Interaction of aggregated native and mutant IgE receptors with the cellular skeleton

Laser scanning confocal fluorescence microscopy: an overview

Innovative and important aspects of laser scanning confocal fluorescence imaging (LSCFI) are presented here as a general overview. We have described and discussed the technology of the procedure in some detail. We also report some of our original work with transmembranous uptake of 5S gamma-globulin on living human leukocytes as an example of one specific application of LSCFI. These original data and results are presented, as well as citing other uses and applications, to show the power of LSCFI technique. The article will hopefully be useful for those not familiar with the methodology and utility of laser scanning confocal fluorescence microscopy. Applications of LSCFI are very diverse, and there are new applications of this technology constantly being developed. Interest is growing in LSCFI, particularly in the pharmacologic and therapeutic areas, as demonstrated in this article.

Inhibition of antigen-induced mediator release from IgE-sensitized cells by a monoclonal anti-Fc epsilon RI alpha-chain receptor antibody: implications for the involvement of the membrane-proximal alpha-chain region in Fc epsilon RI-mediated cell activation

The interaction between human IgE and its high affinity receptor, FcepsilonRI, is a critical event in mediating the allergic response. Aggregation of the alpha-chain of FcepsilonRI (FcepsilonRIalpha) occurs via cross-linking of receptor-bound IgE by Ag, resulting in cell activation and the release of mediators of hypersensitivity. Recently, we mapped the epitopes of two anti-FcepsilonRIalpha mAbs, 15/1 and 5H5F8. In contrast to 15/1, mAb 5H5F8 does not inhibit IgE binding to FcepsilonRIalpha. Here we demonstrate both 5H5F8 binding to FcepsilonRI(+) cells as well as a high level of IgE binding to 5H5F8-saturated cells. At the same time 5H5F8 strongly inhibits hexosaminidase release and Ca(2+) flux after Ag triggering from human IgE-sensitized RBL-2H3 cells stably transfected with human FcepsilonRIalpha. Further, 5H5F8 and its Fab inhibit sulfidoleukotriene and histamine release from primary human peripheral blood leukocytes, including cells bearing endogenous IGE: Furthermore, we confirm that 5H5F8 maps to a linear peptide sequence in close proximity to the cell membrane. Two chemically synthesized peptides containing the 5H5F8 epitope sequence PREKY were selected for detailed analysis of 5H5F8 and 5H5F8 Fab binding and were found to produce K(d) values of similar magnitude to that observed for binding to recombinant FcepsilonRIalpha. These peptides may prove useful as targets for the identification of antagonists of FcepsilonRIalpha-mediated biological activity. Moreover, our data indicate that FcepsilonRIalpha-mediated activation may involve a novel alpha-chain epitope in an early step of the cell-triggering pathway leading to cellular activation.

Structural aspects of the association of FcepsilonRI with detergent-resistant membranes

We recently showed that aggregation of the high affinity IgE receptor on mast cells, FcepsilonRI, causes this immunoreceptor to associate rapidly with specialized regions of the plasma membrane, where it is phosphorylated by the tyrosine kinase Lyn. In this study, we further characterize the detergent sensitivity of this association on rat basophilic leukemia-2H3 mast cells, and we compare the capacity of structural variants of FcepsilonRI and other receptors to undergo this association. We show that this interaction is not mediated by the beta subunit of the receptor or the cytoplasmic tail of the gamma subunit, both of which are involved in signaling. Using chimeric receptor constructs, we found that the extracellular segment of the FcepsilonRI alpha subunit was not sufficient to mediate this association, implicating FcepsilonRI alpha and/or gamma transmembrane segments. To determine the specificity of this interaction, we compared the association of several other receptors. Interleukin-1 type I receptors on Chinese hamster ovary cells and alpha4 integrins on rat basophilic leukemia cells showed little or no association with isolated membrane domains, both before and after aggregation on the cells. In contrast, interleukin-2 receptor alpha (Tac) on Chinese hamster ovary cells exhibited aggregation-dependent membrane domain association similar to FcepsilonRI. These results provide insights into the structural basis and selectivity of lipid-mediated interactions between certain transmembrane receptors and detergent-resistant membranes.

Interaction of IgG immunoglobulins with the guinea pig peritoneal macrophage Fc gamma receptors. Effect on the association of the receptors with the membrane skeleton and the cytoskeleton

Binding of ligands to cell surface receptors may induce an interaction of the receptors with the cytoskeleton and/or membrane skeleton and decrease the solubility of the receptors in nonionic detergents. Cytochalasins, reagents affecting the structure of microfilaments, inhibit some cell functions induced by cross-linking of the receptors with ligands. Information concerning the function of the cytoskeleton in insolubilization of Fc gamma receptors (Fc gamma R) and in Fc gamma R-mediated signal transmission is rather limited. The aim of this work was to investigate the effect of binding of homologous (guinea pig IgG1 and IgG2) and heterologous (rabbit IgG) immunoglobulins to guinea pig peritoneal macrophages on association of the macrophage Fc gamma receptors with the membrane skeleton and cytoskeleton. Cross-linking the macrophage Fc gamma receptors with immunoglobulin ligands induced insolubilization of the receptors in nonionic detergents suggesting association of the receptors with the membrane skeleton and the cytoskeleton. The ligands showed differential effects depending on a subclass and origin of the IgG used. The process of association of the Fc gamma receptors with the skeletons was fast and did not depend on temperature. Treatment of insoluble complexes with cytochalasin D, DNAse I or colchicine showed that actin microfilaments and microtubules play a role, at least partially, in insolubilization of the cross-linked macrophage Fc gamma receptors. Inhibition of insolubilization of the macrophage Fc gamma receptors by genistein indicated that tyrosine kinases are involved in the process of insolubilization. The association with the skeletons might be a part of the process of transduction of a signal which depended on the subclass and origin of IgG used and on the type of the Fc gamma receptor.

Functional heterogeneity of Thy-1 membrane microdomains in rat basophilic leukemia cells

Antibody-mediated cross-linking of Thy-1 glycoprotein on the surface of rat mast cells and rat basophilic leukemia (RBL) cells initiates biochemical events which culminate in secretion of allergy mediators. Thy-1, like some other glycosylphosphatidylinositol (GPI)-anchored proteins, forms detergent-insoluble complexes containing protein tyrosine kinases (PTK) and some other molecules which are implicated in the signaling pathway. On the surface of a rat mast cell there are more than 10(6) Thy-1 molecules; however, it is not known which fraction of them is involved in transmembrane signaling, and what exactly is the heterogeneity of Thy-1 complexes. Using sucrose density gradient ultracentrifugation of detergent-lysed RBL cells we found that the density of Thy-1 complexes depended on the detergent used and the lysis conditions employed. Sepharose 4B gel chromatography fractionation followed by density gradient ultracentrifugation revealed both size and density heterogeneity of Thy-1 and Lyn PTK complexes. Cross-linking of surface Thy-1 caused significant changes in the density of these complexes, and an increase in Lyn kinase activity in low/medium-density fractions. Thy-1 in low-density fractions was relatively resistant to cleavage with phosphatidylinositol-specific phospholipase C (PI-PLC). Interestingly, removal of only a small fraction of surface Thy-1 by PI-PLC abolished the cell activation as determined by tyrosine phosphorylation of certain proteins. When Triton X-100 lysates were fractionated at 12000 x g, about 50 % of Thy-1 remained associated with the nuclear/cytoskeleton pellet; this fraction of Thy-1 exhibited an increased sensitivity to PI-PLC. Confocal laser scanning microscopy on fixed cells revealed that the total Thy-1 was relatively homogeneously distributed over the plasma membrane, whereas the PI-PLC-resistant Thy-1 was found mostly in small clusters. The combined data suggest that specialized membrane microdomains enriched in Thy-1 with increased sensitivity to PI-PLC are directly involved in coupling Thy-1 aggregation to transmembrane signaling.

Potential allergens stimulate the release of mediators of the allergic response from cells of mast cell lineage in the absence of sensitization with antigen-specific IgE

A number of structurally diverse antigens preferentially stimulate the synthesis of IgE antibodies, but no unifying principle has been proposed that explains the nature of isotype selection. In the present study, we show that common allergens present in bee venom, house dust mite emanations and parasite proteins induce mast cell and basophil degranulation and stimulate interleukin-4 synthesis, and secretion in the absence of antigen-specific IgE. These data point to a linkage between the initial activation of cells of the innate immune system and subsequent adaptive immune responses. They suggest that IgE-independent mast cell and basophil degranulation is predictive of potential allergenicity and can be evaluated by means of a cellular assay. Our study indicates that non-immunological degranulation by prototypic allergens, such as bee venom phospholipase A2 or proteases associated with house dust mite emanations, is critically dependent on enzymatic activity. These findings have potentially important implications for vaccine design in allergic and parasitic disease.

Normal T cells express two T cell antigen receptor populations, one of which is linked to the cytoskeleton via zeta chain and displays a unique activation-dependent phosphorylation pattern

The TCR couples antigen recognition and the transmission of activation signals. We report the expression of two TCR populations on the surface of T lymphocytes, one of which is linked to the cytoskeleton via the zeta chain. We also demonstrate that assembly of the CD3 subunits with cytoskeleton-associated zeta is necessary for their maximal localization to the cytoskeleton. The potential significance of these two receptor forms is underscored by differences observed in non-activated T cells; while detergent-soluble phosphorylated zeta appears as a 21-kDa protein, phosphorylated cytoskeleton-associated zeta appears as a 16-kDa form. This dichotomous phosphorylation pattern is rigidly maintained following activation, although each of the receptor populations undergoes different activation-dependent modifications: 1) levels of soluble phosphorylated 21-kDa zeta are enhanced, while phosphorylated 16-kDa cytoskeleton-associated zeta exhibits little change; 2) soluble non-phosphorylated 16-kDa zeta translocates to the cytoskeleton; 3) activation-dependent ubiquitinated zeta forms localize to both fractions, albeit with different kinetics. We also show that the protein tyrosine kinase Lck undergoes activation-dependent modifications and translocates to the cytoskeleton. The phosphorylation profiles of the dichotomous TCR populations in both non-activated and activated lymphocytes suggest that each population could regulate distinct cellular functions, possibly by select intermolecular associations.

Thy-1-mediated activation of rat mast cells: the role of Thy-1 membrane microdomains

The glycosyl-phosphatidylinositol (GPI)-anchored glycoprotein Thy-1 is one of the most abundant molecules expressed on the surface of rat mast cells and rat basophilic leukemia (RBL) cells. The finding that Thy-1 from detergent-solubilized RBL-2H3 cells forms complexes with src-related protein-tyrosine kinase p56/p53lyn suggested that this kinase may play a key role in Thy-1-mediated mast-cell activation. The molecular mechanism of this activation is, however, unknown. Here we show that in RBL-2H3-derived cells extracted by the standard procedure with several non-ionic detergents, the majority of Thy-1 and p56/p53lyn were not released into postnuclear supernatant but remained associated with the detergent-resistant cytoskeletal/nuclear fraction. Pretreatment of the cells with the cholesterol-complexing agents, saponin or digitonin, resulted in complete solubilization of Thy-1 and p56/p53lyn in non-ionic detergents and dissociation of the complexes; this implies that cholesterol plays a crucial role in stabilization of the complexes. This conclusion was supported by double immunofluorescence colocalization experiments which also allowed us to estimate the size of the insoluble complexes to be about 0.1 micron. Sequential treatment with saponin and Nonidet P-40 was used to fractionate tyrosine-phosphorylated proteins during Thy-1-mediated activation of RBL-2H3 cells. Among the soluble cytoplasmic proteins the most dramatic change in tyrosine phosphorylation was found in pp72, whereas pp40 and pp33 were found mainly in the membrane fraction. Our data suggest that surface aggregation of GPI-anchored Thy-1 molecules leads to aggregation of p56/p53lyn kinase located in the same membrane microdomain, followed by transphosphorylation of both soluble and membrane-bound substrates.

Tyrosine phosphorylation of protein kinase C-delta in response to the activation of the high-affinity receptor for immunoglobulin E modifies its substrate recognition

The delta isoform of protein kinase C is phosphorylated on tyrosine in response to antigen activation of the high-affinity receptor for immunoglobulin E. While protein kinase C-delta associates with and phosphorylates this receptor, immunoprecipitation of the receptor revealed that little, if any, tyrosine-phosphorylated protein kinase C-delta is receptor associated. In vitro kinase assays with immunoprecipitated tyrosine-phosphorylated protein kinase C-delta showed that the modified enzyme had diminished activity toward the receptor gamma-chain peptide as a substrate but not toward histones or myelin basic protein peptide. We propose a model in which the tyrosine phosphorylation of protein kinase C-delta regulates the kinase specificity toward a given substrate. This may represent a general mechanism by which in vivo protein kinase activities are regulated in response to external stimuli.

Kinetics of tyrosine phosphorylation when IgE dimers bind to FC epsilon receptors on rat basophilic leukemia cells

Previously, we demonstrated that aggregates of the high affinity receptor for IgE (Fc epsilon RI), formed by the binding of chemically cross-linked oligomers of IgE, continue to signal early and late cellular responses long after the formation of new aggregates is blocked. In the present work, we explore quantitatively the relationship between aggregation of the receptors and one of the earliest biochemical changes this initiates. We compare the time course of aggregate formation, inferred from studies of the binding of dimers of IgE, and the time course of phosphorylation of tyrosines on receptor subunits when the receptors are aggregated. A simple model does not fit the data. It appears that aggregates formed late in the response are less effective signaling units than those formed initially. We propose new explanations for the persistence of the response and the unusual kinetics.

Multisubunit receptors in the immune system and their association with the cytoskeleton: in search of functional significance

Various multisubunit receptors of the immune system share similarities in structure and induce closely related signal transduction pathways upon ligand binding. Examples include the T cell antigen receptor (TCR), the B cell antigen receptor (BCR), and the high-affinity receptor for immunoglobulin E (Fc epsilon RI). Although these receptors are devoid of intrinsic kinase activity, they can associate with a similar array of intracellular kinases, phosphatases and other signaling molecules. Furthermore, these receptor complexes all form an association with the cytoskeletal matrix. In this review, we compare the structural and functional characteristics of the TCR, BCR and Fc epsilon RI. We examine the role of the cytoskeleton in regulating receptor-mediated signal transduction, as analyzed in other well-characterized receptors, including the epidermal growth factor receptor and integrin receptors. On the basis of this evidence, we review the current data depicting a cytoskeletal association for multisubunit immune system receptors and explore the potential bearing of this interaction on signaling function.

Cell-surface-expressed T-cell antigen-receptor zeta chain is associated with the cytoskeleton

The T-cell antigen receptor zeta chain plays an important role in coupling antigen recognition to several intracellular signal-transduction pathways. zeta chain can associate with certain protein tyrosine kinases and retains the capacity to transduce signals independently of the other receptor subunits. Thus, zeta chain could couple cell-surface-expressed T-cell antigen receptors to the intracellular signal-transduction apparatus by its association with various intracellular molecules in addition to tyrosine kinases. In the process of searching for zeta chain-associated molecules we observed that after lysis of resting T cells with Triton X-100, zeta chain is localized in the detergent-insoluble fraction, in addition to its presence in the detergent-soluble fraction. Treatment of T cells with cytochalasin B, an actin-depolymerizing agent, leads to the complete dissociation of zeta chain from the Triton-insoluble fraction, suggesting a linkage between zeta chain and the cytoskeletal matrix. We have also determined that cytoskeletal-associated zeta chain is expressed on the cell surface. Furthermore, a tyrosine-phosphorylated 16-kDa zeta chain was detected only in the Triton-insoluble cytoskeletal fraction of resting T cells. zeta chain also maintains its association with the cytoskeleton when expressed in COS cells, inferring that the cytoskeletal elements involved in this linkage may be ubiquitous. Finally, we have localized a 42-amino acid region in the intracytoplasmic domain of zeta chain, which is crucial for maximal interaction between zeta chain and the cytoskeleton. Anchorage of cell-surface-expressed zeta chain to the cytoskeleton in resting T cells may facilitate recycling of receptor complexes and/or allow the transduction of external stimuli into the cell.

Organization of the transcortin-binding domain on placental plasma membranes

Complex formation between transcortin (corticosteroid-binding globulin) and 20 kDa sialoglycoprotein from human syncytiotrophoblast plasma membranes (presumably a transcortin-recognizing subunit of the transcortin membrane receptor) was studied using FPLC and cross-linking with bifunctional reagents. The action of 1,5-difluoro-2,4-dinitrobenzene (DFDNB) on a solution of the purified 20 kDa sialoglycoprotein and transcortin resulted in formation of covalently linked complexes of 95 kDa and 140 kDa consisting of one transcortin molecule and either two or four molecules of the membrane sialoglycoprotein (the molecular mass of transcortin is 55 kDa). Additionally, cross-linking resulted in the appearance of a 43 kDa species which is the cross-linked dimer of the membrane protein. The dimer was also observed during chromatography on a Superose 12 column in the absence of DFDNB treatment. Treatment of intact syncytiotrophoblast membranes with DFDNB resulted in isolation of the transcortin binding protein dimer as the major portion of total pool of the protein. Formation of the transcortin complexes with two and four molecules of the membrane protein was also observed when the membranes were incubated with 125I-labeled transcortin and treated with DFDNB, but formation of the latter complexes predominated. The results obtained suggest that the recognizing and binding domain for transcortin in placental membranes is organized as dimers consisting of non-covalently linked sialoglycoprotein monomers of a 20 kDa each and that transcortin has two sites for interaction with this dimer. Apparently, binding of two dimers results in the formation of the functional form of the transcortin-receptor complex. The possible biological role of such a complex is discussed.

Immunoglobulin E-binding structures on antigen-presenting cells present in skin and blood

In atopic individuals, cutaneous antigen-presenting cells (APC), i.e., Langerhans cells and dermal dendritic cells, frequently display anti-IgE reactivity. Although earlier observations suggested that this phenomenon results from the binding of (complexed) IgE to the low-affinity IgE receptor (Fc epsilon RII/CD23), we and others demonstrated recently that Langerhans cells, dermal dendritic cells, and peripheral blood monocytes from atopic individuals can bind monomeric IgE via the high-affinity receptor for IgE (Fc epsilon RI). These new observations re-stimulated investigations aiming to unravel the nature and functionality of the relevant in vivo IgE-binding moiety(-ies) on APC. New data demonstrate that Fc epsilon RI, both quantitatively and qualitatively, is the pivotal serum IgE-binding structure on APC of atopics and, even more important, that Fc epsilon RI on APC functions as an allergen-focusing molecule. Thus, it is likely that allergens may be more efficiently taken up, processed, and presented to T cells after targeting to APC via Fc epsilon RI as compared with allergen binding to APC in the conventional manner. In vivo, Fc epsilon RI-IgE-dependent allergen presentation may critically lower atopic individuals' threshold to mount allergen-specific T-cell responses. This would result in the perpetuation of allergen-specific IgE production (type I reactions) and perhaps even the occurrence of T-cell-mediated, delayed-type hypersensitivity reactions in allergen-exposed tissues.

Transphosphorylation as the mechanism by which the high-affinity receptor for IgE is phosphorylated upon aggregation

When aggregated, the high-affinity receptors for IgE on mast cells (Fc epsilon RI) launch a series of phosphorylations, particularly of protein tyrosines. We have analyzed how aggregation initiates this cascade. We examined Fc epsilon RI from unstimulated cells and from cells exposed to a polyvalent hapten conjugate that aggregates the Fc epsilon RI via the receptor-bound anti-hapten IgE. We also examined the latter receptors after they had been disaggregated in vitro with monovalent hapten. By an in vitro kinase assay: (i) Unaggregated and disaggregated receptors are associated with a kinase that phosphorylates an exogenous (peptide) substrate but minimally, or not at all, the subunits of Fc epsilon RI or associated proteins (endogenous substrates). After aggregation, phosphorylation of the exogenous substrate is linear with time, but the modification of the endogenous substrates reaches a plateau, presumably because only those endogenous substrates that are adjacent to the kinase are phosphorylated. (ii) Aggregated receptors and disaggregated receptors have enhanced kinase activity toward exogenous substrate. The state of phosphorylation of the receptor correlates strongly with the yield of enhanced kinase activity. We propose that upon aggregation of Fc epsilon RI, a constitutively associated kinase phosphorylates endogenous substrates by transphosphorylation. As a result, additional kinase activity becomes manifest and this promotes further transphosphorylation. In view of the homology between Fc epsilon RI and other receptors central to the immune response, the latter receptors likely utilize a similar transphosphorylation mechanism.

Dynamics of signal transduction after aggregation of cell-surface receptors: studies on the type I receptor for IgE

Many ligands stimulate cellular responses by aggregating the cell-surface receptors to which they are bound. We investigated several mechanistic questions related to aggregation of receptors by using the high-affinity receptor for IgE (Fc epsilon RI) on mast cells as a model system. We briefly exposed cells to covalently cross-linked oligomers of IgE and then added excess monomeric IgE to prevent further aggregation. Early events were examined by monitoring the phosphorylation of protein tyrosines; later events were examined by monitoring secretion. We found that aggregated receptors continue to signal both late and early events in the absence of formation of new aggregates. Additional experiments suggested that the clustered receptors undergo a dynamic process of phosphorylation and dephosphorylation. Our findings suggest that for these and related receptors that function by aggregation, the persistence of signal transduction is directly related to the intrinsic affinity of the ligand for the individual receptor.

Divalency of the monoclonal antibody 5-1-6 is required for induction of proteinuria in rats

A single i.v. injection of 3 mg of the F(ab')2 fragment of MoAb 5-1-6 into rats induced immediate proteinuria (128.1 +/- 80.7 mg/24 h on day 1) which lasted 1-2 days. In contrast, rats administered 10 mg of the corresponding Fab fragment did not develop abnormal proteinuria even though an equivalent dose of the intact MoAb 5-1-6 far exceeded the nephritogenic dose. The total kidney binding of 125I-Fab fragment was 209.5 +/- 34.3 micrograms/2 kidneys. This exceeded that obtained by injection of 3 mg MoAb 5-1-6 IgG1 (58.9 +/- 12.5 micrograms/2 kidneys at 1 h) and was similar to that obtained following injection of 3 mg F(ab')2 fragment (235.3 +/- 16.9 micrograms/2 kidneys). Immunofluorescence (IF) showed a linear pattern along the glomerular capillary wall at 1 h after the administration of MoAb 5-1-6 IgG1, F(ab')2 or Fab fragment. On day 5, fine to coarse granules were observed scattered in F(ab')2-injected rat glomeruli, whereas granules were densely localized in Fab-injected rat glomeruli. Complement-depleted rats injected with 3 mg of MoAb 5-1-6 IgG1 developed proteinuria with the same time course as non-depleted rats. This observation, together with the ability of F(ab')2 to induce proteinuria, indicates that proteinuria induced by MoAb 5-1-6 is complement-independent. This study suggests that MoAb 5-1-6-induced proteinuria is initiated by cross-linking of the epitopes by divalent MoAb 5-1-6 and is independent of complement activity.

Thy-1 glycoprotein and src-like protein-tyrosine kinase p53/p56lyn are associated in large detergent-resistant complexes in rat basophilic leukemia cells

Thy-1 is a surface glycoprotein that is attached to the plasma membrane by a glycosyl-phosphatidyl-inositol anchor. Crosslinking of Thy-1 in rat mast cells and basophilic leukemia cells (RBL-2H3) induces cell activation including histamine release and tyrosine phosphorylation of several proteins. Here we show that glycosyl-phosphatidylinositol-linked Thy-1 forms noncovalent complexes with src-related protein-tyrosine kinase p53/p56lyn and other protein-tyrosine kinases and/or their substrates. These complexes are resistant to solubilization by a nonionic detergent, sedimentable at 200,000 x g, and very large ( > 10 MDa) as determined by gel chromatography. Activation of RBL-2H3 cells by crosslinking of the high-affinity IgE receptors resulted in decreased recovery of the complexes. The combined data indicate the existence of large detergent-resistant domains in the surface membrane of mast cells that may play an important role in their activation.

Transmembrane signaling by the high-affinity IgE receptor on membrane preparations

Aggregating the receptor with high affinity for IgE (Fc epsilon RI) stimulates a variety of phenomena in mast cells. Previous efforts to reproduce some of these events in broken-cell preparations such as isolated membranes have had limited success, possibly because the phenomena being monitored were too distal from the initial events. One of the earliest responses is now known to be the phosphorylation of tyrosine residues on several proteins, including the beta and gamma subunits of Fc epsilon RI. We show that in cell sonicates or on partially purified membranes derived from tumor mast cells, aggregating Fc epsilon RI stimulates phosphorylation of receptor tyrosine residues. As in the intact cells, receptor-mediated phosphorylation occurs only on receptors that are themselves aggregated. Because even in the unfractionated sonicates the phosphorylation of other cellular components was not detectably enhanced, and because the evidence is against the receptor itself being a kinase, our results suggest that phosphorylation of Fc epsilon RI is one of the earliest events stimulated by the receptor--an event that can now be investigated on simpler biological preparations than previously available.