Raft membrane domains and immunoreceptor functions

Pantethine Alters Lipid Composition and Cholesterol Content of Membrane Rafts, With Down-Regulation of CXCL12-Induced T Cell Migration

Pantethine, a natural low-molecular-weight thiol, shows a broad activity in a large range of essential cellular pathways. It has been long known as a hypolipidemic and hypocholesterolemic agent. We have recently shown that it exerts a neuroprotective action in mouse models of cerebral malaria and Parkinson's disease through multiple mechanisms. In the present study, we looked at its effects on membrane lipid rafts that serve as platforms for molecules engaged in cell activity, therefore providing a target against inappropriate cell response leading to a chronic inflammation. We found that pantethine-treated cells showed a significant change in raft fatty acid composition and cholesterol content, with ultimate downregulation of cell adhesion, CXCL12-driven chemotaxis, and transendothelial migration of various T cell types, including human Jurkat cell line and circulating effector T cells. The mechanisms involved include the alteration of the following: (i) CXCL12 binding to its target cells; (ii) membrane dynamics of CXCR4 and CXCR7, the two CXCL12 receptors; and (iii) cell redox status, a crucial determinant in the regulation of the chemokine system. In addition, we considered the linker for activation of T cells molecule to show that pantethine effects were associated with the displacement from the rafts of the acylated signaling molecules which had their palmitoylation level reduced.. In conclusion, the results presented here, together with previously published findings, indicate that due to its pleiotropic action, pantethine can downregulate the multifaceted process leading to pathogenic T cell activation and migration.

Modulation of lipid rafts by Omega-3 fatty acids in inflammation and cancer: implications for use of lipids during nutrition support

Current understanding of biologic membrane structure and function is largely based on the concept of lipid rafts. Lipid rafts are composed primarily of tightly packed, liquid-ordered sphingolipids/cholesterol/saturated phospholipids that float in a sea of more unsaturated and loosely packed, liquid-disordered lipids. Lipid rafts have important clinical implications because many important membrane-signaling proteins are located within the raft regions of the membrane, and alterations in raft structure can alter activity of these signaling proteins. Because rafts are lipid-based, their composition, structure, and function are susceptible to manipulation by dietary components such as omega-3 polyunsaturated fatty acids and by cholesterol depletion. We review how alteration of raft lipids affects the raft/nonraft localization and hence the function of several proteins involved in cell signaling. We focus our discussion of raft-signaling proteins on inflammation and cancer.

Luteinizing hormone receptors translocate to plasma membrane microdomains after binding of human chorionic gonadotropin

Receptor-mediated signal transduction by G protein-coupled receptors can involve redistribution of plasma membrane receptors into membrane structures that are characterized by insolubility in Triton X-100 and low buoyant density in sucrose gradients. Here we describe the translocation of wild-type (wt) rat LH receptors (LHR-wt) from the bulk membrane into membrane microdomains (rafts) after the binding of human chorionic gonadotropin (hCG). In sucrose gradient ultracentrifugation of plasma membranes from cells stably expressing FLAG-tagged LHR-wt, receptors were located in high-density membrane fractions before binding of hormone and in low-density fractions after hCG treatment. Receptor translocation to low-density sucrose fractions did not occur when cells were pretreated with 1% methyl-beta-cyclodextrin, which reduces membrane cholesterol and disrupts rafts. Single-particle tracking of individual FLAG-LHR-wt receptors showed that hCG-treated receptors become confined in small compartments with a diameter of 86 +/- 36 nm, significantly smaller than 230 +/- 79 nm diameter regions accessed by the untreated receptor. Receptors were no longer confined in these small compartments after disruption of rafts by methyl-beta-cyclodextrin, a treatment that also decreased levels of cAMP in response to hCG. Finally, translocation of LHR into rafts required a functional hormone-receptor complex but did not occur after extensive receptor cross-linking that elevated cAMP levels. Thus, retention of LHR in rafts or small membrane compartments is a characteristic of functional, hormone-occupied LHR-wt. Although raft translocation was not essential for cAMP production, it may be necessary for optimizing hormone-mediated signaling.

CD3 x CD28 cross-interacting bispecific antibodies improve tumor cell dependent T-cell activation

Bispecific antibodies (Bs-Abs) containing an anti-CD3 and an anti-TAA specificity can recruit T cells to the tumor for cancer immunotherapy. To be effective, efficient activation at the tumor site is a prerequisite. This can be achieved by triggering both the T-cell receptor and the co-stimulatory molecule CD28. We engineered two recombinant cross-interacting Bs-Abs (CriBs-Abs) by incorporating a peptide tag and its cognate single-chain variable fragment (scFv), respectively, into a pair of (tumor x CD3) and (tumor x CD28) binding Bs-Abs. A 30-fold lower concentration of the activating CriBs-Ab as compared to non interacting Bs-Ab was sufficient for strong T-cell activation in the presence of tumor cells. One thousand-fold higher concentrations of both CriBs-Abs were required for marginal T-cell activation (70-fold below maximal response) in the absence of tumor cells. An optimized stoichiometry (1 : 1000) of activating versus co-stimulating CriBs-Ab thus allowed low doses of activating CriBs-Ab to induce tumor-cell dependent T-cell activation when used in combination with high concentrations of the pre-targeted co-stimulating CriBs-Ab in vitro. This indicates a large window of operation in which only tumor cell dependent T-cell activation is induced and systemic tumor cell independent T-cell activation is avoided, while ensuring optimal activation with a low concentration of the activating CriBs-Ab, which has the highest potential to induce toxic effects in vivo.

Cyclic beta-1,2-glucan is a Brucella virulence factor required for intracellular survival

Pathogenic brucella bacteria have developed strategies to persist for prolonged periods of time in host cells, avoiding innate immune responses. Here we show that the cyclic beta-1,2-glucans (CbetaG) synthesized by brucella is important for circumventing host cell defenses. CbetaG acted in lipid rafts found on host cell membranes. CbetaG-deficient mutants failed to prevent phagosome-lysosome fusion and could not replicate. However, when treated with purified CbetaG or synthetic methyl-beta-cyclodextrin, the mutants were able to control vacuole maturation by avoiding lysosome fusion, and this allowed intracellular brucella to survive and reach the endoplasmic reticulum. Fusion between the endoplasmic reticulum and the brucella-containing vacuole depended on the brucella virulence type IV secretion system but not on CbetaG. Brucella CbetaG is thus a virulence factor that interacts with lipid rafts and contributes to pathogen survival.

Distinct membrane localization and kinase association of the two isoforms of CD58

The adhesion molecule CD58 is involved in intercellular adhesion and in signal transduction. It is natively expressed in both a transmembrane form and a glycosylphosphatidylinositol (GPI)-anchored form, and hence provides a model for the study of two distinct membrane-anchored forms of the same protein in the same cell. We demonstrate here that the two isoforms of CD58 are localized in distinct membrane compartments. The GPI-anchored form localizes in lipid rafts, while the transmembrane form resides in nonraft domains. In addition to distinct membrane localization, the two isoforms of CD58 differ in their association with protein kinases. GPI-anchored CD58, residing in raft domains, is constitutively associated with protein kinases. However, cross-linking mediates a substantial increase in kinase activity which is predominantly associated with the transmembrane CD58 in nonraft membrane domains. The extensive inducible kinase activity, associated with transmembrane CD58, is demonstrated in wild-type cells as well as in GPI-deficient variant cells. Thus, although the transmembrane CD58 is excluded from rafts, it may trigger signaling independently of the GPI-linked isoform.

Membrane domains in lymphocytes - from lipid rafts to protein scaffolds

Lateral compartmentalization of the plasma membrane into domains is a key feature of immune cell activation and subsequent immune effector functions. Here, we will review the high diversity of membrane domains, ranging from elementary lipid rafts, envisioned as dynamic and small domains (in the tens of nm), to relatively stable microm-scale membrane domains, which form the immunologic synapse of T lymphocytes. We will discuss the relationship between these different types of plasma membrane domains and how raft lipid- and protein-controlled interactions and cell biological processes cooperate to generate functional domains that mediate lymphocyte activity.

Site-Specific Differences in Fatty Acid Composition of Dendritic Cells and Associated Adipose Tissue in Popliteal Depot, Mesentery, and Omentum and Their Modulation by Chronic Inflammation and Dietary Lipids

BACKGROUND

This study explores the role of lymphatics-associated adipocytes in determining the lipid composition of dendritic cells.

METHODS AND RESULTS

Adult male rats were fed plain chow, or chow supplemented with 20% sunflower or fish oil. Chronic local inflammation was induced by subcutaneous injection of 20 microg lipopolysaccharide three times a week for 2 weeks near the popliteal lymph nodes. Chemokine-stimulated dendritic cells were collected over 4 hours from popliteal and mesenteric lymph nodes, and perinodal and other samples of mesenteric, popliteal, and omental adipose tissue. Fatty acids extracted from triacylglycerols and/or phospholipids were separated and quantified by gas chromatography from each sample of dendritic cells and intracellular lipids, membranes, stroma and isolated adipocytes from the adipose tissue. Dendritic cells from lymph nodes and adipose tissue samples differed in fatty acid composition, and were modulated by diet. The site-specific differences of dendritic cells correlated with those of the contiguous adipocytes. Chronic mild stimulation altered the lipid composition of dendritic cells near the inflamed site and elsewhere; changes were minimal after the fish-oil diet. The composition of adipocyte triacylglycerol and phospholipid fatty acids also changed near the stimulation site in ways that counteracted alterations induced by the experimental diets.

CONCLUSIONS

Fatty acids in dendritic cells differed with anatomical site, and were determined by the adjacent adipocytes, which actively regulated their own lipid composition. These findings demonstrated functional bases for the anatomical associations between adipose and lymphoid tissues and may be a mechanism by which dietary lipids modulate the immune system.

The influenza virus ion channel and maturation cofactor M2 is a cholesterol-binding protein

The influenza-virus M2 protein has proton channel activity required for virus uncoating and maturation of hemagglutinin (HA) through low-pH compartments. The proton channel is cytotoxic in heterologous expression systems and can be blocked with rimantadine. In an independent, rimantadine-resistant function, M2, interacting with the M1 protein, controls the shape of virus particles. These bud from cholesterol-rich membrane rafts where viral glycoproteins and matrix (M1)/RNP complexes assemble. We demonstrate that M2 preparations from influenza virus-infected cells and from a baculovirus expression system contain 0.5-0.9 molecules of cholesterol per monomer. Sequence analyses of the membrane-proximal M2 endodomain reveal interfacial hydrophobicity, a cholesterol-binding motif first identified in peripheral benzodiazepine receptor and human immunodeficiency virus gp41, and an overlapping phosphatidylinositol 4,5-bisphosphate-binding motif. M2 induced rimantadine-reversible cytotoxicity in intrinsically cholesterol-free E. coli, and purified E. coli-expressed M2 functionally reconstituted into cholesterol-free liposomes supported rimantadine-sensitive proton translocation. Therefore, cholesterol was nonessential for M2 ion-channel function and cytotoxicity and for the effect of rimantadine. Only about 5-8% of both M2 preparations, regardless of cholesterol content, associated with detergent-resistant membranes. Cholesterol affinity and palmitoylation, in combination with a short transmembrane segment suggest M2 is a peripheral raft protein. Preference for the raft/non-raft interface may determine colocalization with HA during apical transport, the low level of M2 incorporated into the viral envelope and its undisclosed role in virus budding for which a model is presented. M2 may promote clustering and merger of rafts and the pinching-off (fission) of virus particles.

Novel polysulfated galactose-derivatized dendrimers as binding antagonists of human immunodeficiency virus type 1 infection

Evidence indicates that galactosyl ceramide (GalCer) and its 3'-sulfated derivative, sulfatide (SGalCer), may act as alternate coreceptors for human immunodeficiency virus type 1 (HIV-1) in CD4(-) cells. Glycosphingolipids (GSLs) may also be necessary for fusion of HIV-1 and host cell membranes. Using an enzyme-linked immunosorbent assay to determine which GSL was the best ligand for both recombinant and virus-associated gp120, we found that SGalCer was the best ligand for each rgp120 and HIV-1 isolate tested. Therefore, novel multivalent glycodendrimers, which mimic the carbohydrate clustering reportedly found in lipid rafts, were synthesized based on the carbohydrate moiety of SGalCer. Here we describe the synthesis of a polysulfated galactose functionalized, fifth generation DAB dendrimer (PS Gal 64mer), containing on average two sulfate groups per galactose residue. Its ability to inhibit HIV-1 infection of cultured indicator cells was compared to that of dextran sulfate (DxS), a known, potent, binding inhibitor of HIV-1. The results indicate that the PS Gal 64mer inhibited infection by the HIV-1 isolates tested as well as DxS.

Use of detergents to study membrane rafts: the good, the bad, and the ugly

Eukaryotic cell membranes contain microdomains called lipid rafts, which are cholesterol-rich domains in which lipid acyl chains are tightly packed and highly extended. A variety of proteins associate preferentially with rafts, and this raft association is important in a wide range of functions. A powerful and widely-used method for studying lipid rafts takes advantage of their insolubility in non-ionic detergents. Here we describe the basis of detergent insolubility, and review strengths, limitations, and unresolved puzzles regarding this method.

Transmembrane adaptor proteins in membrane microdomains: important regulators of immunoreceptor signaling

Membrane microdomains enriched in glycosphingolipids, cholesterol, glycosylphosphatidylinositol-anchored proteins and Src-family kinases (lipid rafts, GEMs) appear to play many important roles, especially in immunoreceptor signaling. Most transmembrane proteins are excluded from these specialized areas of membranes, notable exceptions being several palmitoylated proteins such as the T cell coreceptors CD4 and CD8, and several recently described transmembrane adaptor proteins, LAT, non-T cell activation linker (NTAL)/linker for activation of B cells (LAB), phosphoprotein associated with GEMs (PAG)/Csk-binding protein (Cbp) and LIME. All these molecules possess a very short N-terminal extracellular peptide (4-17 amino acids), transmembrane segment followed by a palmitoylation motif (CxxC) and cytoplasmic domain containing up to 10 tyrosine residues potentially phosphorylated by the Src- or Syk-family kinases. Tyrosine-phosphorylated transmembrane adaptors bind (directly via SH2 domains or indirectly) other signaling molecules such as several cytoplasmic adaptors and enzymes. LAT is indispensable for TCR signaling (and participates also at signal transduction initiated by some other receptors), NTAL/LAB appears to play a LAT-like role in signaling initiated by BCR and some Fc-receptors; PAG/Cbp cooperates with Csk, the cytoplasmic tyrosine kinase negatively regulating Src-family kinases. Additional transmembrane adaptors exist (TRIM, SIT, LAX) that are however not palmitoylated and therefore excluded from the lipid rafts; structurally and functionally, the zeta-chain family proteins tightly associated with immunoreceptors and activating NK-receptors may be also considered as transmembrane adaptors.

Enrichment of Lck in Lipid Rafts Regulates Colocalized Fyn Activation and the Initiation of Proximal Signals through TCRαβ

Recent results provide insight into the temporal and spatial relationship governing lck-dependent fyn activation and demonstrate TCR/CD4-induced activation and translocation of lck into lipid rafts and the ensuing activation of colocalized fyn. The prediction follows that directly targeting lck to lipid rafts will bypass the requirement for juxtaposing TCR and CD4-lck, and rescue cellular activation mediated by Ab specific for the constant region of TCRbeta chain. The present study uses a family of murine IL-2-dependent CD4(+) T cell clonal variants in which anti-TCRCbeta signaling is impaired in an lck-dependent fashion. Importantly, these variants respond to Ag- and mAb-mediated TCR-CD4 coaggregation, both of which enable the coordinated interaction of CD4-associated lck with the TCR/CD3 complex. We have previously demonstrated that anti-TCRCbeta responsiveness in this system correlates with the presence of kinase-active, membrane-associated lck and preformed hypophosphorylated TCRzeta:zeta-associated protein of 70 kDa complexes, a phenotype recapitulated in primary resting CD4(+) T cells. We show in this study that forced expression of wild-type lck achieved the same basal composition of the TCR/CD3 complex and yet did not rescue anti-TCRCbeta signaling. In contrast, forced expression of C20S/C23S-mutated lck (double-cysteine lck), unable to bind CD4, rescues anti-TCRCbeta proximal signaling and cellular growth. Double-cysteine lck targets lipid rafts, colocalizes with >98% of cellular fyn, and results in a 7-fold increase in basal fyn kinase activity. Coaggregation of CD4 and TCR achieves the same outcome. These results underscore the critical role of lipid rafts in spatially coordinating the interaction between lck and fyn that predicates proximal TCR/CD3 signaling.

Dynamic regulation of Src-family kinases by CD45 in B cells

CD45 is a key protein tyrosine phosphatase regulating Src-family protein tyrosine kinases (Src-PTKs) in lymphocytes; precisely how it exerts its effect remains controversial, however. We previously demonstrated that CD45 negatively regulates Lyn in the WEHI-231 B-cell line. Here we show that negative regulation by CD45 is physiologically significant in B cells and that some CD45 is constitutively associated with glycolipid-enriched microdomains (GEMs), where it inhibits Src-PTKs by dephosphorylating both the negative and the positive regulatory sites. Upon B-cell receptor (BCR) ligation, however, CD45 dissociates from GEMs within 30 seconds, inducing phosphorylation of 2 regulatory sites and activation of Src-PTKs, but subsequently reassociates with the GEMs within 15 minutes. Disruption of GEMs with methyl-β-cyclodextrin results in abrogation of BCR-induced apoptosis in WEHI-231 cells, suggesting GEMs are critical to signals leading to the fate determination. We propose that the primary function of CD45 is inhibition of Src-PTKs and that the level of Src-PTK activation and the B-cell fate are determined in part by dynamic behavior of CD45 with respect to GEMs.

LIME: a new membrane Raft-associated adaptor protein involved in CD4 and CD8 coreceptor signaling

Lymphocyte membrane rafts contain molecules critical for immunoreceptor signaling. Here, we report identification of a new raft-associated adaptor protein LIME (Lck-interacting molecule) expressed predominantly in T lymphocytes. LIME becomes tyrosine phosphorylated after cross-linking of the CD4 or CD8 coreceptors. Phospho-LIME associates with the Src family kinase Lck and its negative regulator, Csk. Ectopic expression of LIME in Jurkat T cells results in an increase of Csk in lipid rafts, increased phosphorylation of Lck and higher Ca²⁺ response to CD3 stimulation. Thus, LIME appears to be involved in regulation of T cell activation by coreceptors.

SKAP-55 regulates integrin adhesion and formation of T cell-APC conjugates

Src kinase-associated phosphoprotein of 55 kDa (SKAP-55; encoded by SCAP1) is a T cell adaptor protein of unknown function that contains a pleckstrin homology and an SH3 domain. Here we show that SKAP-55 regulates integrin-mediated adhesion and conjugate formation between T cells and antigen-presenting cells (APCs). SKAP-55 enhances adhesion to fibronectin and intercellular adhesion molecule-1 (ICAM-1), colocalizes with actin at the T cell-APC synapse and promotes the clustering of lymphocyte-associated antigen-1 (LFA-1). Enhanced conjugation is comparable to that induced by adhesion and degranulation-promoting adaptor protein (ADAP), a binding partner of SKAP-55, and is abrogated by deletion of the SKAP-55 SH3 domain. Conjugate formation is accompanied by the translocation of SKAP-55 to membrane rafts, an event that is regulated by both LFA-1 and T cell receptor ligation. Our findings identify a mechanism by which SKAP-55 modulates T cell responses to antigen.

Glucocorticoids alter the lipid and protein composition of membrane rafts of a murine T cell hybridoma

Glucocorticoids (GC) are widely used anti-inflammatory agents known to suppress T cell activation by interfering with the TCR activation cascade. The attenuation of early TCR signaling events by these compounds has been recently attributed to a selective displacement of key signaling proteins from membrane lipid rafts. In this study, we demonstrate that GC displace the acyl-bound adaptor proteins linker for activation of T cells and phosphoprotein associated with glycosphingolipid-enriched microdomains from lipid rafts of murine T cell hybridomas, possibly by inhibiting their palmitoylation status. Analysis of the lipid content of the membrane rafts revealed that GC treatment led to a significant decrease in palmitic acid content. Moreover, we found an overall decrease in the proportion of raft-associated saturated fatty acids. These changes were consistent with a decrease in fluorescence anisotropy of isolated lipid rafts, indicating an increase in their fluidity. These findings identify the mechanisms underlying the complex inhibitory effects of glucocorticoids on early TCR signaling and suggest that some of the inhibitory properties of GC on T cell responses may be related to their ability to affect the membrane lipid composition and the palmitoylation status of important signaling molecules.

The roles of membrane microdomains (rafts) in T cell activation

Detergent-resistant membrane microdomains enriched in sphingolipids, cholesterol and glycosylphosphatidylinositol-anchored proteins play essential roles in T cell receptor (TCR) signaling. These 'membrane rafts' accumulate several cytoplasmic lipid-modified molecules, including Src-family kinases, coreceptors CD4 and CD8 and transmembrane adapters LAT and PAG/Cbp, essential for either initiation or amplification of the signaling process, while most other abundant transmembrane proteins are excluded from these structures. TCRs in various T cell subpopulations may differ in their use of membrane rafts. Membrane rafts also seem to be involved in many other aspects of T cell biology, such as functioning of cytokine and chemokine receptors, adhesion molecules, antigen presentation, establishing cell polarity or interaction with important pathogens. Although the concept of membrane rafts explains several diverse biological phenomena, many basic issues, such as composition, size and heterogeneity, under native conditions, as well as the dynamics of their interactions with TCRs and other immunoreceptors, remain unclear, partially because of technical problems.

CD45 ectodomain controls interaction with GEMs and Lck activity for optimal TCR signaling

The transmembrane phosphatase CD45 regulates both Lck activity and T cell receptor (TCR) signaling. Here we have tested whether the large ectodomain of CD45 has a role in this regulation. A CD45 chimera containing the large ectodomain of CD43 efficiently rescues TCR signaling in CD45-null T cells, whereas CD45 chimeras containing small ectodomains from other phosphatases do not. Both basal Lck activity in unstimulated cells and the TCR-induced increase in tyrosine phosphorylation of the TCR zeta-chain and in Lck activity depend on the expression of CD45 with a large ectodomain. Unlike CD45 chimeras containing small ectodomains, both the CD45 chimera with a large ectodomain and wild-type CD45 itself are partially localized to glycosphingolipid-enriched membranes (GEMs). Taken together, these data show that the large CD45 ectodomain is required for optimal TCR signaling.

Non-T cell activation linker (NTAL): a transmembrane adaptor protein involved in immunoreceptor signaling

A key molecule necessary for activation of T lymphocytes through their antigen-specific T cell receptor (TCR) is the transmembrane adaptor protein LAT (linker for activation of T cells). Upon TCR engagement, LAT becomes rapidly tyrosine phosphorylated and then serves as a scaffold organizing a multicomponent complex that is indispensable for induction of further downstream steps of the signaling cascade. Here we describe the identification and preliminary characterization of a novel transmembrane adaptor protein that is structurally and evolutionarily related to LAT and is expressed in B lymphocytes, natural killer (NK) cells, monocytes, and mast cells but not in resting T lymphocytes. This novel transmembrane adaptor protein, termed NTAL (non-T cell activation linker) is the product of a previously identified WBSCR5 gene of so far unknown function. NTAL becomes rapidly tyrosine-phosphorylated upon cross-linking of the B cell receptor (BCR) or of high-affinity Fcgamma- and Fc epsilon -receptors of myeloid cells and then associates with the cytoplasmic signaling molecules Grb2, Sos1, Gab1, and c-Cbl. NTAL expressed in the LAT-deficient T cell line J.CaM2.5 becomes tyrosine phosphorylated and rescues activation of Erk1/2 and minimal transient elevation of cytoplasmic calcium level upon TCR/CD3 cross-linking. Thus, NTAL appears to be a structural and possibly also functional homologue of LAT in non-T cells.

Membrane-cytoskeleton interactions during the formation of the immunological synapse and subsequent T-cell activation

Upon antigen recognition, T cells undergo substantial membrane and cytoskeletal rearrangements that lead to the formation of the immunological synapse and are necessary for subsequent T-cell activation. However, little is known about how membrane and cytoskeletal molecules interact during these processes. Here we discuss the involvement of the membrane-microfilament linker ezrin. We propose that ezrin is a component of the cytoskeleton-mediated architecture of the immunological synapse that plays a role in T-cell receptor clustering, protein kinase C theta translocation and intracellular signaling.

The T-cell receptor signalosome: a dynamic structure with expanding complexity

Signal transduction in T cells is a dynamic process involving a large number of membrane and cytosolic proteins. The TCR macromolecular complex (signalosome) is initiated by receptor occupancy and becomes more elaborate over time. This review describes how 'vertical' displacement mechanisms and lateral coalescence of lipid-raft-associated scaffold proteins combine to form distinct signalosomes, which control signal specificity.

TCR signal initiation machinery is pre-assembled and activated in a subset of membrane rafts

Recent studies suggest that rafts are involved in numerous cell functions, including membrane traffic and signaling. Here we demonstrate, using a polyoxyethylene ether Brij 98, that detergent-insoluble microdomains possessing the expected biochemical characteristics of rafts are present in the cell membrane at 37 degrees C. After extraction, these microdomains are visualized as membrane vesicles with a mean diameter of approximately 70 nm. These findings provide further evidence for the existence of rafts under physiological conditions and are the basis of a new isolation method allowing more accurate analyses of raft structure. We found that main components of T cell receptor (TCR) signal initiation machinery, i.e. TCR-CD3 complex, Lck and ZAP-70 kinases, and CD4 co-receptor are constitutively partitioned into a subset of rafts. Functional studies in both intact cells and isolated rafts showed that upon ligation, TCR initiates the signaling in this specialized raft subset. Our data thus strongly indicate an important role of rafts in organizing TCR early signaling pathways within small membrane microdomains, both prior to and following receptor engagement, for efficient TCR signal initiation upon stimulation.