An intact dilysine-like motif in the carboxyl terminus of MAL is required for normal apical transport of the influenza virus hemagglutinin cargo protein in epithelial Madin-Darby canine kidney cells

Clostridium perfringens Epsilon Toxin Binds to and Kills Primary Human Lymphocytes

Clostridium perfringens epsilon toxin (ETX) is the third most lethal bacterial toxin and has been suggested to be an environmental trigger of multiple sclerosis, an immune-mediated disease of the human central nervous system. However, ETX cytotoxicity on primary human cells has not been investigated. In this article, we demonstrate that ETX preferentially binds to and kills human lymphocytes expressing increased levels of the myelin and lymphocyte protein MAL. Using flow cytometry, ETX binding was determined to be time and dose dependent and was highest for CD4+ cells, followed by CD8+ and then CD19+ cells. Similar results were seen with ETX-induced cytotoxicity. To determine if ETX preference for CD4+ cells was related to MAL expression, MAL gene expression was determined by RT-qPCR. CD4+ cells had the highest amount of Mal gene expression followed by CD8+ and CD19+ cells. These data indicate that primary human cells are susceptible to ETX and support the hypothesis that MAL is a main receptor for ETX. Interestingly, ETX bindings to human lymphocytes suggest that ETX may influence immune response in multiple sclerosis.

The MAL Protein, an Integral Component of Specialized Membranes, in Normal Cells and Cancer

The MAL gene encodes a 17-kDa protein containing four putative transmembrane segments whose expression is restricted to human T cells, polarized epithelial cells and myelin-forming cells. The MAL protein has two unusual biochemical features. First, it has lipid-like properties that qualify it as a member of the group of proteolipid proteins. Second, it partitions selectively into detergent-insoluble membranes, which are known to be enriched in condensed cell membranes, consistent with MAL being distributed in highly ordered membranes in the cell. Since its original description more than thirty years ago, a large body of evidence has accumulated supporting a role of MAL in specialized membranes in all the cell types in which it is expressed. Here, we review the structure, expression and biochemical characteristics of MAL, and discuss the association of MAL with raft membranes and the function of MAL in polarized epithelial cells, T lymphocytes, and myelin-forming cells. The evidence that MAL is a putative receptor of the epsilon toxin of Clostridium perfringens, the expression of MAL in lymphomas, the hypermethylation of the MAL gene and subsequent loss of MAL expression in carcinomas are also presented. We propose a model of MAL as the organizer of specialized condensed membranes to make them functional, discuss the role of MAL as a tumor suppressor in carcinomas, consider its potential use as a cancer biomarker, and summarize the directions for future research.

A novel physiological role for ARF1 in the formation of bidirectional tubules from the Golgi

Capitalizing on CRISPR/Cas9 gene-editing techniques and super-resolution nanoscopy, we explore the role of the small GTPase ARF1 in mediating transport steps at the Golgi. Besides its well-established role in generating COPI vesicles, we find that ARF1 is also involved in the formation of long (∼3 µm), thin (∼110 nm diameter) tubular carriers. The anterograde and retrograde tubular carriers are both largely free of the classical Golgi coat proteins coatomer (COPI) and clathrin. Instead, they contain ARF1 along their entire length at a density estimated to be in the range of close packing. Experiments using a mutant form of ARF1 affecting GTP hydrolysis suggest that ARF1[GTP] is functionally required for the tubules to form. Dynamic confocal and stimulated emission depletion imaging shows that ARF1-rich tubular compartments fall into two distinct classes containing 1) anterograde cargoes and clathrin clusters or 2) retrograde cargoes and coatomer clusters.

The Myelin and Lymphocyte Protein MAL Is Required for Binding and Activity of Clostridium perfringens ε-Toxin

Clostridium perfringens ε-toxin (ETX) is a potent pore-forming toxin responsible for a central nervous system (CNS) disease in ruminant animals with characteristics of blood-brain barrier (BBB) dysfunction and white matter injury. ETX has been proposed as a potential causative agent for Multiple Sclerosis (MS), a human disease that begins with BBB breakdown and injury to myelin forming cells of the CNS. The receptor for ETX is unknown. Here we show that both binding of ETX to mammalian cells and cytotoxicity requires the tetraspan proteolipid Myelin and Lymphocyte protein (MAL). While native Chinese Hamster Ovary (CHO) cells are resistant to ETX, exogenous expression of MAL in CHO cells confers both ETX binding and susceptibility to ETX-mediated cell death. Cells expressing rat MAL are ~100 times more sensitive to ETX than cells expressing similar levels of human MAL. Insertion of the FLAG sequence into the second extracellular loop of MAL abolishes ETX binding and cytotoxicity. ETX is known to bind specifically and with high affinity to intestinal epithelium, renal tubules, brain endothelial cells and myelin. We identify specific binding of ETX to these structures and additionally show binding to retinal microvasculature and the squamous epithelial cells of the sclera in wild-type mice. In contrast, there is a complete absence of ETX binding to tissues from MAL knockout (MAL-/-) mice. Furthermore, MAL-/- mice exhibit complete resistance to ETX at doses in excess of 1000 times the symptomatic dose for wild-type mice. We conclude that MAL is required for both ETX binding and cytotoxicity.

Hepatocyte polarity

Hepatocytes, like other epithelia, are situated at the interface between the organism's exterior and the underlying internal milieu and organize the vectorial exchange of macromolecules between these two spaces. To mediate this function, epithelial cells, including hepatocytes, are polarized with distinct luminal domains that are separated by tight junctions from lateral domains engaged in cell-cell adhesion and from basal domains that interact with the underlying extracellular matrix. Despite these universal principles, hepatocytes distinguish themselves from other nonstriated epithelia by their multipolar organization. Each hepatocyte participates in multiple, narrow lumina, the bile canaliculi, and has multiple basal surfaces that face the endothelial lining. Hepatocytes also differ in the mechanism of luminal protein trafficking from other epithelia studied. They lack polarized protein secretion to the luminal domain and target single-spanning and glycosylphosphatidylinositol-anchored bile canalicular membrane proteins via transcytosis from the basolateral domain. We compare this unique hepatic polarity phenotype with that of the more common columnar epithelial organization and review our current knowledge of the signaling mechanisms and the organization of polarized protein trafficking that govern the establishment and maintenance of hepatic polarity. The serine/threonine kinase LKB1, which is activated by the bile acid taurocholate and, in turn, activates adenosine monophosphate kinase-related kinases including AMPK1/2 and Par1 paralogues has emerged as a key determinant of hepatic polarity. We propose that the absence of a hepatocyte basal lamina and differences in cell-cell adhesion signaling that determine the positioning of tight junctions are two crucial determinants for the distinct hepatic and columnar polarity phenotypes.

Structure of Golgi transport proteins

The function of the Golgi has long been recognized to critically depend on vesicular transport from, to, and within its cisternae, involving constant membrane fission and fusion. These processes are mediated by Arf GTPases and coat proteins, and Rabs, tethers and SNARE proteins, respectively. In this article, we describe structural studies of Golgi coats and tethers and their interactions with SNAREs and GTPases as well as insights regarding membrane traffic processes that these have provided.

Viral replication and innate host responses in primary human alveolar epithelial cells and alveolar macrophages infected with influenza H5N1 and H1N1 viruses

Highly pathogenic influenza H5N1 virus continues to pose a threat to public health. Although the mechanisms underlying the pathogenesis of the H5N1 virus have not been fully defined, it has been suggested that cytokine dysregulation plays an important role. As the human respiratory epithelium is the primary target cell for influenza viruses, elucidating the viral tropism and innate immune responses of influenza H5N1 virus in the alveolar epithelium may help us to understand the pathogenesis of the severe pneumonia associated with H5N1 disease. Here we used primary cultures of differentiated human alveolar type II cells, alveolar type I-like cells, and alveolar macrophages isolated from the same individual to investigate viral replication competence and host innate immune responses to influenza H5N1 (A/HK/483/97) and H1N1 (A/HK/54/98) virus infection. The viral replication kinetics and cytokine and chemokine responses were compared by quantitative PCR (qPCR) and enzyme-linked immunosorbent assay (ELISA). We demonstrated that influenza H1N1 and H5N1 viruses replicated productively in type II cells and type I-like cells although with different kinetics. The H5N1 virus replicated productively in alveolar macrophages, whereas the H1N1 virus led to an abortive infection. The H5N1 virus was a more potent inducer of proinflammatory cytokines and chemokines than the H1N1 virus in all cell types. However, higher levels of cytokine expression were observed for peripheral blood monocyte-derived macrophages than for alveolar macrophages in response to H5N1 virus infection. Our findings provide important insights into the viral tropisms and host responses of different cell types found in the lung and are relevant to an understanding of the pathogenesis of severe human influenza disease.

MYADM regulates Rac1 targeting to ordered membranes required for cell spreading and migration

Membrane organization into condensed domains or rafts provides molecular platforms for selective recruitment of proteins. Cell migration is a general process that requires spatiotemporal targeting of Rac1 to membrane rafts. The protein machinery responsible for making rafts competent to recruit Rac1 remains elusive. Some members of the MAL family of proteins are involved in specialized processes dependent on this type of membrane. Because condensed membrane domains are a general feature of the plasma membrane of all mammalian cells, we hypothesized that MAL family members with ubiquitous expression and plasma membrane distribution could be involved in the organization of membranes for cell migration. We show that myeloid-associated differentiation marker (MYADM), a protein with unique features within the MAL family, colocalizes with Rac1 in membrane protrusions at the cell surface and distributes in condensed membranes. MYADM knockdown (KD) cells had altered membrane condensation and showed deficient incorporation of Rac1 to membrane raft fractions and, similar to Rac1 KD cells, exhibited reduced cell spreading and migration. Results of rescue-of-function experiments by expression of MYADM or active Rac1L61 in cells knocked down for Rac1 or MYADM, respectively, are consistent with the idea that MYADM and Rac1 act on parallel pathways that lead to similar functional outcomes.

An essential role for the MAL protein in targeting Lck to the plasma membrane of human T lymphocytes

The MAL protein is an essential component of the specialized machinery for apical targeting in epithelial cells. The src family kinase Lck plays a pivotal role in T cell signaling. We show that MAL is required in T cells for efficient expression of Lck at the plasma membrane and activation of IL-2 transcription. To investigate the mechanism by which MAL regulates Lck targeting, we analyzed the dynamics of Lck and found that it travels to the plasma membrane in specific transport carriers containing MAL. Coimmunoprecipitation experiments indicated an association of MAL with Lck. Both carrier formation and partitioning of Lck into detergent-insoluble membranes were ablated in the absence of MAL. Polarization of T cell receptor for antigen (TCR) and microtubule-organizing center to immunological synapse (IS) were also defective. Although partial correction of the latter defects was possible by forced expression of Lck at the plasma membrane, their complete correction, formation of transport vesicles, partitioning of Lck, and restoration of signaling pathways, which are required for IL-2 transcription up-regulation, were achieved by exogenous expression of MAL. We concluded that MAL is required for recruitment of Lck to specialized membranes and formation of specific transport carriers for Lck targeting. This novel transport pathway is crucial for TCR-mediated signaling and IS assembly.

Morphogenesis of post-Golgi transport carriers

The trans-Golgi network (TGN) is one of the main, if not the main, sorting stations in the process of intracellular protein trafficking. It is therefore of central importance to understand how the key players in the TGN-based sorting and delivery process, the post-Golgi carriers (PGCs), form and function. Over the last few years, modern morphological approaches have generated new insights into the questions of PGC biogenesis, structure and dynamics. Here, we present a view by which the “lifecycle” of a PGC consists of several distinct stages: the formation of TGN tubular export domains (where different cargoes are segregated from each other and from the Golgi enzymes); the docking of these tubular domains onto molecular motors and their extrusion towards the cell periphery along microtubules; the fission of the forming PGC from the donor membrane; and the delivery of the newly formed PGC to its specific acceptor organelle. It is now important to add the many molecular machineries that have been described as operating at the TGN to this “morphofunctional map” of the TGN export process.

Exogenous MAL reroutes selected hepatic apical proteins into the direct pathway in WIF-B cells

Unlike simple epithelial cells that directly target newly synthesized glycophosphatidylinositol (GPI)-anchored and single transmembrane domain (TMD) proteins from the trans-Golgi network to the apical membrane, hepatocytes use an indirect pathway: proteins are delivered to the basolateral domain and then selectively internalized and transcytosed to the apical plasma membrane. Myelin and lymphocyte protein (MAL) and MAL2 have been identified as regulators of direct and indirect apical delivery, respectively. Hepatocytes lack endogenous MAL consistent with the absence of direct apical targeting. Does MAL expression reroute hepatic apical residents into the direct pathway? We found that MAL expression in WIF-B cells induced the formation of cholesterol and glycosphingolipid-enriched Golgi domains that contained GPI-anchored and single TMD apical proteins; polymeric IgA receptor (pIgA-R), polytopic apical, and basolateral resident distributions were excluded. Basolateral delivery of newly synthesized apical residents was decreased in MAL-expressing cells concomitant with increased apical delivery; pIgA-R and basolateral resident delivery was unchanged. These data suggest that MAL rerouted selected hepatic apical proteins into the direct pathway.

Protein sorting in the Golgi complex: Shifting paradigms

The paradigms for transport along the biosynthetic route have changed dramatically over the past 15 years. Unlike the situation 15 years ago, the current paradigm involves sorting signals practically at every step of the pathway. In particular, at the exit from the Golgi complex, apical, basolateral and lysosomal targeting signals result in the generation of a variety of routes. Furthermore, it is now quite clear that not all sorting in the biosynthetic route occurs in the Golgi complex or the Trans Golgi Network (TGN). Sorting may occur distally to the Golgi, in recycling endosomes or in budded tubulosaccular structures, or it may occur proximally to the Golgi complex, at the exit from the ER. Several adaptors are candidates to sort apical and basolateral proteins but only AP1B and AP4 are currently involved. Progress is fast and future work should elucidate many of the open questions.

Organization of vesicular trafficking in epithelia

Experiments using mammalian epithelial cell lines have elucidated biosynthetic and recycling pathways for apical and basolateral plasma-membrane proteins, and have identified components that guide apical and basolateral proteins along these pathways. These components include apical and basolateral sorting signals, adaptors for basolateral signals, and docking and fusion proteins for vesicular trafficking. Recent live-cell-imaging studies provide a real-time view of sorting processes in epithelial cells, including key roles for actin, microtubules and motors in the organization of post-Golgi trafficking.

The subapical compartment: a traffic center in membrane polarity development

Spatially separated apical and basolateral plasma membrane domains that have distinct functions and molecular compositions are a characteristic feature of epithelial cell polarity. The subapical compartment (SAC), also known as the common endosome (CE), where endocytic pathways from both surfaces merge, plays a crucial role in the maintenance and probably the biogenesis of these distinct membrane domains. Although differences in morphology are apparent, the same principal features of a SAC can be distinguished in different types of epithelial cells. As polarity develops, the compartment acquires several distinct machineries that, in conjunction with the cytoskeleton, are necessary for polarized trafficking. Disrupting trafficking via the SAC and hence bypassing its sorting machinery, as occurs upon actin depolymerization, leads to mis-sorting of apical and basolateral molecules, thereby compromising the development of polarity. The structural and functional integrity of the compartment in part depends on microtubules. Moreover, the acquisition of a particular set of Rab proteins, including Rab11 and Rab3, appears to be crucial in regulating molecular sorting and vesicular transport relevant both to recycling to either plasma membrane domain and to de novo assembly of the apical domain. Furthermore, subcompartmentalization of the SAC appears to be key to its various functions.

Features of influenza HA required for apical sorting differ from those required for association with DRMs or MAL

The influenza virus hemagglutinin (HA) is sorted to the apical membrane in polarized epithelial cells and associates with detergent-resistant membranes (DRMs). By systematic mutagenesis of the transmembrane residues, we show that hemagglutinin requires 10 contiguous transmembrane amino acids to enter detergent-resistant membranes and that the surface of the trimeric hemagglutinin transmembrane domain facing the lipid environment as well as that facing the interior of the trimer is important for stable association with detergent-resistant membranes. However, association with detergent-resistant membranes was not required for apical sorting. MAL/VIP17 is a protein that is required for apical transport and a small fraction of hemagglutinin co-precipitates with MAL. Mutations that prevented HA from being isolated in detergent-resistant membranes decreased co-precipitation with MAL. The hemagglutinin and MAL that co-precipitated were contained in a detergent-resistant vesicle. However, most of the co-precipitation of newly synthesized hemagglutinin with MAL occurred only after the majority of hemagglutinin reached the cell surface. Both the timing and the limited extent of co-precipitation suggest that the majority of vesicles containing hemagglutinin and MAL are not the detergent-resistant membrane transport intermediates carrying hemagglutinin from the TGN to the apical surface.

Alpha interferon inhibits human T-cell leukemia virus type 1 assembly by preventing Gag interaction with rafts

Alpha-2a interferon (IFN-alpha2a) has beneficial clinical effects on human T-cell leukemia virus type 1 (HTLV-1) infection, but its antiviral mechanism of action is unknown. Antiviral effects of IFN-alpha2a were studied in 293T cells expressing HTLV-1 proviral DNA and in HTLV-1-infected cells (HOS/PL, MT2, and HUT102). In 293T cells, an 50% inhibitory concentration of 10 U of IFN-alpha2a/ml was determined by p19 antigen ELISA. Analysis of IFN-treated cells demonstrated no defect in viral protein synthesis but did show a decrease in the level of released virus, as determined by immunoblot assays. Electron microscopy studies of IFN-treated cells revealed neither a defect in the site of virus budding nor tethering of virus particles at the plasma membrane, thus arguing against an effect on virus release. Cell fractionation studies and confocal microscopy showed no effect of IFN on Gag association with membranes. However, the level of Gag association with lipid rafts was decreased, suggesting a role of IFN in inhibiting HTLV-1 assembly.

MAL regulates clathrin-mediated endocytosis at the apical surface of Madin-Darby canine kidney cells

MAL is an integral protein component of the machinery for apical transport in epithelial Madin-Darby canine kidney (MDCK) cells. To maintain its distribution, MAL cycles continuously between the plasma membrane and the Golgi complex. The clathrin-mediated route for apical internalization is known to differ from that at the basolateral surface. Herein, we report that MAL depends on the clathrin pathway for apical internalization. Apically internalized polymeric Ig receptor (pIgR), which uses clathrin for endocytosis, colocalized with internalized MAL in the same apical vesicles. Time-lapse confocal microscopic analysis revealed cotransport of pIgR and MAL in the same endocytic structures. Immunoelectron microscopic analysis evidenced colabeling of MAL with apically labeled pIgR in pits and clathrin-coated vesicles. Apical internalization of pIgR was abrogated in cells with reduced levels of MAL, whereas this did not occur either with its basolateral entry or the apical internalization of glycosylphosphatidylinositol-anchored proteins, which does not involve clathrin. Therefore, MAL is critical for efficient clathrin-mediated endocytosis at the apical surface in MDCK cells.

MAL gene expression in esophageal cancer suppresses motility, invasion and tumorigenicity and enhances apoptosis through the Fas pathway

We isolated the MAL (T-lymphocyte maturation associated protein) gene from differentially expressed products of esophageal epithelium relative to esophageal carcinoma tissues. The Mal protein has been demonstrated as being a component of the protein machinery for apical transport in epithelial polarized cells. In this study, we describe the reduced expression of MAL in all 39 cases of esophageal carcinoma tested and 60 other human carcinomas. MAL gene transcription was induced in three out of 13 esophageal carcinoma cell lines by treatment with the demethylating agent 5-aza-2'-deoxycytidine (DAC), and in nine additional cell lines by simultaneous treatment with trichostatin A, an inhibitor of deacetylation, and DAC. We established a stable MAL gene transfectant whose expression was regulated by subcutaneous doxycycline injection in nude mice. Tumor growth was suppressed in cells expressing TE3-MAL compared with TE3 parent cells or cells not expressing TE3-MAL with doxycycline injection (20 microg/body) (P<0.01). Additionally, the TE3-MAL transfectant cells exhibited decreased cellular motility, a G1/S transition block and increased levels of apoptosis, concomitant with increased expression of Fas receptor in vitro. The apoptotic staining in MAL-expressing tumors was confirmed by TUNEL assay. Therefore, we conclude that expression of MAL was frequently decreased or diminished in gastrointestinal tract cancers, and that Mal expression confers reduced tumorigenicity in vivo to tumor TE3 cells through the induction of apoptosis via the Fas signaling pathway.

Expression of MAL, an integral protein component of the machinery for raft-mediated pical transport, in human epithelia

The MAL protein is the only integral membrane protein identified as being an essential component of the machinery necessary for apical transport in the canine MDCK cell line, a paradigm of polarized epithelial cells. To characterize the range of human epithelia that use MAL-mediated pathways of transport, we performed an immunohistochemical survey of normal tissues using a monoclonal antibody (MAb) specific for the MAL protein. For comparison, different types of carcinoma were also analyzed. MAL, with a characteristic strong supranuclear granular distribution, was detected in specific types of normal epithelial cells throughout the respiratory system, the gastrointestinal and genitourinary tracts, and in exocrine and endocrine glands. Absorptive cells (e.g., enterocytes), and many different types of specialized secretory cells, either organized in discrete clusters (e.g., endocrine cells in the pancreas), gathered together in an endocrine gland (e.g., thyroid), interspersed with other cells in glands (e.g., parietal cells), or dispersed singly among other cells (e.g., type 2 pneumocytes) were positive for MAL. We also analyzed a series of epithelial renal and thyroid tumors and found alterations dependent on the particular histological type of tumor. These results open potential applications of the anti-MAL antibody for the characterization of neoplastic tissue.

Independent segregation of human immunodeficiency virus type 1 Gag protein complexes and lipid rafts

Formation of human immunodeficiency virus type 1 (HIV-1) particles takes place at the plasma membrane of cells and is directed by the Pr55Gag polyprotein. A functional assembly domain (the M domain) within the N-terminal portion of Pr55Gag mediates the interaction of Gag with cellular membranes. However, the determinants that provide specificity for assembly on the plasma membrane, as opposed to intracellular membranes, have not been identified. Recently, it was reported that Pr55Gag interacts with lipid raft microdomains of the plasma membrane. We sought to identify the domains within Pr55Gag that contribute to lipid raft association of Gag. Here we demonstrate that the I domain is required for interaction with detergent-resistant membrane fractions (DRMs). Mutation of key I-domain residues or loss of myristylation abrogated the association of Gag with DRMs. Thus, the I domain and the M domain combine to mediate Gag-lipid raft interactions as defined by these biochemical criteria. However, Gag protein complexes defined by flotation studies were much denser than classical lipid rafts, failed to incorporate classical lipid raft marker proteins, and were not disrupted by cholesterol extraction. Large sheets of Gag protein were identified in DRM fractions upon examination by electron microscopy. These results indicate that HIV-1 Pr55Gag forms detergent-resistant complexes at the cellular periphery that are distinct from lipid raft microdomains.

Hantaan virus enters cells by clathrin-dependent receptor-mediated endocytosis

The cellular entry of Hantaan virus (HTN) occurs through interactions with beta(3) integrins as cellular receptors. However, the process of HTN infection following attachment to the cell surface is not well understood. Our data indicate that overexpression of a dominant-negative mutant dynamin inhibits HTN internalization and that compounds that block clathrin- but not caveolae-dependent endocytosis also reduce HTN infectivity. In addition, we show that HTN colocalizes with the clathrin heavy chain but not with caveolae. At the early phase of infection HTN colocalizes with EEA-1, an early endosome marker, and later, HTN colocalizes with LAMP-1, a lysosome marker. Cells treated with lysosomotropic agents are largely resistant to infection, suggesting that a low-pH-dependent step is required for HTN infection. These findings demonstrate that HTN enters cells via the clathrin-coated pit pathway and uses low-pH-dependent intracellular compartments for infectious entry.

MAL mediates apical transport of secretory proteins in polarized epithelial Madin-Darby canine kidney cells

The MAL proteolipid is an integral membrane protein identified as a component of the raft machinery for apical sorting of membrane proteins in Madin-Darby canine kidney (MDCK) cells. Previous studies have implicated lipid rafts in the transport of exogenous thyroglobulin (Tg), the predominant secretory protein of thyroid epithelial cells, to the apical surface in MDCK cells. We have examined the secretion of recombinant Tg and gp80/clusterin, a major endogenous secretory protein not detected in Triton X-100 insoluble rafts, for the investigation of the involvement of MAL in the constitutive apical secretory pathway of MDCK cells. We show that MAL depletion impairs apical secretion of Tg and causes its accumulation in the Golgi. Cholesterol sequestration, which blocks apical secretion of Tg, did not alter the levels of MAL in rafts but created a block proximal to Tg entrance into rafts. Apical secretion of gp80/clusterin was also inhibited by elimination of endogenous MAL. Our results suggest a role for MAL in the transport of both endogenously and exogenously expressed apical secretory proteins in MDCK cells.

The role of lipid rafts in signalling and membrane trafficking in T lymphocytes

Combinatorial association of different lipid species generates microheterogeneity in biological membranes. The association of glycosphingolipids with cholesterol forms membrane microdomains – lipid rafts – that are involved in specialised pathways of protein/lipid transport and signalling. Lipid rafts are normally dispersed in cellular membranes and appear to require specialised machinery to reorganise them to operate. Caveolin-1 and MAL are members of two different protein families involved in reorganisation of lipid rafts for signalling and/or intracellular transport in epithelial cells. T cell activation induces a rapid compartmentalisation of signalling machinery into reorganised rafts that are used as platforms for the assembly of the signalling complex. Costimulatory molecules participate in this process by providing signals that mobilise raft lipids and proteins, and remodel the cytoskeleton to the contact site. As in epithelial cells, rafts are used also as vesicular carriers for membrane trafficking in T lymphocytes. Furthermore, there are potential similarities between the specialised protein machinery underlying raft-mediated processes in T lymphocytes and polarised epithelial cells.