Activation-induced ubiquitination of the T cell antigen receptor

Clathrin controls bidirectional communication between T cells and antigen presenting cells

In circulation, T cells are spherical with selectin enriched dynamic microvilli protruding from the surface. Following extravasation, these microvilli serve another role, continuously surveying their environment for antigen in the form of peptide-MHC (pMHC) expressed on the surface of antigen presenting cells (APCs). Upon recognition of their cognate pMHC, the microvilli are initially stabilized and then flatten into F-actin dependent microclusters as the T cell spreads over the APC. Within 1-5 min, clathrin is recruited by the ESCRT-0 component Hrs to mediate release of T cell receptor (TCR) loaded vesicles directly from the plasma membrane by clathrin and ESCRT-mediated ectocytosis (CEME). After 5-10 min, Hrs is displaced by the endocytic clathrin adaptor epsin-1 to induce clathrin-mediated trans-endocytosis (CMTE) of TCR-pMHC conjugates. Here we discuss some of the functional properties of the clathrin machinery which enables it to control these topologically opposite modes of membrane transfer at the immunological synapse, and how this might be regulated during T cell activation.

CCDC134 facilitates T cell activation through the regulation of early T cell receptor signaling

Modulation of surface T cell antigen receptor (TCR) expression is crucial for proper T cell development and maintenance of mature T cell function at steady state and upon stimulation. We previously determined that CCDC134 (coiled-coil domain containing 134), a cytokine-like molecule that served as a potential member of the γc cytokine family, contributes to antitumor responses by augmenting CD8⁺ T cell-mediated immunity. Here we show that T cell-specific deletion of Ccdc134 decreased peripheral mature CD4⁺ and CD8⁺ T cells, which resulted in impaired T cell homeostasis. Moreover, Ccdc134-deficient T cells exhibited an attenuated response to TCR stimulation in vitro, showing lower activation and proliferative capacity. This was further reflected in vivo, rendering mice refractory to T cell-mediated inflammatory and antitumor responses. More importantly, CCDC134 is associated with TCR signaling components, including CD3ϵ, and attenuated TCR signaling in Ccdc134-deficient T cells via altered CD3ϵ ubiquitination and degradation. Taken together, these findings suggest a role for CCDC134 as a positive regulator of TCR-proximal signaling and provide insight into the cell-intrinsic functional consequences of Ccdc134 deficiency in the attenuation of T cell-mediated inflammatory and antitumor responses.

Termination of TCR-mediated activation signals is regulated by CrkII-dependent Cbl-mediated ubiquitination and degradation of C3G

Crk adaptor proteins are key players in signal transduction from multiple cell surface receptors, including the T cell antigen receptor (TCR). The involvement of CrkII in the early stages of T cell activation is well documented, but little is known about its role during the termination of the activation response. We substantiated findings showing that CrkII utilizes its SH3N and SH2 domains to constitutively associate with C3G and transiently with Cbl in resting and TCR/CD3-stimulated T cells, respectively. Association of CrkII with Cbl peaks within 1 min post-TCR/CD3 stimulation, and involves the formation of multiple CrkII-containing complexes of different molecular mass. Ubiquitination of C3G commences at ∼5 min post TCR/CD3 stimulation concomitantly with its degradation. This entire process conversely correlates with the levels of expression of CrkII and is dependent on the presence of the CrkII-bound Cbl protein. The data suggest that CrkII functions as a scaffold that brings Cbl into close proximity with C3G in TCR/CD3-stimulated T cells and that tyrosine phosphorylation and activation of Cbl promotes C3G ubiquitination and degradation. We suggest that this mechanism contributes to the termination of the TCR/CD3-induced activation signal and helps tune the length and intensity of T cell-mediated immune responses.

Ubiquitination in T-Cell Activation and Checkpoint Inhibition: New Avenues for Targeted Cancer Immunotherapy

The advent of T-cell-based immunotherapy has remarkably transformed cancer patient treatment. Despite their success, the currently approved immunotherapeutic protocols still encounter limitations, cause toxicity, and give disparate patient outcomes. Thus, a deeper understanding of the molecular mechanisms of T-cell activation and inhibition is much needed to rationally expand targets and possibilities to improve immunotherapies. Protein ubiquitination downstream of immune signaling pathways is essential to fine-tune virtually all immune responses, in particular, the positive and negative regulation of T-cell activation. Numerous studies have demonstrated that deregulation of ubiquitin-dependent pathways can significantly alter T-cell activation and enhance antitumor responses. Consequently, researchers in academia and industry are actively developing technologies to selectively exploit ubiquitin-related enzymes for cancer therapeutics. In this review, we discuss the molecular and functional roles of ubiquitination in key T-cell activation and checkpoint inhibitory pathways to highlight the vast possibilities that targeting ubiquitination offers for advancing T-cell-based immunotherapies.

Separate signaling events control TCR downregulation and T cell activation in primary human T cells

Introduction

T‐cell antigen receptor (TCR) interaction with cognate peptide:MHC complexes trigger clustering of TCR:CD3 complexes and signal transduction. Triggered TCR:CD3 complexes are rapidly internalized and degraded in a process called ligand‐induced TCR downregulation. Classic studies in immortalized T‐cell lines have revealed a major role for the Src family kinase Lck in TCR downregulation. However, to what extent a similar mechanism operates in primary human T cells remains unclear.

Methods

Here, we developed an anti‐CD3‐mediated TCR downregulation assay, in which T‐cell gene expression in primary human T cells can be knocked down by microRNA constructs. In parallel, we used CRISPR/Cas9‐mediated knockout in Jurkat cells for validation experiments.

Results

We efficiently knocked down the expression of tyrosine kinases Lck, Fyn, and ZAP70, and found that, whereas this impaired T cell activation and effector function, TCR downregulation was not affected. Although TCR downregulation was marginally inhibited by the simultaneous knockdown of Lck and Fyn, its full abrogation required broad‐acting tyrosine kinase inhibitors.

Conclusions

These data suggest that there is substantial redundancy in the contribution of individual tyrosine kinases to TCR downregulation in primary human T cells. Our results highlight that TCR downregulation and T cell activation are controlled by different signaling events and illustrate the need for further research to untangle these processes.

Coordinating Cytoskeleton and Molecular Traffic in T Cell Migration, Activation, and Effector Functions

Dynamic localization of receptors and signaling molecules at the plasma membrane and within intracellular vesicular compartments is crucial for T lymphocyte sensing environmental cues, triggering membrane receptors, recruiting signaling molecules, and fine-tuning of intracellular signals. The orchestrated action of actin and microtubule cytoskeleton and intracellular vesicle traffic plays a key role in all these events that together ensure important steps in T cell physiology. These include extravasation and migration through lymphoid and peripheral tissues, T cell interactions with antigen-presenting cells, T cell receptor (TCR) triggering by cognate antigen–major histocompatibility complex (MHC) complexes, immunological synapse formation, cell activation, and effector functions. Cytoskeletal and vesicle traffic dynamics and their interplay are coordinated by a variety of regulatory molecules. Among them, polarity regulators and membrane–cytoskeleton linkers are master controllers of this interplay. Here, we review the various ways the T cell plasma membrane, receptors, and their signaling machinery interplay with the actin and microtubule cytoskeleton and with intracellular vesicular compartments. We highlight the importance of this fine-tuned crosstalk in three key stages of T cell biology involving cell polarization: T cell migration in response to chemokines, immunological synapse formation in response to antigen cues, and effector functions. Finally, we discuss two examples of perturbation of this interplay in pathological settings, such as HIV-1 infection and mutation of the polarity regulator and tumor suppressor adenomatous polyposis coli (Apc) that leads to familial polyposis and colorectal cancer.

Ubiquitination of IgG1 cytoplasmic tail modulates B-cell signalling and activation

Upon antigen stimulation, IgG+ B cells rapidly proliferate and differentiate into plasma cells, which has been attributed to the characteristics of membrane-bound IgG (mIgG), but the underlying molecular mechanisms remain elusive. We have found that a part of mouse mIgG1 is ubiquitinated through the two responsible lysine residues (K378 and K386) in its cytoplasmic tail and this ubiquitination is augmented upon antigen stimulation. The ubiquitination of mIgG1 involves its immunoglobulin tail tyrosine (ITT) motif, Syk/Src-family kinases and Cbl proteins. Analysis of a ubiquitination-defective mutant of mIgG1 revealed that ubiquitination of mIgG1 facilitates its ligand-induced endocytosis and intracellular trafficking from early endosome to late endosome, and also prohibits the recycling pathway, thus attenuating the surface expression level of mIgG1. Accordingly, ligation-induced activation of B-cell receptor (BCR) signalling molecules is attenuated by the mIgG1 ubiquitination, except MAP kinase p38 whose activation is up-regulated due to the ubiquitination-mediated prohibition of mIgG1 recycling. Adaptive transfer experiments demonstrated that ubiquitination of mIgG1 facilitates expansion of germinal centre B cells. These results indicate that mIgG1-mediated signalling and cell activation is regulated by ubiquitination of mIgG1, and such regulation may play a role in expansion of germinal centre B cells.

MiR-214 regulates CD3ζ expression in T cells

Introduction

T-cell activation requires the T-cell receptor (TCR)-CD3 complex, which integrates and transduces signals. CD3ζ plays a vital role in TCR signalling by mediating T-cell activation. Abnormal CD3ζ expression is a common characteristic of haematological malignancies with T-cell immune dysfunction or autoimmune diseases. Targeted regulation of CD3ζ expression by either direct or indirect approaches is important for regulating T-cell activation.

Aim of the study

In this study, we focused on identifying miRNAs that may regulate CD3ζ expression.

Material and methods

Three microRNA target search algorithms (TargetScan, PicTar, and microrna.org) were used to identify hypothetical miRNAs that target CD3ζ in T cells. Of the predicted miRNAs, miR-214 was chosen and validated to determine whether miR-214 directly binds to the CD3ζ 3’-UTR and regulates CD3ζ expression by luciferase reporter assays, real-time PCR, and Western blotting.

Results

The results indicate that miR-214 specifically binds the CD3ζ 3’-UTR, and miR-214 mimics remarkably reduce the expression of CD3ζ in MOLT-4 cells.

Conclusions

We identify for the first time that miR-214 targets expression in MOLT-4 cells, suggesting that miR-214 might negatively regulate T-cell activation by targeting CD3ζ.

Coupled feedback regulation of nuclear factor of activated T-cells (NFAT) modulates activation-induced cell death of T cells

A properly functioning immune system is vital for an organism’s wellbeing. Immune tolerance is a critical feature of the immune system that allows immune cells to mount effective responses against exogenous pathogens such as viruses and bacteria, while preventing attack to self-tissues. Activation-induced cell death (AICD) in T lymphocytes, in which repeated stimulations of the T-cell receptor (TCR) lead to activation and then apoptosis of T cells, is a major mechanism for T cell homeostasis and helps maintain peripheral immune tolerance. Defects in AICD can lead to development of autoimmune diseases. Despite its importance, the regulatory mechanisms that underlie AICD remain poorly understood, particularly at an integrative network level. Here, we develop a dynamic multi-pathway model of the integrated TCR signalling network and perform model-based analysis to characterize the network-level properties of AICD. Model simulation and analysis show that amplified activation of the transcriptional factor NFAT in response to repeated TCR stimulations, a phenomenon central to AICD, is tightly modulated by a coupled positive-negative feedback mechanism. NFAT amplification is predominantly enabled by a positive feedback self-regulated by NFAT, while opposed by a NFAT-induced negative feedback via Carabin. Furthermore, model analysis predicts an optimal therapeutic window for drugs that help minimize proliferation while maximize AICD of T cells. Overall, our study provides a comprehensive mathematical model of TCR signalling and model-based analysis offers new network-level insights into the regulation of activation-induced cell death in T cells.

TCellXTalk facilitates the detection of co-modified peptides for the study of protein post-translational modification cross-talk in T cells

MOTIVATION

Protein function is regulated by post-translational modifications (PTMs) that may act individually or interact with others in a phenomenon termed PTM cross-talk. Multiple databases have been dedicated to PTMs, including recent initiatives oriented towards the in silico prediction of PTM interactions. The study of PTM cross-talk ultimately requires experimental evidence about whether certain PTMs coexist in a single protein molecule. However, available resources do not assist researchers in the experimental detection of co-modified peptides.

RESULTS

Herein, we present TCellXTalk, a comprehensive database of phosphorylation, ubiquitination and acetylation sites in human T cells that supports the experimental detection of co-modified peptides using targeted or directed mass spectrometry. We demonstrate the efficacy of TCellXTalk and the strategy presented here in a proof of concept experiment that enabled the identification and quantification of 15 co-modified (phosphorylated and ubiquitinated) peptides from CD3 proteins of the T-cell receptor complex. To our knowledge, these are the first co-modified peptide sequences described in this widely studied cell type. Furthermore, quantitative data showed distinct dynamics for co-modified peptides upon T cell activation, demonstrating differential regulation of co-occurring PTMs in this biological context. Overall, TCellXTalk facilitates the experimental detection of co-modified peptides in human T cells and puts forward a novel and generic strategy for the study of PTM cross-talk.

AVAILABILITY AND IMPLEMENTATION

TCellXTalk is available at https://www.tcellxtalk.org. Source Code is available at https://bitbucket.org/lp-csic-uab/tcellxtalk.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

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.

Control of T cell antigen reactivity via programmed TCR downregulation

The T cell receptor (TCR) is unique in that its affinity for ligand is unknown prior to encounter and can vary by orders of magnitude. How the immune system regulates individual T cells that display highly different reactivity to antigen remains unclear. Here we identified that activated CD4⁺ T cells, at the peak of clonal expansion, persistently downregulate TCR expression in proportion to the strength of initial antigen recognition. This programmed response increases the threshold for cytokine production and recall proliferation in a clone-specific manner, ultimately excluding clones with the highest antigen reactivities. Thus, programmed TCR downregulation represents a negative feedback mechanism to constrain T cell effector function with a suitable time delay, thereby allowing pathogen control while avoiding excess inflammatory damage.

Ubiquitin-dependent endocytosis of NKG2D-DAP10 receptor complexes activates signaling and functions in human NK cells

Cytotoxic lymphocytes share the presence of the activating receptor NK receptor group 2, member D (NKG2D) and the signaling-competent adaptor DNAX-activating protein 10 (DAP10), which together play an important role in antitumor immune surveillance. Ligand stimulation induces the internalization of NKG2D-DAP10 complexes and their delivery to lysosomes for degradation. In experiments with human NK cells and cell lines, we found that the ligand-induced endocytosis of NKG2D-DAP10 depended on the ubiquitylation of DAP10, which was also required for degradation of the internalized complexes. Moreover, through combined biochemical and microscopic analyses, we showed that ubiquitin-dependent receptor endocytosis was required for the activation of extracellular signal-regulated kinase (ERK) and NK cell functions, such as the secretion of cytotoxic granules and the inflammatory cytokine interferon-γ. These results suggest that NKG2D-DAP10 endocytosis represents a means to decrease cell surface receptor abundance, as well as to control signaling outcome in cytotoxic lymphocytes.

A RING finger protein 114 (RNF114) homolog from Chinese sturgeon (Acipenser sinensis) possesses immune-regulation properties via modulating RIG-I signaling pathway-mediated interferon expression

Ubiquitin ligases play important roles in immune regulation. The human RNF114 (RING finger protein 114), an ubiquitin ligase, was recently reported to be involved in immune response to double-stranded RNA in disease pathogenesis. Here, we identified a RNF114 homolog in Chinese sturgeon (Acipenser sinensis) and investigated its potential role in immune response. The full-length cDNA of Chinese sturgeon RNF114 (csRNF114) contains an open reading frame (ORF) of 681 nucleotides coding a protein of 227 amino acids. csRNF114 shares the highest identity of 76% at amino acid level to other RNF114 homologs, clustering with bony fish RNF114s based on phylogenetic analysis. The main structural features of csRNF114, including a C3HC4 (Cys3-His-Cys4) RING domain, a C2HC (Cys2-His-Cys)-type zinc finger motif, a C2H2 (Cys2-His2)-type zinc finger motif, and a UIM (ubiquitin-interacting motif), take csRNF114 as an ubiquitin ligase. csRNF114 mRNA was widely expressed in various tissues and significantly up-regulated in poly(I:C)-treated Chinese sturgeon. Over-expression of csRNF114 in HEK293T cells significantly promoted both basal and poly(I:C)-induced activation of interferon regulatory transcription factor 3 (IRF3) and nuclear factor-κB (NF-κB) downstream retinoic acid inducible gene I (RIG-I) signaling pathway and expression of target genes type I interferon (IFN), which was nearly abolished by knockdown of RIG-I with specific human siRNA and by mutation of the C3HC4 RING domain (C28A/C31A) in csRNF114 as well. Furthermore, csRNF114 associated with ubiquitinated proteins in HEK293T cells, for which the C3HC4 RING domain was essential. These data suggested that an ubiquitin ligase RNF114 homolog with a potential role in antiviral response possibly through modulating RIG-I signaling pathway was cloned from Chinese sturgeon, which might contribute to our understanding of the immune biology of Chinese sturgeon.

Unique ζ-chain motifs mediate a direct TCR-actin linkage critical for immunological synapse formation and T-cell activation

TCR-mediated activation induces receptor microclusters that evolve to a defined immune synapse (IS). Many studies showed that actin polymerization and remodeling, which create a scaffold critical to IS formation and stabilization, are TCR mediated. However, the mechanisms controlling simultaneous TCR and actin dynamic rearrangement in the IS are yet not fully understood. Herein, we identify two novel TCR ζ-chain motifs, mediating the TCR's direct interaction with actin and inducing actin bundling. While T cells expressing the ζ-chain mutated in these motifs lack cytoskeleton (actin) associated (cska)-TCRs, they express normal levels of non-cska and surface TCRs as cells expressing wild-type ζ-chain. However, such mutant cells are unable to display activation-dependent TCR clustering, IS formation, expression of CD25/CD69 activation markers, or produce/secrete cytokine, effects also seen in the corresponding APCs. We are the first to show a direct TCR-actin linkage, providing the missing gap linking between TCR-mediated Ag recognition, specific cytoskeleton orientation toward the T-cell-APC interacting pole and long-lived IS maintenance.

AMPK functions as an adenylate charge-regulated protein kinase

The energy sensor AMP-activated protein kinase (AMPK) is activated by metabolic stress and restores ATP levels in cells by switching off anabolic and switching on catabolic pathways. Recent discoveries demonstrate that AMPK is activated primarily by rising ADP levels and not, as previously thought, by AMP. AMPK activation is dependent on ADP-controlled phosphorylation of Thr172 on its activation loop, a mechanism of protein regulation that represents an example of an allosterically regulated modification (ARM). AMPK embodies many features of an adenylate charge regulatory system envisaged by Atkinson, where anabolic and catabolic pathway regulation is modulated by adenine nucleotide ratios. Here we discuss the current state of AMPK regulation by adenine nucleotides and we propose that AMPK functions as an adenylate charge-regulated protein kinase.

The CD3 zeta subunit contains a phosphoinositide-binding motif that is required for the stable accumulation of TCR-CD3 complex at the immunological synapse

T cell activation involves a cascade of TCR-mediated signals that are regulated by three distinct intracellular signaling motifs located within the cytoplasmic tails of the CD3 chains. Whereas all the CD3 subunits possess at least one ITAM, the CD3 ε subunit also contains a proline-rich sequence and a basic-rich stretch (BRS). The CD3 ε BRS complexes selected phosphoinositides, interactions that are required for normal cell surface expression of the TCR. The cytoplasmic domain of CD3 ζ also contains several clusters of arginine and lysine residues. In this study, we report that these basic amino acids enable CD3 ζ to complex the phosphoinositides PtdIns(3)P, PtdIns(4)P, PtdIns(5)P, PtdIns(3,5)P(2), and PtdIns(3,4,5)P(3) with high affinity. Early TCR signaling pathways were unaffected by the targeted loss of the phosphoinositide-binding functions of CD3 ζ. Instead, the elimination of the phosphoinositide-binding function of CD3 ζ significantly impaired the ability of this invariant chain to accumulate stably at the immunological synapse during T cell-APC interactions. Without its phosphoinositide-binding functions, CD3 ζ was concentrated in intracellular structures after T cell activation. Such findings demonstrate a novel functional role for CD3 ζ BRS-phosphoinositide interactions in supporting T cell activation.

K33-linked polyubiquitination of T cell receptor-zeta regulates proteolysis-independent T cell signaling

Tagging the cell surface receptor with ubiquitin is believed to provide a signal for the endocytic pathway. E3 ubiquitin ligases such as Cbl-b and Itch have been implicated in T cell activation and tolerance induction. However, the underlying mechanisms remain unclear. We describe that in mice deficient in the E3 ubiquitin ligases Cbl-b and Itch, T cell activation was augmented, accompanied by spontaneous autoimmunity. The double-mutant T cells exhibited increased phosphorylation of the T cell receptor-zeta (TCR-zeta) chain, whereas the endocytosis and stability of the TCR complex were not affected. TCR-zeta was polyubiquitinated via a K33-linkage, which affected its phosphorylation and association with the zeta chain-associated protein kinase Zap-70. The juxtamembrane K54 residue in TCR-zeta was identified to be a primary ubiquitin conjugation site, whose mutation increased its phosphorylation and association of TCR-zeta and Zap-70. Thus, the present study reveals unconventional K33-linked polyubiquitination in nonproteolytic regulation of cell-surface-receptor-mediated signal transduction.

Invariant NKT cell development requires a full complement of functional CD3 zeta immunoreceptor tyrosine-based activation motifs

Invariant NKT (iNKT) cells regulate early immune responses to infections, in part because of their rapid release of IFN-gamma and IL-4. iNKT cells are proposed to reduce the severity of Lyme disease following Borrelia burgdorferi infection. Unlike conventional T cells, iNKT cells express an invariant alphabeta TCR that recognizes lipids bound to the MHC class I-like molecule, CD1d. Furthermore, these cells are positively selected following TCR interactions with glycolipid/CD1d complexes expressed on CD4+CD8+ thymocytes. Whereas conventional T cell development can proceed with as few as 4/10 CD3 immunoreceptor tyrosine-based activation motifs (ITAMs), little is known about the ITAM requirements for iNKT cell selection and expansion. We analyzed iNKT cell development in CD3 zeta transgenic lines with various tyrosine-to-phenylalanine substitutions (YF) that eliminated the functions of the first (YF1,2), third (YF5,6), or all three (YF1-6) CD3 zeta ITAMs. iNKT cell numbers were significantly reduced in the thymus, spleen, and liver of all YF mice compared with wild type mice. The reduced numbers of iNKT cells resulted from significant reductions in the expression of the early growth response 2 and promyelocytic leukemia zinc finger transcription factors. In the mice with few to no iNKT cells, there was no difference in the severity of Lyme arthritis compared with wild type controls, following infections with the spirochete B. burgdorferi. These findings indicate that a full complement of functional CD3 zeta ITAMs is required for effective iNKT cell development.

Essential role of ubiquitin and TSG101 protein in formation and function of the central supramolecular activation cluster

Agonist MHC-peptide complexes in the immunological synapse (IS) signal through T cell receptor (TCR) microclusters (MCs) that converge into a central supramolecular activation cluster (cSMAC). The determinants and function of the cSMAC remain unknown. We demonstrate an essential role for ubiquitin (Ub) and TSG101, but less so for HRS, in signal processing events at the cSMAC. Using siRNA in primary T cells, we show that Ub recognition by TSG101 is required for cSMAC formation, TCR MC signal termination, TCR downregulation, and segregation of TCR-MHC-peptide from PKC-theta-enriched signaling complexes. Weak agonist MHC-peptide induced CD80-dependent TCR MCs that dissociated in the center of the IS without recruiting TSG101. These results support TSG101-dependent recognition of CD80-independent TCR MCs as a molecular checkpoint for TCR downregulation.

Tonic ubiquitylation controls T-cell receptor:CD3 complex expression during T-cell development

Expression of the T-cell receptor (TCR):CD3 complex is tightly regulated during T-cell development. The mechanism and physiological role of this regulation are unclear. Here, we show that the TCR:CD3 complex is constitutively ubiquitylated in immature double positive (DP) thymocytes, but not mature single positive (SP) thymocytes or splenic T cells. This steady state, tonic CD3 monoubiquitylation is mediated by the CD3varepsilon proline-rich sequence, Lck, c-Cbl, and SLAP, which collectively trigger the dynamin-dependent downmodulation, lysosomal sequestration and degradation of surface TCR:CD3 complexes. Blocking this tonic ubiquitylation by mutating all the lysines in the CD3 cytoplasmic tails significantly upregulates TCR levels on DP thymocytes. Mimicking monoubiquitylation by expression of a CD3zeta-monoubiquitin (monoUb) fusion molecule significantly reduces TCR levels on immature thymocytes. Moreover, modulating CD3 ubiquitylation alters immunological synapse (IS) formation and Erk phosphorylation, thereby shifting the signalling threshold for positive and negative selection, and regulatory T-cell development. Thus, tonic TCR:CD3 ubiquitylation results in precise regulation of TCR expression on immature T cells, which is required to maintain the fidelity of T-cell development.

Unbiased detection of posttranslational modifications using mass spectrometry

A major challenge in proteomics is to fully identify and characterize posttranslational modification (PTM) patterns present at any given time in cells, tissues, and organisms. Currently, the most frequently used method for identifying PTMs is tandem mass spectrometry combined with searching a protein sequence database. Although, database searching has been highly successful for the identification of proteins, it has a number of significant drawbacks for identification of modifications. The user needs to specify all expected modifications, and the search engine needs to consider all possible combinations of these modifications for all peptide sequences. If several potential modifications are considered, the search can take much longer than the data acquisition, creating a bottleneck in high-throughput analysis. In addition, the many possible assignments that need to be tested increase the noise and require better quality data for confident identification of modifications. Here, we describe a method for identifying both known and unknown PTM using mass spectrometry that does not suffer from these problems. The method is based on the observation that, in many samples, peptides are usually present both with and without modifications. By identifying the unmodified peptide with conventional database searches, the modified species of the peptide can be identified by searching for peptides with common and similar fragments as the unmodified peptide. After identifying both the modified and unmodified peptide, the elemental composition of the modification can be deduced if the mass accuracy of the precursor ion is sufficiently high.

The Sts proteins target tyrosine phosphorylated, ubiquitinated proteins within TCR signaling pathways

The T cell receptor (TCR) detects the presence of infectious pathogens and activates numerous intracellular signaling pathways. Protein tyrosine phosphorylation and ubiquitination serve as key regulatory mechanisms downstream of the TCR. Negative regulation of TCR signaling pathways is important in controlling the immune response, and the Suppressor of TCR Signaling proteins (Sts-1 and Sts-2) have been shown to function as critical negative regulators of TCR signaling. Although their mechanism of action has yet to be fully uncovered, it is known that the Sts proteins possess intrinsic phosphatase activity. Here, we demonstrate that Sts-1 and Sts-2 are instrumental in down-modulating proteins that are dually modified by both protein tyrosine phosphorylation and ubiquitination. Specifically, both naïve and activated T cells derived from genetically engineered mice that lack the Sts proteins display strikingly elevated levels of tyrosine phosphorylated, ubiquitinated proteins following TCR stimulation. The accumulation of the dually modified proteins is transient, and in activated T cells but not naïve T cells is significantly enhanced by co-receptor engagement. Our observations hint at a novel regulatory mechanism downstream of the T cell receptor.

The ubiquitin‐proteasome system—micro target for macro intervention?

The ubiquitin-proteasome system is the two sequential labeling and degradation system that accounts for the degradation of 80-90% of all intracellular proteins. Based on the diversity of its substrates, it is integrated in many different biological processes, especially inflammation and cell proliferation. Given the significance of these two processes for primary atherosclerosis and restenosis, the ubiquitin-proteasome system may be an amendable target in cardiovascular therapy. This review provides background information on the ubiquitin-proteasome system, currently available data on its involvement in cardiovascular diseases, and a future perspective on the targeted use proteasome inhibitors, including drug-eluting stents.

The CD3Z 844 T>A polymorphism within the 3'-UTR of CD3Z confers increased risk of incidence of systemic lupus erythematosus

Recently, a family-based association analysis showed that the haplotype carrying a low expression of the variant CD3Z 844 T>A (rs1052231) polymorphism located in the 3'-untranslated region of CD3Z predisposes to systemic lupus erythematosus (SLE) incidence. We analyzed the prevalence of the CD3Z 844 T>A polymorphism in SLE patients (n = 152) and controls (n = 304) in Poland. We observed that women with the CD3Z AA and CD3Z AT genotypes exhibited a 1.845-fold increased risk of SLE [95% confidence intervals (95% CI) = 1.222-2.787, P = 0.0038]. However, we did not find an increased risk for the homozygous CD3Z AA genotype (odds ratio = 1.204, 95% CI = 0.2838-5.108, P = 1.0000). This observation confers that genetic factors causing a decreased level of CD3-zeta in T cells may predispose to SLE incidence.

Mechanisms involved in the down-regulation of TCR zeta chain in tumor versus peripheral blood of oral cancer patients

Immune dysfunction is the hallmark of patients with oral cancer. Down-regulation of T cell receptor (TCR) zeta chain expression was observed in T cells from patients with oral squamous cell carcinoma. In peripheral blood, the decrease in TCR zeta chain showed an inverse correlation with the tumor stage as demonstrated by western blotting, confocal microscopy and flow cytometry. The mechanism of TCR zeta chain degradation in the peripheral blood involves ubiquitination and subsequent targeting of TCR zeta for degradation in the lysosome. Decreased expression of PKC theta and the subsequent decrease of TCR zeta chain transcription factor Elf-1 and its binding to DNA may contribute to the decreased/or absent TCR zeta chain transcripts in the tumor infiltrating lymphocytes. Oral cancer patients exhibiting TCR zeta chain defect also showed impaired lymphocyte proliferation, cytokine profile and intracellular calcium release upon stimulation with anti CD3 mAb. Our data shows that posttranslational degradation is primarily responsible for decreased TCR zeta chain expression in the peripheral blood, while a transcriptional defect is observed in the tumor compartment. The down-regulation of TCR zeta chain culminates into impaired lymphocyte responses in these patients.

Differential expression of the human CD8beta splice variants and regulation of the M-2 isoform by ubiquitination

The CD8alphabeta heterodimer functions as a coreceptor with the TCR, influencing the outcome of CD8(+) T cell responses to pathogen-infected and tumor cells. In contrast to the murine CD8B gene, the human gene encodes alternatively spliced variants with different cytoplasmic tails (M-1, M-2, M-3, and M-4). At present, little is known about the expression patterns and functional significance of such variants. We used quantitative RT-PCR to demonstrate differential mRNA expression patterns of these splice variants in thymocytes and in resting, memory, and activated primary human CD8(+) T cells. In total CD8(+) T cells, mRNA levels of the M-1 variant were the most predominant and levels of M-3 were the least detected. The M-4 isoform was predominant in effector memory CD8(+) T cells. Upon stimulation of CD8(+) T cells, the M-2 variant mRNA levels were elevated 10-20-fold relative to resting cells in contrast to the other isoforms. Curiously, the M-2 isoform was not expressed on the cell surface in transfected cell lines. Using fluorescent chimeras of the extracellular domain of mouse CD8beta fused to the cytoplasmic tails of each isoform, the M-2 isoform was localized in a lysosomal compartment regulated by ubiquitination of a lysine residue (K215) in its cytoplasmic tail. In contrast, upon short-term stimulation, the M-2 protein localized to the cell surface with the TCR complex. The relatively recent evolution of CD8B gene splice variants in the chimpanzee/human lineage is most likely important for fine-tuning the CD8(+) T cell responses.

Polymorphisms in the CD3Z gene influence TCRzeta expression in systemic lupus erythematosus patients and healthy controls

TCRzeta (CD247) functions as an amplification module in the TCR signaling cascade and is essential for assembly and surface expression of the TCR/CD3 complex. The TCRzeta-chain is down-regulated in many chronic infectious and inflammatory diseases, including systemic lupus erythematosus (SLE). It is unclear whether reduced TCRzeta expression is a cause or a consequence of chronic inflammatory responses. We have addressed this question by adopting a combined genetic and functional approach. We analyzed TCRzeta protein expression using a FACS-based expression index and documented considerable, but longitudinally stable, variation in TCRzeta expression in healthy individuals. The variation in TCRzeta expression was associated with polymorphisms in the CD3Z 3'-untranslated region (UTR) in SLE patients and healthy controls. Detailed mapping of the 3'-UTR revealed that the minor alleles of two single nucleotide polymorphisms (SNPs) in strong disequilibrium (rs1052230 and rs1052231) were the causal variants associated with low TCRzeta expression (p=0.015). Using allelic imbalance analysis, the minor alleles of these 3'-UTR SNPs were associated with one-third of the level of mRNA compared with the major allele. A family-based association analysis showed that the haplotype carrying the low-expression variants predisposes to SLE (p=0.033). This suggests that a genetically determined reduction in TCRzeta expression has functional consequences manifested by systemic autoimmunity.

T-cell regulator RNF125/TRAC-1 belongs to a novel family of ubiquitin ligases with zinc fingers and a ubiquitin-binding domain

The recently identified RNF125 [RING (really interesting new gene) finger protein 125], or TRAC-1 (T-cell RING protein in activation 1), is unique among ubiquitin ligases in being a positive regulator of T-cell activation. In addition, TRAC-1 has been shown to down-modulate HIV replication and to inhibit pathogen-induced cytokine production. However, apart from the presence of an N-terminal C3HC4 (Cys(3)-His-Cys(4)) RING domain, the TRAC-1 protein remains uncharacterized. In the present paper, we report novel interactions and modifications for TRAC-1, and elucidate its domain organization. Specifically, we determine that TRAC-1 associates with membranes and is excluded from the nucleus through myristoylation. Our data are further consistent with a crucial role for the C-terminus in TRAC-1 function. In this region, novel domains were recognized through the identification of three closely related proteins: RNF114, RNF138 and RNF166. TRAC-1 and its relatives were found to contain, apart from the RING domain, a C2HC (Cys(2)-His-Cys)- and two C2H2 (Cys(2)-His(2))-type zinc fingers, as well as a UIM (ubiquitin-interacting motif). The UIM of TRAC-1 binds Lys(48)-linked polyubiquitin chains and is, together with the RING domain, required for auto-ubiquitination. As a consequence of auto-ubiquitination, the half-life of TRAC-1 is shorter than 30 min. The identification of these novel modifications, interactions, domains and relatives significantly widens the contexts for investigating TRAC-1 activity and regulation.

Molecular mechanisms of CD4+ T-cell anergy

Directing both innate and adaptive immune responses against foreign pathogens with correct timing, location and specificity is a fundamental objective for the immune system. Full activation of CD4+ T cells requires the binding of peptide-MHC complexes coupled with accessory signals provided by the antigen-presenting cell. However, aberrant activation of the T-cell receptor alone in mature T cells can produce a long-lived state of functional unresponsiveness, known as anergy. Recent studies probing both immune signalling pathways and the ubiquitin-proteasome system have helped to refine and elaborate current models for the molecular mechanisms underlying T-cell anergy. Controlling anergy induction and maintenance will be a key component in the future to mitigate unwanted T-cell activation that leads to autoimmune disease.

Evaluation of hepatitis B virus replication and proteomic analysis of HepG2.2.15 cell line after cyclosporine A treatment

AIM

The effect of cyclosporine A (CsA) on hepatitis B virus (HBV) replication was investigated, and proteomics expression differentiation after CsA treatment was studied in order to provide clues to explore the effect of CsA on HBV replication.

METHODS

Methyl thiazolyl tetrazolium (MTT) assay was used to evaluate the cytotoxicity of CsA. The HBV replication level in the HBV genomic DNA transfected HepG2.2.15 cell line was determined by an ELISA analysis of hepatitis B surface antigens (HBsAg) and Hepatitis B e antigens (HBeAg) in culture supernatant, while the intracellular HBV DNA replication level was analyzed by slot blot hybridization. Two-dimensional electrophoresis was used to investigate the alteration of protein expression in HepG2.2.15 after CsA treatment in vitro. The differentially-expressed proteins were identified by Matrix-assisted laser desorption/ionization-time of flight mass spectrometry combined with an online database search.

RESULTS

CsA was able to inhibit the expression of HBsAg, HBeAg, and HBV DNA replication in vitro in a dose-dependent manner. A proteomics analysis indicated that the expression of 17 proteins changed significantly in the CsA treatment group compared to the control group. Eleven of the 17 proteins were identified, including the overexpression of eukaryotic translation initiation factors (eIF) 3k, otubain 1, 14.3.3 protein, eIF2-1 alpha, eIF5A, and the tyrosine 3/tryptophan 5-mono-oxygenase activation protein in CsA-treated HepG2.2.15 cells. The downregulation of the ferritin light subunit, erythrocyte cytosolic protein of 51 kDa (ECP-51), stathmin 1/oncoprotein, adenine phosphoribosyl-transferase, and the position of a tumor protein, translationally controlled 1, was shifted, suggesting it had undergone posttranslational modifications.

CONCLUSION

Our study identified the inhibitory effect of CsA on HBV replication, and found that a group of proteins may be responsible for this inhibitory effect.

Phosphorylation of Bcl10 negatively regulates T-cell receptor-mediated NF-kappaB activation

Bcl10 (B-cell lymphoma 10) is an adaptor protein comprised of an N-terminal caspase recruitment domain and a C-terminal serine/threonine-rich domain. Bcl10 plays a critical role in antigen receptor-mediated NF-kappaB activation and lymphocyte development and functions. Our current study has discovered that T-cell activation induced monophosphorylation and biphosphorylation of Bcl10 and has identified S138 within Bcl10 as one of the T-cell receptor-induced phosphorylation sites. Alteration of S138 to an alanine residue impaired T-cell activation-induced ubiquitination and subsequent degradation of Bcl10, ultimately resulting in prolongation of TCR-mediated NF-kappaB activation and enhancement of interleukin-2 production. Taken together, our findings demonstrate that phosphorylation of Bcl10 at S138 down-regulates Bcl10 protein levels and thus negatively regulates T-cell receptor-mediated NF-kappaB activation.

Seven-transmembrane receptors and ubiquitination

Regulation of protein function by posttranslational modification plays an important role in many biological pathways. The most well known among such modifications is protein phosphorylation performed by highly specific protein kinases. In the past decade, however, covalent linkage of the low-molecular-weight protein ubiquitin to substrate proteins (protein ubiquitination) has proven to be yet another widely used mechanism of protein regulation playing a crucial role in virtually all aspects of cellular functions. This review highlights some of the recently discovered and provocative roles for ubiquitination in the regulation of the life cycle and signal transduction properties of 7-transmembrane receptors that serve to integrate many biological functions and play fundamental roles in cardiovascular homeostasis.

Role of Ubiquitylation in Cellular Membrane Transport

Ubiquitylation of membrane proteins has gained considerable interest in recent years. It has been recognized as a signal that negatively regulates the cell surface expression of many plasma membrane proteins both in yeast and in mammalian cells. Moreover, it is also involved in endoplasmic reticulum-associated degradation of membrane proteins, and it acts as a sorting signal both in the secretory pathway and in endosomes, where it targets proteins into multivesicular bodies in the lumen of vacuoles/lysosomes. In this review we discuss the progress in understanding these processes, achieved during the past several years.

Src-like adaptor protein regulates TCR expression on thymocytes by linking the ubiquitin ligase c-Cbl to the TCR complex

The adaptor molecule SLAP and E3 ubiquitin ligase c-Cbl each regulate expression of T cell receptor (TCR)-CD3 on thymocytes. Here we provide genetic and biochemical evidence that both molecules function in the same pathway. TCR-CD3 expression was similar in the absence of SLAP and/or c-Cbl. SLAP and c-Cbl were found to interact, and their expression together downregulated CD3epsilon. This required multiple domains in SLAP and the ring finger of c-Cbl. Furthermore, expression of SLAP and c-Cbl together induced TCRzeta ubiquitination and degradation, preventing the accumulation of fully assembled recycling TCR complexes. These studies indicate that SLAP links the E3 ligase activity of c-Cbl to the TCR, allowing for stage-specific regulation of TCR expression.

Src-like adaptor protein down-regulates T cell receptor (TCR)-CD3 expression by targeting TCRzeta for degradation

Src-like adaptor protein (SLAP) down-regulates expression of the T cell receptor (TCR)–CD3 complex during a specific stage of thymocyte development when the TCR repertoire is selected. Consequently, SLAP−/− thymocytes display alterations in thymocyte development. Here, we have studied the mechanism of SLAP function. We demonstrate that SLAP-deficient thymocytes have increased TCRζ chain expression as a result of a defect in TCRζ degradation. Failure to degrade TCRζ leads to an increased pool of fully assembled TCR–CD3 complexes that are capable of recycling back to the cell surface. We also provide evidence that SLAP functions in a pathway that requires the phosphorylated TCRζ chain and the Src family kinase Lck, but not ZAP-70 (ζ-associated protein of 70 kD). These studies reveal a unique mechanism by which SLAP contributes to the regulation of TCR expression during a distinct stage of thymocyte development.

Evidences for ubiquitination and intracellular trafficking of LAT, the linker of activated T cells

Current evidences indicate that T cells use protein sorting and degradation to control duration and specificity of T cell receptor (TcR) signalling, including the CD3zeta chain which is ubiquitinated upon TcR triggering. In a previous study, we showed that the Linker of activated T cells (LAT) is present at the plasma membrane and in transferrin-labelled intracellular compartments also containing the CD3zeta chain. Here we show that LAT protein levels are tightly regulated in Jurkat lymphoid T cells likely involving proteasome-dependent degradation, recycling through trans-Golgi/endosome compartments and clathrin-dependent internalisation. We further identify a novel post-translational modification of LAT by ubiquitination that is likely to influence LAT protein stability, intracellular localisation and/or recycling. Our results provide novel molecular and regulatory insights into the function of LAT adapter protein in T cell signalling.

Noncovalent interaction of MNSFbeta, a ubiquitin-like protein, with histone 2A

Monoclonal nonspecific suppressor factor (MNSF), a lymphokine produced by murine T cell hybridoma, possesses pleiotrophic antigen-nonspecific suppressive functions. A cDNA clone encoding MNSFbeta, an isoform of the MNSF, has been isolated and characterized. MNSFbeta cDNA encodes a fusion protein consisting of a ubiquitin-like segment (Ubi-L) and ribosomal protein S30. Most recently, we observed that Ubi-L covalently conjugates to Bcl-G, a novel pro-apoptotic protein. In this study, we observed that Ubi-L noncovalently and specifically binds to histone 2A. The maximum binding was observed at a molar ratio equal to 1 for GST-Ubi-L and 2 for histone 2A. Ubi-L formed complex with histone 2A in the presence of 1% Triton X-100. Free Ubi-L was detected in nuclei from unstimulated murine helper T cell line, D10. The increased amounts of free Ubi-L and some Ubi-L adducts were observed in nuclei from mitogen-activated D10 cells. Interestingly, two Ubi-L adducts were unique to the chromatin fraction of nuclei from the activated D10 cells.

A Novel E3 Ubiquitin Ligase TRAC-1 Positively Regulates T Cell Activation

TRAC-1 (T cell RING (really interesting new gene) protein identified in activation screen) is a novel E3 ubiquitin ligase identified from a retroviral vector-based T cell surface activation marker screen. The C-terminal truncated TRAC-1 specifically inhibited anti-TCR-mediated CD69 up-regulation in Jurkat cells, a human T leukemic cell line. In this study, we show that TRAC-1 is a RING finger ubiquitin E3 ligase with highest expression in lymphoid tissues. Point mutations that disrupt the Zn(2+)-chelating ability of its amino-terminal RING finger domain abolished TRAC-1's ligase activity and the dominant inhibitory effect of C-terminal truncated TRAC-1 on TCR stimulation. The results of in vitro biochemical studies indicate that TRAC-1 can stimulate the formation of both K48- and K63-linked polyubiquitin chains and therefore could potentially activate both degradative and regulatory ubiquitin-dependent pathways. Antisense oligonucleotides to TRAC-1 specifically reduced TRAC-1 mRNA levels in Jurkat and primary T cells and inhibited their activation in response to TCR cross-linking. Collectively, these results indicate that the E3 ubiquitin ligase TRAC-1 functions as a positive regulator of T cell activation.

Dissecting the molecular mechanisms of TCR zeta chain downregulation and T cell signaling abnormalities in human systemic lupus erythematosus

Abnormal expression of key signaling molecules and defective function of T lymphocytes play a significant role in the pathogenesis of systemic lupus erythematosus (SLE). Probing on altered expression of genes that may predispose to SLE revealed that the expression of TCR zeta chain is defective in the majority of SLE patients. Current research has been directed towards understanding the molecular basis of TCR zeta chain deficiency and dissecting the T cell signalling abnormalities in SLE T cells. Latest developments suggest that interplay of abnormal transcriptional factor expression, aberrant mRNA processing/editing, unbiquitination, proteolysis, and the effects of oxidative stress as well as changes in chromatin structure invariably contribute to TCR zeta chain deficiency in SLE T cells. On the other hand, multiple factors, including altered receptor structure, modulation of membrane clustering, lipid-raft distribution of signaling molecules, and defective signal silencing mechanisms, play a key role in delivering the increased TCR/CD3-mediated intracellular calcium response in SLE T cells.

T cell receptor engagement leads to phosphorylation of clathrin heavy chain during receptor internalization

T cell receptor (TCR) internalization by clathrin-coated vesicles after encounter with antigen has been implicated in the regulation of T cell responses. We demonstrate that TCR internalization after receptor engagement and TCR signaling involves inducible phosphorylation of clathrin heavy chain (CHC) in both CD4⁺ and CD8⁺ human T cells. Studies with mutant Jurkat T cells implicate the Src family kinase Lck as the responsible enzyme and its activity in this process is influenced by the functional integrity of the downstream signaling molecule ZAP-70. CHC phosphorylation positively correlates with ligand-induced TCR internalization in both CD4⁺ and CD8⁺ T cells, and CHC phosphorylation as a result of basal Lck activity is also implicated in constitutive TCR endocytosis by CD4⁺ T cells. Remarkably, irreversible CHC phosphorylation in the presence of pervanadate reduced both constitutive and ligand-induced TCR internalization in CD4⁺ T cells, and immunofluorescence studies revealed that this inhibition affected the early stages of TCR endocytosis from the plasma membrane. Thus, we propose that CHC phosphorylation and dephosphorylation are involved in TCR internalization and that this is a regulatory mechanism linking TCR signaling to endocytosis.

Ubiquitin ligases and the immune response

Ubiquitin (Ub)-protein conjugation represents a novel means of posttranscriptional modification in a proteolysis-dependent or -independent manner. E3 Ub ligases play a key role in governing the cascade of Ub transfer reactions by recognizing and catalyzing Ub conjugation to specific protein substrates. The E3s, which can be generally classified into HECT-type and RING-type families, are involved in the regulation of many aspects of the immune system, including the development, activation, and differentiation of lymphocytes, T cell-tolerance induction, antigen presentation, immune evasion, and virus budding. E3-promoted ubiquitination affects a wide array of biological processes, such as receptor downmodulation, signal transduction, protein processing or translocation, protein-protein interaction, and gene transcription, in addition to proteasome-mediated degradation. Deficiency or mutation of some of the E3s like Cbl, Cbl-b, or Itch, causes abnormal immune responses such as autoimmunity, malignancy, and inflammation. This review discusses our current understanding of E3 Ub ligases in both innate and adaptive immunity. Such knowledge may facilitate the development of novel therapeutic approaches for immunological diseases.

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.

Regulation of membrane protein transport by ubiquitin and ubiquitin-binding proteins

Ubiquitin regulates protein transport between membrane compartments by serving as a sorting signal on protein cargo and by controlling the activity of trafficking machinery. Monoubiquitin attached to integral plasma membrane proteins or to associated transport modifiers serves as a regulated signal for internalization into the endocytic pathway. Similarly, monoubiquitin attached to biosynthetic and endocytic membrane proteins is a signal for sorting of cargo into vesicles that bud into the late endosome lumen for delivery into the lysosome. Ubiquitination of trans-acting endocytic proteins is also required for transport, and key endocytic proteins are modified by monoubiquitin. Regulatory enzymes of the ubiquitination machinery, ubiquitin ligases, control the timing and specificity of plasma membrane protein downregulation in such diverse biological processes as cell fate specification and neurotransmission. Monoubiquitin signals appended by these ligases are recognized by endocytic proteins carrying ubiquitin-binding motifs, including UBA, UEV, UIM, and CUE domains. The UIM proteins epsins and Hrs are excellent candidates for adaptors that link ubiquitinated cargo to the clathrin-based sorting machinery at appropriate regions of the endosomal or plasma membranes. Other ubiquitin-binding proteins also play crucial roles in cargo transport, although in most cases the role of ubiquitin-binding is not defined. Ubiquitin-binding proteins such as epsins, Hrs, and Vps9 are monoubiquitinated, indicating the general nature of ubiquitin regulation in endocytosis and suggesting new models to explain how recognition of monoubiquitin signals may be regulated.

Immunosuppressory activity of ubiquitin fragments containing retro-RGD sequence

A peptide fragment corresponding to the ubiquitin(50-59) sequence (LEDGRTLSDY) (U50-59) possesses a very high immunosuppressory activity, comparable to that of cyclosporine, both in the cellular and humoral immune responses. We found that the pentapeptide DGRTL (U52-56) is the shortest, effective immunosuppressory fragment of ubiquitin, although its potency is weaker than that of U50-59. Replacement of each consecutive residue with alanine in U52-56 allowed identification of essential amino acids involved in the immunosuppression. We also evaluated the roles of its N- and C-terminal groups by their acetylation and/or amidation, respectively. The active sequence is located in the external loop of the molecule and therefore it may serve as an important functional epitope for intermolecular binding. Based on the crystal structure of ubiquitin molecule, we designed and synthesized the cyclic analogue with a restricted conformation, cyclo(Glt-Gln-Leu-Glu-Asp-Gly-Arg-Thr-Leu-Ser-Asp-Lys)-NH2 (Glt = glutaryl) by reacting the C-terminal Lys side chain with the glutarylated N-terminus. The peptide was designed to mimic the ubiquitin(48-59) loop, in order to obtain the ligand that may interact with hypothetical receptors of the loop. The cyclization product selectively but strongly suppresses the cellular immune response. The results indicate that the 48-59 loop may serve as an important functional epitope in the ubiquitin molecule for intermolecular binding.

Cell surface expression of GluR5 kainate receptors is regulated by an endoplasmic reticulum retention signal

Kainate receptors (KARs) are mediators of excitatory neurotransmission in the mammalian central nervous system, and their efficient targeting and trafficking is critical for normal synaptic function. A key step in the delivery of KARs to the neuronal plasma membrane is the exit of newly assembled receptors from the endoplasmic reticulum (ER). Here we report the identification of a novel ER retention signal in the alternatively spliced C-terminal domain of the GluR5-2b subunit, which controls receptor trafficking in both heterologous cells and neurons. The ER retention motif consists of a critical arginine (Arg-896) and surrounding amino acids, disruption of which promotes ER exit and surface expression of the receptors, as well as altering their physiological properties. The Arg-896-mediated ER retention of GluR5 is regulated by a mutation that mimics phosphorylation of Thr-898, but not by PDZ interactions. Furthermore, two positively charged residues (Arg-900 and Lys-901) in the C terminus were also found to regulate ER export of the receptors. Taken together, our results identify novel trafficking signals in the C-terminal domain of GluR5-2b and demonstrate that alternative splicing is an important mechanism regulating KAR function.

Small glutamine-rich tetratricopeptide repeat-containing protein (SGT) interacts with the ubiquitin-dependent endocytosis (UbE) motif of the growth hormone receptor

Endocytosis of the growth hormone receptor (GHR) is regulated by the ubiquitin-conjugating system. A cytosolic 10 amino acid motif, referred to as the ubiquitin-dependent endocytosis (UbE) motif, is involved in the ubiquitination as well as in the endocytosis of the receptor. Proteins that are implicated in one of these processes have not been identified so far. Using a glutathione S-transferase (GST)-pulldown assay with a GST fusion protein encompassing the UbE motif of the GHR, a 35 kDa protein was purified. The protein was identified by MS as small glutamine-rich tetratricopeptide repeat (TPR)-containing protein (SGT). We found that GHR interacts with SGT. In vivo, both the precursor and the mature form of the receptor interacted with SGT. Inactivation of the ubiquitin-conjugating system did not affect the GHR-SGT interaction. Binding studies showed that the first TPR motif of SGT interacts with the UbE motif of the GHR. Taken together, these data show that SGT is a GHR-interacting protein, which binds independent of the ubiquitin-conjugating system.