had a memory-like phenotype, produced IFN-gamma in response to ex vivo stimulation, and underwent normal homeostatic proliferation in wild-type hosts. Gads(-/-) T cells had defective TCR-mediated calcium responses, but had normal activation of ERK. Gads(-/-) CD4(+) T cells, but not CD8(+) T cells, had a severe block of TCR-mediated proliferation and a high rate of spontaneous cell death and were highly susceptible to CD95-induced apoptosis. This suggests that the rapid turnover of Gads(-/-) CD4(+) T cells is due to a defect in cell survival. The intracellular signaling pathways that regulate homeostasis in CD4(+) and CD8(+) T cells are clearly different, and the Gads adaptor protein is critical for homeostasis of CD4(+) T cells.

Signalling in T-lymphocyte development: integration of signalling pathways is the key

alpha beta T-cell development is restricted to the thymus. Interactions between developing lymphocytes and the thymic stroma, together with bone-marrow-derived monocytes and dendritic cells, are critical for proper development of the T-cell lineage. The developmental sequence through which T-cell progenitors pass on their way to maturity is well established, and can be followed by the sequential acquisition and/or removal of cell surface molecules. Using the combination of modern genetic manipulations, such as transgenesis, gene ablation (knockouts) and targeted mutagenesis (knock-ins), with the ever-improving conditional and inducible manipulation of gene expression, we are beginning to gain an understanding of how intercellular interactions can be relayed via intracellular signalling cascades to bring about nuclear re-organisation and the differentiated mature CD4(+) and CD8(+) subpopulations.

Roles of the Proline-rich Domain in SLP-76 Subcellular Localization and T Cell Function

The adaptor protein Src homology (SH)2 domain-containing and leukocyte-specific phosphoprotein of 76 kDa (SLP-76) is critical for signal transduction in multiple hematopoietic lineages. It links proximal and distal T cell receptor signaling events through its function as a molecular scaffold in the assembly of multimolecular signaling complexes. Here we studied the functional roles of sub-domains within the SLP-76 proline-rich region, specifically the Gads binding domain and the recently defined P1 domain. To gain a further understanding of the functions mediated by this region, we used three complementary approaches as follows: reconstitution of SLP-76-deficient cells with functional domain deletion mutants, blocking molecular associations through the expression of a dominant negative protein fragment, and directed localization of SLP-76 to assess the role of the domains in SLP-76 recruitment. We find the Gads binding domain and the P1 domain are both necessary for optimal SLP-76 function, and in the absence of these two regions, SLP-76 is functionally inert. Furthermore, we provide direct evidence that SLP-76 localization and, in turn, function are dependent upon association with Gads.

Structural basis for differential recognition of tyrosine-phosphorylated sites in the linker for activation of T cells (LAT) by the adaptor Gads

The transmembrane protein, linker for activation of T cells (LAT), is essential for T-cell activation and development. Phosphorylation of LAT at multiple tyrosines creates binding sites for the adaptors Gads and Grb2, leading to nucleation of multiprotein signaling complexes. Human LAT contains five potential binding sites for Gads, of which only those at Tyr171 and Tyr191 appear necessary for T-cell function. We asked whether Gads binds preferentially to these sites, as differential recognition could assist in assembling defined LAT-based complexes. Measured calorimetrically, Gads-SH2 binds LAT tyrosine phosphorylation sites 171 and 191 with higher affinities than the other sites, with the differences ranging from only several fold weaker binding to no detectable interaction. Crystal structures of Gads-SH2 complexed with phosphopeptides representing sites 171, 191 and 226 were determined to 1.8-1.9 A resolutions. The structures reveal the basis for preferential recognition of specific LAT sites by Gads, as well as for the relatively greater promiscuity of the related adaptor Grb2, whose binding also requires asparagine at position +2 C-terminal to the phosphorylated tyrosine.

The importance of three membrane-distal tyrosines in the adaptor protein NTAL/LAB

NTAL (non-T cell activation linker)/LAB (linker for activation of B cells) is a LAT (linker for activation of T cells)-like molecule that is expressed in B cells, mast cells, natural killer cells, and monocytes. Upon engagement of the B cell receptor or Fc receptors, it is phosphorylated and interacts with Grb2. LAB is capable of rescuing thymocyte development in LAT(-/-) mice. In this study, we utilized various LAB Tyr to Phe mutants to map the phosphorylation and Grb2-binding sites of LAB. We also examined the function of these mutants by investigating their ability to rescue signaling defects in LAT-deficient Jurkat cells and thymocyte development in LAT(-/-) mice. Our results indicated that human LAB was primarily phosphorylated on three membrane-distal tyrosines, Tyr(136), Tyr(193), and Tyr(233). Mutation of these three tyrosines abolished Grb2 binding and LAB function. Our data suggested that these tyrosines are the most important tyrosines for LAB function.

Hematopoietic adaptors in T-cell signaling: potential applications to transplantation

Recent advances have been made in understanding the basis of T-cell signaling with the identification of hematopoeitic-specific adaptor proteins, or molecular scaffolds that facilitate protein complex formation and the integration of signals from the surface of T cells. Their potential relevance as targets in the modulation of transplantation relates to their immune-cell-specific expression and their ability to integrate signals needed for T-cell/APC conjugate formation, cytokine production and the clonal expansion of T cells. While LAT, GADS and SLP-76 are needed for TcR-induced cytokine production, the adaptors ADAP, VAV and SKAP-55 play specialized roles in the regulation of integrin adhesion and conjugation. Given the importance of these functions to the reactivity of T cells to allodeterminants of tissue grafts (GvH), and in the recognition and destruction of leukemic cells (GvL), these adaptors represent a new generation of potential targets in the modulation of transplantation.

Positive and negative selection of T cells

A functional immune system requires the selection of T lymphocytes expressing receptors that are major histocompatibility complex restricted but tolerant to self-antigens. This selection occurs predominantly in the thymus, where lymphocyte precursors first assemble a surface receptor. In this review we summarize the current state of the field regarding the natural ligands and molecular factors required for positive and negative selection and discuss a model for how these disparate outcomes can be signaled via the same receptor. We also discuss emerging data on the selection of regulatory T cells. Such cells require a high-affinity interaction with self-antigens, yet differentiate into regulatory cells instead of being eliminated.

Grap-2, a novel RET binding protein, is involved in RET mitogenic signaling

Signal transduction of the RET receptor tyrosine kinase is involved in developmental processes as well as in neoplastic transformation. Activation of RET initiates receptor autophosphorylation on specific tyrosines that act as docking sites for downstream signaling molecules. Using the cytoplasmatic part of RET as bait in a yeast two-hybrid screen, we identified a novel SH2 and SH3 domain containing adaptor protein previously termed Grap-2/Grf40/GrpL/GRID and its murine homologue as Gads/Mona, respectively. This protein, predominantly expressed in cells of hematopoietic origin, is involved in signaling downstream of the T-cell receptor and the receptor for monocyte colony-stimulating factor. Here, we show that Grap-2 is also expressed in neuroendocrine tumors and cell lines known to bear mutated forms of RET. Endogenously expressed RET and Grap-2 coimmunoprecipitate from lysates of a medullary thyroid carcinoma cell line. Grap-2 directly associates with RET in pull-down experiments using in vitro translated proteins. Overexpression of Grap-2 inhibits RET-induced NF-kappaB activation, and cotransfection of Grap-2 significantly reduces focus formation induced by oncogenic RET in NIH 3T3 cells. Taken together, these results suggest that besides being involved in tyrosine kinase signaling in hematopoietic cells, Grap-2 plays a tissue-specific role as an inhibitor of RET mitogenic signaling.

Structural basis for SH3 domain-mediated high-affinity binding between Mona/Gads and SLP-76

SH3 domains are protein recognition modules within many adaptors and enzymes. With more than 500 SH3 domains in the human genome, binding selectivity is a key issue in understanding the molecular basis of SH3 domain interactions. The Grb2-like adaptor protein Mona/Gads associates stably with the T-cell receptor signal transducer SLP-76. The crystal structure of a complex between the C-terminal SH3 domain (SH3C) of Mona/Gads and a SLP-76 peptide has now been solved to 1.7 A. The peptide lacks the canonical SH3 domain binding motif P-x-x-P and does not form a frequently observed poly-proline type II helix. Instead, it adopts a clamp-like shape around the circumfence of the SH3C beta-barrel. The central R-x-x-K motif of the peptide forms a 3(10) helix and inserts into a negatively charged double pocket on the SH3C while several other residues complement binding through hydrophobic interactions, creating a short linear SH3C binding epitope of uniquely high affinity. Interestingly, the SH3C displays ion-dependent dimerization in the crystal and in solution, suggesting a novel mechanism for the regulation of SH3 domain functions.

Adaptor proteins in lymphocyte activation

Adaptor proteins are unique, as they contain modular domains and lack intrinsic enzymatic activity. These proteins are scaffolds for the organization of macromolecular complexes and they recruit other proteins for correct localization during molecular signal transduction. Numerous recent advances have been made through the elucidation of new adaptor proteins and the recognition of novel functions for previously identified molecules. In addition, the roles of adaptors in both the positive and negative regulation of lymphocyte activation have been further clarified.

Gab3-deficient mice exhibit normal development and hematopoiesis and are immunocompetent

Gab proteins are intracellular scaffolding and docking molecules involved in signaling pathways mediated by various growth factor, cytokine, or antigen receptors. Gab3 has been shown to act downstream of the macrophage colony-stimulating factor receptor, c-Fms, and to be important for macrophage differentiation. To analyze the physiological role of Gab3, we used homologous recombination to generate mice deficient in Gab3. Gab3(-/-) mice develop normally, are visually indistinguishable from their wild-type littermates, and are healthy and fertile. To obtain a detailed expression pattern of Gab3, we generated Gab3-specific monoclonal antibodies. Immunoblotting revealed a predominant expression of Gab3 in lymphocytes and bone marrow-derived macrophages. However, detailed analysis demonstrated that hematopoiesis in mice lacking Gab3 is not impaired and that macrophages develop in normal numbers and exhibit normal function. The lack of Gab3 expression during macrophage differentiation is not compensated for by increased levels of Gab1 or Gab2 mRNA. Furthermore, Gab3-deficient mice have no major immune deficiency in T- and B-lymphocyte responses to protein antigens or during viral infection. In addition, allergic responses in Gab3-deficient mice appeared to be normal. Together, these data demonstrate that loss of Gab3 does not result in detectable defects in normal mouse development, hematopoiesis, or immune system function.

Sin: good or bad? A T lymphocyte perspective

Stimulation of T cells through their antigen receptor induces a multitude of signaling networks that regulate T cell activation in the form of cytokine production and T cell proliferation. Multiple signal integration sites exist along these pathways in the form of multiprotein signaling complexes, the formation of which is facilitated by adapter and scaffold molecules. In recent years a number of adapter and scaffold molecules have been described in T cells and shown to play an integral part in T cell function. Among these molecules are proteins that function as positive or negative regulators of T cell activation downstream of the activated T cell receptor (TCR). Here, we discuss the role of a small family of multiadapter proteins on T cell activation, the p130Cas family, with emphasis on one of its members, Sin (Src-interacting protein). Our results suggest that Sin inhibits thymocyte development and T cell activation and is a novel negative regulator of T lymphocyte function.

Negative feedback of T cell activation through inhibitory adapters and costimulatory receptors

Antigen recognition by the T cell receptor (TCR) complex induces the formation of a TCR signalosome by recruiting various signaling molecules, generating the recognition signals for T cell activation. The activation status and functional outcome are positively and negatively regulated by dynamic organization of the signalosome and by costimulation signals. We have studied the negative regulation of T cell activation, particularly through inhibitory adapters and costimulation receptors that are little expressed in resting cells but are induced upon T cell activation. We described Grb-associated binder 2 (Gab2) and cytotoxic T lymphocyte antigen-4 (CTLA-4) as a representative inhibitory adapter and a negative costimulation receptor, respectively, both of which exhibit negative feedback. Gab2 functions as a signal branch for activation vs. inhibition, as phosphorylation of either Src homology 2 (SH2) domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76) or Gab2 by zeta-associated protein of 70 kDa (ZAP-70) determines the fate of the response. As a professional inhibitory receptor, CTLA-4 inhibits T cell response by competition of ligand binding with positive costimulator receptor CD28, and also induces inhibitory signaling. The trafficking and the cell surface expression of CTLA-4 are dynamically regulated and induced. CTLA-4 is accumulated in lysosomes and secreted to the T cell-APC contact site upon TCR stimulation. As T cell activation proceeds, these inhibitory adapters and costimulation receptors are induced and suppress/regulate the responses as negative feedback.

Gads/Grb2-mediated association with LAT is critical for the inhibitory function of Gab2 in T cells

A docking protein, Gab2, is recruited to the vicinity of the TCR complex and inhibits downstream signaling by interaction with negative regulators. However, the molecular mechanisms of this recruitment remain unclear. We have found that Gab2 associates with LAT upon TCR stimulation and that LAT is essential for Gab2 phosphorylation. By analysis of several Gab2 mutants, the c-Met binding domain (MBD) of Gab2 was found to be both necessary and sufficient for stimulation-induced LAT binding. Within the MBD domain, a novel Grb2 SH3 binding motif, PXXXR, is critical for constitutive association with Gads/Grb2. Through this association, Gab2 is recruited to the lipid raft after TCR ligation and exerts inhibitory function. The in vivo significance of this association is illustrated by the fact that T-cell responses are impaired in transgenic mice expressing wild-type Gab2 but not in mice expressing mutant Gab2 lacking the motif. Furthermore, T cells from Gab2-deficient mice showed enhanced proliferative responses upon TCR stimulation. These results indicate that Gads/Grb2-mediated LAT association is critical for the inhibitory function of Gab2, implying that Gab2 induced in stimulated T cells may exert an efficient negative feedback loop by recruiting inhibitory molecules to the lipid raft and competing with SLP-76 through Gads binding.

The TCR ADAPts to integrin-mediated cell adhesion

Adapter proteins regulate leukocyte signal transduction through recruitment of effector molecules to multiprotein complexes. Recent studies in Adhesion and Degranulation promoting Adapter Protein (ADAP)-deficient mice have established that the cytoplasmic phosphoprotein ADAP is required for optimal, mature T-cell proliferation. Furthermore, ADAP plays a key role in T-cell antigen receptor (TCR)-mediated 'inside out' signaling leading to integrin activation and to enhanced cellular adhesion to integrin ligands. ADAP associates physically with molecules known to play roles in the regulation of TCR-stimulated actin polymerization. These associations support the hypothesis that ADAP functions in actin cytoskeletal reorganization leading to cellular adhesion and activation.

Signaling life and death in the thymus: timing is everything

T lymphocytes are generated in the thymus, where developing thymocytes must accept one of two fates: They either differentiate or they die. These fates are chiefly determined by signals that originate from the T cell receptor (TCR), a single receptor complex with a remarkable capacity to decide between distinct cell fates. This review explores TCR signaling in thymocytes and focuses on the kinetic aspects of ligand binding, coreceptor involvement, protein phosphorylation, and mitogen-activated protein kinase (MAPK) activation. Understanding the logic of TCR signaling may eventually explain how thymocytes and T cells distinguish self from nonself, a phenomenon that has fascinated immunologists for 50 years.

Growth factor independence-1B expression leads to defects in T cell activation, IL-7 receptor alpha expression, and T cell lineage commitment

T cell differentiation in the thymus is dependent upon signaling through the TCR and is characterized by the resulting changes in expression patterns of CD4 and CD8 surface coreceptor molecules. Although recent studies have characterized the effects of proximal TCR signaling on T cell differentiation, the downstream integration of these signals remains largely unknown. The growth factor independence-1 (GFI1) and GFI1B transcriptional repressors may regulate cytokine signaling pathways to affect lymphocyte growth and survival. In this study, we show that Gfi1 expression is induced upon induction of the T cell program. Gfi1B expression is low and dynamic during T cell development, but is terminated in mature thymocytes. Transgenic expression of GFI1 and GFI1B in T cells allowed us to determine the functional consequences of constitutive expression. GFI1 potentiates response to TCR stimulation and IL-2, whereas GFI1B-transgenic T cells are defective in T cell activation. Moreover, GFI1B-transgenic thymocytes display reduced expression of the late-activation marker IL-7R alpha, and a decrease in CD4(-)8(+) single-positive T cells that can be mitigated by transgenic expression of BCL2 or GFI1. These data show that GFI1 and GFI1B are functionally unique, and implicate a role for GFI1 in the integration of activation and survival signals.

The Ras/MAPK cascade and the control of positive selection

Immature double positive (DP) thymocytes bearing a T cell receptor (TCR) that interacts with self-major histocompatibility complex (MHC) molecules receive signals that induce either their differentiation (positive selection) or apoptosis (negative selection). Furthermore, those cells that are positively selected develop into two different lineages, CD4 or CD8, depending on whether their TCRs bind to MHC class II or I, respectively. Positive selection therefore involves rescue from the default fate (death), lineage commitment, and progression to the single positive (SP) stage. These are probably temporally distinct events that may require both unique and overlapping signals. Work in the past several years has started to unravel the signaling networks that control these processes. One of the first pathways identified as important for positive selection was Ras and its downstream effector, the Erk mitogen-activated protein kinase (MAPK) cascade. In this review we examine the factors that connect the TCR to the Ras/Erk cascade in DP thymocytes, as well as what we know about the downstream effectors of the Ras/Erk cascade important for positive selection. We also consider the possible role of this cascade in CD4/CD8 lineage development, and the possible interactions of the Ras/Erk cascade with Notch during these cell fate determination processes.

Structural basis for specific binding of the Gads SH3 domain to an RxxK motif-containing SLP-76 peptide: a novel mode of peptide recognition

The SH3 domain, which normally recognizes proline-rich sequences, has the potential to bind motifs with an RxxK consensus. To explore this novel specificity, we have determined the solution structure of the Gads T cell adaptor C-terminal SH3 domain in complex with an RSTK-containing peptide, representing its physiological binding site on the SLP-76 docking protein. The SLP-76 peptide engages four distinct binding pockets on the surface of the Gads SH3 domain and upon binding adopts a unique structure characterized by a right-handed 3(10) helix at the RSTK locus, in contrast to the left-handed polyproline type II helix formed by canonical proline-rich SH3 ligands. The structure, and supporting mutagenesis and peptide binding data, reveal a novel mode of ligand recognition by SH3 domains.

Known and potential functions for the SLP-76 adapter protein in regulating T-cell activation and development

The hematopoietic adapter protein SLP-76 is a critical component of multiple biochemical signaling 'circuits' in T cells that integrate proximal signaling events initiated by ligation of the T-cell receptor (TCR) into more distal pathways. Given the important role ascribed to TCR signaling in directing the outcome of thymocyte selection, it seems likely that SLP-76 may also function in signaling pathways that ultimately impact the establishment of the peripheral T-cell repertoire. It is generally accepted that the peripheral T-cell repertoire is selected in large part during T-cell development in the thymus. Molecular interactions between the TCR and self-peptide/major histocompatibility complexes expressed on thymic stromal elements dictate the fate of developing thymocytes. Thymocyte survival and further maturation (positive selection) require an active signal delivered to the cell as a consequence of TCR ligation. This raises the intriguing question of how a thymocyte can, for a narrow window of developmental time, obtain responsiveness to self while maintaining tolerance to these same determinants upon export to the periphery. This article reviews the current literature describing SLP-76-dependent signaling pathways in mature T cells and developing thymocytes. A potential role for this critical signaling intermediate in integrating signals leading to positive and negative selection of the peripheral T-cell repertoire is also discussed.

Adaptors as central mediators of signal transduction in immune cells

Adaptors are molecular scaffolds that recruit effectors, which are critical for immune cell activation. Recent work has underscored the requirement for adaptors in propagating stimulatory signals as well as their ability to inhibit immune cell function. The mechanisms by which adaptors function rely not only on the intermolecular interactions they mediate, but also on where they are localized within the cell. The use of sophisticated genetic, biochemical, cellular and imaging approaches has provided important new insights into the biology of adaptor protein function. Here we focus on T lymphocytes as a model to illustrate the critical roles adaptors play as regulators of cellular activation.

Characterization of promoter elements directing Mona/Gads molecular adapter expression in T and myelomonocytic cells: involvement of the AML-1 transcription factor

Monocytic adaptor (Mona, also called Gads) is a molecular adaptor implicated in T cell activation and macrophage differentiation. The objective of this study was to identify elements regulating specific expression of Mona/Gads in human T cell and myelomonocytic cell lines. We first confirmed that the -2000 to +150 genomic region relative to the Mona gene transcription start site is sufficient to direct specific reporter gene expression in T cell lines, Jurkat, and MOLT-4 and in the immature myeloid cell lines, KG1a and RC2A. Deletion analysis and electrophoresis mobility shift assay identified several cis regulatory elements: overlapping initiator sequences, one interferon response factor-2 (IRF-2)-binding site at position -154, one GC box recognized by Sp1 and Sp3 at position -52, and two acute myeloid leukemia (AML)-1 binding sites at positions -70 and -13. Site-directed mutagenesis experiments indicated a key role of AML-1 for driving Mona expression in T cells and myeloid cells, and involvement of Sp1/Sp3 and IRF-2 transcription factors to modulate Mona expression in a cell-specific manner.

Minimal requirement of tyrosine residues of linker for activation of T cells in TCR signaling and thymocyte development

Linker for activation of T cells (LAT) is a membrane-associated adaptor protein that is phosphorylated on multiple tyrosines upon TCR cross-linking. Previous studies show that LAT is essential for TCR-mediated signaling and thymocyte development. In this study, we expressed a series of LAT Tyr to Phe mutants in LAT-deficient J.CaM2.5 cells and examined their tyrosine phosphorylation; association with Grb2, Gads, and phospholipase C (PLC)-gamma1; and function in T cell activation. Our results showed that the five membrane-distal tyrosines were phosphorylated upon T cell activation. Grb2, Gads, and PLC-gamma1 associated with LAT preferentially via different sets of tyrosine residues; however, they failed to interact with LAT mutants containing only one tyrosine. We also determined the minimal requirement of LAT tyrosine residues in T cell activation and thymocyte development. Our results showed that a minimum of three tyrosines is required for LAT to function in T cell activation and thymocyte development. LAT mutants that were capable of binding Grb2 and PLC-gamma1 could reconstitute T cell activation in LAT-deficient cells and thymocyte development in LAT-deficient mice.

Expression of the Grb2-related protein of the lymphoid system in B cell subsets enhances B cell antigen receptor signaling through mitogen-activated protein kinase pathways

Adapter proteins play a critical role in regulating signals triggered by Ag receptor cross-linking. These small molecules link receptor proximal events with downstream signaling pathways. In this study, we explore the expression and function of the Grb2-related protein of the lymphoid system (GrpL)/Grb2-related adaptor downstream of Shc adapter protein in human B cells. GrpL is expressed in naive B cells and is down-regulated following B cell Ag receptor ligation. By contrast, germinal center and memory B cells express little or no GrpL. Using human B cell lines, we detected constitutive interactions between GrpL and B cell linker protein, Src homology (SH)2 domain-containing leukocyte protein of 76 kDa, hemopoietic progenitor kinase 1, and c-Cbl. The N-terminal SH3 domain of GrpL binds c-Cbl while the C-terminal SH3 domain binds B cell linker protein and SH2 domain-containing leukocyte protein of 76 kDa. Exogenous expression of GrpL in a GrpL-negative B cell line leads to enhanced Ag receptor-induced extracellular signal-related kinase and p38 mitogen-activated protein kinase phosphorylation. Thus, GrpL expression in human B cell subsets appears to regulate Ag receptor-mediated signaling events.

c-Cbl and Cbl-b regulate T cell responsiveness by promoting ligand-induced TCR down-modulation

How Cbl family proteins regulate T cell responses is unclear. We found that c-Cbl Cbl-b double knock-out (dKO) T cells became hyperresponsive upon anti-CD3 stimulation, even though the major T cell antigen receptor (TCR) signaling pathways were not enhanced. The dKO T cells did not down-modulate surface TCR after ligand engagement, which resulted in sustained TCR signaling. However, these cells showed normal ligand-independent TCR internalization, and trafficking of internalized TCR to the lysosome compartment after ligand engagement was reduced. These findings show that Cbl family proteins negatively regulate T cell activation by promoting clearance of engaged TCR from the cell surface, a process that is apparently essential for the termination of TCR signals.

Molecular cloning of a novel gene encoding a membrane-associated adaptor protein (LAX) in lymphocyte signaling

Membrane-associated adaptors play an important role in coupling antigen receptor engagement to downstream signaling events, such as Ras-MAPK activation, Ca(2+) flux, and nuclear factor of activated T cells (NFAT) activation. Here we identified a novel membrane-associated adaptor protein, LAX. LAX is mainly expressed in B cells, T cells, and other lymphoid-specific cell types. It shares no overall sequence homology with LAT and is not localized to lipid rafts. However, like LAT, LAX has tyrosine motifs for binding Grb2, Gads, and the p85 subunit of phosphatidylinositol 3-kinase. Upon stimulation via the B or T cell receptors, LAX is rapidly phosphorylated by Src and Syk family tyrosine kinases and interacts with Grb2, Gads, and p85. Overexpression of LAX in Jurkat cells specifically inhibits T cell receptor-mediated p38 MAPK activation and NFAT/AP-1 transcriptional activation. Our data suggested that LAX functions to negatively regulate signaling in lymphocytes.

Regulation of T cell receptor-induced activation of the Ras-ERK pathway by diacylglycerol kinase zeta

T cell development in the thymus and activation of mature T cells in the periphery depend on signals stimulated by engagement of the T cell antigen receptor (TCR). Among the second messenger cascades initiated by TCR ligation include the phosphatidylinositol pathway where the membrane phospholipid, phosphatidylinositol 4,5-bisphosphate, is hydrolyzed to inositol 1,4,5-trisphosphate and diacylglycerol (DAG). Inositol 1,4,5-trisphosphate signals a rise in intracellular free calcium, leading to translocation of nuclear factor of activated T cells into the nucleus. DAG activates RasGRP and protein kinase C theta. Because both RasGRP and protein kinase C theta are essential for thymocyte and T cell function, it is critical to understand how DAG is regulated. In this report, we demonstrate expression of DAG kinase zeta (DGKzeta, the enzyme that catalyzes the conversion of DAG to phosphatidic acid) in multiple lymphoid organs, with highest expression observed within the T cell compartment. Overexpression studies in Jurkat T cells indicate that DGKzeta interferes with TCR-induced Ras and ERK activation, AP-1 induction, and expression of the activation marker CD69. In contrast, TCR-stimulated calcium influx is not altered. Mutational analysis indicates that the kinase and DAG binding domains, but not the ankyrin repeats of DGKzeta, are required for its inhibitory effects. Collectively these studies demonstrate a potential role of DGKzeta to function as a selective negative regulator of DAG signaling on T cell activation and provide the first structure/function analysis of this enzyme in T cells.

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.

Dok-related protein negatively regulates T cell development via its RasGTPase-activating protein and Nck docking sites

Downstream of kinase (Dok)-related protein (DokR, also known as p56(dok)/FRIP/Dok-R) is implicated in cytokine and immunoreceptor signaling in myeloid and T cells. Tyrosine phosphorylation induces DokR to bind the signal relay molecules, RasGTPase-activating protein (RasGAP) and Nck. Here, we have examined the function of DokR during hematopoietic development and the requirement for RasGAP and Nck binding sites in its biological function. Retroviral-mediated expression of DokR in bone marrow cells dramatically inhibited their capacity to form colonies in vitro in response to the cytokines macrophage colony-stimulating factor and stem cell factor, whereas responses to interleukin-3 and granulocyte macrophage colony-stimulating factor were only weakly affected. When introduced into lethally irradiated mice, hematopoietic cells expressing DokR showed a drastically reduced capacity to repopulate lymphoid tissues. Most notably, DokR dramatically reduced repopulation of the thymus, in part by reducing the number of T cell precursors seeding in the thymus, but equally, through inhibiting the transition of CD4(-)CD8(-) to CD4(+)CD8(+) T cells. Consequently, the number of mature peripheral T cells was markedly reduced. In contrast, a minimal effect on B cell and myeloid lineage development was observed. Importantly, functional RasGAP and Nck binding sites were found to be essential for the biological effects of DokR in vitro and in vivo.

The T-cell receptor signalosome: a dynamic structure with expanding complexity

Signal transduction in T cells is a dynamic process involving a large number of membrane and cytosolic proteins. The TCR macromolecular complex (signalosome) is initiated by receptor occupancy and becomes more elaborate over time. This review describes how 'vertical' displacement mechanisms and lateral coalescence of lipid-raft-associated scaffold proteins combine to form distinct signalosomes, which control signal specificity.

Induced expression and association of the Mona/Gads adapter and Gab3 scaffolding protein during monocyte/macrophage differentiation

Mona/Gads is a Grb2-related, Src homology 3 (SH3) and SH2 domain-containing adapter protein whose expression is restricted to cells of hematopoietic lineage (i.e., monocytes and T lymphocytes). During monocyte/macrophage differentiation, Mona is induced and interacts with the macrophage colony-stimulating factor receptor, M-CSFR (also called Fms), suggesting that Mona could be involved in developmental signaling downstream of the M-CSFR by recruiting additional signaling proteins to the activated receptor. Our present results identify Mona as a specific partner protein for the DOS/Gab family member Gab3 in monocytic/macrophage development. Mona does not interact with Gab2; however, Gab3 also forms a complex with the Mona-related adapter Grb2. Glutathione S-transferase pull-down experiments demonstrate that the Mona and Gab3 interaction utilizes the carboxy-terminal SH3 domain of Mona and the atypical proline-rich domain of Gab3. Mona is known to interact with the phosphorylated Y697 site of the M-CSFR. The M-CSFR mutation Y697F exhibited qualitative and quantitative abnormalities in receptor and Gab3 tyrosine phosphorylation, and Mona induction was greatly reduced. The Y807F M-CSFR mutation is defective in differentiation signaling, but not growth signaling, and also fails to induce Mona protein expression. During M-CSF-stimulated macrophage differentiation of mouse bone marrow cells, Mona and Gab3 expression is coinduced, these proteins interact, and Mona engages in multimolecular complexes. These data suggest that association of Mona and Gab3 plays a specific role in mediating the M-CSFR differentiation signal.