Trench-shaped binding sites promote multiple classes of interactions between collagen and the adherence receptors, alpha(1)beta(1) integrin and Staphylococcus aureus cna MSCRAMM

Molecular dynamics simulations, docking and MMGBSA studies of newly designed peptide-conjugated glucosyloxy stilbene derivatives with tumor cell receptors

In this work, for the first time, we designed derivatives of beta-D-glucosyloxy-3-hydroxy-trans-stiblene-2-carboxylic acid (GHS), by conjugating GHS with tumor targeting peptides RPARPAR and GGKRPAR to target over-expressed receptors in tumor cells. The sequences RPARPAR and GGKRPAR are known to target the neuropilin1 (NRP1) receptor due to the C-terminal Arg domain; however, their effectiveness has never been examined with other commonly over-expressed receptors in tumor cells, particularly of chronic lymphocytic leukemia that include integrin α1β1 and CD22. By conjugating these peptides with GHS, which is known for its inherent anti-cancer properties, the goal is to further enhance tumor cell targeting by developing compounds that can target multiple receptors. The physicochemical properties of the conjugates and individual peptides were analyzed using Turbomole and COSMOthermX20 in order to determine their hydrogen bond accepting and donating capabilities. The web server POCASA was used in order to determine the surface cavities and binding pockets of the three receptors. To explore the binding affinities, we conducted molecular docking studies with the peptides and the conjugates with each of the receptors. After molecular docking, the complexes were analyzed using Protein-Ligand Interaction Profiler to determine the types of interactions involved. Molecular dynamics simulation studies were conducted to explore the stability of the receptor-ligand complexes. Our results indicated that in most cases the conjugates showed higher binding and stability with the receptors. Additionally, highly stable complexes of conjugates were obtained with CD22, NRP1 and in most cases with the integrin α1β1 receptor as well. The binding energies were calculated for each of the receptor ligand complexes through trajectory analysis using MMGBSA studies. SwissADME studies revealed that the compounds showed low GI absorption and were not found to be CYP inhibitors and had bioavailability score that would allow them to be considered as potential drug candidates. Overall, our results for the first time show that the designed conjugates can target multiple over-expressed receptors in tumor cells and may be potentially developed as future therapeutics for targeting tumor cells.

Bioconjugation of a Collagen-Mimicking Peptide Onto Poly(vinyl alcohol) Encourages Endothelialization While Minimizing Thrombosis

Poly(vinyl alcohol) hydrogel, PVA, is a suitable material for small-diameter vascular grafting. However, the bioinert properties of the material do not allow for in situ endothelialization, which is needed to combat common graft failure mechanisms, such as intimal hyperplasia and thrombosis. In this work, the surface of planar and tubular PVA was covalently modified with a collagen-mimicking peptide, GFPGER. The surface of modified PVA was characterized by measuring contact angle and x-ray photoelectron spectroscopy. Endothelial cell attachment to GFPGER-modified PVA was quantified and qualitatively examined using immunohistochemical staining. Then, in vitro hemocompatibility testing was performed by quantifying platelet attachment, coagulation factor XII activation, and initiation of fibrin formation. Finally, an established ex vivo, non-human primate model was employed to examine platelet attachment and fibrin formation under non-anticoagulated, whole blood flow conditions. GFPGER-modified PVA supported increased EC attachment. In vitro initiation of fibrin formation on the modified material was significantly delayed. Ex vivo thrombosis assessment showed a reduction in platelet attachment and fibrin formation on GFPGER-modified PVA. Overall, GFPGER-modified PVA encouraged cell attachment while maintaining the material’s hemocompatibility. This work is a significant step toward the development and characterization of a modified-hydrogel surface to improve endothelialization while reducing platelet attachment.

Crystal structure of lactobacillar SpaC reveals an atypical five-domain pilus tip adhesin: Exposing its substrate-binding and assembly in SpaCBA pili

Adhesion to cell surfaces is an essential and early prerequisite for successful host colonization by bacteria, and in most instances involves the specificities of various adhesins. Among bacterial Gram-positives, some genera and species mediate attachment to host cells by using long non-flagellar appendages called sortase-dependent pili. A case in point is the beneficial Lactobacillus rhamnosus GG gut-adapted strain that produces the so-called SpaCBA pilus, a structure noted for its promiscuous binding to intestinal mucus and collagen. Structurally, SpaCBA pili are heteropolymers of three different pilin-protein subunits, each with its own location and function in the pilus: backbone SpaA for length, basal SpaB for anchoring, and tip SpaC for adhesion. Previously, we solved the SpaA tertiary structure by X-ray crystallography and also reported on the crystallization of SpaB and SpaC. Here, we reveal the full-length high-resolution (1.9 Å) crystal structure of SpaC, a first for a sortase-dependent pilus-bearing commensal. The SpaC structure, unlike the representative four-domain architecture of other Gram-positive tip pilins, espouses an atypically longer five-domain arrangement that includes N-terminal 'binding' and C-terminal 'stalk' regions of two and three domains, respectively. With the prospect of establishing new mechanistic insights, we provide a structural basis for the multi-substrate binding nature of SpaC, as well as a structural model that reconciles its exclusive localization at the SpaCBA pilus tip.

Magnesium Activates Microsecond Dynamics to Regulate Integrin-Collagen Recognition

Integrin receptors bind collagen via metal-mediated interactions that are modulated by magnesium (Mg²⁺) levels in the extracellular matrix. Nuclear magnetic resonance-based relaxation experiments, isothermal titration calorimetry, and adhesion assays reveal that Mg²⁺ functions as both a structural anchor and dynamic switch of the α₁β₁ integrin I domain (α₁I). Specifically, Mg²⁺ binding activates micro- to millisecond timescale motions of residues distal to the binding site, particularly those surrounding the salt bridge at helix 7 and near the metal ion-dependent adhesion site. Mutagenesis of these residues impacts α₁I functional activity, thereby suggesting that Mg-bound α₁I dynamics are important for collagen binding and consequent allosteric rearrangement of the low-affinity closed to high-affinity open conformation. We propose a multistep recognition mechanism for α₁I-Mg-collagen interactions involving both conformational selection and induced-fit processes. Our findings unravel the multifaceted role of Mg²⁺ in integrin-collagen recognition and assist in elucidating the molecular mechanisms by which metals regulate protein-protein interactions.

Glucan Binding Protein C of Streptococcus mutans Mediates both Sucrose-Independent and Sucrose-Dependent Adherence

The high-resolution structure of glucan binding protein C (GbpC) at 1.14 Å, a sucrose-dependent virulence factor of the dental caries pathogen Streptococcus mutans, has been determined. GbpC shares not only structural similarities with the V regions of AgI/II and SspB but also functional adherence to salivary agglutinin (SAG) and its scavenger receptor cysteine-rich domains (SRCRs). This is not only a newly identified function for GbpC but also an additional fail-safe binding mechanism for S. mutans Despite the structural similarities with S. mutans antigen I/II (AgI/II) and SspB of Streptococcus gordonii, GbpC remains unique among these surface proteins in its propensity to adhere to dextran/glucans. The complex crystal structure of GbpC with dextrose (β-d-glucose; Protein Data Bank ligand BGC) highlights exclusive structural features that facilitate this interaction with dextran. Targeted deletion mutant studies on GbpC's divergent loop region in the vicinity of a highly conserved calcium binding site confirm its role in biofilm formation. Finally, we present a model for adherence to dextran. The structure of GbpC highlights how artfully microbes have engineered the lectin-like folds to broaden their functional adherence repertoire.

Single-Cell and Single-Molecule Analysis Unravels the Multifunctionality of the Staphylococcus aureus Collagen-Binding Protein Cna

The collagen-binding protein Cna is a prototype cell surface protein from Staphylococcus aureus which fulfils important physiological functions during pathogenesis. While it is established that Cna binds to collagen (Cn) via the high-affinity collagen hug mechanism, whether this protein is engaged in other ligand-binding mechanisms is poorly understood. Here, we use atomic force microscopy to demonstrate that Cna mediates attachment to two structurally and functionally different host proteins, i.e., the complement system protein C1q and the extracellular matrix protein laminin (Lam), through binding mechanisms that differ from the collagen hug. We show that single Cna-C1q and Cna-Lam bonds are much weaker than the high-affinity Cna-Cn bond and that their formation does not require the B-region of Cna. At the whole cell level, we find that bacterial adhesion to C1q-substrates involves only one (or two) molecular bond(s), while adhesion to Lam is mediated by multiple bonds, thus suggesting that multivalent or cooperative interactions may enhance the strength of adhesion. Both C1q and Lam interactions can be efficiently blocked by monoclonal antibodies directed against the minimal Cn-binding domain of Cna. These results show that Cna is a multifunctional protein capable of binding to multiple host ligands through mechanisms that differ from the classical collagen hug.

Design and Use of Chimeric Proteins Containing a Collagen-Binding Domain for Wound Healing and Bone Regeneration

Collagen-based biomaterials are widely used in the field of tissue engineering; they can be loaded with biomolecules such as growth factors (GFs) to modulate the biological response of the host and thus improve its potential for regeneration. Recombinant chimeric GFs fused to a collagen-binding domain (CBD) have been reported to improve their bioavailability and the host response, especially when combined with an appropriate collagen-based biomaterial. This review first provides an extensive description of the various CBDs that have been fused to proteins, with a focus on the need for accurate characterization of their interaction with collagen. The second part of the review highlights the benefits of various CBD/GF fusion proteins that have been designed for wound healing and bone regeneration.

Visualisation of Collagen in fixed skeletal muscle tissue using fluorescently tagged Collagen binding protein CNA35

Detection and visualisation of Collagen structure are important to understand the relationship between mechanical behaviour and microstructure in skeletal muscle since Collagen is the main structural protein in animal connective tissues, and is primarily responsible for their passive load-bearing properties. In the current study, the direct detection and visualization of Collagen using fluorescently tagged CNA35 binding protein (fused to EGFP or tdTomato) is reported for the first time on fixed skeletal muscle tissue. This Technical Note also establishes a working protocol by examining tissue preparation, dilution factor, exposure time etc. for sensitivity and specificity. Penetration of the binding protein into intact mature skeletal muscle was found to be very limited, but detection works well on tissue sections with higher sensitivity on wax embedded sections compared to frozen sections. CNA35 fused to tdTomato has a higher sensitivity than CNA35 fused to EGFP but both show specific detection. Best results were obtained with 15μm wax embedded sections, with blocking of non-specific binding in 1% BSA and antigen retrieval in Sodium Citrate. There was a play-off between dilution of the binding protein and time of incubation but both CNA35-tdTomato and CNA35-EGFP worked well with approximately 100μg/ml of purified protein with overnight incubation, while CNA35-tdTomato could be utilized at 5 fold less concentration. This approach can be applied to study the relationship between skeletal muscle micro-structure and to observe mechanical response to applied deformation. It can be used more broadly to detect Collagen in a variety of fixed tissues, useful for structure-functions studies, constitutive modelling, tissue engineering and assessment of muscle tissue pathologies.

Intrinsic local destabilization of the C-terminus predisposes integrin α1 I domain to a conformational switch induced by collagen binding

Integrin-collagen interactions play a critical role in a myriad of cellular functions that include immune response, and cell development and differentiation, yet their mechanism of binding is poorly understood. There is increasing evidence that conformational flexibility assumes a central role in the molecular mechanisms of protein-protein interactions and here we employ NMR hydrogen-deuterium exchange (HDX) experiments to explore the impact of slower timescale dynamic events. To gain insight into the mechanisms underlying collagen-induced conformational switches, we have undertaken a comparative study between the wild type integrin α1 I and a gain-of-function E317A mutant. NMR HDX results suggest a relationship between regions exhibiting a reduced local stability in the unbound I domain and those that undergo significant conformational changes upon binding. Specifically, the αC and α7 helices within the C-terminus are at the center of such major perturbations and present reduced local stabilities in the unbound state relative to other structural elements. Complementary isothermal titration calorimetry experiments have been performed to derive complete thermodynamic binding profiles for association of the collagen-like triple-helical peptide with wild type α1 I and E317A mutant. The differential energetics observed for E317A are consistent with the HDX experiments and support a model in which intrinsically destabilized regions predispose conformational rearrangement in the integrin I domain. This study highlights the importance of exploring different timescales to delineate allosteric and binding events.

Chinese herb medicine against Sortase A catalyzed transformations, a key role in gram-positive bacterial infection progress

Many Gram-positive bacteria can anchor their surface proteins to the cell wall peptidoglycan covalently by a common mechanism with Sortase A (SrtA), thus escaping from the host's identification of immune cells. SrtA can complete this anchoring process by cleaving LPXTG motif conserved among these surface proteins and thus these proteins anchor on the cell wall. Moreover, those SrtA mutants lose this capability to anchor these relative proteins, with these bacteria no longer infectious. Therefore, SrtA inhibitors can be promising anti-infective agents to cure bacterial infections. Chinese herb medicines (CHMs) (chosen from Science Citation Index) have exhibited inhibition on SrtA of Gram-positive pathogens irreversibly or reversibly. In general, CHMs are likely to have important long-term impact as new antibacterial compounds and sought after by academia and the pharmaceutical industry. This review mainly focuses on SrtA inhibitors from CHMs and the potential inhibiting mechanism related to chemical structures of compounds in CHMs.

Structural basis of collagen recognition by human osteoclast-associated receptor and design of osteoclastogenesis inhibitors

Human osteoclast-associated receptor (OSCAR) is an immunoglobulin (Ig)-like collagen receptor that is up-regulated on osteoclasts during osteoclastogenesis and is expressed in a range of myeloid cells. As a member of the leukocyte receptor complex family of proteins, OSCAR shares a high degree of sequence and structural homology with other collagen receptors of this family, including glycoprotein VI, leukocyte-associated Ig-like receptor-1, and leukocyte Ig-like receptor B4, but recognizes a unique collagen sequence. Here, we present the crystal structures of OSCAR in its free form and in complex with a triple-helical collagen-like peptide (CLP). These structures reveal that the CLP peptide binds only one of the two Ig-like domains, the membrane-proximal domain (domain 2) of OSCAR, with the middle and trailing chain burying a total of 661 Å(2) of solvent-accessible collagen surface. This binding mode is facilitated by the unusual topography of the OSCAR protein, which displays an obtuse interdomain angle and a rotation of domain 2 relative to the membrane-distal domain 1. Moreover, the binding of the CLP to OSCAR appears to be mediated largely by tyrosine residues and conformational changes at a shallow Phe pocket. Furthermore, we investigated CLP peptides as inhibitors of osteoclastogenesis and found that a peptide length of 40 amino acids is required to ensure adequate inhibition of osteoclastogenesis in vitro. These findings provide valuable structural insights into the mode of collagen recognition by OSCAR and into the use of synthetic peptide matrikines for osteoclastogenesis inhibition.

Surface biology of collagen scaffold explains blocking of wound contraction and regeneration of skin and peripheral nerves

We review the details of preparation and of the recently elucidated mechanism of biological (regenerative) activity of a collagen scaffold (dermis regeneration template, DRT) that has induced regeneration of skin and peripheral nerves (PN) in a variety of animal models and in the clinic. DRT is a 3D protein network with optimized pore size in the range 20-125 µm, degradation half-life 14 ± 7 d and ligand densities that exceed 200 µM α1β1 or α2β1 ligands. The pore has been optimized to allow migration of contractile cells (myofibroblasts, MFB) into the scaffold and to provide sufficient specific surface for cell-scaffold interaction; the degradation half-life provides the required time window for satisfactory binding interaction of MFB with the scaffold surface; and the ligand density supplies the appropriate ligands for specific binding of MFB on the scaffold surface. A dramatic change in MFB phenotype takes place following MFB-scaffold binding which has been shown to result in blocking of wound contraction. In both skin wounds and PN wounds the evidence has shown clearly that contraction blocking by DRT is followed by induction of regeneration of nearly perfect organs. The biologically active structure of DRT is required for contraction blocking; well-matched collagen scaffold controls of DRT, with structures that varied from that of DRT, have failed to induce regeneration. Careful processing of collagen scaffolds is required for adequate biological activity of the scaffold surface. The newly understood mechanism provides a relatively complete paradigm of regenerative medicine that can be used to prepare scaffolds that may induce regeneration of other organs in future studies.

Chronic Wound Dressings Based on Collagen-Mimetic Proteins

Objective: Chronic wounds are projected to reach epidemic proportions due to the aging population and the increasing incidence of diabetes. There is a strong clinical need for an improved wound dressing that can balance wound moisture, promote cell migration and proliferation, and degrade at an appropriate rate to minimize the need for dressing changes. Approach: To this end, we have developed a bioactive, hydrogel microsphere wound dressing that incorporates a collagen-mimetic protein, Scl2_GFPGER, to promote active wound healing. A redesigned Scl2_GFPGER, engineered collagen (eCol_GFPGER), was created to reduce steric hindrance of integrin-binding motifs and increase overall stability of the triple helical backbone, thereby resulting in increased cell adhesion to substrates. Results: This study demonstrates the successful modification of the Scl2_GFPGER protein to eCol_GFPGER, which displayed enhanced stability and integrin interactions. Fabrication of hydrogel microspheres provided a matrix with adaptive moisture technology, and degradation rates have potential for use in human wounds. Innovation: This collagen-mimetic wound dressing was designed to permit controlled modulation of cellular interactions and degradation rate without impact on other physical properties. Its fabrication into uniform hydrogel microspheres provides a bioactive dressing that can readily conform to irregular wounds. Conclusion: Overall, this new eCol_GFPGER shows strong promise in the generation of bioactive hydrogels for wound healing as well as a variety of tissue scaffolds.

Preferential sites for intramolecular glucosepane cross-link formation in type I collagen: A thermodynamic study

The extracellular matrix (ECM) undergoes progressive age-related stiffening and loss of proteolytic digestibility due to an increase in concentration of advanced glycation end products (AGEs). The most abundant AGE, glucosepane, accumulates in collagen with concentrations over 100 times greater than all other AGEs. Detrimental collagen stiffening properties are believed to play a significant role in several age-related diseases such as osteoporosis and cardiovascular disease. Currently little is known of the potential location of covalently cross-linked glucosepane formation within collagen molecules; neither are there reports on how the respective cross-link sites affect the physical and biochemical properties of collagen. Using fully atomistic molecular dynamics simulations (MD) we have identified six sites where the formation of a covalent intra-molecular glucosepane cross-link within a single collagen molecule in a fibrillar environment is energetically favourable. Identification of these favourable sites enables us to align collagen cross-linking with experimentally observed changes to the ECM. For example, formation of glucosepane was found to be energetically favourable within close proximity of the Matrix Metalloproteinase-1 (MMP1) binding site, which could potentially disrupt collagen degradation.

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We conduct fully atomistic molecular dynamics simulation of fibrillar collagen.

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Glucosepane cross-link formation is energetically favourable at six positions.

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Positions identified are within key collagen biomolecule sites.

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Positioning of sites may have a significant effect on tissue function and integrity.

Integrin α1β1

Integrin α1β1 is widely expressed in mesenchyme and the immune system, as well as a minority of epithelial tissues. Signaling through α1 contributes to the regulation of extracellular matrix composition, in addition to supplying in some tissues a proliferative and survival signal that appears to be unique among the collagen binding integrins. α1 provides a tissue retention function for cells of the immune system including monocytes and T cells, where it also contributes to their long-term survival, providing for peripheral T cell memory, and contributing to diseases of autoimmunity. The viability of α1 null mice, as well as the generation of therapeutic monoclonal antibodies against this molecule, have enabled studies of the role of α1 in a wide range of pathophysiological circumstances. The immune functions of α1 make it a rational therapeutic target.

Molecular Characterization of the Multiple Interactions of SpsD, a Surface Protein from Staphylococcus pseudintermedius, with Host Extracellular Matrix Proteins

Staphylococcus pseudintermedius, a commensal and pathogen of dogs and occasionally of humans, expresses surface proteins potentially involved in host colonization and pathogenesis. Here, we describe the cloning and characterization of SpsD, a surface protein of S. pseudintermedius reported as interacting with extracellular matrix proteins and corneocytes. A ligand screen and Western immunoblotting revealed that the N-terminal A domain of SpsD bound fibrinogen, fibronectin, elastin and cytokeratin 10. SpsD also interfered with thrombin-induced fibrinogen coagulation and blocked ADP-induced platelet aggregation. The binding site for SpsD was mapped to residues 395–411 in the fibrinogen γ-chain, while binding sites in fibronectin were localized to the N- and C-terminal regions. SpsD also bound to glycine- and serine-rich omega loops within the C-terminal tail region of cytokeratin 10. Ligand binding studies using SpsD variants lacking the C-terminal segment or containing an amino-acid substitution in the putative ligand binding site provided insights into interaction mechanism of SpsD with the different ligands. Together these data demonstrate the multi-ligand binding properties of SpsD and illustrate some interesting differences in the variety of ligands bound by SpsD and related proteins from S. aureus.

Collagen-binding microbial surface components recognizing adhesive matrix molecule (MSCRAMM) of Gram-positive bacteria inhibit complement activation via the classical pathway

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.

Dynamic structural changes are observed upon collagen and metal ion binding to the integrin α1 I domain

We have applied hydrogen-deuterium exchange mass spectrometry, in conjunction with differential scanning calorimetry and protein stability analysis, to examine solution dynamics of the integrin α1 I domain induced by the binding of divalent cations, full-length type IV collagen, or a function-blocking monoclonal antibody. These studies revealed features of integrin activation and α1I-ligand complexes that were not detected by static crystallographic data. Mg(2+) and Mn(2+) stabilized α1I but differed in their effects on exchange rates in the αC helix. Ca(2+) impacted α1I conformational dynamics without altering its gross thermal stability. Interaction with collagen affected the exchange rates in just one of three metal ion-dependent adhesion site (MIDAS) loops, suggesting that MIDAS loop 2 plays a primary role in mediating ligand binding. Collagen also induced changes consistent with increased unfolding in both the αC and allosteric C-terminal helices of α1I. The antibody AQC2, which binds to α1I in a ligand-mimetic manner, also reduced exchange in MIDAS loop 2 and increased exchange in αC, but it did not impact the C-terminal region. This is the first study to directly demonstrate the conformational changes induced upon binding of an integrin I domain to a full-length collagen ligand, and it demonstrates the utility of the deuterium exchange mass spectrometry method to study the solution dynamics of integrin/ligand and integrin/metal ion interactions. Based on the ligand and metal ion binding data, we propose a model for collagen-binding integrin activation that explains the differing abilities of Mg(2+), Mn(2+), and Ca(2+) to activate I domain-containing integrins.

Enhancing integrin α1 inserted (I) domain affinity to ligand potentiates integrin α1β1-mediated down-regulation of collagen synthesis

Integrin α1β1 binding to collagen IV, which is mediated by the α1-inserted (I) domain, down-regulates collagen synthesis. When unligated, a salt bridge between Arg(287) and Glu(317) is thought to keep this domain in a low affinity conformation. Ligand binding opens the salt bridge leading to a high-affinity conformation. How modulating integrin α1β1 affinity alters collagen homeostasis is unknown. To address this question, we utilized a thermolysin-derived product of the α1α2α1 network of collagen IV (α1α2α1(IV) truncated protomer) that selectively binds integrin α1β1. We show that an E317A substitution enhanced binding to the truncated protomer, consistent with a previous finding that this substitution eliminates the salt bridge. Surprisingly, we show that an R287A substitution did not alter binding, whereas R287E/E317R substitutions enhanced binding to the truncated protomer. NMR spectroscopy and molecular modeling suggested that eliminating the Glu(317) negative charge is sufficient to induce a conformational change toward the open state. Thus, the role played by Glu(317) is largely independent of the salt bridge. We further show that cells expressing E317A or R287E/E317R substitutions have enhanced down-regulation of collagen IV synthesis, which is mediated by the ERK/MAPK pathway. In conclusion, we have demonstrated that modulating the affinity of the extracellular α1 I domain to collagen IV enhances outside-in signaling by potentiating ERK activation and enhancing the down-regulation of collagen synthesis.

Structure of collagen receptor integrin α(1)I domain carrying the activating mutation E317A

We have analyzed the structure and function of the integrin α(1)I domain harboring a gain-of-function mutation E317A. To promote protein crystallization, a double variant with an additional C139S mutation was used. In cell adhesion assays, the E317A mutation promoted binding to collagen. Similarly, the double mutation C139S/E317A increased adhesion compared with C139S alone. Furthermore, soluble α(1)I C139S/E317A was a higher avidity collagen binder than α(1)I C139S, indicating that the double variant represents an activated form. The crystal structure of the activated variant of α(1)I was solved at 1.9 Å resolution. The E317A mutation results in the unwinding of the αC helix, but the metal ion has moved toward loop 1, instead of loop 2 in the open α(2)I. Furthermore, unlike in the closed αI domains, the metal ion is pentacoordinated and, thus, prepared for ligand binding. Helix 7, which has moved downward in the open α(2)I structure, has not changed its position in the activated α(1)I variant. During the integrin activation, Glu(335) on helix 7 binds to the metal ion at the metal ion-dependent adhesion site (MIDAS) of the β(1) subunit. Interestingly, in our cell adhesion assays E317A could activate collagen binding even after mutating Glu(335). This indicates that the stabilization of helix 7 into its downward position is not required if the α(1) MIDAS is already open. To conclude, the activated α(1)I domain represents a novel conformation of the αI domain, mimicking the structural state where the Arg(287)-Glu(317) ion pair has just broken during the integrin activation.

An engineered alpha1 integrin-binding collagenous sequence

Collagen is an extracellular matrix structural component that can regulate cellular processes through its interaction with the integrins, α1β1, α2β1, α10β1, and α11β1. Collagen-like proteins have been identified in a number of bacterial species. Here, we used Scl2 from Streptococcus pyogenes serotype M28 strain MGAS6274 as a backbone for the introduction of discrete integrin-binding sequences. The introduced sequences GLPGER, GFPGER, or GFPGEN did not affect triple helix stability of the Scl (Streptococcal collagen-like) protein. Using ELISA and surface plasmon resonance, we determined that Scl2(GLPGER) and Scl2(GFPGER) bound to recombinant human α1 and α2 I-domains in a metal ion-dependent manner and without a requirement for hydroxyproline. We predicted a novel and selective integrin-binding sequence, GFPGEN, through the use of computer modeling and demonstrated that Scl2(GFPGEN) shows specificity toward the α1 I-domain and does not bind the α2 I-domain. Using C2C12 cells, we determined that intact integrins interact with the modified Scl2 proteins with the same selectivity as recombinant I-domains. These modified Scl2 proteins also acted as cell attachment substrates for fibroblast, endothelial, and smooth muscle cells. However, the modified Scl2 proteins were unable to aggregate platelets. These results indicate that Scl2 is a suitable backbone for the introduction of mammalian integrin-binding sequences, and these sequences may be manipulated to individually target α1β1 and α2β1.

Structural and functional investigations of Matrilin-1 A-domains reveal insights into their role in cartilage ECM assembly

Matrilin-1 is expressed predominantly in cartilage and co-localizes with matrilin-3 with which it can form hetero-oligomers. We recently described novel structural and functional features of the matrilin-3 A-domain (M3A) and demonstrated that it bound with high affinity to type II and IX collagens. Interactions preferentially occurred in the presence of Zn²⁺ suggesting that matrilin-3 has acquired a requirement for specific metal ions for activation and/or molecular associations. To understand the interdependence of matrilin-1/-3 hetero-oligomers in extracellular matrix (ECM) interactions, we have extended these studies to include the two matrilin-1 A-domains (i.e. M1A1 and M1A2 respectively). In this study we have identified new characteristics of the matrilin-1 A-domains by describing their glycosylation state and the effect of N-glycan chains on their structure, thermal stability, and protein-protein interactions. Initial characterization revealed that N-glycosylation did not affect secretion of these two proteins, nor did it alter their folding characteristics. However, removal of the glycosylation decreased their thermal stability. We then compared the effect of different cations on binding between both M1A domains and type II and IX collagens and showed that Zn²⁺ also supports their interactions. Finally, we have demonstrated that both M1A1 domains and biglycan are essential for the association of the type II·VI collagen complex. We predict that a potential role of the matrilin-1/-3 hetero-oligomer might be to increase multivalency, and therefore the ability to connect various ECM components. Differing affinities could act to regulate the integrated network, thus coordinating the organization of the macromolecular structures in the cartilage ECM.

A collagen-binding adhesin, Acb, and ten other putative MSCRAMM and pilus family proteins of Streptococcus gallolyticus subsp. gallolyticus (Streptococcus bovis Group, biotype I)

Members of the Streptococcus bovis group are important causes of endocarditis. However, factors associated with their pathogenicity, such as adhesins, remain uncharacterized. We recently demonstrated that endocarditis-derived Streptococcus gallolyticus subsp. gallolyticus isolates frequently adhere to extracellular matrix (ECM) proteins. Here, we generated a draft genome sequence of an ECM protein-adherent S. gallolyticus subsp. gallolyticus strain and found, by genome-wide analyses, 11 predicted LPXTG-type cell wall-anchored proteins with characteristics of MSCRAMMs, including a modular architecture of domains predicted to adopt immunoglobulin (Ig)-like folding. A recombinant segment of one of these, Acb, showed high-affinity binding to immobilized collagen, and cell surface expression of Acb correlated with the presence of acb and collagen adherence of isolates. Three of the 11 proteins have similarities to major pilus subunits and are organized in separate clusters, each including a second Ig-fold-containing MSCRAMM and a class C sortase, suggesting that the sequenced strain encodes three distinct types of pili. Reverse transcription-PCR demonstrated that all three genes of one cluster, acb-sbs7-srtC1, are cotranscribed, consistent with pilus operons of other gram-positive bacteria. Further analysis detected expression of all 11 genes in cells grown to mid to late exponential growth phases. Wide distribution of 9 of the 11 genes was observed among S. gallolyticus subsp. gallolyticus isolates with fewer genes present in other S. bovis group species/subspecies. The high prevalence of genes encoding putative MSCRAMMs and pili, including a collagen-binding MSCRAMM, among S. gallolyticus subsp. gallolyticus isolates may play an important role in the predominance of this subspecies in S. bovis endocarditis.

Integrins alpha1beta1 and alpha2beta1 are receptors for the rotavirus enterotoxin

Rotavirus NSP4 is a viral enterotoxin capable of causing diarrhea in neonatal mice. This process is initiated by the binding of extracellular NSP4 to target molecule(s) on the cell surface that triggers a signaling cascade leading to diarrhea. We now report that the integrins alpha1beta1 and alpha2beta1 are receptors for NSP4. NSP4 specifically binds to the alpha1 and alpha2 I domains with apparent K(d) = 1-2.7 muM. Binding is mediated by the I domain metal ion-dependent adhesion site motif, requires Mg(2+) or Mn(2+), is abolished with EDTA, and an NSP4 point mutant, E(120)A, fails to bind alpha2 integrin I domain. NSP4 has two distinct integrin interaction domains. NSP4 amino acids 114-130 are essential for binding to the I domain, and NSP4 peptide 114-135 blocks binding of the natural ligand, collagen I, to integrin alpha2. NSP4 amino acids 131-140 are not associated with the initial binding to the I domain, but elicit signaling that leads to the spreading of attached C2C12-alpha2 cells, mouse myoblast cells stably expressing the human alpha2 integrin. NSP4 colocalizes with integrin alpha2 on the basolateral surface of rotavirus-infected polarized intestinal epithelial (Caco-2) cells as well as surrounding noninfected cells. NSP4 mutants that fail to bind or signal through integrin alpha2 were attenuated in diarrhea induction in neonatal mice. These results indicate that NSP4 interaction with integrin alpha1 and alpha2 is an important component of enterotoxin function and rotavirus pathogenesis, further distinguishing this viral virulence factor from other microbial enterotoxins.

Epitope mapping and use of epitope-specific antisera to characterize the VP5* binding site in rotavirus SA11 NSP4

Rotavirus (RV) is the leading cause of infantile gastroenteritis worldwide. RV nonstructural protein 4 (NSP4), the first characterized viral enterotoxin, is a 28-kDa glycoprotein that has pleiotropic functions in RV infection and pathogenesis. NSP4 has multiple forms enabling it to perform its different functions. Dissecting such functions could be facilitated by use of epitope-specific antibodies. This work mapped the epitopes for the monoclonal antibody B4-2/55 and three polyclonal antisera generated against synthetic SA11 NSP4 peptides corresponding to residues 114-135, 120-147, and 150-175. The epitope for B4-2/55 mapped to residues 100-118, wherein residues E105, R108 and E111 are critical for antibody binding. Antiserum generated to two peptides (aa114-135 and aa120-147) with enterotoxin activity each recognize a single but distinct epitope. The epitope for the peptide antiserum to aa114-135 was mapped to residues 114-125 with highly conserved residues T117/T118, E120, and E122 being critical for antibody binding. The peptide antiserum to aa120-147 binds to NSP4 at residues 130-140 and residues Q137-T138 are critical for this epitope. Finally, the epitope for the antiserum to peptide aa150-175 mapped to residues 155-170, wherein residues E160 and E170 are critical for antibody binding. Knowledge of the binding sites of domain-specific antibodies can aid in further characterizing different functions of NSP4. To demonstrate this, we characterized the interaction between NSP4 and VP5() [K(D)=0.47 microM] and show that binding of NSP4 to VP5* is blocked by antibody to NSP4 aa114-135 and aa120-147, but not aa150-175. The use of single epitope-specific antibodies to differentially block functions of NSP4 is a feasible approach to determine the functional domain structure of this important RV virulence factor.

Structural basis for ligand recognition by integrins

Integrins, the major cell surface receptors mediating cell-extracellular matrix (ECM) adhesion, are central to the basic physiology underlying all multicellular organisms. As the complexity of animal body architecture increased, integrins were forced to acquire recognition capabilities toward the wide variety of ECM ligands and cell surface counter-receptors that emerged during evolution. Structural determination of the integrin-ligand complexes for both I domain-containing and non-I domain-containing integrins revealed two fundamentally different types of integrin-binding surfaces. In addition, recent advances in the biochemical and pharmacological characterization of the integrin-ligand interactions are beginning to reveal how integrins achieve specific recognition of wide variety of ligands using a small binding cleft at the subunit interface common to all integrins.

Imaging Collagen in Intact Viable Healthy and Atherosclerotic Arteries Using Fluorescently Labeled CNA35 and Two-Photon Laser Scanning Microscopy

We evaluated CNA35 as a collagen marker in healthy and atherosclerotic arteries of mice after both ex vivo and in vivo administration and as a molecular imaging agent for the detection of atherosclerosis. CNA35 conjugated with fluorescent Oregon Green 488 (CNA35/OG488) was administered ex vivo to mounted viable muscular (uterine), elastic (carotid), and atherosclerotic (carotid) arteries and fresh arterial rings. Two-photon microscopy was used for imaging. CNA35/OG488 labeling in healthy elastic arteries was compared with collagen type I, III, and IV antibody labeling in histologic sections. For in vivo labeling experiments, CNA35/OG488 was injected intravenously in C57BL6/J and apolipoprotein E−/− mice. Ex vivo CNA35/OG488 strongly labeled collagen in the tunica adventitia, media, and intima of muscular arteries. In healthy elastic arteries, tunica adventitia was strongly labeled, but labeling in tunica media and intima was prevented by endothelium and elastic laminae. Histology confirmed the affinity of CNA35 for type I, III, and IV collagen in arteries. Strong CNA35/OG488 labeling was found in atherosclerotic plaques. In vivo applied CNA35/OG488 minimally labeled the tunica intima of healthy carotid arteries. Atherosclerotic plaques in apolipoprotein E−/− mice exhibited large uptake. CNA35/OG488 imaging in organs revealed endothelium as a limiting barrier for in vivo uptake. CNA35/OG488 is a good molecular imaging agent for atherosclerosis.

Integrin antagonists as therapeutics for inflammatory diseases

Integrins are a family of heterodimeric cell surface receptors that mediate adhesion events crucial to cellular migration, proliferation and activation. Although critical to a normal immune response, integrins can also facilitate the progression of many inflammatory and autoimmune disorders. As such, they have attracted the attention of the pharmaceutical industry. Several humanised monoclonal antibodies directed against integrin targets have proven to be successful in clinical trials and have been approved for use in humans. This has not only resulted in effective therapies for patients, but also has provided important proof-of-concept studies for the development of small-molecule antagonists. This review focuses on those integrin subclasses that are being evaluated for their potential role in pulmonary, dermatological, gastrointestinal or rheumatic diseases. These include the alpha4 and beta2 integrins, as well as an emerging group of targets from the collagen-binding family of integrins. Interfering with integrin signalling pathways represents a future area of interest. The rationale for pursuing these targets, as well as the drugs presently under development, are discussed.

Sortases and the art of anchoring proteins to the envelopes of gram-positive bacteria

The cell wall envelopes of gram-positive bacteria represent a surface organelle that not only functions as a cytoskeletal element but also promotes interactions between bacteria and their environment. Cell wall peptidoglycan is covalently and noncovalently decorated with teichoic acids, polysaccharides, and proteins. The sum of these molecular decorations provides bacterial envelopes with species- and strain-specific properties that are ultimately responsible for bacterial virulence, interactions with host immune systems, and the development of disease symptoms or successful outcomes of infections. Surface proteins typically carry two topogenic sequences, i.e., N-terminal signal peptides and C-terminal sorting signals. Sortases catalyze a transpeptidation reaction by first cleaving a surface protein substrate at the cell wall sorting signal. The resulting acyl enzyme intermediates between sortases and their substrates are then resolved by the nucleophilic attack of amino groups, typically provided by the cell wall cross bridges of peptidoglycan precursors. The surface protein linked to peptidoglycan is then incorporated into the envelope and displayed on the microbial surface. This review focuses on the mechanisms of surface protein anchoring to the cell wall envelope by sortases and the role that these enzymes play in bacterial physiology and pathogenesis.

Fluorescently labeled collagen binding proteins allow specific visualization of collagen in tissues and live cell culture

Visualization of the formation and orientation of collagen fibers in tissue engineering experiments is crucial for understanding the factors that determine the mechanical properties of tissues. In this study, collagen-specific fluorescent probes were developed using a new approach that takes advantage of the inherent specificity of collagen binding protein domains present in bacterial adhesion proteins (CNA35) and integrins (GST-alpha1I). Both collagen binding domains were obtained as fusion proteins from an Escherichia coli expression system and fluorescently labeled using either amine-reactive succinimide (CNA35) or cysteine-reactive maleimide (GST-alpha1I) dyes. Solid-phase binding assays showed that both protein-based probes are much more specific than dichlorotriazinyl aminofluorescein (DTAF), a fluorescent dye that is currently used to track collagen formation in tissue engineering experiments. The CNA35 probe showed a higher affinity for human collagen type I than did the GST-alpha1I probe (apparent K(d) values of 0.5 and 50 microM, respectively) and showed very little cross-reactivity with noncollagenous extracellular matrix proteins. The CNA35 probe was also superior to both GST-alpha1I and DTAF in visualizing the formation of collagen fibers around live human venous saphena cells. Immunohistological experiments on rat tissue showed colocalization of the CNA35 probe with collagen type I and type III antibodies. The fluorescent probes described here have important advantages over existing methods for visualization of collagen, in particular for monitoring the formation of collagen in live tissue cultures over prolonged time periods.

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 beta-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.

A novel binding site in collagen type III for integrins alpha1beta1 and alpha2beta1

Previously identified high affinity integrin-binding motifs in collagens, GFOGER and GLOGER, are not present in type III collagen. Here, we first characterized the binding of recombinant I domains from integrins alpha(1) and alpha(2) (alpha(1)I and alpha(2)I) to fibrillar collagen types I-III and showed that each I domain bound to the three types of collagens with similar affinities. Using rotary shadowing followed by electron microscopy, we identified a high affinity binding region in human type III collagen recognized by alpha(1)I and alpha(2)I. Examination of the region revealed the presence of two sequences that contain the critical GER motif, GROGER and GAOGER. Collagen-like peptides containing these two motifs were synthesized, and their triple helical nature was confirmed by circular dichroism spectroscopy. Experiments show that the GROGER-containing peptide was able to bind both alpha(1)I and alpha(2)I with high affinity and effectively inhibit the binding of alpha(1)I and alpha(2)I to type III and I collagens, whereas the GAOGER-containing peptide was considerably less effective. Furthermore, the GROGER-containing peptide supported adhesion of human lung fibroblast cells when coated on a culture dish. Thus, we have identified a novel high affinity binding sequence for the collagen-binding integrin I domains.

Streptococcus pyogenes collagen type I-binding Cpa surface protein. Expression profile, binding characteristics, biological functions, and potential clinical impact

The Streptococcus pyogenes collagen type I-binding protein Cpa (collagen-binding protein of group A streptococci) expressed by 28 serotypes of group A streptococci has been extensively characterized at the gene and protein levels. Evidence for three distinct families of cpa genes was found, all of which shared a common sequence encoding a 60-amino acid domain that accounted for selective binding to type I collagen. Surface plasmon resonance-based affinity measurements and functional studies indicated that the expression of Cpa was consistent with an attachment role for bacteria to tissue containing collagen type I. A cpa mutant displayed a significantly decreased internalization rate when incubated with HEp-2 cells but had no effect on the host cell viability. By utilizing serum from patients with a positive titer for streptolysin/DNase antibody, an increased anti-Cpa antibody titer was noted for patients with a clinical history of arthritis or osteomyelitis. Taken together, these results suggest Cpa may be a relevant matrix adhesin contributing to the pathogenesis of S. pyogenes infection of bones and joints.

Conversion between three conformational states of integrin I domains with a C-terminal pull spring studied with molecular dynamics

We test with molecular dynamics the hypothesis that interdomain forces in integrins, simulated with a spring attached to the C-terminal alpha 7-helix of an integrin I domain, can allosterically stabilize alternative I domain conformations. Depending on the force applied and timecourse, in alpha(L) and alpha(M) I domains the beta 6-alpha 7 loop moves successively between three ratchet positions; i.e. from closed to intermediate, and then to open. More distal, linked alterations in MIDAS loops and metal coordination closely resemble those seen when the MIDAS becomes ligated. Simulations show that the intermediate state is populated over a wider range of forces for alpha(L) than alpha(M) I domains. Simulations with mutant I domains suggest that specific ratchet residues regulate conformational equilibria. Simulations with alpha(1) and alpha(2) I domains reveal a lack of the intermediate conformation, owing to Phe to Glu substitution at the second ratchet residue. The findings have important implications for biological regulation of integrin adhesiveness.

A Streptococcal Collagen-like Protein Interacts with the α2β1 Integrin and Induces Intracellular Signaling

The streptococcal collagen-like proteins Scl1 and Scl2 are prokaryotic members of a large protein family with domains containing the repeating amino acid sequence (Gly-Xaa-Yaa)(n) that form a collagen-like triple-helical structure. Here, we test the hypothesis that Scl variant might interact with mammalian collagen-binding integrins. We show that the recombinant Scl protein p176 promotes adhesion and spreading of human lung fibroblast cells through an alpha2beta1 integrin-mediated interaction as shown in cell adhesion inhibition assays using anti-alpha2beta1 and anti-beta1 integrins monoclonal antibodies. Accordingly, C2C12 cells stably expressing alpha2beta1 integrin as the only collagen-binding integrin show productive cell adhesion activities on p176 that can be blocked by an anti-alpha2beta1 integrin antibody. In addition, p176 promotes tyrosine phosphorylation of p125(FAK) of C2C12 cells expressing alpha2beta1 integrin, whereas parental cells do not. Furthermore, C2C12 adhesion of human lung fibroblast cells to p176 induces phosphorylation of p125FAK, p130CAS, and p68Paxillin proteins. In a domain swapping experiment, we show that integrin binds to the collagenous domain of the Scl protein. Moreover, the recombinant inserted domain of the alpha2 integrin interacts with p176 with a relatively high affinity (K(D) = 17 nm). Attempts to identify the integrin sites in p176 suggest that more than one site may be involved. These studies, for the first time, suggest that the collagen-like proteins of prokaryotes retained not only structural but also functional characteristics of their eukaryotic counterparts.

Basement membrane reconstruction in human skin equivalents is regulated by fibroblasts and/or exogenously activated keratinocytes

This study was undertaken to examine the role fibroblasts play in the formation of the basement membrane (BM) in human skin equivalents. For this purpose, keratinocytes were seeded on top of fibroblast-free or fibroblast-populated collagen matrix or de-epidermized dermis and cultured in the absence of serum and exogenous growth factors. The expression of various BM components was analyzed on the protein and mRNA level. Irrespective of the presence or absence of fibroblasts, keratin 14, hemidesmosomal proteins plectin, BP230 and BP180, and integrins alpha1beta1, alpha2beta1, alpha3beta1, and alpha6beta4 were expressed but laminin 1 was absent. Only in the presence of fibroblasts or of various growth factors, laminin 5 and laminin 10/11, nidogen, uncein, type IV and type VII collagen were decorating the dermal/epidermal junction. These findings indicate that the attachment of basal keratinocytes to the dermal matrix is most likely mediated by integrins alpha1beta1 and alpha2beta1, and not by laminins that bind to integrin alpha6beta4 and that the epithelial-mesenchymal cross-talk plays an important role in synthesis and deposition of various BM components.

Identification and biochemical characterization of two novel collagen binding MSCRAMMs of Bacillus anthracis

Cell wall-anchored proteins play critical roles in the pathogenesis of infections caused by Gram-positive bacteria. Through the analysis of the genome of Bacillus anthracis Ames strain, we identified two novel putative cell wall-anchored proteins, BA0871 and BA5258, which have sequence homology to CNA, a cell wall-anchored collagen adhesin of Staphylococcus aureus. The two proteins have similar domain organization to that of CNA, with typical signal peptide sequences, a non-repetitive A region followed by repeats, and a characteristic cell wall-anchoring region. They are expressed on the surface of B. anthracis. The A regions of the two proteins were predicted to adopt similar structural folds as CNA. Circular dichroism analysis of the recombinant A regions of the two proteins (rBA0871A and rBA5258A) indicate that their secondary structure compositions are similar to those of the A regions of CNA and other cell wall-anchored adhesins. We demonstrate through solid phase binding assays and surface plasmon resonance analyses that rBA0871A and rBA5258A specifically bound type I collagen in a dose-dependent and saturable manner. Their dissociation constants (KD) for collagen are 1.6-3.2 microm for rBA0871A and 0.6-0.9 microm for rBA5258A, respectively. We further demonstrate that BA0871 and BA5258 can mediate cell attachment to collagen when expressed on the surface of a heterologous host bacterium. To our knowledge these are the first two adhesins of B. anthracis described, which may have important implications for our understanding of the pathogenic mechanisms explored by this organism.

Binding interaction studies of the immobilized Salmonella typhimurium with extracellular matrix and muscle proteins, and polysaccharides

Our research attempts to understand the real-time interactions of immobilized Salmonella typhimurium with extracellular membrane proteins (collagen I, fibronectin and laminin) and muscle proteins (actin and myosin). Salmonella cells were immobilized on the sensor chip of a surface plasmon resonance (SPR) biosensor. Typical results showed that collagen I and myosin had higher binding responses to the S. typhimurium surface but laminin, actin and fibronectin had lower binding responses. The binding kinetics of collagen I and Salmonella cell surface showed an apparent dissociation and association rate constants of 3.90 E-4 s(-1) and 1.07 E+4 mol(-1) s(-1). Using the model system developed in our laboratory, the interactions of carrageenans and other polysaccharides with collagen and the Salmonella sensor surface were evaluated. The kappa-carrageenans blocked 92-100% binding of collagen to the Salmonella surface, while sodium alginate and low methoxy pectin blocked 50% and 18% binding, respectively. These biosensor studies allowed the rapid evaluation of compounds that may prevent bacterial attachment to poultry skin and carcasses, thus reducing pathogen contamination of poultry foods.

Multiple binding sites in fibrinogen for integrin alphaMbeta2 (Mac-1)

The leukocyte integrin alphaMbeta2 (Mac-1) is a multiligand receptor that mediates a range of adhesive reactions of leukocytes during the inflammatory response. This integrin binds the coagulation protein fibrinogen providing a key link between thrombosis and inflammation. However, the mechanism by which alphaMbeta2 binds fibrinogen remains unknown. Previous studies indicated that a model in which two fibrinogen gammaC domain sequences, P1 (gamma190-202) and P2 (gamma377-395), serve as the alphaMbeta2 binding sites cannot fully account for recognition of fibrinogen by integrin. Here, using surface plasmon resonance, we examined the interaction of the ligand binding alphaMI-domain of alphaMbeta2 with the D fragment of fibrinogen and showed that this ligand is capable of associating with several alphaMI-domain molecules. To localize the alternative alphaMI-domain binding sites, we screened peptide libraries covering the complete sequences of the gammaC and betaC domains, comprising the majority of the D fragment structure, for alphaMI-domain binding. In addition to the P2 and P1 peptides, the alphaMI-domain bound to many other sequences in the gammaC and betaC scans. Similar to P1 and P2, synthetic peptides derived from gammaC and betaC were efficient inhibitors of alphaMbeta2-mediated cell adhesion and were able to directly support adhesion suggesting that they contain identical recognition information. Analyses of recognition specificity using substitutional peptide libraries demonstrated that the alphaMI-domain binding depends on basic and hydrophobic residues. These findings establish a new model of alphaMbeta2 binding in which the alphaMI-domain interacts with multiple sites in fibrinogen and has the potential to recognize numerous sequences. This paradigm may have implications for mechanisms of promiscuity in ligand binding exhibited by integrin alphaMbeta2.

Crystal Structure of EMS16 in Complex with the Integrin α2-I Domain

Snake venoms contain a number of heterodimeric C-type lectin-like proteins (CLPs) that interact specifically with components of the haemostatic system. EMS16 from the venom of Echis multisquamatus binds to the collagen receptor, integrin alpha2beta1, also known as glycoprotein (GP) Ia/IIa, and specifically inhibits collagen binding. Here we report the crystal structure of EMS16 in complex with recombinant integrin alpha2-I domain that plays a central role in collagen binding. The structure of the complex at 1.9 Angstrom resolution reveals that the collagen-binding site of the alpha2-I domain is covered completely by the bound EMS16. This blockage by EMS16 appears to spatially inhibit collagen binding to the alpha2-I domain. The bound alpha2-I domain adopts a closed conformation, which is seen in the absence of ligand, suggesting that EMS16 stabilizes a closed conformation corresponding to the less active structure of the alpha2-I domain. EMS16 does not directly bind to the manganese ion and residues of the metal ion-dependent adhesion site (MIDAS) of the alpha2-I domain, suggesting that EMS16 may have the potential to bind specifically to the alpha2-I domain in a metal ion-independent fashion.

Exploring the Collagen-binding Site of the DDR1 Tyrosine Kinase Receptor

Discoidin domain receptors 1 and 2 (DDR1 and DDR2) are tyrosine kinase receptors activated by triple-helical collagens. Aberrant expression and signaling of these receptors have been implicated in several human diseases linked to accelerated matrix degradation and remodeling including tumor invasion, atherosclerosis and liver fibrosis. The objective of this study is to characterize the collagen-binding sites in the discoidin domains of DDR1 and DDR2 at a molecular level. We expressed glutathione S-transferase fusion proteins containing the discoidin and extracellular domains of DDR1 and DDR2 in insect cells and subjected them to a solid-phase collagen-binding assay. We found high affinity binding of the DDR extracellular domains to immobilized type I collagen and confirmed the discoidin-collagen interaction with an enzyme-linked immunosorbent assay-based read-out. Furthermore, we created a three-dimensional model of the DDR1 discoidin domain based on the related domains of blood coagulation factors V and VIII. This model predicts the presence of four neighboring, surface-exposed loops that are topologically equivalent to a major phospholipid-binding site in factors V and VIII. To test the involvement of these loops in collagen binding, we mutated individual amino acid residues to alanine or deleted short sequence stretches within these loops. We found that several residues within loop 1 (Ser-52-Thr-57) and loop 3 (Arg-105-Lys-112) as well as Ser-175 in loop 4 are critically involved in collagen binding. Our structure-function analysis of the DDR discoidin domains provides new insights into this non-integrin-mediated collagen-signaling mechanism and may ultimately lead to the design of small molecule inhibitors that interfere with aberrant DDR function.

Synthetic heterotrimeric collagen peptides as mimics of cell adhesion sites of the basement membrane

Collagen type IV forms a network in the basement membrane into which other constituents of the tissue are incorporated. It also provides cell-adhesion sites that are specifically recognized by cell-surface receptors, i.e., the integrins. Different from the ubiquitous sequential RGD adhesion motif found in most of the matrix proteins, in collagen type IV, the responsible binding sites for alpha1beta1 integrin have been identified as Asp461 of the two alpha1 chains and Arg461 of the alpha2 chain. Because of the heterotrimeric character of this collagen, the spatial geometry of the binding epitope depends not only on the triple-helical fold, but decisively even on the stagger of the chains. To investigate the effects of chain registration on the conformational properties and binding affinities of this adhesion epitope, two synthetic heterotrimeric collagen peptides consisting of the identical three chains were assembled by an artificial cystine knot in two different registers, i.e., in the most plausible alpha2alpha1alpha1' and less probable alpha1alpha2alpha1' chain alignment. A detailed conformational characterization of both trimers allowed to correlate their different binding affinities for alpha1beta1 integrin with the degree of local plasticity of the two different triple helices. Optimal local breathing of the rod-shaped collagens is apparently crucial for selective recognition by proteins interacting with these main components of the extracellular matrix.

Host collagen signal induces antigen I/II adhesin and invasin gene expression in oral Streptococcus gordonii

Microbial interactions with host molecules, and programmed responses to host environmental stimuli, are critical for colonization and initiation of pathogenesis. Bacteria of the genus Streptococcus are primary colonizers of the human mouth. They express multiple cell-surface adhesins that bind salivary components and other oral bacteria and enable the development of polymicrobial biofilms associated with tooth decay and periodontal disease. However, the mechanisms by which streptococci invade dentine to infect the tooth pulp and periapical tissues are poorly understood. Here we show that production of the antigen I/II (AgI/II) family polypeptide adhesin and invasin SspA in Streptococcus gordonii is specifically upregulated in response to a collagen type I signal, minimally the tri-peptide Gly-Pro-Xaa (where Xaa is hydroxyproline or alanine). Increased AgI/II polypeptide expression promotes bacterial adhesion and extended growth of streptococcal cell chains along collagen type I fibrils that are characteristically found within dentinal tubules. These observations define a new model of host matrix signal-induced tissue penetration by bacteria and open the way for novel therapy opportunities for oral invasive diseases.