Serine 6 of Lck tyrosine kinase: a critical site for Lck myristoylation, membrane localization, and function in T lymphocytes

Computational design and genetic incorporation of lipidation mimics in living cells

Protein lipidation, which regulates numerous biological pathways and plays crucial roles in the pharmaceutical industry, is not encoded by the genetic code but synthesized post-translationally. In the present study, we report a computational approach for designing lipidation mimics that fully recapitulate the biochemical properties of natural lipidation in membrane association and albumin binding. Furthermore, we establish an engineered system for co-translational incorporation of these lipidation mimics into virtually any desired position of proteins in Escherichia coli and mammalian cells. We demonstrate the utility of these length-tunable lipidation mimics in diverse applications, including improving the half-life and activity of therapeutic proteins in living mice, anchoring functional proteins to membrane by substituting natural lipidation, functionally characterizing proteins carrying different lengths of lipidation and determining the plasma membrane-binding capacity of a given compound. Our strategy enables gain-of-function studies of lipidation in hundreds of proteins and facilitates the creation of superior therapeutic candidates.

B cell receptor-induced protein dynamics and the emerging role of SUMOylation revealed by proximity proteomics

Successful B cell activation, which is critical for high-affinity antibody production, is controlled by the B cell antigen receptor (BCR). However, we still lack a comprehensive protein-level view of the very dynamic multi-branched cellular events triggered by antigen binding. Here, we employed APEX2 proximity biotinylation to study antigen-induced changes, 5-15 min after receptor activation, at the vicinity of the plasma membrane lipid rafts, wherein BCR enriches upon activation. The data reveals dynamics of signaling proteins, as well as various players linked to the subsequent processes, such as actin cytoskeleton remodeling and endocytosis. Interestingly, our differential expression analysis identified dynamic responses in various proteins previously not linked to early B cell activation. We demonstrate active SUMOylation at the sites of BCR activation in various conditions and report its functional role in BCR signaling through the AKT and ERK1/2 axes.

Regulating the discriminatory response to antigen by T-cell receptor

The cell-mediated immune response constitutes a robust host defense mechanism to eliminate pathogens and oncogenic cells. T cells play a central role in such a defense mechanism and creating memories to prevent any potential infection. T cell recognizes foreign antigen by its surface receptors when presented through antigen-presenting cells (APCs) and calibrates its cellular response by a network of intracellular signaling events. Activation of T-cell receptor (TCR) leads to changes in gene expression and metabolic networks regulating cell development, proliferation, and migration. TCR does not possess any catalytic activity, and the signaling initiates with the colocalization of several enzymes and scaffold proteins. Deregulation of T cell signaling is often linked to autoimmune disorders like severe combined immunodeficiency (SCID), rheumatoid arthritis, and multiple sclerosis. The TCR remarkably distinguishes the minor difference between self and non-self antigen through a kinetic proofreading mechanism. The output of TCR signaling is determined by the half-life of the receptor antigen complex and the time taken to recruit and activate the downstream enzymes. A longer half-life of a non-self antigen receptor complex could initiate downstream signaling by activating associated enzymes. Whereas, the short-lived, self-peptide receptor complex disassembles before the downstream enzymes are activated. Activation of TCR rewires the cellular metabolic response to aerobic glycolysis from oxidative phosphorylation. How does the early event in the TCR signaling cross-talk with the cellular metabolism is an open question. In this review, we have discussed the recent developments in understanding the regulation of TCR signaling, and then we reviewed the emerging role of metabolism in regulating T cell function.

T cell receptor (TCR) signaling in health and disease

Interaction of the T cell receptor (TCR) with an MHC-antigenic peptide complex results in changes at the molecular and cellular levels in T cells. The outside environmental cues are translated into various signal transduction pathways within the cell, which mediate the activation of various genes with the help of specific transcription factors. These signaling networks propagate with the help of various effector enzymes, such as kinases, phosphatases, and phospholipases. Integration of these disparate signal transduction pathways is done with the help of adaptor proteins that are non-enzymatic in function and that serve as a scaffold for various protein-protein interactions. This process aids in connecting the proximal to distal signaling pathways, thereby contributing to the full activation of T cells. This review provides a comprehensive snapshot of the various molecules involved in regulating T cell receptor signaling, covering both enzymes and adaptors, and will discuss their role in human disease.

Ionic CD3-Lck interaction regulates the initiation of T-cell receptor signaling

Antigen-triggered T-cell receptor (TCR) phosphorylation is the first signaling event in T cells to elicit adaptive immunity against invading pathogens and tumor cells. Despite its physiological importance, the underlying mechanism of TCR phosphorylation remains elusive. Here, we report a key mechanism regulating the initiation of TCR phosphorylation. The major TCR kinase Lck shows high selectivity on the four CD3 signaling proteins of TCR. CD3ε is the only CD3 chain that can efficiently interact with Lck, mainly through the ionic interactions between CD3ε basic residue-rich sequence (BRS) and acidic residues in the Unique domain of Lck. We applied a TCR reconstitution system to explicitly study the initiation of TCR phosphorylation. The ionic CD3ε-Lck interaction controls the phosphorylation level of the whole TCR upon antigen stimulation. CD3ε BRS is sequestered in the membrane, and antigen stimulation can unlock this motif. Dynamic opening of CD3ε BRS and its subsequent recruitment of Lck thus can serve as an important switch of the initiation of TCR phosphorylation.

Grouper (Epinephelus coioides) TCR signaling pathway was involved in response against Cryptocaryon irritans infection

T cell activation is a complicated process accompanying with the activation of T cell receptor (TCR) signaling pathway, which is not well described in teleost fish. The initiation of this pathway depends on the interaction of membrane TCR co-receptors (e.g. CD4/8, CD3 and CD45) and a series of cytoplasmic protein tyrosine kinases (e.g. Lck, Fyn and ZAP70). Cyptocaryon irritans is a ciliate pathogen of marine fish white spot disease causing huge economic lost in marine aquaculture. This parasite can infect fish gill and skin and is considered to be a good pathogen model for fish gill and skin mucosal immunity. Our previous studies showed the locally mucosal antibody response was important for fish defense against this parasite. While how TCR signaling pathway involved in T cell activation to help B cell activation in C. irritans infected fish is still not known. In the present study, we cloned a grouper TCR co-receptor gene EcCD3ε (537 bp) and its three kinase genes, including EcLck (1512 bp), EcFyn (1605 bp) and EcZAP70 (1893 bp). Homology analysis showed that they all shared the highest identity with corresponding genes from Takifugu rubripes (EcCD3ε 41%, EcLck 88%, EcFyn 98% and EcZAP70 93%), and their conserved motifs involved in the signaling transduction were analyzed. The tissue distribution analysis showed these four genes were high expressed in thymus, and it is interesting to find their comparative high expression in skin, gill and midgut mucosal immune tissues. In C. irritans infected grouper, the expression of three TCR co-receptors (EcCD4-1, EcCD3ε and EcCD45) and three kinases (EcLck, EcFyn and EcZAP70) was tested in skin, gill, head kidney and spleen at 0, 12 h, 24 h, 2 d, 3 d, 5 d and 7 d. All six genes were significantly up-regulated in skin at most tested time points, which indicate the possibility of skin local T cell activation to support the local antibody response. Compared to three TCR co-receptors, significantly up-regulation of three kinases were seen in the spleen, and the spleen fold changes of these three kinases were much higher than head kidney, which indicates spleen maybe the major systematic immune organs for T cell activation in C. irritans infected fish.

Molecular cloning and characterization of lymphocyte cell kinase from humphead snapper (Lutjanus sanguineus)

Lymphocyte cell kinase (LCK) belongs to the Src family of tyrosine kinases, which involves in the proliferation control of lymphocytes. In this study, we cloned the LCK gene of humphead snapper (Lutjanus sanguineus) (designed as LsLCK). Sequence analysis showed that the full-length cDNA of LsLCK was 2279 bp, contained a 1506-bp open reading frame (ORF), encoding a polypeptide of 501 amino acids. The deduced amino acid possessed the typical structural features of known LCK proteins, including four Src homology (SH) domains arranged as the SH1 domain followed by a regulatory C-terminal tail (COOH-domain), SH2 and SH3 adapter domains and SH4 domain which required for membrane attachment and CD4/CD8 binding. Fluorescent quantitative real-time PCR analysis indicated that LsLCK transcripts were expressed mainly in thymus, spleen and head kidney in healthy fish. Moreover, the mRNA expressions in these tissues were significantly up-regulated after challenge with Vibrio harveyi. The results of immunohistochemistry showed that LsLCK protein localized distinctly in cytoplasm of cell in thymus, spleen and head kidney. Taken together, these findings indicated that LsLCK may play an important role in the immune response of humphead snapper against bacterial infection.

Rapid and transient palmitoylation of the tyrosine kinase Lck mediates Fas signaling

Palmitoylation is the posttranslational modification of proteins with a 16-carbon fatty acid chain through a labile thioester bond. The reversibility of protein palmitoylation and its profound effect on protein function suggest that this modification could play an important role as an intracellular signaling mechanism. Evidence that palmitoylation of proteins occurs with the kinetics required for signal transduction is not clear, however. Here we show that engagement of the Fas receptor by its ligand leads to an extremely rapid and transient increase in palmitoylation levels of the tyrosine kinase Lck. Lck palmitoylation kinetics are consistent with the activation of downstream signaling proteins, such as Zap70 and PLC-γ1. Inhibiting Lck palmitoylation not only disrupts proximal Fas signaling events, but also renders cells resistant to Fas-mediated apoptosis. Knockdown of the palmitoyl acyl transferase DHHC21 eliminates activation of Lck and downstream signaling after Fas receptor stimulation. Our findings demonstrate highly dynamic Lck palmitoylation kinetics that are essential for signaling downstream of the Fas receptor.

Systems Analysis of Protein Fatty Acylation in Herpes Simplex Virus-Infected Cells Using Chemical Proteomics

Protein fatty acylation regulates diverse aspects of cellular function and organization and plays a key role in host immune responses to infection. Acylation also modulates the function and localization of virus-encoded proteins. Here, we employ chemical proteomics tools, bio-orthogonal probes, and capture reagents to study myristoylation and palmitoylation during infection with herpes simplex virus (HSV). Using in-gel fluorescence imaging and quantitative mass spectrometry, we demonstrate a generalized reduction in myristoylation of host proteins, whereas palmitoylation of host proteins, including regulators of interferon and tetraspanin family proteins, was selectively repressed. Furthermore, we found that a significant fraction of the viral proteome undergoes palmitoylation; we identified a number of virus membrane glycoproteins, structural proteins, and kinases. Taken together, our results provide broad oversight of protein acylation during HSV infection, a roadmap for similar analysis in other systems, and a resource with which to pursue specific analysis of systems and functions.

Lymphocyte-specific protein tyrosine kinase (Lck) interacts with CR6-interacting factor 1 (CRIF1) in mitochondria to repress oxidative phosphorylation

BACKGROUND

Many cancer cells exhibit reduced mitochondrial respiration as part of metabolic reprogramming to support tumor growth. Mitochondrial localization of several protein tyrosine kinases is linked to this characteristic metabolic shift in solid tumors, but remains largely unknown in blood cancer. Lymphocyte-specific protein tyrosine kinase (Lck) is a key T-cell kinase and widely implicated in blood malignancies. The purpose of our study is to determine whether and how Lck contributes to metabolic shift in T-cell leukemia through mitochondrial localization.

METHODS

We compared the human leukemic T-cell line Jurkat with its Lck-deficient derivative Jcam cell line. Differences in mitochondrial respiration were measured by the levels of mitochondrial membrane potential, oxygen consumption, and mitochondrial superoxide. Detailed mitochondrial structure was visualized by transmission electron microscopy. Lck localization was evaluated by subcellular fractionation and confocal microscopy. Proteomic analysis was performed to identify proteins co-precipitated with Lck in leukemic T-cells. Protein interaction was validated by biochemical co-precipitation and confocal microscopy, followed by in situ proximity ligation assay microscopy to confirm close-range (<16 nm) interaction.

RESULTS

Jurkat cells have abnormal mitochondrial structure and reduced levels of mitochondrial respiration, which is associated with the presence of mitochondrial Lck and lower levels of mitochondrion-encoded electron transport chain proteins. Proteomics identified CR6-interacting factor 1 (CRIF1) as the novel Lck-interacting protein. Lck association with CRIF1 in Jurkat mitochondria was confirmed biochemically and by microscopy, but did not lead to CRIF1 tyrosine phosphorylation. Consistent with the role of CRIF1 in functional mitoribosome, shRNA-mediated silencing of CRIF1 in Jcam resulted in mitochondrial dysfunction similar to that observed in Jurkat. Reduced interaction between CRIF1 and Tid1, another key component of intramitochondrial translational machinery, in Jurkat further supports the role of mitochondrial Lck as a negative regulator of CRIF1 through competitive binding.

CONCLUSIONS

This is the first report demonstrating the role of mitochondrial Lck in metabolic reprogramming of leukemic cells. Mechanistically, it is distinct from other reported mitochondrial protein tyrosine kinases. In a kinase-independent manner, mitochondrial Lck interferes with mitochondrial translational machinery through competitive binding to CRIF1. These findings may reveal novel approaches in cancer therapy by targeting cancer cell metabolism.

The role of membrane rafts in Lck transport, regulation and signalling in T-cells

Tyrosine phosphorylation is one of the key covalent modifications that occur in multicellular organisms. Since its discovery more than 30 years ago, tyrosine phosphorylation has come to be understood as a fundamentally important mechanism of signal transduction and regulation in all eukaryotic cells. The tyrosine kinase Lck (lymphocyte-specific protein tyrosine kinase) plays a crucial role in the T-cell response by transducing early activation signals triggered by TCR (T-cell receptor) engagement. These signals result in the phosphorylation of immunoreceptor tyrosine-based activation motifs present within the cytosolic tails of the TCR-associated CD3 subunits that, once phosphorylated, serve as scaffolds for the assembly of a large supramolecular signalling complex responsible for T-cell activation. The existence of membrane nano- or micro-domains or rafts as specialized platforms for protein transport and cell signalling has been proposed. The present review discusses the signals that target Lck to membrane rafts and the importance of these specialized membranes in the transport of Lck to the plasma membrane, the regulation of Lck activity and the phosphorylation of the TCR.

A plasma membrane-targeted cytosolic domain of STIM1 selectively activates ARC channels, an arachidonate-regulated store-independent Orai channel

The Orai family of calcium channels includes the store-operated CRAC channels and store-independent, arachidonic acid (AA)-regulated ARC channels. Both depend on STIM1 for their activation but, whereas CRAC channel activation involves sensing the depletion of intracellular calcium stores via a luminal N terminal EF-hand of STIM1 in the endoplasmic reticulum (ER) membrane, ARC channels are exclusively activated by the pool of STIM1 that constitutively resides in the plasma membrane (PM). Here, the EF-hand is extracellular and unlikely to ever lose its bound calcium, suggesting that STIM1-dependent activation of ARC channels is very different from that of CRAC channels. We now show that attachment of the cytosolic portion of STIM1 to the inner face of the PM via an N terminal Lck-domain sequence is sufficient to enable normal AA-dependent activation of ARC channels, while failing to allow activation of store-operated CRAC channels. Introduction of a point mutation within the Lck-domain resulted in the loss of both PM localization and ARC channel activation. Reversing the orientation of the PM-anchored STIM1 C terminus via a C-terminal CAAX-box fails to support either CRAC or ARC channel activation. Finally, the Lck-anchored STIM1 C-terminal domain also enabled the exclusive activation of the ARC channels following physiological agonist addition. These data demonstrate that simple tethering of the cytosolic C-terminal domain of STIM1 to the inner face of the PM is sufficient to allow the full, normal and exclusive activation of ARC channels, and that the N-terminal regions of STIM1 (including the EF-hand domain) play no significant role in this activation.

Lowering glycosphingolipid levels in CD4+ T cells attenuates T cell receptor signaling, cytokine production, and differentiation to the Th17 lineage

Lipid rafts reportedly have a role in coalescing key signaling molecules into the immunological synapse during T cell activation, thereby modulating T cell receptor (TCR) signaling activity. Recent findings suggest that a correlation may exist between increased levels of glycosphingolipids (GSLs) in the lipid rafts of T cells and a heightened response of those T cells toward activation. Here, we show that lowering the levels of GSLs in CD4(+) T cells using a potent inhibitor of glucosylceramide synthase (Genz-122346) led to a moderation of the T cell response toward activation. TCR proximal signaling events, such as phosphorylation of Lck, Zap70 and LAT, as well as early Ca(2+) mobilization, were attenuated by treatment with Genz-122346. Concomitant with these events were significant reductions in IL-2 production and T cell proliferation. Similar findings were obtained with CD4(+) T cells isolated from transgenic mice genetically deficient in GM3 synthase activity. Interestingly, lowering the GSL levels in CD4(+) T cells by either pharmacological inhibition or disruption of the gene for GM3 synthase also specifically inhibited the differentiation of T cells to the Th(17) lineage but not to other Th subsets in vitro. Taken together with the recently reported effects of Raftlin deficiency on Th(17) differentiation, these results strongly suggest that altering the GSL composition of lipid rafts modulates TCR signaling activity and affects Th(17) differentiation.

Dynamic palmitoylation and the role of DHHC proteins in T cell activation and anergy

Although protein S-palmitoylation was first characterized >30 years ago, and is implicated in the function, trafficking, and localization of many proteins, little is known about the regulation and physiological implications of this posttranslational modification. Palmitoylation of various signaling proteins required for TCR-induced T cell activation is also necessary for their proper function. Linker for activation of T cells (LAT) is an essential scaffolding protein involved in T cell development and activation, and we found that its palmitoylation is selectively impaired in anergic T cells. The recent discovery of the DHHC family of palmitoyl acyl transferases and the establishment of sensitive and quantitative proteomics-based methods for global analysis of the palmitoyl proteome led to significant progress in studying the biology and underlying mechanisms of cellular protein palmitoylation. We are using these approaches to explore the palmitoyl proteome in T lymphocytes and, specifically, the mechanistic basis for the impaired palmitoylation of LAT in anergic T cells. This chapter reviews the history of protein palmitoylation and its role in T cell activation, the DHHC family and new methodologies for global analysis of the palmitoyl proteome, and summarizes our recent work in this area. The new methodologies will accelerate the pace of research and provide a greatly improved mechanistic and molecular understanding of the complex process of protein palmitoylation and its regulation, and the substrate specificity of the novel DHHC family. Reversible protein palmitoylation will likely prove to be an important posttranslational mechanism that regulates cellular responses, similar to protein phosphorylation and ubiquitination.

Reversible phosphorylation as a molecular switch to regulate plasma membrane targeting of acylated SH4 domain proteins

Acylated SH4 domains represent N-terminal targeting signals that anchor peripheral membrane proteins such as Src kinases in the inner leaflet of plasma membranes. Here we provide evidence for a novel regulatory mechanism that may control the levels of SH4 proteins being associated with plasma membranes. Using a fusion protein of the SH4 domain of Leishmania HASPB and GFP as a model system, we demonstrate that threonine 6 is a substrate for phosphorylation. Substitution of threonine 6 by glutamate (to mimic a phosphothreonine residue) resulted in a dramatic redistribution from plasma membranes to intracellular sites with a particular accumulation in a perinuclear region. As shown by both pharmacological inhibition and RNAi-mediated down-regulation of the threonine/ serine-specific phosphatases PP1 and PP2A, recycling back to the plasma membrane required dephosphorylation of threonine 6. We provide evidence that a cycle of phosphorylation and dephosphorylation may also be involved in intracellular targeting of other SH4 proteins such as the Src kinase Yes.

Tyrosine-phosphorylation-dependent translocation of the SLAT protein to the immunological synapse is required for NFAT transcription factor activation

SWAP-70-like adaptor of T cells (SLAT) is a guanine nucleotide exchange factor for Rho GTPases that regulates the development of T helper 1 (Th1) and Th2 cell inflammatory responses by controlling the Ca(2+)-NFAT signaling pathway. However, the mechanism used by SLAT to regulate these events is unknown. Here, we report that the T cell receptor (TCR)-induced translocation of SLAT to the immunological synapse required Lck-mediated phosphorylation of two tyrosine residues located in an immunoreceptor tyrosine-based activation motif-like sequence but was independent of the SLAT PH domain. This subcellular relocalization was coupled to, and necessary for, activation of the NFAT pathway. Furthermore, membrane targeting of the SLAT Dbl-homology (catalytic) domain was sufficient to trigger TCR-mediated NFAT activation and Th1 and Th2 differentiation in a Cdc42-dependent manner. Therefore, tyrosine-phosphorylation-mediated relocalization of SLAT to the site of antigen recognition is required for SLAT to exert its pivotal role in NFAT-dependent CD4(+) T cell differentiation.

The Yersinia effector protein YpkA induces apoptosis independently of actin depolymerization

The pathogenicity of the plague agent Yersinia pestis is largely due to the injection of effector proteins that potently block immune responses into host cells through a type III secretion apparatus. One Yersinia effector protein, YpkA, a putative serine/threonine kinase, has been reported to act by depolymerizing actin and disrupting actin microfilament organization. Using YpkA-GFP fusion proteins to directly visualize cells expressing YpkA, we found instead that YpkA triggered rapid cell death that can be blocked by caspase inhibitors and Bcl-xL, but was not dependent on caspase-8. The actin depolymerization promoted by YpkA was only seen in cells with other features of apoptosis, and was blocked by inhibiting apoptosis, indicating that actin filament disruption is likely to be a result, rather than a cause of YpkA-induced apoptosis. A region including aa 133-262 in YpkA was sufficient for inducing apoptosis independent of localization to the plasma membrane. These data suggest that YpkA can act as a direct inducer of cell death.

Molecular and biochemical analysis of rainbow trout LCK suggests a conserved mechanism for T-cell signaling in gnathostomes

Two genes were identified in rainbow trout that display high sequence identity to vertebrate Lck. Both of the trout Lck transcripts are associated with lymphoid tissues and were found to be highly expressed in IgM-negative lymphocytes. In vitro analysis of trout lymphocytes indicates that trout Lck mRNA is up-regulated by T-cell mitogens, supporting an evolutionarily conserved function for Lck in the signaling pathways of T-lymphocytes. Here, we describe the generation and characterization of a specific monoclonal antibody raised against the N-terminal domains of recombinant trout Lck that can recognize Lck protein(s) from trout thymocyte lysates that are similar in size ( approximately 57kDa) to mammalian Lck. This antibody also reacted with permeabilized lymphocytes during FACS analysis, indicating its potential usage for cellular analyses of trout lymphocytes, thus representing an important tool for investigations of salmonid T-cell function.

Substitutions in the conserved C2C domain of otoferlin cause DFNB9, a form of nonsyndromic autosomal recessive deafness

DFNB, the nonsyndromic hearing loss with an autosomal recessive mode of inheritance constitutes the majority of severe to profound prelingual forms of hearing impairment, usually leading to inability of speech acquisition. We analyzed a consanguineous family with autosomal recessive deafness which has been shown to segregate within chromosomal region 2p23.1 (DFNB9; MIM 601071). By SSCP analysis and DNA sequencing of the 48 exons of the DFNB9 gene, coding for otoferlin, previously reported mutations in OTOF were excluded. Next to a frequent T > C single nucleotide polymorphism in exon 8, two novel mutations linked in exon 15 of the OTOF long splice form were identified comprising substitutions at positions 490 (Pro > Gln) and 515 (Ile > Thr), both located in the conserved Ca(2+) binding C2C domain of this peptide. Comparisons of homology using human and mice otoferlins and closely related peptides and computer simulation analyses suggest that changes in the mutated segment's secondary structure affect the Ca(2+) binding capacity of the C2C domain in otoferlin.

CD8beta endows CD8 with efficient coreceptor function by coupling T cell receptor/CD3 to raft-associated CD8/p56(lck) complexes

The extraordinary sensitivity of CD8+ T cells to recognize antigen impinges to a large extent on the coreceptor CD8. While several studies have shown that the CD8beta chain endows CD8 with efficient coreceptor function, the molecular basis for this is enigmatic. Here we report that cell-associated CD8alphabeta, but not CD8alphaalpha or soluble CD8alphabeta, substantially increases the avidity of T cell receptor (TCR)-ligand binding. To elucidate how the cytoplasmic and transmembrane portions of CD8beta endow CD8 with efficient coreceptor function, we examined T1.4 T cell hybridomas transfected with various CD8beta constructs. T1.4 hybridomas recognize a photoreactive Plasmodium berghei circumsporozoite (PbCS) peptide derivative (PbCS (4-azidobezoic acid [ABA])) in the context of H-2K(d), and permit assessment of TCR-ligand binding by TCR photoaffinity labeling. We find that the cytoplasmic portion of CD8beta, mainly due to its palmitoylation, mediates partitioning of CD8 in lipid rafts, where it efficiently associates with p56(lck). In addition, the cytoplasmic portion of CD8beta mediates constitutive association of CD8 with TCR/CD3. The resulting TCR-CD8 adducts exhibit high affinity for major histocompatibility complex (MHC)-peptide. Importantly, because CD8alphabeta partitions in rafts, its interaction with TCR/CD3 promotes raft association of TCR/CD3. Engagement of these TCR/CD3-CD8/lck adducts by multimeric MHC-peptide induces activation of p56(lck) in rafts, which in turn phosphorylates CD3 and initiates T cell activation.

Down-regulation of N-myristoyl transferase expression in human T-cell line CEM by human immunodeficiency virus type-1 infection

The present study focuses on the expression level of N-myristoyl transferase (NMT) in the course of human immunodeficiency virus type-1 (HIV-1) infection. HIV-1 structural proteins were gradually expressed during the process of infection of the human T-cell line CEM, whereas the expression levels of NMT subsequently decreased under the same conditions. In addition, the chronically HIV-1-infected T-cell line CEM/LAV-1 exhibited low expression levels of NMT. We hypothesize that the decrease in the expression level of NMT due to HIV-1 infection may be related to the virus' strategy that leads to its persistent replication.

High dose of dexamethasone upregulates TCR/CD3-induced calcium response independent of TCR zeta chain expression in human T lymphocytes

Glucocorticoids are very potent anti-inflammatory and immunosuppressive agents that modulate cellular immune responses, although, the molecular mechanisms that impart their complex effects have not been completely defined. We have previously demonstrated that dexamethasone (Dex), a synthetic glucocorticoid, biphasically modulates the expression of TCR (T cell receptor) zeta chain in human T cells. At 10 nM, it induced the expression of TCR zeta chain whereas at 100 nM, it inhibited its expression. In parallel to the upregulation of TCR zeta chain, the TCR/CD3-mediated [Ca(2+)](i) response was enhanced in 10 nM Dex-treated cells. However, at 100 nM, Dex treatment enhanced TCR/CD3-mediated [Ca(2+)](i) response without the induction of TCR zeta chain expression. Because the classical transcriptional model of glucocorticoid action cannot account for the effects of high dose of Dex, here we studied alternative mechanisms of action. We show that, increased and more sustained TCR/CD3-mediated [Ca(2+)](i) response was also observed in 100 nM Dex-treated cells in the presence of actinomycin D or cycloheximide suggesting that cellular transcription and/or de novo protein synthesis are not required for the induction. The TCR/CD3-mediated hyper [Ca(2+)](i) response in 100 nM Dex-treated cells was readily reversible by short-term culture in steroid-free medium. RU-486, a competitive antagonist of Dex, inhibited the increase in [Ca(2+)](i) response suggesting that the effect of Dex is mediated through the glucocorticoid receptor. Although the lipid-raft association of the TCR zeta chain was not significantly increased, high-dose of Dex increased the amount of ubiquitinated form of the TCR zeta chain in the cell membrane along with increased levels of actin. Fluorescence microscopy showed that high-dose of Dex alters the distribution of the TCR zeta chain and form more distinct clusters upon TCR/CD3 stimulation. These results suggest that high dose of Dex perturbs the membrane distribution of TCR zeta chain leading to more functional signaling clusters that result in increased TCR/CD3-mediated [Ca(2+)](i) response independent of TCR zeta chain expression.

Proximal protein tyrosine kinases in immunoreceptor signaling

Immunoreceptor engagement results in the sequential activation of several classes of protein tyrosine kinases, including the Src and Syk/Zap-70 families. Recent progress has been made in our understanding of the regulation and function of these molecules. First, it was revealed that membrane compartmentation of protein tyrosine kinases may be essential for their proper biological function. Second, Src family kinases were found to act not only as positive regulators, but also as inhibitors of cell activation. Third, it was appreciated that Csk, a potent inhibitor of Src kinases, is regulated by an assortment of protein-protein interactions. Fourth, differences in the regulation of Syk and Zap-70 were observed, suggesting significant distinctions in the purpose of these two kinases in immunoreceptor signaling. And fifth, it was suggested that proximal kinases implicated in immunoreceptor-mediated signal transduction may be regulated by protein degradation via binding to c-Cbl, a ubiquitin ligase.