Lysine 271 in the transmembrane domain of the T-cell antigen receptor beta chain is necessary for its assembly with the CD3 complex but not for alpha/beta dimerization

Identification, expression and molecular polymorphism of T-cell receptors α and β from the glacial relict Hucho bleekeri

The T-cell receptor (TCR) is a specific molecule on the surface of all T cells that mediates cellular adaptive immune responses to antigens. Hucho bleekeri is a critically endangered species and is regarded as a glacial relict that has the lowest-latitude distribution compared with any Eurasian salmonid. In the present study, two TCR genes, namely, TCR α and β, were identified and characterized in H. bleekeri. Both TCR α and TCR β have typical TCR structures, including the IgV domain, IgC domain, connecting peptide, transmembrane and cytoplasmic domains. The two TCR genes were constitutionally expressed in various tissues, with the highest expression found in the spleen for TCR α and in the trunk kidney for TCR β. Challenge of H. bleekeri with LPS or poly(I:C) resulted in significant upregulation of both TCR α and β expression in headkidney and spleen primary cells, indicating their potential roles in the immune response. Molecular polymorphism analysis of the whole ORF regions of TCR α and β in different individuals revealed high diversity of IgV domains of these two genes, especially in complementarity-determining region (CDR) 3. The ratio of nonsynonymous substitution occurred at a significantly higher frequency than synonymous substitution in the CDR of TCR α and β, demonstrating the existence of positive selection. The results obtained in the present study enhance our understanding of TCR roles in regulating immune mechanisms and provide new information for the study of TCR lineage diversity in fish.

Allosteric activation of T cell antigen receptor signaling by quaternary structure relaxation

The mechanism of T cell antigen receptor (TCR-CD3) signaling remains elusive. Here, we identify mutations in the transmembrane region of TCRβ or CD3ζ that augment peptide T cell antigen receptor (pMHC)-induced signaling not explicable by enhanced ligand binding, lateral diffusion, clustering, or co-receptor function. Using a biochemical assay and molecular dynamics simulation, we demonstrate that the gain-of-function mutations loosen the interaction between TCRαβ and CD3ζ. Similar to the activating mutations, pMHC binding reduces TCRαβ cohesion with CD3ζ. This event occurs prior to CD3ζ phosphorylation and at 0°C. Moreover, we demonstrate that soluble monovalent pMHC alone induces signaling and reduces TCRαβ cohesion with CD3ζ in membrane-bound or solubilised TCR-CD3. Our data provide compelling evidence that pMHC binding suffices to activate allosteric changes propagating from TCRαβ to the CD3 subunits, reconfiguring interchain transmembrane region interactions. These dynamic modifications could change the arrangement of TCR-CD3 boundary lipids to license CD3ζ phosphorylation and initiate signal propagation.

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Mutations in TCRβ and CD3ζ TMRs that reduce their interaction augment signaling

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pMHC and anti-CD3 binding to TCR-CD3 induce similar quaternary structure relaxation

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Soluble monovalent pMHC alone signals and reduces TCRαβ cohesion with CD3ζ

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Allosteric changes in TCR-CD3 dynamics instigate T cell activation

Lysosomal targeting of the ABC transporter TAPL is determined by membrane-localized charged residues

The human lysosomal polypeptide ABC transporter TAPL (ABC subfamily B member 9, ABCB9) transports 6-59-amino-acid-long polypeptides from the cytosol into lysosomes. The subcellular localization of TAPL depends solely on its N-terminal transmembrane domain, TMD0, which lacks conventional targeting sequences. However, the intracellular route and the molecular mechanisms that control TAPL localization remain unclear. Here, we delineated the route of TAPL to lysosomes and investigated the determinants of single trafficking steps. By synchronizing trafficking events by a retention using selective hooks (RUSH) assay and visualizing individual intermediate steps through immunostaining and confocal microscopy, we demonstrate that TAPL takes the direct route to lysosomes. We further identified conserved charged residues within TMD0 transmembrane helices that are essential for individual steps of lysosomal targeting. Substitutions of these residues retained TAPL in the endoplasmic reticulum (ER) or Golgi. We also observed that for release from the ER, a salt bridge between Asp-17 and Arg-57 is essential. An interactome analysis revealed that Yip1-interacting factor homolog B membrane-trafficking protein (YIF1B) interacts with TAPL. We also found that YIF1B is involved in ER-to-Golgi trafficking and interacts with TMD0 of TAPL via its transmembrane domain and that this interaction strongly depends on the newly identified salt bridge within TMD0. These results expand our knowledge about lysosomal trafficking of TAPL and the general function of extra transmembrane domains of ABC transporters.

The T Cell Antigen Receptor α Transmembrane Domain Coordinates Triggering through Regulation of Bilayer Immersion and CD3 Subunit Associations

Initial molecular details of cellular activation following αβT cell antigen receptor (TCR) ligation by peptide-major histocompatibility complexes (pMHC) remain unexplored. We determined the nuclear magnetic resonance (NMR) structure of the TCRα subunit transmembrane (TM) domain revealing a bipartite helix whose segmentation fosters dynamic movement. Positively charged TM residues Arg251 and Lys256 project from opposite faces of the helix, with Lys256 controlling immersion depth. Their modification caused stepwise reduction in TCR associations with CD3ζζ homodimers and CD3εγ plus CD3εδ heterodimers, respectively, leading to an activated transcriptome. Optical tweezers revealed that Arg251 and Lys256 mutations altered αβTCR-pMHC bond lifetimes, while mutations within interacting TCRα connecting peptide and CD3δ CxxC motif juxtamembrane elements selectively attenuated signal transduction. Our findings suggest that mechanical forces applied during pMHC ligation initiate T cell activation via a dissociative mechanism, shifting disposition of those basic sidechains to rearrange TCR complex membrane topology and weaken TCRαβ and CD3 associations.

Cell Biology of T Cell Receptor Expression and Regulation

T cell receptors (TCRs) are protein complexes formed by six different polypeptides. In most T cells, TCRs are composed of αβ subunits displaying immunoglobulin-like variable domains that recognize peptide antigens associated with major histocompatibility complex molecules expressed on the surface of antigen-presenting cells. TCRαβ subunits are associated with the CD3 complex formed by the γ, δ, ε, and ζ subunits, which are invariable and ensure signal transduction. Here, we review how the expression and function of TCR complexes are orchestrated by several fine-tuned cellular processes that encompass (a) synthesis of the subunits and their correct assembly and expression at the plasma membrane as a single functional complex, (b) TCR membrane localization and dynamics at the plasma membrane and in endosomal compartments, (c) TCR signal transduction leading to T cell activation, and (d) TCR degradation. These processes balance each other to ensure efficient T cell responses to a variety of antigenic stimuli while preventing autoimmunity.

T cell adhesion triggers an early signaling pole distal to the immune synapse

The immunological synapse forms at the interface between a T cell and an antigen-presenting cell after foreign antigen recognition. The immunological synapse is considered to be the site where the signaling cascade leading to T lymphocyte activation is triggered. Here, we show that another signaling region can be detected before formation of the synapse at the opposite pole of the T cell. This structure appears during the first minute after the contact forms, is transient and contains all the classic components that have been previously described at the immunological synapse. Its formation is independent of antigen recognition but is driven by adhesion itself. It constitutes a reservoir of signaling molecules that are potentially ready to be sent to the immunological synapse through a microtubule-dependent pathway. The antisynapse can thus be considered as a pre-synapse that is triggered independently of antigen recognition.

Impact of Exogenous Galectin-9 on Human T Cells: CONTRIBUTION OF THE T CELL RECEPTOR COMPLEX TO ANTIGEN-INDEPENDENT ACTIVATION BUT NOT TO APOPTOSIS INDUCTION

Galectin-9 (gal-9) is a multifunctional β-galactoside-binding lectin, frequently released in the extracellular medium, where it acts as a pleiotropic immune modulator. Despite its overall immunosuppressive effects, a recent study has reported bimodal action of gal-9 on human resting blood T cells with apoptosis occurring in the majority of them, followed by a wave of activation and expansion of Th1 cells in the surviving population. Our knowledge of the signaling events triggered by exogenous gal-9 in T cells remains limited. One of these events is cytosolic calcium (Ca(2+)) release reported in some murine and human T cells. The aim of this study was to investigate the contribution of Ca(2+) mobilization to apoptotic and nonapoptotic effects of exogenous gal-9 in human T cells. We found that the T cell receptor (TCR)-CD3 complex and the Lck kinase were required for Ca(2+) mobilization but not for apoptosis induction in Jurkat cells. These data were confirmed in human CD4(+) T cells from peripheral blood as follows: a specific Lck chemical inhibitor abrogated Ca(2+) mobilization but not apoptosis induction. Moreover, Lck activity was also required for the production of Th1-type cytokines, i.e. interleukin-2 and interferon-γ, which resulted from gal-9 stimulation in peripheral CD4(+) T cells. These findings indicate that gal-9 acts on T cells by two distinct pathways as follows: one mimicking antigen-specific activation of the TCR with a mandatory contribution of proximal elements of the TCR complex, especially Lck, and another resulting in apoptosis that is independent of this complex.

Structure of the chicken CD3εδ/γ heterodimer and its assembly with the αβT cell receptor

In mammals, the αβT cell receptor (TCR) signaling complex is composed of a TCRαβ heterodimer that is noncovalently coupled to three dimeric signaling molecules, CD3εδ, CD3εγ, and CD3ζζ. The nature of the TCR signaling complex and subunit arrangement in different species remains unclear however. Here we present a structural and biochemical analysis of the more primitive ancestral form of the TCR signaling complex found in chickens. In contrast to mammals, chickens do not express separate CD3δ and CD3γ chains but instead encode a single hybrid chain, termed CD3δ/γ, that is capable of pairing with CD3ε. The NMR structure of the chicken CD3εδ/γ heterodimer revealed a unique dimer interface that results in a heterodimer with considerable deviation from the distinct side-by-side architecture found in human and murine CD3εδ and CD3εγ. The chicken CD3εδ/γ heterodimer also contains a unique molecular surface, with the vast majority of surface-exposed, nonconserved residues being clustered to a single face of the heterodimer. Using an in vitro biochemical assay, we demonstrate that CD3εδ/γ can assemble with both chicken TCRα and TCRβ via conserved polar transmembrane sites. Moreover, analogous to the human TCR signaling complex, the presence of two copies of CD3εδ/γ is required for ζζ assembly. These data provide insight into the evolution of this critical receptor signaling apparatus.

Lck mediates signal transmission from CD59 to the TCR/CD3 pathway in Jurkat T cells

The glycosylphosphatidylinositol (GPI)-anchored molecule CD59 has been implicated in the modulation of T cell responses, but the underlying molecular mechanism of CD59 influencing T cell signaling remained unclear. Here we analyzed Jurkat T cells stimulated via anti-CD3ε- or anti-CD59-coated surfaces, using time-resolved single-cell Ca²⁺ imaging as a read-out for stimulation. This analysis revealed a heterogeneous Ca²⁺ response of the cell population in a stimulus-dependent manner. Further analysis of T cell receptor (TCR)/CD3 deficient or overexpressing cells showed that CD59-mediated signaling is strongly dependent on TCR/CD3 surface expression. In protein co-patterning and fluorescence recovery after photobleaching experiments no direct physical interaction was observed between CD59 and CD3 at the plasma membrane upon anti-CD59 stimulation. However, siRNA-mediated protein knock-downs of downstream signaling molecules revealed that the Src family kinase Lck and the adaptor molecule linker of activated T cells (LAT) are essential for both signaling pathways. Furthermore, flow cytometry measurements showed that knock-down of Lck accelerates CD3 re-expression at the cell surface after anti-CD59 stimulation similar to what has been observed upon direct TCR/CD3 stimulation. Finally, physically linking Lck to CD3ζ completely abolished CD59-triggered Ca²⁺ signaling, while signaling was still functional upon direct TCR/CD3 stimulation. Altogether, we demonstrate that Lck mediates signal transmission from CD59 to the TCR/CD3 pathway in Jurkat T cells, and propose that CD59 may act via Lck to modulate T cell responses.

Diversity, molecular characterization and expression of T cell receptor γ in a teleost fish, the sea bass (Dicentrarchus labrax, L)

Two lineages of T cells, expressing either the αβ T cell receptor (TR) or the γδ TR, exist in Gnathostomes. The latter type of T cells account for 1–10 % of T cells in blood and up to 30 % in the small intestine. They may recognize unconventional antigens (phosphorylated microbial metabolites, lipid antigens) without the need of major histocompatibility class I (MH1) or class II (MH2) presentation. In this work we have described cloning and structural characterization of TR -chain (TRG) from the teleost Dicentrarchus labrax. Further, by means of quantitative PCR analysis, we analyzed TRG expression levels both in poly I:C stimulated leukocytes in vitro, and following infection with betanodavirus in vivo. Two full length cDNAs relative to TRG, with the highest peptide and nucleotide identity with Japanese flounder, were identified. A multiple alignment analysis showed the conservation of peptides fundamental for TRG biological functions, and of the FGXG motif in the FR4 region, typical of most TR and immunoglobulin light chains. A 3D structure consisting of two domains mainly folded as beta strands with a sandwich architecture for each domain was also reported. TRG CDR3 of 8–18 AA in length and diversity in the TRG rearrangements expressed in thymus and intestine for a given V/C combination were evidenced by junction length spectratyping. TRG mRNA expression levels were high in basal conditions both in thymus and intestine, while in kidney and gut leukocytes they were up-regulated after in vitro stimulation by poly I:C. Finally, in juveniles the TRG expression levels were up-regulated in the head kidney and down-regulated in intestine after in vivo infection with betanodavirus. Overall, in this study the involvement of TRG-bearing T cells during viral stimulation was described for the first time, leading to new insights for the identification of T cell subsets in fish.

Molecular characterization and increased expression of the Nile tilapia, Oreochromis niloticus (L.), T-cell receptor beta chain in response to Streptococcus agalactiae infection

The complete cDNA sequence of the Nile tilapia T-cell receptor (TCR) β chain was cloned using 5' RACE. The full-length, 1263-bp cDNA contained a 942-bp open reading frame (ORF) encoding a 314-amino-acid protein. Sequence analyses revealed that the Nile tilapia TCR β chain contains four conserved cysteine residues involved in the formation of disulphide bridges and a conserved amino acid motif believed to be important for assembly and signalling of the TCR αβ/CD3 complex, both of which are normally found in the TCR β chain of other vertebrates. As detected using semi-quantitative and quantitative RT-PCR, the highest expression level of TCR β was detected in the thymus. Interestingly, Streptococcus agalactiae significantly induced the up-regulation of the TCR β chain, and the strongest up-regulation was detected in the brain and peripheral blood leucocytes (PBLs). In in vitro experiments, concanavalin A and Aeromonas hydrophila were found to significantly increase the expression of the TCR β chain in PBLs after 48 h (P < 0.01) and 72 h (P < 0.05), respectively. Furthermore, real-time PCR analysis showed that intraperitoneal injection (IP) of 10(7) cfu mL(-1) of S. agalactiae could induce TCR β expression that was greater than the expression observed following administration of 10(9) cfu mL(-1). The presence of the TCR β chain in fish detected in this study suggests the presence of T-cell populations that have been found in higher vertebrates, which may play a crucial functional role in the response to fish pathogens.

Identification of CD3ε, CD4, CD8β splice variants of Atlantic salmon

In vertebrates, CD3 complex and CD4 and CD8 co-receptors are essential for signal transduction during T cell activation. In the present study, we report the mRNA spliced variants of the Atlantic salmon CD3ε, CD4 and CD8β and the effect of pathogen encounter on the expression of these variants. CD3ε is alternatively spliced in thymus, head kidney, spleen and gills to give rise to the complete mRNA sequence and to an alternative product that lacks the transmembrane exon. CD4 is also alternatively spliced in the thymus, head kidney, spleen and gills to form two variants, although the alternative product is barely detectable. The alternative product lacks the exon 1B encoding the D1 domain, which is essential for binding to MHC class II proteins. Two amplicons were also found for the CD8β gene; sequencing analysis revealed that the main PCR product corresponds to the previously reported CD8β sequence, whereas the variant sequence encodes a potential protein that lacks the Ig-like domain. The expression of CD3, CD4, CD8β genes also analyzed in head kidney of LPS-treated and IPNV infected salmon and different patterns of expression were observed. The presence and balance of the different variants of T cell co-receptors could be related to the ability of fish to induce a particular type of immune response, as well as, the ability of the pathogen to modify the fish immune response.

Structural characterization of the TCR complex by electron microscopy

Structural information on how the TCR transmits signals upon binding of its antigen peptide MHC molecule ligand is still lacking. The ectodomains of the TCRα/β, CD3εγ and CD3εδ dimers, as well as the transmembrane domain of CD3ζ, have been characterized by X-ray crystallography and nuclear magnetic resonance (NMR). However, no structural data have been obtained for the entire TCR complex. In this study, we have purified the TCR from T cells under native conditions and used electron microscopy to derive a three-dimensional structure. The TCR complex appears as a pear-shaped structure of 180 × 120 × 65 . Furthermore, the use of mAbs has allowed to determine the orientation of the TCRα/β and CD3 subunits and to suggest a model of interactions. Interestingly, the reconstructed TCR is larger than expected for a complex with a αβγεδεζζ stoichiometry. The accommodation of a second TCRαβ to fill in the extra volume is discussed.

T-cell antigen receptor (TCR) transmembrane peptides: A new paradigm for the treatment of autoimmune diseases

Cell surface membranes are generally considered as inert and hydrophobic providing a stable physical barrier that anchor proteins and maintain cellular homeostasis between the intra- and the extra-cellular environment. The integral proteins that transverse membranes do so once or multiple times and can function alone or as part of a larger complex. Far from being inert, there is a multiplicity of biophysical factors that drive protein-protein and protein-lipid interactions within membranes that are being increasingly recognised as very important for cellular function. Unravelling these "hot-spots" on the contact surface of transmembrane (TM) proteins and targeting peptides to these sites to interrupt the cohesive interaction between the proteins provides both an enormous challenge and a huge therapeutic potential that as yet remains unrecognized. Indeed, with biopharmaceutical research on the rise, TM peptides may prove a useful innovation. Using the T-cell antigen receptor (TCR) as a model system of multi-subunits interacting at the TM via electrostatic charges the potential for peptides as therapeutic agents to interfere with normal immune responses is discussed. The principles of such can be extended to other similar receptor systems including those involved in cancer or infection.

Significant functional heterogeneity among KIR2DL1 alleles and a pivotal role of arginine 245

Killer immunoglobulin-like receptors (KIRs) play an essential role in the regulation of natural killer cell functions. KIR genes are highly polymorphic in nature, showing both haplotypic and allelic variations among people. We demonstrated in both in vitro and in vivo models a significant heterogeneity in function among different KIR2DL1 alleles, including their ability to inhibit YT-Indy cells from degranulation, interferon gamma production, and cytotoxicity against target cells expressing the HLA-Cw6 ligand. Subsequent experiments showed that the molecular determinant was an arginine residue at position 245 (R245) in its transmembrane domain that mechanistically affects both the efficiency of inhibitory signaling and durability of surface expression. Specifically, in comparison with R245-negative alleles, KIR2DL1 that included R245 recruited more Src-homology-2 domain-containing protein tyrosine phosphatase 2 and beta-arrestin 2, showed higher inhibition of lipid raft polarization at immune synapse, and had less down-regulation of cell-surface expression upon interaction with its ligand. Thus, our findings provide novel insights into the molecular determinant of KIR2DL1 and conceivably a fundamental understanding of KIR2DL1 allelic polymorphism in human disease susceptibility, transplant outcome, and donor selection.

Transmembrane interactions are needed for KAI1/CD82-mediated suppression of cancer invasion and metastasis

In transmembrane (TM) domains, tetraspanin KAI1/CD82 contains an Asn, a Gln, and a Glu polar residue. A mutation of all three polar residues largely disrupts the migration-, invasion-, and metastasis-suppressive activities of KAI1/CD82. Notably, KAI1/CD82 inhibits the formation of microprotrusions and the release of microvesicles, while the mutation disrupts these inhibitions, revealing the connections of microprotrusion and microvesicle to KAI1/CD82 function. The TM polar residues are needed for proper interactions between KAI1/CD82 and tetraspanins CD9 and CD151, which also regulate cell movement, but not for the association between KAI1/CD82 and alpha3beta1 integrin. However, KAI1/CD82 still efficiently inhibits cell migration when either CD9 or CD151 is absent. Hence, KAI1/CD82 interacts with tetraspanin and integrin by different mechanisms and is unlikely to inhibit cell migration through its associated proteins. Moreover, without significantly affecting the glycosylation, homodimerization, and global folding of KAI1/CD82, the TM interactions maintain the conformational stability of KAI1/CD82, evidenced by the facts that the mutant is more sensitive to denaturation and less associable with tetraspanins and supported by the modeling analysis. Thus, the TM interactions mediated by these polar residues determine a conformation either in or near the tightly packed TM region and this conformation and/or its change are needed for the intrinsic activity of KAI1/CD82. In contrast to immense efforts to block the signaling of cancer progression, the perturbation of TM interactions may open a new avenue to prevent cancer invasion and metastasis.

Common themes in the assembly and architecture of activating immune receptors

Each of the many different cell types of the immune system expresses one or several activating receptors which serve a central role in the cell's surveillance function. Many of these cell-surface receptors share a distinctive modular design that consists of a ligand-binding module with no intrinsic signalling capability that is non-covalently associated with one or more dimeric signalling modules. Receptor assembly is directed by unique polar contacts within the transmembrane domains, whereas extracellular contacts can contribute to stability and specificity. This Review discusses the structural basis of receptor assembly and the implications of these findings for the mechanisms of receptor triggering.

Convergence of immunoreceptor and integrin signaling

A common signaling pathway is known to operate downstream of immunoreceptors, such as the T-cell, B-cell, or Fc receptors, following engagement by their respective ligands. This pathway involves Src family kinase-mediated tyrosine phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) that recruit and activate spleen tyrosine kinase (Syk) or Zap70 (zeta-associated protein of 70 kDa) kinases, which in turn activate a variety of downstream signals. Evidence has been building from a variety of sources, particularly mouse models, that molecules involved in the immunoreceptor signaling pathway are also required for signals initiated by integrins. Integrins are the major cell surface receptors that mediate adhesion of leukocytes to a variety of extracellular matrix proteins and counter-receptors expressed on endothelial cells. Integrin ligation is a critical step in the activation of leukocyte effector functions (such as neutrophil degranulation or lymphocyte proliferation). Integrin signaling through pathways common to those utilized by immunoreceptors provides a mechanism by which leukocyte adhesion can regulate activation of cellular responses. In animal models, integrin-mediated signal transduction plays a critical role in inflammatory disease. In this review, we discuss the convergence of immunoreceptor and integrin signaling, focusing on how these pathways modulate leukocyte activation.

The structure of the zetazeta transmembrane dimer reveals features essential for its assembly with the T cell receptor

The T cell receptor (TCR) alphabeta heterodimer communicates ligand binding to the cell interior via noncovalently associated CD3gammaepsilon, CD3deltaepsilon, and zetazeta dimers. While structures of extracellular components of the TCR-CD3 complex are known, the transmembrane (TM) domains that mediate assembly have eluded structural characterization. Incorporation of the zetazeta signaling module is known to require one basic TCRalpha and two zetazeta aspartic acid TM residues. We report the NMR structure of the zetazeta(TM) dimer, a left-handed coiled coil with substantial polar contacts. Mutagenesis experiments demonstrate that three polar positions are critical for zetazeta dimerization and assembly with TCR. The two aspartic acids create a single structural unit at the zetazeta interface stabilized by extensive hydrogen bonding, and there is evidence for a structural water molecule (or molecules) within close proximity. This structural unit, representing only the second transmembrane dimer interface solved to date, serves as a paradigm for the assembly of all modules involved in TCR signaling.

A membrane-proximal tetracysteine motif contributes to assembly of CD3deltaepsilon and CD3gammaepsilon dimers with the T cell receptor

Assembly of the T cell receptor (TCR) with its dimeric signaling modules, CD3deltaepsilon, CD3gammaepsilon, and zetazeta, is organized by transmembrane (TM) interactions. Each of the three assembly steps requires formation of a three-helix interface involving one particular basic TCR TM residue and two acidic TM residues of the respective signaling dimer. The extracellular domains of CD3deltaepsilon and CD3gammaepsilon contribute to assembly, but TCR interaction sites on CD3 dimers have not been defined. The structures of the extracellular domains of CD3deltaepsilon and CD3gammaepsilon demonstrated parallel beta-strands ending at the first cysteine in the CXXCXEXXX motif present in the stalk segment of each CD3 chain. Mutation of the membrane-proximal cysteines impaired assembly of either CD3 dimer with TCR, and little complex was isolated when all four membrane-proximal cysteines were mutated to alanine. These mutations had, however, no discernable effect on CD3deltaepsilon or CD3gammaepsilon dimerization. CD3deltaepsilon assembled with a TCRalpha mutant that lacked both immunoglobulin domains, but shortening of the TCRalpha connecting peptide reduced assembly, consistent with membrane-proximal TCRalpha-CD3deltaepsilon interactions. Chelation of divalent cations did not affect assembly, indicating that coordination of a cation by the tetracysteine motif was not required. The membrane-proximal cysteines were within close proximity but only formed covalent CD3 dimers when one cysteine was mutated. The four cysteines may thus form two intrachain disulfide bonds integral to the secondary structure of CD3 stalk regions. The three-chain interaction theme first established for the TM domains thus extends into the membrane-proximal domains of TCRalpha-CD3deltaepsilon and TCRbeta-CD3gammaepsilon.

Recognition of fresh human tumor by human peripheral blood lymphocytes transduced with a bicistronic retroviral vector encoding a murine anti-p53 TCR

The p53 protein is markedly up-regulated in a high proportion of human malignancies. Using an HLA-A2 transgenic mouse model, it was possible to isolate high-avidity murine CTLs that recognize class I-restricted human p53 epitopes. We isolated the alpha- and beta-chain of a TCR from a highly avid murine CTL clone that recognized the human p53(264-272) epitope. These genes were cloned into a retroviral vector that mediated high efficiency gene transfer into primary human lymphocytes. Efficiencies of >90% for gene transfer into lymphocytes were obtained without selection for transduced cells. The p53 TCR-transduced lymphocytes were able to specifically recognize with high-avidity, peptide-pulsed APCs as well as HLA-A2.1+ cells transfected with either wild-type or mutant p53 protein. p53 TCR-transduced cells demonstrated recognition and killing of a broad spectrum of human tumor cell lines as well as recognition of fresh human tumor cells. Interestingly, both CD8+ and CD4+ subsets were capable of recognizing and killing target cells, stressing the potential application of such a CD8-independent TCR molecule that can mediate both helper and cytotoxic responses. These results suggest that lymphocytes genetically engineered to express anti-p53 TCR may be of value for the adoptive immunotherapy of patients with a variety of common malignancies.

The T cell receptor: critical role of the membrane environment in receptor assembly and function

Recent studies have demonstrated that cell membranes provide a unique environment for protein-protein and protein-lipid interactions that are critical for the assembly and function of the T cell receptor (TCR)-CD3 complex. Highly specific polar interactions among transmembrane (TM) domains that are uniquely favorable in the lipid environment organize the association of the three signaling dimers with the TCR. Each of these three assembly steps depends on the formation of a three-helix interface between one basic and two acidic residues in the membrane environment. The same polar TM residues that drive assembly also play a central role in quality control and export by directing the retention and degradation of free subunits and partial complexes, while membrane proximal cytoplasmic signals control recycling and degradation of surface receptors. Recent studies also suggest that interactions between the membrane and the cytoplasmic domains of CD3 proteins may be important for receptor triggering.

Crystal structure of a human CD3-epsilon/delta dimer in complex with a UCHT1 single-chain antibody fragment

The alpha/beta T cell receptor complex transmits signals from MHC/peptide antigens through a set of constitutively associated signaling molecules, including CD3-epsilon/gamma and CD3-epsilon/delta. We report the crystal structure at 1.9-A resolution of a complex between a human CD3-epsilon/delta ectodomain heterodimer and a single-chain fragment of the UCHT1 antibody. CD3-epsilon/delta and CD3-epsilon/gamma share a conserved interface between the Ig-fold ectodomains, with parallel packing of the two G strands. CD3-delta has a more electronegative surface and a more compact Ig fold than CD3-gamma; thus, the two CD3 heterodimers have distinctly different molecular surfaces. The UCHT1 antibody binds near an acidic region of CD3-epsilon opposite the dimer interface, occluding this region from direct interaction with the TCR. This immunodominant epitope may be a uniquely accessible surface in the TCR/CD3 complex, because there is overlap between the binding site of the UCHT1 and OKT3 antibodies. Determination of the CD3-epsilon/delta structure completes the set of TCR/CD3 globular ectodomains and contributes information about exposed CD3 surfaces.

Constitutive and Ligand-Induced TCR Degradation

Modulation of TCR expression levels is a central event during T cell development and activation, and it probably plays an important role in adjusting T cell responsiveness. Conflicting data have been published on down-regulation and degradation rates of the individual TCR subunits, and several divergent models for TCR down-regulation and degradation have been suggested. The aims of this study were to determine the rate constants for constitutive and ligand-induced TCR degradation and to determine whether the TCR subunits segregate or are processed as an intact unit during TCR down-regulation and degradation. We found that the TCR subunits in nonstimulated Jurkat cells were degraded with rate constants of approximately 0.0011 min(-1), resulting in a half-life of approximately 10.5 h. Triggering of the TCR by anti-TCR Abs resulted in a 3-fold increase in the degradation rate constants to approximately 0.0033 min(-1), resulting in a half-life of approximately 3.5 h. The subunits of the TCR complex were down-regulated from the cell surface and degraded with identical kinetics, and most likely remained associated during the passage throughout the endocytic pathway from the cell surface to the lysosomes. Similar results were obtained in studies of primary human Vbeta8+ T cells stimulated with superantigen. Based on these results, the simplest model for TCR internalization, sorting, and degradation is proposed.

Molecular mechanisms for the assembly of the T cell receptor-CD3 complex

The T cell receptor (TCR)-CD3 complex represents on of the most intricate membrane receptor structures since it is built from six distinct chains. This complexity led to a number of different proposals for the arrangement of the receptor subunits, its stoichiometry and the mechanisms responsible for receptor triggering. Early work had demonstrated that basic and acidic transmembrane (TM) residues were involved in the assembly but the molecular arrangement could not be deduced due to the complexity of the receptor. Using a novel method for the isolation of intact radiolabeled protein complexes, we demonstrated that the complex assembled in the ER contains only a single TCRalphabeta heterodimer and one copy of each of the CD3deltaepsilon, CD3gammaepsilon and zeta-zeta signaling dimers. Surprisingly, assembly of each of the three signaling dimers with TCR was dependent on one of the three basic TCR TM residues as well as both acidic residues located in the TM domains of the interacting signaling dimer. Each assembly step thus results in the formation of a three-helix interface in the membrane that involves one basic and two acidic TM residues, and this arrangement effectively shields these ionizable residues at protein-protein interfaces from the lipid. Since proteins whose TM domains have exposed ionizable residues are not stably integrated into the lipid bilayer, assembly based on shielding of ionizable residues permits full equilibration of the receptor into the lipid bilayer and prevents degradation. Assembly, export of intact receptor complexes and degradation of unassembled components thus rely on the same organizing principle.

Stoichiometry of the T-cell receptor-CD3 complex and key intermediates assembled in the endoplasmic reticulum

The T-cell receptor (TCR)-CD3 complex is critical for T-cell development and function, and represents one of the most complex transmembrane receptors. Models of different stoichiometry and valency have been proposed based on cellular experiments and these have important implications for the mechanisms of receptor triggering. Since determination of receptor stoichiometry in T-cells is not possible due to the presence of previously synthesized, unlabeled receptor components with different half-lives, we examined the stoichiometry of the receptor assembled in endoplasmic reticulum (ER) microsomes of B-cell origin. The stoichiometric relationship among all subunits was directly determined using intact radiolabeled TCR-CD3 complexes that were isolated with a sequential, non-denaturing immunoprecipitation method, and identical results were obtained with two detergents belonging to different structural classes. The results firmly establish that the alphabeta TCR-CD3 complex assembled in the ER is monovalent and composed of one copy of the TCRalphabeta, CD3deltaepsilon, CD3gammaepsilon and zeta-zeta dimers.

Mutational analysis of conserved amino acids in the T cell receptor alpha-chain transmembrane region: a critical role of leucine 112 and phenylalanine 127 for assembly and surface expression

Correct assembly of all TCR complex polypeptides is essential for its cell surface expression and function. The transmembrane region of the TCRalpha chain is highly conserved and to gain insight into the structural and functional role of these residues, single amino acid substitutions were introduced and surface expression and signaling ability studied in T hybridoma cells. Introduction of acid residues within the TCRalpha chain transmembrane region were mostly tolerated, indicating that the net charge within this region of the TCR complex is not crucial to either assembly or signaling. However, mutations of leucine 112 or phenylalanine 127 to aspartic acids (L112D or F127D, respectively) resulted in dramatic loss of surface expression and, therefore, their signaling ability. Intracellular flow cytometry showed that the mutant TCRalpha polypeptides were present at levels comparable to wild-type, indicating that the reduced surface expression was not a consequence of impaired protein survival. The defect was characterized by immunoprecipitation and showed that residues L112 and F127 were involved in early interactions with the CD3 complex. A large proportion of the TCRalpha chain mutants L112D and F127D consisted of immature protein, indicative of a problem during early assembly of the TCR. Our findings provide evidence for the involvement of the conserved L112 and F127 residues of the TCRalpha chain transmembrane region in the assembly process of the TCR complex.

Vav-1 and the IKK alpha subunit of I kappa B kinase functionally associate to induce NF-kappa B activation in response to CD28 engagement

We have recently observed that CD28 engagement initiates a signaling pathway leading to the activation of I kappa B kinase (IKK) complex and, consequently, to NF-kappa B activation, and we identified Vav-1 as an important mediator of this function. Here we report for the first time that Vav-1 constitutively associates with IKK alpha in both Jurkat and primary CD4(+) T cells. Vav-1/IKK alpha association is mediated by their helix-loop-helix domains, does not involve IKK beta, and is functionally relevant in that Vav-1-associated IKK alpha kinase activity is increased following CD28 engagement by B7. Moreover, we demonstrate that CD28-induced NF-kappa B activation is augmented by both IKK alpha and Vav-1, but not IKK beta. Confocal microscopy showed that endogenous Vav-1 and IKK alpha, but not IKK beta, were recruited to the membrane and colocalized in response to CD28 stimulation. Taken together, these data evidence that Vav-1 plays a key role in the control of NF-kappa B pathway by targeting IKK alpha in the T cell membrane and favoring its activation in response to CD28 stimulation.

The organizing principle in the formation of the T cell receptor-CD3 complex

The T cell receptor (TCR) serves a critical function in the immune system and represents one of the most complex receptor structures. A striking feature is the presence of nine highly conserved, potentially charged residues in the transmembrane helices. Previous models have attempted to explain assembly based on pairwise interactions of these residues. Using a novel method for the isolation of intact radiolabeled protein complexes, we demonstrate that one basic and two acidic transmembrane residues are required for the assembly of each of the three signaling dimers with the TCR. This remarkable three-helix arrangement applies to all three assembly steps and represents the organizing principle for the formation of this intricate receptor structure.

CD4-Lck through TCR and in the absence of Vav exchange factor induces Bax increase and mitochondrial damage

In the present study, we aimed to demonstrate that CD4 may represent a critical turning point that governs the apoptotic and survival programs in T cells, without modifying the physical association with the TCR-CD3 complex. To address this issue, we have explored the possibility that the activation of CD4 may transduce apoptotic signals unless signaling effectors neutralize them. Our data show that in Jurkat T cells CD4 engagement by Leu3a mAb results in a rapid and strong increase of Lck kinase activity, subsequent alterations of mitochondrial membrane potential, and apoptosis. Critical parameters are coassociation of CD4/Lck with TCR/CD3 and up-regulation of the proapoptotic protein Bax. Indeed, Leu3a-mediated Lck activation failed to induce apoptotic features in Jurkat cells either defective for TCR/CD3 or overexpressing the antiapoptotic protein Bcl-2. Furthermore, we demonstrate that Leu3a treatment of Jurkat cells overexpressing Vav results in the inhibition of mitochondrial damage and apoptosis; this rescue effect is accompanied with a significant decrease of Bax expression observed in apoptotic cells. Our evidence that the activation of Lck activates in T cells apoptotic pathways which are counteracted by Vav, a signaling molecule that cooperates with CD28 to boost TCR signals, suggests a novel role for costimulation in protecting T cells from CD4-mediated cell death.

Some hints concerning the shape of T-cell receptor structures

Several models are proposed for T-cell antigen receptor (TCR) assembly and structure. However, there is little experimental data favouring directly either one or the other(s). The minimal complex appears to be composed of a TCRalphabeta/CD3deltaepsilon,gammaepsilon/zeta2 structure but at the cell membrane, multimers of this minimal structure may be formed. Quantitative cytofluometry has suggested three CD3epsilon chains for two TCRbeta (or TCRdelta) chains/complex. Such data should be repeated with monoclonal antibodies (MoAb) against extracellular (EC) parts of CD3delta or CD3gamma chains. In the present review, we have compared the TCR/CD3 assembly of pre-TCR, TCRgammadelta and TCRalphabeta containing complexes, and analysed the reactivity of antibodies (Abs) against the EC part of CD3delta chains. Our data suggest an alternative assembly pathway and structure of TCR/CD3 complexes.

T cell antigen receptor (TCR) transmembrane peptides colocalize with TCR, not lipid rafts, in surface membranes

We have previously shown that a synthetic peptide termed core peptide (CP), which corresponds to a sequence within the transmembrane domain of the alpha chain of the T cell antigen receptor (TCR), can inhibit IL-2 production in antigen-stimulated T cells and can suppress inflammation in several T cell-mediated animal models of disease. As the first step in determining the mechanism of CP action, we examined the association of CP with the plasma membrane of human T cells using confocal microscopy. A homogeneous distribution of CP was observed in the plasma membrane of human T cells. This membrane localization was dependent on the presence of positive charges in the CP sequence. CP analogs, containing either neutral or negatively charged amino acids in place of the positive amino acid charges, did not localize within TCR membranes. Following antibody-induced TCR clustering, there was specific colocalization of CP with surface TCR. No association was observed with other cell surface receptors when similarly clustered. Since TCR activation leads to an increased movement of the receptor complex to cholesterol/glycosphingolipid (GSL) plasma membrane microdomains (rafts) we examined whether the association of CP with TCR was raft-driven. TCR clustering led only to a partial colocalization of TCRs with raft GSL, ganglioside GM1, and a complete colocalization of CP with TCRs. We conclude that CP associates specifically with plasma membrane TCRs and not raft lipids.

TCR dynamics on the surface of living T cells

T lymphocyte activation by specific antigen requires prolonged TCR occupancy and sustained signaling. This is accomplished by the formation of a specialized signaling domain, the immunological synapse, at the T cell-antigen-presenting cell contact site. Surface receptors and signaling components are progressively recruited into this domain where they are organized in defined three-dimensional structures. To better understand how TCR are supplied to the signaling domain during the activation process, we measured (using confocal microscopy and photo-bleaching recovery techniques) lateral mobility of GFP-tagged TCR on living Jurkat cell surface. We show that: (i) surface-expressed TCR exhibit an intrinsic, actin cytoskeleton-independent, lateral mobility which allows them to passively diffuse over the entire T cell surface within approximately 60 min and (ii) non-stimulated TCR rapidly enter the signaling domain. Our results indicate that TCR lateral mobility per se is sufficient to ensure TCR supply to the immunological synapse in the course of sustained T cell activation.

Importance of the T cell receptor alpha-chain transmembrane distal region for assembly with cognate subunits

Antigen recognition by alphabeta T lymphocytes is mediated via the multisubunit TCR complex consisting of invariant CD3gamma,delta,epsilon and zeta chains associated with clonotypic TCRalpha and beta molecules. Charged amino acids located centrally within the TCRalpha transmembrane region are necessary and sufficient for assembly with the CD3deltaepsilon heterodimer. Previously, we have shown that deletion of 6-12 amino acids from the carboxy terminus of the TCRalpha-chain dramatically abrogates surface TCR expression, suggesting that the distal portion of the TCRalpha transmembrane region contains information that regulates the assembly and/or intracellular transport of TCR complexes. We have examined in more detail the molecular basis for reduced TCR expression in T cells bearing truncated TCRalpha chains. We found that in contrast to wild-type (wt), variant TCRalpha proteins missing the last nine C-terminal amino acids did not associate with core CD3gamma,delta,epsilon chains and were not assembled into disulphide-linked alphabeta heterodimers. The stability of newly synthesised wt and variant TCRalpha molecules was similar, showing that the abrogated surface TCR expression was not a consequence of impaired protein survival. Nevertheless, truncated TCRalpha chains still assembled with the chaperon protein calnexin in the endoplasmic reticulum, indicating that the distal portion of the TCRalpha transmembrane region is not essential for calnexin interaction. These data document a role for the distal portion of the TCRalpha transmembrane region in the assembly of TCR complexes and provide a molecular basis for reduced TCR expression in cells bearing truncated TCRalpha chains.

Molecular modelling and endoplasmic reticulum retention of mutated TCR/CD3 complexes

T cell receptor (TCR)/CD3 complex assembly takes place in the endoplasmic reticulum (ER). Normal TCR/CD3 complexes egress from the ER to the cis-Golgi, where the interaction with zeta2 homodimers occurs. This interaction leads to further uncontrolled transport of TCR/CD3/zeta molecules to the cell surface. The purpose of the present experiments was to determine firstly the basis for the impact of the Phe195/216 --> Val mutations on TCR/CD3 expression in Jurkat cells, and secondly why mutated J79-cell TCRalphabeta/CD3 hexamers are prevented from interacting with zeta2 homodimers. We found that Phe --> Val mutations cause serious perturbations in a so far undefined hydrophobic area formed by the two Phe195/216 on beta-strand F and aromatic/large hydrophobic amino acids on neighboring beta-strands B and A in Calpha and Cbeta domains, respectively. In addition, TCR/CD3 hexamers and zeta2 homodimers colocalize in normal Jurkat T cells, in revertant J79r58 cells, and in J79 cells transfected with wild-type TCRalpha cDNA but not in J79 mutant cells (confocal microscopy). Furthermore, mutated TCR/CD3 complexes seem to be actively retained in the ER in J79 cells but not in revertant J79r58 cells by a nondominant mechanism. We propose that a hitherto undefined ER-retention molecule controls both the protein structure and egress of TCR/CD3 complexes from the ER of alphabeta and gammadelta T cells.

Further characterization of T cell receptor chains of marsupials

cDNA clones encoding T cell receptor alpha (TCRalpha) and beta (TCRbeta) from the South American opossum, Monodelphis domestica were isolated and characterized. A single clone isolated encoding a TCRalpha chain was full length, containing the complete V (variable), J (joining) and C (constant) regions. Three partial cDNA clones were isolated for TCRbeta which contained complete C sequences. Phylogenetic analysis of the TCR Valpha revealed that the M. domestica sequence and a sequence from the Australian brushtail possum, Trichosurus vulpecula, belong to separate Valpha families and intersperse with sequences from eutherian mammals. Similar to results described for marsupial and eutherian light chains, diversity at the V region of the TCR is ancient and maintained. In contrast phylogenetic analysis of the TCR Calpha and Cbeta sequences from M. domestica, T. vulpecula, and other vertebrates revealed that the marsupial TCR C grouped together forming a sister group to eutherian mammals.

T-cell antigen receptors in Atlantic cod (Gadus morhua l.): structure, organisation and expression of TCR alpha and beta genes

By using short degenerate primers complementing conserved T-cell antigen receptor (TCR) variable and constant region segments for PCR, we were able to isolate putative TCRalpha and beta chain full length cDNAs in Atlantic cod. The Valpha and Vbeta domains have the canonical features of known teleost and mammalian TCR V domains, including conserved residues in the beginning of FR2 and at the end of FR3. The Jalpha and Jbeta region possess the conserved Phe-Gly-X-Gly motif found in nearly all TCR and immunoglobulin light chain J regions. Similar to other vertebrates, the Atlantic cod Calpha and Cbeta sequences exhibit distinct immunoglobulin, connecting peptide, transmembrane and cytoplasmic regions. The Atlantic cod Cbeta sequence lacks a cysteine in its connecting peptide region, but other motifs proposed to be important for dimerisation and cell surface expression are observed. Four different cod Cbeta sequences were identified, two of which share 3' untranslated regions different from one of the other two sequences, suggesting the existence of isotypic gene variants of Cbeta. Based on Southern blot analyses, the TCRalpha and beta gene loci appear to be arranged in translocon organisation (as opposed to multicluster) with multiple V gene segments, some (D) and J gene segments and a single or few C gene segments. Northern blot analyses show expression of the TCRalpha and beta chains in thymus, spleen and head kidney, expression of the TCRbeta chain was also detected in the ovary. Interestingly, no expression was detected in intestine even though the existence of T-cells in intestine has been proposed in other teleost species.

Conserved transmembrane tyrosine residues of the TCR beta chain are required for TCR expression and function in primary T cells and hybridomas

The T cell receptor (TCR) beta chain transmembrane domain contains two evolutionarily conserved tyrosines (Y). In this study, the functional basis for the evolutionary conservation is addressed by mutation of the residues, expression of the mutants in hybridoma and primary T cells, and examination of TCR signaling function. We find that the phenotype of the mutants, both surface expression and ability to signal for IL-2 production, is highly variable in different mouse T hybridoma lines. Although we have not been able to determine the basis for these differences in the hybridomas, expression of the mutants in primary T cells provides a definitive assessment of mutant phenotype. We show that mutation of the N-terminal Y to either leucine (L) or alanine (A) results in low surface expression in primary T cells, while mutation of both N- and C-terminal Y to A or L abrogates surface expression. However, the more conservative mutation of both transmembrane Y to phenylalanine maintained receptor surface expression and assembly while severely disrupting signaling in primary T cells. Our data demonstrate that TCR beta chain transmembrane Y are essential for TCR signal transduction as well as complex assembly. These findings suggest that protein-protein interactions involving membrane-spanning domains are likely relevant for TCR signal transduction mechanisms.

CD28 utilizes Vav-1 to enhance TCR-proximal signaling and NF-AT activation

The mechanism through which CD28 costimulation potentiates TCR-driven gene expression is still not clearly defined. Vav-1, an exchange factor for Rho GTPases thought to regulate, mainly through Rac-1, various signaling components leading to cytokine gene expression, is tyrosine phosphorylated upon CD28 engagement. Here, we provide evidence for a key role of Vav-1 in CD28-mediated signaling. Overexpression of Vav-1 in Jurkat cells in combination with CD28 ligation strongly reduced the concentration of staphylococcus enterotoxin E/MHC required for TCR-induced NF-AT activation. Surprisingly, upon Vav-1 overexpression CD28 ligation sufficed to activate NF-AT in the absence of TCR engagement. This effect was not mediated by overexpression of ZAP-70 nor of SLP-76 but necessitated the intracellular tail of CD28, the intactness of the TCR-proximal signaling cascade, the Src-homology domain 2 (SH2) domain of Vav-1, and SLP-76 phosphorylation, an event which was favored by Vav-1 itself. Cells overexpressing Vav-1 formed lamellipodia and microspikes reminiscent of Rac-1 and Cdc42 activation, respectively, for which the SH2 domain of Vav-1 was dispensable. Together, these data suggest that CD28 engagement activates Vav-1 to boost TCR signals through a synergistic cooperation between Vav-1 and SLP-76 and probably via cortical actin changes to facilitate the organization of a signaling zone.

Kinetics of T-cell receptor binding by bivalent HLA-DR. Peptide complexes that activate antigen-specific human T-cells

Monovalent major histocompatibility complex-peptide complexes dissociate within seconds from the T-cell receptor (TCR), indicating that dimerization/multimerization may be important during early stages of T-cell activation. Soluble bivalent HLA-DR2.myelin basic protein (MBP) peptide complexes were expressed by replacing the F(ab) arms of an IgG2a antibody with HLA-DR2.MBP peptide complexes. The binding of bivalent HLA-DR2.peptide complexes to recombinant TCR was examined by surface plasmon resonance. The bivalent nature greatly enhanced TCR binding and slowed dissociation from the TCR, with a t((1)/(2)) of 2.1 to 4.6 min. Soluble bivalent HLA-DR2.MBP peptide complexes activated antigen-specific T-cells in the absence of antigen presenting cells. In contrast, soluble antibodies to the TCR.CD3 complex were ineffective, indicating that they failed to induce an active TCR dimer. TCR/CD3 antibodies induced T-cell proliferation when bound by antigen presenting cells that expressed Fc receptors. In the presence of dendritic cells, bivalent HLA-DR2. MBP peptide complexes induced T-cell activation at >100-fold lower concentrations than TCR/CD3 antibodies and were also superior to peptide or antigen. These results demonstrate that bivalent HLA-DR. peptide complexes represent effective ligands for activation of the TCR. The data support a role for TCR dimerization in early TCR signaling and kinetic proofreading.

Tyrosine 319 in the interdomain B of ZAP-70 is a binding site for the Src homology 2 domain of Lck

T-cell antigen receptor-induced signaling requires both ZAP-70 and Lck protein-tyrosine kinases. One essential function of Lck in this process is to phosphorylate ZAP-70 and up-regulate its catalytic activity. We have previously shown that after T-cell antigen receptor stimulation, Lck binds to ZAP-70 via its Src homology 2 (SH2) domain (LckSH2) and, more recently, that Tyr319 of ZAP-70 is phosphorylated in vivo and plays a positive regulatory role. Here, we investigated the possibility that Tyr319 mediates the SH2-dependent interaction between Lck and ZAP-70. We show that a phosphopeptide encompassing the motif harboring Tyr319, YSDP, interacted with LckSH2, although with a lower affinity compared with a phosphopeptide containing the optimal binding motif, YEEI. Moreover, mutation of Tyr319 to phenylalanine prevented the interaction of ZAP-70 with LckSH2. Based on these results, a gain-of-function mutant of ZAP-70 was generated by changing the sequence Y319SDP into Y319EEI. As a result of its increased ability to bind LckSH2, this mutant induced a dramatic increase in NFAT activity in Jurkat T-cells, was hyperphosphorylated, and displayed a higher catalytic activity compared with wild-type ZAP-70. Collectively, our findings indicate that Tyr319-mediated binding of the SH2 domain of Lck is crucial for ZAP-70 activation and consequently for the propagation of the signaling cascade leading to T-cell activation.

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

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

T-cell receptors in ectothermic vertebrates

The structure and expression of genes encoding molecules homologous to mammalian T-cell receptors (TCR) have been recently studied in ectothermic vertebrate species representative of chondrychthians, teleosts, and amphibians. The overall TCR chain structure is well conserved in phylogeny: TCR beta- and TCR alpha-like chains were detected in all the species analyzed; TCR gamma- and TCR delta-like chains were also present in a chondrychthian species. The diversity potential of the variable (V) and joining (J) segments is rather large and, as in mammals, conserved diversity (D) segments are associated to the TCR beta and TCR delta chains. An important level of junctional diversity occurred at the V-(D)-J junctions, with the potential addition of N- and P-nucleotides. Thus, the conservation of the structure and of the potential of diversity of TCR molecules have been under a permanent selective pressure during vertebrate evolution. The structure of MHC class I and class II molecules was also well conserved in jawed vertebrates. TCR and MHC molecules are strongly functionally linked and play a determinant role in the initiation and the regulation of the specific immune responses; thus, it is not surprising that their structures have been reciprocally frozen during evolution.

T cell receptor alpha-chain tail is required for protein kinase C-mediated down-regulation, but not for signaling

Antigen stimulation through the T cell receptor (TCR) induces phosphorylation of the associated CD3 gamma delta epsilon- and zeta-chain cytoplasmic tails. These events lead to the induction of the intracellular signaling pathways with concomitant receptor down-regulation. The TCR is down-regulated from the cell surface by the activation of protein kinase, C (PKC) and subsequent serine phosphorylation of the CD3 gamma-chain. We report here that the TCR alpha-chain cytoplasmic tail is also necessary for PKC-mediated internalization of the TCR complex. The requirement for the TCR alpha-chain cytoplasmic tail is specific for internalization of the TCR complex, since down-regulation of CD4 is still intact in hybridoma cells expressing a tailless TCR alpha-chain. The absence of TCR internalization directly correlates with defective PKC-mediated phosphorylation of the CD3 gamma-chain. Despite deficient PKC-mediated TCR down-regulation, the tailless alpha beta TCR still transduces antigenic signals resulting in the production of interleukin-2. Although the TCR tails are not obviously required for signal transduction, the TCR alpha-tail may serve as a targeting domain for PKC-mediated down-regulation of the TCR complex.

Identification of the sites of interaction between lymphocyte phosphatase-associated phosphoprotein (LPAP) and CD45

Human lymphocyte phosphatase-associated phospho-protein (LPAP) is a phosphoprotein of unknown function that noncovalently associates with CD45 in lymphocytes. In CD45-deficient human T cells, LPAP protein is synthesized at normal levels but is more rapidly degraded than in wild-type cells. Expression of CD45 cDNA rescues LPAP protein expression. This strongly suggests that LPAP is protected from degradation through its interaction with CD45. We have mapped the sites of interaction between LPAP and CD45 employing chimeric CD45 molecules and LPAP deletion mutants. Our data demonstrate that the interaction between LPAP and CD45 is mediated via the transmembrane regions of both molecules. In addition, the intracytoplasmic amino acids adjacent to the transmembrane region of LPAP may influence its binding to CD45.

The T cell antigen receptor beta chain interacts with the extracellular domain of CD3-gamma

Selective pairwise interactions between a number of CD3 chains and the clonotypic T cell antigen receptor (TCR-alpha, -beta) chains have recently been reported. What still remains unanswered is the site of interaction between TCR-beta and CD3-gamma chains. To examine the region of interaction between TCR-beta and CD3-gamma chains, a variety of genetically altered TCR-beta and CD3-gamma chains were constructed using recombinant cDNA techniques. Non-T cells (COS-7) were transfected with cDNA constructs, metabolically labelled, and immunoprecipitates were analysed for assembly using non-equilibrium pH gel electrophoresis (NEPHGE)/SDS-PAGE. The results demonstrated that assembly between TCR-beta and CD3-gamma chains was localized to their extracellular domain. These findings, when coupled with the information on pairwise interactions and formation of higher order subcomplexes, extend our model of the structure of the TCR complex.

Distinct domains of the CD3-gamma chain are involved in surface expression and function of the T cell antigen receptor

The T cell antigen receptor (TcR) is a multisubunit complex that consists of at least six different polypeptides. We have recently demonstrated that the CD3-delta subunit cannot substitute for the CD3-gamma subunit in TcR cell surface expression, in spite of significant amino acid homology between these two subunits. To identify CD3-gamma-specific domains that are required for assembly of the complete TcR and for surface expression and function of the TcR, chimeric CD3-gamma/CD3-delta molecules were constructed and expressed in T cells devoid of endogenous CD3-gamma. Substitution of the extracellular domain of CD3-gamma with that of CD3-delta did not allow cell surface expression of the TcR. In contrast, substitution of the transmembrane and/or the intracellular domains of CD3-gamma with those of CD3-delta did allow TcR cell surface expression. These results conclusively demonstrate that the extracellular domain of CD3-gamma plays a unique role in TcR assembly. Functional analyses of the transfectants demonstrated that the intracellular domain of CD3-gamma is required for protein kinase C-mediated down-regulation of TcR but is dispensable for the pattern of tyrosine phosphorylation observed following activation through TcR.