Association between B-lymphocyte membrane immunoglobulin and multiple members of the Src family of protein tyrosine kinases

The B-cell receptor pathway: a critical component of healthy and malignant immune biology

The pathogenesis and progression of normal B-cell development to malignant transformation of chronic lymphocytic leukemia (CLL) is still poorly understood and has hampered attempts to develop targeted therapeutics for this disease. The dependence of CLL cells on B-cell receptor signaling has fostered a new area of basic and therapeutic research interest. In particular, identification of the dependence of CLL cells on both phosphatidylinositol 3-kinase delta and Bruton's tyrosine kinase signaling for survival and proliferation has come forth through well-performed preclinical studies and subsequent trials demonstrating dramatic efficacy. This review outlines essential components of B-cell receptor signaling and briefly addresses therapeutics that are emerging to target these in patients with CLL and related lymphoid malignancies.

Multiplex analysis of Src family kinase signaling by microbead suspension arrays

There is renewed interest in the Src family of protein tyrosine kinases (SFKs) as a result of their potential utility as molecular targets for cancer therapy. This protein family consists of 9 nonreceptor tyrosine kinases that, although implicated in a diverse array of cellular functions, possess a similar modular structure. Here we describe a simple and efficient multiplex microbead immunoassay (MMIA), based on Luminex xMAP technology, which allows for the simultaneous detection of 8 phosphorylated SFKs in a single assay. Microbead sets identifiable by unique fluorescence were individually coated with antibodies specific for an individual SFK member. Detection of phosphorylated SFKs was accomplished using a secondary antibody directed against phosphotyrosine. The assay requires < or = 10 microg of cell lysate or nanogram amounts of purified SFK. The use of a generic secondary antibody allows for the expansion of the assay to include any other tyrosine kinase for which a specific antibody exists. Using either mammalian cell lines or purified, recombinant kinases as the SFK source, we demonstrate the utility of the assay by evaluating the phosphorylation status of SFK members following several in vitro manipulations designed to modulate the phosphotyrosine content of the kinases. These results show that the SFK multiplex assay is a robust tool to investigate the function of SFKs in basic and potentially in clinical research.

B-cell receptor

The subunit structure of the B-cell antigen receptor (BCR) and its associated compartmentalization of function confer enormous flexibility for generating signals and directing these toward specific and divergent cell fate decisions. Like all the multichain immune recognition receptors discussed in this volume, assembly of these multi-unit complexes sets these receptors apart from almost all other cell surface signal transduction proteins and affords them the ability to participate in almost all of the diverse aspects of, in this case, B-cell biology. We discuss here the structural aspects of the BCR and its associated coreceptors and relate these mechanistically to how BCR signaling can be directed towards specific fate decisions. By doing so, the BCR plays a pivotal role in ensuring the effective and appropriate B-cell response to antigen.

Identification of genes deregulated during serum-free medium adaptation of a Burkitt's lymphoma cell line

OBJECTIVE

Serum is usually added to growth media when mammalian cells are cultured in vitro to supply the cells with growth factors, hormones, nutrients and trace elements. Defined proteins and metal ions, such as insulin, growth factors, transferrin and sodium selenite, are sometimes also included and can in some cases substitute serum components. How adaptation to serum free media influences cells has not been studied in detail.

MATERIALS AND METHODS

We have adapted the Burkitt's lymphoma line Ramos to a serum-free medium that supports long-term survival and studied gene expression changes that occurred during the adaptation process.

RESULTS AND CONCLUSIONS

The adaptation process was characterized by initial cell population growth arrest, and after that extensive cell death, followed by proliferation and long-term survival of clonal cultures. Proliferation and cell cycle progression of the serum-free cultures closely mimicked that of serum-dependent cells. Affymetrix micro-array technology was used to identify gene expression alterations that had occurred during the adaptation. Most changes were subtle, but frequently the genes with altered expression were involved in basal cellular functions such as cell division, cell cycle regulation, apoptosis and cell signalling. Some alterations were restored when the cells were transferred back to serum-containing medium, indicating that expression of these genes was controlled by components in serum. Others were not, and may represent changes that were selected during the adaptation process. Among these were, for example, several genes within the Wnt signalling pathway.

Functional and structural requirements for the internalization of distinct BCR-ligand complexes

Antigen (Ag) binding to the BCR rapidly initiates two important events: a phosphorylation cascade that results in the production of secondary signaling intermediaries and the internalization of Ag-BCR complexes. Previous studies using anti-BCR antibodies (Ab) have suggested that BCR signaling is an essential requirement for BCR endocytosis and have further implicated lipid rafts as essential platforms for both BCR functions. However, published data from our laboratory indicate that lipid rafts and consequently raft-mediated signaling are dispensable for BCR-mediated internalization of Ag-specific BCR. Therefore, we investigated the relationship between BCR signaling and endocytosis by defining the role of early kinase signaling in the BCR-mediated internalization of a model Ag (haptenated protein). The results demonstrate that Src kinases and Syk-mediated BCR signaling are not essential for BCR-mediated Ag internalization. Moreover, by comparing Ag and Ab, it was determined that while both localize to clathrin-coated pits, the internalization of Ab-BCR complexes is more susceptible to inhibition of signaling and highly sensitive to disruption of lipid rafts and the actin cytoskeleton compared to Ag-BCR complexes. Thus, these results demonstrate that the nature of the ligand ultimately determines the functional requirements and relative contribution of lipid rafts and other membrane structures to the internalization of BCR-ligand complexes.

ITAM-mediated tonic signalling through pre-BCR and BCR complexes

Studies carried out over the past few years provide strong support for the idea that Ig alpha-Ig beta-containing complexes such as the pre-B-cell receptor and the B-cell receptor can signal independently of ligand engagement, and this has been termed tonic signalling. In this Review, I discuss recent literature that is relevant to the potential mechanisms by which tonic signals are initiated and regulated, and discuss views on how tonic and ligand-dependent (aggregation-mediated) signalling differ. These mechanisms are relevant to the possibility that tonic signals generated through immunoreceptor tyrosine-based activation motif (ITAM)-containing proteins that are expressed by oncogenic viruses induce transformation in non-haematopoietic cells.

Inhibition of Src Kinases Combined with CD40 Ligand Blockade Prolongs Murine Cardiac Allograft Survival

BACKGROUND

Members of the Src family of tyrosine kinases (SFKs) are requisite signaling molecules activated by multiple receptors during immune responses. Their expression and catalytic activity has not been characterized in allograft rejection in vivo.

METHODS

We measured expression and catalytic activity of SFKs in MHC- mismatched murine cardiac allografts. We also examined the effects of a Src inhibitor (CGP77675) with or without anti-CD154 mAb on graft survival, histology, and expression and catalytic activity of SFKs within the grafts.

RESULTS

In acutely rejecting allografts from untreated controls, total activity of Hck and Lyn increased 10-fold, predominantly reflecting increases in the amount of protein. Total activity of Lck increased only fourfold, reflecting small changes in both the amount of protein and specific activity. One dose of anti-CD154 plus CGP77675 markedly diminished cellular infiltration, but survival was only moderately prolonged despite inhibition of all SFKs in the rejected grafts. Two doses of anti-CD154 plus CGP77675 allowed permanent graft acceptance in 60% of recipients even after discontinuation of the inhibitor. Both rejected and long surviving grafts showed increased activity of all SFKs. Recipients that rejected their grafts showed serum alloantibody production, and grafts rejected during treatment demonstrated deposition of complement indicating the contribution of antibody to rejection.

CONCLUSIONS

The myeloid and B cell Src family kinases, Hck and Lyn, rather than the T cell Src kinase Lck, show the greatest increase in expression and total activity in rejecting allografts. Both rejected and long-surviving grafts show significant increases in SFK expression and acitivity.

The strength of receptor signaling is centrally controlled through a cooperative loop between Ca2+ and an oxidant signal

Activation of cell-surface receptors stimulates generation of intracellular signals that, in turn, direct the cellular response. However, mechanisms that ensure combinatorial control of these signaling events are not well understood. We show here that the Ca2+ and reactive oxygen intermediates generated upon BCR activation rapidly engage in a cooperative interaction that acts in a feedback manner to amplify the early signal generated. This cooperativity acts by regulating the concentration of the oxidant produced. The latter exerts its influence through a pulsed inactivation of receptor-coupled phosphatases, where the amplitude of this pulse is determined by oxidant concentration. The extent of phosphatase inhibition, in turn, dictates what proportion of receptor-proximal kinases are activated and, as a result, the net strength of the initial signal. It is the strength of this initial signal that finally determines the eventual duration of BCR signaling and the rate of its transmission through downstream pathways.

Membrane-targeted peptides derived from Igalpha attenuate B-cell antigen receptor function

Within the B-cell antigen receptor (BCR), heterodimers of Igalpha/Igbeta couple the receptor to intracellular signaling pathways. In the resting state, Igalpha associates with Src-family tyrosine kinases (SFTKs) which contain some basal activity. Upon engagement of the receptor, the SFTKs phosphorylate tyrosine residues in the BCR that recruit and activate the tyrosine kinase Syk, initiating signaling pathways. To test the hypothesis that disrupting the association between the resting receptor and the SFTKs would attenuate both basal and induced receptor activities, we expressed non-phosphorylatable membrane-targeted analogs of Igalpha (Igalpha/M) or Igbeta (Igbeta/M) in B lymphocytes. Both Igalpha/M and Igbeta/M inhibited BCR-induced calcium mobilization, but only Igalpha/M was able to diminish tyrosine phosphorylation. In an immature B-cell line, Igalpha/M attenuated both receptor-induced and basal apoptosis. Taken together, these data demonstrate the importance of the resting receptor complex and suggest therapeutic strategies for regulating receptor-mediated functions.

Abrogation of autoimmune disease in Lyn-deficient mice by the deletion of IL-5 receptor alpha chain gene

Lyn, the src-family protein tyrosine kinase, plays a crucial role in the regulation of B cell antigen receptor (BCR)- and IL-5-receptor (IL-5R)-mediated signaling. Lyn-deficient mice have been reported to exhibit an increase in B-1 cell numbers, splenomegaly and accumulation of lymphoblast-like cells in the spleen with age, resulting in hyperimmunoglobulinemia and glomerulonephritis caused by the deposition of autoantibody complexes. To elucidate the role of IL-5 in B-1 cell activation, autoantibody production and autoimmune diseases, Lyn-deficient mice were crossed with IL-5Ralpha chain (IL-5Ralpha)-deficient mice and generated Lyn- and IL-5Ralpha-deficient (DKO) mice. In contrast to Lyn-deficient mice, DKO mice showed significantly reduced splenomegaly and lymphoadenopathy and reduced B-1 cell number in the peritoneal cavity. DKO mice also secreted low levels of IgM and IgG autoantibodies. Biochemical and histological analyses revealed that DKO mice showed milder pathogenesis of autoimmune-like disorders than Lyn-deficient mice. These results suggest involvement of IL-5 in enhanced B-1 cell activation, autoantibody production, and development of autoimmune disease in Lyn-deficient mice.

Microarray analysis of Lyn-deficient B cells reveals germinal center-associated nuclear protein and other genes associated with the lymphoid germinal center

Lyn is the only member of the Src family expressed in DT40 B cells, which provide a unique model to study the singular contribution of this protein tyrosine kinase (PTK) family to cell signaling. In these cells, gene ablation of Lyn leads to defective B cell receptor signaling. Complementary DNA array analysis of Lyn-deficient DT40 cells shows that the absence of Lyn leads to down-regulation of numerous genes encoding proteins involved in B cell receptor signaling, proliferation, control of transcription, immunity/inflammation response, and cytoskeletal organization. Most of these expression changes have not been previously associated with Lyn PTK signaling. They include alterations in mRNA levels of germinal center-associated nuclear protein (germinal center-associated DNA primase) (GANP), CD74, CD22, NF-kappaB, elongation factor 1alpha, CD79b, octamer binding factor 1, Ig H chain, stathmin, and gamma-actin. Changes in GANP expression were also confirmed in Lyn-deficient mice, suggesting that Lyn PTK has a unique function not compensated for by other Src kinases. Because Lyn-deficient mice have impaired development of germinal centers in spleen, the decreased expression of GANP in the Lyn-deficient DT40 cell line and Lyn-deficient mice suggests that Lyn controls the formation and proliferation of germinal centers via GANP. GANP promoter activity was higher in wild-type vs Lyn-deficient cells. Mutation of the PU.1 binding site reduced activity in wild-type cells and had no effect in Lyn-deficient cells. The presence of Lyn enhanced PU.1 expression in a Northern blot. Thus, the following new signaling pathway has been described: Lyn-->PU.1-->GANP.

The tyrosine kinase Lyn is required for B cell development beyond the T1 stage in the spleen: rescue by over-expression of Bcl-2

We have analyzed the effects of deficiency in the tyrosine kinase Lyn on B cell development using transgenic mice that express a B cell antigen receptor (BCR) of defined specificity (3-83,anti-H-2K(k or b)). In the absence of Lyn, immature B cells are abundant in the bone marrow and spleen up until the T1 stage (IgM(hi) IgD(-) CD21(-)CD23(-)), after which B cell development is severely impaired. The small number of mature B cells that do develop in Lyn-deficient mice express normal levels of the transgenic BCR and lack expression of CD80 and CD86, suggesting they are not activated. In Lyn-deficient animals the presence of a Bcl-2 transgene leads to a dramatic increase in B cell numbers and restores T2 stage (IgM(hi) IgD(hi) CD21(hi) CD23(int)) and mature populations. In 3-83 lyn-/- Bcl-2 Tg mice, a population of lambda-positive cells that also express the 383 idiotype is evident, suggesting that in the absence of lyn isotype exclusion by the transgenic BCR is less efficient. The results indicate that Lyn plays a positive role in the selection and survival of mature B cells in addition to its previously documented negative role in tolerance and B cell activation.

Tyrosine residues in phospholipase Cgamma 2 essential for the enzyme function in B-cell signaling

Phospholipase Cgamma (PLCgamma) isoforms are regulated through activation of tyrosine kinase-linked receptors. The importance of growth factor-stimulated phosphorylation of specific tyrosine residues has been documented for PLCgamma1; however, despite the critical importance of PLCgamma2 in B-cell signal transduction, neither the tyrosine kinase(s) that directly phosphorylate PLCgamma2 nor the sites in PLCgamma2 that become phosphorylated after stimulation are known. By measuring the ability of human PLCgamma2 to restore calcium responses to the B-cell receptor stimulation or oxidative stress in a B-cell line (DT40) deficient in PLCgamma2, we have demonstrated that two tyrosine residues, Tyr(753) and Tyr(759), were important for the PLCgamma2 signaling function. Furthermore, the double mutation Y753F/Y759F in PLCgamma2 resulted in a loss of tyrosine phosphorylation in stimulated DT40 cells. Of the two kinases that previously have been proposed to phosphorylate PLCgamma2, Btk, and Syk, purified Btk had much greater ability to phosphorylate recombinant PLCgamma2 in vitro, whereas Syk efficiently phosphorylated adapter protein BLNK. Using purified proteins to analyze the formation of complexes, we suggest that function of Syk is to phosphorylate BLNK, providing binding sites for PLCgamma2. Further analysis of PLCgamma2 tyrosine residues phosphorylated by Btk and several kinases from the Src family has suggested multiple sites of phosphorylation and, in the context of a peptide incorporating residues Tyr(753) and Tyr(759), shown preferential phosphorylation of Tyr(753).

Ligand-independent signaling functions for the B lymphocyte antigen receptor and their role in positive selection during B lymphopoiesis

Signal transduction through the B cell antigen receptor (BCR) is determined by a balance of positive and negative regulators. This balance is shifted by aggregation that results from binding to extracellular ligand. Aggregation of the BCR is necessary for eliciting negative selection or activation by BCR-expressing B cells. However, ligand-independent signaling through intermediate and mature forms of the BCR has been postulated to regulate B cell development and peripheral homeostasis. To address the importance of ligand-independent BCR signaling functions and their regulation during B cell development, we have designed a model that allows us to isolate the basal signaling functions of immunoglobulin (Ig)alpha/Igbeta-containing BCR complexes from those that are dependent upon ligand-mediated aggregation. In vivo, we find that basal signaling is sufficient to facilitate pro-B --> pre-B cell transition and to generate immature/mature peripheral B cells. The ability to generate basal signals and to drive developmental progression were both dependent on plasma membrane association of Igalpha/Igbeta complexes and intact immunoregulatory tyrosine activation motifs (ITAM), thereby establishing a correlation between these processes. We believe that these studies are the first to directly demonstrate biologically relevant basal signaling through the BCR where the ability to interact with both conventional as well as nonconventional extracellular ligands is eliminated.

A CD19-dependent signaling pathway regulates autoimmunity in Lyn-deficient mice

CD19 and the Src family protein tyrosine kinases (PTKs) are important regulators of intrinsic signaling thresholds in B cells. Regulation is achieved by cross-talk between Src family PTKs and CD19; Lyn is essential for CD19 phosphorylation, while CD19 establishes an Src family PTK activation loop that amplifies kinase activity. However, CD19-deficient (CD19(-/-)) B cells are hyporesponsive to transmembrane signals, while Lyn-deficient (Lyn(-/-)) B cells exhibit a hyper-responsive phenotype resulting in autoimmunity. To identify the outcome of interactions between CD19 and Src family PTKs in vivo, B cell function was examined in mice deficient for CD19 and Lyn (CD19/Lyn(-/-)). Remarkably, CD19 deficiency suppressed the hyper-responsive phenotype of Lyn(-/-) B cells and autoimmunity characterized by serum autoantibodies and immune complex-mediated glomerulonephritis in Lyn(-/-) mice. Consistent with Lyn and CD19 each regulating conventional B cell development, B1 cell development was markedly reduced by Lyn deficiency, with further reductions in the absence of CD19 expression. Tyrosine phosphorylation of Fyn and other cellular proteins induced following B cell Ag receptor ligation was dramatically reduced in CD19/Lyn(-/-) B cells relative to Lyn(-/-) B cells, while Syk phosphorylation was normal. In addition, the enhanced intracellular Ca(2+) responses following B cell Ag receptor ligation that typify Lyn deficiency were delayed by the loss of CD19 expression. BCR-induced proliferation and humoral immune responses were also markedly inhibited by CD19/Lyn deficiency. These findings demonstrate that while the CD19/Lyn amplification loop is a major regulator of signal transduction thresholds in B lymphocytes, CD19 regulation of other Src family PTKs also influences B cell function and the development of autoimmunity.

B cell receptor signaling involves physical and functional association of FAK with Lyn and IgM

B cell receptor (BCR) stimulation induces phosphorylation of a number of proteins, leading to functional activation of B lymphocytes. Focal adhesion kinase (FAK) is a non-receptor protein tyrosine kinase, involved in a variety of signaling pathways. In this study, we show that FAK is tyrosine-phosphorylated and activated following BCR stimulation. We also demonstrate constitutive association of FAK with the Src-family kinase Lyn and with components of the BCR. Association of Lyn with FAK which was not correlated with BCR-induced activation of both kinases, appeared to be mediated via the binding of Lyn to the COOH-terminal part of the FAK molecule. Our results indicate that FAK is a component of the BCR complex and that it participates in BCR signaling.

Novel roles for Lyn in B cell migration and lipopolysaccharide responsiveness revealed using anti-double-stranded DNA Ig transgenic mice

Lyn-deficient mice produce Abs against dsDNA, yet exhibit exaggerated tolerance to the model Ag hen-egg lysozyme. To investigate this apparent contradiction, and to further examine the function of Lyn in Ag-engaged cells, we have used an anti-dsDNA Ig transgenic model. Previously, looking at these anti-dsDNA B cells in Lyn-sufficient BALB/c mice, we showed that they are regulated by functional inactivation (anergy). In the absence of Lyn, these anti-dsDNA B cells remain unable to secrete Ab. This suggests that functional inactivation of anti-dsDNA B cells does not depend on Lyn, and that the anti-dsDNA Abs that are produced in lyn(-/-) mice arise from a defect in another mechanism of B cell tolerance. Although the anti-dsDNA B cells remain anergic, Lyn deficiency does restore their ability to proliferate to LPS. This reveals a novel role for Lyn in mediating the LPS unresponsiveness that normally follows surface Ig engagement. Furthermore, Lyn deficiency leads to an altered splenic localization and EBV-induced molecule 1 ligand chemokine responsiveness of anti-dsDNA B cells, as well as an absence of marginal zone B cells, suggesting additional roles for Lyn in controlling the migration and development of specific B cell populations.

The effects of the Epstein-Barr virus latent membrane protein 2A on B cell function

Epstein-Barr Virus (EBV) infects B-lymphocytes circulating through the oral epithelium and establishes a lifelong latent infection in a subset of mature-memory B cells. In these latently infected B cells, EBV exhibits limited gene expression with the latent membrane protein 2A (LMP2A) being the most consistently detected transcript. This persistent expression, coupled with many studies ofthe function of LMP2A in vitro and invivo, indicates that LMP2A is functioning to control some aspect of viral latency. Establishment and maintenance of viral latency requires exquisite manipulation of normal B cell signaling and function. LMP2A is capable of blocking normal B cell signal transduction in vitro, suggesting that LMP2A may act to regulate lytic activation from latency in vivo. Furthermore, LMP2A is capable of providing B cells with survival signals in the absence of normal BCR signaling. These data show that LMP2A may help EBV-infected cells to persist in vivo. This review discusses the advances that have been made in our understanding of LMP2A and the effects it has on B cell development, activation, and viral latency.

Interaction of folate receptor with signaling molecules lyn and G(alpha)(i-3) in detergent-resistant complexes from the ovary carcinoma cell line IGROV1

Using as a model the ovary carcinoma cell line IGROV1, we analyzed the partitioning of the glycosyl-phosphatidylinositol-anchored folate receptor into lipid rafts based on its relative detergent insolubility, with a focus on physically and functionally associated signaling molecules. A variable amount (40-60%) of folate receptor was found in low-density Triton X-100 insoluble complexes together with subunits of heterotrimeric G-proteins and the src-family non-receptor tyrosine kinases p53-56 lyn. In the same fraction the structural component of caveolae, caveolin, was not detected at the protein level, although the corresponding mRNA was detected in trace amounts. Comodulation of folate receptor and signalling molecules was observed in the detergent-insoluble complexes during cell proliferation or induced by phosphatidylinositol-specific phospholipase C treatment or by interaction with anti-folate receptor monoclonal antibodies. Moreover, complexes of folate receptor, lyn and the G(α)(i-3) subunit were immunoprecipitated using either anti-folate receptor or anti-lyn antibodies. In vitro kinase assay of the immunoprecipitates revealed stimulation of phosphorylation of common and specific proteins. In particular, the p53 form of lyn appeared to be enriched and phosphorylated in the anti-folate receptor MOv19 monoclonal antibody immunoprecipitate, whereas a 40 kDa band common to anti-folate receptor and anti-lyn immunoprecipitates was the phosphorylated form of the G(α)(i-3) subunit. These findings point to the functional interaction between folate receptor and associated signaling molecules.

Characteristic expression of Hck in human B-cell precursors

OBJECTIVE

To identify molecules involved in signaling for early B-cell development, we investigated the expression of signal transduction-related proteins in B-cell progenitors.

MATERIALS AND METHODS

[corrected] Normal as well as leukemic B-cell progenitors were examined by immunoblotting and immunofluorescence study.

RESULTS

[corrected] In a survey of the expression of a broad range of signal transduction molecules, the Src-family protein tyrosine kinases were found to be differentially expressed in early B-cell differentiation. [corrected] Analysis of freshly prepared precursor-B acute lymphoblastic leukemia cells and B-lineage cell lines showed Hck and Lyn are major Src-family protein tyrosine kinases expressed in this type of leukemic blasts. [corrected] However, heterogeneity of Hck and Lyn expression was found in these cells, and precursor-B acute lymphoblastic leukemia cells subsequently were classified according to the expression pattern of Hck and Lyn as Hck/Lyn dual-negative, Hck-predominant, Hck/Lyn dual-positive, and Lyn-predominant. Further studies on normal B-lineage cells indicated that the Src-family protein tyrosine kinases are expressed sequentially in a differentiation-dependent fashion during B-cell ontogeny and that the predominant expression of Hck is a common feature in B-cell progenitors, whereas Lyn expression is more significant in mature B cells.

CONCLUSIONS

Although the biologic significance remains unknown, sequential expression of Src-family protein tyrosine kinases should play a role in regulation of early B-cell differentiation.

Modifications of Igalpha and Igbeta expression as a function of B lineage differentiation

Transcription of the mb1 and B29 genes is initiated when lymphoid progenitors enter the B cell differentiation pathway, and their transmembrane Igalpha and Igbeta products constitute essential signaling components of pre-B and B cell antigen receptors. We analyzed Igalpha/Igbeta biosynthesis, heterogeneity, and molecular interactions as a function of human B lineage differentiation in cell lines representative of the pro-B, pre-B, and B cell stages. All B lineage representatives produced a 36-kDa Igbeta form and three principal Igalpha forms, transient 33/40-kDa species and a mature 44-kDa glycoprotein. Deglycosylation revealed a major Igalpha core protein of 25 kDa and a minor 21-kDa Igalpha protein, apparently the product of an alternatively spliced mRNA. In pro-B cells, the Igalpha and Igbeta molecules existed primarily in separate unassembled pools, exhibited an immature glycosylation pattern, did not associate with surrogate light chain proteins, and were retained intracellularly. Their unanticipated association with the Lyn protein-tyrosine kinase nevertheless suggests functional potential for the Igalpha/Igbeta molecules in pro-B cells. Greater heterogeneity of the Igalpha and Igbeta molecules in pre-B and B cell lines was attributable to increased glycosylation complexity. Finally, the Igalpha/Igbeta heterodimers associated with fully assembled IgM molecules as a terminal event in B cell receptor assembly.

Phosphorylation- and activation-independent association of the tyrosine kinase Syk and the tyrosine kinase substrates Cbl and Vav with tubulin in B-cells

Aggregation of the B-cell antigen receptor leads to the activation of the 72-kDa Syk protein-tyrosine kinase and the phosphorylation of tubulin on tyrosine. To explore the requirement of Syk catalytic activity for tubulin phosphorylation, tubulin was isolated from cytosolic fractions from anti-IgM-activated B-cells (DT40) that lacked endogenous Syk and immunoblotted with anti-phosphotyrosine antibodies. Tubulin was not tyrosine-phosphorylated in Syk- B-cells. Phosphorylation could be restored by the expression of wild-type, but not catalytically inactive, Syk. However, both catalytically inactive and wild-type Syk were capable of constitutive association with tubulin, indicating that tubulin phosphorylation is not required for this interaction. Anti-phosphotyrosine antibody immunoblotting of proteins adsorbed to colchicine-agarose revealed the presence of three major tubulin-associated phosphoproteins of 110, 90, and 74 kDa, the phosphorylation of which was dependent on Syk expression. The proteins of 110 and 90 kDa were identified as Cbl and Vav, two proto-oncogene products known to become prominently phosphorylated following receptor engagement. Both proteins were shown to be constitutively associated with tubulin.

Role of transmembrane domains in the functions of B- and T-cell receptors

The antigen receptors on the surface of B- and T-lymphocytes are complexes of several integral membrane proteins, essential for their proper expression and function. Recent studies demonstrated that transmembrane (TM) domains of the components of these receptors play a critical role in their association and function. It was specifically demonstrated that in many cases point mutations in the TM domains can partially or completely disrupt the receptor surface expression and function. Here we review studies of the TM domains of B- and T-cell receptors. Furthermore, we use a novel method, PHDtopology, to provide estimates of the exact locations and lengths of the TM domains of the subunit components of these receptors. Most previous studies used single residue hydrophobicity as a criterion for determining the position and length of the TM domains. In contrast, PHDtopology utilizes a system of neural networks and the evolutionary information contained in multiple alignments of related sequences to predict the location, length, and orientation of transmembrane helices. Present results significantly differ from most published estimates of the TM domains of the B- and T-cell receptor components, primarily in the length of the TM domains. These results may lead to modification of putative TM motifs and re-interpretation of the results of studies using mutated TM domains. The availability of PHDtopology on the Internet would make it a valuable tool in the future studies of the TM domains of integral membrane proteins.

RA70 is a src kinase-associated protein expressed ubiquitously

RA70, which is expressed during neuronal differentiation of P19 EC, is highly homologous to human src kinase-associated phosphoprotein (SKAP55). Here we isolated human full-length RA70 cDNA. Unlike SKAP55, which is specifically expressed in thymus and T cells, RA70 was expressed ubiquitously in various tissues including lung, skeletal muscle, and spleen, and in various cell lines including human monocytic leukemia (U937) cells, but RA70 was undetectable in thymus and T cell lymphoma (Jurkat) cells. RA70 as well as SKAP55 proved to be a protein with molecular weight 55 kDa associated with SH2 domain of Fyn. Interaction between RA70 and src family kinases, Fyn, Hck and Lyn, was detected during monocytes/macrophage-differentiation of U937 cells. Thus, like SKAP55, RA70 is an adaptor protein of the src family kinases. RA70 may play an essential role in the src signaling pathway in various cells.

Early function of Pax5 (BSAP) before the pre-B cell receptor stage of B lymphopoiesis

The formation of the pre-B cell receptor (BCR) corresponds to an important checkpoint in B cell development that selects pro-B (pre-BI) cells expressing a functionally rearranged immunoglobulin mu (Igmu) heavy chain protein to undergo the transition to the pre-B (pre-BII) cell stage. The pre-BCR contains, in addition to Igmu, the surrogate light chains lambda5 and VpreB and the signal transducing proteins Igalpha and Igbeta. The absence of one of these pre-BCR components is known to arrest B cell development at the pre-BI cell stage. Disruption of the Pax5 gene, which codes for the B cell-specific activator protein (BSAP), also blocks adult B lymphopoiesis at the pre-BI cell stage. Moreover, expression of the mb-1 (Igalpha) gene and VH-to-DHJH recombination at the IgH locus are reduced in Pax5-deficient B lymphocytes approximately 10- and approximately 50-fold, respectively. Here we demonstrate that complementation of these deficiencies in pre-BCR components by expression of functionally rearranged Ig mu and chimeric Igmu-Igbeta transgenes fails to advance B cell development to the pre-BII cell stage in Pax5 (-/-) mice in contrast to RAG2 (-/-) mice. Furthermore, the pre-BCR is stably expressed on cultured pre-BI cells from Igmu transgenic, Pax5-deficient bone marrow, but is unable to elicit its normal signaling responses. In addition, the early developmental block is unlikely to be caused by the absence of a survival signal, as it could not be rescued by expression of a bcl2 transgene in Pax5-deficient pre-BI cells. Together, these data demonstrate that the absence of Pax5 arrests adult B lymphopoiesis at an early developmental stage that is unresponsive to pre-BCR signaling.

Protein tyrosine kinases p53/56lyn and p72syk in MHC class I-mediated signal transduction in B lymphoma cells

Crosslinking of major histocompatibility complex class I (MHC-I) molecules on the surface of human B lymphoma cells was shown to induce protein tyrosine phosphorylation and mobilization of intracellular free calcium. Immunoprecipitations indicated that the protein tyrosine kinases p53/56lyn and p72syk are among the tyrosine-phosphorylated proteins. The kinetics of phosphorylation of these kinases after MHC-I crosslinking differ from the kinetics observed after crosslinking of the B cell antigen receptor (BCR). Additional experiments were performed with chicken lyn- and syk-negative DT40 B cells and the results indicate that these two kinases have different substrate specificity and regulate intracellular free calcium differently in response to MHC-I crosslinking. In addition MHC-I crosslinking of a sIgM-negative DT40 chicken B cell variant results in less activity of tyrosine kinases and less mobilization of intracellular free calcium compared with MHC-I crosslinking of wild-type DT40 cells. Thus, expression of BCR at the cell surface is likely to be important for the signal cascade initiated by MHC-I crosslinking. Our data suggest that signal transduction initiated through ligation of the MHC-I molecule plays a role in the regulation of B cell homeostasis.

Polygenic autoimmune traits: Lyn, CD22, and SHP-1 are limiting elements of a biochemical pathway regulating BCR signaling and selection

A B lymphocyte hyperactivity syndrome resembling systemic lupus erythematosus characterizes mice lacking the src-family kinase Lyn. Lyn is not required to initiate B cell antigen receptor (BCR) signaling but is an essential inhibitory component. lyn-/- B cells have a delayed but increased calcium flux and exaggerated negative selection responses in the presence of antigen and spontaneous hyperactivity in the absence of antigen. As in invertebrates, genetic effects of loci with only one functional allele can be used to analyze signaling networks in mice, demonstrating that negative regulation of the BCR is a complex quantitative trait in which Lyn, the coreceptor CD22, and the tyrosine phosphatase SHP-1 are each limiting elements. The biochemical basis of this complex trait involves a pathway requiring Lyn to phosphorylate CD22 and recruit SHP-1 to the CD22/BCR complex.

Glycosylated extracellular domains of membrane immunoglobulin M contribute to its association with mb-1/B29 gene products and the B cell receptor complex

It has recently become clear that the B cell antigen receptor, membrane immunoglobulin (mIg) is part of a complex composed of a number of different polypeptides. In a manner analogous to the T cell receptor, mIg has been found to be associated with several tyrosine kinases, and other proteins, which although not kinases themselves become targets of kinase activity upon binding of mIg to antigen. Thus the B cell receptor complex appears to be a structure whose function during signal transduction is to facilitate the interaction of tyrosine kinases with their proper substrates, and to coordinate the phosphorylation of these proteins with the binding of antigen to mIg. In an effort to understand the nature of the interactions which mediate the organization of the B cell receptor complex, we have explored binding of components of the complex including Ig-alpha and Ig-beta to IgM. Previous results have indicated that binding was mediated by transmembrane domains. Our results indicate that extracellular domains of IgM may also contribute to its association with Ig alpha and beta and other members of the B cell receptor complex.

Expression and chromosomal localization of the human alpha 4/IGBP1 gene, the structure of which is closely related to the yeast TAP42 protein of the rapamycin-sensitive signal transduction pathway

To study the function of the B cell signal transduction molecule alpha 4 (IGBP1), we isolated a human alpha 4 (IGBP1) gene that has sequence similarity to the yeast protein (TAP42) involved in the rapamycin-sensitive signal transduction pathway. The human alpha 4 has sequence identities with murine alpha 4 of 83.4% nucleotide and 82.9% amino acid sequence, and a stretch of consensus motifs in the carboxyl terminal is conserved among the related genes of human, mouse, yeast, and rice. The gene is expressed as a 1.4-kb mRNA in the spleen, lymph node, thymus, appendix, peripheral blood leukocytes, bone marrow, fetal liver, heart, brain, placenta, skeletal muscle, kidney, and pancreas. The anti-human alpha 4 antibody detected a 45-kDa protein in human lymphoid cell lines. Moreover, human alpha 4 (IGBP1) gene is located at q13.1-q13.3 on chromosome X.

Expression of p56lck in B-cell neoplasias

The protooncogene p56lck is considered to participate in malignant transformation of lymphoid cells. In order to evaluate the role of this tyrosine kinase in B cell neoplasias, we investigated the expression of p56lck by Western blot analysis. In 12/16 Burkitt's lymphoma derived cell lines, 3/3 lymphoblastoid cell lines, 1/6 Hodgkin's disease derived cell lines, and 10/10 freshly isolated chronic lymphocytic leukemia cells constitutive expression of the protein was detected. Protein tyrosine kinase assays detected a catalytic active form of p56lck in all p56lck expressing samples. Stimulation experiments of the different cell lines and primary tumour cells by the phorbol ester TPA and the B-cell specific stimulation with SAC/anti-IgM respectively indicated a change of the expression level in comparison with the unstimulated cells and, a higher molecular weight species of the protein tyrosine kinase p56lck was observed. This was probably due to hyperphosphorylation of p56lck. No correlation between an infection with the Epstein-Barr virus and the expression of p56lck was found in the cell lines used and in primary tumour cells. Inhibition of p56lck activity by the specific inhibitor 4-amino-6-hydroxyflavone revealed a decrease of proliferation of the T-cell line Jurkat, but not of the Burkitt's lymphoma cell lines. In the analysed cell lines we found a reduction of the kinase activity of p56lck of approximately 70%. These results suggest that lck may contribute to the maintenance of the transformation of the analysed B cell neoplasias but that lck does not support a model for an initial event in B cell transformation.

B cell antigen receptor desensitization: disruption of receptor coupling to tyrosine kinase activation

Antigen binding to the B cell receptor (BCR) induces receptor desensitization, a condition characterized by cellular unresponsiveness to subsequent Ag stimulation despite the continued ability to bind Ag. To better understand the molecular mechanism of this unresponsiveness, we have used complementary lymphoma (K46 mu) and Ig transgenic (3-83 mu delta) mouse models to study regulation of BCR signaling. Our findings in the lymphoma model show that an initial Ag encounter renders receptors unresponsive to subsequent Ag challenge, as measured by their inability to mobilize Ca2+ and to mediate phosphorylation of receptor-proximal kinases, including Lyn, Blk, and Syk. Most importantly, the Ig alpha and Ig beta components of desensitized receptors are not phosphorylated, and receptor-associated kinases are not activated upon Ag challenge. The molecular defect does not appear to result from Lyn inactivation, sequestration, or repression, since Lyn from desensitized cell lysates is activated in vitro by synthetic doubly phosphorylated immunoreceptor tyrosine-based activation motif peptides. A similar deficit in Ag-induced receptor phosphorylation was observed in desensitized B cells from 3-83 mu delta transgenic mice. These studies indicate that Ag receptor desensitization reflects an inability to initiate activation of receptor-associated kinases that normally phosphorylate receptor Ig alphabeta subunits, leading to signal propagation.

Initiation and processing of signals from the B cell antigen receptor

Current models of signal transduction from the antigen receptors on B and T cells still resemble equations with several unknown elements. Data from recent knockout experiments in cell lines and mice contradict the assumption that Src-family kinase and tyrosine kinases of the Syk/Zap-70 family are the transducer elements that set signaling from these receptors in motion. Using a functional definition of signaling elements, we discuss the current knowledge of signaling events from the BCR and suggest the existence of an as-yet-unknown BCR transducer complex.

Leukocyte protein tyrosine kinases: potential targets for drug discovery

Intracellular signal transduction following the extracellular ligation of a wide variety of different types of surface molecules on leukocytes involves the activation of protein tyrosine kinases. The dependence of successful intracellular signaling on the functions of the nontransmembrane class of protein tyrosine kinases coupled with the cell type-specific expression patterns for several of these enzymes makes them appealing targets for therapeutic intervention. Development of drugs that can interfere with the catalytic functions of the nontransmembrane protein tyrosine kinases or that can disrupt critical interactions with regulatory molecules and/or substrates should find clinical applications in the treatment of allergic diseases, autoimmunity, transplantation rejection, and cancer.

Regulation of ITAM signaling by specific sequences in Ig-beta B cell antigen receptor subunit

B cell antigen receptors (BCR) are composed of an antigen binding subunit, the membrane Ig, and Ig-alpha/Ig-beta heterodimers, that contain a transducing motif named ITAM for "immuno-receptor tyrosine-based activation motif." Ig-alpha and Ig-beta ITAMs only differ by four amino acids located before the second conserved tyrosine (DCSM in Ig-alpha and QTAT in Ig-beta), which determine the in vitro association of Ig-alpha with the src kinase fyn. We have previously shown that Ig-alpha and Ig-beta BCR subunits activate different signaling pathways by expressing, in B cells, FcgammaRII chimeras containing the cytoplasmic tails of Ig-alpha or Ig-beta. We report here that the signaling capacity of Ig-beta ITAM is regulated by peptide sequences located inside (QTAT region) or outside the ITAM (flanking sequences). Furthermore, when isolated, Ig-alpha and Ig-beta ITAM have similar abilities as the entire Ig-alpha tail and the whole BCR in triggering tyrosine kinase activation, an increase of intracellular calcium concentration as well as late events of cell activation as assessed by cytokine secretion. These data show that alterations that modify the ability of Ig-alpha and Ig-beta to interact in vitro with the src kinase fyn (switch between QTAT and DCSM) also determine signal transduction capabilities of these molecules expressed in B cells.

Use of isolated immature-stage B cells to understand negative selection and tolerance induction at the molecular level

Encounter with antigen by newly developing antigen receptor-positive B cells leads to negative selection. This process positions the B cell antigen receptor (BCR) in a central role for initiating the process of negative selection and suggests developmental regulation of its signaling. The observation that immature B cells are more susceptible to negative selection than are mature B cells has been demonstrated in a number of in vitro and in vivo model systems and support the idea of developmental regulation of BCR-initiated responses. Since identical antigen receptors are expressed on immature and mature B cells, the critical fate-determining distinction between these developmental stages must lie downstream of the receptor-ligand interaction itself, in the form of different BCR-linked signaling processes or with different secondary events occurring subsequent to BCR cross-linking. To address the first possibility, our laboratory and others have sought to define the differences in BCR-mediated signal transduction in immature and mature B lymphocytes. In this review article we will discuss current in vitro systems to study this question in primary, nontransformed murine B lymphocytes. In addition, we will discuss our previously published work in order to illustrate how these model systems have been useful in beginning to unravel the molecular basis for immune B cell negative selection and tolerance.

Negative signaling via FcgammaRIIB1 in B cells blocks phospholipase Cgamma2 tyrosine phosphorylation but not Syk or Lyn activation

Crosslinking of the B cell antigen receptor surface immunoglobulin induces tyrosine phosphorylation and activation of the Src family and Syk tyrosine protein kinases, tyrosine phosphorylation of phospholipase Cgamma2 (PLCgamma2) and increases in intracellular second messengers inositol phosphates and Ca2+. These activation events, in conjunction with other pathways, culminate in the induction of B cell proliferation and differentiation. In contrast, co-crosslinking surface Ig with the B cell IgG Fc receptor prevents many of these activation events, including B cell proliferation and differentiation. The precise nature of the negative signal(s) derived from Fc receptors that prevent B cell activation is not known. Here, early activation events were examined in B cells stimulated via the antigen receptor alone or under co-crosslinking conditions. The data indicated a selective block in the tyrosine phosphorylation and activation of PLCgamma2 but not in activation of the upstream kinases, Syk and Lyn, under co-crosslinking conditions. We conclude that the negative signal acts directly on PLCgamma2 and is consistent with recent studies describing an activation-induced association of a phosphotyrosine phosphatase with tyrosine-phosphorylated B cell Fc receptor.

Regulation, substrates and functions of src

Src is the best understood member of a family of 9 tyrosine kinases that regulates cellular responses to extracellular stimuli. Activated mutants of Src are oncogenic. Using Src as an example, and referring to other Src family members where appropriate, this review describes the structure of Src, the functions of the individual domains, the regulation of Src kinase activity in the cell, the selection of substrates, and the biological functions of Src. The review concentrates on developments in the last 6-7 years, and cites data resulting from the isolation and characterization of Src mutants, crystallographic studies of the structures of SH2, SH3 and tyrosine kinase domains, biochemical studies of Src kinase activity and binding properties, and the biology of transgenic and knockout mouse strains.

Cooperativity and segregation of function within the Ig-alpha/beta heterodimer of the B cell antigen receptor complex

The B cell antigen receptor complex contains heterodimers of Ig-alpha and Ig-beta. The cytoplasmic tails of each of these chains contain two conserved tyrosines, phosphorylation of which initiates the signal transduction cascades activated by the receptor complex. Although the cytoplasmic domains of Ig-alpha and Ig-beta have been expressed individually and demonstrated to be competent signal transduction units, we postulated that within the context of a heterodimer, Ig-alpha and Ig-beta could have new, complementary or even synergistic functions. Therefore we developed a system to compare the signal transducing capacities of dimers of Ig-alpha/Ig-alpha, Ig-beta/Ig-beta, or Ig-alpha/Ig-beta. This was done by fusing the extracellular and transmembrane domains of either human platelet-derived growth factor receptor (PDGFR) alpha or beta to the cytoplasmic tail of either Ig-alpha or Ig-beta. Three cell lines expressing PDGFRbeta/Ig-alpha, PDGFRbeta/Ig-beta, or PDGFRalpha/Ig-beta together with PDGFRbeta/Ig-alpha were established in the murine B cell line A20 IIA1.6. While aggregation of each dimer by itself could induce the tyrosine phosphorylation of cellular substrates, only aggregation of the heterodimer induced the phosphorylation of substrates similar in range and intensity to that induced by the endogenous B cell antigen receptor complex. Interestingly, Ig-beta remarkably enhanced the rapidity (Tmax decreased from 5 to 1 min) and intensity (greater than 10-fold enhancement) of Ig-alpha phosphorylation. Conversely, the phosphorylation of Ig-beta was reduced to undetectable levels when co-aggregated with Ig-alpha. The enhancement of Ig-alpha phosphorylation by Ig-beta correlated with a lowering of the stimulation threshold for tyrosine kinase activation.

p120cbl is a major substrate of tyrosine phosphorylation upon B cell antigen receptor stimulation and interacts in vivo with Fyn and Syk tyrosine kinases, Grb2 and Shc adaptors, and the p85 subunit of phosphatidylinositol 3-kinase

We and others have demonstrated that the c-cbl proto-oncogene product is one of the earliest targets of tyrosine phosphorylation upon T cell receptor stimulation. Given the similarities in the B and T lymphocyte antigen receptors, and the induction of pre-B leukemias in mice by the v-cbl oncogene, we examined the potential involvement of Cbl in B cell receptor signaling. We demonstrate prominent and early tyrosine phosphorylation of Cbl upon stimulation of human B cell lines through surface IgM. Cbl was associated in vivo with Fyn and, to a lesser extent, other Src family kinases. B cell activation also induced a prominent association of Cbl with Syk tyrosine kinase. A substantial fraction of Cbl was constitutively associated with Grb2 and this interaction was mediated by Grb2 SH3 domains. Tyrosine-phosphorylated Shc, which prominently associated with Grb2, was detected in association with Cbl in activated B cells. Thus, Grb2 and Shc adaptors, which associate with immunoreceptor tyrosine based activation motifs, may link Cbl to the B cell receptor. B cell activation also induced a prominent association between Cbl and the p85 subunit of phosphatidylinositol (PI) 3-kinase resulting in the association of a substantial fraction of PI 3-kinase activity with Cbl. Thus, Cbl is likely to play an important role to couple the B cell receptor to the PI 3-kinase pathway. Our results strongly suggest a role for p120cbl in signaling downstream of the B cell receptor and support the idea that Cbl participates in a general signal transduction function downstream of the immune cell surface receptors.

Altered calcium signal transduction in B-1 malignant cells

Lymphocyte proliferation is guided by receptor-mediated signal transduction pathways that dictate the immunological response/clonality of that cell. We have previously reported that NZB-derived malignant B-1 cells, which serve as a murine model for human chronic lymphocytic leukaemia, demonstrate altered expression of surface IgM and CD45 signalling molecules, and a failure to proliferate following membrane IgM stimulation. To examine receptor-mediated cytosolic calcium (Cai) signalling in B cell leukaemia, we studied IgM-induced Cai responses in malignant B-1 cells and B cells from non-leukaemic mice. Basal Cai was slightly lower in malignant B-1 cells than in non-leukaemic cells. Anti-IgM stimulation induced a sustained increase in Cai to levels 1.3-fold greater than basal Cai in conventional B cells. In contrast, leukaemic B-1 cells demonstrated a sharp but transient rise in Cai followed by a gradual increase to levels 2.3-fold greater than basal [Ca]i Ca influx from extracellular sources contributed to the early and late Cai signal in both sets of cells. Pre-incubation (2-30 min) with anti-CD45 had no effect on basal Cai or the anti-IgM Cai signal in B cells, but reduced the Cai transient in malignant B-1 cells. Additional experiments characterized the effects of phosphorylation/dephosphorylation events on the Cai profile following anti-IgM stimulation. Protein tyrosine kinase inhibitors decreased the anti-IgM-induced Cai transient in malignant B-1 cells by 80%, but only moderately affected (40%) of the Cai response in non-leukaemic B cells. Protein tyrosine phosphatase inhibitors and protein kinase C (PKC) activators attenuated the Cai response to the same degree in normal and leukaemic B cells. These results show that Cai signalling differs widely between non-malignant B cells and malignant B-1 cells, and that tyrosine phosphorylation and CD45 modulation of IgM signalling are involved in the altered Cai responses in malignant B-1 cells.

Selective regulation of Lyn tyrosine kinase by CD45 in immature B cells

It has been well established that protein-tyrosine phosphatase CD45 is critically involved in the regulation of initial tyrosine phosphorylation and effector functions of T and B cells. However, the signaling pathway governed by CD45 is not completely understood. In B cells, it has not been unequivocally resolved as to which protein-tyrosine kinases (PTKs) associated with B cell antigen receptor are regulated by CD45 in intact cells. As a first step toward the elucidation of CD45-initiated signaling events, we have tried to identify physiological substrates for CD45 by analyzing PTK activity in CD45-deficient clones recently generated from the immature B cell line WEHI-231. The results clearly demonstrated that among PTKs examined (Lyn, Lck, and Syk), only Lyn kinase is dysregulated in the absence of CD45 such that without B cell antigen receptor ligation, Lyn is hyperphosphorylated and activated in CD45-negative clones. Thus, Lyn seems to be a selective in vivo substrate for CD45 in immature B cells.

Association of activated phosphatidylinositol 3-kinase with p120cbl in antigen receptor-ligated B cells

A 120-kDa protein that is tyrosine-phosphorylated upon antigen receptor ligation in B lymphocytes has been identified as the product of the c-cbl protooncogene. Tyrosine phosphorylation of Cbl depends on the efficient association of membrane immunoglobulin heavy chains with the Ig alpha/beta heterodimer but is unimpaired in splenic B cells from the Xid mouse. Cross-linking of membrane IgM and membrane IgG, but not of CD40, leads to the tyrosine phosphorylation of Cbl. In receptor-ligated B lymphocytes, p120cbl associates with an 85-kDa protein that has been identified as the 85-kDa subunit of phosphatidylinositol 3-kinase.

Reconstitution of the B cell antigen receptor signaling components in COS cells

To elucidate interactions occurring between B cell protein tyrosine kinases and the signaling components of the B cell antigen receptor, we have co-transfected into COS cells individual tyrosine kinases together with chimeric cell surface receptors containing the cytoplasmic domains of Ig alpha or Ig beta. Of the tyrosine kinases transfected (Lyn, Blk, Hck, Syk, Fyn), only Blk was able to phosphorylate and subsequently associate with cotransfected Ig alpha and Ig beta chimeras in vivo. Association between Blk and the Ig alpha and Ig beta cytoplasmic domains was shown by mutational analyses to be the result of an SH2-phosphotyrosine interaction. We identified the tyrosine residues of the Ig alpha and Ig beta cytoplasmic domains was shown by mutational analyses to be the result of an SH2-phosphotyrosine interaction. We identified the tyrosine residues of the Ig alpha and Ig beta cytoplasmic domains phosphorylated by Blk. The enzymatic activity and membrane association of Blk were required for the observed phosphorylation of the Ig alpha and Ig beta chimeras. Sequences within the amino-terminal unique domain of Blk are responsible for recognition and subsequent phosphorylation of the Ig alpha chimera since transfer of the unique region of Blk to Fyn results in the chimeric kinase's ability to phosphorylate the cytoplasmic domain of Ig alpha. These findings indicate that the unique domain of Src family kinases may direct recognition of certain substrates leading to their phosphorylation.