The tyrosine activation motif as a target of protein tyrosine kinases and SH2 domains

Genomic view of the origins of cell-mediated immunity

NKp30 is an activating natural killer cell receptor (NKR) with a single-exon variable (VJ)-type immunoglobulin superfamily (IgSF) domain. Such VJ-IgSF domains predate the emergence of the antigen receptors (immunoglobulin and T cell receptor), which possess the same domain but undergo gene rearrangement. NCR3, the gene encoding NKp30, is present in jawed vertebrates from sharks to mammals; thus, unlike most NKR that are highly divergent among vertebrate taxa, NKp30 is uniquely conserved. We previously hypothesized that an ancestral NCR3 gene was encoded in the proto-major histocompatibility complex (MHC), the region where many immune-related genes have accumulated. Herein, we searched in silico databases to identify NCR3 paralogues and examined their genomic locations. We found a paralogue, NCR3H, in many vertebrates but was lost in mammals. Additionally, we identified a set of voltage-gated sodium channel beta (SCNB) genes as NCR3-distantly-related genes. Like NCR3, both NCR3H and SCNB proteins contain a single VJ-IgSF domain followed by a transmembrane region. These genes map to MHC paralogous regions, originally described in an invertebrate, along with genes encoding cell adhesion molecules involved in NK cell recognition networks. Other genes having no obvious relationship to immunity also map to these paralogous regions. These gene complexes were traced to several invertebrates, suggesting that the foundation of these cellular networks emerged before the genome-wide duplications in early gnathostome history. Here, we propose that this ancestral region was involved in cell-mediated immunity prior to the emergence of adaptive immunity and that NCR3 piggybacked onto this primordial complex, heralding the emergence of vertebrate NK cell/T cells.

Synthetic biology of B cell activation: understanding signal amplification at the B cell antigen receptor using a rebuilding approach

Upon activation of the B cell antigen receptor (BCR), the spleen tyrosine kinase (Syk) and the Src family kinase Lyn phosphorylate tyrosines of the immunoreceptor tyrosine-based activation motif (ITAM) of Igα and Igβ which further serve as binding sites for the SH2 domains of these kinases. Using a synthetic biology approach, we dissect the roles of different ITAM residues of Igα in Syk activation. We found that a leucine to glycine mutation at the Y+3 position after the first ITAM tyrosine prevents Syk binding and activation. However, a pre-activated Syk can still phosphorylate this tyrosine in trans. Our data show that the formation of a Syk/ITAM initiation complex and trans-ITAM phosphorylation is crucial for BCR signal amplification. In contrast, the interaction of Lyn with the first ITAM tyrosine is not altered by the leucine to glycine mutation. In addition, our study suggests that an ITAM-bound Syk phosphorylates the non-ITAM tyrosine Y204 of Igα only in cis. Collectively, our reconstitution experiments suggest a model whereby first trans-phosphorylation amplifies the BCR signal and subsequently cis-phosphorylation couples the receptor to downstream signaling elements.

Conformational Changes in the Cytoplasmic Region of KIR3DL1 upon Interaction with SHP-2

KIR3DL1 is an inhibitory killer cell immunoglobulin-like receptor (KIR) that negatively regulates natural killer cell cytotoxicity. The KIR3DL1 cytoplasmic region (3DL1-cyto) is disordered and can be dissected into three segments: (I) H340-V351; (II) M352-D371; and (III) P372-P423. NMR studies indicate that segment II can dynamically adopt a loop-like conformation, and segments I and III can form dynamic helices that may mediate binding to membranes, particularly in the region around the N-terminal (N) immunoreceptor tyrosine-based inhibitory motif (ITIM), consistent with its role in signaling. Furthermore, individual SH2 domains of SHP-2 strongly engage with the unphosphorylated N-ITIM of 3DL1-cyto, while binding of the tandem SHP-2 SH2 domains to the bis-phosphorylated ITIMs results in more extensive conformational changes in segments I and III. The findings enhance our understanding of KIR function and how ITIMs in a target receptor operate in concert to engage the tandem SH2 domains of SHP-2.

SYK inhibition blocks proliferation and migration of glioma cells and modifies the tumor microenvironment

Background

Glioblastoma (GBM) is one of the most aggressive human brain tumors, with a median survival of 15-18 months. There is a desperate need to find novel therapeutic targets. Various receptor protein kinases have been identified as potential targets; however, response rates in clinical studies have been somewhat disappointing. Targeting the spleen tyrosine kinase (SYK), which acts downstream of a range of oncogenic receptors, may therefore show more promising results.

Methods

Kinase expression of brain tumor samples including GBM and low-grade tumors were compared with normal brain and normal human astrocytes by microarray analysis. Furthermore, SYK, LYN, SLP76, and PLCG2 protein expressions were analyzed by immunohistochemistry, western blot, and immunofluorescence of additional GBM patient samples, murine glioma samples, and cell lines. SYK was then blocked chemically and genetically in vitro and in vivo in 2 different mouse models. Multiphoton intravital imaging and multicolor flow cytometry were performed in a syngeneic immunocompetent C57BL/6J mouse GL261 glioma model to study the effect of these inhibitors on the tumor microenvironment.

Results

SYK, LYN, SLP76, and PLCG2 were found expressed in human and murine glioma samples and cell lines. SYK inhibition blocked proliferation, migration, and colony formation. Flow cytometric and multiphoton imaging imply that targeting SYK in vivo attenuated GBM tumor growth and invasiveness and reduced B and CD11b+ cell mobility and infiltration.

Conclusions

Our data suggest that gliomas express a SYK signaling network important in glioma progression, inhibition of which results in reduced invasion with slower tumor progression.

Ancient Origin of the CARD-Coiled Coil/Bcl10/MALT1-Like Paracaspase Signaling Complex Indicates Unknown Critical Functions

The CARD–coiled coil (CC)/Bcl10/MALT1-like paracaspase (CBM) signaling complexes composed of a CARD–CC family member (CARD-9, -10, -11, or -14), Bcl10, and the type 1 paracaspase MALT1 (PCASP1) play a pivotal role in immunity, inflammation, and cancer. Targeting MALT1 proteolytic activity is of potential therapeutic interest. However, little is known about the evolutionary origin and the original functions of the CBM complex. Type 1 paracaspases originated before the last common ancestor of planulozoa (bilaterians and cnidarians). Notably in bilaterians, Ecdysozoa (e.g., nematodes and insects) lacks Bcl10, whereas other lineages have a Bcl10 homolog. A survey of invertebrate CARD–CC homologs revealed such homologs only in species with Bcl10, indicating an ancient common origin of the entire CBM complex. Furthermore, vertebrate-like Syk/Zap70 tyrosine kinase homologs with the ITAM-binding SH2 domain were only found in invertebrate organisms with CARD–CC/Bcl10, indicating that this pathway might be related to the original function of the CBM complex. Moreover, the type 1 paracaspase sequences from invertebrate organisms that have CARD–CC/Bcl10 are more similar to vertebrate paracaspases. Functional analysis of protein–protein interactions, NF-κB signaling, and CYLD cleavage for selected invertebrate type 1 paracaspase and Bcl10 homologs supports this scenario and indicates an ancient origin of the CARD–CC/Bcl10/paracaspase signaling complex. By contrast, many of the known MALT1-associated activities evolved fairly recently, indicating that unknown functions are at the basis of the protein conservation. As a proof-of-concept, we provide initial evidence for a CBM- and NF-κB-independent neuronal function of the Caenorhabditis elegans type 1 paracaspase malt-1. In conclusion, this study shows how evolutionary insights may point at alternative functions of MALT1.

Swiprosin-1: Its Expression and Diverse Biological Functions

Swiprosin-1/EFhd2 is a Ca²⁺ binding adapter protein involved in the various cellular functions. Swiprosin-1 is significantly upregulated in a number of pathological conditions of inflammation, neurodegeneration, and cancer. Swiprosin-1 associated with actin and its expression level amplifies the production of proinflammatory mediators and modulates the activation of transcription factor during immune cells activation. This review aims at providing an overview of the expression and function of swiprosin-1/EFhd2 in various pathophysiological conditions. We also discussed the key role of swiprosin-1 in immune cell activation, cell migration, apoptosis, humoral immunity, cancer invasion and metastasis, neuronal transport, and major signaling cascades. J. Cell. Biochem. 119: 150-156, 2018. © 2017 Wiley Periodicals, Inc.

Coevolution of MHC genes (LMP/TAP/class Ia, NKT-class Ib, NKp30-B7H6): lessons from cold-blooded vertebrates

Comparative immunology provides the long view of what is conserved across all vertebrate taxa versus what is specific to particular organisms or group of organisms. Regarding the major histocompatibility complex (MHC) and coevolution, three striking cases have been revealed in cold-blooded vertebrates: lineages of class Ia antigen-processing and -presenting genes, evolutionary conservation of NKT-class Ib recognition, and the ancient emergence of the natural cytotoxicity receptor NKp30 and its ligand B7H6. While coevolution of transporter associated with antigen processing (TAP) and class Ia has been documented in endothermic birds and two mammals, lineages of LMP7 are restricted to ectotherms. The unambiguous discovery of natural killer T (NKT) cells in Xenopus demonstrated that NKT cells are not restricted to mammals and are likely to have emerged at the same time in evolution as classical α/β and γ/δ T cells. NK cell receptors evolve at a rapid rate, and orthologues are nearly impossible to identify in different vertebrate classes. By contrast, we have detected NKp30 in all gnathostomes, except in species where it was lost. The recently discovered ligand of NKp30, B7H6, shows strong signs of coevolution with NKp30 throughout evolution, i.e. coincident loss or expansion of both genes in some species. NKp30 also offers an attractive IgSF candidate for the invasion of the RAG transposon, which is believed to have initiated T-cell receptor/immunoglobulin adaptive immunity. Besides reviewing these intriguing features of MHC evolution and coevolution, we offer suggestions for future studies and propose a model for the primordial or proto MHC.

IL-4 enhances expression and function of surface IgM in CLL cells

Kinase inhibitors targeting the B-cell receptor (BCR) are now prominent in the treatment of chronic lymphocytic leukemia (CLL). We have focused here on interleukin 4 (IL-4), a cytokine that protects normal and malignant B cells from apoptosis and increases surface immunoglobulin M (sIgM) expression on murine splenic B cells. First, we have demonstrated that IL-4 treatment increased sIgM expression in vitro on peripheral blood B cells obtained from healthy individuals. In CLL, IL-4 target genes are overexpressed in cells purified from the lymph nodes of patients compared with cells derived from matched blood and bone marrow samples. As for normal B cells, IL-4 increased sIgM expression on CLL cells in vitro, especially in samples expressing unmutated V-genes. IL-4-induced sIgM expression was associated with increased receptor signalling activity, measured by anti-IgM-induced calcium mobilization, and with increased expression of CD79B messenger RNA and protein, and the "mature" glycoform of sIgM. Importantly, the ability of the BCR-associated kinase inhibitors idelalisib and ibrutinib, approved for treatment of CLL and other B-cell malignancies, to inhibit anti-IgM-induced signalling was reduced following IL-4 pretreatment in samples from the majority of patients. In contrast to stimulatory effects on sIgM, IL-4 decreased CXCR4 and CXCR5 expression; therefore, CLL cells, particularly within the progressive unmutated V-gene subset, may harness the ability of IL-4 to promote BCR signalling and B-cell retention within lymph nodes. Effects of IL-4 were mediated via JAK3/STAT6 and we propose a potential role for JAK inhibitors in combination with BCR kinase inhibitors for the treatment of CLL.

TCR ITAM multiplicity is required for the generation of follicular helper T-cells

The T-cell antigen receptor (TCR) complex contains 10 copies of a di-tyrosine Immunoreceptor-Tyrosine-based-Activation-Motif (ITAM) that initiates TCR signalling by recruiting protein tyrosine kinases. ITAM multiplicity amplifies TCR signals, but the importance of this capability for T-cell responses remains undefined. Most TCR ITAMs (6 of 10) are contributed by the CD3ζ subunits. We generated 'knock-in' mice that express non-signalling CD3ζ chains in lieu of wild-type CD3ζ. Here we demonstrate that ITAM multiplicity is important for the development of innate-like T-cells and follicular helper T-cells, events that are known to require strong/sustained TCR-ligand interactions, but is not essential for 'general' T-cell responses including proliferation and cytokine production or for the generation of a diverse antigen-reactive TCR repertoire.

Mouse mammary tumor virus suppresses apoptosis of mammary epithelial cells through ITAM-mediated signaling

Many receptors in hematopoietic cells use a common signaling pathway that relies on a highly conserved immunoreceptor tyrosine-based activation motif (ITAM), which signals through Src family tyrosine kinases. ITAM-bearing proteins are also found in many oncogenic viruses, including the mouse mammary tumor virus (MMTV) envelope (Env). We previously showed that MMTV Env expression transformed normal mammary epithelial cells and that Src kinases were important mediators in this transformation. To study how ITAM signaling affects mammary cell transformation, we utilized mammary cell lines expressing two different ITAM-containing proteins, one encoding a MMTV provirus and the other a B cell receptor fusion protein. ITAM-expressing cells were resistant to both serum starvation- and chemotherapeutic drug-induced apoptosis, whereas cells transduced with these molecules bearing ITAM mutations were indistinguishable from untransduced cells in their sensitivity to these treatments. We also found that Src kinase was activated in the MMTV-expressing cells and that MMTV-induced apoptosis resistance was completely restored by the Src inhibitor PP2. In vivo, MMTV infection delayed involution-induced apoptosis in the mouse mammary gland. Our results show that MMTV suppresses apoptosis through ITAM-mediated Src tyrosine kinase signaling. These studies could lead to the development of effective treatment of nonhematopoietic cell cancers in which ITAM-mediated signaling plays a role.

FcγRIIa requires lipid rafts, but not co-localization into rafts, for effector function

OBJECTIVE

To determine if receptor localization into lipid rafts, or the lipid rafts themselves, are important for FcγRIIa effector functions.

MATERIAL

Wild-type FcγRIIa or mutant FcγRIIa(C208A) that does not translocate to lipid rafts were transfected into Chinese hamster ovary (CHO) cells which have been shown to be reliable cells for studying FcγR function.

TREATMENT

Cells were treated with buffer or methyl-β-cyclodextrin (MβCD) to deplete cholesterol and dissolve the structure of lipid rafts.

METHODS

To evaluate lipid raft association, transfected CHO cells were lysed and centrifuged over a sucrose gradient. Fractions were run on SDS-PAGE and blotted for FcγRIIa or sphingolipid GM1 to illustrate the lipid raft fractions. Lateral mobility of GFP-tagged wild-type or mutant FcγRIIa was assessed using fluorescence recovery after photobleaching (FRAP) microscopy. Internalization of IgG-opsonized erythrocytes was assessed by fluorescence microscopy and uptake of heat-aggregated IgG (haIgG) was measured using flow cytometry.

RESULTS

We observed that FcγRIIa(C208A) did not localize into lipid rafts. However, the mutant FcγRIIa retained lateral mobility and effector function similar to wild-type FcγRIIa. However, mutant FcγRIIa function was abolished upon treatment with MβCD.

CONCLUSIONS

Lipid rafts provide an essential component required for effector activities independent of receptor localization.

The SH2-domain of SHIP1 interacts with the SHIP1 C-terminus: impact on SHIP1/Ig-α interaction

The SH2-containing inositol 5'-phosphatase, SHIP1, negatively regulates signal transduction from the B cell antigen receptor (BCR). The mode of coupling between SHIP1 and the BCR has not been elucidated so far. In comparison to wild-type cells, B cells expressing a mutant IgD- or IgM-BCR containing a C-terminally truncated Ig-α respond to pervanadate stimulation with markedly reduced tyrosine phosphorylation of SHIP1 and augmented activation of protein kinase B. This indicates that SHIP1 is capable of interacting with the C-terminus of Ig-α. Employing a system of fluorescence resonance energy transfer in S2 cells, we can clearly demonstrate interaction between the SH2-domain of SHIP1 and Ig-α. Furthermore, a fluorescently labeled SH2-domain of SHIP1 translocates to the plasma membrane in an Ig-α-dependent manner. Interestingly, whereas the SHIP1 SH2-domain can be pulled-down with phospho-peptides corresponding to the immunoreceptor tyrosine-based activation motif (ITAM) of Ig-α from detergent lysates, no interaction between full-length SHIP1 and the phosphorylated Ig-α ITAM can be observed. Further studies show that the SH2-domain of SHIP1 can bind to the C-terminus of the SHIP1 molecule, most probably by inter- as well as intra-molecular means, and that this interaction regulates the association between different forms of SHIP1 and Ig-α.

Fraternal twins: Swiprosin-1/EFhd2 and Swiprosin-2/EFhd1, two homologous EF-hand containing calcium binding adaptor proteins with distinct functions

Changes in the intracellular calcium concentration govern cytoskeletal rearrangement, mitosis, apoptosis, transcriptional regulation or synaptic transmission, thereby, regulating cellular effector and organ functions. Calcium binding proteins respond to changes in the intracellular calcium concentration with structural changes, triggering enzymatic activation and association with downstream proteins. One type of calcium binding proteins are EF-hand super family proteins. Here, we describe two recently discovered homologous EF-hand containing adaptor proteins, Swiprosin-1/EF-hand domain containing 2 (EFhd2) and Swiprosin-2/EF-hand domain containing 1 (EFhd1), which are related to allograft inflammatory factor-1 (AIF-1). For reasons of simplicity and concision we propose to name Swiprosin-1/EFhd2 and Swiprosin-2/EFhd1 from now on EFhd2 and EFhd1, according to their respective gene symbols. AIF-1 and Swiprosin-1/EFhd2 are already present in Bilateria, for instance in Drosophila melanogaster and Caenhorhabditis elegans. Swiprosin-2/EFhd1 arose later from gene duplication in the tetrapodal lineage. Secondary structure prediction of AIF-1 reveals disordered regions and one functional EF-hand. Swiprosin-1/EFhd2 and Swiprosin-2/EFhd1 exhibit a disordered region at the N-terminus, followed by two EF-hands and a coiled-coil domain. Whereas both proteins are similar in their predicted overall structure they differ in a non-homologous stretch of 60 amino acids just in front of the EF-hands. AIF-1 controls calcium-dependent cytoskeletal rearrangement in innate immune cells by means of its functional EF-hand. We propose that Swiprosin-1/EFhd2 as well is a cytoskeleton associated adaptor protein involved in immune and brain cell function. Pro-inflammatory conditions are likely to modulate expression and function of Swiprosin-1/EFhd2. Swiprosin-2/EFhd1, on the other hand, modulates apoptosis and differentiation of neuronal and muscle precursor cells, probably through an association with mitochondria. We suggest furthermore that Swiprosin-2/EFhd1 is part of a cellular response to oxidative stress, which could explain its pro-survival activity in neuronal, muscle and perhaps some malignant tissues.

Differential requirement of lipid rafts for FcγRIIA mediated effector activities

Immunoglobulin G (IgG) dependent activities are important in host defense and autoimmune diseases. Various cell types including macrophages and neutrophils contribute to pathogen destruction and tissue damage through binding of IgG to Fcγ receptors (FcγR). One member of this family, FcγRIIA, is a transmembrane glycoprotein known to mediate binding and internalization of IgG-containing targets. FcγRIIA has been observed to translocate into lipids rafts upon binding IgG-containing targets. We hypothesize that lipid rafts participate to different extents in binding and internalizing targets of different sizes. We demonstrate that disruption of lipid rafts with 8mM methyl-β-cyclodextrin (MβCD) nearly abolishes binding (91% reduction) and phagocytosis (60% reduction) of large IgG-coated targets. Conversely, binding and internalization of small IgG-complexes is less dependent on lipid rafts (49% and 17% inhibition at 8mM MβCD, respectively). These observations suggest that differences between phagocytosis and endocytosis may arise as early as the initial stages of ligand recognition.

Molecular mechanism of the Syk activation switch

Many immune signaling pathways require activation of the Syk tyrosine kinase to link ligation of surface receptors to changes in gene expression. Despite the central role of Syk in these pathways, the Syk activation process remains poorly understood. In this work we quantitatively characterized the molecular mechanism of Syk activation in vitro using a real time fluorescence kinase assay, mutagenesis, and other biochemical techniques. We found that dephosphorylated full-length Syk demonstrates a low initial rate of substrate phosphorylation that increases during the kinase reaction due to autophosphorylation. The initial rate of Syk activity was strongly increased by either pre-autophosphorylation or binding of phosphorylated immune tyrosine activation motif peptides, and each of these factors independently fully activated Syk. Deletion mutagenesis was used to identify regions of Syk important for regulation, and residues 340-356 of the SH2 kinase linker region were identified to be important for suppression of activity before activation. Comparison of the activation processes of Syk and Zap-70 revealed that Syk is more readily activated by autophosphorylation than Zap-70, although both kinases are rapidly activated by Src family kinases. We also studied Syk activity in B cell lysates and found endogenous Syk is also activated by phosphorylation and immune tyrosine activation motif binding. Together these experiments show that Syk functions as an "OR-gate" type of molecular switch. This mechanism of switch-like activation helps explain how Syk is both rapidly activated after receptor binding but also sustains activity over time to facilitate longer term changes in gene expression.

Compartmentalization of ITAM and integrin signaling by adapter molecules

Adapters are multidomain molecules that recruit effector proteins during signal transduction by immunoreceptors and integrins. The absence of these scaffolding molecules profoundly affects development and function of various hematopoietic lineages, underscoring their importance as regulators of signaling cascades. An emerging aspect of the mechanism by which engaged immunoreceptors and integrins transmit signals within the cell is by differential usage of various adapters that function to nucleate formation of distinct signaling complexes in a specific location within the cell. In this review, we discuss the mechanisms by which adapter proteins coordinate signal transduction with an emphasis on the role of subcellular compartmentalization in adapter function.

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.

A proteomics strategy for the enrichment of receptor-associated complexes

Multimeric protein complexes are important for cell function and are being identified by proteomics approaches. Enrichment strategies, such as those employing affinity matrices, are required for the characterization of such complexes, for example, those containing growth factor receptors. The receptor for the macrophage lineage growth factor, macrophage-colony stimulating factor (M-CSF or CSF-1), is the tyrosine kinase, c-Fms. There is evidence that the CSF-1 receptor (CSF-1R) forms distinct multimeric complexes involving autophosphorylated tyrosines in its cytoplasmic region; however, these complexes are difficult to identify by immunoprecipitation, making enrichment necessary. We report here the use of a tyrosine-phosphorylated, GST-fusion construct of the entire CSF-1R cytoplasmic region to characterize proteins putatively associating with the activated CSF-1R. Besides signalling molecules known to associate with the receptor or be involved in CSF-1R-dependent signalling, mass spectrometry identified a number of other molecules binding to the construct. So far among these candidate proteins, dynein, claudin and silencer of death domains co-immunoprecipitated with the CSF-1R, suggesting association. This affinity matrix method, using an entire cytoplasmic region, may have relevance for other growth factor receptors.

Transmembrane sequences are determinants of immunoreceptor signaling

To investigate structural features critical for signal initiation by Ag-stimulated immunoreceptors, we constructed a series of single-chain chimeric receptors that incorporate extracellular human Fc epsilonRIalpha for IgE binding, a variable transmembrane (TM) segment, and the ITAM-containing cytoplasmic tail of the TCR zeta-chain. We find that functional responses mediated by these receptors are strongly dependent on their TM sequences, and these responses are highly correlated to cross-link-dependent association with detergent-resistant lipid rafts. For one chimera designated alpha Fzeta, mutation of a TM cysteine abolishes robust signaling and lipid raft association. In addition, TM disulfide-mediated oligomerization of another chimeric receptor, alpha zetazeta, enhances signaling. These results demonstrate an important role for TM segments in immunoreceptor signaling and a strong correspondence between strength of signaling and cross-link-dependent partitioning into ordered membrane domains.

Protein phosphatase subunit G5PR is needed for inhibition of B cell receptor-induced apoptosis

B cell receptor (BCR) cross-linking induces B cell proliferation and sustains survival through the phosphorylation-dependent signals. We report that a loss of the protein phosphatase component G5PR increased the activation-induced cell death (AICD) and thus impaired B cell survival. G5PR associates with GANP, whose expression is up-regulated in mature B cells of the peripheral lymphoid organs. To study G5PR function, the G5pr gene was conditionally targeted with the CD19-Cre combination (G5pr^−/− mice). The G5pr^−/− mice had a decreased number of splenic B cells (60% of the controls). G5pr^−/− B cells showed a normal proliferative response to lipopolysaccharide or anti-CD40 antibody stimulation but not to BCR cross-linking with or without IL-4 in vitro. G5pr^−/− B cells did not show abnormalities in the BCR-mediated activation of Erks and NF-κB, cyclin D2 induction, or Akt activation. However, G5pr^−/− B cells were sensitive to AICD caused by BCR cross-linking. This was associated with an increased depolarization of the mitochondrial membrane and the enhanced activation of c-Jun NH₂-terminal protein kinase and Bim. These results suggest that G5PR is required for the BCR-mediated proliferation associated with the prevention of AICD in mature B cells.

Neisseria gonorrhoeae kills carcinoembryonic antigen-related cellular adhesion molecule 1 (CD66a)-expressing human B cells and inhibits antibody production

Neisseria gonorrhoeae cells (gonococci [GC]), the etiological agents for gonorrhea, can cause repeated infections. During and after gonococcal infection, local and systemic antigonococcal antibody levels are low. These clinical data indicate the possibility that GC may suppress immune responses during infection. Carcinoembryonic antigen-related cellular adhesion molecule 1 (CEACAM1 or CD66a), a receptor for GC opacity (Opa) proteins, was shown to mediate inhibitory signals. In the present study, human B cells were activated by interleukin-2 to express CEACAM1 and then stimulated to secrete antibodies and simultaneously coincubated with Opa- and OpaI GC of strain MS11. Our results show that this OpaI GC has the ability to inhibit antibody production. The interaction of GC and CEACAM1 with human peripheral B cells also results in induction of cell death. The same findings were observed in DT40 B cells. This CEACAM1-promoted cell death pathway does not involve the inhibitory signals or the tyrosine phosphatases SHP-1 and SHP-2 but depends on Bruton's tyrosine kinase in DT40 cells. Our results suggest that Neisseria gonorrhoeae possesses the ability to suppress antibody production by killing CEACAM1-expressing B cells.

Perspectives on the nature of BCR-mediated survival signals

In the June 11 issue of Cell, Kraus et al. (2004) show, through conditional mutagenesis, that mature B cells have a drastically reduced life span in the absence of normal B cell antigen receptor (BCR) surface expression or tonic signal transduction. These studies support a role for signal transduction downstream of the BCR, rather than continued surface expression per se for the maintenance and survival of mature B cells in the periphery. Further, these studies exclude transient INF(gamma)-induced activation as a prerequisite to apoptosis in receptor-less cells.

The role of the FcepsilonRI beta-chain in allergic diseases

The high affinity receptor for IgE, FcepsilonRI, is a multimeric surface receptor that is expressed exclusively as a tetramer on rodent cells, but exists as a tetramer or trimer on human cells. The tetrameric form is expressed on effector cells of allergic responses such as mast cells and basophils and is composed of an IgE-binding alpha-subunit, a beta-subunit and a gamma-subunit dimer. Complexes lacking the beta-subunit are found on human antigen-presenting cells. On mast cells and basophils, FcepsilonRI is essential for IgE-mediated acute allergic reactions. Crosslinking of FcepsilonRI by IgE and multivalent antigen induces a signaling cascade that culminates in the release of preformed mediators and the synthesis of lipid mediators and cytokines. The beta-subunit functions as an amplifier of FcepsilonRI expression and signaling. As a consequence, strongly enhanced mast cell effector functions and in vivo allergic reactions can be observed in the presence of FcepsilonRIbeta. In contrast, a truncated beta-isoform (betaT) that is produced by alternative splicing acts as an inhibitor of FcepsilonRI surface expression. Thus, by producing two proteins with antagonistic functions, the FcepsilonRIbeta gene could serve as a potent regulator of allergic responses. In addition, the genomic region encompassing the beta-chain has been linked to atopy and a number of polymorphisms within the FcepsilonRIbeta gene are associated with various atopic diseases. It remains to be elucidated how these polymorphisms might affect the allergic phenotype. These functions of the beta-chain together with the described genetic linkages to atopy make it a candidate for a role in the pathophysiology of allergic diseases.

Dileucine and YXXL motifs in the cytoplasmic tail of the bovine leukemia virus transmembrane envelope protein affect protein expression on the cell surface

Several retroviruses downmodulate the cell surface expression of envelope (Env) proteins through peptide sequences located in the cytoplasmic tail of the transmembrane (TM) subunit. We investigated whether cell surface expression of a chimeric protein containing the cytoplasmic domain of the TM protein (CTM) of bovine leukemia virus (BLV) was regulated by two membrane-proximal dileucine motifs or by tyrosine Y487 or Y498 in YXXL motifs. A chimeric protein composed of the extracellular and membrane-spanning portions of human CD8-alpha plus a wild-type (wt) BLV CTM was detectable on the surface of only 40% of the cells in which it was transiently expressed. Replacement of either dileucine pair with alanines increased the level of surface display of chimeric proteins. Nearly all cells became surface positive when both dileucine motifs were altered simultaneously and when either an N-terminal segment containing both dileucine motifs or a C-terminal segment containing all YXXL motifs was deleted. In contrast, replacement of Y487 or Y498 with alanine or phenylalanine enabled only small increases in surface display compared with wt levels. Chimeric proteins had similar stabilities but were downmodulated from the cell surface at three different rates. Point mutants segregated into each of the three groups of proteins categorized according to these different rates. Interestingly, Y487 mutants were downmodulated less efficiently than Y498 mutants, which behaved like wt. CD8-CTM chimeric proteins were phosphorylated on serine residues, but the native BLV Env protein was not phosphorylated either in transfected cells or in a lymphoid cell line constitutively producing BLV. Thus, both dileucine and YXXL motifs within the BLV CTM contribute to downmodulation of a protein containing this domain. Interactions with other proteins may influence surface exposure of Env protein complexes in virus-infected cells, assisting in viral evasion of adaptive immunity.

Protein kinase C-alpha and -delta are required for FcalphaR (CD89) trafficking to MHC class II compartments and FcalphaR-mediated antigen presentation

Studies have demonstrated that receptor-mediated signaling, receptor/antigen complex trafficking, and major histocompatibility complex class II compartments (MIIC) are critically related to antigen presentation to CD4+ T cells. In this study, we investigated the role of protein kinase C (PKC) in FcalphaR/gammagamma (CD89, human IgA receptor)-mediated internalization of immune complexes and subsequent antigen presentation. The classical and novel PKC inhibitor, Calphostin C, inhibits FcalphaR-mediated antigen presentation and interaction of MIIC and cargo vesicle (receptor and antigen). PKC-alpha, PKC-delta, and PKC-epsilon were recruited to lipid rafts following FcalphaR crosslinking, the extent of which was determined by the phenotype of the gamma chain. Mutant gamma chain with an FcgammaRIIA ITAM (immunoreceptor tyrosine-based activation motif) insert was less able to recruit PKC and trigger antigen presentation. Both PKC isoform-specific peptide inhibitors and short interfering RNA (siRNA) showed that PKC-alpha and PKC-delta, but not PKC-epsilon, were required for association of cargo vesicle and MIIC and for FcalphaR-mediated and soluble antigen presentation. Inhibition of PKC (classical and novel) did not alter major histocompatibility class II biosynthesis, assembly, transport, or plasma membrane stability. PKC's role in facilitating interaction of cargo vesicle and MIIC is likely due to regulation of vesicle biology required for fusion of cargo vesicles to MIIC.

IgSF13, a novel human inhibitory receptor of the immunoglobulin superfamily, is preferentially expressed in dendritic cells and monocytes

A novel inhibitory receptor of immunoglobin superfamily (IgSF), IgSF member 13 (IgSF13), has been identified from human dendritic cells (DC). IgSF13 is a type I transmembrane protein containing an N-terminal signal peptide, a extracellular region with a single Ig V-like domain, a transmembrane region, and a cytoplasmic tail with two classical immunoreceptor tyrosine-based inhibitory motifs (ITIM), suggesting its inhibitory function. IgSF13 shows significant homology to human CMRF35 and pIgR. IgSF13 gene is mapped to chromosome 17q25.2, very close to that of CMRF35. IgSF13 is preferentially expressed in myelo-monocytic cells, including monocytes, monocyte-derived DC, and monocyte-related cell lines. Upon pervanadate treatment, IgSF13 was hyper-phosphorylated and associated with Src homology-2 domain-containing phosphatases SHP-1 and SHIP, but not SHP-2. The identification of IgSF13 as a novel ITIM-bearing receptor selectively expressed by DC and monocytes suggests that it may be potentially involved in the negative regulation of specific leukocyte population.

Signaling molecules as therapeutic targets in allergic diseases

A molecular understanding of physiologic and pathologic processes requires complete knowledge about the signal transduction mechanism of involved cells. Signal transduction research is a rapidly growing field in basic science. Unlike intercellular inflammatory mediators, signaling molecules show less functional redundancy. This allows inhibition of multiple cytokines/mediators by blocking one common signaling molecule. Interference with signaling pathways has shown significant potential for inhibition of fundamental processes as well as clinical phenotype of allergic diseases. The purpose of this review was to provide a theoretical classification of signaling molecules based on their function and to analyze various strategies for developing effective signaling inhibitors for allergic diseases.

Bovine leukemia virus gp30 transmembrane (TM) protein is not tyrosine phosphorylated: examining potential interactions with host tyrosine-mediated signaling

Bovine leukemia virus (BLV) causes persistent lymphocytosis, a preneoplastic, polyclonal expansion of B lymphocytes. The expansion increases viral transmission to new hosts, but the mechanisms of this expansion have not been determined. We hypothesized that BLV infection contributes to B-cell expansion by signaling initiated via viral transmembrane protein motifs undergoing tyrosine phosphorylation. Viral mimicry of host cell proteins is a well-demonstrated mechanism by which viruses may increase propagation or decrease recognition by the host immune system. The cytoplasmic tail of BLV transmembrane protein gp30 (TM) has multiple areas of homology to motifs of host cell signaling proteins, including two immunoreceptor tyrosine-based activation motifs (ITAMs) and two immunoreceptor tyrosine-based inhibition motifs (ITIMs), which are homologous to B-cell receptor and inhibitory co-receptor motifs. Signaling by these motifs in B cells typically relies on tyrosine phosphorylation, followed by interactions with Src-homology-2 (SH2) domains of nonreceptor protein tyrosine kinases or phosphatases. Phosphorylation of tyrosine residues in the cytoplasmic tail of TM was tested in four systems including ex vivo cultured peripheral blood mononuclear cells from BLV infected cows, BLV-expressing fetal lamb kidney cell and bat lung cell lines, and DT40 B cells transfected with a fusion of mouse extracellular CD8alpha and cytoplasmic TM. No phosphorylation of TM was detected in our experiments in any of the cell types utilized, or with various stimulation methods. Detection was attempted by immunoblotting for phosphotyrosines, or by metabolic labeling of cells. Thus BLV TM is not likely to modify host signal pathways through interactions between phosphorylated tyrosines of the ITAM or ITIM motifs and host-cell tyrosine kinases or phosphatases.

Amplification of B cell antigen receptor signaling by a Syk/ITAM positive feedback loop

We have established a protocol allowing transient and inducible coexpression of many foreign genes in Drosophila S2 Schneider cells. With this powerful approach of reverse genetics, we studied the interaction of the protein tyrosine kinases Syk and Lyn with the B cell antigen receptor (BCR). We find that Lyn phosphorylates only the first tyrosine whereas Syk phosphorylates both tyrosines of the BCR immunoreceptor tyrosine-based activation motif (ITAM). Furthermore, we show that Syk is a positive allosteric enzyme, which is strongly activated by the binding to the phosphorylated ITAM tyrosines, thus initiating a positive feedback loop at the receptor. The BCR-dependent Syk activation and signal amplification is efficiently counterbalanced by protein tyrosine phosphatases, the activity of which is regulated by H(2)O(2) and the redox equilibrium inside the cell.

PTEN controls immunoreceptor (immunoreceptor tyrosine-based activation motif) signaling and the activation of Rac

Fcgamma receptor-mediated phagocytosis is a model for the study of immunoreceptor (immunoreceptor tyrosine-based activation motif [ITAM]) signaling and involves the activation of protein tyrosine kinases, protein tyrosine phosphatases, and downstream effectors including phosphatidylinositol-3 (PI-3) kinase. Relatively little is known of the role of lipid phosphatases in the control of ITAM signaling and inflammation. A heterologous COS7 cell system was used to examine the roles played by PI-3 kinase and the dual-specificity phosphatase, phosphatase and tensin homolog deleted on chromosome 10 (PTEN), in the signal transduction pathway leading to Fcgamma receptor IIA-mediated phagocytosis and the activation of Rac. The expression of wildtype PTEN completely abrogated the phagocytosis of immunoglobulin-G-sensitized sheep red blood cells, as compared with the catalytically inactive mutant of PTEN, which had no effect. This is the first direct evidence that PTEN, an inositol 3' phosphatase, regulates Fcgamma receptor-mediated phagocytosis, an ITAM-based signaling event. The data suggest that PTEN exerts control over phagocytosis potentially by controlling the downstream conversion of guanosine diphosphate-Rac to guanosine triphosphate-Rac following ITAM stimulation.

The CGM1a (CEACAM3/CD66d)-mediated phagocytic pathway of Neisseria gonorrhoeae expressing opacity proteins is also the pathway to cell death

Phagocytosis of Opa+ Neisseria gonorrhoeae (gonococcus, GC) by neutrophils is in part dependent on the interaction of Opa proteins with CGM1a (CEACAM3/CD66d) antigens, a neutrophil-specific receptor. However, the signaling pathways leading to phagocytosis have not been characterized. Here we show that interaction of OpaI bacteria with neutrophils or CGM1a-transfected DT40 cells induces calcium flux, which correlates with phagocytosis of bacteria. We identified an immunoreceptor tyrosine-based activation motif (ITAM) in CGM1a, and showed that the ability of CGM1a to transduce signals and mediate phagocytosis was abolished by mutation of the ITAM tyrosines. We also demonstrated that CGM1a-ITAM-mediated bacterial phagocytosis is dependent on Syk and phospholipase C activity in DT40 cells. Unexpectedly, the activation of the CGM1a-ITAM phagocytic pathway by Opa+ GC results in induction of cell death.

Fc alpha receptor cross-linking causes translocation of phosphatidylinositol-dependent protein kinase 1 and protein kinase B alpha to MHC class II peptide-loading-like compartments

A20 IIA1.6 B cells cotransfected with FcalphaR and wild-type gamma-chain (wt-ITAM (immunoreceptor tyrosine-based activation motif)) or FcalphaR and gamma-chain, in which the wt-ITAM was substituted with the FcgammaRIIA ITAM (IIA-ITAM), were used to investigate cell signaling events influencing presentation of FcalphaR-targeted exogenous Ag in the context of MHC class II. wt-ITAM cells presented FcalphaR-targeted OVA more efficiently than IIA-ITAM transfectants to OVA-specific T cell hybridomas. Phosphatidylinositol 3-kinase (PI 3-kinase) inhibition abrogated Ag presentation, suggesting that FcalphaR may trigger a PI 3-kinase-dependent signal transduction pathway, and thus phosphatidylinositol-dependent protein kinase (PDK1) and protein kinase B alpha (PKBalpha) activation. Cross-linking FcalphaR on wt-ITAM or IIA-ITAM cells triggered equivalent PI 3-kinase-dependent activation of PKBalpha. Furthermore, FcalphaR cross-linking triggered recruitment of PDK1 and serine-phosphorylated PKBalpha to capped cell surface FcalphaR irrespective of the gamma-chain ITAM. Although FcalphaR endocytosis was accompanied by translocation of PDK1 and phospho-PKBalpha to FcalphaR-containing vesicles in both transfectants, this was decreased in IIA-ITAM cells, and a significant proportion of PDK1 and PKBalpha remained at the plasma membrane. In wt-ITAM cells, PDK1 and serine-phosphorylated PKBalpha translocated to lysosomal-associated membrane glycoprotein 1- and cathepsin B-containing vesicles, consistent with MHC class II peptide-loading compartments (MIIC) described by other groups. Our data indicate that translocation of signal transduction mediators to MIIC-like compartments accompanies efficient presentation of receptor-targeted Ag, and suggest a mechanism connecting signaling to the Ag-processing pathway.

Spontaneous phosphorylation of the receptor with high affinity for IgE in transfected fibroblasts

Receptors with high affinity for IgE, FcepsilonRI, which had been transfected into Chinese hamster ovary fibroblasts exhibit an over 20-fold greater spontaneous phosphorylation at physiological temperatures than the same receptors on the widely studied rat mucosal mast cell line, RBL-2H3. This enhanced phosphorylation was not accounted for either by changes in the src-family kinase responsible for the phosphorylation, by reduced activity of phosphatases, or by spontaneous association of the receptors with microdomains. A variety of approaches failed to detect evidence for stable spontaneous aggregates of the receptor. Whereas the altered posttranslational glycosylation of the receptor's principal ectodomain we detected could promote transient spontaneous aggregation and explain the observed effect, other changes in the membrane milieu cannot be excluded. The functional consequences of such spontaneous phosphorylation are considered.

NMR structure of the N-SH2 of the p85 subunit of phosphoinositide 3-kinase complexed to a doubly phosphorylated peptide reveals a second phosphotyrosine binding site

The N-terminal src homology 2 (SH2) domain of the p85 subunit of phosphoinositide 3-kinase (PI3K) has a higher affinity for a peptide with two phosphotyrosines than for the same peptide with only one. This unexpected result was not observed for the C-terminal SH2 from the same protein. NMR structural analysis has been used to understand the behavior of the N-SH2. The structure of the free SH2 domain has been compared to that of the SH2 complexed with a doubly phosphorylated peptide derived from polyomavirus middle T antigen (MT). The structure of the free SH2 domain shows some differences from previous NMR and X-ray structures. In the N-SH2 complexed with a doubly phosphorylated peptide, a second site for phosphotyrosine interaction has been identified. Further, line shapes of NMR signals showed that the SH2 protein-ligand complex is subject to temperature-dependent conformational mobility. Conformational mobility is also supported by the spectra of the ligand peptide. A binding model which accounts for these results is developed.

The ins and outs of getting in: structures and signals that enhance BCR or Fc receptor-mediated antigen presentation

Antigen-presenting cells internalize antigen by fluid-phase pinocytosis or by endocytosis via surface receptors such as the B cell receptor (BCR) and Fc receptors for IgG, IgA and IgE (FcR). While both modes of internalization lead to antigen presentation it is recognized that receptor-mediated endocytosis greatly enhances the efficiency of processing and antigen presentation. Receptors facilitate the entry of antigen into the endocytic pathway by interaction of their internalization motifs with the endocytic machinery. These motifs include tyrosine-based, dileucine and casein kinase-like motifs. However these structures appear insufficient to support processing of cryptic epitopes, leading to a limited immune response. Cryptic epitope processing appears dependent on receptor signaling which is mediated by immunoreceptor tyrosine activation motifs (ITAMs). The signaling cascade which follows receptor crosslinking promotes reorganization and acidification of the late endocytic compartment or MIIC. Signaling events downstream of Syk, in particular calcium flux and protein kinase C activation, are necessary for MIIC induction. PI(3) kinase is also involved at multiple steps in antigen presentation, including production of PIP3 and transport of cathepsins. PIP3 is crucial both as a binding substrate for proteins implicated in vesicle transport and for the recruitment of signaling molecules to the plasma membrane. Among PIP3 activated molecules, protein kinase B (PKB) has been linked to endocytic function. We observe association of activated PKB with the MIIC after signaling through antigen presentation-competent receptors, but not mutant, presentation-defective receptors.

Immunoreceptor tyrosine-based inhibition motif-bearing receptors regulate the immunoreceptor tyrosine-based activation motif-induced activation of immune competent cells

ITIM-bearing receptors, a family which only recently has been recognized, play a key role in the regulation of the ITAM-induced activation of immune competent cells. The mechanism of ITM-mediated regulation in various cells was recently clarified. The present review focuses on ITIM bearing membrane proteins that negatively regulate the activation of cells when co-crosslinked with ITAM containing receptors, illustrates the inhibitory processes by the negative regulation of B-, NK-, T-cells and mast cells and summarizes current views on the mechanism of ITIM-mediated inhibition.

Negative regulation of Fc epsilon RI-mediated degranulation by CD81

Signaling through the high affinity receptor for immunoglobulin E (Fc epsilon RI) results in the coordinate activation of tyrosine kinases before calcium mobilization. Receptors capable of interfering with the signaling of antigen receptors, such as Fc epsilon RI, recruit tyrosine and inositol phosphatases that results in diminished calcium mobilization. Here, we show that antibodies recognizing CD81 inhibit Fc epsilon RI-mediated mast cell degranulation but, surprisingly, without affecting aggregation-dependent tyrosine phosphorylation, calcium mobilization, or leukotriene synthesis. Furthermore, CD81 antibodies also inhibit mast cell degranulation in vivo as measured by reduced passive cutaneous anaphylaxis responses. These results reveal an unsuspected calcium-independent pathway of antigen receptor regulation, which is accessible to engagement by membrane proteins and on which novel therapeutic approaches to allergic diseases could be based.

Several carcinoembryonic antigens (CD66) serve as receptors for gonococcal opacity proteins

Neisseria gonorrhoeae (GC) is a human pathogen that adheres to and invades genital surfaces. Although pili are required for the initial adherence, the interaction of GC with epithelial cells is also promoted by a family of outer membrane proteins, the opacity (Opa) proteins such as OpaA protein from strain MS11. Studies have demonstrated that the interaction of the OpaA GC with epithelial cells involves binding to heparan sulfate attached to syndecan receptors. However, other Opa proteins interact with CEA gene family member 1 (CGM1) or biliary glycoprotein (BGP), members of the CD66 antigen family. In this study, we demonstrate that, in addition, the 180-kD carcinoembryonic antigen (CEA) is a receptor for Opa proteins. This conclusion was based on the following observations. First, transfected HeLa cells expressing CEA (HeLa-CEA) and the CEA-expressing colon cancer cell line (LS 174T) bound and subsequently engulfed the Opa+ bacteria. These interactions were inhibited by anti-CEA antibody, but could not be inhibited by addition of heparin. Furthermore, OpaI E. coli directly bound purified CEA. We also compared the adherence and invasion by Opa+ bacteria of CD66 transfected HeLa cells: HeLa-BGPa, HeLa-CGM6, HeLa-NCA, HeLa-CGM1a, HeLa-CEA, and HeLa-Neo serving as negative control. Using OpaI as the prototype, the relative ability of the transfected HeLa cell lines to support adherence was (CEA = BGPa >CGM1a >NCA >>CGM6 = Neo). The ability to mediate invasion of the transfectant cells was (CGM1a >CEA >BGPa >NCA >CGM6 = Neo). Among the Opa proteins tested, OpaC proved to be bifunctional, able to mediate adherence to both syndecan receptors and to CD66 antigens.

"Peptabody": a new type of high avidity binding protein

A new type of high avidity binding molecule, termed "peptabody" was created by harnessing the effect of multivalent interaction. A short peptide ligand was fused via a semi-rigid hinge region with the coiled-coil assembly domain of the cartilage oligomeric matrix protein, resulting in a pentameric multivalent binding molecule. In the first peptabody (Pab-S) described here, a peptide (S) specific for the mouse B-cell lymphoma BCL1 surface Ig idiotype, was selected from a phage display library. A fusion gene was constructed encoding peptide S, followed by the 24 aa hinge region from camel IgG and a modified 55 aa cartilage oligomeric matrix protein pentamerization domain. The Pab-S fusion protein was expressed in Escherichia coli in a soluble form at high levels and purified in a single step by metal-affinity chromatography. Pab-S specifically bound the BCL1 surface idiotype with an avidity of about 1 nM, which corresponds to a 2 x 10(5)-fold increase compared with the affinity of the synthetic peptide S itself. Biochemical characterization showed that Pab-S is a stable homopentamer of about 85 kDa, with interchain disulfide bonds. Pab-S can be dissociated under denaturing and reducing conditions and reassociated as a pentamer with full-binding activity. This intrinsic feature provides an easy way to combine Pab molecules with two different peptide specificities, thus producing heteropentamers with bispecific and/or chelating properties.

ITIMs and ITAMs. The Yin and Yang of antigen and Fc receptor-linked signaling machinery

The initial stages of an immune response are regulated at the level of the cell-surface antigen and Fc receptors. The extracellular portions of these receptors provide immune specificity and determine the nature of the responding effector cells, whereas the intracellular portion transduces signals into the cell and determines the intensity and duration of the immune response. Recent studies led to the identification of two types of modules within the cytoplasmic region of receptor subunits that are critical for the activation and termination of signal transduction pathways. Phosphorylation of the conserved tyrosine residues within the two modules, the immunoreceptor tyrosine-based activation motif (ITAM) and the immunoreceptor tyrosine-based inhibition motif (ITIM), is followed by the recruitment of different sets of SH2-containing molecules to the receptor site. These proteins regulate the receptor-linked signal transduction pathways in a positive or a negative fashion, which is a reminiscent of the ancestral Yin-Yang principle.

Identification of a cytoplasmic Tyr-X-X-Leu motif essential for down regulation of the human cell receptor CD46 in persistent measles virus infection

To investigate the sequence requirements for measles virus (MV)-induced receptor down regulation, we transfected the human CD46 gene into simian cells persistently infected by the Biken strain of MV. Surface expression of CD46 is drastically reduced in these cells. Deletion analysis has shown that the juxtamembrane region of the CD46 cytoplasmic domain is essential for down regulation. Deleting a Tyr-Arg-Tyr-Leu sequence in this region or changing these residues to Ala prevents CD46 down regulation from the infected cell surface. Alanine-scanning mutagenesis has identified two amino acid residues, Tyr and Leu, forming a Tyr-X-X-Leu motif critical for CD46 down regulation. Mutations that prevent CD46 down regulation enhance syncytium formation. These results indicate that CD46 down regulation limits the cytopathic effects in a persistent MV infection and that CD46 down regulation requires a cytoplasmic Tyr-X-X-Leu sequence which resembles known motifs for membrane protein trafficking and receptor signalling.

Identification and characterization of a substrate specific for the T cell protein tyrosine kinase ZAP-70

ZAP-70 is a protein tyrosine kinase (PTK) that plays a critical role in T cell activation. To study the role of ZAP-70 catalytic activity in this process, a substrate capable of distinguishing between the activities of ZAP-70 and other PTKs would be useful, especially since it has recently been shown that ZAP-70 interacts with another T cell PTK, Lck. We have thus identified a site of phosphorylation on the cytoplasmic fragment of the erythrocyte band 3 protein that is recognized by ZAP-70, but not Lck. A synthetic peptide based on this site has been demonstrated to be a good in vitro substrate for ZAP-70 and a poor substrate for the T cell PTKs Lck and Itk. This peptide molecule should thus prove useful to many investigators working in the field of T cell activation.

In vivo and in vitro specificity of protein tyrosine kinases for immunoglobulin G receptor (FcgammaRII) phosphorylation

Human B cells express four immunoglobulin G receptors, FcgammaRIIa, FcgammaRIIb1, FcgammaRIIb2, and FcgammaRIIc. Coligation of either FcgammaRII isoform with the B-cell antigen receptor (BCR) results in the abrogation of B-cell activation, but only the FcgammaRIIa/c and FcgammaIIb1 isoforms become phosphorylated. To identify the FcgammaRII-phosphorylating protein tyrosine kinase (PTK), we used the combination of an in vitro and an in vivo approach. In an in vitro assay using recombinant cytoplasmic tails of the different FcgammaRII isoforms as well as tyrosine exchange mutants, we show that each of the BCR-associated PTKs (Lyn, Blk, Fyn, and Syk) shows different phosphorylation patterns with regard to the different FcgammaR isoforms and point mutants. While each PTK phosphorylated FcgammaRIIa/c, FcgammaRIIb1 was phosphorylated by Lyn and Blk whereas FcgammaRIIb2 became phosphorylated only by Blk. Mutants lacking both tyrosine residues of the immune receptor tyrosine-based activation motif (ITAM) of FcgammaRIIa/c were not phosphorylated by Blk and Fyn, while Lyn-mediated phosphorylation was dependent on the presence of the C-terminal tyrosine of the ITAM. Results obtained in assays using an FcgammaR- B-cell line transfected with wild-type or mutated FcgammaRIIa demonstrated that exchange of the C-terminal tyrosine of the ITAM of FcgammaRIIa/c was sufficient to abolish FcgammaRIIa/c phosphorylation in B cells. Additionally, we could show that Lyn and Fyn bind to FcgammaRIIa/c, with the ITAM being necessary for association. Comparison of the phosphorylation pattern of each PTK observed in vitro with the phosphorylation pattern observed in vivo suggests that Lyn is the most likely candidate for FcgammaRIIa/c and FcgammaRIIb1 phosphorylation in vivo.