A sweet new role for LCP enzymes in protein glycosylation

The peptidoglycan that surrounds Gram‐positive bacteria is affixed with a range of macromolecules that enable the microbe to effectively interact with its environment. Distinct enzymes decorate the cell wall with proteins and glycopolymers. Sortase enzymes covalently attach proteins to the peptidoglycan, while LytR‐CpsA‐Psr (LCP) proteins are thought to attach teichoic acid polymers and capsular polysaccharides. Ton‐That and colleagues have discovered a new glycosylation pathway in the oral bacterium Actinomyces oris in which sortase and LCP enzymes operate on the same protein substrate. The A. oris LCP protein has a novel function, acting on the cell surface to transfer glycan macromolecules to a protein, which is then attached to the cell wall by a sortase. The reactions are tightly coupled, as elimination of the sortase causes the lethal accumulation of glycosylated protein in the membrane. Since sortase enzymes are attractive drug targets, this novel finding may provide a convenient cell‐based tool to discover inhibitors of this important enzyme family.     

Construction of a Novel <Emphasis Type="Italic">Pichia pastoris</Emphasis> Cell-Surface Display System Based on the Cell Wall Protein Pir1

A novel system based on Pir1 from Saccharomyces cerevisiae was developed for cell-surface display of heterologous proteins in Pichia pastoris with the alpha-factor secretion signal sequence. As a model protein, enhanced green fluorescence protein (EGFP) was fused to the N-terminal of the mature peptide of Pir1 (Pir1-a). The expression of fusion protein EGFP-Pir1-a was irregular throughout the P. pastoris cell surface per detection by confocal laser scanning microscopy. A truncated sequence containing only the internal repetitive sequences of Pir1-a (Pir1-b) was used as a new anchor protein in further study. The fusion protein EGFP-Pir1-b was expressed uniformly on the cell surface. The fluorescence intensity of the whole yeast was measured by spectrofluorometer. Western blot confirmed that the fusion proteins were released from cell walls after mild alkaline treatment. The results indicate that a Pir1-based system can express proteins on the surface of P. pastoris and that the fusion proteins do not affect the manner in which Pir1 attaches to the cell wall. The repetitive sequences of Pir1 are required for cell wall retention, and the C-terminal sequence contributes to the irregular distribution of fusion proteins in P. pastoris.     

A cell wall protein (YqgA) is genetically related to the cell wall-degrading dl-endopeptidases in Bacillus subtilis

The Gram-positive bacterium Bacillus subtilis has a thick cell wall. The cell wall contains various proteins, both for secretion and for peptidoglycan (PG) maintenance. Penicillin-binding proteins for PG synthesis, PG hydrolases (autolysins), and regulator proteins for the autolysins are the known components of the PG maintenance system. YqgA was identified as an abundant protein attached to the cell wall of B. subtilis through a proteomics analysis. The YqgA protein was localized at cell division sites during the transition period between the exponential and the stationary phases. YqgA localization was affected by mutations in the dl-endopeptidases (DLEPases), which are the autolysins involved in cell morphogenesis. Furthermore, yqgA mutations on a background of defective DLEPases led to delays in cell growth and cell morphological changes. These results demonstrate that yqgA is genetically related to the genes encoding DLEPases involved in cell morphogenesis.     

A putative catabolite-repressed cell wall protein from the mycoparasitic fungus Trichoderma harzianum

A cDNA clone encoding a putative cell wall protein (Qid3) was isolated from a library prepared from chitin-induced mRNA in cultures of the mycoparasitic fungus Trichoderma harzianum. The predicted 14 kDa protein shows a potential signal peptide, several hydrophobic domains and certain motifs that are structurally similar to proline-rich and glycine-rich plant cell wall proteins. Expression of the qid3 gene is derepressed in the absence of glucose. When introduced in yeast, qid3 expression causes cell division arrest into cytokinesis and cell separation, probably due to its cell wall localization.     

Domain structure and conserved epitopes of Sfb protein, the fibronectin-binding adhesin of Streptococcus pyogenes

Streptococcus pyogenes expresses a fibronectin-binding surface protein (Sfb protein) which mediates adherence to human epithelial cells. The nucleotide sequence of the sfb gene was determined and the primary sequence of the Sfb protein was analysed. The protein consists of 638 amino acids and comprises five structurally distinct domains. The protein starts with an N-terminal signal peptide followed by an aromatic domain. The central part of the protein is formed by four proline-rich repeats which are flanked by non-repetitive spacer sequences. A second repeat region, consisting of four repeats that are distinct from the proline repeats and have been shown to form the fibronectin-binding domain, is located in the Cterminal part of the protein. The protein ends with a typical cell wall and membrane anchor region. Comparative sequence analysis of the N-terminal aromatic domain revealed similarities with carbohydrate-binding sites of other proteins. The proline repeat region of the Sfb protein shares characteristic features with proline-rich repeats of functionally distinct surface proteins from pathogenic Gram-positive cocci. Immunoelectron microscopy revealed an even distribution of the fibronectin-binding domain of Sfb protein on the surface of streptococcal cells. Analyses of 38 sfb genes originating from different S. pyogenes isolates revealed primary sequence variability in regions coding for the N-termini of mature Sfb proteins, whereas sequences coding for the central and C-terminal repeats were highly conserved. The repeat sequences are postulated to act as target sites for intragenic recombination events that result in variable numbers of repeats within the different sfb genes. A model of the Sfb protein is presented.     

Mapping HA‐tagged protein at the surface of living cells by atomic force microscopy

Single‐molecule force spectroscopy using atomic force microscopy (AFM) is more and more used to detect and map receptors, enzymes, adhesins, or any other molecules at the surface of living cells. To be specific, this technique requires antibodies or ligands covalently attached to the AFM tip that can specifically interact with the protein of interest. Unfortunately, specific antibodies are usually lacking (low affinity and specificity) or are expensive to produce (monoclonal antibodies). An alternative strategy is to tag the protein of interest with a peptide that can be recognized with high specificity and affinity with commercially available antibodies. In this context, we chose to work with the human influenza hemagglutinin (HA) tag (YPYDVPDYA) and labeled two proteins: covalently linked cell wall protein 12 (Ccw12) involved in cell wall remodeling in the yeast Saccharomyces cerevisiae and the β2‐adrenergic receptor (β2‐AR), a G protein‐coupled receptor (GPCR) in higher eukaryotes. We first described the interaction between HA antibodies, immobilized on AFM tips, and HA epitopes, immobilized on epoxy glass slides. Using our system, we then investigated the distribution of Ccw12 proteins over the cell surface of the yeast S. cerevisiae. We were able to find the tagged protein on the surface of mating yeasts, at the tip of the mating projections. Finally, we could unfold multimers of β2‐AR from the membrane of living transfected chinese hamster ovary cells. This result is in agreement with GPCR oligomerization in living cell membranes and opens the door to the study of the influence of GPCR ligands on the oligomerization process. Copyright © 2014 John Wiley & Sons, Ltd.     

Proteolytic cleavage and cell wall anchoring at the LPXTG motif of surface proteins in Gram-positive bacteria

Many surface proteins are thought to be anchored to the cell wall of Gram-positive bacteria via their C-terminus. Cell wall anchoring requires a specific sorting signal, normally located at the predicted C-terminus of surface proteins. Here we show that when placed into the middle of a polypeptide chain, the sorting signal causes the specific cleavage of the precursor as well as the cell wall anchoring of its N- terminal fragment, while the C-terminal fragment remains within the cytoplasm. N-terminal sequencing of the C-terminal cleavage fragment suggests that the cleavage site is located between threonine (T) and glycine (G) of the LPXTG motif, the signature sequence of cell wall sorting signals. All surface proteins harbouring an LPXTG sequence motif may therefore be cleaved and anchored by a universal mechanism. We also propose a novel hypothesis for the cell wall linkage of surface proteins in Gram-positive bacteria.     

An oat coleoptile wall protein that induces wall extension in vitro and that is antigenically related to a similar protein from cucumber hypocotyls

Plant cell walls expand considerably during cell enlargement, but the biochemical reactions leading to wall expansion are unknown. McQueen-Mason et al. (1992, Plant Cell 4, 1425) recently identified two proteins from cucumber (Cucumis sativus L.) that induced extension in walls isolated from dicotyledons, but were relatively ineffective on grass coleoptile walls. Here we report the identification and partial characterization of an oat (Avena sativa L.) coleoptile wall protein with similar properties. The oat protein has an apparent molecular mass of 29 kDa as revealed by sodium dodecyl sulfate-polyacrylamide gel eletrophoresis. Activity was optimal between pH 4.5 and 5.0, which makes it a suitable candidate for acid growth responses of plant cell walls. The oat protein induced extension in walls from oat coleoptiles, cucumber hypocotyls and pea (Pisum sativum L.) epicotyls and was specifically recognized by an antibody raised against the 29-kDa wall-extension-inducing protein from cucumber hypocotyls. Contrary to the situation in cucumber walls, the acid-extension response in heat-inactivated oat walls was only partially restored by oat or cucumber wall-extension proteins. Our results show that an antigenically conserved protein in the walls of cucumber and oat seedlings is able to mediate a form of acid-induced wall extension. This implies that dicotyledons and grasses share a common biochemical mechanism for at least part of acid-induced wall extensions, despite the significant differences in wall composition between these two classes of plants.Abbreviations ConA concanavalin A - CM carboxymethyl - DEAE diethylaminoethyl - DTT dithiothreitol - Ex29 29-kDa expansin     

External cell surface protein phosphorylation in normal and Rous sarcoma virus transformed chick embryo fibroblasts

Normal and Rous sarcoma virus (RSV)-transformed chick embryo fibroblasts growing on plastic dishes were incubated with ATP (γ³²P) in situ to detect external cell surface protein kinase activity. Under the conditions employed, ³²P was incorporated exclusively into proteins, specifically those at the external cell surface, as radioactivity was removed by tryspin treatment of labeled whole cells. In addition, exogenous histones were phosphorylated when added to the reaction mixture. Cyclic nucleotides had virtually no effect on ³²P incorporation, suggesting that little or no cyclic nucleotide-dependent protein kinase activity was present at the external cell surface. Cell surface protein kinase activity was higher in transformed than in normal cells, and, using a temperature-sensitive RSV src mutant, this difference was shown to be transformation-specific. Several differences were observed in the cell surface proteins phosphoryllated in normal and transformed cells and at least two of these were transformation-specific. These data suggest that changes in external cell surface protein physphorylation are associated with RSV transformation and thus could play a role in the formation of the transformed cell phenotype.     

Expression of an apoplast‐localized BURP‐domain protein from soybean (GmRD22) enhances tolerance towards abiotic stress

The BURP‐domain protein family comprises a diverse group of plant‐specific proteins that share a conserved BURP domain at the C terminus. However, there have been only limited studies on the functions and subcellular localization of these proteins. Members of the RD22‐like subfamily are postulated to associate with stress responses due to the stress‐inducible nature of some RD22‐like genes. In this report, we used different transgenic systems (cells and in planta) to show that the expression of a stress‐inducible RD22‐like protein from soybean (GmRD22) can alleviate salinity and osmotic stress. We also performed detailed microscopic studies using both fusion proteins and immuno‐electron microscopic techniques to demonstrate the apoplast localization of GmRD22, for which the BURP domain is a critical determinant of the subcellular localization. The apoplastic GmRD22 interacts with a cell wall peroxidase and the ectopic expression of GmRD22 in both transgenic Arabidopsis thaliana and transgenic rice resulted in increased lignin production when subjected to salinity stress. It is possible that GmRD22 regulates cell wall peroxidases and hence strengthens cell wall integrity under such stress conditions.     

Construction of an <Emphasis Type="Italic">Aspergillus oryzae</Emphasis> cell-surface display system using a putative GPI-anchored protein

A novel cell-surface display system was constructed in Aspergillus oryzae. Each of the five genes encoding the putative cell-wall-localized protein from the A. oryzae genome was cloned and these cell-surface anchor functions were examined by fusion to the C-terminal of the green fluorescent protein (GFP). Using the MP1 and CWP proteins as anchor proteins, GFP signals were strongly observed on the cell surface of recombinant A. oryzae. When these proteins were used as anchor proteins for cell-surface display of β-glucosidase from A. oryzae, enzyme activity was detected on the cell surface. In particular, β-glucosidase activity of recombinant A. oryzae using MP1, a putative glycosylphosphatidylinositol (GPI) anchor protein was higher than CWP. Based on these results, it was concluded that the MP1 protein can act as a GPI-anchor protein in A. oryzae, and the proposed cell-surface display system using MP1 allows for the display of heterogeneous and endogenous proteins.     

Characterization of mutations in divlB of Bacillus subtilis and cellular localization of the DivlB protein

Four temperature-sensitive mutations in the divlB gene of Bacillus subtilis have been localized to the region corresponding to the C-terminal half of the 263-residue DivlB protein. Antiserum was raised to the 80%C-terminal portion lying on one side of a putative transmembrane (hydrophobic) segment, and used to examine aspects of the nature and localization of the DivIB protein in the cell. A single DivIB species of a size equal to the full-length protein encoded by the divIB gene was detected in wild-type cells. Cell fractionation studies established that DivIB is associated preferentially with the cell envelope (membrane plus cell wall), with approximately 50% being released into solution upon treatment of cells with lysozyme under conditions that yield protoplasts. Of the remaining 50%, approximately half remained firmly associated with the membrane fraction. On the basis of the‘positive-inside rule’of von Heijne (1986) it is suggested that the topology of membrane-bound DivlB is such that the long C-terminal portion is directed to the outside and the smaller N-terminal portion to the inside of the cell. DivlB in protoplasts was rapidly degraded by proteinase K under conditions where there was no general proteolysis of the cytoplasmic proteins. This is consistent with its absence from the cytoplasm, and with the predicted membrane topology. Septum positioning in a divIB null mutant, which grows as filaments at temperatures of 30^°C and below, was found to be normal. It appears that DivlB is needed for achieving the appropriate rate of initiation of septum formation at normal division sites. It is proposed that the C-terminal portion of DivlB, localized on the exterior surface of the membrane and in juxtaposition to the peptidoglycan, normally interacts with another protein (or proteins) to initiate septum formation.     

Characterization of the Sclerotinia sclerotiorum cell wall proteome

We used a proteomic analysis to identify cell wall proteins released from Sclerotinia sclerotiorum hyphal and sclerotial cell walls via a trifluoromethanesulfonic acid (TFMS) digestion. Cell walls from hyphae grown in Vogel's glucose medium (a synthetic medium lacking plant materials), from hyphae grown in potato dextrose broth and from sclerotia produced on potato dextrose agar were used in the analysis. Under the conditions used, TFMS digests the glycosidic linkages in the cell walls to release intact cell wall proteins. The analysis identified 24 glycosylphosphatidylinositol (GPI)‐anchored cell wall proteins and 30 non‐GPI‐anchored cell wall proteins. We found that the cell walls contained an array of cell wall biosynthetic enzymes similar to those found in the cell walls of other fungi. When comparing the proteins in hyphal cell walls grown in potato dextrose broth with those in hyphal cell walls grown in the absence of plant material, it was found that a core group of cell wall biosynthetic proteins and some proteins associated with pathogenicity (secreted cellulases, pectin lyases, glucosidases and proteases) were expressed in both types of hyphae. The hyphae grown in potato dextrose broth contained a number of additional proteins (laccases, oxalate decarboxylase, peroxidase, polysaccharide deacetylase and several proteins unique to Sclerotinia and Botrytis) that might facilitate growth on a plant host. A comparison of the proteins in the sclerotial cell wall with the proteins in the hyphal cell wall demonstrated that sclerotia formation is not marked by a major shift in the composition of cell wall protein. We found that the S. sclerotiorum cell walls contained 11 cell wall proteins that were encoded only in Sclerotinia and Botrytis genomes.     

Changes in apoplast protein pattern suggest an early role of cell wall structure remodelling in flagellin-triggered basal immunity

The leaf apoplast is a dynamic compartment in contact with plant pathogenic bacteria after infection. Among the very first interaction events is the receptor-mediated perception of bacterial surface molecules such as flagellin or other conserved microbe-associated molecular patterns (MAMPs). Apoplast proteins likely play a role in basal resistance (BR) or pattern-triggered immunity (PTI). Here, a proteomic approach was carried out on water soluble — potentially the most mobile — apoplast proteins from flagellin-treated tobacco (Nicotiana tabacum) leaves. As the quickness of BR/PTI seems crucial for its efficacy, samples were taken as early as 2.5 and 7 h post inoculation. Proteins were separated by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and identified by liquid chromatography tandem mass spectrometry (LC-MS/MS). Forty-nine different proteins from 28 protein spots changed in their density compared to the water-inoculated control. Eleven protein spots appeared de novo in response to EBR induction. There are glycohydrolases and redox-active proteins besides pathogenesis-related proteins among them, predicting plant cell wall structural modifications and more direct antimicrobial effectors as earliest changes related to BR/PTI.     

Protein cell surface display in Gram-positive bacteria: from single protein to macromolecular protein structure

In the course of evolution, Gram-positive bacteria, defined here as prokaryotes from the domain Bacteria with a cell envelope composed of one biological membrane (monodermita) and a cell wall composed at least of peptidoglycan and covalently linked teichoic acids, have developed several mechanisms permitting to a cytoplasmic synthesized protein to be present on the bacterial cell surface. Four major types of cell surface displayed proteins are currently recognized: (i) transmembrane proteins, (ii) lipoproteins, (iii) LPXTG-like proteins and (iv) cell wall binding proteins. The subset of proteins exposed on the bacterial cell surface, and thus interacting with extracellular milieu, constitutes the surfaceome. Here, we review exhaustively the current molecular mechanisms involved in protein attachment within the cell envelope of Gram-positive bacteria, from single protein to macromolecular protein structure.     

Cell wall of Thermoanaerobacterium thermosulfurigenes EM1: isolation of its components and attachment of the xylanase XynA

Thermoanaerobacterium thermosulfurigenes EM1 has a gram-positive type cell wall completely covered by a surface layer (S-layer) with hexagonal lattice symmetry. The components of the cell envelope were isolated, and the S-layer protein was purified and characterized. S-layer monomers assembled in vitro into sheets with the same hexagonal symmetry as in vivo. Monosaccharide analysis revealed that the S-layer is associated with fucose, rhamnose, mannosamine, glucosamine, galactose, and glucose. The N-terminal 31 amino acid residues of the S-layer protein showed significant similarity to SLH (S-layer homology) domains found in S-layer proteins of different bacteria and in the exocellular enzymes pullulanase, polygalacturonate hydrolase, and xylanase of T. thermosulfurigenes EM1. The xylanase from T. thermosulfurigenes EM1 was copurified with the S-layer protein during isolation of cell wall components. Since SLH domains of some structural proteins have been shown to anchor these proteins noncovalently to the cell envelope, we propose a common anchoring mechanism for the S-layer protein and exocellular enzymes via their SLH domains in the peptidoglycan-containing layer of T. thermosulfurigenes EM1. Received: 23 October 1998 / Accepted: 21 December 1998     

Heterologous protein display on the cell surface of lactic acid bacteria mediated by the s-layer protein

Background

Previous studies have revealed that the C-terminal region of the S-layer protein from Lactobacillus is responsible for the cell wall anchoring, which provide an approach for targeting heterologous proteins to the cell wall of lactic acid bacteria (LAB). In this study, we developed a new surface display system in lactic acid bacteria with the C-terminal region of S-layer protein SlpB of Lactobacillus crispatus K2-4-3 isolated from chicken intestine.

Results

Multiple sequence alignment revealed that the C-terminal region (LcsB) of Lb. crispatus K2-4-3 SlpB had a high similarity with the cell wall binding domains S_A and CbsA of Lactobacillus acidophilus and Lb. crispatus. To evaluate the potential application as an anchoring protein, the green fluorescent protein (GFP) or beta-galactosidase (Gal) was fused to the N-terminus of the LcsB region, and the fused proteins were successfully produced in Escherichia coli, respectively. After mixing them with the non-genetically modified lactic acid bacteria cells, the fused GFP-LcsB and Gal-LcsB were functionally associated with the cell surface of various lactic acid bacteria tested. In addition, the binding capacity could be improved by SDS pretreatment. Moreover, both of the fused proteins could simultaneously bind to the surface of a single cell. Furthermore, when the fused DNA fragment of gfp:lcsB was inserted into the Lactococcus lactis expression vector pSec:Leiss:Nuc, the GFP could not be secreted into the medium under the control of the nisA promoter. Western blot, in-gel fluorescence assay, immunofluorescence microscopy and SDS sensitivity analysis confirmed that the GFP was successfully expressed onto the cell surface of L. lactis with the aid of the LcsB anchor.

Conclusion

The LcsB region can be used as a functional scaffold to target the heterologous proteins to the cell surfaces of lactic acid bacteria in vitro and in vivo, and has also the potential for biotechnological application.

     

Cell surface display of highly pathogenic avian influenza virus hemagglutinin on the surface of Pichia pastoris cells using α‐agglutinin for production of oral vaccines

Yeast is an ideal organism to express viral antigens because yeast glycosylate proteins more similarly to mammals than bacteria. Expression of proteins in yeast is relatively fast and inexpensive. In addition to the convenience of production, for purposes of vaccination, yeast has been shown to have natural adjuvant activity making the expressed proteins more immunogenic when administered along with yeast cell wall components. Development of genetic systems to display foreign proteins on the surface of yeast via fusion to glycosylphosphatidylinositol‐anchored (GPI) proteins has further simplified the purification of recombinant proteins by not requiring harsh treatments for cellular lysis or protein purification. We have expressed the hemagglutinin protein from a highly pathogenic avian influenza (HPAI) virus [A/Egret/HK/757.2/02], subtype H5N1, on the surface of the yeast strain Pichia pastoris, as an anchored C‐terminal fusion with the Saccharomyces cerevisiae GPI‐anchored cell wall protein, α‐agglutinin. Surface expression of the hemagglutinin fusion protein was demonstrated by immunofluorescence microscopy. Functionally, the fusion protein retained hemagglutinin agglutinating activity, and oral vaccination with the yeast resulted in production of virus neutralizing antibodies. This study represents the first steps in the generation of a yeast‐based vaccine for protection against highly pathogenic strains of avian influenza. Published 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2010     

Atomic force microscopy reveals a dual collagen‐binding activity for the staphylococcal surface protein SdrF

Staphylococcus epidermidis causes nosocomial infections by colonizing and forming biofilms on indwelling medical devices. This process involves specific interactions between cell wall‐anchored (CWA) proteins and host proteins adsorbed onto the biomaterial. Here, we have explored the molecular forces by which the S. epidermidis CWA protein serine‐aspartate repeat protein F (SdrF) binds to type I collagen, by means of advanced atomic force microscopy (AFM) techniques. Using single‐cell force spectroscopy, we found that SdrF mediates bacterial adhesion to collagen‐coated substrates through both weak and strong bonds. Single‐molecule force spectroscopy demonstrated that these bonds involve the A and B regions of SdrF, thus revealing that the protein is capable of dual ligand‐binding activity. Both weak and strong bonds showed high dissociation rates, indicating they are much less stable than those formed by the well‐characterized ‘dock, lock and latch’ mechanism. Collectively, our results show that CWA proteins can bind to ligands by novel mechanisms. We anticipate that AFM will greatly contribute to the identification of novel binding partners and binding mechanisms in staphylococcal CWA proteins.