Fusion of a bacterial cadherin-like domain and green fluorescent protein as a specific probe to study biofilm matrix formation in Rhizobium spp.

Rhizobium adhering proteins or 'Raps' are secreted proteins identified in a very restricted group of rhizobial strains, specifically those belonging to R. leguminosarum and R. etli. The distinctive feature of members of the Rap family is the presence of one or two cadherin-like domains or CHDLs that are also present in numerous extracellular bacterial and archaeal proteins and were proposed to confer carbohydrate binding ability. We have previously made an in-depth characterization of RapA2, a calcium-binding lectin, composed by two CHDLs, involved in biofilm matrix remodelling in R. leguminosarum bv. viciae 3841. In this study, CHDLs derived from RapA2 were analysed in detail, finding significant structural and functional differences despite their considerable sequence similarity. Only the carboxy-terminal CHDL retained properties similar to those displayed by RapA2. Our findings were used to obtain a novel fluorescent probe to study biofilm matrix development by confocal laser scanning microscopy, and also to shed some light on the role of the ubiquitous CHDL domains in bacterial secreted proteins.

A Rhizobium leguminosarum CHDL- (Cadherin-Like-) Lectin Participates in Assembly and Remodeling of the Biofilm Matrix.

In natural environments most bacteria live in multicellular structures called biofilms. These cell aggregates are enclosed in a self-produced polymeric extracellular matrix, which protects the cells, provides mechanical stability and mediates cellular cohesion and adhesion to surfaces. Although important advances were made in the identification of the genetic and extracellular factors required for biofilm formation, the mechanisms leading to biofilm matrix assembly, and the roles of extracellular proteins in these processes are still poorly understood. The symbiont Rhizobium leguminosarum requires the synthesis of the acidic exopolysaccharide and the PrsDE secretion system to develop a mature biofilm. PrsDE is responsible for the secretion of the Rap family of proteins that share one or two Ra/CHDL (cadherin-like-) domains. RapA2 is a calcium-dependent lectin with a cadherin-like ß sheet structure that specifically recognizes the exopolysaccharide, either as a capsular polysaccharide (CPS) or in its released form [extracellular polysaccharide (EPS)]. In this study, using gain and loss of function approaches combined with phenotypic and microscopic studies we demonstrated that RapA lectins are involved in biofilm matrix development and cellular cohesion. While the absence of any RapA protein increased the compactness of bacterial aggregates, high levels of RapA1 expanded distances between cells and favored the production of a dense matrix network. Whereas endogenous RapA(s) are predominantly located at one bacterial pole, we found that under overproduction conditions, RapA1 surrounded the cell in a way that was reminiscent of the capsule. Accordingly, polysaccharide analyses showed that the RapA lectins promote CPS formation at the expense of lower EPS production. Besides, polysaccharide analysis suggests that RapA modulates the EPS size profile. Collectively, these results show that the interaction of RapA lectins with the polysaccharide is involved in rhizobial biofilm matrix assembly and remodeling.

Lipopolysaccharide O-chain core region required for cellular cohesion and compaction of in vitro and root biofilms developed by Rhizobium leguminosarum.

The formation of biofilms is an important survival strategy allowing rhizobia to live on soil particles and plant roots. Within the microcolonies of the biofilm developed by Rhizobium leguminosarum, rhizobial cells interact tightly through lateral and polar connections, forming organized and compact cell aggregates. These microcolonies are embedded in a biofilm matrix, whose main component is the acidic exopolysaccharide (EPS). Our work shows that the O-chain core region of the R. leguminosarum lipopolysaccharide (LPS) (which stretches out of the cell surface) strongly influences bacterial adhesive properties and cell-cell cohesion. Mutants defective in the O chain or O-chain core moiety developed premature microcolonies in which lateral bacterial contacts were greatly reduced. Furthermore, cell-cell interactions within the microcolonies of the LPS mutants were mediated mostly through their poles, resulting in a biofilm with an altered three-dimensional structure and increased thickness. In addition, on the root epidermis and on root hairs, O-antigen core-defective strains showed altered biofilm patterns with the typical microcolony compaction impaired. Taken together, these results indicate that the surface-exposed moiety of the LPS is crucial for proper cell-to-cell interactions and for the formation of robust biofilms on different surfaces.

The BtaF trimeric autotransporter of Brucella suis is involved in attachment to various surfaces, resistance to serum and virulence.

The adhesion of bacterial pathogens to host cells is an event that determines infection, and ultimately invasion and intracellular multiplication. Several evidences have recently shown that this rule is also truth for the intracellular pathogen Brucella. Brucella suis displays the unipolar BmaC and BtaE adhesins, which belong to the monomeric and trimeric autotransporter (TA) families, respectively. It was previously shown that these adhesins are involved in bacterial adhesion to host cells and components of the extracellular matrix (ECM). In this work we describe the role of a new member of the TA family of B. suis (named BtaF) in the adhesive properties of the bacterial surface. BtaF conferred the bacteria that carried it a promiscuous adhesiveness to various ECM components and the ability to attach to an abiotic surface. Furthermore, BtaF was found to participate in bacterial adhesion to epithelial cells and was required for full virulence in mice. Similar to BmaC and BtaE, the BtaF adhesin was expressed in a small subpopulation of bacteria, and in all cases, it was detected at the new pole generated after cell division. Interestingly, BtaF was also implicated in the resistance of B. suis to porcine serum. Our findings emphasize the impact of TAs in the Brucella lifecycle.

BtaE, an adhesin that belongs to the trimeric autotransporter family, is required for full virulence and defines a specific adhesive pole of Brucella suis.

Brucella is responsible for brucellosis, one of the most common zoonoses worldwide that causes important economic losses in several countries. Increasing evidence indicates that adhesion of Brucella spp. to host cells is an important step to establish infection. We have previously shown that the BmaC unipolar monomeric autotransporter mediates the binding of Brucella suis to host cells through cell-associated fibronectin. Our genome analysis shows that the B. suis genome encodes several additional potential adhesins. In this work, we characterized a predicted trimeric autotransporter that we named BtaE. By expressing btaE in a nonadherent Escherichia coli strain and by phenotypic characterization of a B. suis ΔbtaE mutant, we showed that BtaE is involved in the binding of B. suis to hyaluronic acid. The B. suis ΔbtaE mutant exhibited a reduction in the adhesion to HeLa and A549 epithelial cells compared with the wild-type strain, and it was outcompeted by the wild-type strain in the binding to HeLa cells. The knockout btaE mutant showed an attenuated phenotype in the mouse model, indicating that BtaE is required for full virulence. BtaE was immunodetected on the bacterial surface at one cell pole. Using old and new pole markers, we observed that both the BmaC and BtaE adhesins are consistently associated with the new cell pole, suggesting that, in Brucella, the new pole is functionally differentiated for adhesion. This is consistent with the inherent polarization of this bacterium, and its role in the invasion process.

RapA2 is a calcium-binding lectin composed of two highly conserved cadherin-like domains that specifically recognize Rhizobium leguminosarum acidic exopolysaccharides.

In silico analyses have revealed a conserved protein domain (CHDL) widely present in bacteria that has significant structural similarity to eukaryotic cadherins. A CHDL domain was shown to be present in RapA, a protein that is involved in autoaggregation of Rhizobium cells, biofilm formation, and adhesion to plant roots as shown by us and others. Structural similarity to cadherins suggested calcium-dependent oligomerization of CHDL domains as a mechanistic basis for RapA action. Here we show by circular dichroism spectroscopy, light scattering, isothermal titration calorimetry, and other methods that RapA2 from Rhizobium leguminosarum indeed exhibits a cadherin-like ß-sheet conformation and that its proper folding and stability are dependent on the binding of one calcium ion per protein molecule. By further in silico analysis we also reveal that RapA2 consists of two CHDL domains and expand the range of CHDL-containing proteins in bacteria and archaea. However, light scattering assays at various concentrations of added calcium revealed that RapA2 formed neither homo-oligomers nor hetero-oligomers with RapB (a distinct CHDL protein), indicating that RapA2 does not mediate cellular interactions through a cadherin-like mechanism. Instead, we demonstrate that RapA2 interacts specifically with the acidic exopolysaccharides (EPSs) produced by R. leguminosarum in a calcium-dependent manner, sustaining a role of these proteins in the development of the biofilm matrix made of EPS. Because EPS binding by RapA2 can only be attributed to its two CHDL domains, we propose that RapA2 is a calcium-dependent lectin and that CHDL domains in various bacterial and archaeal proteins confer carbohydrate binding activity to these proteins.

Structure and mechanism of GumK, a membrane-associated glucuronosyltransferase.

Xanthomonas campestris GumK (beta-1,2-glucuronosyltransferase) is a 44-kDa membrane-associated protein that is involved in the biosynthesis of xanthan, an exopolysaccharide crucial for this bacterium's phytopathogenicity. Xanthan also has many important industrial applications. The GumK enzyme is the founding member of the glycosyltransferase family 70 of carbohydrate-active enzymes, which is composed of bacterial glycosyltransferases involved in exopolysaccharide synthesis. No x-ray structures have been reported for this family. To better understand the mechanism of action of the bacterial glycosyltransferases in this family, the x-ray crystal structure of apo-GumK was solved at 1.9 angstroms resolution. The enzyme has two well defined Rossmann domains with a catalytic cleft between them, which is a typical feature of the glycosyltransferase B superfamily. Additionally, the crystal structure of GumK complexed with UDP was solved at 2.28 angstroms resolution. We identified a number of catalytically important residues, including Asp157, which serves as the general base in the transfer reaction. Residues Met231, Met273, Glu272, Tyr292, Met306, Lys307, and Gln310 interact with UDP, and mutation of these residues affected protein activity both in vitro and in vivo. The biological and structural data reported here shed light on the molecular basis for donor and acceptor selectivity in this glycosyltransferase family. These results also provide a rationale to obtain new polysaccharides by varying residues in the conserved alpha/beta/alpha structural motif of GumK.

The capsular polysaccharide biosynthesis of Streptococcus pneumoniae serotype 8: functional identification of the glycosyltransferase WciS (Cap8H).

CPS (capsular polysaccharide) is a major virulence factor in Streptococcus pneumoniae. Biosynthesis of CPS RU (repeat unit) proceeds by sequential transfer of sugar residues from the appropriate sugar donor to an activated lipid carrier by committed GTs (glycosyltransferases). While the nucleotide sequence of many cps loci is already known, the real substrate specificity of the hypothetical GTs, as well as the sequence of sugar addition is unclear. In the present paper, we report the biochemical characterization of one alpha-galactosyltransferase, WciS (Cap8H), a member of GT family 4. This enzyme is implicated in the tetrasaccharide RU biosynthetic pathway of Strep. pneumoniae CPS 8 ([-->4)-alpha-D-Glcp-(1-->4)-alpha-D-Galp-(1-->4)-beta-D-GlcAp-(1-->4)-beta-D-Glcp-(1-->]n). Expression of WciS-His6 in Escherichia coli BL21 (DE3) strains or BL21 (DE3)/DeltagalU strain resulted in synthesis of a 39 kDa membrane-associated protein identified by N-terminal sequencing and recognized by anti-His6-tag antibody. This protein was capable of adding a galactose residue cellobiuronic acid [beta-D-GlcAp-(1-->4)-D-Glcp]-pyrophosphate-polyprenol from UDP-Gal. The newly added galactose residue is removed by alpha-galactosidase, indicating that WciS is a retaining GT. Our results suggest that WciS catalyses the addition of the third sugar residue of the CPS 8 RU. The recombinant WciS-His6 was solubilized and purified as a soluble multimer, opening the way for structural studies.

Functional characterization of GumK, a membrane-associated beta-glucuronosyltransferase from Xanthomonas campestris required for xanthan polysaccharide synthesis.

Xanthomonas campestris is a Gram-negative bacterium that produces an exopolysaccharide known as xanthan gum. Xanthan is involved in a variety of biological functions, including pathogenesis, and is widely used in the industry as thickener and viscosifier. Although the genetics and biosynthetic process of xanthan are well documented, the enzymatic components have not been examined and no data on glycosyltransferases have been reported. We describe the functional characterization of the gumK gene product, an essential protein for xanthan synthesis. Immunoblots and complementation studies showed that GumK is a 44-kDa protein associated to the membrane fraction. This value corresponds to the expected molecular mass for GumK encoded by an extended open reading frame than proposed from previous genetic data and in X. campestris published complete genome. The protein was expressed in Escherichia coli cells. The purified protein catalyzed the transfer of a glucuronic acid residue from UDP-glucuronic acid to mannose-alpha-1,3-glucose-beta-1,4-glucose-P-P-polyisoprenyl with formation of a glucuronic acid-beta-mannose linkage. We examined the acceptor substrate specificity. GumK was unable to use the trisaccharide acceptor freed from the pyrophosphate lipid moiety. Replacement of the natural lipid moiety by phytanyl showed that the catalytic function could proceed with glucuronic acid transfer. These results suggest the enzyme does not show specificity for the lipidic portion of the acceptor. GumK showed diminished activity when tested with 6-O-acetyl-mannose-alpha-1,3-glucose-beta-1,4-glucose-P-P-polyisoprenyl, a putative intermediate in the synthesis of xanthan. This could indicate that acetylation of the internal mannose takes place after the formation of the GumK product.