Lactobacillus S-layer Proteins Research Articles

The molecular architecture of Lactobacillus S-layer: Assembly and attachment to teichoic acids

S-layers are crystalline arrays found on bacterial and archaeal cells. Lactobacillus is a diverse family of bacteria known especially for potential gut health benefits. This study focuses on the S-layer proteins from Lactobacillus acidophilus and Lactobacillus amylovorus common in the mammalian gut. Atomic resolution structures of Lactobacillus S-layer proteins SlpA and SlpX exhibit domain swapping, and the obtained assembly model of the main S-layer protein SlpA aligns well with prior electron microscopy and mutagenesis data. The S-layer's pore size suggests a protective role, with charged areas aiding adhesion. A highly similar domain organization and interaction network are observed across the Lactobacillus genus. Interaction studies revealed conserved binding areas specific for attachment to teichoic acids. The structure of the SlpA S-layer and the suggested incorporation of SlpX as well as its interaction with teichoic acids lay the foundation for deciphering its role in immune responses and for developing effective treatments for a variety of infectious and bacteria-mediated inflammation processes, opening opportunities for targeted engineering of the S-layer or lactobacilli bacteria in general.

Identification and Characterization of Domains Responsible for Cell Wall Binding, Self-Assembly, and Adhesion of S-layer Protein from Lactobacillus acidophilus CICC 6074.

Lactobacillus S-layer protein (SLP) is a biologically active protein on the cell surface. To further elucidate the structures and functions of SLP in Lactobacillus acidophilus CICC 6074, this study was conducted to identify the functional domains of SLP which is responsible for cell wall anchoring, self-assembly, and adhesion. The gene (slpA) of L. acidophilus CICC 6074 SLP was amplified by polymerase chain reaction and speculated functional domains. Fusion proteins of C-terminal truncations from SLP were exogenously expressed in Escherichia coli BL21 (DE3). FITC-labeling N-terminal truncations of SLP were synthesized. The C-terminal domain was more likely to be the binding region, and the cell wall-anchored receptor of SLP was teichoic acid. Furthermore, N-terminal truncations could self-assemble to milk fat globule membrane polar lipid liposomes observed using a fluorescence microscope. Notably, SAN1 (region 32-55) of N-terminal truncations was mainly responsible for the adhesion of SLP to HT-29 cells. These results showed that SLP played a crucial role in the functions of L. acidophilus CICC 6074, which might be of significant reference value for future studies.

Lipoteichoic acid mediates binding of a Lactobacillus S-layer protein.

The Gram-positive lactic acid bacterium Lactobacillus buchneri CD034 is covered by a two-dimensional crystalline, glycoproteinaceous cell surface (S-) layer lattice. While lactobacilli are extensively exploited as cell surface display systems for applied purposes, questions about how they stick their cell wall together are remaining open. This also includes the identification of the S-layer cell wall ligand. In this study, lipoteichoic acid was isolated from the L. buchneri CD034 cell wall as a significant fraction of the bacterium’s cell wall glycopolymers, structurally characterized and analyzed for its potential to mediate binding of the S-layer to the cell wall. Combined component analyses and 1D- and 2D-nuclear magnetic resonance spectroscopy (NMR) revealed the lipoteichoic acid to be composed of on average 31 glycerol-phosphate repeating units partially substituted with α-d-glucose, and with an α-d-Galp(1→2)-α-d-Glcp(1→3)−1,2-diacyl-sn-Gro glycolipid anchor. The specificity of binding between the L. buchneri CD034 S-layer protein and purified lipoteichoic acid as well as their interaction force of about 45 pN were obtained by single-molecule force spectroscopy; this value is in the range of typical ligand–receptor interactions. This study sheds light on a functional implication of Lactobacillus cell wall architecture by showing direct binding between lipoteichoic acid and the S-layer of L. buchneri CD034.

Mucosal Immunogenicity of Genetically Modified Lactobacillus acidophilus Expressing an HIV-1 Epitope within the Surface Layer Protein.

Surface layer proteins of probiotic lactobacilli are theoretically efficient epitope-displaying scaffolds for oral vaccine delivery due to their high expression levels and surface localization. In this study, we constructed genetically modified Lactobacillus acidophilus strains expressing the membrane proximal external region (MPER) from human immunodeficiency virus type 1 (HIV-1) within the context of the major S-layer protein, SlpA. Intragastric immunization of mice with the recombinants induced MPER-specific and S-layer protein-specific antibodies in serum and mucosal secretions. Moreover, analysis of systemic SlpA-specific cytokines revealed that the responses appeared to be Th1 and Th17 dominant. These findings demonstrated the potential use of the Lactobacillus S-layer protein for development of oral vaccines targeting specific peptides.

Protective action of S-layer proteins from Lactobacillus paracasei M7 against Salmonella infection and mediated inhibition of Salmonella-induced apoptosis

Surface layer proteins had been proposed that it could protect human intestine from pathogens and aid in maintaining cellular integrity. However, the functions of S-layer proteins have not yet been fully revealed, and it has been proposed that these structures protected the intestinal microbes from hostile environmental agents. In this study, the ability of the Lactobacilli to inhibit adhesion of Salmonella to intestinal mucosa was studied on cultured HT-29 cells. Lactobacillus paracasei M7 was shown to inhibit the adhesion of Salmonella to epithelial cells, a process which may be related to the specific component of the bacterial surface. Human epithelial HT-29 cells were treated with S-layer proteins after Salmonella injection and pretreated with S-layer proteins to determine their importance in the inhibition of pathogen adherence. According to the analysis of cell cycle by flow cytometry, the HT-29 cells treated by Salmonella in S phase were 26.88 % compared with 36.70 % in the normal HT-29 cells. However, the addition of S-layer proteins made the HT-29 cells in S phase increase to 30.13 %. The activity of caspase-3 in the HT-29 cell was determined using a caspase-3 activity kit, which indicated the degree of apoptosis. Moreover, we also found that Lactobacillus S-layer proteins could protect Salmonella-induced apoptosis through reducing the caspase-3 activity. This mechanism might represent a novel approach for antagonizing Salmonella infection.

Lactobacillus surface layer proteins: structure, function and applications

Bacterial surface (S) layers are the outermost proteinaceous cell envelope structures found on members of nearly all taxonomic groups of bacteria and Archaea. They are composed of numerous identical subunits forming a symmetric, porous, lattice-like layer that completely covers the cell surface. The subunits are held together and attached to cell wall carbohydrates by non-covalent interactions, and they spontaneously reassemble in vitro by an entropy-driven process. Due to the low amino acid sequence similarity among S-layer proteins in general, verification of the presence of an S-layer on the bacterial cell surface usually requires electron microscopy. In lactobacilli, S-layer proteins have been detected on many but not all species. Lactobacillus S-layer proteins differ from those of other bacteria in their smaller size and high predicted pI. The positive charge in Lactobacillus S-layer proteins is concentrated in the more conserved cell wall binding domain, which can be either N- or C-terminal depending on the species. The more variable domain is responsible for the self-assembly of the monomers to a periodic structure. The biological functions of Lactobacillus S-layer proteins are poorly understood, but in some species S-layer proteins mediate bacterial adherence to host cells or extracellular matrix proteins or have protective or enzymatic functions. Lactobacillus S-layer proteins show potential for use as antigen carriers in live oral vaccine design because of their adhesive and immunomodulatory properties and the general non-pathogenicity of the species.

Lactobacillus S-layer protein inhibition of Salmonella-induced reorganization of the cytoskeleton and activation of MAPK signalling pathways in Caco-2 cells

Surface layer (S-layer) proteins are crystalline arrays of proteinaceous subunits that are present as the outermost component of the cell wall in several Lactobacillus species. The S-layer proteins have been shown to play a role in the antimicrobial activity of certain lactobacilli. However, it is not fully understood how the S-layer proteins exert this biological function. The aim of this study was to test the hypothesis that Lactobacillus acidophilus S-layer proteins antagonize Salmonella Typhimurium (S. Typhimurium) infection by protecting against F-actin cytoskeleton rearrangements and the activation of mitogen-activated protein kinase (MAPK) signalling pathways. Monolayer transepithelial electrical resistance (TER) was measured after S. Typhimurium infection in Caco-2 cultured human intestinal cells with L. acidophilus S-layer proteins. F-actin rearrangement and MAPK activation were also assessed by immunofluorescence staining or Western blotting. The results showed that when S. Typhimurium was co-incubated with S-layer proteins, the S. Typhimurium-induced Caco-2 cell F-actin rearrangement was reduced, and the S. Typhimurium-induced TER decrease and interleukin 8 (IL-8) secretion were attenuated. Additionally, L. acidophilus S-layer proteins could inhibit S. Typhimurium-induced phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun amino-terminal kinase (JNK) and p38. This study indicates that L. acidophilus S-layer proteins are able to inhibit S. Typhimurium infection through blocking S. Typhimurium-induced F-actin rearrangements and S. Typhimurium-induced ERK1/2, JNK and p38 activation in Caco-2 cells. These data provide a rationale for the use of lactobacillus S-layer proteins as therapeutic and preventative agents, at least in infectious diarrhoea.

Antagonistic activity of Lactobacillus acidophilus ATCC 4356 S-layer protein on Salmonella enterica subsp. enterica serovar Typhimurium in Caco-2 cells

To gain insight into the mechanism of Lactobacillus S-layer protein in antimicrobial activity, we examined how the Lactobacillus acidophilus ATCC 4356 S-layer protein inhibited the adhesion and invasion of Salmonella enterica subsp. enterica serovar Typhimurium SL1344 (Salmonella Typhimurium SL1344) in vitro in cultured Caco-2 cells. The results showed that S-layer protein reduced Salmonella Typhimurium SL1344 association in Caco-2 cells by 69–88% in the adhesive experiments (competition, exclusion, and displacement assays). Although the antagonistic activity was demonstrated in the exclusion assay (preincubated with the S-layer protein), a greater effect for the S-layer protein on Salmonella Typhimurium was observed when Salmonella Typhimurium was coincubated with the S-layer protein. The S-layer protein could be bound directly to the Caco-2 cell line or be associated with Salmonella Typhimurium surface, thereby blocking Salmonella attachment. The data support the antimicrobial mechanisms of Lactobacillus S-layer protein, which are involved not only in competition for binding sites on the surface of host epithelial cells, but also in a direct interaction between this protein and Salmonella Typhimurium surface.

Lactobacillusacidophilus S-layer protein-mediated inhibition of Salmonella-induced apoptosis in Caco-2 cells

Surface layer (S-layer) proteins are crystalline arrays of proteinaceous subunits present as the outermost component of the cell wall in several Lactobacillus species. The underlying mechanism for how S-layer proteins inhibit pathogen infections remains unclear. To gain insights into the mechanism of the antimicrobial activity of Lactobacillus S-layer proteins, we examined how Lactobacillus S-layer proteins impact Salmonella Typhimurium-induced apoptosis in vitro in Caco-2 human colon epithelial cells. When Caco-2 cells infected with Salmonella Typhimurium SL1344, we found that apoptosis was mediated by activation of caspase-3, but not caspase-1. When Salmonella Typhimurium SL1344 and S-layer proteins were coincubated simultaneously, Caco-2 cell apoptosis was markedly decreased and the cell damage was modified, as evaluated by flow cytometry and microscopy. Detailed analyses showed that the S-layer proteins inhibited the caspase-3 activity and activated the extracellular signal-regulated kinases 1 and 2 (ERK1/2) signaling pathway. Taken together, these findings suggest that Lactobacillus S-layer proteins protected against Salmonella-induced apoptosis through reduced caspase-3 activation. In addition, Salmonella-induced apoptotic cell damage was modified by S-layer proteins through the ERK1/2 signaling pathway. This mechanism may represent a novel approach for antagonizing Salmonella infection.

Characterization of Surface Layer Proteins in Lactobacillus crispatus Isolate ZJ001

Lactobacillus crispatus (L. crispatus) ZJ001 is highly adhesive to epithelial cells and expresses S-layer proteins. In this study, S-layer genes were sequenced and expressed in E. coli to characterize the function of S-layer proteins with this particular strain. L. crispatus ZJ001 harbored two Slayer genes slpA and slpB, and only slpA gene was expressed in the bacterium, as revealed by RT-PCR and immunoassays. The mature SlpA showed 47% amino acid sequence identity to SlpB. The SlpA and SlpB of L. crispatus ZJ001 were highly homologous at the C-terminal region to other Lactobacillus S-layer proteins, but were substantially variable at N-terminal and middle regions. Electron microscopic analysis indicated that His-slpA expressed in E. coli was able to form a sheet-like structure similar to the natural S-layer, but His-slpB formed as disclike structures. In the cell binding experiments, HeLa cells were able to bind to both recombinant His-slpA and HisslpB proteins to the extent similar to the natural S-layer. The cell binding domains remain mostly in the N-terminal regions in SlpA and SlpB, as shown by high binding of truncated peptides SlpA2-228 and SlpB2-249. Our results indicated that SlpA was active and high binding to HeLa cells, and that the slpA gene could be targeted to display foreign proteins on the bacterial surface of ZJ001 as a potential mucosal vaccine vector.

Identification and characterization of domains responsible for self-assembly and cell wall binding of the surface layer protein of Lactobacillus brevisATCC 8287

BackgroundLactobacillus brevis ATCC 8287 is covered by a regular surface (S-) layer consisting of a 435 amino acid protein SlpA. This protein is completely unrelated in sequence to the previously characterized S-layer proteins of Lactobacillus acidophilus group.ResultsIn this work, the self-assembly and cell wall binding domains of SlpA were characterized. The C-terminal self-assembly domain encompassed residues 179–435 of mature SlpA, as demonstrated by the ability of N-terminally truncated recombinant SlpA to form a periodic structure indistinguishable from that formed by full length SlpA. Furthermore, a trypsin degradation analysis indicated the existence of a protease resistant C-terminal domain of 214 amino acids. By producing a set of C-terminally truncated recombinant SlpA (rSlpA) proteins the cell wall binding region was mapped to the N-terminal part of SlpA, where the first 145 amino acids of mature SlpA alone were sufficient for binding to isolated cell wall fragments of L. brevis ATCC 8287. The binding of full length rSlpA to the cell walls was not affected by the treatment of the walls with 5% trichloroacetic acid (TCA), indicating that cell wall structures other than teichoic acids are involved, a feature not shared by the Lactobacillus acidophilus group S-layer proteins characterized so far. Conserved carbohydrate binding motifs were identified in the positively charged N-terminal regions of six Lactobacillus brevis S-layer proteins.ConclusionThis study identifies SlpA as a two-domain protein in which the order of the functional domains is reversed compared to other characterized Lactobacillus S-layer proteins, and emphasizes the diversity of potential cell wall receptors despite similar carbohydrate binding sequence motifs in Lactobacillus S-layer proteins.

Characterization of S-layer proteins of Lactobacillus by FTIR spectroscopy and differential scanning calorimetry

FTIR spectroscopy was used for the characterization of S-layer proteins extracted from microorganisms isolated from kefir grains. S-layer from Lactobacillus brevis ATCC 8287 has been already characterized [G. Vidgren, I. Palva, R. Pakkanen, K. Lounatmaa, A. Palva, J. Bacteriol. 174 (1992) 7419] and therefore it was used for the validation of FTIR as a method to investigate the secondary structure of the S-layer proteins of the studied kefir strains. A correlation between the secondary structures of S-layer proteins with surface properties of Lactobacillus kefir strains was found: a high percentage of β-sheet contents (40–50%) was found for non-aggregating strains, whereas this percentage decreased to 25–30% for aggregating ones. A quantitative comparison of the S-layers was performed by means of cluster analysis based on the obtained spectroscopic data. This analysis enabled the strains to be grouped in clusters according to the spectral diversity in the Amide I region. The non-aggregating strains of L. kefir cluster at S sm > 0.943 and the aggregating strains form another cluster, with S sm > 0.769. L. brevis ATCC 8287 appears clearly separated from these two clusters: the similarity with the aggregating strains is 0.658 and the similarity with the non-aggregating ones, 0.665. The thermal analysis of the lyophilized S-layer proteins was performed by means of differential scanning calorimetry (DSC) and FTIR. DSC analysis within the 30–130 °C range showed two phase transitions with maxima located at ca. 58 and 98 °C for L. brevis and in the 67–70 and 110–119 °C ranges for the different strains of L. kefir (CIDCA 8344 only shows the lowest temperature phase transition). FTIR spectra obtained reveal that for all the L. kefir S-layer proteins the major secondary structure modifications upon heating occur nearly at the first phase transitions observed by DSC, with the thermal stability increasing with the percentage of β-sheets structures. The S-layer protein of L. brevis ATICC 8287, which among all protein studied is that with maximum β-sheet contents (and no α-helix structure) was then found to be the protein showing a greater thermal stability.

Inhibition of the adherence of Escherichia coli strains to basement membrane by Lactobacillus crispatus expressing an S-layer.

This study aimed to evaluate the efficiency with which Lactobacillus crispatus JCM 5810 inhibited the adhesion of enteric pathogens to a synthetic basement membrane and to elucidate the mechanism underlying the inhibition. Lactobacillus crispatus JCM 5810 inhibited the adhesion of three diarrhoeagenic Escherichia coli strains to a reconstituted basement membrane preparation called Matrigel, used as a model of a damaged intestinal tissue site. Inhibition was also observed with the use of immobilized laminin, a major component of Matrigel, but diminished after the removal of S-layer protein (CbsA) from JCM 5810 cells. The isolated CbsA inhibited the adhesion of E. coli to both Matrigel and immobilized laminin. Lactobacillus crispatus JCM 5810 and CbsA seem to inhibit pathogenic E. coli from adhering to basement membrane via competition with laminin molecules for binding sites. These results suggested that not only Lact. crispatus JCM 5810 cells but CbsA alone might prevent pathogens from colonizing damaged intestinal tissues. This is the first study to show the applied aspect of Lactobacillus S-layer protein.