Thus, during the course of evolution cells have "learned" a variety of strategies to achieve volume. I Jakarta Buka situs. Cooper In Volume 23 we considered, in seven chapters, the basic armamentarium of the invertebrate immune system and its cells, as well as an. This volume constitutes. The basic thesis for this study was that the telencephalon is needed to make decisions in new situations.
Subsidary hypotheses were that the. Introduction During the past 20 years, whilst electron microscopy and histochemistry have made great advances in many fields, their application to. Bacterial CPSs have been shown to be potent immunomodulating molecules; they have largely been characterized in pathogenic species [ ] and are considered to be virulence factors that act by preventing phagocytosis. The L. These results highlight the immunosuppressive function of WPSs [ ]. Similar results were obtained with L. Finally, the galactose-rich PSs of L. In Gram-positive bacteria, most of these proteins are secreted by the universally conserved and essential Sec pathway.
This pathway has been extensively studied in E. Almost all proteins that are targeted by this secretory pathway have an N-terminal signal peptide composed of approximately 30 amino acids. Once the proteins have been translocated across the cytoplasmic membrane, this signal peptide is cleaved off by the appropriate signal peptidase.
Then, the protein is either released into the extracellular medium or, alternatively, it is retained in the cell envelope, if it contains a specific sequence ensuring its attachment to the cytoplasmic membrane or the components of the cell wall in addition to the signal peptide. Secreted proteins can be covalently attached to the cell surface by sortase-mediated reactions or non-covalently attached via i transmembrane anchors; ii lipid anchors; or iii different cell wall binding domains CWBD [ , ].
We will review here LAB proteins which are linked to cell wall components through covalent or non covalent binding. A portion of a given cell wall protein is covalently bound to PG by a transpeptidation mechanism that is catalyzed by sortase A SrtA, also called housekeeping sortase. In addition to an N-terminal signal peptide, they also contain, at their C-terminal, a conserved LPXTG motif that is followed by a stretch of hydrophobic residues and a positively charged tail [ , , ].
Inactivation of srtA in L. These proteins contain mucus-binding domains MUB or MucBP that are thought to play an important role in the adhesion of LAB to the mucus layer that covers intestinal epithelial cells [ ]. Other functionally important LPXTG proteins are the pilins, which are the structural components of pili.
Pili or fimbriae are long filamentous structures that extend from the surfaces of various Gram-negative and Gram-positive bacteria. Most studies on pili in Gram-positive bacteria have been conducted on pathogenic species, including streptococci, enterococci, corynebacteria, and bacilli [ — ].
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Pili have been shown to be involved in adhesion to host cells and tissues and are thus considered to promote host colonization and invasion [ ]. In Gram-positive bacteria, the sortase-dependent pili Spa-type for sortase-mediated pilus assembly are composed of a major backbone pilin, whose subunits are covalently assembled by sortase C, and of one or two accessory pilins.
The minor pilins are located at the base and the tip of the pilus and are possibly also dispersed along the shaft.
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The pili structures are anchored to PG by housekeeping sortase A [ ]. The presence of pili in LAB and in bifidobacteria has also been described and has been linked to the ability of these bacteria to colonize the guts of their hosts and persist in their gastrointestinal tracts [ , ]. More recently, L. In a natural L.
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This strain produces thin pili that are rather short averaging nm length. These proteins contain specific CWBDs that have been described in several reviews [ , ].
The LysM sequence Lys motif, PF is about 40 amino acid residues long and is present in more than 2, eukaryotic and prokaryotic proteins. Several LysM sequences linked by intervening sequences constitute a LysM domain [ 70 , , ]. Studies examining the binding patterns of different PG chemotypes have found that LysM non-covalently binds to the GlcNAc moiety of glycan chains [ 70 ]. The presence of an optimal number of LysM sequences is crucial for the enzymatic activity of PGHs and, as a consequence, for the different functions of these bacterial enzymes: they are involved in cell growth, cell separation, and autolysis.
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The main lactococcal autolysin AcmA, which is one of the best studied PGHs, has a modular structure and a C-terminal LysM domain that contains three LysM sequences and an N-terminal N-acetyl-glucosaminidase catalytic domain [ 68 ]. AcmA also binds to PG in other bacteria, and even to the cells of different Gram-positive species in mixed communities [ ]. It has been shown that, in L.
In the case of the lactococcal autolysin AcmA, it has been proposed that attachment to the cell wall can be hindered by CW constituents; LTAs are suggested candidates [ 70 , ]. This domain is the bacterial equivalent of the well-characterized SH3 domain that is found in eukaryotes and viruses. Conflicting results have been obtained when it comes to the PG motif recognized by this domain. In staphylococci with a five-Gly PG crossbridge, the length and amino acid composition of the cross-bridge have been found to have a significant impact on the binding of the SH3-containing homolog of lysostaphin ALE-1 [ ].
However, more recently, single-molecule AFM experiments using tips functionalized with the L. This domain was initially identified based on in silico analysis of gene clusters that encode the cell surface proteins of lactobacilli, enterococcoci, and listeria species [ ]. Proteins containing the WxL domain are present in L.
This domain was recently discovered in the C-terminal of the endolysins Lc-Lys and Lc-Lys2 of prophages found in the complete genome sequence of L. It does not share amino acid sequence similarity with any known CWBDs. The domain can bind to PG and can specifically recognize the amidated D-Asp cross-bridge that occurs in L. This domain is also present in the endolysins of other L.
The surface S layer entirely coats the bacterial surface and is composed of glyco proteins that intrinsically form a two-dimensional paracrystalline structure.
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Most prokaryotic S-layer proteins possess a signal peptide. These proteins bind non-covalently via their N- or C-terminus to PG or secondary cell wall polymers. The attachment is mediated by S-layer homologous domains SLHDs , which can also be found in other enzymes of Gram-positive bacteria [ , ]. S-layer proteins are present in lactobacilli, and their structure and functions have already been extensively reviewed [ ]. The cell wall ligands of the S-layer proteins isolated from different Lactobacillus species have been proposed to be carbohydrates either teichoic acids or neutral polysaccharides [ ].
The surface proteins of probiotic or commensal bacteria are thought to facilitate mucosal colonization and persistence in the gastrointestinal tract; they may also favor cross-talk with immune cells by mediating direct contact with the intestinal mucosa. The role of pili appendages and mucus-binding proteins as surface determinants in certain LAB strains has been underscored: they allow bacteria to adhere to intestinal epithelial cells or mucus. Notably, the pili identified in L.
Furthermore, pili synthetized by a natural isolate of L. Two types of surface determinants--pili and mucus-binding proteins--have also been shown to play a role in bacterial adhesion to model mucins, and mucus-binding proteins make a greater contribution under shear flow conditions [ ]. Moreover, L. Other cell wall-associated or secreted proteins of probiotic strains have also been shown to be involved in modulating the response of the host immune system.
The attachment of bacteria to DCs has been shown to stimulate immature DCs and regulate T-cell function.
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Also, p40 and p75, two proteins described above section 1. From an applied perspective, LAB, because of their GRAS status, are considered to be convenient vectors for delivering therapeutic proteins or antigens to gastrointestinal tract mucosa. In an alternative approach to vector creation that avoids the use of genetically modified bacteria, proteins of interest can be fused with CWBDs found in cell wall proteins and then anchored on the surfaces of LAB.
The ability of LysM and SLH domains to bind to bacterial cell walls has been exploited to display protein antigens on LAB surfaces when developing oral vaccines [ , ]. Remarkable advances have been made in the last two decades in terms of understanding the structure and function of LAB cell walls.
Cell wall components have been purified from several LAB species, which has allowed the elucidation of fine-scale cell wall structure as well as interspecific and intraspecific variation. In tandem, genes involved in cell wall biosynthesis, modification, and degradation pathways have been identified, which has allowed for the construction of mutants that can be used to investigate the role of such genes in bacterial physiology; the results obtained can also inform technological and health applications of LAB.