Machaerium acutifolium lectin alters membrane structure and induces ROS production in Candida parapsilosis

Lectins are a group of widely distributed and structurally heterogeneous proteins of nonimmune origin. These proteins have the ability to interact with glycans present on cell surfaces and elicit diverse biological activities. Machaerium acutifolium lectin (MaL) is an N-acetyl-D-glucosamine-binding lectin that exhibits antinociceptive activity via transient receptor potential cation channel subfamily V member 1 (TRPV1). Lectins that have the ability to recognize and interact with N-acetyl-D-glucosamine residues are potential candidates for studies of fungicidal activity. In this work, we show that MaL has antifungal activity against Candida species, and we describe its mode of action towards Candida parapsilosis. MaL inhibited the growth of C. albicans and C. parapsilosis. However, MaL was more potent against C. parapsilosis. The candidacidal mode of action of MaL on C. parapsilosis involves enhanced cell permeabilization, alteration of the plasma membrane proton-pumping ATPase function (H+-ATPase), induction of oxidative stress, and DNA damage. MaL also exhibited antibiofilm activity and noncytotoxicity to Vero cells. These results indicate that MaL is a promising candidate for the future development of a new, natural, and safe drug for the treatment of infections caused by C. parapsilosis.

Peripheral Protein Unfolding Drives Membrane Bending

Dynamic modulation of lipid membrane curvature can be achieved by a number of peripheral protein binding mechanisms such as hydrophobic insertion of amphipathic helices and membrane scaffolding. Recently, an alternative mechanism was proposed in which crowding of peripherally bound proteins induces membrane curvature through steric pressure generated by lateral collisions. This effect was enhanced using intrinsically disordered proteins that possess high hydrodynamic radii, prompting us to explore whether membrane bending can be triggered by the folding-unfolding transition of surface-bound proteins. We utilized histidine-tagged human serum albumin bound to Ni-NTA-DGS containing liposomes as our model system to test this hypothesis. We found that reduction of the disulfide bonds in the protein resulted in unfolding of HSA, which subsequently led to membrane tubule formation. The frequency of tubule formation was found to be significantly higher when the proteins were unfolded while being localized to a phase-separated domain as opposed to randomly distributed in fluid phase liposomes, indicating that the steric pressure generated from protein unfolding can drive membrane deformation. Our results are critical for the design of peripheral membrane protein-immobilization strategies and open new avenues for exploring mechanisms of membrane bending driven by conformational changes of peripheral membrane proteins.

USE OF VIBRIO CHOLERAE SURFACE STRUCTURES FOR SPECIFIC PROPHY- LAXIS AND DIAGNOSTICS OF CHOLERA

The need for efficient and cost-effective cholera vaccine hasn't lost its actuality in view of the emergence of new strains leading to severe clinical forms of cholera and capable to replace strains of the seventh.cholera pandemic, and in connection with the threat of cholera spreading beyond the borders of endemic countries. In this review data from literature sources are presented about the use of outer membrane proteins, vesicles, cell ghosts of the cholera causative agent in specific prophylaxis and diagnostics of the disease.

The neuronal porosome complex in health and disease

Cup-shaped secretory portals at the cell plasma membrane called porosomes mediate the precision release of intravesicular material from cells. Membrane-bound secretory vesicles transiently dock and fuse at the base of porosomes facing the cytosol to expel pressurized intravesicular contents from the cell during secretion. The structure, isolation, composition, and functional reconstitution of the neuronal porosome complex have greatly progressed, providing a molecular understanding of its function in health and disease. Neuronal porosomes are 15 nm cup-shaped lipoprotein structures composed of nearly 40 proteins, compared to the 120 nm nuclear pore complex composed of >500 protein molecules. Membrane proteins compose the porosome complex, making it practically impossible to solve its atomic structure. However, atomic force microscopy and small-angle X-ray solution scattering studies have provided three-dimensional structural details of the native neuronal porosome at sub-nanometer resolution, providing insights into the molecular mechanism of its function. The participation of several porosome proteins previously implicated in neurotransmission and neurological disorders, further attest to the crosstalk between porosome proteins and their coordinated involvement in release of neurotransmitter at the synapse.

Surface Density-Induced Pleating of a Lipid Monolayer Drives Nascent High-Density Lipoprotein Assembly

Biogenesis of high-density lipoproteins (HDL) is coupled to the transmembrane protein, ATP-binding cassette transporter A1 (ABCA1), which transports phospholipid (PL) from the inner to the outer membrane monolayer. Using a combination of computational and experimental approaches, we show that increased outer lipid monolayer surface density, driven by excess PL or membrane insertion of amphipathic helices, results in pleating of the outer monolayer to form membrane-attached discoidal bilayers. Apolipoprotein (apo)A-I accelerates and stabilizes the pleats. In the absence of apoA-I, pleats collapse to form vesicles. These results mimic cells overexpressing ABCA1 that, in the absence of apoA-I, form and release vesicles. We conclude that the basic driving force for nascent discoidal HDL assembly is a PL pump-induced surface density increase that produces lipid monolayer pleating. We then argue that ABCA1 forms an extracellular reservoir containing an isolated pressurized lipid monolayer decoupled from the transbilayer density buffering of cholesterol.

A receptor-interacting protein 1 (RIP1)-independent necrotic death under the control of protein phosphatase PP2A that involves the reorganization of actin cytoskeleton and the action of cofilin-1

Cell death by necrosis is emerging not merely as a passive phenomenon but as a cell-regulated process. Here, by using different necrotic triggers, we prove the existence of two distinct necrotic pathways. The mitochondrial reactive oxygen species generator 2,3-dimethoxy-1,4-naphthoquinone elicits necrosis characterized by the involvement of RIP1 and Drp1. However, G5, a non-selective isopeptidase inhibitor, triggers a distinct necrotic pathway that depends on the protein phosphatase PP2A and the actin cytoskeleton. PP2A catalytic subunit is stabilized by G5 treatment, and its activity is increased. Furthermore, PP2Ac accumulates into the cytoplasm during necrosis similarly to HMGB1. We have also defined in the actin-binding protein cofilin-1 a link between PP2A, actin cytoskeleton, and necrotic death. Cofilin-1-severing/depolymerization activity is negatively regulated by phosphorylation of serine 3. PP2A contributes to the dephosphorylation of serine 3 elicited by G5. Finally, a cofilin mutant that mimics phosphorylated Ser-3 can partially rescue necrosis in response to G5.

Comparison of the physical characteristics of chlorosomes from three different phyla of green phototrophic bacteria

Chlorosomes, the major antenna complexes in green sulphur bacteria, filamentous anoxygenic phototrophs, and phototrophic acidobacteria, are attached to the cytoplasmic side of the inner cell membrane and contain thousands of bacteriochlorophyll (BChl) molecules that harvest light and channel the energy to membrane-bound reaction centres. Chlorosomes from phototrophs representing three different phyla, Chloroflexus (Cfx.) aurantiacus, Chlorobaculum (Cba.) tepidum and the newly discovered "Candidatus (Ca.) Chloracidobacterium (Cab.) thermophilum" were analysed using PeakForce Tapping atomic force microscopy (PFT-AFM). Gentle PFT-AFM imaging in buffered solutions that maintained the chlorosomes in a near-native state revealed ellipsoids of variable size, with surface bumps and undulations that differ between individual chlorosomes. Cba. tepidum chlorosomes were the largest (133×57×36nm; 141,000nm(3) volume), compared with chlorosomes from Cfx. aurantiacus (120×44×30nm; 84,000nm(3)) and Ca. Cab. thermophilum (99×40×31nm; 65,000nm(3)). Reflecting the contributions of thousands of pigment-pigment stacking interactions to the stability of these supramolecular assemblies, analysis by nanomechanical mapping shows that chlorosomes are highly stable and that their integrity is disrupted only by very strong forces of 1000-2000pN. AFM topographs of Ca. Cab. thermophilum chlorosomes that had retained their attachment to the cytoplasmic membrane showed that this membrane dynamically changes shape and is composed of protrusions of up to 30nm wide and 6nm above the mica support, possibly representing different protein domains. Spectral imaging revealed significant heterogeneity in the fluorescence emission of individual chlorosomes, likely reflecting the variations in BChl c homolog composition and internal arrangements of the stacked BChls within each chlorosome.

Effects of low doses of gamma rays on the stability of normal and diabetic erythrocytes

We studied the influence of low doses of γ radiation (from 0.04 to 1.8 mGy) on the stability of human red blood cells (RBC) from healthy donors and diabetic patients using absorption spectroscopy. Because of the alteration of many enzymatic pathways in diabetic RBCs resulting in strong modification of the lipid and protein membrane components one could expect that the ionizing γ-radiation should influence the stability of the healthy and diabetic cells in a different way. Indeed, distinct discontinuities and monotonic changes of hemolysis detected in the healthy and diabetic RBCs suggest that various enzymatic and chemical processes are activated in these membranes by γ radiation. Mössbauer measurements showed that only the highest applied dose of γ radiation caused modification of hemoglobin in both types of RBCs.

The phospholipid monolayer associated with perilipin-enriched lipid droplets is a highly organized rigid membrane structure

The significance of lipid droplets (LD) in lipid metabolism, cell signaling, and membrane trafficking is increasingly recognized, yet the role of the LD phospholipid monolayer in LD protein targeting and function remains unknown. To begin to address this issue, two populations of LD were isolated by ConA sepharose affinity chromatography: 1) functionally active LD enriched in perilipin, caveolin-1, and several lipolytic proteins, including ATGL and HSL; and 2) LD enriched in ADRP and TIP47 that contained little to no lipase activity. Coimmunoprecipitation experiments confirmed the close association of caveolin and perilipin and lack of interaction between caveolin and ADRP, in keeping with the separation observed with the ConA procedure. The phospholipid monolayer structure was evaluated to reveal that the perilipin-enriched LD exhibited increased rigidity (less fluidity), as shown by increased cholesterol/phospholipid, Sat/Unsat, and Sat/MUFA ratios. These results were confirmed by DPH-TMA, NBD-cholesterol, and NBD-sphingomyelin fluorescence polarization studies. By structure and organization, the perilipin-enriched LD most closely resembled the adipocyte PM. In contrast, the ADRP/TIP47-enriched LD contained a more fluid monolayer membrane, reflecting decreased polarizations and lipid order based on phospholipid fatty acid analysis. Taken together, results indicate that perilipin and associated lipolytic enzymes target areas in the phospholipid monolayer that are highly organized and rigid, similar in structure to localized areas of the PM where cholesterol and fatty acid uptake and efflux occur.

Membrane nanotubes induced by aqueous phase separation and stabilized by spontaneous curvature

Tubular membrane structures are widespread in eukaryotic cells, but the mechanisms underlying their formation and stability are not well understood. Previous work has focused on tube extrusion from cells and model membranes under the application of external forces. Here, we present novel membrane/polymer systems, where stable tubes form in the absence of externally applied forces. Solutions of two water-soluble polymers, polyethylene glycol and dextran, were encapsulated in giant lipid vesicles, cell-size model systems. Hypertonic deflation induced phase separation of the enclosed solution. The excess membrane area created during the deflation process was stored in a large number of membrane nanotubes inside the vesicle. The tubes had a diameter below optical resolution and became visible only when fluorescently labeled. The tubes were rather stable: In the absence of external forces, they existed for several days. A theoretical analysis of the shapes of the deflated vesicles reveals that these shapes would be unstable if the membranes had no spontaneous curvature. Using the large separation of length scales between the tube diameter and the overall size of the vesicles, the spontaneous curvature can be calculated and is found to be about -1/(240 nm) for a certain range of polymer concentrations. The nanotubes could also be retracted back into the mother vesicle by increasing the membrane tension via micropipette aspiration of the vesicle. Membrane tubes, which can form and be retracted easily, should be relevant for lipid storage in cells.

Global proteomic profiling of native outer membrane vesicles derived from Escherichia coli

Gram-negative bacteria constitutively secrete native outer membrane vesicles (OMVs) into the extracellular milieu. Although recent progress in this area has revealed that OMVs are essential for bacterial survival and pathogenesis, the mechanism of vesicle formation and the biological roles of OMVs have not been clearly defined. Using a proteomics approach, we identified 141 protein components of Escherichia coli-derived native OMVs with high confidence; two separate analyses yielded identifications of 104 and 117 proteins, respectively, with 80 proteins overlapping between the two trials. In the group of identified proteins, the outer membrane proteins were highly enriched, whereas inner membrane proteins were lacking, suggesting that a specific sorting mechanism for vesicular proteins exists. We also identified proteins involved in vesicle formation, the removal of toxic compounds and attacking phage, and the elimination of competing organisms, as well as those involved in facilitating the transfer of genetic material and protein to other bacteria, targeting host cells, and modulating host immune responses. This study provides a global view of native bacterial OMVs. This information will help us not only to elucidate the biogenesis and functions of OMV from nonpathogenic and pathogenic bacteria but also to develop vaccines and antibiotics effective against pathogenic strains.

Stimulation of actin polymerization by vacuoles via Cdc42p-dependent signaling

We have previously shown that actin ligands inhibit the fusion of yeast vacuoles in vitro, which suggests that actin remodeling is a subreaction of membrane fusion. Here, we demonstrate the presence of vacuole-associated actin polymerization activity, and its dependence on Cdc42p and Vrp1p. Using a sensitive in vitro pyrene-actin polymerization assay, we found that vacuole membranes stimulated polymerization, and this activity increased when vacuoles were preincubated under conditions that support membrane fusion. Vacuoles purified from a VRP1-gene deletion strain showed reduced polymerization activity, which could be recovered when reconstituted with excess Vrp1p. Cdc42p regulates this activity because overexpression of dominant-negative Cdc42p significantly reduced vacuole-associated polymerization activity, while dominant-active Cdc42p increased activity. We also used size-exclusion chromatography to directly examine changes in yeast actin induced by vacuole fusion. This assay confirmed that actin undergoes polymerization in a process requiring ATP. To further confirm the need for actin polymerization during vacuole fusion, an actin polymerization-deficient mutant strain was examined. This strain showed in vivo defects in vacuole fusion, and actin purified from this strain inhibited in vitro vacuole fusion. Affinity isolation of vacuole-associated actin and in vitro binding assays revealed a polymerization-dependent interaction between actin and the SNARE Ykt6p. Our results suggest that actin polymerization is a subreaction of vacuole membrane fusion governed by Cdc42p signal transduction.

Biological Soft Materials

With one or two exceptions, biological materials are "soft", meaning that they combine viscous and elastic elements. This mechanical behavior results from self-assembled supramolecular structures that are stabilized by noncovalent interactions. It is an ongoing and profound challenge to understand the self-organization of biological materials. In many cases, concepts can be imported from soft-matter physics and chemistry, which have traditionally focused on materials such as colloids, polymers, surfactants, and liquid crystals. Using these ideas, it is possible to gain a new perspective on phenomena as diverse as DNA condensation, protein and peptide fibrillization, lipid partitioning in rafts, vesicle fusion and budding, and others, as discussed in this selective review of recent highlights from the literature.

Cell surface lectin-binding glycoconjugates on marine planktonic protists

Carbohydrate-protein interactions appear to play an important role in the phagocytosis of microbial prey by free-living protozoa. The present study utilizes FITC-labelled plant lectins to investigate the presence and localization of cell surface glycoconjugates on live and fixed planktonic protists (Dunaliella primolecta, Oxyrrhis marina, Goniomonas amphinema, Paraphysomonas vestita and Euplotes vannus). With live flagellate preparations, lectins primarily bound to external cell surfaces, with minimal internal staining observed. In contrast, cell fixation permeabilized cell membranes, allowing lectins to bind to internal structures, such as nuclear membranes and food vacuoles, interfering with the characterization of cell surface glycoconjugates. The method developed to label cell surface sugar moieties of live planktonic protists successfully overcomes the problems associated with fixation, and thus provides a useful protocol for future studies on protistan cell surface carbohydrate characterization.

Characterization of outer membrane vesicles released by the psychrotolerant bacterium Pseudoalteromonas antarctica NF3

Pseudoalteromonas antarctica NF3 is an Antarctic psychrotolerant Gram-negative bacterium that accumulates large amounts of an extracellular polymeric substance (EPS) with high protein content. Transmission electron microscopy analysis after high-pressure freezing and freeze substitution (HPF-FS) shows that the EPS is composed of a capsular polymer and large numbers of outer membrane vesicles (OMVs). These vesicles are bilayered structures and predominantly spherical in shape, with an average diameter of 25-70 nm, which is similar to what has been observed in OMVs from other Gram-negative bacteria. Analyses of lipopolysaccharide (LPS), phospholipids and protein profiles of OMVs are consistent with the bacterial outer membrane origin of these vesicles. In an initial attempt to elucidate the functions of OMVs proteins, we conducted a proteomic analysis on 1D SDS-PAGE bands. Those proteins putatively identified match with outer membrane proteins and proteins related to nutrient processing and transport in Gram-negative bacteria. This approach suggests that OMVs present in the EPS from P. antarctica NF3, might function to deliver proteins to the external media, and therefore play an important role in the survival of the bacterium in the extreme Antarctic environment.

Herpes simplex virus-type 2 infectivity and agents that block gap junctional intercellular communication

In rat liver epithelial (WB) cells, the protein kinase C inhibitor H7 blocked gap junctional intercellular communication (GJIC) and reduced virus infectivity. Octanol, 18-beta-glycyrrhetinic acid, and staurosporine, agents that reduce GJIC, had no effect upon virus infectivity. Previous studies demonstrated that herpes simplex virus-type 2 (HSV-2) infection was accompanied by attenuated GJIC. Of agents tested, only H7 reduced plaque forming unit (pfu) ability in a dose-dependent manner with 100% plaque reduction at 40 microM without evidence of cytotoxicity. Dye transfer indicated that H7 decreased GJIC, although Western blotting revealed that it did not alter phosphorylation of the gap junction protein, connexin 43 (Cx43). Using indirect immunofluorescence, Cx43 was found to localize in membrane plaques in uninfected cells and H7 did not alter this distribution. However, Cx43 was lost from the membrane at 24h in both H7-treated and untreated cells infected with HSV-2. Viral infection increased serine phosphorylation, particularly in the nuclear region, and this effect was reduced following H7 treatment. Thus, H7 attenuated both GJIC and infectivity of HSV-2 in WB cells but the anti-viral effects were due to reduced nuclear protein phosphorylation rather than alterations in phosphorylation or localization of Cx43.

A novel role for phagocytosis-like uptake in herpes simplex virus entry

It is becoming increasingly clear that herpesviruses can exploit the endocytic pathway to infect cells, yet several important features of this process remain poorly defined. Using herpes simplex virus-1 (HSV-1) as a model, we demonstrate that endocytosis of the virions mimic many features of phagocytosis. During entry, HSV-1 virions associated with plasma membrane protrusions followed by a phagocytosis-like uptake involving rearrangement of actin cytoskeleton and trafficking of the virions in large phagosome-like vesicles. RhoA GTPase was activated during this process and the mode of entry was cell type-specific. Clathrin-coated vesicles had no detectable role in virion trafficking as Eps15 dominant-negative mutants failed to affect HSV-1 uptake. Binding and fusion of the virion envelope with the phagosomal membrane is likely facilitated by clustering of nectin-1 (or HVEM) in phagosomes, which was observed in infected cells. Collectively, our data suggests a novel mode of uptake by which the virus can infect both professional and nonprofessional phagocytes.

Circulating leukocyte-derived microparticles predict subclinical atherosclerosis burden in asymptomatic subjects

OBJECTIVE

To clarify circulating microparticles (MP) relationships with preclinical atherosclerosis.

METHODS AND RESULTS

In 216 subjects without cardiovascular disease, we assessed: (1) annexin V-positive, platelet-derived, endothelium-derived and leukocyte-derived circulating MP by capture on annexin V, anti-GPIb, anti-CD105, and anti-CD11a antibody-coated wells, respectively; (2) Framingham risk, metabolic syndrome, and low-grade inflammation by risk factors measurement including hsCRP; and (3) subclinical atherosclerosis by ultrasound examination of carotid, abdominal aorta, and femoral arteries. Number of sites with plaque ranged from 0 to 3 and plaque burden was classified into 0 to 1 or 2 to 3 sites disease. Leukocyte-derived MP level was higher in the presence than in the absence of moderate to high Framingham risk (P<0.05), metabolic syndrome (P<0.01), high C-reactive protein (CRP) (P<0.05), or 2- to 3-sites disease (P<0.01), and correlated positively with number of metabolic syndrome components (P<0.001), tertiles of fibrinogen (P<0.001), and number of diseased sites (P<0.01). In multivariate analysis, 2- to 3-sites disease was independently associated with leukocyte-derived MP level (P<0.05), Framingham risk (P<0.001), and metabolic syndrome (P<0.01). None of the other MP types correlated with risk markers or atherosclerosis.

CONCLUSIONS

Leukocyte-derived MP, identified by affinity for CD11a, are increased in subjects with ultrasound evidence of subclinical atherosclerosis, unveiling new directions for atherosclerosis research.

The exocyst defrocked, a framework of rods revealed

The exocyst complex is required for the interaction of vesicles with the plasma membrane in preparation for exocytic fusion. Recent crystallographic studies indicate that at least four of the eight subunits contain long, rod-like domains formed from helical bundles. These rods may pack against one another to generate the framework of the complex. How this complex assembles, how it responds to various GTPases and how it is ultimately displaced to allow bilayer fusion are key questions for the future.

PV-1 labels trans-cellular openings in mouse endothelial cells and is negatively regulated by VEGF

The PV-1 protein is endogenously expressed from a single mRNA in the mouse pancreatic MS-1 endothelial cell line as a 60-kDa N-glycosylated and 50-kDa non-glycosylated protein that form DTT sensitive oligomers. In the absence of cell permeabilization, PV-1 antibodies label transcellular openings of variable size, many that penetrate through the cytosol with circular openings on the free and attached surface of the plasma membrane. Intracellular PV-1 is localized in perinuclear aggregates that can extend as a fibrous network through the cytosol and often surround the nuclear compartment. In some cells, PV-1 is organized as a large unipolar spindle-like structure that is often associated with severe deformation of the nucleus. The VEGF-R2 inhibitor SU5614 increased the PV-1 protein levels in a dose-dependent manner and inhibited MS-1 cell growth, without inducing apoptosis. This report provides compelling evidence for a functional role of PV-1 in the formation of large transendothelial channels and modulation of nuclear shape. Moreover, these data suggest the PV-1 protein is negatively regulated by VEGF.

Magnetosomes Are Cell Membrane Invaginations Organized by the Actin-Like Protein MamK

Magnetosomes are membranous bacterial organelles sharing many features of eukaryotic organelles. Using electron cryotomography, we found that magnetosomes are invaginations of the cell membrane flanked by a network of cytoskeletal filaments. The filaments appeared to be composed of MamK, a homolog of the bacterial actin-like protein MreB, which formed filaments in vivo. In a mamK deletion strain, the magnetosome-associated cytoskeleton was absent and individual magnetosomes were no longer organized into chains. Thus, it seems that prokaryotes can use cytoskeletal filaments to position organelles within the cell.

Functional analysis of detergent-solubilized and membrane-reconstituted ATP-binding cassette transporters

ATP-binding cassette (ABC) transporters are vital to any living system and are involved in the translocation of a wide variety of substances, from ions and nutrients to high molecular weight proteins. This chapter describes methods used to purify and membrane reconstitute ABC transporters in a fully functional state. The procedures are largely based on our experience with substrate-binding protein-dependent ABC uptake systems from bacteria, but the approaches should be applicable to multisubunit membrane complexes in general. Also, we present simple methods, based on substrate binding or translocation, to follow the activity of the protein complexes in detergent-solubilized and/or membrane-reconstituted state(s).

Involvement of Cdc42 and Rac small G proteins in invadopodia formation of RPMI7951 cells

BACKGROUND

Invadopodia are membrane protrusions into the extracellular matrix by aggressive tumour cells. These structures are associated with sites of matrix degradation and invasiveness of malignant tumour cells in an in vitro fibronectin degradation/invasion assay. The Rho family small G proteins, consisting of the Rho, Rac and Cdc42 subfamilies, are implicated in various cell functions, such as cell shape change, adhesion, and motility, through reorganization of the actin cytoskeleton. We studied the roles of the Rho family small G proteins in invadopodia formation.

RESULTS

We first demonstrated that invadopodia of RPMI7951 human melanoma cells extended into the matrix substratum on a vertical view using a laser scanning confocal microscope system. We confirmed that invadopodia were rich in actin filaments (F-actin) and visualized clearly with F-actin staining on a vertical view as well as on a horizontal view. We then studied the roles of Rho, Rac, and Cdc42 in invasiveness of the same cell line. In the in vitro fibronectin degradation/invasion assay, a dominant active mutant of Cdc42 enhanced dot-like degradation, whereas a dominant active mutant of Rac enhanced diffuse-type degradation. Furthermore, frabin, a GDP/GTP exchange protein for Cdc42 with F-actin-binding activity, enhanced both dot-like and diffuse-type degradation. However, a dominant active mutant of Rho did not affect the fibronectin degradation. Moreover, inhibition of phosphatidylinositol-3 kinase (PI3K) disrupted the Rac and Cdc42-dependent actin structures and blocked the fibronectin degradation.

CONCLUSION

These results suggest that Cdc42 and Rac play important roles in fibronectin degradation and invasiveness in a coordinate manner through the frabin-Cdc42/Rac-PI3K signalling pathway.

Transmembrane segments of syntaxin line the fusion pore of Ca2+-triggered exocytosis

The fusion pore of regulated exocytosis is a channel that connects and spans the vesicle and plasma membranes. The molecular composition of this important intermediate structure of exocytosis is unknown. Here, we found that mutations of some residues within the transmembrane segment of syntaxin (Syx), a plasma membrane protein essential for exocytosis, altered neurotransmitter flux through fusion pores and altered pore conductance. The residues that influenced fusion-pore flux lay along one face of an alpha-helical model. Thus, the fusion pore is formed at least in part by a circular arrangement of 5 to 8 Syx transmembrane segments in the plasma membrane.

Lipid composition of membrane domains

The isolation of subfractions of cell membranes on the basis of their solubility in non-ionic detergents has led to the discovery of lipid domain structure in membranes. Detergents used for this purpose include Triton, Brij, Lubrol and CHAPS. Different lipid constituents are known to resist solubilization by different detergents and the resulting fractions may associate with different membrane proteins. In general, the detergent-resistant membrane fractions tend to be dominated by saturated molecular species of sphingomyelin and phosphatidylcholine and invariably include significant proportions of cholesterol. The lipid composition is consistent with formation of liquid-ordered phases. The present evidence favours a model in which the lateral segregation of membrane proteins takes place on the basis of their affinity for liquid-ordered lipid domains within the membrane.

Mycomembrane and S-layer: two important structures of Corynebacterium glutamicum cell envelope with promising biotechnology applications

Corynebacteria belong to a distinct Gram-positive group of bacteria including mycobacteria and nocardia, which are characterized by the presence of mycolic acids in their cell wall. These bacteria share the property of having an unusual cell envelope structural organization close to Gram-negative bacteria. In addition to the inner membrane, the cell envelope is constituted of a thick arabinogalactan-peptidoglycan polymer covalently linked to an outer lipid layer, which is mainly composed of mycolic acids and probably organized in an outer membrane like structure. In some species, the cell is covered by a crystalline surface layer composed of a single protein species, which is anchored in the outer membrane like barrier. An increasing number of reports have led to a better understanding of the structure of the cell wall of Corynebacterium glutamicum. These works included the characterization of several cell wall proteins like S-layer protein and porins, genetic and biochemical characterization of mycolic acids biosynthesis, ultrastructural description of the cell envelope, and chemical analysis of its constituents. All these data address new aspects regarding cell wall permeability towards macromolecules and amino acids but also open new opportunities for biotechnology applications.

pH, serum proteins and ionic strength influence the uptake of merocyanine 540 by WiDr cells and its interaction with membrane structures

It has been suggested that selective uptake of photosensitizers is due to significantly lower pH of the interstitial fluid in tumors compared to normal tissue. Therefore, the cellular uptake of merocyanine 540 (MC 540) was examined at two pH values: 6.8+/-0.1 and 7.4+/-0.1. There was no difference in spectral properties (absorption and fluorescence maxima positions, fluorescence intensity) of the drug in the presence of increasing amounts of either human blood plasma or FCS (0-2%) at the two pH values investigated. Nevertheless, significantly higher amounts of the drug were taken up by WiDr cells at pH 6.8+/-0.1, both in the presence of 10% FCS and in the absence of FCS. The absorption spectra of MC 540 in the presence of egg phosphatidylcholine (PC) liposomes turned out to be NaCl concentration-dependent (0.00-0.30 mol l(-1)). Membrane fluidity, as measured by fluorescence anisotropy of diphenylhexatriene (DPH), was unchanged within the experimental error in the NaCl concentration range 0.01-0.30 mol l(-1). The spectral changes indicated an enhancement of the incorporation of MC 540 into lipid membranes with increasing ionic strength. Such a salt concentration dependence suggests a possible involvement of the surface potential in the interaction of MC 540 with lipid membranes. The results might provide an explanation of the pH dependency of the cellular uptake of MC 540 observed in this study.

Elevated numbers of tissue-factor exposing microparticles correlate with components of the metabolic syndrome in uncomplicated type 2 diabetes mellitus

BACKGROUND

Type 2 diabetes is associated with accelerated atherosclerosis. Because cell-derived microparticles support coagulation and inflammation, they may be involved in atherogenesis. We characterized circulating microparticles both in patients with uncomplicated, well-regulated type 2 diabetes and in healthy subjects, as well as their relationship with coagulation and metabolic control.

METHODS AND RESULTS

Microparticles were isolated from plasma, stained with annexin V, cell-specific monoclonal antibodies (MoAbs) and a MoAb directed against tissue factor (TF), and analyzed by flow cytometry. Microparticle numbers and origin were comparable in the two groups, but the median number of TF-positive microparticles was twice as high in patients than in controls (P=0.018). Patients had higher percentages of TF-positive microparticles from T-helper cells (P=0.045), granulocytes (P=0.004), and platelets (P=0.002). Subpopulations of TF-positive microparticles from platelets and T-helper cells exposed granulocytic markers. Correlations were found between the numbers of various TF-positive microparticle subpopulations and body mass index, fasting plasma glucose and insulin, or tumor necrosis factor-alpha and serum HDL cholesterol. Microparticles from patients generated less thrombin in vitro (P=0.007). Microparticle numbers did not correlate with in vivo coagulation markers prothrombin fragment F(1+2) and thrombin-antithrombin complexes.

CONCLUSIONS

TF, possibly of granulocytic origin, is exposed on microparticle subpopulations in asymptomatic patients with well-regulated type 2 diabetes. TF-positive microparticles are associated with components of the metabolic syndrome but not with coagulation. Thus, TF on microparticles may be involved in processes other than coagulation, including transcellular signaling or angiogenesis.

Membrane self-assembly processes: steps toward the first cellular life

This review addresses the question of the origin of life, with emphasis on plausible boundary structures that may have initially provided cellular compartmentation. Some form of compartmentation is a necessary prerequisite for maintaining the integrity of interdependent molecular systems that are associated with metabolism, and for permitting variations required for speciation. The fact that lipid-bilayer membranes define boundaries of all contemporary living cells suggests that protocellular compartments were likely to have required similar, self-assembled boundaries. Amphiphiles such as short-chain fatty acids, which were presumably available on the early Earth, can self-assemble into stable vesicles that encapsulate hydrophilic solutes with catalytic activity. Their suspensions in aqueous media have therefore been used to investigate nutrient uptake across simple membranes and encapsulated catalyzed reactions, both of which would be essential processes in protocellular life forms.

Proteins of peripheral myelin are associated with glycosphingolipid/cholesterol-enriched membranes

A characteristic feature of the vertebrate nervous system is the ensheathment of axons by myelin, a multilamellar membrane specialization produced by polarized glial cells. Although the main protein and lipid components of the myelin sheath are well characterized, relatively little is known about the mechanisms of their intracellular distribution to the respective sites of assembly within the myelin sheath. To analyze whether peripheral myelin protein trafficking is mediated by glycosphingolipid/cholesterol-enriched membranes (GEMs), we studied the association of established myelin proteins, peripheral myelin protein 22 (PMP22), protein zero (P0), plasmolipin, and myelin basic protein (MBP), with these membrane microdomains. To examine the association of the selected peripheral myelin proteins with detergent-insoluble GEMs, purified myelin from sciatic nerve of adult rat was extracted with Triton X-100 at 4 degrees C and 37 degrees C and, in additional experiments, was pretreated with the cholesterol chelator methyl-beta-cyclodextrin. The material was then centrifuged to equilibrium in sucrose gradients, and fractions were analyzed by Western blotting. Here we demonstrate for the first time that PMP22, P0, and plasmolipin prepared from purified peripheral myelin are associated with GEMs. To characterize whether the association of these proteins is a specialized feature of myelinating Schwann cells, we studied the distribution of PMP22, P0, and plasmolipin in transiently transfected HeLa cells. These experiments confirm the specific association of these proteins with GEMs in both neural and nonneural cell types.

Recent advances in membrane microdomains: rafts, caveolae, and intracellular cholesterol trafficking

Cellular cholesterol homeostasis is a balance of influx, catabolism and synthesis, and efflux. Unlike vascular lipoprotein cholesterol transport, intracellular cholesterol trafficking is only beginning to be resolved. Exogenous cholesterol and cholesterol ester enter cells via the low-density lipoprotein (LDL) receptor/lysosomal and less so by nonvesicular, high-density lipoprotein (HDL) receptor/caveolar pathways. However, the mechanism(s) whereby cholesterol enters the lysosomal membrane, translocates, and transfers out of the lysosome to the cell interior are unknown. Likewise, the steps whereby cholesterol enters the cytofacial leaflet of the plasma membrane caveolae, rapidly translocates, leaves the exofacial leaflet, and transfers to extracellular HDL are unclear. Increasing evidence obtained with model and isolated cell membranes, transfected cells, genetic mutants, and gene-ablated mice suggests that proteins such as caveolin, sterol carrier protein-2 (SCP-2), Niemann-Pick C1 protein, steroidogenic acute regulatory protein (StAR), and other intracellular proteins mediate intracellular cholesterol transfer. While these proteins bind cholesterol and/or interact with cholesterol-rich membrane microdomains (e.g., caveolae, rafts, and annuli), their relative contributions to direct molecular versus vesicular cholesterol transfer remain to be resolved. The formation, regulation, and role of membrane microdomains in regulating cholesterol uptake/efflux and trafficking are unclear. Some cholesterol-binding proteins exert opposing effects on cellular cholesterol uptake/efflux, transfer of cholesterol out of the lysosomal membrane, and/or intracellular cholesterol trafficking to select membranous organelles. Resolving these cholesterol pathways and the role of membrane cholesterol microdomains is essential to our understanding not only of processes that affect cholesterol metabolism, but also of the abnormal regulation that may lead to disease (diabetes, obesity, atherosclerosis, neutral lipid storage, Niemann-Pick C, congenital lipoid adrenal hyperplasia, etc.).

Life without a cell membrane: regeneration of protoplasts from disintegrated cells of the marine green alga Bryopsis plumosa

When the multi-nucleate giant cells of the green alga Bryopsis plumosa (Huds.) Ag. are injured, the protoplasm is extruded from the cells and can generate spontaneously numerous new cells. The cell organelles aggregate rapidly in seawater and become covered with a gelatinous envelope within 15 minutes. A lipid cell membrane is formed inside the envelope within 9 to 12 hours and about 15% of the original cell membrane is recycled to make the membrane of new protoplasts. Cytochemical studies using Nile Red and various enzymes revealed that the primary envelope is initially composed of polysaccharides, and then transformed into a polysaccharide-lipid complex. Fluorescein diacetate staining showed that the primary envelope has some characteristics of a cell membrane including semi-permeability and selective transport of materials. The aggregation of cell organelles appears to be mediated by two kinds of materials, one present in vacuolar sap and the other on the surface of the cell organelles. About a thousand new cells were generated from a single disintegrated branch and 40% of them eventually developed into mature plants.

Oligomerization of NhaA, the Na+/H+ antiporter of Escherichia coli in the membrane and its functional and structural consequences

Recently, a two-dimensional crystal structure of NhaA, the Na+/H+ antiporter of Escherichia coli has been obtained [Williams, K. A., Kaufer, U. G., Padan, E., Schuldiner, S. and Kühlbrandt, W. (1999) EMBO J., 18, 3558-3563]. In these crystals NhaA exists as a dimer. Using biochemical and genetic approaches here we show that NhaA exists in the native membrane as a homooligomer. Functional complementation between the polypeptides of NhaA was demonstrated by coexpression of pairs of conditional lethal (at high pH in the presence of Na+) mutant alleles of nhaA in EP432, a strain lacking antiporters. Physical interaction in the membrane was shown between the His-tagged NhaA polypeptide which is readily affinity purified from DM-solubilized membranes with a Ni2+-NTA column and another which is not; only when coexpressed did both copurify on the column. The organization of the oligomer in the membrane was studied in situ by site-directed cross-linking experiments. Cysteine residues were introduced--one per NhaA--into certain loops of Cys-less NhaA, so that only intermolecular cross-linking could take place. Different linker-size cross-linkers were applied to the membranes, and the amount of the cross-linked protein was analyzed by mobility shift on SDS-PAGE. The results are consistent with homooligomeric NhaA and the location of residue 254 in the interface between monomers. Intermolecular cross-linking of V254C caused an acidic shift in the pH profile of NhaA.

Molecular transport and organization in supported lipid membranes

The mechanism by which vesicles spontaneously form supported lipid bilayer membranes on glass surfaces is becoming better understood and this knowledge is the basis of a technology of patterning membrane arrays and controlling composition. Controlled interactions between supported membranes and cells, particularly from the immune system, provide direct insight into cell-cell surface interactions.

Myosin-X, a novel myosin with pleckstrin homology domains, associates with regions of dynamic actin

Myosin-X is the founding member of a novel class of unconventional myosins characterized by a tail domain containing multiple pleckstrin homology domains. We report here the full-length cDNA sequences of human and bovine myosin-X as well as the first characterization of this protein's distribution and biochemical properties. The 235 kDa myosin-X contains a head domain with &lt;45% protein sequence identity to other myosins, three IQ motifs, and a predicted stalk of coiled coil. Like several other unconventional myosins and a plant kinesin, myosin-X contains both a myosin tail homology 4 (MyTH4) domain and a FERM (band 4.1/ezrin/radixin/moesin) domain. The unique tail domain also includes three pleckstrin homology domains, which have been implicated in phosphatidylinositol phospholipid signaling, and three PEST sites, which may allow cleavage of the myosin tail. Most intriguingly, myosin-X in cultured cells is present at the edges of lamellipodia, membrane ruffles, and the tips of filopodial actin bundles. The tail domain structure, biochemical features, and localization of myosin-X suggest that this novel unconventional myosin plays a role in regions of dynamic actin.

Coordinated gating of TRP-dependent channels in rhabdomeral membranes from Drosophila retinas

Using a newly developed dissociation procedure, we isolated the specialized rhabdomeral membranes from Drosophila retinal photoreceptors. From these membranes, we have recorded spontaneous active currents in excised patch, voltage-clamp recordings. We observed rapid opening events that closely resembled those ascribed to one class of light-activated channels, TRP. All activity exhibited Ba(2+) permeability, little voltage dependence, and sensitivity to La(3+) block. Mutational analysis indicated that the spontaneous activity present in these membranes was TRP-dependent. Excised patches from wild-type rhabdomeral membranes exhibited a wide range of conductance amplitudes. In addition, large conductance events exhibited many conductance levels in the open state. Block of activity by La(3+) both developed and recovered in a stepwise manner. Our results indicate that TRP-dependent channels have a small unitary conductance and that many channels can be gated coordinately.

Effect of leucinyl-phenylalanyl-valine on DMPC liposome membrane

Leucinyl-phenylalanyl-valine (LFV) is a hydrophobic tripeptide with a flat egg shaped structure with the long axis dimension of about 12 A. The effect of LFV on dimyristoylphosphatidylcholine (DMPC) liposome membrane has been studied by differential scanning calorimetry (DSC) and fluorescence spectroscopy. Calorimetric studies shows that incorporation of LFV completely abolishes the pretransition temperature with broadening of main transition temperature. Four conceptually different fluorescence probes, 1-naphthol (1-ROH) an excited state proton transfer probe, 8-anilino-1-naphthalenesulphonate (ANS) a solvent polarity probe, 1-6-diphenylhexatriene (DPH) an anisotropy probe and pyrene an excimer-forming probe have been used for fluorescence spectroscopic studies. For 1-ROH, ANS and DPH, a decreased partitioning with increasing mol.% of LFV was observed. Increasing LFV mol.% caused a decrease in the neutral form emission of 1-ROH, and a decrease in fluorescence intensity with red shift in ANS. The excimer formation ability of pyrene also decreased. The phase transition behavior of DMPC membrane in the presence of LFV was similar to the known effect of cholesterol on lipid bilayers. These results suggest that LFV cause an increased compactness of membrane.