Syk is activated by phosphotyrosine-containing peptides representing the tyrosine-based activation motifs of the high affinity receptor for IgE

Dominant-negative zeta-associated protein 70 inhibits T cell antigen receptor signaling

Zeta-associated protein (ZAP)-70 is a cytoplasmic protein tyrosine required for T cell antigen receptor (TCR) signaling and development. Mutations in ZAP-70 result in severe combined immunodeficiency in humans. ZAP-70 interacts with the TCR by binding to tyrosine-phosphorylated immunoreceptor tyrosine-based activation motifs (ITAMs) present in the invariant subunits of the TCR complex. Here we report that two ZAP-70 mutants devoid of kinase activity, generated either by a point mutation in the kinase domain to create an inactive kinase, or by truncation of the entire kinase domain (SH2[N+C]), functioned as dominant-negative mutants to specifically suppress TCR-mediated activation of NFAT, a nuclear factor essential for inducible interleukin 2 gene expression. Biochemical studies with the SH2(N+C) mutant showed that it also blocked early TCR signaling events, such as p95vav tyrosine phosphorylation, extracellular signal-regulated kinase 2 activation, and the association of a number of tyrosine-phosphorylated proteins with growth factor receptor-binding protein 2 (GRB2). The inhibitory effects of the SH2(N+C) mutant revealed that it requires an intact phosphotyrosine-binding site in its COOH-terminal SH2 domain. Using a CD8-zeta chimeric receptor to analyze the interaction of the SH2(N+C) mutant with ITAMs of TCR-zeta, we found that this mutant was constitutively bound to the hyperphosphorylated CD8-zeta chimera. These results indicate that tyrosine-phosphorylated ITAM is the target for the action of this dominant-negative mutant, suggesting that the assembly of a functional receptor signaling complex on ITAMs is a critical proximal TCR signaling event leading to downstream activation.

SH2 domains: a question of independence

The three-dimensional structures of parts of two enzymes that contain tandem Src homology 2 (SH2) domains have recently been determined. The structures suggest how the SH2 domains function in concert to regulate enzymatic activity and localization.

Spatial constraints on the recognition of phosphoproteins by the tandem SH2 domains of the phosphatase SH-PTP2

The domain organization of many signalling proteins facilitates a segregation of binding, catalytic and regulatory functions. The mammalian SH2 domain protein tyrosine phosphatases (PTPs) contain tandem SH2 domains and a single carboxy-terminal catalytic domain. SH-PTP1 (PTP1C, HCP) and SH-PTP2 (Syp, PTP2C, PTP1D) function downstream from tyrosine kinase-linked insulin, growth factor, cytokine and antigen receptors. As well as directing subcellular localization by binding to receptors and their substrates, the two SH2 domains of these PTPs function together to regulate catalysis. Here we report the structure of the tandem SH2 domains of SH-PTP2 in complex with monophosphopeptides. A fixed relative orientation of the two domains, stabilized by a disulphide bond and a small hydrophobic patch within the interface, separates the peptide binding sites by approximately 40 A. The defined orientation of the SH2 domains in the structure, and data showing that peptide orientation and spacing between binding sites is critical for enzymatic activation, suggest that spatial constraints are important in this multidomain protein-protein interaction.

The yeast tribrid system--genetic detection of trans-phosphorylated ITAM-SH2-interactions

Protein-protein interactions are often dependent on the post-translational modification of one component of a complex. To facilitate the study of these interactions in signal transduction, we have developed the yeast tribrid system, a modification of the yeast two-hybrid system. We demonstrate that the interactions are dependent upon the presence of a tyrosine kinase, an SH2 domain and a tyrosine containing substrate. Using the gamma subunit of the high-affinity IgE receptor, Fc epsilon RI, this approach has been used to isolate a novel SH2-containing family member. The mRNA encoding this novel protein is differentially expressed in rat tissues. The yeast tribrid system can be readily adapted for the characterization of novel tyrosine kinases or substrates, as well as the study of protein-protein interactions which involve other post-translational modifications.

Role of the Syk autophosphorylation site and SH2 domains in B cell antigen receptor signaling

To explore the mechanism(s) by which the Syk protein tyrosine kinase participates in B cell antigen receptor (BCR) signaling, we have studied the function of various Syk mutants in B cells made Syk deficient by homologous recombination knockout. Both Syk SH2 domains were required for BCR-mediated Syk and phospholipase C (PLC)-gamma 2 phosphorylation, inositol 1,4,5-triphosphate release, and Ca2+ mobilization. A possible explanation for this requirement was provided by findings that recruitment of Syk to tyrosine-phosphorylated immunoglobulin (Ig) alpha and Ig beta requires both Syk SH2 domains. A Syk mutant in which the putative autophosphorylation site (Y518/Y519) of Syk was changed to phenylalanine was also defective in signal transduction; however, this mutation did not affect recruitment to the phosphorylated immunoreceptor family tyrosine-based activation motifs (ITAMs). These findings not only confirm that both SH2 domains are necessary for Syk binding to tyrosine-phosphorylated Ig alpha and Ig beta but indicate that this binding is necessary for Syk (Y518/519) phosphorylation after BCR ligation. This sequence of events is apparently required for coupling the BCR to most cellular protein tyrosine phosphorylation, to the phosphorylation and activation of PLC-gamma 2, and to Ca2+ mobilization.

Syk mutation in Jurkat E6-derived clones results in lack of p72syk expression

The human leukemic Jurkat cell line is commonly used as a model cellular system to study T lymphocyte signal transduction. Various clonal derivatives of Jurkat T cells exist which display different characteristics with regard to responses to external stimuli. Among these, the E6-1 clone of Jurkat T cells has been used as a parental line from which numerous important somatic mutant clones have been generated. During the course of experiments examining signals initiated by the T cell antigen receptor in an E6-1-derived Jurkat cell clone J.CaM1, we observed that the 72-kilodalton Syk protein tyrosine kinase previously found in other Jurkat cells was not detected. Upon further analysis it was determined that Syk transcripts from the J.CaM1 cells as well as the parental E6-1 cells contain a single guanine nucleotide insertion at position 92. This nucleotide insertion results in a shift in the Syk open reading frame leading to alternate codon usage as well as the generation of a termination codon at position 109. Thus, Syk transcripts in E6-1 cells and E6-1-derived clones are predicted to be capable of encoding only the first 33 amino acids of the 630-amino acid wild type Syk. These findings are incompatible with a recently proposed model of T cell antigen receptor signal transduction based, in part, on experiments conducted using E6-1-derived cells, suggesting that Syk might play a role upstream of Lck and Zap70.

Why two heads are better

The tandem SH2 domains of the ZAP-70 kinase complexed with a doubly phosphorylated ligand reveal a mechanism for building a high-affinity interaction with very low non-specific binding.

Molecular basis for interaction of the protein tyrosine kinase ZAP-70 with the T-cell receptor

The crystal structure of the tandem SH2 domains of human ZAP-70 in complex with a peptide derived from the zeta-subunit of the T-cell receptor reveals an unanticipated interaction between the two domains. A coiled coil of alpha-helices connects the two SH2 domains, producing an interface that constitutes one of the two critical phosphotyrosine binding sites. These and other unique features provide the molecular basis for highly selective association of ZAP-70 with the T-cell receptor.