GAG-binding Sites Research Articles

Paradigms in the structural biology of the mitogenic ternary complex FGF:FGFR:heparin.

The main achievements regarding the molecular interaction involving fibroblast growth factors (FGFs), canonical receptors (FGFRs) and the glycosaminoglycans (GAGs) heparan sulfate (HS)/heparin (Hp) are overviewed. Despite the recent works concerning the subject, conflicting paradigms in the structural biology of the resultant ternary complex FGF:FGFR:HS/Hp seem to persist up to these days. The principal dilemma, centered on the functional intermolecular complex of mitogenesis and angiogenesis, has been lasting for approximately a decade and a half since the publications of the two contradicting crystal structures, the asymmetric 2:2:1 versus the symmetric 2:2:2 complex model. When the principal results regarding this ternary complex are analyzed as a whole and through an impartial manner, conclusion heavily and reliably supports the existence and activity of both complex models. Selection of each complex is driven by multiple factors of different degrees of impact. Specificity in protein-binding motifs in ligands (although the minimal binding sequences are yet controversial), slight differences on the structure of the GAG-binding sites of FGF and of FGFR isoforms as well as on the possible ligand-induced conformational changes of FGFR are examples of these factors. Here, the structural biology of the mitogenic FGF:FGFR:HS/Hp ternary complex is revisited. Discussion is focused on the major attributes of this intermolecular complex including the existing conflicts about the righter biologically active model and information regarding ligand structure, conformation and minimal length required for binding to the growth factors and receptors. This review is very timely in light of the 100(th) anniversary of the discovery of Hp.

The Anti-inflammatory Protein TSG-6 Regulates Chemokine Function by Inhibiting Chemokine/Glycosaminoglycan Interactions

TNF-stimulated gene-6 (TSG-6) is a multifunctional protein secreted in response to pro-inflammatory stimuli by a wide range of cells, including neutrophils, monocytes, and endothelial cells. It has been shown to mediate anti-inflammatory and protective effects when administered in disease models, in part, by reducing neutrophil infiltration. Human TSG-6 inhibits neutrophil migration by binding CXCL8 through its Link module (Link_TSG6) and interfering with the presentation of CXCL8 on cell-surface glycosaminoglycans (GAGs), an interaction that is vital for the function of many chemokines. TSG-6 was also found to interact with chemokines CXCL11 and CCL5, suggesting the possibility that it may function as a broad specificity chemokine-binding protein, functionally similar to those encoded by viruses. This study was therefore undertaken to explore the ability of TSG-6 to regulate the function of other chemokines. Herein, we demonstrate that Link_TSG6 binds chemokines from both the CXC and CC families, including CXCL4, CXCL12, CCL2, CCL5, CCL7, CCL19, CCL21, and CCL27. We also show that the Link_TSG6-binding sites on chemokines overlap with chemokine GAG-binding sites, and that the affinities of Link_TSG6 for these chemokines (KD values 1–85 nm) broadly correlate with chemokine-GAG affinities. Link_TSG6 also inhibits chemokine presentation on endothelial cells not only through a direct interaction with chemokines but also by binding and therefore masking the availability of GAGs. Along with previous work, these findings suggest that TSG-6 functions as a pluripotent regulator of chemokines by modulating chemokine/GAG interactions, which may be a major mechanism by which TSG-6 produces its anti-inflammatory effects in vivo.

CXCL1/MGSA Is a Novel Glycosaminoglycan (GAG)-binding Chemokine: STRUCTURAL EVIDENCE FOR TWO DISTINCT NON-OVERLAPPING BINDING DOMAINS

In humans, the chemokine CXCL1/MGSA (hCXCL1) plays fundamental and diverse roles in pathophysiology, from microbial killing to cancer progression, by orchestrating the directed migration of immune and non-immune cells. Cellular trafficking is highly regulated and requires concentration gradients that are achieved by interactions with sulfated glycosaminoglycans (GAGs). However, very little is known regarding the structural basis underlying hCXCL1-GAG interactions. We addressed this by characterizing the binding of GAG heparin oligosaccharides to hCXCL1 using NMR spectroscopy. Binding experiments under conditions at which hCXCL1 exists as monomers and dimers indicate that the dimer is the high-affinity GAG ligand. NMR experiments and modeling studies indicate that lysine and arginine residues mediate binding and that they are located in two non-overlapping domains. One domain, consisting of N-loop and C-helical residues (defined as α-domain) has also been identified previously as the GAG-binding domain for the related chemokine CXCL8/IL-8. The second domain, consisting of residues from the N terminus, 40s turn, and third β-strand (defined as β-domain) is novel. Eliminating β-domain binding by mutagenesis does not perturb α-domain binding, indicating two independent GAG-binding sites. It is known that N-loop and N-terminal residues mediate receptor activation, and we show that these residues are also involved in extensive GAG interactions. We also show that the GAG-bound hCXCL1 completely occlude receptor binding. We conclude that hCXCL1-GAG interactions provide stringent control over regulating chemokine levels and receptor accessibility and activation, and that chemotactic gradients mediate cellular trafficking to the target site.

Functional und structural insights into TIMP-3- glycosaminoglycan interactions - implications for the design of functional biomaterials improving chronic wound treatment

Event Abstract Back to Event Functional und structural insights into TIMP-3- glycosaminoglycan interactions - implications for the design of functional biomaterials improving chronic wound treatment Sandra Rother1, Sergey A. Samsonov2, Tommy Hofmann3, Ute Hempel4, Joanna Blaszkiewicz5, Jörg Rademann5, Stephanie Moeller6, Matthias Schnabelrauch6, Stefan Kalkof3, Martin Von Bergen3, 7, Mayte T. Pisabarro2, Dieter Scharnweber1 and Vera Hintze1 1 TU Dresden, Max Bergmann Center of Biomaterials, Germany 2 BIOTEC, TU Dresden, Structural Bioinformatics, Germany 3 Helmholtz-Center for Environmental Research-UFZ, Department of Proteomics, Germany 4 TU Dresden, Institute of Physiological Chemistry, Carl Gustav Carus Faculty of Medicine, Germany 5 Freie Universität Berlin, Institute of Pharmacy & Institute of Chemistry and Biochemistry, Germany 6 INNOVENT e.V., Biomaterials Department, Germany 7 Helmholtz-Center for Environmental Research-UFZ, Department of Metabolomics, Germany Introduction: In developed countries the life expectancy increases and therefore the number of multi-morbidities, which lead to impaired wound healing. One important aspect of remodeling during wound healing is related to extracellular matrix (ECM) degradation. This process is regulated via the interplay of matrix metalloproteinases (MMPs) and their counterparts, tissue inhibitors of metalloproteinases (TIMPs). Hence, chronic wounds are often associated with increased MMP and decreased TIMP levels[1]. Among TIMPs, TIMP‑3 is unique as it is bound to sulfated glycosaminoglycans (sGAGs) in the ECM and thought to be the main regulator of matrix turnover. Its cellular up-take is mediated by the endocytic receptor LRP-1[2]. GAGs, as functional components of the ECM, also influence ECM turnover, since sGAGs like heparin modulate the activities of both, TIMP-3[2] and MMPs[3]. Sulfated hyaluronans (sHAs) are promising in this respect as they also support e.g. fibroblast proliferation when used in collagen-based artificial ECMs (aECMs)[4]. Hence, the purpose of this study was to elucidate the influence of chemically sulfated GAGs on ECM homeostasis and in particular the molecular mechanisms by which they affect TIMP activity and up-take. Experimental Methods: Enzyme kinetic analyses were used to determine the residual activity of MMP-1/-2 after pre-incubation with native and chemically sGAGs, TIMP‑3 or combinations of both. The molecular binding mechanism was analyzed via surface plasmon resonance (SPR). TIMP-3 or LRP-1 were immobilized on sensor chips and GAG derivatives were injected. Furthermore, GAGs pre-incubated with TIMP-3 were injected over the LRP-1 surface. Computational approaches and H/D exchange experiments were performed to determine GAG possible binding sites on TIMP-3. Finally in vitro studies with human mesenchymal stromal cells (hMSCs) were supposed to reveal the biological consequence of the interaction between TIMP-3 and GAG derivatives. Results and Discussion: SPR analysis reveals a dose- and sulfation-dependent binding of GAGs to immobilized TIMP-3. This interaction does however not affect the inhibitory potential of TIMP-3 against MMP‑1/-2, even after pre-incubation with GAGs, indicating that GAGs do not affect the activity of TIMP-3. This is in line with the respective GAG binding sites on TIMP-3 determined by molecular modelling and H/D exchange mass spectroscopy. The former indicates that the GAG-binding regions of TIMP-3 do not overlap with the MMP-binding region. At the same time sGAGs interfere with the TIMP-3/LRP‑1 complex formation leading to a reduced binding of TIMP-3 to this endocytic receptor. This is supported by in vitro experiments with hMSCs revealing the accumulation of TIMP-3 in the media after treatments with sHA and a co-localization of TIMP-3 and sHA. Conclusion: Sulfated GAGs are promising candidates for the treatment of chronic wounds since they sequester TIMP-3 and inhibit its internalization without altering the inhibitory potential of TIMP-3 towards MMPs. By slowing down matrix degradation processes sGAGs as part of biomaterials could prolong their beneficial presence in the wound and thereby improve chronic wound healing. We acknowledge financial support by the DFG [TRR 67 A2, A3, A7, A8, B1, Z4].

Basic chemokine-derived glycosaminoglycan binding peptides exert antiviral properties against dengue virus serotype 2, herpes simplex virus-1 and respiratory syncytial virus

Chemokines attract leukocytes to sites of infection in a G protein-coupled receptor (GPCR) and glycosaminoglycan (GAG) dependent manner. Therefore, chemokines are crucial molecules for proper functioning of our antimicrobial defense mechanisms. In addition, some chemokines have GPCR-independent defensin-like antimicrobial activities against bacteria and fungi. Recently, high affinity for GAGs has been reported for the positively charged COOH-terminal region of the chemokine CXCL9. In addition to CXCL9, also CXCL12γ has such a positively charged COOH-terminal region with about 50% positively charged amino acids. In this report, we compared the affinity of COOH-terminal peptides of CXCL9 and CXCL12γ for GAGs and KD values in the low nM range were detected. Several enveloped viruses such as herpesviruses, hepatitis viruses, human immunodeficiency virus (HIV), dengue virus (DENV), etc. are known to bind to GAGs such as the negatively charged heparan sulfate (HS). In this way GAGs are important for the initial contacts between viruses and host cells and for the infection of the cell. Thus, inhibiting the virus-cell interactions, by blocking GAG-binding sites on the host cell, might be a way to target multiple virus families and resistant strains. This article reports that the COOH-terminal peptides of CXCL9 and CXCL12γ have antiviral activity against DENV serotype 2, clinical and laboratory strains of herpes simplex virus (HSV)-1 and respiratory syncytial virus (RSV). Moreover, we show that CXCL9(74-103) competes with DENV envelope protein domain III for binding to heparin. These short chemokine-derived peptides may be lead molecules for the development of novel antiviral agents.

A Simple Method for Discovering Druggable, Specific Glycosaminoglycan-Protein Systems. Elucidation of Key Principles from Heparin/Heparan Sulfate-Binding Proteins.

Glycosaminoglycans (GAGs) affect human physiology and pathology by modulating more than 500 proteins. GAG-protein interactions are generally assumed to be ionic and nonspecific, but specific interactions do exist. Here, we present a simple method to identify the GAG-binding site (GBS) on proteins that in turn helps predict high specific GAG–protein systems. Contrary to contemporary thinking, we found that the electrostatic potential at basic arginine and lysine residues neither identifies the GBS consistently, nor its specificity. GBSs are better identified by considering the potential at neutral hydrogen bond donors such as asparagine or glutamine sidechains. Our studies also reveal that an unusual constellation of ionic and non-ionic residues in the binding site leads to specificity. Nature engineers the local environment of Asn45 of antithrombin, Gln255 of 3-O-sulfotransferase 3, Gln163 and Asn167 of 3-O-sulfotransferase 1 and Asn27 of basic fibroblast growth factor in the respective GBSs to induce specificity. Such residues are distinct from other uncharged residues on the same protein structure in possessing a significantly higher electrostatic potential, resultant from the local topology. In contrast, uncharged residues on nonspecific GBSs such as thrombin and serum albumin possess a diffuse spread of electrostatic potential. Our findings also contradict the paradigm that GAG-binding sites are simply a collection of contiguous Arg/Lys residues. Our work demonstrates the basis for discovering specifically interacting and druggable GAG-protein systems based on the structure of protein alone, without requiring access to any structure-function relationship data.

A novel combined RNA-protein interaction analysis distinguishes HIV-1 Gag protein binding sites from structural change in the viral RNA leader.

RNA-protein interactions govern many viral and host cell processes. Conventional ‘footprinting’ to examine RNA-protein complex formation often cannot distinguish between sites of RNA-protein interaction and sites of RNA structural remodelling. We have developed a novel technique combining photo crosslinking with RNA 2′ hydroxyl reactivity (‘SHAPE’) that achieves rapid and hitherto unachievable resolution of both RNA structural changes and the sites of protein interaction within an RNA-protein complex. ‘XL-SHAPE’ was validated using well-characterized viral RNA-protein interactions: HIV-1 Tat/TAR and bacteriophage MS2 RNA/Coat Binding Protein. It was then used to map HIV-1 Gag protein interactions on 2D and 3D models of the viral RNA leader. Distinct Gag binding sites were identified on exposed RNA surfaces corresponding to regions identified by mutagenesis as important for genome packaging. This widely applicable technique has revealed a first view of the stoichiometry and structure of the initial complex formed when HIV captures its genome.

Structure of decorin binding protein B from Borrelia burgdorferi and its interactions with glycosaminoglycans

Decorin-binding proteins (DBPs), DBPA and DBPB, are surface lipoproteins on Borrelia burgdorferi, the causative agent of Lyme disease. DBPs bind to the connective tissue proteoglycan decorin and facilitate tissue colonization by the bacterium. Although structural and biochemical properties of DBPA are well understood, little is known about DBPB. In current work, we determined the solution structure of DBPB from strain B31 of B. burgdorferi and characterized its interactions with glycosaminoglycans (GAGs). Our structure shows that DBPB adopts the same topology as DBPA, but possesses a much shorter terminal helix, resulting in a longer unstructured C-terminal tail, which is also rich in basic amino acids. Characterization of DBPB–GAG interactions reveals that, despite similar GAG affinities of DBPA and DBPB, the primary GAG-binding sites in DBPB are different from DBPA. In particular, our results indicate that lysines in the C-terminus of DBPB are vital to DBPB's ability to bind GAGs whereas C-terminal tail for DBPA from strain B31 only plays a minor role in facilitating GAG bindings. Furthermore, the traditional GAG-binding pocket important to DBPA–GAG interactions is only secondary to DBPB's GAG-binding ability.

Flexible Linker Modulates Glycosaminoglycan Affinity of Decorin Binding Protein A.

Decorin binding protein A (DBPA) is a glycosaminoglycan (GAG)-binding adhesin found on the surface of the bacterium Borrelia burgdorferi (B. burgdorferi), the causative agent of Lyme disease. DBPA facilitates bacterial adherence to extracellular matrices of human tissues and is crucial during the early stage of the infection process. Interestingly, DBPA from different strains (B31, N40, and PBr) show significant differences in GAG affinities, but the structural basis for the differences is not clear. In this study, we show that GAG affinity of N40 DBPA is modulated in part by flexible segments that control access to the GAG binding site, such that shortening of the linker leads to higher GAG affinity when analyzed using ELISA, gel mobility shift assay, solution NMR, and isothermal titration calorimetry. Our observation that GAG affinity differences among different B. burgdorferi strains can be attributed to a flexible linker domain regulating access to the GAG-binding domain is novel. It also provides a rare example of how neutral amino acids and dynamic segments in GAG binding proteins can have a large influence on GAG affinity and provides insights into why the number of basic amino acids in the GAG-binding site may not be the only factor determining GAG affinity of proteins.

Mucin-like Region of Herpes Simplex Virus Type 1 Attachment Protein Glycoprotein C (gC) Modulates the Virus-Glycosaminoglycan Interaction

Glycoprotein C (gC) mediates the attachment of HSV-1 to susceptible host cells by interacting with glycosaminoglycans (GAGs) on the cell surface. gC contains a mucin-like region located near the GAG-binding site, which may affect the binding activity. Here, we address this issue by studying a HSV-1 mutant lacking the mucin-like domain in gC and the corresponding purified mutant protein (gCΔmuc) in cell culture and GAG-binding assays, respectively. The mutant virus exhibited two functional alterations as compared with native HSV-1 (i.e. decreased sensitivity to GAG-based inhibitors of virus attachment to cells and reduced release of viral particles from the surface of infected cells). Kinetic and equilibrium binding characteristics of purified gC were assessed using surface plasmon resonance-based sensing together with a surface platform consisting of end-on immobilized GAGs. Both native gC and gCΔmuc bound via the expected binding region to chondroitin sulfate and sulfated hyaluronan but not to the non-sulfated hyaluronan, confirming binding specificity. In contrast to native gC, gCΔmuc exhibited a decreased affinity for GAGs and a slower dissociation, indicating that once formed, the gCΔmuc-GAG complex is more stable. It was also found that a larger number of gCΔmuc bound to a single GAG chain, compared with native gC. Taken together, our data suggest that the mucin-like region of HSV-1 gC is involved in the modulation of the GAG-binding activity, a feature of importance both for unrestricted virus entry into the cells and release of newly produced viral particles from infected cells.

Structural mechanisms underlying sequence-dependent variations in GAG affinities of decorin binding protein A, a Borrelia burgdorferi adhesin.

Decorin-binding protein A (DBPA) is an important surface adhesin of the bacterium Borrelia burgdorferi, the causative agent of Lyme disease. DBPA facilitates the bacteria's colonization of human tissue by adhering to glycosaminoglycan (GAG), a sulfated polysaccharide. Interestingly, DBPA sequence variation among different strains of Borrelia spirochetes is high, resulting in significant differences in their GAG affinities. However, the structural mechanisms contributing to these differences are unknown. We determined the solution structures of DBPAs from strain N40 of B. burgdorferi and strain PBr of Borrelia garinii, two DBPA variants whose GAG affinities deviate significantly from strain B31, the best characterized version of DBPA. Our structures revealed that significant differences exist between PBr DBPA and B31/N40 DBPAs. In particular, the C-terminus of PBr DBPA, unlike C-termini from B31 and N40 DBPAs, is positioned away from the GAG-binding pocket and the linker between helices one and two of PBr DBPA is highly structured and retracted from the GAG-binding pocket. The repositioning of the C-terminus allowed the formation of an extra GAG-binding epitope in PBr DBPA and the retracted linker gave GAG ligands more access to the GAG-binding epitopes than other DBPAs. Characterization of GAG ligands' interactions with wild-type (WT) PBr and mutants confirmed the importance of the second major GAG-binding epitope and established the fact that the two epitopes are independent of one another and the new epitope is as important to GAG binding as the traditional epitope.

Designing "high-affinity, high-specificity" glycosaminoglycan sequences through computerized modeling.

The prediction of high-affinity and/or high-specificity protein-glycosaminoglycan (GAG) interactions is an inherently difficult task, due to several factors including the shallow nature of the typical GAG-binding site and the inherent size, flexibility, diversity, and polydisperse nature of the GAG molecules. Here, we present a generally applicable methodology termed Combinatorial Library Virtual Screening (CVLS) that can identify potential high-affinity, high-specificity protein-GAG interactions from very large GAG combinatorial libraries and a suitable GAG-binding protein. We describe the CVLS approach along with the rationale behind it and provide validation for the method using the well-known antithrombin-thrombin-heparin system.

Specific recognition of the HIV-1 genomic RNA by the Gag precursor.

During assembly of HIV-1 particles in infected cells, the viral Pr55(Gag) protein (or Gag precursor) must select the viral genomic RNA (gRNA) from a variety of cellular and viral spliced RNAs. However, there is no consensus on how Pr55(Gag) achieves this selection. Here, by using RNA binding and footprinting assays, we demonstrate that the primary Pr55(Gag) binding site on the gRNA consists of the internal loop and the lower part of stem-loop 1 (SL1), the upper part of which initiates gRNA dimerization. A double regulation ensures specific binding of Pr55(Gag) to the gRNA despite the fact that SL1 is also present in spliced viral RNAs. The region upstream of SL1, which is present in all HIV-1 RNAs, prevents binding to SL1, but this negative effect is counteracted by sequences downstream of SL4, which are unique to the gRNA.

A Small Molecule Glycosaminoglycan Mimetic Blocks Plasmodium Invasion of the Mosquito Midgut

Malaria transmission-blocking (T-B) interventions are essential for malaria elimination. Small molecules that inhibit the Plasmodium ookinete-to-oocyst transition in the midgut of Anopheles mosquitoes, thereby blocking sporogony, represent one approach to achieving this goal. Chondroitin sulfate glycosaminoglycans (CS-GAGs) on the Anopheles gambiae midgut surface are putative ligands for Plasmodium falciparum ookinetes. We hypothesized that our synthetic polysulfonated polymer, VS1, acting as a decoy molecular mimetic of midgut CS-GAGs confers malaria T-B activity. In our study, VS1 repeatedly reduced midgut oocyst development by as much as 99% (P<0.0001) in mosquitoes fed with P. falciparum and Plasmodium berghei. Through direct-binding assays, we observed that VS1 bound to two critical ookinete micronemal proteins, each containing at least one von Willebrand factor A (vWA) domain: (i) circumsporozoite protein and thrombospondin-related anonymous protein-related protein (CTRP) and (ii) vWA domain-related protein (WARP). By immunofluorescence microscopy, we observed that VS1 stains permeabilized P. falciparum and P. berghei ookinetes but does not stain P. berghei CTRP knockouts or transgenic parasites lacking the vWA domains of CTRP while retaining the thrombospondin repeat region. We produced structural homology models of the first vWA domain of CTRP and identified, as expected, putative GAG-binding sites on CTRP that align closely with those predicted for the human vWA A1 domain and the Toxoplasma gondii MIC2 adhesin. Importantly, the models also identified patches of electropositive residues that may extend CTRP's GAG-binding motif and thus potentiate VS1 binding. Our molecule binds to a critical, conserved ookinete protein, CTRP, and exhibits potent malaria T-B activity. This study lays the framework for a high-throughput screen of existing libraries of safe compounds to identify those with potent T-B activity. We envision that such compounds when used as partner drugs with current antimalarial regimens and with RTS,S vaccine delivery could prevent the transmission of drug-resistant and vaccine-breakthrough strains.

Structural insights into proteoglycan-shaped Hedgehog signaling

Hedgehog (Hh) morphogens play fundamental roles during embryogenesis and adulthood, in health and disease. Multiple cell surface receptors regulate the Hh signaling pathway. Among these, the glycosaminoglycan (GAG) chains of proteoglycans shape Hh gradients and signal transduction. We have determined crystal structures of Sonic Hh complexes with two GAGs, heparin and chondroitin sulfate. The interaction determinants, confirmed by site-directed mutagenesis and binding studies, reveal a previously not identified Hh site for GAG binding, common to all Hh proteins. The majority of Hh residues forming this GAG-binding site have been previously implicated in developmental diseases. Crystal packing analysis, combined with analytical ultracentrifugation of Sonic Hh-GAG complexes, suggests a potential mechanism for GAG-dependent Hh multimerization. Taken together, these results provide a direct mechanistic explanation of the observed correlation between disease and impaired Hh gradient formation. Moreover, GAG binding partially overlaps with the site of Hh interactions with an array of protein partners including Patched, hedgehog interacting protein, and the interference hedgehog protein family, suggesting a unique mechanism of Hh signaling modulation.

Homogeneous, Sulfated Oligosaccharides from Glycosaminoglycans Activate Promatrilysin (proMMP‐7) Largely by Co‐localization

Human matrix metalloproteinase‐7 (MMP‐7 or matrilysin) and glycosaminolycans (GAGs) are two important players in extracellular matrix (ECM), participating in innate immunity, aging and cancer. MMP‐7 is over‐expressed in tumor cells and secreted into the ECM as a proenzyme (proMMP‐7). It has been found that MMP‐7 forms complexes with glycoproteins CD44 and heparin‐binding EGF via GAG carbohydrate chains. Recent in vitro studies showed that some GAGs regulate MMP‐7 activities by accelerating proMMP‐7 autolytic cleavage of the pro‐domain (Ra et al., 2009, JBC). We have found recombinant proMMP‐7 to be activated by homogeneous oligosaccharides from heparin in a saccharide length‐dependent manner. 6‐mer, 8‐mer, and 12‐mer heparin oligosaccharides accelerate proMMP‐7 activation at a similar rate while 16‐mer and 20‐mer oligosaccharides accelerate proMMP‐7 activation several‐fold more dramatically. ProMMP‐7 free or bound to the 6‐mer appear to be monomeric by atomic force microscopy. By contrast, proMMP‐7 and the 20‐mer formed large, elongated assemblies. Interface mapping by NMR spectroscopy suggests GAG binding sites in the catalytic and pro domains and potential modes of binding. Our results suggest GAGs regulate proMMP‐7 activation by drawing these protease molecules together. Supported by NIH grant R01GM57289.

Molecular Mechanism by Which Surface Antigen HP0197 Mediates Host Cell Attachment in the Pathogenic Bacteria Streptococcus suis

Streptococcus suis, one of the most important and prevalent pathogens in swine, presents a major challenge to global public health. HP0197 is an S. suis surface antigen that was previously identified by immunoproteomics and can bind to the host cell surface. Here, we investigated the interaction between HP0197 and the host cell surface glycosaminoglycans (GAGs) using indirect immunofluorescence and cell adhesion inhibition assays. In addition, we determined that a novel 18-kDa domain in the N-terminal region of HP0197 functions as the GAG-binding domain. We then solved the three-dimensional structures of the N-terminal 18-kDa and C-terminal G5 domains using x-ray crystallography. Based on this structural information, the GAG-binding sites in HP0197 were predicted and subsequently verified using site-directed mutagenesis and indirect immunofluorescence. The results indicate that the positively charged residues on the exposed surface of the 18-kDa domain, which are primarily lysines, likely play a critical role in the HP0197-heparin interaction that mediates bacterium-host cell adhesion. Understanding this molecular mechanism may provide a basis for the development of effective drugs and therapeutic strategies for treating streptococcal infections.

The cellular protein lyric interacts with HIV-1 Gag.

Human immunodeficiency virus type 1 (HIV-1) Gag is the main structural protein driving assembly and release of virions from infected cells. Gag alone is capable of self-assembly in vitro, but host factors have been shown to play a role in efficient viral replication and particle morphogenesis within the living cell. In a series of affinity purification experiments, we identified the cellular protein Lyric to be an HIV-1 Gag-interacting protein. Lyric was previously described to be an HIV-inducible gene and is involved in various signaling pathways. Gag interacts with endogenous Lyric via its matrix (MA) and nucleocapsid (NC) domains. This interaction requires Gag multimerization and Lyric amino acids 101 to 289. Endogenous Lyric is incorporated into HIV-1 virions and is cleaved by the viral protease. Gag-Lyric interaction was also observed for murine leukemia virus and equine infectious anemia virus, suggesting that it represents a conserved feature among retroviruses. Expression of the Gag binding domain of Lyric increased Gag expression levels and viral infectivity, whereas expression of a Lyric mutant lacking the Gag binding site resulted in lower Gag expression and decreased viral infectivity. The results of the current study identify Lyric to be a cellular interaction partner of HIV-1 Gag and hint at a potential role in regulating infectivity. Further experiments are needed to elucidate the precise role of this interaction.

Glycosaminoglycans mediate retention of the poxvirus type I interferon binding protein at the cell surface to locally block interferon antiviral responses

Eradication of smallpox was accomplished 30 yr ago, but poxviral infections still represent a public health concern due to the potential release of variola virus or the emergence of zoonotic poxviruses, such as monkeypox virus. A critical determinant of poxvirus virulence is the inhibition of interferons (IFNs) by the virus-encoded type I IFN-binding protein (IFNα/βBP). This immunomodulatory protein is secreted and has the unique property of interacting with the cell surface in order to prevent IFN-mediated antiviral responses. However, the mechanism of its attachment to the cell surface remains unknown. Using surface plasmon resonance and cell-binding assays, we report that the IFNα/βBP from vaccinia virus, the smallpox vaccine, interacts with cell surface glycosaminoglycans (GAGs). Analysis of the contribution of different regions of the protein to cell surface binding demonstrated that clusters of basic residues in the first immunoglobulin domain mediate GAG interactions. Furthermore, mutation of the GAG-interaction motifs does not affect its IFN-binding and -blocking capacity. Functional conservation of GAG-binding sites is demonstrated for the IFNα/βBP from variola and monkeypox viruses, extending our understanding of immune modulation by the most virulent human poxviruses. These results are relevant for the design of improved vaccines and intervention strategies.

Characterization of the Human Sulfatase Sulf1 and Its High Affinity Heparin/Heparan Sulfate Interaction Domain

The extracellular sulfatases Sulf1 and Sulf2 remodel the 6O-sulfation state of heparan sulfate proteoglycans on the cell surface, thereby modulating growth factor signaling. Different from all other sulfatases, the Sulfs contain a unique, positively charged hydrophilic domain (HD) of about 320 amino acid residues. Using various HD deletion mutants and glutathione S-transferase (GST)-HD fusion proteins, this study demonstrates that the HD is required for enzymatic activity and acts as a high affinity heparin/heparan sulfate interaction domain. Association of the HD with the cell surface is sensitive to heparinase treatment, underlining specificity toward heparan sulfate chains. Correspondingly, isolated GST-HD binds strongly to both heparin and heparan sulfate in vitro and also to living cells. Surface plasmon resonance studies indicate nanomolar affinity of GST-HD toward immobilized heparin. The comparison of different mutants reveals that especially the outer regions of the HD mediate heparan sulfate binding, probably involving "tandem" interactions. Interestingly, binding to heparan sulfate depends on the presence of 6O-sulfate substrate groups, suggesting that substrate turnover facilitates release of the enzyme from its substrate. Deletion of the inner, less conserved region of the HD drastically increases Sulf1 secretion without affecting enzymatic activity or substrate specificity, thus providing a tool for the in vitro modulation of HS-dependent signaling as demonstrated here for the signal transduction of fibroblast growth factor 2. Taken together, the present study shows that specific regions of the HD influence different aspects of HS binding, cellular localization, and enzyme function.