Xiaowen Liang

A ‘Collagen Hug’ Model for Staphylococcus aureus CNA binding to collagen

The structural basis for the association of eukaryotic and prokaryotic protein receptors and their triple‐helical collagen ligand remains poorly understood. Here, we present the crystal structures of a high affinity subsegment of the Staphylococcus aureus collagen‐binding CNA as an apo‐protein and in complex with a synthetic collagen‐like triple helical peptide. The apo‐protein structure is composed of two subdomains (N1 and N2), each adopting a variant IgG‐fold, and a long linker that connects N1 and N2. The structure is stabilized by hydrophobic inter‐domain interactions and by the N2 C‐terminal extension that complements a β‐sheet on N1. In the ligand complex, the collagen‐like peptide penetrates through a spherical hole formed by the two subdomains and the N1–N2 linker. Based on these two structures we propose a dynamic, multistep binding model, called the ‘Collagen Hug’ that is uniquely designed to allow multidomain collagen binding proteins to bind their extended rope‐like ligand.

Identification and Characterization of the C3 Binding Domain of the Staphylococcus aureus Extracellular Fibrinogen-binding Protein (Efb)*

The secreted Staphylococcus aureus extracellular fibrinogen-binding protein (Efb) is a virulence factor that binds to both the complement component C3b and fibrinogen. Our laboratory previously reported that by binding to C3b, Efb inhibited complement activation and blocked opsonophagocytosis. We have now located the Efb binding domain in C3b to the C3d fragment and determined a disassociation constant (Kd) of 0.24 μm for the Efb-C3d binding using intrinsic fluorescence quenching assays. Using truncated, recombinant forms of Efb, we also demonstrate that the C3b binding region of Efb is located within the C terminus, in contrast to the fibrinogen binding domains that are located at the N-terminal end of the protein. Enzyme-linked immunosorbent assay-type binding assays demonstrated that recombinant Efb could bind to both C3b and fibrinogen simultaneously, forming a trimolecular complex and that the C-terminal region of Efb could inhibit complement activity in vitro. In addition, secondary structure analysis using circular dichroism spectroscopy revealed that the C-terminal, C3b binding region of Efb is composed primarily of α-helices, suggesting that this domain of Efb represents a novel type of C3b-binding protein.

Virulence potential of the staphylococcal adhesin CNA in experimental arthritis is determined by its affinity for collagen.

BACKGROUND Staphylococcus aureus is a major cause of bacterial arthritis, which often results in severe joint damage. CNA, a collagen adhesin of S. aureus, was shown to be a virulence factor in several animal models. However, the precise molecular mechanism by which CNA contributes to virulence remains unclear. METHODS We examined the role of the collagen-binding function of CNA in a mouse model of septic arthritis by comparing the virulence of isogenic strains of S. aureus expressing (1) wild-type CNA, (2) a truncated form of CNA (CNA35) with a higher affinity for collagen than the wild type, (3) CNA35 containing a single point mutation resulting in loss of collagen binding, (4) CNA lacking the collagen-binding domain, and (5) the collagen-binding domain of ACE (adhesin of collagen from Enterococcus faecalis). RESULTS AND CONCLUSIONS The results provide, for the first time, direct evidence that the virulence of CNA depends on its collagen-binding ability. Collagen binding facilitated early colonization of the joints of mice. Furthermore, the virulence potential of the adhesin is determined by the adhesins affinity for its ligand, as well as its binding kinetics.

Systemic combinatorial peptide selection yields a non-canonical iron-mimicry mechanism for targeting tumors in a mouse model of human glioblastoma

The management of CNS tumors is limited by the blood-brain barrier (BBB), a vascular interface that restricts the passage of most molecules from the blood into the brain. Here we show that phage particles targeted with certain ligand motifs selected in vivo from a combinatorial peptide library can cross the BBB under normal and pathological conditions. Specifically, we demonstrated that phage clones displaying an iron-mimic peptide were able to target a protein complex of transferrin and transferrin receptor (TfR) through a non-canonical allosteric binding mechanism and that this functional protein complex mediated transport of the corresponding viral particles into the normal mouse brain. We also showed that, in an orthotopic mouse model of human glioblastoma, a combination of TfR overexpression plus extended vascular permeability and ligand retention resulted in remarkable brain tumor targeting of chimeric adeno-associated virus/phage particles displaying the iron-mimic peptide and carrying a gene of interest. As a proof of concept, we delivered the HSV thymidine kinase gene for molecular-genetic imaging and targeted therapy of intracranial xenografted tumors. Finally, we established that these experimental findings might be clinically relevant by determining through human tissue microarrays that many primary astrocytic tumors strongly express TfR. Together, our combinatorial selection system and results may provide a translational avenue for the targeted detection and treatment of brain tumors.

Collagen-binding Microbial Surface Components Recognizing Adhesive Matrix Molecule (MSCRAMM) of Gram-positive Bacteria Inhibit Complement Activation via the Classical Pathway

Background: Collagen-binding MSCRAMMs from Gram-positive bacteria are adhesins and are virulence factors in several infectious diseases models. Results: Cna and related collagen-binding MSCRAMMs bind C1q and block activation of the classical complement pathway. Conclusion: Collagen-binding MSCRAMMs are novel classical complement pathway inhibitors. Significance: The novel function of the Cna-like collagen-binding MSCRAMMs represents an immune evasion strategy potentially used by numerous Gram-positive pathogens. Members of a family of collagen-binding microbial surface components recognizing adhesive matrix molecules (MSCRAMMs) from Gram-positive bacteria are established virulence factors in several infectious diseases models. Here, we report that these adhesins also can bind C1q and act as inhibitors of the classical complement pathway. Molecular analyses of Cna from Staphylococcus aureus suggested that this prototype MSCRAMM bound to the collagenous domain of C1q and interfered with the interactions of C1r with C1q. As a result, C1r2C1s2 was displaced from C1q, and the C1 complex was deactivated. This novel function of the Cna-like MSCRAMMs represents a potential immune evasion strategy that could be used by numerous Gram-positive pathogens.

Fibrinogen Is a Ligand for the Staphylococcus aureus Microbial Surface Components Recognizing Adhesive Matrix Molecules (MSCRAMM) Bone Sialoprotein-binding Protein (Bbp)

Microbial surface components recognizing adhesive matrix molecules (MSCRAMMs) are bacterial surface proteins mediating adherence of the microbes to components of the extracellular matrix of the host. On Staphylococci, the MSCRAMMs often have multiple ligands. Consequently, we hypothesized that the Staphylococcus aureus MSCRAMM bone sialoprotein-binding protein (Bbp) might recognize host molecules other than the identified bone protein. A ligand screen revealed that Bbp binds human fibrinogen (Fg) but not Fg from other mammals. We have characterized the interaction between Bbp and Fg. The binding site for Bbp was mapped to residues 561–575 in the Fg Aα chain using recombinant Fg chains and truncation mutants in Far Western blots and solid-phase binding assays. Surface plasmon resonance was used to determine the affinity of Bbp for Fg. The interaction of Bbp with Fg peptides corresponding to the mapped residues was further characterized using isothermal titration calorimetry. In addition, Bbp expressed on the surface of bacteria mediated adherence to immobilized Fg Aα. Also, Bbp interferes with thrombin-induced Fg coagulation. Together these data demonstrate that human Fg is a ligand for Bbp and that Bbp can manipulate the biology of the Fg ligand in the host.

Molecular characterization of the interaction of staphylococcal microbial surface components recognizing adhesive matrix molecules (MSCRAMM) ClfA and Fbl with fibrinogen.

The ligand-binding domain of Fbl (the fibrinogen binding protein from Staphylococcus lugdunensis) shares 60% sequence identity with ClfA (clumping factor A) of Staphylococcus aureus. Recombinant Fbl corresponding to the minimum fibrinogen-binding region (subdomains N2N3) was compared with ClfA for binding to fibrinogen. Fbl and ClfA had very similar affinities for fibrinogen by surface plasmon resonance. The binding site for Fbl in fibrinogen was localized to the extreme C terminus of the fibrinogen γ-chain at the same site recognized by ClfA. Isothermal titration calorimetry showed that Fbl and ClfA had very similar affinities for a peptide mimicking the C-terminal segment of the fibrinogen γ-chain. The peptide also inhibited binding of Fbl and ClfA to fibrinogen. A series of substituted γ-chain variant peptides behaved very similarly when used to inhibit ClfA and Fbl binding to immobilized fibrinogen. Both ClfA and Fbl bound to bovine fibrinogen with a lower affinity compared with human fibrinogen and did not bind detectably to ovine fibrinogen. The structure of the N2N3 subdomains of Fbl in complex with the fibrinogen γ-chain peptide was modeled based on the crystal structure of the N2N3 subdomains of the ClfA-γ-chain peptide complex. Residues in the putative binding trench likely to be involved in fibrinogen binding were identified. Fbl variant proteins with alanine substitutions in key residues had reduced affinities for fibrinogen. Thus Fbl and ClfA bind the same site in fibrinogen by similar mechanisms.

Procollagen C Proteinase Enhancer 1 Genes Are Important Determinants of the Mechanical Properties and Geometry of Bone and the Ultrastructure of Connective Tissues

ABSTRACT Procollagen C proteinases (pCPs) cleave type I to III procollagen C propeptides as a necessary step in assembling the major fibrous components of vertebrate extracellular matrix. The protein PCOLCE1 (procollagen C proteinase enhancer 1) is not a proteinase but can enhance the activity of pCPs ∼10-fold in vitro and has reported roles in inhibiting other proteinases and in growth control. Here we have generated mice with null alleles of the PCOLCE1 gene, Pcolce, to ascertain in vivo roles. Although Pcolce− / − mice are viable and fertile, Pcolce − / − male, but not female, long bones are more massive and have altered geometries that increase resistance to loading, compared to wild type. Mechanical testing indicated inferior material properties of Pcolce − / − male long bone, apparently compensated for by the adaptive changes in bone geometry. Male and female Pcolce − / − vertebrae both appeared to compensate for inferior material properties with thickened and more numerous trabeculae and had a uniquely altered morphology in deposited mineral. Ultrastructurally, Pcolce − / − mice had profoundly abnormal collagen fibrils in both mineralized and nonmineralized tissues. In Pcolce − / − tendon, 100% of collagen fibrils had deranged morphologies, indicating marked functional effects in this tissue. Thus, PCOLCE1 is an important determinant of bone mechanical properties and geometry and of collagen fibril morphology in mammals, and the human PCOLCE1 gene is identified as a candidate for phenotypes with defects in such attributes in humans.

Binding of Efb from Staphylococcus aureus to fibrinogen blocks neutrophil adherence.

In addition to its pivotal role in hemostasis, fibrinogen (Fg) and provisional fibrin matrices play important roles in inflammation and regulate innate immune responses by interacting with leukocytes. Efb (the extracellular fibrinogen-binding protein) is a secreted Staphylococcus aureus protein that engages host Fg and complement C3. However, the molecular details underlying the Efb-Fg interaction and the biological relevance of this interaction have not been determined. In the present study, we characterize the interaction of Efb with Fg. We demonstrate that the Fg binding activity is located within the intrinsically disordered N-terminal half of Efb (Efb-N) and that the D fragment of Fg is the region that mediates Efb-N binding. More detailed studies of the Efb-N-Fg interactions using ELISA and surface plasmon resonance analyses revealed that Efb-N exhibits a much higher affinity for Fg than typically observed with Fg-binding MSCRAMMs (microbial surface components recognizing adhesive matrix molecules), and data obtained from ELISA analyses using truncated Efb-N constructs demonstrate that Efb-N contains two binding sites located within residues 30–67 and 68–98, respectively. Efb-N inhibits neutrophil adhesion to immobilized Fg by binding to Fg and blocking the interaction of the protein with the leukocyte integrin receptor, αMβ2. A motif in the Fg γ chain previously shown to be central to the αMβ2 interaction was shown to be functionally distinguishable from the Efb-N binding site, suggesting that the Fg-Efb interaction indirectly impedes Fg engagement by αMβ2. Taken together, these studies provide insights into how Efb interacts with Fg and suggest that Efb may support bacterial virulence at least in part by impeding Fg-driven leukocyte adhesion events.

A Novel Fibronectin Binding Motif in MSCRAMMs Targets F3 Modules

Background BBK32 is a surface expressed lipoprotein and fibronectin (Fn)-binding microbial surface component recognizing adhesive matrix molecule (MSCRAMM) of Borrelia burgdorferi, the causative agent of Lyme disease. Previous studies from our group showed that BBK32 is a virulence factor in experimental Lyme disease and located the Fn-binding region to residues 21–205 of the lipoprotein. Methodology/Principal Findings Studies aimed at identifying interacting sites between BBK32 and Fn revealed an interaction between the MSCRAMM and the Fn F3 modules. Further analysis of this interaction showed that BBK32 can cause the aggregation of human plasma Fn in a similar concentration-dependent manner to that of anastellin, the superfibronectin (sFn) inducing agent. The resulting Fn aggregates are conformationally distinct from plasma Fn as indicated by a change in available thermolysin cleavage sites. Recombinant BBK32 and anastellin affect the structure of Fn matrices formed by cultured fibroblasts and inhibit endothelial cell proliferation similarly. Within BBK32, we have located the sFn-forming activity to a region between residues 160 and 175 which contains two sequence motifs that are also found in anastellin. Synthetic peptides mimicking these motifs induce Fn aggregation, whereas a peptide with a scrambled sequence motif was inactive, suggesting that these motifs represent the sFn-inducing sequence. Conclusions/Significance We conclude that BBK32 induces the formation of Fn aggregates that are indistinguishable from those formed by anastellin. The results of this study provide evidence for how bacteria can target host proteins to manipulate host cell activities.