Biomedical subjects

R P Siraganian

Publications and source records attributed to R P Siraganian.

At least 19 recordsLinked to original sources

PubMed2001

SH2 domain-mediated targeting, but not localization, of Syk in the plasma membrane is critical for FcepsilonRI signaling.

Aggregation of the high-affinity IgE receptor induces the tyrosine phosphorylation of subunits of the receptor and the subsequent association with the receptor of the cytosolic protein tyrosine kinase Syk. The current experiments examined the functional importance of membrane association of Syk and the role of the SH2 domain in receptor-mediated signal transduction. Wild-type Syk and chimeric Syk molecules with the c-Src myristylation sequence at the amino-terminus were expressed in a Syk-negative mast cell line. Chimeric Syk with the myristylation sequence was membrane associated, and a small fraction was constitutively colocalized with FcepsilonRI, Lyn, and LAT (linker for T-cell activation) in the glycolipid-enriched microdomains or rafts. However, even under these conditions, the tyrosine phosphorylation of Syk and the downstream propagation of signals required FcepsilonRI aggregation. This chimeric Syk was less active than wild-type Syk in FcepsilonRI-mediated signal transduction. In contrast, a truncated membrane-associated form of Syk that lacked the SH2 domains was not tyrosine phosphorylated by receptor aggregation and failed to transduce intracellular signals. These findings suggest that SH2 domain-mediated membrane translocation of Syk is essential for the FcepsilonRI-mediated activation of Syk for downstream signaling events leading to histamine release. Furthermore, the localization of Syk in glycolipid-enriched microdomains by itself is not enough to generate or enhance signaling events.

Animals↗

PubMed2001

Activated Cdc42/Rac reconstitutes Fcepsilon RI-mediated Ca2+ mobilization and degranulation in mutant RBL mast cells.

Antigen stimulation of mast cells via FcepsilonRI, the high-affinity receptor for IgE, triggers a signaling cascade that requires Ca(2+) mobilization for exocytosis of secretory granules during an allergic response. This study investigates critical signaling components by using mutant RBL mast cells that are defective in antigen-stimulated phospholipase Cgamma (PLCgamma) activation, as well as other signaling activities downstream of stimulated tyrosine phosphorylation. We show that the expression of activated versions of the Cdc42 or Rac1 GTPase restores antigen-stimulated Ca(2+) mobilization necessary for degranulation in these mutant cells. Wild-type Cdc42 and Rac1, as well as activated Cdc42 containing effector domain mutations, all fail to restore antigen-stimulated signaling leading to exocytosis. Expression of oncogenic Dbl, a guanine nucleotide exchange factor for Cdc42 and Rac1, partially restores sustained Ca(2+) mobilization and degranulation, suggesting that activation of endogenous Cdc42 and/or Rac1 is impaired in the mutant cells. Overexpression of PLCgamma1 with either activated Cdc42 or Rac1 synergistically stimulates degranulation, consistent with a critical defect in PLCgamma activation in these cells. Thus, our results point to activation of Cdc42 and/or Rac1 playing an essential role in antigen stimulation of early events that culminate in mast cell degranulation.

Animals↗

PubMed2001

Monomeric IgE stimulates signaling pathways in mast cells that lead to cytokine production and cell survival.

Although IgE binding to mast cells is thought to be a passive presensitization step, we demonstrate herein that monomeric IgE (mIgE) in the absence of antigen (Ag) stimulates multiple phosphorylation events in normal murine bone marrow-derived mast cells (BMMCs). While mIgE does not induce degranulation or leukotriene synthesis, it leads to a more potent production of cytokines than IgE + Ag. Moreover, mIgE prevents the apoptosis of cytokine-deprived BMMCs, likely by maintaining Bcl-X(L) levels and producing autocrine-acting cytokines. The addition of Ag does not increase this IgE-induced survival. Since IgE concentrations as low as 0.1 microg/ml enhance BMMC survival, elevated plasma IgE levels in humans with atopic disorders may contribute to the elevated mast cell numbers seen in these individuals.

Animals↗

PubMed2001

Identification and isolation of rat bone marrow-derived mast cells using the mast cell-specific monoclonal antibody AA4.

Previous studies of mast cell maturation, structure, and function have been hampered by the lack of mast cell-specific markers. In this study, using a well-characterized mast cell-specific monoclonal antibody, MAb AA4, mast cells from rat bone marrow in various stages of maturation were isolated and characterized. The very immature mast cells, which have not been previously described, contained few granules and would not be recognized as mast cells by standard cytological methods. Pure populations of mast cells were isolated from the bone marrow using MAb AA4-conjugated magnetic beads. The same stages of maturation were observed in the isolated mast cells as were seen in the unfractionated bone marrow. All of these cells were immunopositive for the alpha-subunit of Fc epsilon RI, IgE, and c-kit, confirming their identity as mast cells. By direct counting of immunolabled cells and by flow cytometry, approximately 2.4% of the cells in the bone marrow are mast cells. Staining with toluidine blue and berberine sulfate, as well as RT-PCR of the cells, indicates that these cells are connective tissue-type mast cells. The use of immunological methods for identification of mast cell precursors should facilitate the study of these cells. (J Histochem Cytochem 49:219-228, 2001)

Animals↗

PubMed2000

Phosphorylation of Syk activation loop tyrosines is essential for Syk function. An in vivo study using a specific anti-Syk activation loop phosphotyrosine antibody.

Syk is an important protein-tyrosine kinase in immunoreceptor signaling. FcepsilonRI aggregation in mast cells induces tyrosine phosphorylation and increased enzymatic activity of Syk. The two adjacent tyrosines in the Syk activation loop are thought to be important for the propagation of FcepsilonRI signaling. To evaluate the phosphorylation of these tyrosines in vivo and further understand the relationship of Syk tyrosine phosphorylation with its function, an antibody was developed specific for phosphorylated tyrosines in the activation loop of Syk. FcepsilonRI aggregation on mast cells induced the phosphorylation of both tyrosine residues of the activation loop. The kinase activity of Syk played the major role in phosphorylating its activation loop tyrosines both in vivo and in vitro. In FcepsilonRI-stimulated mast cells, the total Syk tyrosine phosphorylation paralleled the phosphorylation of its activation loop tyrosines and downstream propagation of signals for histamine release. In contrast, the cell surface binding of anti-ganglioside monoclonal antibody AA4 induced only strong general tyrosine phosphorylation of Syk and minimal histamine release and weak phosphorylation of activation loop tyrosines. These results demonstrate that phosphorylation of the activation loop tyrosines is important for mediating receptor signaling and is a better marker of Syk function than is total Syk tyrosine phosphorylation.

Animals↗

PubMed2000

The NH2-terminal region of focal adhesion kinase reconstitutes high affinity IgE receptor-induced secretion in mast cells.

Focal adhesion kinase (FAK) is tyrosine-phosphorylated by adherence of cells and also by FcepsilonRI aggregation in RBL-2H3 mast cells. Using phosphorylation site-specific antibodies, we observed that FcepsilonRI activation in these cells led to an increase in FAK phosphorylation at the same tyrosine residues that are phosphorylated by integrin-induced activation. Previous studies in the 3B6 line, a FAK-deficient variant of the RBL-2H3 cells, suggest that FAK plays a role in FcepsilonRI-induced secretion. Stable cell lines expressing either full-length or truncated forms of FAK were isolated after transfection of the FAK-deficient 3B6 variant cells. The NH(2) domain of FAK, which lacks the enzymatic and the COOH-terminal regions, was sufficient to reconstitute secretion. The different truncated forms of FAK were still tyrosine-phosphorylated after FcepsilonRI aggregation. Therefore, the kinase domain and the COOH-terminal region are not essential for FcepsilonRI-induced tyrosine phosphorylation of FAK or for secretion. Taken together, our data demonstrate that the reconstitution of secretion is dissociated from FAK activation and that the NH(2)-terminal region of FAK is the only critical element that may play a role in FcepsilonRI-induced secretion by acting as an adaptor or linker molecule.

Animals↗

PubMed2000

Regulation of protein kinase CbetaI by two protein-tyrosine kinases, Btk and Syk.

Two protein-tyrosine kinases, Bruton's tyrosine kinase (Btk) and Syk, and members of the protein kinase C (PKC) subfamily of serine/threonine kinases play crucial roles in signal transduction through antigen receptors in B lymphocytes and high-affinity IgE receptors (FcepsilonRI) in mast cells. The present study provides genetic, biochemical, and pharmacological evidence that, on FcepsilonRI stimulation, Syk regulates Btk, and Btk selectively regulates the membrane translocation and enzymatic activity of PKCbetaI among the conventional PKC isoforms (alpha, betaI, and betaII) expressed in mast cells. Syk/Btk-mediated PKCbetaI regulation is involved in transcriptional activation of the IL-2 and tumor necrosis factor alpha genes through the JNK pathway induced by FcepsilonRI stimulation. Accordingly, FcepsilonRI-induced production of these cytokines is inhibited by specific inhibitors of Btk and Syk, as well as broad-specificity inhibitors of PKC and a selective inhibitor of PKCbeta. Specific regulation of PKCbetaI by Btk is consistent with the selective association of Btk with PKCbetaI. Components of this signaling pathway may represent an attractive set of potential targets of pharmaceutical interference for the treatment of allergic and other immunologic diseases.

Agammaglobulinaemia Tyrosine Kinase↗

PubMed2000

Positive regulation of c-Jun N-terminal kinase and TNF-alpha production but not histamine release by SHP-1 in RBL-2H3 mast cells.

The SH2-containing protein tyrosine phosphatase1 (SHP-1) is important for signaling from immune receptors. To investigate the role of SHP-1 in mast cells we overexpressed the wild-type and the phosphatase-inactive forms of SHP-1 in rat basophilic leukemia 2H3 (RBL-2H3) mast cell line. The phosphatase-inactive SHP-1 (C453S or D419A) retains its ability to bind tyrosine phosphorylated substrates and thereby competes with the endogenous wild-type enzyme. Overexpression of wild-type SHP-1 decreased the FcepsilonRI aggregation-induced tyrosine phosphorylation of the beta and gamma subunits of the receptor whereas the dominant negative SHP-1 enhanced phosphorylation. There were also similar changes in the tyrosine phosphorylation of Syk. However, receptor-induced histamine release in the cells expressing either wild-type or dominant negative SHP-1 was similar to that in the parental control cells. In contrast, compared with the parental RBL-2H3 cells, FcepsilonRI-induced c-Jun N-terminal kinase phosphorylation and the level of TNF-alpha mRNA was increased in the cells overexpressing wild-type SHP-1 whereas the dominant negative SHP-1 had the opposite effect. The substrate-trapping mutant SHP1/D419A identified pp25 and pp30 as two major potential substrates of SHP-1 in RBL-2H3 cells. Therefore, SHP-1 may play a role in allergy and inflammation by regulating mast cell cytokine production.

Animals↗

PubMed2000

The Src homology 2 domain of Vav is required for its compartmentation to the plasma membrane and activation of c-Jun NH(2)-terminal kinase 1.

Vav is a hematopoietic cell-specific guanine nucleotide exchange factor (GEF) whose activation is mediated by receptor engagement. The relationship of Vav localization to its function is presently unclear. We found that Vav redistributes to the plasma membrane in response to Fcin receptor I (FcinRI) engagement. The redistribution of Vav was mediated by its Src homology 2 (SH2) domain and required Syk activity. The FcinRI and Vav were found to colocalize and were recruited to glycosphingolipid-enriched microdomains (GEMs). The scaffold protein, linker for activation of T cells (LAT), and Rac1 (a target of Vav activity) were constitutively present in GEMs. Expression of an SH2 domain-containing COOH-terminal fragment of Vav inhibited Vav phosphorylation and movement to the GEMs but had no effect on the tyrosine phosphorylation of the adaptor protein, SLP-76 (SH2 domain-containing leukocyte protein of 76 kD), and LAT. However, assembly of the multiprotein complex containing these proteins was inhibited. In addition, FcinRI-dependent activation of c-Jun NH(2)-terminal kinase 1 (JNK1) was also inhibited. Thus, Vav localization to the plasma membrane is mediated by its SH2 domain and may serve to regulate downstream effectors like JNK1.

Adaptor Proteins, Signal Transducing↗

PubMed2000

Point mutation of a tyrosine in the linker region of Syk results in a gain of function.

The protein tyrosine kinase Syk plays an essential role in Fc epsilon RI-mediated histamine release in mast cells by regulating the phosphorylation of other proteins. We investigated the functional role of a putative Syk phosphorylation site, Tyr317. This tyrosine in the linker region of Syk is a possible site for binding by the negative regulator Cbl. Syk with Tyr317 mutated to Phe (Y317F) was expressed in a Syk-negative variant of the RBL-2H3 mast cells. Compared with cells expressing wild-type Syk, expression of the Y317F mutant resulted in an increase in the Fc epsilon RI-mediated tyrosine phosphorylation of phospholipase C-gamma and a dramatic enhancement of histamine release. The in vivo Fc epsilon RI-induced tyrosine phosphorylation of wild-type Syk and that of the Y317F mutant were similar. Although the Fc epsilon RI-induced tyrosine phosphorylation of total cellular proteins was enhanced in the cells expressing the Y317F Syk, the phosphorylation of some other molecules, including the receptor subunits, Vav and mitogen-activated protein kinase, was not increased. The Fc epsilon RI-induced phosphorylation of Cbl was downstream of Syk kinase activity and was unchanged by expression of the Y317F mutation. These data indicate that Tyr317 in the linker region of Syk functions to negatively regulate the signals leading to degranulation.

Animals↗

PubMed2000

Mutant RBL mast cells defective in Fc epsilon RI signaling and lipid raft biosynthesis are reconstituted by activated Rho-family GTPases.

Characterization of defects in a variant subline of RBL mast cells has revealed a biochemical event proximal to IgE receptor (Fc epsilon RI)-stimulated tyrosine phosphorylation that is required for multiple functional responses. This cell line, designated B6A4C1, is deficient in both Fc epsilon RI-mediated degranulation and biosynthesis of several lipid raft components. Agents that bypass receptor-mediated Ca(2+) influx stimulate strong degranulation responses in these variant cells. Cross-linking of IgE-Fc epsilon RI on these cells stimulates robust tyrosine phosphorylation but fails to mobilize a sustained Ca(2+) response. Fc epsilon RI-mediated inositol phosphate production is not detectable in these cells, and failure of adenosine receptors to mobilize Ca(2+) suggests a general deficiency in stimulated phospholipase C activity. Antigen stimulation of phospholipases A(2) and D is also defective. Infection of B6A4C1 cells with vaccinia virus constructs expressing constitutively active Rho family members Cdc42 and Rac restores antigen-stimulated degranulation, and active Cdc42 (but not active Rac) restores ganglioside and GPI expression. The results support the hypothesis that activation of Cdc42 and/or Rac is critical for Fc epsilon RI-mediated signaling that leads to Ca(2+) mobilization and degranulation. Furthermore, they suggest that Cdc42 plays an important role in the biosynthesis and expression of certain components of lipid rafts.

Actins↗

PubMed1999

CD45 is essential for Fc epsilon RI signaling by ZAP70, but not Syk, in Syk-negative mast cells.

The ZAP70/Syk family of protein tyrosine kinases plays an important role in Ag receptor signaling. Structural similarity of Syk and ZAP70 suggests their functional overlap. Previously, it was observed that expression of either ZAP70 or Syk reconstitutes Ag receptor signaling in Syk-negative B cells. However, in CD45-deficient T cells, Syk, but not ZAP70, restores T cell receptor-signaling pathway. To study the function of Syk, ZAP70, and CD45 in mast cells, a Syk/CD45 double-deficient variant of RBL-2H3 cells was characterized. After transfection, stable cell lines were isolated that expressed ZAP70, Syk, CD45, ZAP70 plus CD45, and Syk plus CD45. IgE stimulation did not induce degranulation in parental double-deficient cells, nor in the cells expressing only CD45. ZAP70 expression did not restore Fc epsilon RI signaling unless CD45 was coexpressed in the cells. However, Syk alone restored the IgE signal transduction pathway. The coexpression of CD45 with Syk had no significant effects on the responses to FcepsilonRI-aggregation. There was much better binding of Syk than ZAP70 to the phosphorylated Fc epsilon RI gamma-ITAM. Furthermore, unlike Syk, ZAP70 required CD45 to display receptor-induced increase in kinase activity. Therefore, in mast cells, ZAP70, but not Syk, requires CD45 for Ag receptor-induced signaling.

Animals↗

PubMed1998

Mutations in the activation loop tyrosines of protein tyrosine kinase Syk abrogate intracellular signaling but not kinase activity.

The protein tyrosine kinase Syk plays a pivotal role in mediating the high-affinity IgE receptor (Fc epsilonRI)-induced degranulation of mast cells. To examine the mechanism of Syk regulation, the two tyrosine residues at 519 and 520 in the putative activation loop of rat Syk were mutated to phenylalanine either singly or in combination. The various mutants were expressed in a Syk-negative variant of the RBL-2H3 (rat basophilic leukemia 2H3) mast cell line. In these transfected cell lines, mutant Syk did show increased tyrosine phosphorylation in vivo and increased enzymatic activity in vitro after Fc epsilonRI aggregation. There were conformational changes detected by an Ab when the wild-type and mutant Syk were either tyrosine phosphorylated or bound to tyrosine-phosphorylated immunoreceptor tyrosine-based activation motif peptides. However, these mutant Syk were incapable of transducing Fc epsilonRI signaling. In cells in which the expression level of mutant Syk was similar to that of the wild-type Syk, Fc epsilonRI cross-linking induced no increase in cellular protein tyrosine phosphorylation, no increase in tyrosine phosphorylation of phospholipase C-gamma2 and mitogen-activated protein kinase, and no histamine release. Overexpression of Y519F or Y520F Syk mutants partially reconstituted the signaling pathways. These results indicate that these tyrosines in the putative activation loop are not essential for the enzymatic activity of Syk or for the conformational changes induced by binding of tyrosine-phosphorylated immunoreceptor tyrosine-based activation motif peptides. However, these tyrosines are necessary for Syk-mediated propagation of Fc epsilonRI signaling.

Animals↗

PubMed1998

A unique insert in the linker domain of Syk is necessary for its function in immunoreceptor signalling.

Accumulating data indicate that the 'linker' region of Syk, which lies between its tandem Src homology 2 (SH2) domains and kinase region, provides a critical function for the biological activity of Syk. This importance has been ascribed to the presence of tyrosine phosphorylation sites capable of mediating the recruitment of cellular effectors. We and others previously identified an alternatively spliced variant of Syk, termed SykB, which lacks a 23 amino acid sequence in the linker domain. As this 'linker insert' is also not present in the closely related enzyme Zap-70, it seems plausible that Syk possesses this unique sequence for functional reasons. To understand its role better, we have compared the abilities of Syk and SykB to participate in immunoreceptor-triggered signal transduction. The results of our experiments revealed that, unlike Syk, SykB was inefficient at coupling stimulation of FcepsilonRI on basophils or the antigen receptor on T cells to the early and late events of cellular activation. Further studies showed that the functional defect in SykB was not caused by the absence of crucial tyrosine phosphorylation sites, or by a reduced intrinsic kinase activity. Rather, it correlated with the reduced ability of SykB to bind phosphorylated immunoreceptor tyrosine-based activation motifs (ITAMs) in vitro and in vivo. In combination, these results demonstrated that the unique insert in the linker domain of Syk is crucial for its capacity to participate in immunoreceptor signalling. Furthermore, they provided evidence that the linker region can regulate the ability of Syk to bind ITAMs, thus identifying a novel function for this domain.

Amino Acid Sequence↗

PubMed1997

Activation of protein-tyrosine kinase Pyk2 is downstream of Syk in FcepsilonRI signaling.

Aggregation of the FcepsilonRI, a member of the immune receptor family, induces the activation of proteintyrosine kinases and results in tyrosine phosphorylation of proteins that are involved in downstream signaling pathways. Here we report that Pyk2, another member of the focal adhesion kinase family, was present in the RBL-2H3 mast cell line and was rapidly tyrosine-phosphorylated and activated after FcepsilonRI aggregation. Tyrosine phosphorylation of Pyk2 was also induced by the calcium ionophore A23187, by phorbol myristate acetate, or by stimulation of G-protein-coupled receptors. Adherence of cells to fibronectin dramatically enhanced the induced tyrosine phosphorylation of Pyk2. Although Src family kinases are activated by FcepsilonRI stimulation and tyrosine-phosphorylate the receptor subunits, the activation and tyrosine phosphorylation of Pyk2 were downstream of Syk. In contrast, tyrosine phosphorylation of Pyk2 by stimulation of G-protein-coupled receptors was independent of Syk. Therefore, the FcepsilonRI-induced tyrosine phosphorylation of Pyk2 is downstream of Syk and may play a role in cell secretion.

Cell Adhesion↗

PubMed1997

The protein-tyrosine phosphatase SHP-2 associates with tyrosine-phosphorylated adhesion molecule PECAM-1 (CD31).

Aggregation of many cell-surface receptors results in tyrosine phosphorylation of numerous proteins. We previously observed the tyrosine phosphorylation of the platelet/endothelial cell adhesion molecule, PECAM-1 (CD31), after FcepsilonRI stimulation in rat basophilic leukemia RBL-2H3 cells. Here we found that PECAM-1 was also transiently tyrosine-phosphoryated after adherence of these cells to fibronectin. Similarly aggregation of the T cell receptor on Jurkat cells also induced this tyrosine phosphorylation. The protein-tyrosine phosphatase SHP-2 is a widely expressed cytosolic enzyme with two Src homology 2 (SH2) domains. SHP-2, but not the related protein-tyrosine phosphatase SHP-1, associated with PECAM-1. This association of the two proteins correlated with the extent of the tyrosine phosphorylation of PECAM-1. A fusion protein containing the two SH2 domains of SHP-2 precipitated PECAM-1 from cell lysates and also directly bound to phosphorylated PECAM-1. In immune precipitate phosphatase assays, there was tyrosine dephosphorylation of PECAM-1. Therefore, integrin and immune receptor activation results in tyrosine phosphorylation of PECAM-1 and the binding of the protein-tyrosine phosphatase SHP-2, which could regulate receptor-mediated signaling in cells.

Animals↗

PubMed1997

Reconstitution of high affinity IgE receptor-mediated secretion by transfecting protein tyrosine kinase pp125FAK.

To study the role of the focal adhesion tyrosine kinase (FAK) in receptor-mediated secretion, we transfected FAK cDNA into a variant (3B6) of the RBL-2H3 mast cell line. This 3B6 cell line expressed low levels of FAK and was defective in high affinity IgE receptor (FcepsilonRI) but not Ca2+ ionophore-mediated secretion. FcepsilonRI-mediated secretion was reconstituted after transfection of wild-type FAK. Histamine release was also enhanced by the stable expression of two mutants of FAK: a kinase-inactive form in which the ATP binding site Lys-454 was replaced by Arg or a mutant in which the autophosphorylation site Tyr-397 was replaced by Phe. Therefore, the catalytic activity and the autophosphorylation site of FAK are not essential for secretion. FcepsilonRI aggregation increased the tyrosine phosphorylation of both mutants of FAK to the same extent as wild-type FAK. Therefore, tyrosine kinases activated by FcepsilonRI aggregation are phosphorylating FAK and some of these phosphorylation sites are other than Tyr-397. These results strongly suggest that FAK plays a role in FcepsilonRI-induced secretion by functioning as an adapter or linker molecule.

Amino Acid Substitution↗

PubMed1997

Syk-independent tyrosine phosphorylation and association of the protein tyrosine phosphatases SHP-1 and SHP-2 with the high affinity IgE receptor.

Aggregation of the high affinity IgE receptor (Fc epsilonRI), a member of the immune receptor family, results in the activation of protein tyrosine kinases and downstream signaling pathways. The two cytoplasmic Src homology 2 domain-containing protein tyrosine phosphatases, SHP-1 (also called SH-PTP1, PTP1C or HCP) and SHP-2 (also known as SH-PTP2, PTP1D, PTP2C, or Syp), are expressed in the RBL-2H3 rat mast cell line. Here we report that aggregation of Fc epsilonRI induced the tyrosine phosphorylation of both SHP-1 and SHP-2. This phosphorylation was independent of the presence of the protein tyrosine kinase Syk. Both SHP-1 and SHP-2 associated with Fc epsilonRI. Whereas SHP-1 was constitutively associated with the receptor, SHP-2 coprecipitated with Fc epsilonRI only after receptor aggregation. Fusion proteins containing either the full-length or the Src homology 2 domains of SHP-2 directly bound to the tyrosine-phosphorylated beta, but not the gamma, subunit of Fc epsilonRI. In the reciprocal experiments, synthetic phosphorylated peptides based on the immunoreceptor tyrosine-based activation motif of the beta, but not the gamma, subunit precipitated SHP-2. In contrast, neither fusion proteins nor synthetic peptides detected interaction between SHP-1 and Fc epsilonRI. In vitro, both SHP-1 and SHP-2 dephosphorylated tyrosine-phosphorylated beta and gamma subunits of Fc epsilonRI. Therefore, SHP-1 and SHP-2 associate with Fc epsilonRI by different mechanisms and can regulate the extent of the tyrosine phosphorylation of the receptor subunits. Thus, unlike other immune cells in which inhibitory molecules are recruited by accessory proteins, Fc epsilonRI bind molecules that both activate and inhibit signal transduction.

Amino Acid Sequence↗