Functional mapping of an oligomeric autotransporter adhesin of Aggregatibacter actinomycetemcomitans

Virulence-linked adhesin drives mutualist colonization of the bee gut via biofilm formation

Bacterial biofilms are stable multicellular structures that can enable long term host association. Yet, the role of biofilms in supporting gut mutualism is still not fully understood. Here, we investigate Snodgrassella alvi, a beneficial bacterial symbiont of honey bees, and find that biofilm formation is required for its colonization of the bee gut. We constructed fifteen S. alvi mutants containing knockouts of genes known to promote colonization with putative roles in biofilm formation. Genes required for colonization included staA and staB, encoding trimeric autotransporter adhesins (TAAs) and mltA, encoding a lytic transglycosylase. Intriguingly, TAAs are considered virulence factors in pathogens but support mutualism by the symbiont S. alvi. In vitro, biofilm formation was reduced in ΔstaB cells and abolished in the other two mutants. Loss of staA also reduced auto-aggregation and cell-cell connections. Based on structural predictions, StaA/B are massive (>300 nm) TAAs with many repeats in their stalk regions. Further, we find that StaA/B are conserved across Snodgrassella species, suggesting that StaA/B-dependent colonization is characteristic of this symbiont lineage. Finally, staA deletion increases sensitivity to bactericidal antimicrobials, suggesting that the biofilm indirectly buffers against antibiotic stress. In all, the inability of two biofilm-deficient strains (ΔstaA and ΔmltA) to effectively mono-colonize bees indicates that S. alvi biofilm formation is required for colonization of the bee gut. We envision the bee gut system as a genetically tractable model for studying the physical basis of biofilm-mutualist-gut interactions.

The Trimeric Autotransporter Adhesin EmaA and Infective Endocarditis

Infective endocarditis (IE), a disease of the endocardial surface of the heart, is usually of bacterial origin and disproportionally affects individuals with underlying structural heart disease. Although IE is typically associated with Gram-positive bacteria, a minority of cases are caused by a group of Gram-negative species referred to as the HACEK group. These species, classically associated with the oral cavity, consist of bacteria from the genera Haemophilus (excluding Haemophilus influenzae), Aggregatibacter, Cardiobacterium, Eikenella, and Kingella. Aggregatibacter actinomycetemcomitans, a bacterium of the Pasteurellaceae family, is classically associated with Aggressive Periodontitis and is also concomitant with the chronic form of the disease. Bacterial colonization of the oral cavity serves as a reservoir for infection at distal body sites via hematological spreading. A. actinomycetemcomitans adheres to and causes disease at multiple physiologic niches using a diverse array of bacterial cell surface structures, which include both fimbrial and nonfimbrial adhesins. The nonfimbrial adhesin EmaA (extracellular matrix binding protein adhesin A), which displays sequence heterogeneity dependent on the serotype of the bacterium, has been identified as a virulence determinant in the initiation of IE. In this chapter, we will discuss the known biochemical, molecular, and structural aspects of this protein, including its interactions with extracellular matrix components and how this multifunctional adhesin may contribute to the pathogenicity of A. actinomycetemcomitans.

Serotype-Specific Sugars Impact Structure but Not Functions of the Trimeric Autotransporter Adhesin EmaA of Aggregatibacter actinomycetemcomitans

The human oral pathobiont Aggregatibacter actinomycetemcomitans expresses multiple virulence factors, including the trimeric, extracellular matrix protein adhesin A (EmaA). The posttranslational modification of EmaA is proposed to be dependent on the sugars and enzymes associated with O-polysaccharide (O-PS) synthesis of the lipopolysaccharide (LPS). This modification is important for the structure and function of this adhesin. To determine if the composition of the sugars alters structure and/or function, the prototypic 202-kDa protein was expressed in a non-serotype b, emaA mutant strain. The transformed strain displayed EmaA adhesins similar in appearance to the prototypic adhesin as observed by two-dimensional (2D) electron microscopy of whole-mount negatively stained bacterial preparations. Biochemical analysis indicated that the protein monomers were posttranslationally modified. 3D electron tomographic reconstruction and structure analyses of the functional domain revealed three well-defined subdomains (SI, SII, and SIII) with a linker region between SII and SIII. Structural changes were observed in all three subdomains and the linker region of the adhesins synthesized compared with the known structure. These changes, however, did not affect the ability of the strain to bind collagen or form biofilms. The data suggest that changes in the composition of the glycan moiety alter the 3D structure of the molecule without negatively affecting the function(s) associated with this adhesin. IMPORTANCE The human oral pathogen A. actinomycetemcomitans is a causative agent of periodontal and several systemic diseases. EmaA is a trimeric autotransporter protein adhesin important for colonization by this pathobiont in vivo. This adhesin is modified with sugars associated with the O-polysaccharide (O-PS), and the modification is mediated using the enzymes involved in lipopolysaccharide (LPS) biosynthesis. The interaction with collagen is not mediated by the specific binding between the glycans and collagen but is attributed to changes in the final quaternary structure necessary to maintain an active adhesin. In this study, we have determined that the composition of the sugars utilized in the posttranslational modification of this adhesin is exchangeable without compromising functional activities.

The serotype a-EmaA adhesin of Aggregatibacter actinomycetemcomitans does not require O-PS synthesis for collagen binding activity

Aggregatibacter actinomycetemcomitans, a causative agent of periodontitis and non-oral diseases, synthesizes a trimeric extracellular matrix protein adhesin A (EmaA) that mediates collagen binding and biofilm formation. EmaA is found as two molecular forms, which correlate with the serotype of the bacterium. The canonical protein (b-EmaA), associated with serotypes b and c, has a monomeric molecular mass of 202 kDa. The collagen binding activity of b-EmaA is dependent on the presence of O-polysaccharide (O-PS), whereas biofilm activity is independent of O-PS synthesis. The EmaA associated with serotype a strains (a-EmaA) has a monomeric molecular mass of 173 kDa and differs in the amino acid sequence of the functional domain of the protein. In this study, a-emaA was confirmed to encode a protein that forms antenna-like appendages on the surface of the bacterium, which were found to be important for both collagen binding and biofilm formation. In an O-PS-deficient talose biosynthetic (tld) mutant strain, the electrophoretic mobility of the a-EmaA monomers was altered and the amount of membrane-associated EmaA was decreased when compared to the parent strain. The mass of biofilm formed remained unchanged. Interestingly, the collagen binding activity of the mutant strain was similar to the activity associated with the parent strain, which differs from that observed with the canonical b-EmaA isoform. These data suggest that the properties of the a-EmaA isoform are like those of b-EmaA, with the exception that collagen binding activity is independent of the presence or absence of the O-PS.

Herpesvirus-bacteria synergistic interaction in periodontitis

The etiopathogenesis of severe periodontitis includes herpesvirus-bacteria coinfection. This article evaluates the pathogenicity of herpesviruses (cytomegalovirus and Epstein-Barr virus) and periodontopathic bacteria (Aggregatibacter actinomycetemcomitans and Porphyromonas gingivalis) and coinfection of these infectious agents in the initiation and progression of periodontitis. Cytomegalovirus and A. actinomycetemcomitans/P. gingivalis exercise synergistic pathogenicity in the development of localized ("aggressive") juvenile periodontitis. Cytomegalovirus and Epstein-Barr virus are associated with P. gingivalis in adult types of periodontitis. Periodontal herpesviruses that enter the general circulation may also contribute to disease development in various organ systems. A 2-way interaction is likely to occur between periodontal herpesviruses and periodontopathic bacteria, with herpesviruses promoting bacterial upgrowth, and bacterial factors reactivating latent herpesviruses. Bacterial-induced gingivitis may facilitate herpesvirus colonization of the periodontium, and herpesvirus infections may impede the antibacterial host defense and alter periodontal cells to predispose for bacterial adherence and invasion. Herpesvirus-bacteria synergistic interactions, are likely to comprise an important pathogenic determinant of aggressive periodontitis. However, mechanistic investigations into the molecular and cellular interaction between periodontal herpesviruses and bacteria are still scarce. Herpesvirus-bacteria coinfection studies may yield significant new discoveries of pathogenic determinants, and drug and vaccine targets to minimize or prevent periodontitis and periodontitis-related systemic diseases.

Interactions between the Trimeric Autotransporter Adhesin EmaA and Collagen Revealed by Three-Dimensional Electron Tomography

Bacterial adhesion to host tissues is considered the first and critical step of microbial infection. The extracellular matrix protein adhesin A (EmaA) is a collagen-binding adhesin of the periodontal pathogen Aggregatibacter actinomycetemcomitans Three 202-kDa EmaA monomers form antenna-like structures on the bacterial surface with the functional domain located at the apical end. The structure of the 30-nm functional domain has been determined by three-dimensional (3D) electron tomography and subvolume averaging. The region exhibits a complex architecture composed of three subdomains (SI to SIII) and a linker between subdomains SII and SIII. However, the molecular interaction between the adhesin receptor complexes has yet to be revealed. This study provides the first detailed 3D structure of reconstituted EmaA/collagen complexes obtained using 3D electron tomography and image processing techniques. The observed interactions of EmaA with collagen were not to whole, intact fibrils, but rather to individual collagen triple helices dissociated from the fibrils. The majority of the contacts with the EmaA functional domain encompassed subdomains SII and SIII and in some cases the tip of the apical domain, involving SI. These data suggest a multipronged mechanism for the interaction of Gram-negative bacteria with collagen.IMPORTANCE Bacterial adhesion is a crucial step for bacterial colonization and infection. In recent years, the number of antibiotic-resistant strains has dramatically increased; therefore, there is a need to search for novel antimicrobial agents. Thus, great efforts are being devoted to develop a clear understanding of the bacterial adhesion mechanism for preventing infections. In host/pathogen interactions, once repulsive forces are overcome, adhesins recognize and tightly bind to specific receptors on the host cell or tissue components. Here, we present the first 3D structure of the interaction between the collagen-binding adhesin EmaA and collagen, which is critical for the development of endocarditis in humans.

A Nonfimbrial Adhesin of Aggregatibacter actinomycetemcomitans Mediates Biofilm Biogenesis

Periodontitis is an inflammatory disease caused by polymicrobial biofilms. The periodontal pathogen Aggregatibacter actinomycetemcomitans displays two proteinaceous surface structures, the fimbriae and the nonfimbrial extracellular matrix binding protein A (EmaA), as observed by electron microscopy. Fimbriae participate in biofilm biogenesis and the EmaA adhesins mediate collagen binding. However, in the absence of fimbriae, A. actinomycetemcomitans still retains the potential to form robust biofilms, suggesting that other surface macromolecules participate in biofilm development. Here, isogenic mutant strains lacking EmaA structures, but still expressing fimbriae, were observed to have reduced biofilm potential. In strains lacking both EmaA and fimbriae, biofilm mass was reduced by 80%. EmaA enhanced biofilm formation in different strains, independent of the fimbriation state or serotype. Confocal microscopy revealed differences in cell density within microcolonies between the EmaA positive and mutant strains. EmaA-mediated biofilm formation was found to be independent of the glycosylation state and the precise three-dimensional conformation of the protein, and thus this function is uncorrelated with collagen binding activity. The data suggest that EmaA is a multifunctional adhesin that utilizes different mechanisms to enhance bacterial binding to collagen and to enhance biofilm formation, both of which are important for A. actinomycetemcomitans colonization and subsequent infection.

Utilization of Variant and Fusion Proteins To Functionally Map the Aggregatibacter actinomycetemcomitans Trimeric Autotransporter Protein ApiA

The Gram-negative bacterium Aggregatibacter actinomycetemcomitans is a causative agent of localized aggressive periodontitis. Critical to its infection process is the first and essential step of attachment, which is related to the coordinated functions of surface components comprised of proteins and extracellular polysaccharides. One such protein is the outer membrane trimeric autotransporter protein ApiA, a versatile virulence factor with numerous functions, including cell binding, invasion, serum resistance, autoaggregation, and induction of cytokine release. Here we report on the use of Escherichia coli strains expressing protein variants to define the separate functions ascribed to the N terminus and those related to the C terminus. Importantly, a hybrid protein that comprised the N terminus of trimeric ApiA and the β-barrel domain of monomeric autotransporter Aae was constructed, which allowed the expression of a monomer surface-exposed domain of ApiA. Functional and phenotypic analyses demonstrated that the C terminus of ApiA forms an independent domain that is crucial for general stability and trimer formation, which appears to be associated with autoaggregation, biofilm formation, and surface expression. Importantly, the results show that the monomeric form of the N-terminal passenger domain of ApiA, while surface exposed, is sufficient for binding to buccal epithelial cells; however, it is not sufficient to allow aggregation and biofilm formation, strengthening the importance of the role of trimerization in these phenotypes.

A novel intrinsically disordered outer membrane lipoprotein of Aggregatibacter actinomycetemcomitans binds various cytokines and plays a role in biofilm response to interleukin-1β and interleukin-8

Intrinsically disordered proteins (IDPs) do not have a well-defined and stable 3-dimensional fold. Some IDPs can function as either transient or permanent binders of other proteins and may interact with an array of ligands by adopting different conformations. A novel outer membrane lipoprotein, bacterial interleukin receptor I (BilRI) of the opportunistic oral pathogen Aggregatibacter actinomycetemcomitans binds a key gatekeeper proinflammatory cytokine interleukin (IL)-1β. Because the amino acid sequence of the novel lipoprotein resembles that of fibrinogen binder A of Haemophilus ducreyi, BilRI could have the potential to bind other proteins, such as host matrix proteins. However, from the tested host matrix proteins, BilRI interacted with neither collagen nor fibrinogen. Instead, the recombinant non-lipidated BilRI, which was intrinsically disordered, bound various pro/anti-inflammatory cytokines, such as IL-8, tumor necrosis factor (TNF)-α, interferon (IFN)-γ and IL-10. Moreover, BilRI played a role in the in vitro sensing of IL-1β and IL-8 because low concentrations of cytokines did not decrease the amount of extracellular DNA in the matrix of bilRI⁻ mutant biofilm as they did in the matrix of wild-type biofilm when the biofilms were exposed to recombinant cytokines for 22 hours. BilRI played a role in the internalization of IL-1β in the gingival model system but did not affect either IL-8 or IL-6 uptake. However, bilRI deletion did not entirely prevent IL-1β internalization, and the binding of cytokines to BilRI was relatively weak. Thus, BilRI might sequester cytokines on the surface of A. actinomycetemcomitans to facilitate the internalization process in low local cytokine concentrations.

Physiological adaptations of key oral bacteria

Oral colonising bacteria are highly adapted to the various environmental niches harboured within the mouth, whether that means while contributing to one of the major oral diseases of caries, pulp infections, or gingival/periodontal disease or as part of a commensal lifestyle. Key to these infections is the ability to adhere to surfaces via a range of specialised adhesins targeted at both salivary and epithelial proteins, their glycans and to form biofilm. They must also resist the various physical stressors they are subjected to, including pH and oxidative stress. Possibly most strikingly, they have developed the ability to harvest both nutrient sources provided by the diet and those derived from the host, such as protein and surface glycans. We have attempted to review recent developments that have revealed much about the molecular mechanisms at work in shaping the physiology of oral bacteria and how we might use this information to design and implement new treatment strategies.

Ultrastructural analysis of the rugose cell envelope of a member of the Pasteurellaceae family

Bacterial membranes serve as selective environmental barriers and contain determinants required for bacterial colonization and survival. Cell envelopes of Gram-negative bacteria consist of an outer and an inner membrane separated by a periplasmic space. Most Gram-negative bacteria display a smooth outer surface (e.g., Enterobacteriaceae), whereas members of the Pasteurellaceae and Moraxellaceae families show convoluted surfaces. Aggregatibacter actinomycetemcomitans, an oral pathogen representative of the Pasteurellaceae family, displays a convoluted membrane morphology. This phenotype is associated with the presence of morphogenesis protein C (MorC). Inactivation of the morC gene results in a smooth membrane appearance when visualized by two-dimensional (2D) electron microscopy. In this study, 3D electron microscopy and atomic force microscopy of whole-mount bacterial preparations as well as 3D electron microscopy of ultrathin sections of high-pressure frozen and freeze-substituted specimens were used to characterize the membranes of both wild-type and morC mutant strains of A. actinomycetemcomitans. Our results show that the mutant strain contains fewer convolutions than the wild-type bacterium, which exhibits a higher curvature of the outer membrane and a periplasmic space with 2-fold larger volume/area ratio than the mutant bacterium. The inner membrane of both strains has a smooth appearance and shows connections with the outer membrane, as revealed by visualization and segmentation of 3D tomograms. The present studies and the availability of genetically modified organisms with altered outer membrane morphology make A. actinomycetemcomitans a model organism for examining membrane remodeling and its implications in antibiotic resistance and virulence in the Pasteurellaceae and Moraxellaceae bacterial families.

Pathogenicity of Treponema denticola Wild-Type and Mutant Strain Tested by an Active Mode of Periodontal Infection Using Microinjection

The available passive mode of periodontal infections in mice requires high efficiency of bacterial attachment and invasiveness and is not always suitable to test the pathogenicity of genetically engineered mutant strains. We developed an active mode of oral infection, using microinjection in the marginal gingiva of mice, to test the pathogenicity of a genetically engineered Treponema denticola mutant strain deficient in intermediate-like filaments, compared to the wild-type strain. This targeted mode of infection inoculates the bacterial strain to be tested directly at a lesion site (needle entry point) located at the future periodontal lesion site. The efficiency of T. denticola wild-type strain to elicit bone loss contrasted with the lack of pathogenicity of the intermediate-like filament deficient mutant strain in comparison to the sham infection. The periodontal microinjection oral model in mice can be used for a variety of applications complementary to the passive mode of periodontal infection in context of pathogenicity testing.

O-polysaccharide glycosylation is required for stability and function of the collagen adhesin EmaA of Aggregatibacter actinomycetemcomitans

Aggregatibacter actinomycetemcomitans is hypothesized to colonize through the interaction with collagen and establish a reservoir for further dissemination. The trimeric adhesin EmaA of A. actinomycetemcomitans binds to collagen and is modified with sugars mediated by an O-antigen polysaccharide ligase (WaaL) that is associated with lipopolysaccharide (LPS) biosynthesis (G. Tang and K. Mintz, J. Bacteriol. 192:1395-1404, 2010). This investigation characterized the function and cellular localization of EmaA glycosylation. The interruption of LPS biogenesis by using genetic and pharmacological methods changed the amount and biophysical properties of EmaA molecules in the outer membrane. In rmlC and waaL mutant strains, the membrane-associated EmaA was reduced by 50% compared with the wild-type strain, without changes in mRNA levels. The membrane-associated EmaA protein levels were recovered by complementation with the corresponding O-polysaccharide (O-PS) biosynthetic genes. In contrast, another trimeric autotransporter, epithelial adhesin ApiA, was not affected in the same mutant background. The inhibition of undecaprenyl pyrophosphate recycling by bacitracin resulted in a similar decrease in the membrane-associated EmaA protein. This effect was reversed by removal of the compound. A significant decrease in collagen binding activity was observed in strains expressing the nonglycosylated form of EmaA. Furthermore, the electrophoretic mobility shifts of the EmaA monomers found in the O-PS mutant strains were associated only with the membrane-associated protein and not with the cytoplasmic pre-EmaA protein, suggesting that this modification does not occur in the cytoplasm. The glycan modification of EmaA appears to be required for collagen binding activity and protection of the protein against degradation by proteolytic enzymes.

Characterization of the secretion pathway of the collagen adhesin EmaA of Aggregatibacter actinomycetemcomitans

The extracellular matrix protein adhesin A (EmaA) surface antennae-like structures of the periodontal pathogen Aggregatibacter actinomycetemcomitans are composed of three identical protein monomers. Recently, we have demonstrated that the protein is synthesized with an extended signal peptide of 56 amino acids necessary for membrane targeting and protein translocation. In this study, EmaA secretion was demonstrated to be reliant on a chaperone-dependent secretion pathway. Deletion of secB partially reduced but did not abolish the amount of EmaA in the membrane. This observation was attributed to an increase in the synthesis of DnaK in the ΔsecB strain. Overexpression of a DnaK substitution mutant (A174T), with diminished activity, in the ΔsecB strain further reduced the amount of EmaA in the membrane. Expression of dnaK A174T in the wild-type strain did not affect the amount of EmaA in the membrane when grown under optimal growth conditions at 37°C. However, EmaA was found to be reduced when this strain was grown at heat-shock temperature. A chromosomal deletion of amino acids 16-39 of the EmaA extended signal peptide, transformed with either the wild-type or dnaK A174T-expressing plasmid, did not affect the amount of EmaA in the membrane. In addition, the level of EmaA in a ΔsecB/emaA(-) double mutant strain expressing EmaAΔ16-39 was unchanged when grown at both temperatures. The data suggest that chaperones are required for the targeting of EmaA to the membrane and a specific region of the signal peptide is necessary for secretion under stress conditions.

Lipopolysaccharides mediate leukotoxin secretion in Aggregatibacter actinomycetemcomitans

We previously reported that lipopolysaccharide (LPS) -related sugars are associated with the glycosylation of the collagen adhesin EmaA, a virulence determinant of Aggregatibacter actinomycetemcomitans. In this study, the role of LPS in the secretion of other virulence factors was investigated. The secretion of the epithelial adhesin Aae, the immunoglobulin Fc receptor Omp34 and leukotoxin were examined in a mutant strain with inactivated TDP-4-keto-6-deoxy-d-glucose 3,5-epimerase (rmlC), which resulted in altered O-antigen polysaccharides (O-PS) of LPS. The secretion of Aae and Omp34 was not affected. However, the leukotoxin secretion, which is mediated by the TolC-dependent type I secretion system, was altered in the rmlC mutant. The amount of secreted leukotoxin in the bacterial growth medium was reduced nine-fold, with a concurrent four-fold increase of the membrane-bound toxin in the mutant compared with the wild-type strain. The altered leukotoxin secretion pattern was restored to the wild-type by complementation of the rmlC gene in trans. Examination of the ltxA mRNA levels indicated that the leukotoxin secretion was post-transcriptionally regulated in the modified O-PS containing strain. The mutant strain also showed increased resistance to vancomycin, an antibiotic dependent on TolC for internalization, indicating that TolC was affected. Overexpression of TolC in the rmlC mutant resulted in an increased TolC level in the outer membrane but did not restore the leukotoxin secretion profile to the wild-type phenotype. The data suggest that O-PS mediate leukotoxin secretion in A. actinomycetemcomitans.

Correlation of the amino-acid sequence and the 3D structure of the functional domain of EmaA from Aggregatibacter actinomycetemcomitans

Adhesion to collagen is an important virulence determinant for the periodontal pathogen Aggregatibacter actinomycetemcomitans. Binding to collagen is mediated by the extracellular-matrix protein adhesin-A (EmaA). EmaA is a homotrimeric autotransporter protein that forms flexible antenna-like appendages on the bacterium surface. An ellipsoidal structure at the distal end of the appendage, composed of three subdomains, contains the functional domain of the molecule. A correlation between amino-acid sequence and subdomain structure (SI and SII) was proposed based on an analysis of the volume/molecular weight ratio. EmaA from three mutant strains (deletions of amino-acids 70-206 and 70-386 and a substitution mutation G162S) has been studied by electron microscopy to test this hypothesis. 3D structures were analyzed using single-axis tilt tomography of negatively stained preparations of bacteria combined with subvolume averaging. Additionally, a large number of 2D images of the apical domain of the adhesins from the mutants were extracted from micrographs of the bacterial surface, aligned and classified. The combined data showed that amino-acids 70-206 localize to subdomain SI and 70-386 comprise subdomains SI and SII. Moreover, we showed that the substitution mutation G162S, which abolishes collagen binding activity, does not affect the overall structural integrity of the functional domain. However, the structure of subdomain SI in this mutant is slightly altered with respect to the wild-type strain. These data also have allowed us to interpret the architectural features of each subdomain of EmaA in more detail and to correlate the 3D structure of the functional domain of EmaA with the amino-acid sequence.

The structure of E. coli IgG-binding protein D suggests a general model for bending and binding in trimeric autotransporter adhesins

The Escherichia coli Ig-binding (Eib) proteins are trimeric autotransporter adhesins (TAAs) and receptors for IgG Fc. We present the structure of a large fragment of the passenger domain of EibD, the first TAA structure to have both a YadA-like head domain and the entire coiled-coil stalk. The stalk begins as a right-handed superhelix, but switches handedness halfway down. An unexpected β-minidomain joins the two and inserts a ∼120° rotation such that there is no net twist between the beginning and end of the stalk. This may be important in folding and autotransport. The surprisingly large cavities we found in EibD and other TAAs may explain how TAAs bend to bind their ligands. We identified how IgA and IgG bind and modeled the EibD-IgG Fc complex. We further show that EibD promotes autoagglutination and biofilm formation and forms a fibrillar layer covering the cell surface making zipper-like contacts between cells.

The extended signal peptide of the trimeric autotransporter EmaA of Aggregatibacter actinomycetemcomitans modulates secretion

The extracellular matrix protein adhesin A (EmaA) of the Gram-negative bacterium Aggregatibacter actinomycetemcomitans is a fibrillar collagen adhesin belonging to the family of trimeric autotransporters. The protein forms antenna-like structures on the bacterial surface required for collagen adhesion. The 202-kDa protein monomers are proposed to be targeted and translocated across the inner membrane by a long signal peptide composed of 56 amino acids. The predicted signal peptide was functionally active in Escherichia coli and A. actinomycetemcomitans using truncated PhoA and Aae chimeric proteins, respectively. Mutations in the signal peptide were generated and characterized for PhoA activity in E. coli. A. actinomycetemcomitans strains expressing EmaA with the identical mutant signal peptides were assessed for cellular localization, surface expression, and collagen binding activity. All of the mutants impaired some aspect of EmaA structure or function. A signal peptide mutant that promoted alkaline phosphatase secretion did not allow any cell surface presentation of EmaA. A second mutant allowed for cell surface exposure but abolished protein function. A third mutant allowed for the normal localization and function of EmaA at 37°C but impaired localization at elevated temperatures. Likewise, replacement of the long EmaA signal peptide with a typical signal peptide also impaired localization above 37°C. The data suggest that the residues of the EmaA signal peptide are required for protein folding or assembly of this collagen adhesin.

Stick to your gums: mechanisms of oral microbial adherence

Studies on the adherence properties of oral bacteria have been a major focus in microbiology research for several decades. The ability of bacteria to adhere to the variety of surfaces present in the oral cavity, and to become integrated within the resident microbial communities, confers growth and survival properties. Molecular analyses have revealed several families of Gram-positive bacterial surface proteins, including serine-rich repeat, antigen I/II, and pilus families, that mediate adherence to a variety of salivary and oral bacterial receptors. In Gram-negative bacteria, pili, auto-transporters, and extracellular matrix-binding proteins provide components for host tissue recognition and building of complex microbial communities. Future studies will reveal in greater detail the binding pockets for these adhesin families and their receptors. This information will be crucial for the development of new inhibitors or vaccines that target the functional regions of bacterial proteins that are involved in colonization and pathogenesis.

Adhesive activity of the haemophilus cryptic genospecies cha autotransporter is modulated by variation in tandem Peptide repeats

The Haemophilus cryptic genospecies is an important cause of maternal genital tract and neonatal systemic infections and initiates infection by colonizing the genital or respiratory epithelium. In recent work, we identified a unique Haemophilus cryptic genospecies protein called Cha, which mediates efficient adherence to genital and respiratory epithelia. The Cha adhesin belongs to the trimeric autotransporter family and contains an N-terminal signal peptide, an internal passenger domain that harbors adhesive activity, and a C-terminal membrane anchor domain. The passenger domain in Cha contains clusters of YadA-like head domains and neck motifs as well as a series of tandem 28-amino-acid peptide repeats. In the current study, we report that variation in peptide repeat number gradually modulates Cha adhesive activity, associated with a direct effect on the length of Cha fibers on the bacterial cell surface. The N-terminal 404 residues of the Cha passenger domain mediate binding to host cells and also facilitate bacterial aggregation through intermolecular Cha-Cha binding. As the tandem peptide repeats expand, the Cha fiber becomes longer and Cha adherence activity decreases. The expansion and contraction of peptide repeats represent a novel mechanism for modulating adhesive capacity, potentially balancing the need of the organism to colonize the genital and respiratory tracts with the ability to attach to alternative substrates, disperse within the host, or evade the host immune system.

Glycosylation of the collagen adhesin EmaA of Aggregatibacter actinomycetemcomitans is dependent upon the lipopolysaccharide biosynthetic pathway

The human oropharyngeal pathogen Aggregatibacter actinomycetemcomitans synthesizes multiple adhesins, including the nonfimbrial extracellular matrix protein adhesin A (EmaA). EmaA monomers trimerize to form antennae-like structures on the surface of the bacterium, which are required for collagen binding. Two forms of the protein have been identified, which are suggested to be linked with the type of O-polysaccharide (O-PS) of the lipopolysaccharide (LPS) synthesized (G. Tang et al., Microbiology 153:2447-2457, 2007). This association was investigated by generating individual mutants for a rhamnose sugar biosynthetic enzyme (rmlC; TDP-4-keto-6-deoxy-d-glucose 3,5-epimerase), the ATP binding cassette (ABC) sugar transport protein (wzt), and the O-antigen ligase (waaL). All three mutants produced reduced amounts of O-PS, and the EmaA monomers in these mutants displayed a change in their electrophoretic mobility and aggregation state, as observed in sodium dodecyl sulfate (SDS)-polyacrylamide gels. The modification of EmaA with O-PS sugars was suggested by lectin blots, using the fucose-specific Lens culinaris agglutinin (LCA). Fucose is one of the glycan components of serotype b O-PS. The rmlC mutant strain expressing the modified EmaA protein demonstrated reduced collagen adhesion using an in vitro rabbit heart valve model, suggesting a role for the glycoconjugant in collagen binding. These data provide experimental evidence for the glycosylation of an oligomeric, coiled-coil adhesin and for the dependence of the posttranslational modification of EmaA on the LPS biosynthetic machinery in A. actinomycetemcomitans.

Analysis of a collagen-binding trimeric autotransporter adhesin from Mannheimia haemolytica A1

A locus that codes for a high-molecular-weight adhesin was previously isolated from Mannheimia haemolytica A1. In this study, we showed that this locus, named ahs, codes for two proteins (AhsA and AhsB) that exhibit characteristics of a trimeric autotransporter adhesin. Sequence analysis of AhsA showed the presence of 21 collagen-binding motifs in the protein. Collagen-binding assays showed that M. haemolytica A1 binds to collagen in a dose-dependent manner. This binding activity is trypsin sensitive and can be inhibited by anti-AhsA antibody. AhsB is the cognate transporter for AhsA. The C-terminal of AhsB showed highly conserved amino acids typical of trimeric autotransporters. Experimental data showed that the C-terminal 120 amino acids of AhsB could indeed form trimeric molecules. Western immunoblots showed the presence of anti-AhsA antibodies in the sera of calves that had been challenged with M. haemolytica A1, suggesting that AhsA is expressed and immunogenic in cattle.

Investigation of the three-dimensional architecture of the collagen adhesin EmaA of Aggregatibacter actinomycetemcomitans by electron tomography

The periodontal pathogen Aggregatibacter actinomycetemcomitans displays on the bacterial surface a nonfimbrial adhesin, EmaA, which is required for collagen binding. In this study, electron tomography was used to characterize the three-dimensional (3D) architecture of this adhesin. The antenna-like surface appendages, corresponding to EmaA, were found to be composed of an ellipsoidal domain capping a rod-like domain that adopts either a straight or a bent conformation at various positions along the length. The most common flexible point along the length of the EmaA appendage was localized 29.4 nm away from the distal end. One-fifth of the appendages were straight and the remaining showed angles distributed between 140 degrees and 170 degrees at this location. Deletion analysis mapped this bend to amino acids 611 to 640 of the protein sequence. The 3D structure of the collagen binding domain of EmaA was generated by alignment and averaging of 9 subvolumes of the adhesin extracted from tomograms. The structure contains three subdomains: a globular structure with a diameter of approximately 5 nm and a cylindrical domain ( approximately 4.4 nm by 5.8 nm) separated by a linker region with a diameter of approximately 3 nm, followed by a cylindrical domain ( approximately 4.6 nm by 6.6 nm). This is the first 3D structure of a trimeric autotransporter protein of A. actinomycetemcomitans.

Membrane morphology and leukotoxin secretion are associated with a novel membrane protein of Aggregatibacter actinomycetemcomitans

Gram-negative bacteria display either a flat or an irregular outer membrane. The periodontal pathogen Aggregatibacter (Actinobacillus) actinomycetemcomitans has an irregular outer membrane. We have identified a gene that is associated with the biogenesis of this morphology. The gene is part of a three-gene operon and codes for a 141-kDa protein designated morphogenesis protein C (MorC), which is conserved in several gram-negative bacteria including Haemophilus influenzae and Pasteurella multocida. Insertional inactivation of this gene resulted in the conversion of an irregularly shaped membrane to a flat membrane. Associated with this morphological change were the autoaggregation of the bacteria during planktonic growth and a concomitant increase in the surface hydrophobicity of the bacterium. The absence of MorC also resulted in the loss of the secretion of leukotoxin but not the ltxA transcription. Our findings suggest that MorC is critical for membrane morphology and leukotoxin secretion in A. actinomycetemcomitans.