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

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.

Folding Control in the Path of Type 5 Secretion

The type 5 secretion system (T5SS) is one of the more widespread secretion systems in Gram-negative bacteria. Proteins secreted by the T5SS are functionally diverse (toxins, adhesins, enzymes) and include numerous virulence factors. Mechanistically, the T5SS has long been considered the simplest of secretion systems, due to the paucity of proteins required for its functioning. Still, despite more than two decades of study, the exact process by which T5SS substrates attain their final destination and correct conformation is not totally deciphered. Moreover, the recent addition of new sub-families to the T5SS raises additional questions about this secretion mechanism. Central to the understanding of type 5 secretion is the question of protein folding, which needs to be carefully controlled in each of the bacterial cell compartments these proteins cross. Here, the biogenesis of proteins secreted by the Type 5 secretion system is discussed, with a focus on the various factors preventing or promoting protein folding during biogenesis.

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.

The extended leader peptide of Haemophilus parasuis trimeric autotransporters conditions their protein expression in Escherichia coli

Trimeric autotransporters are surface-exposed proteins of Gram-negative bacteria belonging to the type V secretion system. They are involved in virulence and are targets for vaccine and diagnostic tool development, so optimal systems for their expression and purification are required. In the present study, the impact of the extended leader peptide of the Haemophilus parasuis virulence-associated trimeric autotransporters (VtaA) in its production as recombinant proteins in Escherichia coli was evaluated. The 13 genes encoding the VtaA1 to VtaA13 passenger domains of the strain Nagasaki were cloned in the pASK-IBA33plus plasmid and expressed in E. coli. Recombinant protein production was higher for truncated forms in which the entire leader peptide was deleted, and the recombinant protein accumulated in the cytoplasm of the cells. The yield of protein production of the different VtaAs was size dependent, and reached maximal amount at 2-4 h post -induction. The optimization of these conditions allowed to scale-up the production to obtain enough recombinant protein to immunize large animals.

Type V Secretion Systems in Bacteria

Type V secretion denotes a variety of secretion systems that cross the outer membrane in Gram-negative bacteria but that depend on the Sec machinery for transport through the inner membrane. They are possibly the simplest bacterial secretion systems, because they consist only of a single polypeptide chain (or two chains in the case of two-partner secretion). Their seemingly autonomous transport through the outer membrane has led to the term "autotransporters" for various subclasses of type V secretion. In this chapter, we review the structure and function of these transporters and review recent findings on additional factors involved in the secretion process, which have put the term "autotransporter" to debate.

The Respiratory Pathogen Moraxella catarrhalis Targets Collagen for Maximal Adherence to Host Tissues

Moraxella catarrhalis is a human respiratory pathogen that causes acute otitis media in children and is associated with exacerbations in patients suffering from chronic obstructive pulmonary disease (COPD). The first step in M. catarrhalis colonization is adherence to the mucosa, epithelial cells, and extracellular matrix (ECM). The objective of this study was to evaluate the role of M. catarrhalis interactions with collagens from various angles. Clinical isolates (n= 43) were tested for collagen binding, followed by a detailed analysis of protein-protein interactions using recombinantly expressed proteins.M. catarrhalis-dependent interactions with collagen produced by human lung fibroblasts and tracheal tissues were studied by utilizing confocal immunohistochemistry and high-resolution scanning electron microscopy. A mouse smoke-induced chronic obstructive pulmonary disease (COPD) model was used to estimate the adherence of M. catarrhalis in vivo. We found that all M. catarrhalis clinical isolates tested adhered to fibrillar collagen types I, II, and III and network-forming collagens IV and VI. The trimeric autotransporter adhesins ubiquitous surface protein A2(UspA2) and UspA2H were identified as major collagen-binding receptors.M. catarrhalis wild type adhered to human tracheal tissue and collagen-producing lung fibroblasts, whereas UspA2 and UspA2H deletion mutants did not. Moreover, in the COPD mouse model, bacteria devoid of UspA2 and UspA2H had a reduced level of adherence to the respiratory tract compared to the adherence of wild-type bacteria. Our data therefore suggest that theM. catarrhalisUspA2 and UspA2H-dependent interaction with collagens is highly critical for adherence in the host and, furthermore, may play an important role in the establishment of disease.

IMPORTANCE

The respiratory tract pathogen Moraxella catarrhalis adheres to the host by interacting with several components, including the ECM. Collagen accounts for 30% of total body proteins, and therefore, bacterial adherence to abundant host collagens mediates bacterial persistence and colonization. In this study, we characterized previously unknown M. catarrhalis-dependent interactions with host collagens and found that the trimeric autotransporter adhesins ubiquitous surface protein A2(UspA2) and UspA2H are highly important. Our observations also suggested that collagen-mediated adherence ofM. catarrhalis is indispensable for bacterial survival in the host, as exemplified by a mouse COPD model.

New findings on the function and potential applications of the trimeric autotransporter adhesin

Trimeric autotransporter adhesins (TAAs) are located on the surface of many pathogenic Gram-negative bacteria. TAAs belong to the autotransporter protein family and consist of three identical monomers. These obligate homotrimeric proteins are secreted through the bacterial type Vc secretion system and share a common molecular organization that each monomer consists of a N-terminal "passenger" domain and a C-terminal translocation domain. TAAs are important virulence factors that are involved in bacterial life cycle and participate in mediating infection, invasion, dissemination and evasion of host immune responses. TAAs have also proved to be useful for many applications, such as vaccines and disease biomarkers. We here mainly focused on new findings on bio-function and application of TAAs in addition to their common structure and secretion mechanisms.

Breaking the bacterial protein targeting and translocation model: oral organisms as a case in point

Insights into the membrane biogenesis of oral and throat bacteria have highlighted key differences in protein localization by the general secretion pathway compared with the well-studied Escherichia coli model system. These intriguing novelties have advanced our understanding of both how these microorganisms have adapted to survive and cause disease in the oral cavity, and the field of protein translocation as a whole. This review focuses on findings that highlight where oral bacteria differ from the E. coli paradigm, why these differences are biologically important, and what questions remain about the differences in pathway function. The majority of insight into protein translocation in microbes of the oral cavity has come from streptococcal species, which will be the main topic of this review. However, other bacteria will be discussed when relevant. An overview of the E. coli model of protein targeting and translocation is provided for comparison.

Autotransporter secretion: varying on a theme

Autotransporters are widely distributed among Gram-negative bacteria. They can have a large variety of functions and many of them have a role in virulence. They are synthesized as large precursors with an N-terminal signal sequence that mediates transport across the inner membrane via the Sec machinery and a translocator domain that mediates the transport of the connected passenger domain across the outer membrane to the bacterial cell surface. Like integral outer membrane proteins, the translocator domain folds in a β-barrel structure and requires the Bam machinery for its insertion into the outer membrane. After transport across the outer membrane, the passenger may stay connected via the translocator domain to the bacterial cell surface or it is proteolytically released into the extracellular milieu. Based on the size of the translocator domain and its position relative to the passenger in the precursor, autotransporters are divided into four sub-categories. We review here the current knowledge of the biogenesis, structure and function of various autotransporters.

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.

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.