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

Dynamic control of signaling by modular adaptor proteins

Adaptor proteins are composed exclusively of domains and motifs that mediate molecular interactions, and can thereby link signaling proteins such as activated cell-surface receptors to downstream effectors. Recent data supports the notion that adaptors are not simply coupling devices that hard-wire successive components of signaling pathways. Rather, they display highly dynamic properties that direct the flow of information through signaling networks. The binding activity of adaptors can be regulated by conformational reorganization, and by the cooperative association of domains within the same adaptor. Furthermore, an individual adaptor can deliver different outputs by utilizing distinct combinations of binding partners. Adaptors can also control the oligomerization of receptor signaling complexes, and the subcellular location and duration of signaling events, and act as coincidence detectors to enhance specificity in cellular responses.

Spleen tyrosine kinase (Syk) as a novel target for allergic asthma and rhinitis

Allergic asthma and rhinitis are prevalent diseases in the modern world, both marked by inflammation of the airways. The spleen tyrosine kinase (Syk) plays a critical role in the regulation of such immune and inflammatory responses. Although Syk is best known as a key component of immunoreceptor signalling complexes in leukocytes, recent studies demonstrated Syk expression in cells outside the haematopoietic lineage. Moreover, in recent years, it has been established that Syk is involved in various signalling cascades including those originating from integrin and cytokine receptors. Thus, Syk likely has a much wider biological role than previously recognised. Specific inhibition of Syk using aerosolised antisense oligonucleotides in liposome complexes significantly decreased lung inflammatory responses in experimental asthma and acute lung injury models. In addition, pharmacological inhibitors of Syk have been recently developed with potential for use as therapeutics. However, in the development and the rational delivery of drugs targeting Syk, it is important to consider the multiple cell types that express this kinase and the potential effects of its inhibition on various physiological functions. This review focuses on the recent data and the emerging ideas about Syk as a therapeutic target.

Measurement of the formation of complexes in tyrosine kinase-mediated signal transduction

The use of isothermal titration calorimetry (ITC) provides a full thermodynamic characterization of an interaction in one experiment. The determination of the affinity is an important value; however, the additional layer of information provided by the change in enthalpy and entropy can help in understanding the biology. This is demonstrated with respect to tyrosine kinase-mediated signal transduction.

Isothermal titration calorimetry (ITC) provides highly complementary data to high-resolution structural detail. An overview of the methodology of the technique is provided. Ultimately, the correlation of the thermodynamic parameters determined by ITC with structural perturbation observed on going from the free to the bound state should be possible at an atomic level. Currently, thermodynamic data provide some insight as to potential changes occurring on complex formation. Here, this is demonstrated in the context of in vitro quantification of intracellular tyrosine kinase-mediated signal transduction and the issue of specificity of the important interactions. The apparent lack of specificity in the interactions of domains of proteins involved in early signalling from membrane-bound receptors is demonstrated using data from ITC.

Application of azide-alkyne cycloaddition 'click chemistry' for the synthesis of Grb2 SH2 domain-binding macrocycles

Copper (I) promoted [3+2] Huisgen cycloaddition of azides with terminal alkynes was used to prepare triazole-containing macrocycles based on the Grb2 SH2 domain-binding motif, 'Pmp-Ac(6)c-Asn', where Pmp and Ac(6)c stand for 4-phosphonomethylphenylalanine and 1-aminocyclohexanecarboxylic acid, respectively. When cycloaddition reactions were conducted at 1mM substrate concentrations, cyclization of monomeric units occurred. At 2mM substrate concentrations the predominant products were macrocyclic dimers. In Grb2 SH2 domain-binding assays the monomeric (S)-Pmp-containing macrocycle exhibited a K(d) value of 0.23microM, while the corresponding dimeric macrocycle was found to have greater than 50-fold higher affinity. The open-chain dimer was also found to have affinity equal to the dimeric macrocycle. This work represents the first application of 'click chemistry' to the synthesis of SH2 domain-binding inhibitors and indicates its potential utility.

CXCR4 physically associates with the T cell receptor to signal in T cells

SDF-1alpha (CXCL12) signaling via its receptor, CXCR4, stimulates T cell chemotaxis and gene expression. The ZAP-70 tyrosine kinase critically mediates SDF-1alpha-dependent migration and prolonged ERK mitogen-activated protein (MAP) kinase activation in T cells. However, the molecular mechanism by which CXCR4 or other G protein-coupled receptors activate ZAP-70 has not been characterized. Here we show that SDF-1alpha stimulates the physical association of CXCR4 and the T cell receptor (TCR) and utilizes the ZAP-70 binding ITAM domains of the TCR for signal transduction. This pathway is responsible for several of the effects of SDF-1alpha on T cells, including prolonged ERK MAP kinase activity, increased intracellular calcium ion concentrations, robust AP-1 transcriptional activity, and SDF-1alpha costimulation of cytokine secretion. These results suggest new paradigms for understanding the effects of SDF-1alpha and other chemokines on immunity.

Reading protein modifications with interaction domains

Proteins are controlled by a vast and dynamic array of post-translational modifications, many of which create binding sites for specific protein-interaction domains. We propose that these domains, working together, read the state of the proteome and therefore couple post-translational modifications to cellular organization. We also identify common strategies through which modification-dependent interactions synergize to regulate cell behaviour.

Spontaneous development of autoimmune arthritis due to genetic anomaly of T cell signal transduction: Part 1

A point mutation of the gene encoding ZAP-70, a key signal transduction molecule in T cells, results in spontaneous development of T cell-mediated autoimmune arthritis in mice homozygous for the mutation. The genetic anomaly alters differentiation and selection of T cells in the thymus, leading to thymic production of arthritogenic autoimmune T cells. The arthritogenic T cells persist in the periphery and elicit arthritis when activated by microbial agents that stimulate innate immunity. This model is instrumental in understanding how genetic variations in T cell signal transduction, together with environmental influences, contribute to the development of autoimmune disease.

Type II phosphatidylinositol 4-kinase β associates with TCR-CD3 ζ chain in Jurkat cells

Phosphatidylinositol lipid signaling cascades are integral part of TCR-CD3 signaling. The mechanisms by which phosphatidylinositol kinases are coupled to TCR-CD3 complex remain elusive. Here we report an association of type II PtdIns 4-kinase with TCR-CD3 zeta chain upon cross-linking. Mapping studies have revealed that the C-terminal ITAM is critical for docking of the enzyme on the zeta chain. The association is shown to be tyrosyl phosphorylation dependent as mutation of Y-151 and Y-142 on the C-terminal ITAM disrupts interaction of the two proteins. Identification of the associated type II PtdIns 4-kinase revealed that the beta isoform of the enzyme interacts with the zeta chain in vivo.

A role of kinase inactive ZAP-70 in altered peptide ligand stimulated T cell activation

T cell activation signals induced by altered peptide ligands (APLs) are different from those induced by the original agonistic peptide. The characteristics of the former are partial phosphorylation of TCR-zeta and no tyrosine-phosphorylation of zeta-associated protein-70 (ZAP-70). To analyze further those signaling pathways, we introduced a dominant negative (DN) form of ZAP-70 into a human CD4(+) T cell clone in which fully and partially agonistic peptide ligands have been well characterized. We found that some over-expressed partially agonistic ligands (OPALs) induced T cell responses without tyrosine-phosphorylation and kinase activation of ZAP-70. However, those responses were inhibited in T cells expressing DN ZAP-70, which could associate with partially phosphorylated TCR-zeta. In OPAL-stimulated T cells, PLC-gamma1 was phosphorylated and it was suppressed by DN ZAP-70 expression, suggesting that the ZAP-70-TCR-zeta association mediates the activation of PLC-gamma1 leading to T cell responses even in the absence of kinase activation of ZAP-70.

WW or WoW: The WW domains in a union of bliss

WW domains are small protein modules that recognize proline-rich peptide motifs or phosphorylated-serine/threonine proline sites in cognate proteins. Within host proteins these modules are joined to other protein domains or to a variety of catalytic domains acting together as adaptors or targeting anchors of enzymes. An important aspect of signaling by WW domains is their ability to recognize their cognate ligands in tandem. Tandem WW domains not only act in a synergistic manner but also appear to chaperone the function of each other. In this review, we focus on structure, function, and mechanism of the tandem WW domains co-operativity as well as independent actions. We emphasize here the implications of tandem arrangement and cooperative function of the domains for signaling pathways.

A quantitative protein interaction network for the ErbB receptors using protein microarrays

Although epidermal growth factor receptor (EGFR; also called ErbB1) and its relatives initiate one of the most well-studied signalling networks, there is not yet a genome-wide view of even the earliest step in this pathway: recruitment of proteins to the activated receptors. Here we use protein microarrays comprising virtually every Src homology 2 (SH2) and phosphotyrosine binding (PTB) domain encoded in the human genome to measure the equilibrium dissociation constant of each domain for 61 peptides representing physiological sites of tyrosine phosphorylation on the four ErbB receptors. This involved 77,592 independent biochemical measurements and provided a quantitative protein interaction network that reveals many new interactions, including ones that fall outside of our current view of domain selectivity. By slicing through the network at different affinity thresholds, we found surprising differences between the receptors. Most notably, EGFR and ErbB2 become markedly more promiscuous as the threshold is lowered, whereas ErbB3 does not. Because EGFR and ErbB2 are overexpressed in many human cancers, our results suggest that the extent to which promiscuity changes with protein concentration may contribute to the oncogenic potential of receptor tyrosine kinases, and perhaps other signalling proteins as well.

Protein Flexibility and Ligand Rigidity: A Thermodynamic and Kinetic Study of ITAM-Based Ligand Binding to Syk Tandem SH2

The Syk tandem Src homology 2 domain (Syk tSH2) constitutes a flexible protein module involved in the regulation of Syk kinase activity. The Syk tSH2 domain is assumed to function by adapting the distance between its two SH2 domains upon bivalent binding to diphosphotyrosine ligands. A thermodynamic and kinetic analysis of ligand binding was performed by using surface plasmon resonance (SPR). Furthermore, the effect of binding on the Syk tSH2 structural dynamics was probed by hydrogen/deuterium exchange and electrospray mass spectrometry (ESI-MS). Two ligands were studied: 1, a flexible peptide derived from the tSH2 recognition ITAM sequence at the gamma chain of the FcepsilonRI-receptor, and 2, a ligand in which the amino acids between the two SH2 binding motifs in ligand 1 have been replaced by a rigid linker of comparable length. Both ligands display comparable affinity for Syk tSH2 at 25 degrees C, yet a major difference in thermodynamics is observed. Upon binding of the rigid ligand, 2, the expected entropy advantage is not realized. On the contrary, 2 binds with a considerably higher entropy price of approximately 9 kcal mol-1, which is attributed to a further decrease in protein flexibility upon binding to this rigid ligand. The significant reduction in deuterium incorporation in the Syk tSH2 protein upon binding of either 1 or 2, as monitored by ESI-MS, indicates a major reduction in protein dynamics upon binding. The results are consistent with a two-step binding model: after an initial binding step, a rapid structural change of the protein occurs, followed by a second binding step. Such a bivalent binding model allows high affinity and fast dissociation kinetics, which are very important in transient signal-transduction processes.

Differential Src family kinase activity requirements for CD3 zeta phosphorylation/ZAP70 recruitment and CD3 epsilon phosphorylation

The current model of T cell activation is that TCR engagement stimulates Src family tyrosine kinases (SFK) to phosphorylate CD3zeta. CD3zeta phosphorylation allows for the recruitment of the tyrosine kinase ZAP70, which is phosphorylated and activated by SFK, leading to the phosphorylation of downstream targets. We stimulated mouse CTLs with plate-bound anti-CD3 and, after cell lysis, recovered proteins that associated with the CD3 complex. The protein complexes were not preformed, and a number of tyrosine-phosphorylated proteins were inducibly and specifically associated with the TCR/CD3 complex. These results suggest that complex formation only occurs at the site of TCR engagement. The recruitment and tyrosine phosphorylation of most proteins were abolished when T cells were stimulated in the presence of the SFK inhibitor PP2. Surprisingly, CD3zeta, but not CD3epsilon, was inducibly tyrosine phosphorylated in the presence of PP2. Furthermore, ZAP70 was recruited, but not phosphorylated, after TCR stimulation in the presence of PP2, thus confirming the phosphorylation status of CD3zeta. These data suggest that there is a differential requirement for SFK activity in phosphorylation of CD3zeta vs CD3epsilon. Consistent with this possibility, ZAP70 recruitment was also detected with anti-CD3-stimulated, Lck-deficient human Jurkat T cells. We conclude that TCR/CD3-induced CD3zeta phosphorylation and ZAP70 recruitment do not absolutely require Lck or other PP2-inhibitable SFK activity, but that SFK activity is absolutely required for CD3epsilon and ZAP70 phosphorylation. These data reveal the potential for regulation of signaling through the TCR complex by the differential recruitment or activation of SFK.

Binding of a diphosphorylated-ITAM peptide to spleen tyrosine kinase (Syk) induces distal conformational changes: a hydrogen exchange mass spectrometry study

Structural flexibility plays a crucial role in protein function. To assess whether specific structural changes are associated with the binding of an immunoreceptor tyrosine-based activation motif (ITAM) to the tandem Src homology-2 domains (tSH2) of the spleen tyrosine kinase [EC 2.7.7.112] (Syk), we used an approach based on protein hydrogen/deuterium exchange in the presence and absence of the diphosphorylated ITAM peptide. The protein deuterium uptake by the intact Syk protein was monitored in time by electrospray mass spectrometry, which revealed a dramatic relative decrease in deuterium uptake when the protein was bound to the ITAM peptide, suggesting an overall change in protein dynamics. Subsequently, the deuterium incorporation of individual segments of the protein was investigated using proteolysis and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) peptide mass-analysis, which revealed that several regions of Syk tSH2 are significantly more protected from exchange in the presence of the ITAM peptide. Four protected regions encompass the phosphotyrosine and hydrophobic binding sites on the SH2 domains, whereas two other protected regions are located in the inter-SH2 linker motif and do not make any direct contacts with the peptide. Interestingly, our data suggest that binding of the ITAM peptide to Syk tSH2 induces distal structural effects on the protein that stabilize the inter-SH2 linker region, possibly by raising the degree of helical structure upon binding.

The target discovery process

In order to minimise attrition rates in drug development projects, a target discovery process is implemented to select and characterise the most suitable candidate kinase targets, before lead identification and lead optimisation are embarked upon. The process consists of 1) target selection, 2) target assessment, and 3) target validation. This rational approach to target discovery, as a prerequisite for lead discovery, ensures that new therapeutic targets fulfil a set of general criteria, as well as indication-specific, descriptive and functional ones. The approach should ultimately maximise the likelihood of achieving target-selective inhibition by small-molecule inhibitors with minimal in vivo side effects and a therapeutic effect based on a sound biological hypothesis.

SYK expression in human breast cancer

Background

Syk (Splenic Tyrosine Kinase) is an intracellular receptor protein kinase involved in cell proliferation, differentiation and phagocytosis. It has been studied in T and B lymphocytes, NK cells and platelets. The strong expression of Syk in mammary gland prompted research into its potential role in mammary carcinogenesis. There have been very few studies about its role in breast cancer with conflicting results. This study aims to investigate the hypothesis that Syk expression is down-regulated in breast cancer compared with ANCT and the association between its expression and clinicopathological parameters.

Materials and methods

mRNA was extracted from 48 breast cancer specimens. Relative Syk to ribosomal RNA expression was determined by RT-PCR and Taqman methodology. Mann-Whitney U test was used to examine the association between Syk expression in cancer and ANCT. Spearman's rank correlation test was used to examine the association between Syk expression in tumours and patients' age, tumour size, tumour grade, estrogen and progesterone receptor status, lymph node metastasis, vascular invasion and clinical outcome.

Results

The median for the relative value of Syk expression was 0.17 and 0.18 (range: 0.12 – 0.56 and 0.0 – 1.77) for tumours and ANCT respectively. There was no significant association between Syk expression in cancers and ANCT (p= 0.598) nor between Syk expression in tumours and patients' age, tumour size, tumour grade, estrogen and progesterone receptor status, lymph node metastasis, vascular invasion or prognosis.

Conclusion

This study shows that Syk mRNA expression does not seem to vary between breast tumours and ANCT. Furthermore, we observed no significant association between Syk expression and clinicopathological parameters.

Syk tyrosine kinase participates in beta1-integrin signaling and inflammatory responses in airway epithelial cells

The protein tyrosine kinase Syk is critically involved in immunoreceptor signaling in hematopoietic cells. Recent studies demonstrate Syk expression in nonhematopoietic cells, including fibroblasts, endothelial cells, hepatocytes, and breast epithelium. However, the role of Syk in these cells is uncertain. We hypothesized that Syk is expressed in respiratory epithelial cells (EC) and that it functions as a signaling molecule involved in inflammatory responses in the epithelium. With the use of immunohistochemistry, Western blot, PCR, and laser scanning confocal microscopy, Syk was detected in human, rat, and mouse bronchial epithelium in situ and in cultured human bronchial EC in primary cells and the cell lines HS-24 and BEAS-2B. Syk-dependent signaling pathways in EC were initiated by engagement of beta1-integrin receptors. Stimulation of beta1-integrin receptors by fibronectin or antibody cross-linking caused redistribution of Syk from a cytoplasmic to plasma membrane localization. In stimulated cells, Syk and beta1-integrin colocalized. In addition, following beta1-integrin receptor engagement, tyrosine phosphorylation of Syk was observed. Expression of the intercellular adhesion molecule-1 (ICAM-1) and production of IL-6, both important molecules in lung inflammation, was downregulated in EC treated with Syk small interfering RNA or Syk inhibitor piceatannol. We propose that Syk is involved in signaling pathways induced by integrin engagement in airway EC. Syk-mediated signaling regulates IL-6 and ICAM-1 expression and may be important in the pathophysiology of lung inflammation.

The Tudor Tandem of 53BP1

53BP1 is a key transducer of the DNA damage checkpoint signal, which is required for phosphorylation of a subset of ATM substrates and p53 accumulation. After cell irradiation, the 53BP1 N-terminal region is phosphorylated. Its two C-terminal BRCT motifs interact with p53. Its central region is required and sufficient for 53BP1 foci formation at DNA strand breaks and for 53BP1 binding to the kinetochore. It contains an RG-rich segment and interacts with DNA in vitro. Here we show that the major globular domain of the 53BP1 central region adopts a new structural motif composed of two tightly packed Tudor domains and a C-terminal alpha helix. A unique surface essentially located on the first Tudor domain is involved in the binding to 53BP1 RG-rich sequence and to DNA, suggesting that the Tudor tandem can act as an adaptor mediating intramolecular as well as intermolecular protein-protein interactions and protein-nucleic acid associations.

The three-dimensional structure of the ZAP-70 kinase domain in complex with staurosporine: implications for the design of selective inhibitors

The ZAP-70 tyrosine kinase plays a critical role in T cell activation and the immune response and therefore is a logical target for immunomodulatory therapies. Although the crystal structure of the tandem Src homology-2 domains of human ZAP-70 in complex with a peptide derived from the zeta subunit of the T cell receptor has been reported (Hatada, M. H., Lu, X., Laird, E. R., Green, J., Morgenstern, J. P., Lou, M., Marr, C. S., Phillips, T. B., Ram, M. K., Theriault, K., Zoller, M. J., and Karas, J. L. (1995) Nature 377, 32-38), the structure of the kinase domain has been elusive to date. We crystallized and determined the three-dimensional structure of the catalytic subunit of ZAP-70 as a complex with staurosporine to 2.3 A resolution, utilizing an active kinase domain containing residues 327-606 identified by systematic N- and C-terminal truncations. The crystal structure shows that this ZAP-70 kinase domain is in an active-like conformation despite the lack of tyrosine phosphorylation in the activation loop. The unique features of the ATP-binding site, identified by structural and sequence comparison with other kinases, will be useful in the design of ZAP-70-selective inhibitors.

Conformation-dependent intermolecular interaction energies of the triphosphate anion with divalent metal cations. Application to the ATP-binding site of a binuclear bacterial enzyme. A parallel quantum chemical and polarizable molecular mechanics investigation

We have explored the conformation-dependent interaction energy of the triphosphate moiety, a key constituent of ATP and GTP, with a closed-shell divalent cation, Zn2+, used as a probe. This was done using the SIBFA polarizable molecular mechanics procedure. We have resorted to a previously developed approach in which triphosphate is built out from its elementary constitutive fragments, and the intramolecular, interfragment, interaction energies are computed simultaneously with their intermolecular interactions with the divalent cation. This approach has enabled reproduction of the values of the intermolecular interaction energies from ab initio quantum-chemistry with relative errors <3%. It was extended to the complex of a nonhydrolyzable analog of ATP with the active site of a bacterial enzyme having two Mg2+ cations as cofactors. We obtained following energy-minimization a very close overlap of the ATP analog over its position from X-ray crystallography. For models of the ATP analog-enzyme complex encompassing up to 169 atoms, the values of the SIBFA interaction energies were found to match their DFT counterparts with relative errors of <2%.

Identification of a novel isoform of ZAP-70, truncated ZAP kinase

We identified a novel cDNA encoding truncated ZAP-70, which lacked the SH2 domain and a part of interdomain B, and named it truncated ZAP kinase (TZK). TZK was expressed in the thymus, spleen, and lymph nodes with ZAP-70. TZK was expressed in CD44+CD25- thymocytes up to mature T cells, but ZAP-70 was not expressed in CD44+CD25- or CD44+CD25+ thymocytes. ZAP-70 or TZK was transfected into P116 cells derived from a Jurkat T-cell line deficient in ZAP-70. The P116 cells with ZAP-70 induced the T-cell receptor-mediated signal transduction, but the cells expressing TZK did not. While ZAP-70 was accumulated at the immune synapse, TZK was not. Meanwhile, impaired phosphorylation of SLP-76, one of the substrates of ZAP-70, in P116 cells upon pervanadate stimulation was rescued in the cells expressing TZK. These findings show that TZK is a novel isoform of ZAP-70, which is expressed in pre-T-cell receptor-minus thymocytes and functions as a kinase not associated with T-cell receptor.

ζ-Associated Protein of 70 kDa (ZAP-70), but Not Syk, Tyrosine Kinase Can Mediate Apoptosis of T Cells through the Fas/Fas Ligand, Caspase-8 and Caspase-3 Pathways

The TCR zeta-chain-associated protein of 70 kDA (ZAP-70) and Syk tyrosine kinases play critical roles in regulating TCR-mediated signal transduction. They not only share some overlapped functions but also may play unique roles in regulating the function and development of T cells. However, it is not known whether they have different effects on the activation and activation-induced cell death of T cells. To address this question, we generated cDNAs encoding chimeric molecules that a tailless TCR zeta-chain was directly linked to truncated ZAP-70 (Z/ZAP) or Syk (Z/Syk) molecules lacking the two Src homology 2 domains. Transfection of these molecules into zeta-chain-deficient cells restored their TCR expression. In addition, Z/ZAP and Z/Syk transfectants but not control cells demonstrated kinase activities in phosphorylating an exogenous substrate specific for ZAP-70 and Syk kinases. Z/ZAP transfectants activated through TCRs underwent a faster time course of apoptosis and had a greater percentage of apoptotic cells than that of Z/Syk and control cells. Activated Z/ZAP transfectants increased Fas and Fas ligand (FasL) expression 3- and 40-fold, respectively. Blocking of the Fas/FasL interaction could inhibit the apoptosis of Z/ZAP transfectants. In contrast, although activated Z/Syk transfectants could increase FasL expression, their Fas expression actually decreased and the percentage of apoptotic cells did not increase. Further studies of the mechanisms revealed that activation of Z/ZAP but not Z/Syk transfectants resulted in rapid activation of caspase-3 and caspase-8 that could also be inhibited by blocking Fas/FasL interaction. These results demonstrated that ZAP-70 and Syk play distinct roles in T cell activation and activation-induced cell death.

T-cell receptor signal transmission: who gives an ITAM?

T cells have an amazing ability to discern and differentially respond to MHC-embedded peptides that can differ by only a single amino acid. This potential involves a combination of the precise ligand-binding specificities of the T-cell receptor (TCR) and the distinct intracellular signaling processes it transmits. Signaling processes are controlled by the ten immunoreceptor tyrosine-based activation motifs (ITAMs) present in the invariant chains of the TCR complex (TCR zeta and CD3-gamma, -delta and -epsilon ). Here, we discuss recent studies of the functions of TCR invariant chains and the contribution of the ten ITAMs to T-cell signal transmission. We incorporate these results into two non-exclusive models of TCR signal transduction: the ITAM multiplicity model, which describes a functional redundancy within the TCR zeta and CD3 ITAMs; and the differential signaling model, which proposes distinct functions for the CD3-gamma, -delta and -epsilon and TCR zeta modules.

PTPH1 is a predominant protein-tyrosine phosphatase capable of interacting with and dephosphorylating the T cell receptor zeta subunit

Protein-tyrosine phosphatases (PTPases) play key roles in regulating tyrosine phosphorylation levels in cells, yet the identity of their substrates remains limited. We report here on the identification of PTPases capable of dephosphorylating the phosphorylated immune tyrosine-based activation motifs present in the T cell receptor zeta subunit. To characterize these PTPases, we purified enzyme activities directed against the phosphorylated T cell receptor zeta subunit by a combination of anion and cation chromatography procedures. A novel ELISA-based PTPase assay was developed to rapidly screen protein fractions for enzyme activity following the various chromatography steps. We present data that SHP-1 and PTPH1 are present in highly enriched protein fractions that exhibit PTPase activities toward a tyrosine-phosphorylated TCR zeta substrate (specific activity ranging from 0.23 to 40 pmol/min/microg). We also used a protein-tyrosine phosphatase substrate-trapping library comprising the catalytic domains of 47 distinct protein-tyrosine phosphatases, representing almost all the tyrosine phosphatases identified in the human genome. PTPH1 was the predominant phosphatase capable of complexing phospho-zeta. Subsequent transfection assays indicated that SHP-1 and PTPH1 are the two principal PTPases capable of regulating the phosphorylation state of the TCR zeta ITAMs, with PTPH1 directly dephosphorylating zeta. This is the first reported demonstration that PTPH1 is a candidate PTPase capable of interacting with and dephosphorylating TCR zeta.

Critical role of Ser-520 phosphorylation for membrane recruitment and activation of the ZAP-70 tyrosine kinase in T cells

Regulation of protein tyrosine kinases (PTKs) by tyrosine phosphorylation is well recognized; in fact, nearly all PTKs require phosphorylation of tyrosine residues in their "activation loop" for catalytic activity. In contrast, the phosphorylation of PTKs on serine and threonine residues has not been studied nearly as much. We report that the ZAP-70 PTK contains predominately phosphoserine in normal T lymphocytes as well as in Jurkat T leukemia cells. We have identified one site of phosphorylation as Ser-520 and find this site to be important for the recruitment and activation of ZAP-70 in T cells. Mutant ZAP-70-S520A had reduced ability to autophosphorylate and to mediate antigen receptor-induced interleukin 2 gene activation and was not enriched at the plasma membrane. These defects were rescued by addition of a myristylation signal to the N terminus of ZAP-70-S520A to force its plasma membrane and lipid raft localization. We conclude that phosphorylation of ZAP-70 at Ser-520 plays an important role in the correct localization of ZAP-70 and in priming ZAP-70 for its acute recruitment and activation upon antigen receptor ligation.

Tandem PDZ repeats in glutamate receptor–interacting proteins have a novel mode of PDZ domain–mediated target binding

The interaction of the glutamate receptor subunits 2 and 3 (GluR2/3) with multi-PDZ domain glutamate receptor-interacting protein (GRIP) is important for the synaptic trafficking and clustering of the receptors. Binding of GluR2/3 to GRIP requires both the fourth and fifth PDZ domains (PDZ4 and PDZ5) to be covalently linked, although only one PDZ domain is directly involved in binding to the receptor tail. To elucidate the molecular basis of this mode of PDZ domain-mediated target recognition, we solved the solution structures of the PDZ45 tandem and the isolated PDZ4 of GRIP. The two PDZ domains form a compact structure with a fixed interdomain orientation. The interdomain packing and the stable folding of both PDZ domains require a short stretch of amino acids N-terminal to PDZ4 and a conserved linker connecting PDZ4 and PDZ5. PDZ4 contains a deformed aB-bB groove that is unlikely to bind to carboxyl peptides. Instead, the domain stabilizes the structure of PDZ5.

SH2 and PTB domains in tyrosine kinase signaling

Intracellular signaling pathways that involve protein tyrosine kinases (PTKs) are critical for the control of most cellular processes. Dysfunctions in PTKs, or in the signaling pathways that they regulate, result in a variety of diseases such as cancer, diabetes, immune deficiency, and many others. SH2 (Src homology region 2) and PTB (phosphotyrosine-binding) domains are small protein modules that mediate protein-protein interactions involved in many signal transduction pathways. Both domains were initially identified as modules that recognize phosphorylated tyrosines in receptor tyrosine kinases and other signaling proteins. Subsequent studies have shown that, while binding of SH2 domains to their target proteins is strictly regulated by tyrosine phosphorylation, most PTB domains actually bind to their (nonphosphorylated) targets constitutively. The functions of SH2 and PTB domains include targeting of their host proteins to different cellular compartments, assembly of key components of signaling pathways in response to extracellular signals, and the control of autoinhibition, activation and dimerization of their host proteins. The information flow from the cell surface to different cellular compartments to regulate the cell cycle, cell shape and movement, cell proliferation, differentiation and cell survival are all controlled in part by SH2 and PTB domains that can recognize phosphotyrosine or particular amino acid sequence motifs in a wide variety of target molecules.

Molecular recognition by SH2 domains

In this chapter, we have described the biophysical investigations which have dissected the mechanisms of SH2 domain function. Due to nearly a decade and a half of investigation on SH2 domains, much about their binding mechanism has been characterized. SH2 domains have been found to have a positively charged binding cavity, largely conserved between different SH2 domains, which coordinates binding of the pTyr in the target. The ionic interactions between this pocket and the pTyr, in particular, between Arg beta B5 and the phosphate, provide the majority of the binding energy stabilizing SH2 domain-target interactions. The specificity in SH2 domain-target interactions emanates most often from the interactions between the residues C-terminal to the pTyr in the target and the specificity determining residues in the C-terminal half of the SH2 domain. However, the interactions in the specificity determining region of SH2 domains are weak, and hence single SH2 domains show only a modest level of specificity for tyrosine phosphorylated targets. Greater specificity in SH2 domain-containing protein-tyrosine phosphorylated target interactions can be achieved by placing SH2 domains in tandem (as is often found) or possibly through specific localization of SH2 domain-containing proteins within the cell. Although a relatively good understanding of how SH2 domains function in isolation has been obtained, the ways in which SH2 domain binding is coupled to allosteric transmission of signals in larger SH2 domain-containing proteins are still not clear. Hence, the future should bring further investigations of the mechanisms by which SH2 domain ligation alters the enzymatic activity and cellular localization of SH2 domain-containing proteins.

Supramodular structure and synergistic target binding of the N-terminal tandem PDZ domains of PSD-95

PDZ domain proteins play critical roles in binding, clustering and subcellular targeting of membrane receptors and ion channels. PDZ domains in multi-PDZ proteins often are arranged in groups with highly conserved spacing and intervening sequences; however, the functional significance of such tandem arrangements of PDZs is unclear. We have solved the three-dimensional structure of the first two PDZ domains of postsynaptic density protein-95 (PSD-95 PDZ1 and PDZ2), which are closely linked to each other in the PSD-95 family of scaffold proteins. The two PDZs have limited freedom of rotation and their C-terminal peptide-binding grooves are aligned with each other with an orientation preference for binding to pairs of C termini extending in the same direction. Increasing the spacing between PDZ1 and PDZ2 resulted in decreased binding between PDZ12 and its dimeric targets. The same mutation impaired the functional ability of PSD-95 to cluster Kv1.4 potassium channels in heterologous cells. The data presented provide a molecular basis for preferential binding of PSD-95 to multimeric membrane proteins with appropriate C-terminal sequences.

The formation and functions of the 21- and 23-kDa tyrosine-phosphorylated TCR zeta subunits

The interaction between the T cell receptor (TCR) and its cognate antigen/major histocompatibility complex (MHC) complex activates a cascade of intracellular protein phosphorylations within the T cell. The signals are initiated by the specific phosphorylation of two tyrosine residues located in a conserved sequence motif termed an ITAM (immune receptor-based tyrosine activation motif). There are 10 ITAMs in the TCR complex, and 6 of these ITAMs are present in the TCR zeta homodimer. Following TCR stimulation, the TCR zeta subunit forms two tyrosine-phosphorylated intermediates of 21- and 23-kDa, respectively. The dramatic and diverse biological responses of T cells are proposed to be partly regulated by the relative ratios of the 21- vs. 23-kDa phosphorylated forms of TCR zeta that are induced following TCR ligation. In this review, we describe a stepwise model of zeta phosphorylation required for the formation of these two phosphorylated derivatives. We describe the kinases and phosphatases controlling these phosphorylation processes. In addition, we present some preliminary findings from ongoing studies that discuss the contributions of each phosphorylated form of zeta on T cell development, TCR signaling, T cell anergy induction, and T cell survival.

Crystal structure and NMR studies of the apo SH2 domains of ZAP-70: two bikes rather than a tandem

The protein kinase ZAP-70 is involved in T-cell activation, and interacts with tyrosine-phosphorylated peptide sequences known as immunoreceptor tyrosine activation motifs (ITAMs), which are present in three of the subunits of the T-cell receptor. We have studied the tandem SH2 (tSH2) domains of ZAP-70, by both X-ray and NMR. Here, we present the crystal structure of the apoprotein, i.e., the tSH2 domain in the absence of ITAM. Comparison with the previously reported complex structure reveals that binding to the ITAM peptide induces surprisingly large movements between the two SH2 domains and within the actual binding sites. The conformation of the ITAM-free protein is partly governed by a hydrophobic cluster between the linker region and the C-terminal SH2 domain. Our data suggest that the two SH2 domains are able to undergo large interdomain movements. The proposed relative flexibility of the SH2 domains is further supported by the finding that no NMR signals could be detected for the two helices connecting the SH2 domains; these are likely to be broadened beyond detection due to conformational exchange. It is likely that this conformational reorientation induced by ITAM binding is the main signaling event activating the kinase domain in ZAP-70. Another NMR observation was that the N-terminal SH2 domain could bind tetrapeptides derived from the ITAM sequence, apparently without the need to interact with the C-terminal domain. In contrast, the C-terminal domain has little affinity for tetrapeptides. The opposite situation is true for binding to plain phosphotyrosine, where the C-terminal domain has a higher affinity. Distinct features in the crystal structure, showing the interdependence of both domains, explain these binding data.

Phosphorylation of Tyr342 in the linker region of Syk is critical for Fc epsilon RI signaling in mast cells

The linker region of Syk and ZAP70 tyrosine kinases plays an important role in regulating their function. There are three conserved tyrosines in this linker region; Tyr317 of Syk and its equivalent residue in ZAP70 were previously shown to negatively regulate the function of Syk and ZAP70. Here we studied the roles of the other two tyrosines, Tyr342 and Tyr346 of Syk, in Fc epsilon RI-mediated signaling. Antigen stimulation resulted in Tyr342 phosphorylation in mast cells. Syk with Y342F mutation failed to reconstitute Fc epsilon RI-initiated histamine release. In the Syk Y342F-expressing cells there was dramatically impaired receptor-induced phosphorylation of multiple signaling molecules, including LAT, SLP-76, phospholipase C-gamma2, but not Vav. Compared to wild-type Syk, Y342F Syk had decreased binding to phosphorylated immunoreceptor tyrosine-based activation motifs and reduced kinase activity. Surprisingly, mutation of Tyr346 had much less effect on Fc epsilon RI-dependent mast cell degranulation. An anti-Syk-phospho-346 tyrosine antibody indicated that antigen stimulation induced only a very minor increase in the phosphorylation of this tyrosine. Therefore, Tyr342, but not Tyr346, is critical for regulating Syk in mast cells and the function of these tyrosines in immune receptor signaling appears to be different from what has been previously reported for the equivalent residues of ZAP70.

The N-terminal SH2 domains of Syk and ZAP-70 mediate phosphotyrosine-independent binding to integrin beta cytoplasmic domains

Syk and ZAP-70 form a subfamily of nonreceptor tyrosine kinases that contain tandem SH2 domains at their N termini. Engagement of these SH2 domains by tyrosine-phosphorylated immunoreceptor tyrosine-based activation motifs leads to kinase activation and downstream signaling. These kinases are also regulated by beta3 integrin-dependent cell adhesion via a phosphorylation-independent interaction with the beta3 integrin cytoplasmic domain. Here, we report that the interaction of integrins with Syk and ZAP-70 depends on the N-terminal SH2 domain and the interdomain A region of the kinase. The N-terminal SH2 domain alone is sufficient for weak binding, and this interaction is independent of tyrosine phosphorylation of the integrin tail. Indeed, phosphorylation of tyrosines within the two conserved NXXY motifs in the integrin beta3 cytoplasmic domain blocks Syk binding. The tandem SH2 domains of these kinases bind to multiple integrin beta cytoplasmic domains with varying affinities (beta3 (Kd = 24 nm) > beta2 (Kd = 38 nm) > beta1 (Kd = 71 nm)) as judged by both affinity chromatography and surface plasmon resonance. Thus, the binding of Syk and ZAP-70 to integrin beta cytoplasmic domains represents a novel phosphotyrosine-independent interaction mediated by their N-terminal SH2 domains.

Signal transduction mediated by the T cell antigen receptor: the role of adapter proteins

Engagement of the T cell antigen receptor (TCR) leads to a complex series of molecular changes at the plasma membrane, in the cytoplasm, and at the nucleus that lead ultimately to T cell effector function. Activation at the TCR of a set of protein tyrosine kinases (PTKs) is an early event in this process. This chapter reviews some of the critical substrates of these PTKs, the adapter proteins that, following phosphorylation on tyrosine residues, serve as binding sites for many of the critical effector enzymes and other adapter proteins required for T cell activation. The role of these adapters in binding various proteins, the interaction of adapters with plasma membrane microdomains, and the function of adapter proteins in control of the cytoskeleton are discussed.

Dancing with multiple partners

Transmembrane proteins, such as G protein-coupled receptors (GPCRs) and integrins, activate intracellular signaling pathways through interactions with downstream binding partners. Woodside discusses two examples in which GPCRs and integrins interact in a noncompeting manner with more than one partner. The specific GPCR described is the thrombin receptor, in experiments where G protein peptides selectively block signaling through a particular G protein that does not appear to inhibit coupling of the receptor to other G proteins. The second system described is the alphaIIbbeta3 integrin and its activation of the nonreceptor tyrosine kinase Syk. Syk appeared capable of interacting with both the integrin and intracellular domains of immune response receptors, because binding of Syk to the integrin was not inhibited by peptides based on the Syk binding site in immune response receptors. Thus, multiple, noncompeting binding partners add to the complexity of signal transduction outputs from a single receptor complex.

Molecular mechanisms for protein kinase A-mediated modulation of immune function

Protein kinase A (PKA) is a serine/threonine kinase that regulates a number of cellular processes important for immune activation and control. Modulation of signal transduction by PKA is a complex and diverse process, and differential isozyme expression, holoenzyme composition and subcellular localization contribute specificity to the PKA signalling pathway. In lymphocytes, phosphorylation by PKA has been demonstrated to regulate antigen receptor-induced signalling both by altering protein-protein interactions and by changing the enzymatic activity of target proteins. PKA substrates involved in immune activation include transcription factors, members of the MAP kinase pathway and phospholipases. The ability of PKA type I to regulate activation of signalling components important for formation of the immunological synapse, demonstrates that the cAMP signalling pathway can directly modulate proximal events in lymphocyte activation. Furthermore, the recent discovery that PKA regulates Src kinases through modulation of Csk, supports the notion that PKA is involved in the fine-tuning of immune receptor signalling in lipid rafts.

A repertoire library that allows the selection of synthetic SH2s with altered binding specificities

Tyrosine phosphorylation is one of the major mechanisms involved in the intracellular propagation of external signals. Strategies aimed at interfering with this process might allow the control of several cellular phenotypes. SH2 domains mediate protein-protein interactions by recognizing phosphotyrosine (pY) residues in the context of specific phosphopeptides. We created an SH2-scaffolded repertoire library by randomly mutagenizing five critical amino acid positions in the specificity-determining region of the PLCgamma C-terminal SH2 domain. Synthetic SH2 domains were selected from the library using biotinylated phosphopeptides derived from a natural PLCgamma-SH2 ligand as well as unrelated SH2 ligands. The isolated SH2s displayed high binding affinity constants for the selecting peptides and were capable of interacting with the corresponding proteins.

Requirement for tyrosine residues 315 and 319 within zeta chain-associated protein 70 for T cell development

Engagement of the T cell antigen receptor (TCR) induces the transphosphorylation of the zeta chain-associated protein of 70,000 Mr (ZAP-70) protein tyrosine kinase (PTK) by the CD4/8 coreceptor associated Lck PTK. Phosphorylation of Tyr 493 within ZAP-70's activation loop results in the enzymatic activation of ZAP-70. Additional tyrosines (Tyrs) within ZAP-70 are phosphorylated that play both positive and negative regulatory roles in TCR function. Phosphorylation of Tyr residues (Tyrs 315 and 319) within the Interdomain B region of the ZAP-70 PTK plays important roles in the generation of second messengers after TCR engagement. Here, we demonstrate that phosphorylation of these two Tyr residues also play important roles in mediating the positive and negative selection of T cells in the thymus.

Release from tonic inhibition of T cell activation through transient displacement of C-terminal Src kinase (Csk) from lipid rafts

In resting peripheral T cells, Csk is constitutively present in lipid rafts through an interaction with the Csk SH2-binding protein, PAG, also known as Cbp. Upon triggering of the T cell antigen receptor (TCR), PAG/Cbp is rapidly dephosphorylated leading to dissociation of Csk from lipid rafts. However, tyrosine phosphorylation of PAG/Cbp resumes after 3--5 min, at which time Csk reassociates with the rafts. Cells overexpressing a mutant Csk that lacks the catalytic domain, but displaces endogenous Csk from lipid rafts, have elevated basal levels of TCR-zeta-chain phosphorylation and spontaneous activation of an NFAT-AP1 reporter from the proximal interleukin-2 promoter as well as stronger and more sustained responses to TCR triggering than controls. We suggest that a transient release from Csk-mediated inhibition by displacement of Csk from lipid rafts is important for normal T cell activation.

Src inhibitors: genomics to therapeutics

Following the milestone discoveries that identified Src as the first known protein tyrosine kinase and as a prototype oncogene, as well as Src transgenic studies to validate it as a promising therapeutic target for osteoporosis, intense efforts are being made to create Src inhibitor drugs. Drug discovery strategies focused on both the non-catalytic and catalytic domains of Src have successfully resulted in promising Src inhibitor lead compounds with potential therapeutic applications for osteoporosis, cancer, and other diseases. Some noteworthy examples of Src inhibitors are described, and their chemical diversity, structure-based design, and biological activities in vitro and in vivo are illustrated. The potency, selectivity, and in vivo efficacy of key Src inhibitors are being investigated in molecular, cellular and animal models. Consequently, Src inhibitor drug development is imminent, and current studies are well-poised to achieve the ultimate milestone of a Src inhibitor therapeutic.

Intracellular signaling by the killer immunoglobulin-like receptors and Ly49

Once thought to be promiscuous killers, it is now known that natural killer (NK) cells possess an elaborate array of receptors that regulate NK cytotoxic and secretory functions upon interaction with target cell MHC class I proteins. These receptors, known as killer cell immunoglobulin-like receptors (KIRs) in humans, and Ly49 receptors in the mouse, have become the focus of intense study in an effort to discern the underlying biology of these large receptor families. These receptor families include both inhibitory and activating receptors. Interrogation of a target expressing KIR ligands leads to coengagement of the inhibitory receptor with as-yet poorly defined activation receptors. Kinases activated during engagement mediate the phosphorylation of the KIR or Ly49 cytoplasmic immunoreceptor tyrosine-based inhibitory motifs (ITIMs). The phosphorylated ITIMs serve as efficient recruitment points for the cytosolic protein tyrosine phosphatases, SHP-1 and SHP-2, resulting in the dephosphorylation of substrates critical for cellular activation. In contrast, some KIRs and Ly49s lack the ITIM and possess a charged residue in their transmembrane domains that mediates interaction with the DAP12 signal transduction chain. DAP12 uses its cytoplasmic immunoreceptor tyrosine-based activation motif (ITAM) to mediate cellular activation. Engagement of a DAP12 coupled KIR or Ly49 results in phosphorylation of DAP12, and other key substrates, including the Syk tryosine kinase, phospholipase C, and c-Cbl. DAP12 activation then leads to the Mapk cascade and ultimately to enhanced degranulation, and production of cytokines and chemokines. Although the context in which inhibitory and activating KIR and Ly49s function is not yet known, the dissection of the activating and inhibitory signal transduction pathways should shed light on their method of integration into the activation sequela of NK cells. Ultimately, this work will lead to concrete understanding of the immunobiology of these seemingly antagonistic receptor systems.

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

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

Coevolution of the domains of cytoplasmic tyrosine kinases

Many signaling molecules are multidomain proteins that have other domains in addition to the catalytic kinase domain. Protein tyrosine kinases almost without exception contain Src homology 2 (SH2) and/or SH3 domains that can interact with other signaling proteins. Here, we studied evolution of the tyrosine kinases containing SH2 and/or SH3 and kinase domains. The three domains seem to have duplicated together, since the phylogenetic analysis using parsimony gave almost identical evolutionary trees for the separate domains and the multidomain complexes. The congruence analysis of the sequences for the separate domains also suggested that the domains have coevolved. There are several reasons for the domains to appear in a cluster. Kinases are regulated in many ways, and the presence of SH2 and SH3 domains at proper positions is crucial. Because all three domains can recognize different parts of ligands and substrates, their evolution has been interconnected. The reasons for the clustering and coevolution of the three domains in protein tyrosine kinases (PTKs) are discussed.

A soluble LAT deletion mutant inhibits T-cell activation: reduced recruitment of signalling molecules to glycolipid-enriched microdomains

The type III transmembrane adaptor protein linker for activation of T cells (LAT) is essential for membrane recruitment of signalling molecules following TCR activation. Here we show that although LAT deleted in the transmembrane domain is completely soluble, it can be tyrosine phosphorylated after anti-CD3 stimulation or pervanadate treatment. Overexpression of this deletion mutant in transiently transfected Jurkat TAg cells inhibits transcriptional activation of nuclear factor of activated T cells (NF-AT)/AP-1 reporter construct in a concentration-dependent manner. Furthermore, by selection of transiently transfected cells, a clear reduction of TCR-induced CD69 expression was observed in cells expressing the mutant. These dominant negative effects seemed to be dependent both on the ability of the membrane deletion mutant to reduce phosphorylation of endogenous LAT and to reduce interaction of endogenous LAT with PLC-gamma1 and Grb2. Consistent with this, the redistribution of PLC-gamma1 and Grb2 to glycolipid-enriched microdomains, called lipid rafts, after stimulation was inhibited when the soluble form of LAT was overexpressed. We suggest that the dominant negative effect is caused by the ability of the mutant to sequester signalling molecules in cytosol and thereby inhibit redistribution of signalling molecules to lipid rafts upon T-cell activation.

Signal-transduction defects in T cells

In conclusion, multiple receptors and signal transduction cascades influence T-cell function and fate. During the past few years many of these important aspects of T-cell biology were identified. The complexity of the various signaling pathways has made appreciation of their clinical significance difficult. One way of studying the function of these molecules is to create mice deficient of these components. However, frequently the murine phenotype is far from reflecting the homologous human deficiency. It is therefore beneficial to define the human immunodeficiencies in order to understand the role of a certain signaling molecule in humans. Further, mutations that result in partial deficiencies may result in a different phenotype from null mutations. This information may aid in improving structure/function analysis of these signaling components.