Split-GFP complementation at the bacterial cell surface for antibody-free labeling and quantification of heterologous protein display

The split-GFP system is a versatile tool with numerous applications, but it has been underutilized for the labeling of heterologous surface-displayed proteins. By inserting the 16 amino acid sequence of the GFP11-tag between a protein of interest and an autotransporter protein, it is possible to present a protein at the outer membrane of gram-negative bacteria and to fluorescently label it by complementation with externally added GFP1-10. The labeled cells could be clearly discerned from cells without the protein of interest using flow cytometry and the insertion of the GFP11-tag caused no significant alteration of the catalytic activity for the tested model enzyme CsBglA. Furthermore, the amount of the protein of interest on the cells could be quantified by comparing the green fluorescence resulting from the complementation to that of standards with known concentrations. This allows a precise characterization of whole-cell biocatalysts, which is difficult with existing methods. The split-GFP complementation approach was shown to be specific, in a similar manner as commercial antibodies. It is cost-efficient, minimizes the possibility of adverse effects on protein expression or solubility, and can be performed at high throughput.

Professor Luiz R. Travassos and the study of surface structures of fungal pathogens

Brazilian medical mycology considerably expanded in the last decades due to the efforts of several pioneers who started and expanded mycology during the twentieth century. In this manuscript, we highlight some of the contributions of one of these pioneers: Professor Luiz R. Travassos, who started his career in the field of microbiology in the 1960s. We will discuss his contributions to the areas of medical mycology and glycobiology, with a focus on glycosphingolipids, sialic acids, and surface enzymes.

STIM2 regulates NMDA receptor endocytosis that is induced by short-term NMDA receptor overactivation in cortical neurons

Recent findings suggest an important role for the dysregulation of stromal interaction molecule (STIM) proteins, activators of store-operated Ca2+ channels, and the prolonged activation of N-methyl-D-aspartate receptors (NMDARs) in the development of neurodegenerative diseases. We previously demonstrated that STIM silencing increases Ca2+ influx through NMDAR and STIM-NMDAR2 complexes are present in neurons. However, the interplay between NMDAR subunits (GluN1, GluN2A, and GluN2B) and STIM1/STIM2 with regard to intracellular trafficking remains unknown. Here, we found that the activation of NMDAR endocytosis led to an increase in STIM2-GluN2A and STIM2-GluN2B interactions in primary cortical neurons. STIM1 appeared to migrate from synaptic to extrasynaptic sites. STIM2 silencing inhibited post-activation NMDAR translocation from the plasma membrane and synaptic spines and increased NMDAR currents. Our findings reveal a novel molecular mechanism by which STIM2 regulates NMDAR synaptic trafficking by promoting NMDAR endocytosis after receptor overactivation, which may suggest protection against excessive uncontrolled Ca2+ influx through NMDARs.

Defining the Cell Surface Cysteinome using Two-step Enrichment Proteomics

The plasma membrane proteome is a rich resource of functional and therapeutically relevant protein targets. Distinguished by high hydrophobicity, heavy glycosylation, disulfide-rich sequences, and low overall abundance, the cell surface proteome remains undersampled in established proteomic pipelines, including our own cysteine chemoproteomics platforms. Here we paired cell surface glycoprotein capture with cysteine chemoproteomics to establish a two-stage enrichment method that enables chemoproteomic profiling of cell Surface Cysteinome. Our "Cys-Surf" platform captures >2,800 total membrane protein cysteines in 1,046 proteins, including 1,907 residues not previously captured by bulk proteomic analysis. By pairing Cys-Surf with an isotopic chemoproteomic readout, we uncovered 821 total ligandable cysteines, including known and novel sites. Cys-Surf also robustly delineates redox-sensitive cysteines, including cysteines prone to activation-dependent changes to cysteine oxidation state and residues sensitive to addition of exogenous reductants. Exemplifying the capacity of Cys-Surf to delineate functionally important cysteines, we identified a redox sensitive cysteine in the low-density lipoprotein receptor (LDLR) that impacts both the protein localization and uptake of LDL particles. Taken together, the Cys-Surf platform, distinguished by its two-stage enrichment paradigm, represents a tailored approach to delineate the functional and therapeutic potential of the plasma membrane cysteinome.

Detection of Chemokine Binding Proteins Association to Cell Surface Glycosaminoglycans by Flow Cell Cytometry and Indirect Immunofluorescence

Viruses encode secreted proteins that bind chemokines to modulate their activity. Viral proteins may simultaneously interact with glycosaminoglycans allowing these proteins to be anchored at the cell surface to increase their anti-chemokine activity in the proximity of infection. Here we describe methodology to evaluate the interaction of viral secreted proteins with cell-surface glycosaminoglycans by immunofluorescence and detection by flow cytometry or microscopy. These methods could be equally applied to other chemokine binding proteins that do not have viral origin.

Plant lectins: Handymen at the cell surface

Lectins are carbohydrate-binding proteins and are involved in a multitude of biological functions. Lectins at the surface of plant cells often occur as lectin receptor-like kinases (LecRLK) anchored to the plasma membrane. These LecRLKs are part of the plant's pattern-recognition receptor (PRR) system enabling the plant to perceive threats and respond adequately. Furthermore, plant lectins also occur as secreted proteins, which are associated with stress signalling and defence. The aim of this short review is to provide a general perspective on plant lectins and their role at the cell surface.

Targeting acute myeloid cell surface using a recombinant antibody isolated from whole-cell biopanning of a phage display human scFv antibody library

To discover new therapeutic antibodies for treatment of acute myeloid leukemia (AML) without the requirement of a known antigen, a human single-chain variable fragment (scFv) library was used to isolate novel antibody fragments recognizing HL-60 AML cells. After three rounds of biopanning, scFv-expressing phages were selected to test for binding to the target cell by flow cytometry. The clone with highest binding specificity to HL-60 cells (designated y1HL63D6) was further investigated. Fluorescent staining indicated that y1HL63D6 scFv bound to a target located on the cell surface. Whole immunoglobulin, IgG-y1HL63D6 was then generated and tested for the binding against bone marrow mononuclear cells (BMMCs) from AML patients. Significantly higher fluorescent signals were observed for some patient samples when compared to normal BMMCs or non-AML patients' BMMCs. Next, the IgG-y1HL63D6 format was tested for antibody-dependent cell cytotoxicity (ADCC). The results demonstrated that IgG-y1HL63D6 but not the control antibody, trastuzumab, could mediate specific killing of HL-60 target cells. In conclusion, our results indicate that specific antibodies can be isolated by biopanning whole cells with a non-immunized human scFv antibody phage display library and that the isolated antibody against HL-60 cells showed therapeutic potential.

Microbial cell surface engineering for high-level synthesis of bio-products

Microbial cell surface layers, which mainly include the cell membrane, cell wall, periplasmic space, outer membrane, capsules, S-layers, pili, and flagella, control material exchange between the cell and the extracellular environment, and have great impact on production titers and yields of various bio-products synthesized by microbes. Recent research work has made exciting achievements in metabolic engineering using microbial cell surface components as novel regulation targets without direct modifications of the metabolic pathways of the desired products. This review article will summarize the accomplishments obtained in this emerging field, and will describe various engineering strategies that have been adopted in bacteria and yeasts for the enhancement of mass transfer across the cell surface, improvement of protein expression and folding, modulation of cell size and shape, and re-direction of cellular resources, all of which contribute to the construction of more efficient microbial cell factories toward the synthesis of a variety of bio-products. The existing problems and possible future directions will also be discussed.

Mapping Cell Surface Proteolysis with Plasma Membrane-Targeted Subtiligase

N terminomics methods combine selective isolation of protein N-terminal peptides with mass spectrometry (MS)-based proteomics for global profiling of proteolytic cleavage sites. However, traditional N terminomics workflows require cell lysis before N-terminal enrichment and provide poor coverage of N termini derived from cell surface proteins. Here, we describe application of subtiligase-TM, a plasma membrane-targeted peptide ligase, for selective biotinylation of cell surface N termini, enabling their enrichment and analysis by liquid chromatography-tandem MS (LC-MS/MS). This method provides increased coverage of and specificity for cell surface N termini and is compatible with existing quantitative LC-MS/MS workflows.

NMR chemical shift assignments of a module of unknown function in the cellulosomal secondary scaffoldin ScaF from Clostridium thermocellum

The cellulosome is a highly efficient cellulolytic complex containing cellulolytic enzymes and non-catalytic subunits, i.e. scaffoldins, which are assembled by the interactions between the dockerin modules of the enzymes and the cohesin modules of the primary scaffoldins. The cellulosome attaches to the cell surface via the S-layer homology (SLH) modules of the anchoring scaffoldins. Clostridium thermocellum DSM1313 is a thermophilic cellulosome-producing bacterium with great potential in lignocellulose bioconversion and biofuel production. The bacterium contains four anchoring scaffoldins ScaB, ScaC, ScaD and ScaF, among which ScaF is the only one that contains an additional module of unknown function (ScaF-X) between the cohesin and SLH modules. The gene of ScaF is located outside the scaffoldin gene cluster of scaA, scaB, scaC and scaD. Previous studies showed unique regulation properties and function of ScaF compared to other anchoring scaffoldins, which might be related to the additional ScaF-X module. Here we report the NMR chemical shift assignments of ScaF-X from C. thermocellum DSM1313. The well-dispersed NMR spectrum and the secondary structure prediction based on the chemical shifts of ScaF-X indicated that ScaF-X is a well-folded protein module. The chemical shift assignments provide the basis for future studies on the structure of this module and its function in cellulosomes.

Yeast cell surface displaying VP28 antigen and its potential application for shrimp farming

VP28 is an envelope protein of White Spot Syndrome Virus (WSSV), which has been shown in previous studies to induce a high immune response in shrimp. VP28 has been produced in some host systems such as Escherichia coli, Bacillus subtilis, and Pichia pastoris as free protein. Here we showed a new strategy of anchoring VP28 on the Saccharomyces cerevisiae yeast surface and using the yeast cell extract combined with probiotic as an oral vaccine for shrimp farming. We have successfully constructed a recombinant yeast cell capable of expressing VP28 on the cell surface. The feeding diet combined with VP28 anchored yeast cell extract provided significant assurance to Litopenaeus vannamei, challenged by WSSV, resulting in a relative percent survival (RPS) of 87.10 ± 2.15%. Interestingly, the utilization of VP28 anchored yeast cell extract could enhance the efficiency of probiotic strains like Lactobacillus and Bacillus on shrimp farming. The results in both laboratory scales and field trials using extract of VP28 displaying Saccharomyces showed a growth-promoting effect in shrimp, assessed through average shrimp weight. Taken together, our results in this study demonstrated a new successful strategy of using yeast cell surface as a tool to produce VP28-based oral vaccine for shrimp aquaculture. KEY POINTS: • A new strategy of using VP28 antigen as anchored protein on S. cerevisiae yeast cell surface (S. cerevisiae::VP28) • The utilization of VP28 antigen and yeast as S. cerevisiae::VP28 extract enhanced potential protection of Litopenaeus vannamei against White Spot Syndrome Virus (RPS 87.10%) • The use of S. cerevisiae::VP28 extract increased efficiency of probiotic on shrimp growth-promoting effect either lab-scale or field trial.

Recovery of Sea Star Egg Cell Surface Proteins Released at Fertilization

To provide a better understanding of the composition of the egg cell membrane, we describe a method in which proteins and peptides that are either naturally released by the egg or cleaved by sperm proteases can be collected, analyzed, and identified. Such molecules are captured and isolated from the surrounding seawater via biotinylation, before being concentrated by an affinity interaction and subsequently analyzed by western blotting and mass spectrometry.

Autodisplay of human PIP5K1α lipid kinase on Escherichia coli and inhibitor testing

The human phosphatidylinositol 4-phosphate 5-kinase type I α (hPIP5K1α) plays a major role in the PI3K/AKT/mTOR signaling pathway. As it has been shown before that hPIP5K1α is involved in the development of different types of cancer in particular prostate cancer, inhibitors of the enzyme might be a new option for the treatment of this disease. Here we report on the expression of hPIP5K1α on the surface of E. coli using Autodisplay. Autodisplay is defined as the surface display of a recombinant protein on a gramnegative bacterium by the autotransporter secretion pathway. After verification of surface expression, enzyme activity of whole cells displaying hPIP5K1α was determined by a capillary electrophoresis based assay. When using cells at an OD₅₇₈ of 2.5, the artificial substrate phosphatidylinositol4-phosphate (PI(4)P) fluorescein was converted by a rate of 10.7 ± 0.2 fmol/min. Using this substrate inhibition of three pyranobenzoquinone type compounds was tested. The most active compound was 4-(2-amino-3-cyano-6-hydroxy-5,8-dioxo-7-undecyl-5,8-dihydro-4H-chromen-4-yl) benzoic acid with an IC₅₀ value of 8.6 μM. Because until now, all attempts to purify hPIP5K1α failed, we suggest the use of whole cells of E. coli displaying the enzyme as a convenient tool for inhibitor identification.

Quantification of The Surface Expression of G Protein-coupled Receptors Using Intact Live-cell Radioligand Binding Assays

G protein-coupled receptors (GPCRs) are the most structurally diverse family of signaling proteins and regulate a variety of cell function. For most GPCRs, the cell surface is their functional destination where they are able to respond a wide range of extracellular stimuli, leading to the activation of intracellular signal transduction cascades. Thus, the quantity of receptor expression at the cell surface is a crucial factor regulating the functionality of the receptors. Over the past decades, many methods have been developed to measure the cell surface expression of GPCRs. Here, we describe an intact live-cell radioligand binding assay to quantify the surface expression of GPCRs at the endogenous levels or after overexpression. In this assay, cell cultures will be incubated with specific cell-nonpermeable radioligands which selectively and stoichiometrically bind to individual GPCRs and the receptor numbers at the cell surface are quantified by the radioactivity of receptor-bound ligands. This method is highly specific for measuring the functional GPCRs at the surface of intact live cells and is particularly useful for endogenous, low-abundant GPCRs.

Natural display of nuclear-encoded RNA on the cell surface and its impact on cell interaction

BACKGROUND

Compared to proteins, glycans, and lipids, much less is known about RNAs on the cell surface. We develop a series of technologies to test for any nuclear-encoded RNAs that are stably attached to the cell surface and exposed to the extracellular space, hereafter called membrane-associated extracellular RNAs (maxRNAs).

RESULTS

We develop a technique called Surface-seq to selectively sequence maxRNAs and validate two Surface-seq identified maxRNAs by RNA fluorescence in situ hybridization. To test for cell-type specificity of maxRNA, we use antisense oligos to hybridize to single-stranded transcripts exposed on the surface of human peripheral blood mononuclear cells (PBMCs). Combining this strategy with imaging flow cytometry, single-cell RNA sequencing, and maxRNA sequencing, we identify monocytes as the major type of maxRNA+ PBMCs and prioritize 11 candidate maxRNAs for functional tests. Extracellular application of antisense oligos of FNDC3B and CTSS transcripts inhibits monocyte adhesion to vascular endothelial cells.

CONCLUSIONS

Collectively, these data highlight maxRNAs as functional components of the cell surface, suggesting an expanded role for RNA in cell-cell and cell-environment interactions.

S-layer associated proteins contribute to the adhesive and immunomodulatory properties of Lactobacillus acidophilus NCFM

BACKGROUND

Surface layers (S-layers) are two-dimensional crystalline arrays of repeating proteinaceous subunits that form the outermost layer of many bacterial cell envelopes. Within the Lactobacillus genus, S-layer presence is frequently associated with probiotic-relevant properties such as improved adherence to host epithelial cells and modulation of the immune response. However, recent studies have demonstrated that certain S-layer functions may be supplemented by a novel subset of proteins embedded within its lattice, termed S-layer associated proteins (SLAPs). In the following study, four Lactobacillus acidophilus NCFM SLAPs (LBA0046, LBA0864, LBA1426, and LBA1539) were selected for in silico and phenotypic assessment.

RESULTS

Despite lacking any sequence similarity or catalytic domains that may indicate function, the genes encoding the four proteins of interest were shown to be unique to S-layer-forming, host-adapted lactobacilli species. Likewise, their corresponding deletion mutants exhibited broad, host-relevant phenotypes including decreased inflammatory profiles and reduced adherence to Caco-2 intestinal cells, extracellular matrices, and mucin in vitro.

CONCLUSIONS

Overall, the data presented in this study collectively links several previously uncharacterized extracellular proteins to roles in the underlying host adaptive mechanisms of L. acidophilus.

AFM dendritips functionalized with molecular probes specific to cell wall polysaccharides as a tool to investigate cell surface structure and organization

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Functionalisation of AFM dendritips with conA, WGA and anti-β-1,3/β-1, 6-glucan antibodies.

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Cell wall polysaccharides were immobilized on epoxy-activated glass slides.

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Specific binding of immobilized polysaccharides to functionalized dendritips.

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Functionalized dendritips used as a new tool to probe yeast cell surface.

The yeast cell wall is composed of mannoproteins, β-1,3/β-1, 6-glucans and chitin. Each of these components has technological properties that are relevant for industrial and medical applications. To address issues related to cell wall structure and alteration in response to stress or conditioning processes, AFM dendritips were functionalized with biomolecules that are specific for each of the wall components, which was wheat germ agglutinin (WGA) for chitin, concanavalin A (ConA) for mannans and anti-β-1,3/anti-β-1,6-glucan antibodies for β-1,3/β-1,6-glucans. Binding specificity of these biomolecules were validated using penta-N-acetylchitopentaose, α-mannans, laminarin (short β-1,3-glucan chain) and gentiobiose (2 glucose units linked in β 1→6) immobilized on epoxy glass slides. Dynamic force spectroscopy was employed to obtain kinetic and thermodynamic information on the intermolecular interaction of the binary complexes using the model of Friddle-Noy-de Yoreo. Using this model, transition state distance x_t, dissociate rate k_off and the lowest force (f_eq) required to break the intermolecular bond of the complexes were approximated. These functionalized dendritips were then used to probe the yeast cell surface treated with a bacterial protease. As expected, this treatment, which removed the outer layer of the cell wall, gave accessibility to the inner layer composed of β-glucans. Likewise, bud scars were nicely localized using AFM dendritip bearing the WGA probe. To conclude, these functionalized AFM dendritips constitute a new toolbox that can be used to investigate cell surface structure and organization in response to a wide arrays of cultures and process conditions.

Narrowing the focus: Therapeutic cell surface targets for refractory triple-negative breast cancer

Triple-negative breast cancer (TNBC) is defined as a type of breast cancer with lack of expression of estrogen receptor, progesterone receptor and human epidermal growth factor 2 protein. In comparison to other types of breast cancer, TNBC characterizes for its aggressive behavior, more prone to early recurrence and a disease with poor response to molecular target therapy. Although TNBC is identified in only 25%-30% of American breast cancer cases annually, these tumors continue to be a therapeutic challenge for clinicians for several reasons: Tumor heterogeneity, limited and toxic systemic therapy options, and often resistance to current standard therapy, characterized by progressive disease on treatment, residual tumor after cytotoxic chemotherapy, and early recurrence after complete surgical excision. Cell-surface targeted therapies have been successful for breast cancer in general, however there are currently no approved cell-surface targeted therapies specifically indicated for TNBC. Recently, several cell-surface targets have been identified as candidates for treatment of TNBC and associated targeted therapies are in development. The purpose of this work is to review the current clinical challenges posed by TNBC, the therapeutic approaches currently in use, and provide an overview of developing cell surface targeting approaches to improve outcomes for treatment resistant TNBC.

A quantitative N-glycoproteomics study of cell-surface N-glycoprotein markers of MCF-7/ADR cancer stem cells

Isotopic-labeling quantitative N-glycoproteomics characterization of cell-surface differentially expressed N-glycosylation in MCF-7/ADR cancer stem cells (CSCs) relative to MCF-7/ADR cells was carried out at the intact N-glycopeptide level with trypsin digestion, ZIC-HILIC enrichment, isotopic diethyl labeling, RPLC-MS/MS analysis of the 1:1 mixture, and GPSeeker DB search. With a spectrum-level false discovery rate of ≤ 1%, 1,336 intact N-glycopeptides from the combination of 301 unique peptide backbones and 169 putative N-glycan linkages (52 monosaccharide compositions) were identified; the corresponding intact N-glycoproteins and N-glycosites were 289 and 305, respectively, among which 176 N-glycosites were confirmed with GlcNAc-containing site-determining b/y fragment ion pairs. The N-glycan moieties in 546 intact N-glycopeptide IDs were identified with more than one structure-diagnostic fragment ions where multiple linkage structures exist for each of the monosaccharide compositions. With the criteria of ≥ 1.5-fold change and p value < 0.05, 72 cell-surface differentially expressed intact N-glycopeptides (DEGPs) were found in MCF-7/ADR CSCs relative to MCF-7/ADR cells, where 8 and 64 were downregulated and upregulated, respectively. Graphical abstract.

Automated Analysis of Cell Surface Ruffling: Ruffle Quantification Macro

Cell surface protrusions include F-actin rich, wave-like ruffles that are erected transiently in response to stimuli and during cell migration. Macrophages are innate immune cells that ruffle constitutively and more dramatically in cells activated by pathogens. Dorsal ruffles and their resulting macropinosomes are key sites for environmental sampling, pathogen detection and immune signaling. Quantitative assessment of ruffling is important for assessing pathogen responses in macrophages and for analysis of growth factor responses in other cell types but automated and quantitative methods are lacking, and rely on manual and qualitative assessments. Here we present an automated ImageJ macro for quantifying dorsal cell surface protrusions from 3D microscope images. The assay presented here is suitable for high-throughput screening applications to detect drug, pathogen, or growth factor induced changes in cell ruffling by measuring ruffle area and intensity and providing normalized values in an easy to read combined spreadsheet.

Systematic and site-specific analysis of N-glycoproteins on the cell surface by integrating bioorthogonal chemistry and MS-based proteomics

Glycoproteins on the cell surface are essential for various cellular activities including cell-cell communication and cell-matrix interaction. Alterations of glycosylation are correlated with many diseases such as cancer and infectious diseases. However, it is greatly challenging to systematically and site-specially analyze glycoproteins only located on cell surface because of the heterogeneity of glycans, the low abundance of many surface glycoproteins and the requirement of effective methods to separate surface glycoproteins. In this chapter, we briefly review existing mass spectrometry (MS)-based methods for global analysis of surface glycoproteins. Then we discuss an effective method integrating metabolic labeling, click and enzymatic reactions, and MS-based proteomics to comprehensively and site-specifically investigate cell surface N-glycoproteins. A detailed protocol for this method is also included. In combination with quantitative proteomics, we applied this method to quantify cell surface N-glycoproteins and study the relationship between cell invasiveness and N-sialoglycoproteins on the cell surface. Considering the importance of surface glycoproteins, this method can be extensively applied to advance glycoscience, which leads to a better understanding of the molecular mechanisms of human diseases, and the discovery of surface glycoproteins as biomarkers for disease detection.

Copper-Induced Expression of a Transmissible Lipoprotein Intramolecular Transacylase Alters Lipoprotein Acylation and the Toll-Like Receptor 2 Response to Listeria monocytogenes

Bacterial lipoproteins are globular proteins anchored to the extracytoplasmic surfaces of cell membranes through lipidation at a conserved N-terminal cysteine. Lipoproteins contribute to an array of important cellular functions for bacteria, as well as being a focal point for innate immune system recognition through binding to Toll-like receptor 2 (TLR2) heterodimer complexes. Although lipoproteins are conserved among nearly all classes of bacteria, the presence and type of α-amino-linked acyl chain are highly variable and even strain specific within a given bacterial species. The reason for lyso-lipoprotein formation and N-acylation variability in general is presently not fully understood. In Enterococcus faecalis, lipoproteins are anchored by an N-acyl-S-monoacyl-glyceryl cysteine (lyso form) moiety installed by a chromosomally encoded lipoprotein intramolecular transacylase (Lit). Here, we describe a mobile genetic element common to environmental isolates of Listeria monocytogenes and Enterococcus spp. encoding a functional Lit ortholog (Lit2) that is cotranscribed with several well-established copper resistance determinants. Expression of Lit2 is tightly regulated, and induction by copper converts lipoproteins from the diacylglycerol-modified form characteristic of L. monocytogenes type strains to the α-amino-modified lyso form observed in E. faecalis Conversion to the lyso form through either copper addition to media or constitutive expression of lit2 decreases TLR2 recognition when using an activated NF-κB secreted embryonic alkaline phosphatase reporter assay. While lyso formation significantly diminishes TLR2 recognition, lyso-modified lipoprotein is still predominantly recognized by the TLR2/TLR6 heterodimer.IMPORTANCE The induction of lipoprotein N-terminal remodeling in response to environmental copper in Gram-positive bacteria suggests a more general role in bacterial cell envelope physiology. N-terminal modification by lyso formation, in particular, simultaneously modulates the TLR2 response in direct comparison to their diacylglycerol-modified precursors. Thus, use of copper as a frontline antimicrobial control agent and ensuing selection raises the potential of diminished innate immune sensing and enhanced bacterial virulence.

Externally Controlled Cellular Transport of Magnetic Iron Oxide Particles with Polysaccharide Surface Coatings

Recently, due to their promising applications in biomedicine, magnetic iron oxide nanoparticles (MPs) have become one of the research hotspots in the nanomedicine field. Since various synthetic modifications have been widely applied to these nanoparticles for better targeting behaviors, it is meaningful to apply the optimal magnetic field condition for each case. This will enable creating a safe and efficient drug targeting using different types of MPs. In the present study, we aimed to find out any changes of transepithelial transport of polysaccharide-coated MPs by applying the continuous or the pulsatile magnetic field condition. Our results with heparin-functionalized MPs indicate that the particle concentrations and the external magnetic field could influence the transepithelial permeability of the particles. In the presence of a continuously applied magnetic density, heparin-MPs at high concentrations, by forming magnetically-induced aggregation of particles over the cell surface layer, showed a lower cellular transport than those at low concentrations. Furthermore, the results from the quantitative chemical assays and imaging analyses showed that transepithelial transport of heparin-MPs (negatively charged) under the pulsatile magnetic field was higher than that under the continuous magnetic field (CP), whereas the starch-MPs (neutrally charged) showed a small difference in transepithelial transport or cell retention between pulsatile vs. continuous magnetic field conditions. Taken together, our results suggest that the external magnetic field should be differentially applied to control the cellular drug transport depending on the physicochemical properties of the surface chemistry of magnetic particles.

KDEL Receptor 1 Contributes to Cell Surface Association of Protein Disulfide Isomerases

BACKGROUND/AIMS

Endoplasmic reticulum (ER)-resident proteins with a C-terminal KDEL ERretention sequence are captured in the Golgi apparatus by KDEL receptors (KDELRs). The binding of such proteins to these receptors induces their retrograde transport. Nevertheless, some KDEL proteins, such as Protein Disulfide Isomerases (PDIs), are found at the cell surface. PDIs target disulfide bridges in the extracellular domains of proteins, such as integrins or A Disintegrin And Metalloprotease 17 (ADAM17) leading to changes in the structure and function of these molecules. Integrins become activated and ADAM17 inactivated upon disulfide isomerization. The way that PDIs escape from retrograde transport and reach the plasma membrane remains far from clear. Various mechanisms might exist, depending on whether a local cell surface association or a more global secretion is required.

METHODS

To get a more detailed insight in the transport of PDIs to the cell surface, methods such as cell surface biotinylation, flow cytometric analysis, immunoprecipitation, fluorescence microscopy as well as labeling of cells with fluorescence labled recombinant PDIA6 was performed.

RESULTS

Here, we show that the C-terminal KDEL ER retention sequence is sufficient to prevent secretion of PDIA6 into the extracellular space but is mandatory for its association with the cell surface. The cell surface trafficking of PDIA1, PDIA3, and PDIA6 is dependent on KDELR1, which travels in a dynamic manner to the cell surface. This transport is assumed to result in PDI cell surface association, which differs from PDI inducible secretion into the extracellular space. Distinct PDIs differ in their trafficking properties. Endogenous KDELR1, detectable at the cell surface, might be involved not only in the transport of cell-surface-associated PDIs, but also in their retrieval and internalization from the extracellular space.

CONCLUSION

Beside their ER retention motive PDIs travel to the cell surface. Here they target different proteins to render their function. To escape the ER PDIs travel via various pathways. One of them depends on the KDELR1, which can transport its target to the cell surface, where it is to be expected to release its cargo in close vicinity to its target molecules. Hence, the KDEL sequence is needed for cell surface association of PDIs, such as PDIA6.

Tumor-derived nanovesicles promote lung distribution of the therapeutic nanovector through repression of Kupffer cell-mediated phagocytosis

Tumor-derived nanovesicles have been widely used as a biomarker or therapeutic target in various tumor types. However, these nanovesicles have limited use in therapy due to the risk of advancing tumor development. Methods: Exosome-like nanovesicles (ENVs) were developed from metastatic breast cancer 4T1 cells-derived exosomes. The distribution of ENVs and their impact on macrophage-mediated phagocytosis were evaluated. The effect of ENVs pretreatment on anti-lung metastasis therapeutic effects of chemotherapeutic drugs delivered by DOTAP/DOPE liposomes in breast cancer-bearing mice was also examined. Results: We demonstrated that, following intravenous injection in mice, ENVs were preferentially uptaken by Kupffer cells and repressed phagocytosis. The decreased uptake appeared to be due to the translocation of membrane nucleolin from the inner face of the plasma membrane to the cell surface and intercellular Ca2+ fluxes, leading to altered expression of genes involved in phagocytosis by macrophages. Mice pretreated with 4T1-derived ENVs led to the decreased uptake of DOTAP: DOPE liposomes (DDL) in the liver. Consequently, doxorubicin-loaded DDL transported to the lungs instead of the liver, effectively inhibiting breast cancer lung metastasis. Importantly, 4T1 cells exosome-derived ENVs had no detectable toxicity in vivo and low-risk to promote tumor growth and metastasis compared to 4T1 cells exosomes. Conclusion: Our results suggested that pretreatment with 4T1 ENVs represents a strategy to escape Kupffer cell-mediated phagocytosis effectively targeting drug delivery vehicles to tumor metastasis, reducing the IC50 of the chemotherapeutic drugs, and avoiding adverse side effects.

LimeSeg: a coarse-grained lipid membrane simulation for 3D image segmentation

Background

3D segmentation is often a prerequisite for 3D object display and quantitative measurements. Yet existing voxel-based methods do not directly give information on the object surface or topology. As for spatially continuous approaches such as level-set, active contours and meshes, although providing surfaces and concise shape description, they are generally not suitable for multiple object segmentation and/or for objects with an irregular shape, which can hamper their adoption by bioimage analysts.

Results

We developed LimeSeg, a computationally efficient and spatially continuous 3D segmentation method. LimeSeg is easy-to-use and can process many and/or highly convoluted objects. Based on the concept of SURFace ELements (“Surfels”), LimeSeg resembles a highly coarse-grained simulation of a lipid membrane in which a set of particles, analogous to lipid molecules, are attracted to local image maxima. The particles are self-generating and self-destructing thus providing the ability for the membrane to evolve towards the contour of the objects of interest.

The capabilities of LimeSeg: simultaneous segmentation of numerous non overlapping objects, segmentation of highly convoluted objects and robustness for big datasets are demonstrated on experimental use cases (epithelial cells, brain MRI and FIB-SEM dataset of cellular membrane system respectively).

Conclusion

In conclusion, we implemented a new and efficient 3D surface reconstruction plugin adapted for various sources of images, which is deployed in the user-friendly and well-known ImageJ environment.

Enrichment of Cell Surface-Associated Proteins in Gram-Positive Bacteria by Biotinylation or Trypsin Shaving for Mass Spectrometry Analysis

In microbial cells surface-exposed proteins represent a physiologically important class of molecules as they enable these cells to interact with their environment both as free-living organisms and during interactions with a host. However, the characteristics of these proteins are quite divergent, which makes attempts to enrich, analyze, and quantify these molecules a challenging task. In this chapter two complementary methods for the enrichment and identification of cell surface-associated proteins, namely the biotinylation and the shaving approaches, are presented. Both protocols have been optimized for Gram-positive bacteria, and we provide a step-by-step guide for sample generation. Possible pitfalls during protein preparation are discussed.

Mass Spectrometry-Based Identification of Extracellular Domains of Cell Surface N-Glycoproteins: Defining the Accessible Surfaceome for Immunophenotyping Stem Cells and Their Derivatives

Human stem cells and their progeny are valuable for a variety of research applications and have the potential to revolutionize approaches to regenerative medicine. However, we currently have limited tools to permit live isolation of homogeneous populations of cells apt for mechanistic studies or cellular therapies. While these challenges can be overcome through the use of immunophenotyping based on accessible cell surface markers, the success of this process depends on the availability of reliable antibodies and well-characterized markers, which are lacking for most stem cell lineages. This chapter outlines an iterative process for the development of new cell surface marker barcodes for identifying and selecting stem cell derived progeny of specific cell types, subtypes, and maturation stages, where antibody-independent identification of cell surface proteins is achieved using a modern chemoproteomic approach to specifically identify N-glycoproteins localized to the cell surface. By taking advantage of a large repository of available cell surfaceome data, proteins that are unlikely to confer cell type specificity can be rapidly eliminated from consideration. Subsequently, targeted quantitation by mass spectrometry can be used to refine candidates of interest, and a bioinformatic visualization tool is key to mapping experimental data to candidate protein sequences for the purpose of epitope selection during the antibody development phase. Overall, the process of developing cell surface barcodes for immunophenotyping is iterative and can include multiple rounds of discovery, refinement, and validation depending on the phenotypic resolution required.

Similarities and differences in surface receptor expression by THP-1 monocytes and differentiated macrophages polarized using seven different conditioning regimens

Macrophages are key in orchestrating immune responses to micro-environmental stimuli, sensed by a complex set of surface receptors. The human cell line THP-1 has a monocytic phenotype, including the ability to differentiate into macrophages, providing a tractable, standardised surrogate for human monocyte-derived macrophages. Here we assessed the expression of 49 surface markers including Fc, complement, C-type lectin and scavenger receptors; TIMs; Siglecs; and co-stimulatory molecules by flow cytometry on both THP-1 monocytes and macrophages and following macrophage activation with seven standard conditioning/polarizing stimuli. Of the 34 surface markers detected on macrophages, 18 altered expression levels on activation. From these, expression of 9 surface markers were consistently altered by all conditioning regimens, while 9 were specific to individual polarizing stimuli. This study provides a resource for the study of macrophages and highlights that macrophage polarization states share much in common and the differences do not easily fit a simple classification system.

Consolidated bioprocessing for cellulosic ethanol conversion by cellulase-xylanase cell-surfaced yeast consortium

Consolidated bioprocessing (CBP) strategy was developed to construct a cell-surface displayed consortium for heterologously expressing functional lignocellulytic enzymes. The reaction system composed of two engineered yeast strains: Y5/XynII-XylA (co-displaying two types of xylanases) and Y5/EG-CBH-BGL (co-displaying three types of cellulases). The immobilization of recombinant fusion proteins and their cell-surface accessibility of were analyzed by flow cytometry and immunofluorescence. The feasibility of consolidated bioprocessing by using pretreated corn stover (CS) as substrate for direct bioconversion was further investigated, and the synergistic activity and proximity effect between cellulases and xylanases on lignocelluloses degradation were also discussed in this work. Without any commercial enzyme addition, the combined yeast consortium produced 1.61 g/L ethanol which achieved 64.7% of the theoretical ethanol yield during 144 h from steam-exploded CS. The results indicated that the assembly of cellulases and xylanases using a synthetic consortium capable of combined displaying lignocellulytic enzymes is a promising approach for simultaneous saccharification and fermentation to ethanol from lignocellulosic biomass.

Click Chemistry-mediated Biotinylation Reveals a Function for the Protease BACE1 in Modulating the Neuronal Surface Glycoproteome

The cell surface proteome is dynamic and has fundamental roles in cell signaling. Many surface membrane proteins are proteolytically released into a cell's secretome, where they can have additional functions in cell-cell-communication. Yet, it remains challenging to determine the surface proteome and to compare it to the cell secretome, under serum-containing cell culture conditions. Here, we set up and evaluated the 'surface-spanning protein enrichment with click sugars' (SUSPECS) method for cell surface membrane glycoprotein biotinylation, enrichment and label-free quantitative mass spectrometry. SUSPECS is based on click chemistry-mediated labeling of glycoproteins, is compatible with labeling of living cells and can be combined with secretome analyses in the same experiment. Immunofluorescence-based confocal microscopy demonstrated that SUSPECS selectively labeled cell surface proteins. Nearly 700 transmembrane glycoproteins were consistently identified at the surface of primary neurons. To demonstrate the utility of SUSPECS, we applied it to the protease BACE1, which is a key drug target in Alzheimer's disease. Pharmacological BACE1-inhibition selectively remodeled the neuronal surface glycoproteome, resulting in up to 7-fold increased abundance of the BACE1 substrates APP, APLP1, SEZ6, SEZ6L, CNTN2, and CHL1, whereas other substrates were not or only mildly affected. Interestingly, protein changes at the cell surface only partly correlated with changes in the secretome. Several altered proteins were validated by immunoblots in neurons and mouse brains. Apparent nonsubstrates, such as TSPAN6, were also increased, indicating that BACE1-inhibition may lead to unexpected secondary effects. In summary, SUSPECS is broadly useful for determination of the surface glycoproteome and its correlation with the secretome.

Effect of U(VI) aqueous speciation on the binding of uranium by the cell surface of Rhodotorula mucilaginosa, a natural yeast isolate from bentonites

This study presents the effect of aqueous uranium speciation (U-hydroxides and U-hydroxo-carbonates) on the interaction of this radionuclide with the cells of the yeast Rhodotorula mucigilanosa BII-R8. This strain was isolated from Spanish bentonites considered as reference materials for the engineered barrier components of the future deep geological repository of radioactive waste. X-ray absorption and infrared spectroscopy showed that the aqueous uranium speciation has no effect on the uranium binding process by this yeast strain. The cells bind mobile uranium species (U-hydroxides and U-hydroxo-carbonates) from solution via a time-dependent process initiated by the adsorption of uranium species to carboxyl groups. This leads to the subsequent involvement of organic phosphate groups forming uranium complexes with a local coordination similar to that of the uranyl mineral phase meta-autunite. Scanning transmission electron microscopy with high angle annular dark field analysis showed uranium accumulations at the cell surface associated with phosphorus containing ligands. Moreover, the effect of uranium mobile species on the cell viability and metabolic activity was examined by means of flow cytometry techniques, revealing that the cell metabolism is more affected by higher concentrations of uranium than the cell viability. The results obtained in this work provide new insights on the interaction of uranium with bentonite natural yeast from genus Rhodotorula under deep geological repository relevant conditions.

Protein arginine methyltransferase 5 mediates enolase-1 cell surface trafficking in human lung adenocarcinoma cells

OBJECTIVES

Enolase-1-dependent cell surface proteolysis plays an important role in cell invasion. Although enolase-1 (Eno-1), a glycolytic enzyme, has been found on the surface of various cells, the mechanism responsible for its exteriorization remains elusive. Here, we investigated the involvement of post-translational modifications (PTMs) of Eno-1 in its lipopolysaccharide (LPS)-triggered trafficking to the cell surface.

RESULTS

We found that stimulation of human lung adenocarcinoma cells with LPS triggered the monomethylation of arginine 50 (R50me) within Eno-1. The Eno-1R50me was confirmed by its interaction with the tudor domain (TD) from TD-containing 3 (TDRD3) protein recognizing methylarginines. Substitution of R50 with lysine (R50K) reduced Eno-1 association with epithelial caveolar domains, thereby diminishing its exteriorization. Similar effects were observed when pharmacological inhibitors of arginine methyltransferases were applied. Protein arginine methyltransferase 5 (PRMT5) was identified to be responsible for Eno-1 methylation. Overexpression of PRMT5 and caveolin-1 enhanced levels of membrane-bound extracellular Eno-1 and, conversely, pharmacological inhibition of PRMT5 attenuated Eno-1 cell-surface localization. Importantly, Eno-1R50me was essential for cancer cell motility since the replacement of Eno-1 R50 by lysine or the suppression of PRMT 5 activity diminished Eno-1-triggered cell invasion.

CONCLUSIONS

LPS-triggered Eno-1R50me enhances Eno-1 cell surface levels and thus potentiates the invasive properties of cancer cells. Strategies to target Eno-1R50me may offer novel therapeutic approaches to attenuate tumor metastasis in cancer patients.

Analysis of multiple components involved in the interaction between Cryptococcus neoformans and Acanthamoeba castellanii

Cryptococcus neoformans is an environmental fungus that can cause lethal meningoencephalitis in immunocompromised individuals. The mechanisms by which environmental microbes become pathogenic to mammals are still obscure, but different studies suggest that fungal virulence evolved from selection imposed by environmental predators. The soil-living Acanthamoeba castellanii is a well-known predator of C. neoformans. In this work, we evaluated the participation of C. neoformans virulence-associated structures in the interaction of fungal cells with A. castellanii. Fungal extracellular vesicles (EVs) and the polysaccharide glucuronoxylomannan (GXM) were internalized by A. castellanii with no impact on the viability of amoebal cells. EVs, but not free GXM, modulated antifungal properties of A. castellanii by inducing enhanced yeast survival. Phagocytosis of C. neoformans by amoebal cells and the pathogenic potential in a Galleria mellonella model were not affected by EVs, but previous interactions with A. castellanii rendered fungal cells more efficient in killing this invertebrate host. This observation was apparently associated with marked amoeba-induced changes in surface architecture and increased resistance to both oxygen- and nitrogen-derived molecular species. Our results indicate that multiple components with the potential to impact pathogenesis are involved in C. neoformans environmental interactions.

Preparation of an Oral Vaccine by Proteome Analysis and Molecular Display Technology

In recent years, genetic engineering and protein expression technologies have promoted the development of recombinant protein vaccines. To accelerate the development of efficient vaccines for mycosis, screening candidate antigens, and determining the optimal route of administration are indispensable steps. Two methods for identifying novel antigens and producing antigens specific to Candida albicans, as a model causative pathogen of mycosis, are discussed in this chapter. Specifically, the application of liquid chromatography/tandem mass spectrometry using a long monolithic column for proteome analysis to identify virulence factors of C. albicans, followed by molecular display technology to produce an oral vaccine using antigens found by the proteomic study, is described.

Virus Binding and Internalization Assay for Adeno-associated Virus

The binding and internalization of adeno-associated virus (AAV) is an important determinant of viral infectivity and tropism. The ability to dissect these two tightly connected cellular processes would allow better understanding and provide insight on virus entry and trafficking. In the following protocol, we describe a quantitative PCR (qPCR) based method to determine the amount of vector bound to the cell surface and the amount of subsequent virus internalization based on viral genome quantification. This protocol is optimized for studying AAV. Nevertheless, it can serve as a backbone for studying other viruses with careful modification.

Glycolipid substrates for ABC transporters required for the assembly of bacterial cell-envelope and cell-surface glycoconjugates

Glycoconjugates, molecules that contain sugar components, are major components of the cell envelopes of bacteria and cover much of their exposed surfaces. These molecules are involved in interactions with the surrounding environment and, in pathogens, play critical roles in the interplay with the host immune system. Despite the remarkable diversity in glycoconjugate structures, most are assembled by glycosyltransferases that act on lipid acceptors at the cytosolic membrane. The resulting glycolipids are then transported to the cell surface in processes that frequently begin with ATP-binding cassette transporters. This review summarizes current understanding of the structure and biosynthesis of glycolipid substrates and the structure and functions of their transporters. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

Hemagglutinin of influenza A virus binds specifically to cell surface nucleolin and plays a role in virus internalization

The hemagglutinin (HA) protein of influenza A virus initiates cell entry by binding to sialic acids on target cells. In the current study, we demonstrated that in addition to sialic acids, influenza A/Puerto Rico/8/34 H1N1 (PR8) virus HA specifically binds to cell surface nucleolin (NCL). The interaction between HA and NCL was initially revealed with virus overlay protein binding assay (VOPBA) and subsequently verified with co-immunoprecipitation. Importantly, inhibiting cell surface NCL with NCL antibody, blocking PR8 viruses with purified NCL protein, or depleting endogenous NCL with siRNA all substantially reduced influenza virus internalization. We further demonstrated that NCL was a conserved cellular factor required for the entry of multiple influenza A viruses, including H1N1, H3N2, H5N1, and H7N9. Overall, our findings identified a novel role of NCL in influenza virus life cycle and established NCL as one of the host cell surface proteins for the entry of influenza A virus.

CSF-1 receptor signalling is governed by pre-requisite EHD1 mediated receptor display on the macrophage cell surface

Colony stimulating factor-1 receptor (CSF-1R), a receptor tyrosine kinase (RTK), is the master regulator of macrophage biology. CSF-1 can bind CSF-1R resulting in receptor activation and signalling essential for macrophage functions such as proliferation, differentiation, survival, polarization, phagocytosis, cytokine secretion, and motility. CSF-1R activation can only occur after the receptor is presented on the macrophage cell surface. This process is reliant upon the underlying macrophage receptor trafficking machinery. However, the mechanistic details governing this process are incompletely understood. C-terminal Eps15 Homology Domain-containing (EHD) proteins have recently emerged as key regulators of receptor trafficking but have not yet been studied in the context of macrophage CSF-1R signalling. In this manuscript, we utilize primary bone-marrow derived macrophages (BMDMs) to reveal a novel function of EHD1 as a regulator of CSF-1R abundance on the cell surface. We report that EHD1-knockout (EHD1-KO) macrophages cell surface and total CSF-1R levels are significantly decreased. The decline in CSF-1R levels corresponds with reduced downstream macrophage functions such as cell proliferation, migration, and spreading. In EHD1-KO macrophages, transport of newly synthesized CSF-1R to the macrophage cell surface was reduced and was associated with the shunting of the receptor to the lysosome, which resulted in receptor degradation. These findings reveal a novel and functionally important role for EHD1 in governing CSF-1R signalling via regulation of anterograde transport of CSF-1R to the macrophage cell surface.

Surface proteome analysis identifies platelet derived growth factor receptor-alpha as a critical mediator of transforming growth factor-beta-induced collagen secretion

Fibroblasts are extracellular matrix-producing cells in the lung. Fibroblast activation by transforming growth factor-beta leads to myofibroblast-differentiation and increased extracellular matrix deposition, a hallmark of pulmonary fibrosis. While fibroblast function with respect to migration, invasion, and extracellular matrix deposition has been well-explored, little is known about the surface proteome of lung fibroblasts in general and its specific response to fibrogenic growth factors, in particular transforming growth factor-beta. We thus performed a cell-surface proteome analysis of primary human lung fibroblasts in presence/absence of transforming growth factor-beta, followed by characterization of our findings using FACS analysis, Western blot, and siRNA-mediated knockdown experiments. We identified 213 surface proteins significantly regulated by transforming growth factor-beta, platelet derived growth factor receptor-alpha being one of the top down-regulated proteins. Transforming growth factor beta-induced downregulation of platelet derived growth factor receptor-alpha induced upregulation of platelet derived growth factor receptor-beta expression and phosphorylation of Akt, a downstream target of platelet derived growth factor signaling. Importantly, collagen type V expression and secretion was strongly increased after forced knockdown of platelet derived growth factor receptor-alpha, an effect that was potentiated by transforming growth factor-beta. We therefore show previously underappreciated cross-talk of transforming growth factor-beta and platelet derived growth factor signaling in human lung fibroblasts, resulting in increased extracellular matrix deposition in a platelet derived growth factor receptor-alpha dependent manner. These findings are of particular importance for the treatment of lung fibrosis patients with high pulmonary transforming growth factor-beta activity.

Specific binding of a naturally occurring amyloidogenic fragment of Streptococcus mutans adhesin P1 to intact P1 on the cell surface characterized by solid state NMR spectroscopy

The P1 adhesin (aka Antigen I/II or PAc) of the cariogenic bacterium Streptococcus mutans is a cell surface-localized protein involved in sucrose-independent adhesion and colonization of the tooth surface. The immunoreactive and adhesive properties of S. mutans suggest an unusual functional quaternary ultrastructure comprised of intact P1 covalently attached to the cell wall and interacting with non-covalently associated proteolytic fragments thereof, particularly the ~57-kDa C-terminal fragment C123 previously identified as Antigen II. S. mutans is capable of amyloid formation when grown in a biofilm and P1 is among its amyloidogenic proteins. The C123 fragment of P1 readily forms amyloid fibers in vitro suggesting it may play a role in the formation of functional amyloid during biofilm development. Using wild-type and P1-deficient strains of S. mutans, we demonstrate that solid state NMR (ssNMR) spectroscopy can be used to (1) globally characterize cell walls isolated from a Gram-positive bacterium and (2) characterize the specific binding of heterologously expressed, isotopically-enriched C123 to cell wall-anchored P1. Our results lay the groundwork for future high-resolution characterization of the C123/P1 ultrastructure and subsequent steps in biofilm formation via ssNMR spectroscopy, and they support an emerging model of S. mutans colonization whereby quaternary P1-C123 interactions confer adhesive properties important to binding to immobilized human salivary agglutinin.

An update on cell surface proteins containing extensin-motifs

In recent years it has become clear that there are several molecular links that interconnect the plant cell surface continuum, which is highly important in many biological processes such as plant growth, development, and interaction with the environment. The plant cell surface continuum can be defined as the space that contains and interlinks the cell wall, plasma membrane and cytoskeleton compartments. In this review, we provide an updated view of cell surface proteins that include modular domains with an extensin (EXT)-motif followed by a cytoplasmic kinase-like domain, known as PERKs (for proline-rich extensin-like receptor kinases); with an EXT-motif and an actin binding domain, known as formins; and with extracellular hybrid-EXTs. We focus our attention on the EXT-motifs with the short sequence Ser-Pro(3-5), which is found in several different protein contexts within the same extracellular space, highlighting a putative conserved structural and functional role. A closer understanding of the dynamic regulation of plant cell surface continuum and its relationship with the downstream signalling cascade is a crucial forthcoming challenge.

Oral Vaccine Development by Molecular Display Methods Using Microbial Cells

Oral vaccines are easier to administer than injectable vaccines. To induce an adequate immune response using vaccines, antigenic proteins are usually combined with adjuvant materials. This chapter presents methodologies for the design of oral vaccines using molecular display technology. In molecular display technology, antigenic proteins are displayed on a microbial cell surface with adjuvant ability. This technology would provide a quite convenient process to produce oral vaccines when the DNA sequence of an efficient antigenic protein is available. As an example, oral vaccines against candidiasis were introduced using two different molecular display systems with Saccharomyces cerevisiae and Lactobacillus casei.

Cell Surface Interference with Plasma Membrane and Transport Processes in Yeasts

The wall of the yeast Saccharomyces cerevisiae is a shell of about 120 nm thick, made of two distinct layers, which surrounds the cell. The outer layer is constituted of highly glycosylated proteins and the inner layer is composed of β-glucan and chitin. These two layers are interconnected through covalent linkages leading to a supramolecular architecture that is characterized by physical and chemical properties including rigidity, porosity and biosorption. The later property results from the presence of highly negative charged phosphate and carboxylic groups of the cell wall proteins, allowing the cell wall to act as an efficient barrier to metals ions, toxins and organic compounds. An intimate connection between cell wall and plasma membrane is indicated by the fact that changes in membrane fluidity results in change in cell wall nanomechanical properties. Finally, cell wall contributes to transport processes through the use of dedicated cell wall mannoproteins, as it is the case for Fit proteins implicated in the siderophore-iron bound transport and the Tir/Dan proteins family in the uptake of sterols.

Bacterial Surfaces: Front Lines in Host-Pathogen Interaction

All bacteria are bound by at least one membrane that acts as a barrier between the cell's interior and the outside environment. Surface components within and attached to the cell membrane are essential for ensuring that the overall homeostasis of the cell is maintained. However, many surface components of the bacterial cell also have an indispensable role mediating interactions of the bacteria with their immediate environment and as such are essential to the pathogenesis of infectious disease. During the course of an infection, bacterial pathogens will encounter many different ecological niches where environmental conditions such as salinity, temperature, pH, and the availability of nutrients fluctuate. It is the bacterial cell surface that is at the front-line of these host-pathogen interactions often protecting the bacterium from hostile surroundings but at the same time playing a critical role in the adherence to host tissues promoting colonization and subsequent infection. To deal effectively with the changing environments that pathogens may encounter in different ecological niches within the host many of the surface components of the bacterial cell are subject to phenotypic variation resulting in heterogeneous subpopulations of bacteria within the clonal population. This dynamic phenotypic heterogeneity ensures that at least a small fraction of the population will be adapted for a particular circumstance should it arise. Diversity within the clonal population has often been masked by studies on entire bacterial populations where it was often assumed genes were expressed in a uniform manner. This chapter, therefore, aims to highlight the non-uniformity in certain cell surface structures and will discuss the implication of this heterogeneity in bacterial-host interaction. Some of the recent advances in studying bacterial surface structures at the single cell level will also be reviewed.

The adsorption potential and recovery of thallium using green micro-algae from eutrophic water sources

Thallium (Tl) is a highly volatile and toxic heavy metal regarded to cause pollution even at very low concentrations of several parts per million. Despite the extremely high risk of Tl in the environment, limited information on removal/recovery exists. The study focussed on the use of green algae to determine the sorption potential and recovery of Tl. From the study, removal efficiency was achieved at 100% for lower concentrations of ≥150 mg/L of Tl. At higher concentrations in a range of 250-500 mg/L, the performance of algae was still higher with sorption capacity (qmax) between 830 and 1000 mg/g. Generally, Chlorella vulgaris was the best adsorbent with a high qmax and lower affinity of 1000 mg/g and 1.11 L/g, respectively. When compared to other studies on Tl adsorption, the tested algae showed a better qmax than most adsorbents. The kinetic studies showed better correlation co-efficient of ≤0.99 for Pseudo-second order model than the first order model. Recovery was achieved highest for C. vulgaris using nitric acid at 93.3%. The strongest functional groups responsible for Tl binding on the algal cell wall were carboxyl and phenols. Green algae from freshwater bodies showed significant potential for Tl removal/recovery from industrial wastewater.

Endocytic membrane turnover at the leading edge is driven by a transient interaction between Cdc42 and GRAF1

Changes in cell morphology require coordination of plasma membrane turnover and cytoskeleton dynamics, processes that are regulated by Rho GTPases. Here, we describe how a direct interaction between the Rho GTPase Cdc42 and the GTPase-activating protein (GAP) GRAF1 (also known as ARHGAP26), facilitates rapid cell surface turnover at the leading edge. Both Cdc42 and GRAF1 were required for fluid-phase uptake and regulated the generation of transient GRAF1-coated endocytic carriers, which were distinct from clathrin-coated vesicles. GRAF1 was found to transiently assemble at discrete Cdc42-enriched punctae at the plasma membrane, resulting in a corresponding decrease in the microdomain association of Cdc42. However, Cdc42 captured in its active state was, through a GAP-domain-mediated interaction, localised together with GRAF1 on accumulated internal structures derived from the cell surface. Correlative fluorescence and electron tomography microscopy revealed that these structures were clusters of small membrane carriers with defective endosomal processing. We conclude that a transient interaction between Cdc42 and GRAF1 drives endocytic turnover and controls the transition essential for endosomal maturation of plasma membrane internalised by this mechanism.

Summary: A transient interaction between Cdc42 and GRAF1 drives endocytic turnover at the leading edge, and controls the transition essential for endosomal maturation of plasma membrane internalised by this mechanism.

Rapid analysis of cell surface N-glycosylation from living cells using mass spectrometry

Cell surfaces are covered with a dense carbohydrate layer referred to as the glycocalyx. Because different cell types express different glycan signatures, it is of paramount importance to have robust methods to analyze the glycome of living cells. To achieve this, a common procedure involves cell lysis and extraction of membrane (glyco)proteins and yields a major proportion of high-mannose N-glycans that most likely stem from intracellular proteins derived from the ER. Using HEK 293 cells as a model system, we developed a reproducible, sensitive, and fast method to profile surface N-glycosylation from living cells. We directly released glycopeptides from cell surfaces through tryptic digestion of freshly harvested and vital cells, thereby improving the detection and quantification of complex-type N-glycans by increasing their relative amount from 14 to 85%. It was also possible to detect 25 additional structures in HEK 293, 48 in AGE1.HN, 42 in CHO-K1, and 51 in Hep G2 cells. The additional signals provided deeper insight into cell-type-specific N-glycan features such as antennarity, fucosylation, and sialylation. Thus, this protocol, which can potentially be applied to any cells, will be useful in the fields of glycobiotechnology and biomarker discovery.

Candida albicans Niche Specialization: Features That Distinguish Biofilm Cells from Commensal Cells

The fungus Candida albicans is a frequent commensal colonizer of the human gastrointestinal (GI) tract, but is also an opportunistic pathogen. This review explores features that distinguish the colonizing and pathogenic forms of C. albicans. Candida albicans in a biofilm is used as an example of a pathogenic form of the organism, because biofilms are a common feature of device-associated C. albicans infections. Biofilms (complex, sessile communities of cells) have been the subject of several large-scale gene expression studies. Biofilms and commensal C. albicans colonizing the murine GI tract show a variety of differentially expressed genes. Cell surface proteins encoded by these differentially expressed genes are especially attractive as targets for new clinical prevention, diagnosis, or treatment tools that are specific for C. albicans in its pathogenic biofilm state.

TLR9 expressed on plasma membrane acts as a negative regulator of human B cell response

Toll-like receptors (TLRs) are positioned at the interface between innate and adaptive immunity. Unlike others, those such as TLR9, that recognize nucleic acids, are confined to the endosomal compartment and are scarce on the cell surface. Here, we present evidence for TLR9 expression on the plasma membrane of B cells. In contrast to endosomal TLR9, cell surface TLR9 does not bind CpG-B oligodeoxynucleotides. After B cell-receptor (BCR) stimulation, TLR9 was translocated into lipid rafts with the BCR, suggesting that it could serve as a co-receptor for BCR. Nevertheless, stimulation of B cells with anti-TLR9 antibodies did not modify the BCR-induced responses despite up-regulation of tyrosine phosphorylation of proteins. However, CpG-B activation of B cells, acting synergistically with BCR signals, was inhibited by anti-TLR9 stimulation. Induction of CD25 expression and proliferation of B cells were thus down-regulated by the engagement of cell surface TLR9. Overall, our results indicate that TLR9 expressed on the plasma membrane of B cells might be a negative regulator of endosomal TLR9, and could provide a novel control by which activation of autoreactive B cells is restrained.