Cell fusion experiments reveal distinctly different association characteristics of cell-surface receptors

Monomeric agonist peptide/MHCII complexes activate T-cells in an autonomous fashion

Molecular crowding of agonist peptide/MHC class II complexes (pMHCIIs) with structurally similar, yet per se non-stimulatory endogenous pMHCIIs is postulated to sensitize T-cells for the recognition of single antigens on the surface of dendritic cells and B-cells. When testing this premise with the use of advanced live cell microscopy, we observe pMHCIIs as monomeric, randomly distributed entities diffusing rapidly after entering the APC surface. Synaptic TCR engagement of highly abundant endogenous pMHCIIs is low or non-existent and affects neither TCR engagement of rare agonist pMHCII in early and advanced synapses nor agonist-induced TCR-proximal signaling. Our findings highlight the capacity of single freely diffusing agonist pMHCIIs to elicit the full T-cell response in an autonomous and peptide-specific fashion with consequences for adaptive immunity and immunotherapeutic approaches.

Characterization of the Effect of Sphingolipid Accumulation on Membrane Compactness, Dipole Potential, and Mobility of Membrane Components

Communication between cells and their environment is carried out through the plasma membrane including the action of most pharmaceutical drugs. Although such a communication typically involves specific binding of a messenger to a membrane receptor, the biophysical state of the lipid bilayer strongly influences the outcome of this interaction. Sphingolipids constitute an important part of the lipid membrane, and their mole fraction modifies the biophysical characteristics of the membrane. Here, we describe methods that can be used for measuring how sphingolipid accumulation alters the compactness, microviscosity, and dipole potential of the lipid bilayer and the mobility of membrane components.

Particle Simulation of Oxidation Induced Band 3 Clustering in Human Erythrocytes

Oxidative stress mediated clustering of membrane protein band 3 plays an essential role in the clearance of damaged and aged red blood cells (RBCs) from the circulation. While a number of previous experimental studies have observed changes in band 3 distribution after oxidative treatment, the details of how these clusters are formed and how their properties change under different conditions have remained poorly understood. To address these issues, a framework that enables the simultaneous monitoring of the temporal and spatial changes following oxidation is needed. In this study, we established a novel simulation strategy that incorporates deterministic and stochastic reactions with particle reaction-diffusion processes, to model band 3 cluster formation at single molecule resolution. By integrating a kinetic model of RBC antioxidant metabolism with a model of band 3 diffusion, we developed a model that reproduces the time-dependent changes of glutathione and clustered band 3 levels, as well as band 3 distribution during oxidative treatment, observed in prior studies. We predicted that cluster formation is largely dependent on fast reverse reaction rates, strong affinity between clustering molecules, and irreversible hemichrome binding. We further predicted that under repeated oxidative perturbations, clusters tended to progressively grow and shift towards an irreversible state. Application of our model to simulate oxidation in RBCs with cytoskeletal deficiency also suggested that oxidation leads to more enhanced clustering compared to healthy RBCs. Taken together, our model enables the prediction of band 3 spatio-temporal profiles under various situations, thus providing valuable insights to potentially aid understanding mechanisms for removing senescent and premature RBCs.

In order to maintain a steady internal environment, our bodies must be able to specifically recognize old and damaged red blood cells (RBCs), and remove them from the circulation in a timely manner. Clusters of membrane protein band 3, which form in response to elevated oxidative damage, serve as essential molecular markers that initiate this cell removal process. However, little is known about the details of how these clusters are formed and how their properties change under different conditions. To understand these mechanisms in detail, we developed a computational model that enables the prediction of the time course profiles of metabolic intermediates, as well as the visualization of the resulting band 3 distribution during oxidative treatment. Our model predictions were in good agreement with previous published experimental data, and provided predictive insights on the key factors of cluster formation. Furthermore, simulation experiments of the effects of multiple oxidative pulses and cytoskeletal defect using the model also suggested that clustering is enhanced under such conditions. Analyses using our model can provide hypotheses and suggest experiments to aid the understanding of the physiology of anemia-associated RBC disorders, and optimization of quality control of RBCs in stored blood.

Distinct spatial relationship of the interleukin-9 receptor with interleukin-2 receptor and major histocompatibility complex glycoproteins in human T lymphoma cells

The interleukin-9 receptor (IL-9R) consists of an α subunit and a γ(c) chain that are shared with other cytokine receptors, including interleukin-2 receptor (IL-2R), an important regulator of T cells. We previously showed that IL-2R is expressed in common clusters with major histocompatibility complex (MHC) glycoproteins in lipid rafts of human T lymphoma cells, which raised the question about what the relationship between clusters of IL-2R/MHC and IL-9R is. Confocal microscopy colocalization and fluorescence resonance energy transfer experiments capable of detecting membrane protein organization at different size scales revealed nonrandom association of IL-9R with IL-2R/MHC clusters at the surface of human T lymphoma cells. Accommodation of IL-9Rα in membrane areas segregated from the IL-2R/MHC domains was also detected. The bipartite nature of IL-9R distribution was mirrored by signal transducer and activator of transcription (STAT) activation results. Our data indicate that co-compartmentalization with MHC glycoproteins is a general property of γ(c) receptors. Distribution of receptor chains between different membrane domains may regulate their function.

Crohn's disease alters MHC-rafts in CD4+ T-cells

Clusters of MHCI, ICAM-1, CD44, CD59, IL-2R, and IL-15R molecules have been studied on the surface of CD4(+) T-cells from peripheral blood and lymph nodes of patients in Crohn's disease and healthy individuals as controls by using a dual-laser flow cytometric fluorescence resonance energy transfer (FRET) technique and fluorescently stained Fabs. When cells from patients in Crohn's disease are compared to those of controls, the surface expression level for the MHCI reduced by ∼45%, for CD44 enhanced by ∼100%, and for IL-2Rα, IL-15Rα, and common γ(c) enhanced by ∼50%, ∼70%, and ∼130%, respectively. Efficiencies of FRET monitoring homoassociation for the MHCI and CD44 reduced, that for IL-2Rα enhanced. While efficiencies of FRET monitoring the association of γ(c) and ICAM-1 with the MHCI reduced, those monitoring association of IL-2/15Rα, CD44, and CD59 with MHCI enhanced. Efficiencies of FRET measured between the MHCI and IL-2Rα, IL-15Rα differently enhanced to the advantage of IL-15Rα, the one measured between γ(c) and IL-2Rα reduced, suggesting modulations in the strength of interaction of MHCI with IL-2R, IL-15R, and γ(c). The increases in density of surface bound cTx and in the associations of the receptors with the G(M1)-ganglioside lipid molecules suggest stronger lipid raft interactions of the receptors. The observed alterations of MHC-rafts in Crohn's disease--summarized in models of receptor patterns of diseased and control cells--may have functional consequences regarding signaling by the raft components.

Coclustering of ErbB1 and ErbB2 revealed by FRET-sensitized acceptor bleaching

Classical theory states that ligand binding induces the dimerization of ErbB proteins, leading to their activation. Although we and other investigators have shown the existence of preformed homoclusters of ErbB receptors and analyzed their composition, the stoichiometry of their heteroclusters has not been quantitatively described. Here, we report the development of the fluorescence resonance energy transfer (FRET)-sensitized acceptor bleaching (FSAB) technique to quantitate the ratio of ErbB1 and ErbB2 in their heteroclusters. In FSAB, photolabile acceptors within FRET distance from photostable donors are excited and photobleached by FRET, and the fraction of acceptors that are participating in FRET is determined. In quiescent SKBR-3 breast cancer cells, approximately 35% of ErbB1 and approximately 10% of ErbB2 have been found in heteroclusters. Epidermal growth factor (ligand of ErbB1) increased the fraction of ErbB2 heteroclustering with ErbB1, whereas the ratio of heteroclustered ErbB1 did not change significantly. The fractions of heteroclustered ErbB1 and ErbB2 were independent of their expression levels, indicating that the formation of these clusters is not driven by the law of mass action. In contrast, the FRET efficiency depended on the donor/acceptor ratio as expected. We present a model in which preformed receptor clusters are rearranged upon ligand stimulation, and report that the composition of these clusters can be quantitatively described by the FSAB technique.

High Q optical fiber tips for NC-AFM in liquid

Non-contact atomic force microscopy is rapidly expanding from ultra-high vacuum to include the study of surfaces and biomolecules in liquids by high resolution imaging and force spectroscopy. This is despite the additional frequency shift noise due to the inherently low Q factor of the cantilever oscillating in a liquid. In this paper we present a tip based on an optical fiber which can operate in liquid with Q factors in excess of 100 using a 'diving bell' arrangement which allows only a small portion of the tip to be submerged. We demonstrate stable imaging and force spectroscopy using this set-up. The tips are based on scanning near-field optical microscopy tips and, when used with NC-AFM, provide a method of combining both high resolution mechanical and fluorescence studies of biomolecules and cells.

Cytometry of raft and caveola membrane microdomains: from flow and imaging techniques to high throughput screening assays

The evolutionarily developed microdomain structure of biological membranes has gained more and more attention in the past decade. The caveolin-free "membrane rafts," the caveolin-expressing rafts (caveolae), as well as other membrane microdomains seem to play an essential role in controlling and coordinating cell-surface molecular recognition, internalization/endocytosis of the bound molecules or pathogenic organisms and in regulation of transmembrane signal transduction processes. Therefore, in many research fields (e.g. neurobiology and immunology), there is an ongoing need to understand the nature of these microdomains and to quantitatively characterize their lipid and protein composition under various physiological and pathological conditions. Flow and image cytometry offer many sophisticated and routine tools to study these questions. In this review, we give an overview of the past efforts to detect and characterize these membrane microdomains by the use of classical cytometric technologies, and finally we will discuss the results and perspectives of a new line of raft cytometry, the "high throughput screening assays of membrane microdomains," based on "lipidomic" and "proteomic" approaches.

Nanometer-scale organization of the alpha subunits of the receptors for IL2 and IL15 in human T lymphoma cells

Interleukin 2 and interleukin 15 (IL2 and IL15, respectively) provide quite distinct contributions to T-cell-mediated immunity, despite having similar receptor composition and signaling machinery. As most of the proposed mechanisms underlying this apparent paradox attribute key significance to the individual alpha-chains of IL2 and IL15 receptors, we investigated the spatial organization of the receptors IL2Ralpha and IL15Ralpha at the nanometer scale expressed on a human CD4+ leukemia T cell line using single-molecule-sensitive near-field scanning optical microscopy (NSOM). In agreement with previous findings, we here confirm clustering of IL2Ralpha and IL15Ralpha at the submicron scale. In addition to clustering, our single-molecule data reveal that a non-negligible percentage of the receptors are organized as monomers. Only a minor fraction of IL2Ralpha molecules reside outside the clustered domains, whereas approximately 30% of IL15Ralpha molecules organize as monomers or small clusters, excluded from the main domain regions. Interestingly, we also found that the packing densities per unit area of both IL2Ralpha and IL15Ralpha domains remained constant, suggesting a 'building block' type of assembly involving repeated structures and composition. Finally, dual-color NSOM demonstrated co-clustering of the two alpha-chains. Our results should aid understanding the action of the IL2R-IL15R system in T cell function and also might contribute to the more rationale design of IL2R- or IL15R-targeted immunotherapy agents for treating human leukemia.

Vertebrate membrane proteins: structure, function, and insights from biophysical approaches

Membrane proteins are key targets for pharmacological intervention because they are vital for cellular function. Here, we analyze recent progress made in the understanding of the structure and function of membrane proteins with a focus on rhodopsin and development of atomic force microscopy techniques to study biological membranes. Membrane proteins are compartmentalized to carry out extra- and intracellular processes. Biological membranes are densely populated with membrane proteins that occupy approximately 50% of their volume. In most cases membranes contain lipid rafts, protein patches, or paracrystalline formations that lack the higher-order symmetry that would allow them to be characterized by diffraction methods. Despite many technical difficulties, several crystal structures of membrane proteins that illustrate their internal structural organization have been determined. Moreover, high-resolution atomic force microscopy, near-field scanning optical microscopy, and other lower resolution techniques have been used to investigate these structures. Single-molecule force spectroscopy tracks interactions that stabilize membrane proteins and those that switch their functional state; this spectroscopy can be applied to locate a ligand-binding site. Recent development of this technique also reveals the energy landscape of a membrane protein, defining its folding, reaction pathways, and kinetics. Future development and application of novel approaches during the coming years should provide even greater insights to the understanding of biological membrane organization and function.

A biophysical approach to IL-2 and IL-15 receptor function: localization, conformation and interactions

Interleukin-2 and interleukin-15 (IL-2, IL-15) are key participants in T and NK cell activation and function. Sharing the beta and gamma receptor subunits results in several common functions: e.g. the promotion of T cell proliferation. On the other hand, due to their distinct alpha receptor subunits, they also play opposing roles in immune processes such as activation induced cell death and immunological memory. Divergence of signaling pathways must ensue already at the plasma membrane where the cytokines interact with their receptors. Therefore understanding molecular details of receptor organization and mapping interactions with other membrane proteins that might influence receptor conformation and function, are of key importance. Biophysical/advanced microscopic methods (fluorescence resonance energy transfer (FRET), fluorescence crosscorrelation spectroscopy (FCCS), near-field scanning optical microscopy (NSOM), X-ray crystallography, surface plasmon resonance, NMR spectroscopy) have been instrumental in clarifying the details of receptor structure and organization from the atomic level to the assembly and dynamics of supramolecular clusters. In this short review some important contributions shaping our current view of IL-2 and IL-15 receptors are presented.

ICAM-1 inhibits the homocluster formation of MHC-I in colon carcinoma cells

ICAM-1 and MHC-I proteins play fundamental roles in antigen presentation, activation of T lymphocytes, and immune responses against tumor cells. Both of them participate in the formation of lipid raft-associated membrane protein clusters. We found significant colocalization between ICAM-1 and MHC-I at the level of large-scale associations. We combined RNA interference and fluorescence resonance energy transfer studies to show that ICAM-1 promotes the partial disassembly of MHC-I homoclusters on LS-174T colon carcinoma cells. Interferon-gamma (IFN-gamma) treatment induced an increase in the expression of MHC-I and ICAM-1 resulting in decreased MHC-I homoassociation. Small interfering RNAs directed against ICAM-1 restored the homoassociation of MHC-I without influencing the expression level of MHC-I by eliminating ICAM-1 molecules interspersed in MHC-I clusters. We conclude that the composition of membrane protein clusters is dynamically altered in response to both physiological and experimentally elicited changes in antigen expression levels.

The role of supramolecular protein complexes and membrane potential in transmembrane signaling processes of lymphocytes

The formation of protein patterns in lymphocyte plasma membranes is analyzed in the light of past and, also, very recent experiments. The analysis surveys the lateral organization of major histocompatibility complex glycoproteins, intercellular adhesion molecule-1, interleukin-2 and -15 receptors, Kv1.3 K+ ion channels and the T-cell receptor as well as their behavior under different conditions. These molecules form small- and large-scale clusters in the membrane of human lymphocytes. Many of the association motifs occur in other investigated cell types. The conclusions point toward a possible role for ion channel activities, membrane potential changes and alterations of the lateral organization of proteins in transmembrane signaling and cytotoxic interactions. In our outlook new factors that potentially affect membrane protein cluster formation and interactions are discussed. A role for MHC glycoproteins in concentrating membrane proteins and organizing protein patterns is suggested, and the possibility that the membrane potential may modulate protein conformation and, thereby, affect protein-protein interactions is pointed out. A well-defined role for the presence of ion channels in the immune synapse is offered, which could explain the significance of ion channel accumulation in the immune synapse together with the T-cell receptor.

Cholesterol and sphingolipids as lipid organizers of the immune cells’ plasma membrane: Their impact on the functions of MHC molecules, effector T-lymphocytes and T-cell death

The possible regulatory mechanisms by which glycosphingolipid- and cholesterol-rich membrane microdomains, caveolar and non-caveolar lipid rafts, control the immune response are continuously expanding. In the present overview we will focus on how these membrane-organizing lipids are involved, in collaboration with tetraspanin proteins, in the formation of distinct MHC-I and MHC-II microdomains at the cell surface and will analyze the possible roles of MHC compartmentation in the processes of antigen presentation and regulation of various stages of the cellular immune response. Some basic, lipid raft- and tetraspan mediated mechanisms involved in the formation and function of immunological synapses between various APCs and T-cells will also be discussed. Finally, a new aspect of immune regulation by sphingolipids will be briefly described, namely how can the death or stress signals, leading to ceramide accumulation, result in raft-associated regulatory platforms controlling cell death or antigen-induced, TCRmediated signaling of T-lymphocytes. The influence of these signals and their cross-talk on the fate (death or survival) of T-cells and the outcome of T-cell response will also be discussed.

Detection of channel proximity by nanoparticle-assisted delaying of toxin binding; a combined patch-clamp and flow cytometric energy transfer study

Gold nanoparticles of 30 nm diameter bound to cell-surface receptor major histocompatibility complex glycoproteins (MHCI and MHCII), interleukin-2 receptor alpha subunit (IL-2Ralpha), very late antigen-4 (VLA-4) integrin, transferrin receptor, and the receptor-type protein tyrosin phosphatase CD45 are shown by the patch-clamp technique to selectively modulate binding characteristics of Pi(2) toxin, an efficient blocker of K(v)1.3 channels. After correlating the electrophysiological data with those on the underlying receptor clusters obtained by simultaneously conducted flow cytometric energy transfer measurements, the modulation was proved to be sensitive to the density and size of the receptor clusters, and to the locations of the receptors as well. Based on the observation that engagement of MHCII by a monoclonal antibody down-regulates channel current and based on the close nanometer-scale proximity of the MHCI and MHCII glycoproteins, an analogous experiment was carried out when gold nanoparticles bound to MHCI delayed down-regulation of the K(v)1.3 current initiated by ligation of MHCII. Localization of K(v)1.3 channels in the nanometer-scale vicinity of the MHC-containing lipid rafts is demonstrated for the first time. A method is proposed for detecting receptor-channel or receptor-receptor proximity by observing nanoparticle-induced increase in relaxation times following concentration jumps of ligands binding to channels or to receptors capable of regulating channel currents.

Near-field scanning optical microscopy in liquid for high resolution single molecule detection on dendritic cells

Clustering of cell surface receptors into micro-domains in the plasma membrane is an important mechanism for regulating cellular functions. Unfortunately, these domains are often too small to be resolved with conventional optical microscopy. Near-field scanning optical microscopy (NSOM) is a relatively new technique that combines ultra high optical resolution, down to 70 nm, with single molecule detection sensitivity. As such, the technique holds great potential for direct visualisation of domains at the cell surface. Yet, NSOM operation under liquid conditions is far from trivial. In this contribution, we show that the performance of NSOM can be extended to measurements in liquid environments using a diving bell concept. For the first time, individual fluorescent molecules on the membrane of cells in solution are imaged with a spatial resolution of 90 nm. Furthermore, using this technique we have been able to directly visualise nanometric sized domains of the C-type lectin DC-SIGN on the membrane of dendritic cells, both in air and in liquid.

Rafting MHC-II domains in the APC (presynaptic) plasma membrane and the thresholds for T-cell activation and immunological synapse formation

Glycosphingolipid- and cholesterol-rich membrane microdomains (rafts) in T-cells are important in triggering and regulation of T(H)-cell activation in immunological synapses (IS), which in turn may control the T-cell repertoire in lymph nodes and at the periphery. It is less known, however, how the "presynaptic side" controls formation and function of IS. We investigated here activation signals and synapse formation frequency of murine IP12-7 T(H) hybridoma cell specific to influenza virus HA-peptide upon stimulation with two B-lymphoma cells, A20 and 2PK3, pulsed with peptide antigen. Confocal microscopic colocalization and FRET data consonantly revealed clustered distribution and constitutive raft-association of a major fraction of MHC-II molecules in both APCs. Costimulatory molecules (CD80 and CD86), not associated constitutively with rafts, were expressed at much lower level in A20 cells. T-cells responded to 2PK3 APC with much higher signal strength than to A20 cells, in good correlation with the frequency of IS formation, as assessed by microscopic conjugation assay. Disruption of rafts by cholesterol depletion in 2PK3 cells largely decreased the magnitude of T(H) cell activation signals, especially at low peptide antigen doses, similarly to masking CD4 with mAb on T-cells. The frequency of IS formation was reduced by blocking LFA-1 on T-cells and CD80 on APCs, by lowering the temperature below the phase transition of the membrane or by disrupting actin cytoskeleton. These data together suggest that the surface density and affinity/stability of peptide-MHC-II complexes and the costimulatory level are primary determinants for an efficient TCR recognition and the strength of the subsequent T-cell signals, as well as of the IS formation, which additionally requires a cytoskeleton-dependent remodeling of APC surface after the initial TCR signal. The threshold of T-cell activation can be further set by rafting MHC-II domains via concentrating high affinity ligands and promoting thereby T-cells for sensing low density antigen. Our data also demonstrate that B-cells, similarly to dendritic cells, could also provide T-cells with antigen-independent weak survival signals, likely associated with integrin engagement.

Dynamic, yet structured: The cell membrane three decades after the Singer-Nicolson model

The fluid mosaic membrane model proved to be a very useful hypothesis in explaining many, but certainly not all, phenomena taking place in biological membranes. New experimental data show that the compartmentalization of membrane components can be as important for effective signal transduction as is the fluidity of the membrane. In this work, we pay tribute to the Singer-Nicolson model, which is near its 30th anniversary, honoring its basic features, "mosaicism" and "diffusion," which predict the interspersion of proteins and lipids and their ability to undergo dynamic rearrangement via Brownian motion. At the same time, modifications based on quantitative data are proposed, highlighting the often genetically predestined, yet flexible, multilevel structure implementing a vast complexity of cellular functions. This new "dynamically structured mosaic model" bears the following characteristics: emphasis is shifted from fluidity to mosaicism, which, in our interpretation, means nonrandom codistribution patterns of specific kinds of membrane proteins forming small-scale clusters at the molecular level and large-scale clusters (groups of clusters, islands) at the submicrometer level. The cohesive forces, which maintain these assemblies as principal elements of the membranes, originate from within a microdomain structure, where lipid-lipid, protein-protein, and protein-lipid interactions, as well as sub- and supramembrane (cytoskeletal, extracellular matrix, other cell) effectors, many of them genetically predestined, play equally important roles. The concept of fluidity in the original model now is interpreted as permissiveness of the architecture to continuous, dynamic restructuring of the molecular- and higher-level clusters according to the needs of the cell and as evoked by the environment.

New trends in studying structure and function of biological membranes

Thirty years ago Singer and Nicolson constructed the "fluid mosaic model" of the membrane, which described the structural and functional characteristics of the plasma membrane of non-polarized cells like circulating blood lymphocytes as a fluid lipid phase accommodating proteins with a relatively free mobility. It is a rare phenomenon in biology that such a model could survive 30 years and even today it has a high degree of validity. However, in the light of new data it demands some modifications. In this minireview we present a new concept, which revives the SN model, by shifting the emphasis from fluidity to mosaicism, i.e. to lipid microdomains and rafts. A concise summary of data and key methods is given, proving the existence of non-random co-distribution patterns of different receptor kinds in the microdomain system of the plasma membrane. Furthermore we present evidence that proteins are not only accommodated by the lipid phase, but they are integral structural elements of it. Novel suggestions to the SN model help to develop a modernized version of the old paradigm in the light of new data.

Colocalization and nonrandom distribution of Kv1.3 potassium channels and CD3 molecules in the plasma membrane of human T lymphocytes

Distribution and lateral organization of Kv1.3 potassium channels and CD3 molecules were studied by using electron microscopy, confocal laser scanning microscopy, and fluorescence resonance energy transfer. Immunogold labeling and electron microscopy showed that the distribution of FLAG epitope-tagged Kv1.3 channels (Kv1.3/FLAG) significantly differs from the stochastic Poisson distribution in the plasma membrane of human T lymphoma cells. Confocal laser scanning microscopy images showed that Kv1.3/FLAG channels and CD3 molecules accumulated in largely overlapping membrane areas. The numerical analysis of crosscorrelation of the spatial intensity distributions yielded a high correlation coefficient (C = 0.64). A different hierarchical level of molecular proximity between Kv1.3/FLAG and CD3 proteins was reported by a high fluorescence resonance energy transfer efficiency (E = 51%). These findings implicate that reciprocal regulation of ion-channel activity, membrane potential, and the function of receptor complexes may contribute to the proper functioning of the immunological synapse.

Does mosaicism of the plasma membrane at molecular and higher hierarchical levels in human lymphocytes carry information on the immediate history of cells?

A theoretical analysis of experimental data is presented in this mini-review on non-random homo- and hetero-associations of cell surface receptors, which can be recruited in the plasma membrane or at the surface of the rough endoplasmic reticulum during the protein synthesis. In the latter case, the likely genetic origin of these supramolecular formations is analyzed, contrasting this concept to the mobility of the cell surface proteins. A model is offered which, on the one hand, allows the mobility in a restricted way even among microdomain-confined receptor proteins through 'swapping partners'. On the other hand, the lack of mixing molecular components of protein clusters will be analyzed, when homo-and hetero-associations are studied through cell fusion experiments. The most frequently studied cell surface patterns have included lipid raft organized HLA class I and II, ICAM-1, tetraspan molecules, IL2 and IL15 and other receptors, as well. On the contrary coated pit-associated transferrin receptors would not mix with the above lipid raft associated receptor patterns, although transferrin receptor would readily oligomerize into homo-associates. The functional consequences of these superstructures are also analyzed. On the 30th anniversary of the Singer-Nicolson fluid mosaic membrane model one has to pay tribute to the authors, because of their deep insight emphasizing also the mosaicism of the membranes in general and that of the plasma membrane, in particular.

Endocytosis of lipid rafts: an identity crisis

Membrane rafts enriched in cholesterol and sphingolipids have been hypothesized to be key mediators of sorting and signaling functions of associated molecules. Apart from a limited number of biophysical studies in living cell membranes, raft-association has been defined by a simple biochemical criterion, namely the ability to partition with detergent-resistant membranes (DRMs). Here we examine the evidence for the specification of internalization mechanisms and endocytic pathways by rafts as defined by this criterion. We have surveyed the endocytic trafficking of a variety of molecules such as lipids, toxins, glycosylphosphatidylinositol (GPI)-anchored proteins, and DRM-associated transmembrane proteins.