Solution structure of human myeloid-derived growth factor suggests a conserved function in the endoplasmic reticulum.

Human myeloid-derived growth factor (hMYDGF) is a 142-residue protein with a C-terminal endoplasmic reticulum (ER) retention sequence (ERS). Extracellular MYDGF mediates cardiac repair in mice after anoxic injury. Although homologs of hMYDGF are found in eukaryotes as distant as protozoans, its structure and function are unknown. Here we present the NMR solution structure of hMYDGF, which consists of a short α-helix and ten ß-strands distributed in three ß-sheets. Conserved residues map to the unstructured ERS, loops on the face opposite the ERS, and the surface of a cavity underneath the conserved loops. The only protein or portion of a protein known to have a similar fold is the base domain of VNN1. We suggest, in analogy to the tethering of the VNN1 nitrilase domain to the plasma membrane via its base domain, that MYDGF complexed to the KDEL receptor binds cargo via its conserved residues for transport to the ER.

A disintegrin-like and metalloproteinase domain with thrombospondin type 1 motif 9 (ADAMTS9) regulates fibronectin fibrillogenesis and turnover.

The secreted metalloprotease ADAMTS9 has dual roles in extracellular matrix (ECM) turnover and biogenesis of the primary cilium during mouse embryogenesis. Its gene locus is associated with several human traits and disorders, but ADAMTS9 has few known interacting partners or confirmed substrates. Here, using a yeast two-hybrid screen for proteins interacting with its C-terminal Gon1 domain, we identified three putative ADAMTS9-binding regions in the ECM glycoprotein fibronectin. Using solid-phase binding assays and surface plasmon resonance experiments with purified proteins, we demonstrate that ADAMTS9 and fibronectin interact. ADAMTS9 constructs, including those lacking Gon1, co-localized with fibronectin fibrils formed by cultured fibroblasts lacking fibrillin-1, which co-localizes with fibronectin and binds several ADAMTSs. We observed no fibrillar ADAMTS9 staining after blockade of fibroblast fibronectin fibrillogenesis with a peptide based on the functional upstream domain of a Staphylococcus aureus adhesin. These findings indicate that ADAMTS9 binds fibronectin dimers and fibrils directly through multiple sites in both molecules. Proteolytically active ADAMTS9, but not a catalytically inactive variant, disrupted fibronectin fibril networks formed by fibroblasts in vitro, and ADAMTS9-deficient RPE1 cells assembled a robust fibronectin fibril network, unlike WT cells. Targeted LC-MS analysis of fibronectin digested by ADAMTS9-expressing cells identified a semitryptic peptide arising from cleavage at Gly2196-Leu2197 We noted that this scissile bond is in the linker between fibronectin modules III17 and I10, a region targeted also by other proteases. These findings, along with stronger fibronectin staining previously observed in Adamts9 mutant embryos, suggest that ADAMTS9 contributes to fibronectin turnover during ECM remodeling.

The calcium-binding type III repeats domain of thrombospondin-2 binds to fibroblast growth factor 2 (FGF2).

Thrombospondin (TSP)-1 and TSP-2 share similar structures and functions, including a remarkable antiangiogenic activity. We have previously demonstrated that a mechanism of the antiangiogenic activity of TSP-1 is the interaction of its type III repeats domain with fibroblast growth factor-2 (FGF2), affecting the growth factor bioavailability and angiogenic activity. Since the type III repeats domain is conserved in TSP-2, this study aimed at investigating whether also TSP-2 retained the ability to interact with FGF2. The FGF2 binding properties of TSP-1 and TSP-2 and their recombinant domains were analyzed by solid-phase binding and surface plasmon resonance assays. TSP-2 bound FGF2 with high affinity (Kd = 1.3 nM). TSP-2/FGF2 binding was inhibited by calcium and heparin. The FGF2-binding domain of TSP-2 was located in the type III repeats and the minimal interacting sequence was identified as the GVTDEKD peptide in repeat 3C, corresponding to KIPDDRD, the active sequence of TSP-1. A second putative FGF2 binding sequence was also identified in repeat 11C of both TSPs. Computational docking analysis predicted that both the TSP-2 and TSP-1-derived heptapeptides interacted with FGF2 with comparable binding properties. Accordingly, small molecules based on the TSP-1 active sequence blocked TSP-2/FGF2 interaction. Binding of TSP-2 to FGF2 impaired the growth factor ability to interact with its cellular receptors, since TSP-2-derived fragments prevented the binding of FGF2 to both heparin (used as a structural analog of heparan sulfate proteoglycans) and FGFR-1. These findings identify TSP-2 as a new FGF2 ligand that shares with TSP-1 the same molecular requirements for interaction with the growth factor and a comparable capacity to block FGF2 interaction with proangiogenic receptors. These features likely contribute to TSP-2 antiangiogenic and antineoplastic activity, providing the rationale for future therapeutic applications.

Pentraxin 3 regulates synaptic function by inducing AMPA receptor clustering via ECM remodeling and ß1-integrin.

Control of synapse number and function in the developing central nervous system is critical to the formation of neural circuits. Astrocytes play a key role in this process by releasing factors that promote the formation of excitatory synapses. Astrocyte-secreted thrombospondins (TSPs) induce the formation of structural synapses, which however remain post-synaptically silent, suggesting that completion of early synaptogenesis may require a two-step mechanism. Here, we show that the humoral innate immune molecule Pentraxin 3 (PTX3) is expressed in the developing rodent brain. PTX3 plays a key role in promoting functionally-active CNS synapses, by increasing the surface levels and synaptic clustering of AMPA glutamate receptors. This process involves tumor necrosis factor-induced protein 6 (TSG6), remodeling of the perineuronal network, and a ß1-integrin/ERK pathway. Furthermore, PTX3 activity is regulated by TSP1, which directly interacts with the N-terminal region of PTX3. These data unveil a fundamental role of PTX3 in promoting the first wave of synaptogenesis, and show that interplay of TSP1 and PTX3 sets the proper balance between synaptic growth and synapse function in the developing brain.

Control of cytokine-driven eosinophil migratory behavior by TGF-beta-induced protein (TGFBI) and periostin.

Periostin, which is induced by interleukin (IL)-13, is an extracellular matrix (ECM) protein that supports αMß2 integrin-mediated adhesion and migration of IL-5-stimulated eosinophils. Transforming growth factor (TGF)-ß-induced protein (TGFBI) is a widely expressed periostin paralog known to support monocyte adhesion. Our objective was to compare eosinophil adhesion and migration on TGFBI and periostin in the presence of IL-5-family cytokines. Eosinophil adhesion after 1 h and random motility over 20 h in the presence of various concentrations of IL-5, IL-3, or granulocyte macrophage-colony stimulating factor (GM-CSF) were quantified in wells coated with various concentrations of TGFBI or periostin. Results were compared to video microscopy of eosinophils. Cytokine-stimulated eosinophils adhered equivalently well to TGFBI or periostin in a coating concentration-dependent manner. Adhesion was blocked by anti-αMß2 and stimulated at the lowest concentration by GM-CSF. In the motility assay, periostin was more potent than TGFBI, the coating-concentration effect was bimodal, and IL-3 was the most potent cytokine. Video microscopy revealed that under the optimal coating condition of 5 µg/ml periostin, most eosinophils migrated persistently and were polarized and acorn-shaped with a ruffling forward edge and granules gathered together, in front of the nucleus. On 10 µg/ml periostin or TGFBI, more eosinophils adopted a flattened pancake morphology with dispersed granules and nuclear lobes, and slower migration. Conversion between acorn and pancake morphologies were observed. We conclude that TGFBI or periostin supports two modes of migration by IL-5 family cytokine-activated eosinophils. The rapid mode is favored by intermediate protein coatings and the slower by higher coating concentrations. We speculate that eosinophils move by haptotaxis up a gradient of adhesive ECM protein and then slow down to surveil the tissue.

Myeloid-derived growth factor is a resident endoplasmic reticulum protein.

Human myeloid-derived growth factor (MYDGF; also known as C19orf10) is named based on its identification as a secreted monocyte/macrophage-derived mediator of cardiac repair following myocardial infarction in mice. Homologs of MYDGF, however, are present in organisms throughout and outside of the animal kingdom, some of which lack hematopoietic and circulatory systems. Moreover, the UPF0556 protein domain, which defines these homologs, lacks a known structure. As a result, the functions and properties of MYDGF are unclear. Our current work was initiated to test whether MYDGF is present in secretory vesicles of eosinophils as it was recently reported to be abundant in these cells. However, we could not demonstrate secretion and unexpectedly discovered that MYDGF colocalizes with P4HB in the nuclear envelope, which comprises the bulk of endoplasmic reticulum (ER) in eosinophils, and with P4HB and RCAS1 in Golgi. We noted a ubiquitous C-terminal sequence, BXEL (B, basic; X, variable residue; E, Glu; L, Leu), that has the potential to retain human MYDGF and its homologs in the ER. To test the functionality of this sequence, we expressed full-length human MYDGF or MYDGF lacking the C-terminal Glu-Leu residues in monolayers of human embryonic kidney 293 (HEK293) cells. Full-length MYDGF accumulated in cells, whereas truncated MYDGF appeared in the medium. These observations reveal that MYDGF resides in the ER and Golgi and provide a new framework for investigating and understanding this intriguing protein.

A Selective Extracellular Matrix Proteomics Approach Identifies Fibronectin Proteolysis by A Disintegrin-like and Metalloprotease Domain with Thrombospondin Type 1 Motifs (ADAMTS16) and Its Impact on Spheroid Morphogenesis.

Secreted and cell-surface proteases are major mediators of extracellular matrix (ECM) turnover, but their mechanisms and regulatory impact are poorly understood. We developed a mass spectrometry approach using a cell-free ECM produced in vitro to identify fibronectin (FN) as a novel substrate of the secreted metalloprotease ADAMTS16. ADAMTS16 cleaves FN between its (I)5 and (I)6 modules, releasing the N-terminal 30 kDa heparin-binding domain essential for FN self-assembly. ADAMTS16 impairs FN fibrillogenesis as well as fibrillin-1 and tenascin-C assembly, thus inhibiting formation of a mature ECM by cultured fibroblasts. Furthermore ADAMTS16 has a marked morphogenetic impact on spheroid formation by renal tubule-derived MDCKI cells. The N-terminal FN domain released by ADAMTS16 up-regulates MMP3, which cleaves the (I)5-(I)6 linker of FN similar to ADAMTS16, therefore creating a proteolytic feed-forward mechanism. Thus, FN proteolysis not only regulates FN turnover, but also FN assembly, with potential long-term consequences for ECM assembly and morphogenesis.

Adaptation in a Fibronectin Binding Autolysin of Staphylococcus saprophyticus.

Human-pathogenic bacteria are found in a variety of niches, including free-living, zoonotic, and microbiome environments. Identifying bacterial adaptations that enable invasive disease is an important means of gaining insight into the molecular basis of pathogenesis and understanding pathogen emergence. Staphylococcus saprophyticus, a leading cause of urinary tract infections, can be found in the environment, food, animals, and the human microbiome. We identified a selective sweep in the gene encoding the Aas adhesin, a key virulence factor that binds host fibronectin. We hypothesize that the mutation under selection (aas_2206A>C) facilitates colonization of the urinary tract, an environment where bacteria are subject to strong shearing forces. The mutation appears to have enabled emergence and expansion of a human-pathogenic lineage of S. saprophyticus. These results demonstrate the power of evolutionary genomic approaches in discovering the genetic basis of virulence and emphasize the pleiotropy and adaptability of bacteria occupying diverse niches. IMPORTANCEStaphylococcus saprophyticus is an important cause of urinary tract infections (UTI) in women; such UTI are common, can be severe, and are associated with significant impacts to public health. In addition to being a cause of human UTI, S. saprophyticus can be found in the environment, in food, and associated with animals. After discovering that UTI strains of S. saprophyticus are for the most part closely related to each other, we sought to determine whether these strains are specially adapted to cause disease in humans. We found evidence suggesting that a mutation in the gene aas is advantageous in the context of human infection. We hypothesize that the mutation allows S. saprophyticus to survive better in the human urinary tract. These results show how bacteria found in the environment can evolve to cause disease.

Absence of Vitamin K-Dependent γ-Carboxylation in Human Periostin Extracted from Fibrotic Lung or Secreted from a Cell Line Engineered to Optimize γ-Carboxylation.

Periostin (PN, gene name POSTN) is an extracellular matrix protein that is up-regulated in bronchial epithelial cells and lung fibroblasts by TH-2 cytokines. Its paralog, TGF-ß-induced protein (ßig-h3, gene name TGFBI), is also expressed in the lung and up-regulated in bronchial myofibroblasts by TGF-ß. PN and ßig-h3 contain fasciclin 1 modules that harbor putative recognition sequences for γ-glutamyl carboxylase and are annotated in UniProt as undergoing vitamin K-dependent γ-carboxylation of multiple glutamic acid residues. γ-carboxylation profoundly alters activities of other proteins subject to the modification, e.g., blood coagulation factors, and would be expected to alter the structure and function of PN and ßig-h3. To analyze for the presence of γ-carboxylation, proteins extracted from fibrotic lung were reacted with monoclonal antibodies specific for PN, ßig-h3, or modification with γ-carboxyglutamic acid (Gla). In Western blots of 1-dimensional gels, bands stained with anti-PN or -ßig-h3 did not match those stained with anti-Gla. In 2-dimensional gels, anti-PN-positive spots had pIs of 7.0 to >8, as expected for the unmodified protein, and there was no overlap between anti-PN-positive and anti-Gla-positive spots. Recombinant PN and blood coagulation factor VII were produced in HEK293 cells that had been transfected with vitamin K 2, 3-epoxide reductase C1 to optimize γ-carboxylation. Recombinant PN secreted from these cells did not react with anti-Gla antibody and had pIs similar to that found in extracts of fibrotic lung whereas secreted factor VII reacted strongly with anti-Gla antibody. Over 67% coverage of recombinant PN was achieved by mass spectrometry, including peptides with 19 of the 24 glutamates considered targets of γ-carboxylation, but analysis revealed no modification. Over 86% sequence coverage and three modified glutamic acid residues were identified in recombinant fVII. These data indicate that PN and ßig-h3 are not subject to vitamin K-dependent γ-carboxylation.

Periostin and TGF-ß-induced protein: Two peas in a pod?

Periostin (PN) and TGF-ß-induced protein (ßig-h3) are paralogs that contain a single emilin and four fasciclin-1 modules and are secreted from cells. PN receives attention because of its up-regulation in cancer and degenerative and allergic diseases. ßig-h3 is highly enriched in cornea and best known for harboring mutations in humans associated with corneal dystrophies. Both proteins are expressed widely, and many functions, some over-lapping, have been attributed to PN and ßig-h3 based on biochemical, cell culture, and whole animal experiments. We attempt to organize this knowledge so as to facilitate research on these interesting and incompletely understood proteins. We focus particularly on whether PN and ßig-h3 are modified by vitamin K-dependent γ-glutamyl carboxylation, a question of considerable importance given the profound effects of γ-carboxylation on structure and function of other proteins. We consider the roles of PN and ßig-h3 in formation of extracellular matrix and as ligands for integrin receptors. We attempt to reconcile the contradictory results that have arisen concerning the role of PN, which has emerged as a marker of TH2 immunity, in murine models of allergic asthma. Finally, when possible we compare and contrast the structures and functions of the two proteins.

Thrombospondin-1 (TSP1) contributes to the development of vascular inflammation by regulating monocytic cell motility in mouse models of abdominal aortic aneurysm.

RATIONALE: Histological examination of abdominal aortic aneurysm (AAA) tissues demonstrates extracellular matrix destruction and infiltration of inflammatory cells. Previous work with mouse models of AAA has shown that anti-inflammatory strategies can effectively attenuate aneurysm formation. Thrombospondin-1 is a matricellular protein involved in the maintenance of vascular structure and homeostasis through the regulation of biological functions, such as cell proliferation, apoptosis, and adhesion. Expression levels of thrombospondin-1 correlate with vascular disease conditions. OBJECTIVE: To use thrombospondin-1-deficient (Thbs1(-/-)) mice to test the hypothesis that thrombospondin-1 contributes to pathogenesis of AAAs. METHODS AND RESULTS: Mouse experimental AAA was induced through perivascular treatment with calcium phosphate, intraluminal perfusion with porcine elastase, or systemic administration of angiotensin II. Induction of AAA increased thrombospondin-1 expression in aortas of C57BL/6 or apoE-/- mice. Compared with Thbs1(+/+) mice, Thbs1(-/-) mice developed significantly smaller aortic expansion when subjected to AAA inductions, which was associated with diminished infiltration of macrophages. Thbs1(-/-) monocytic cells had reduced adhesion and migratory capacity in vitro compared with wild-type counterparts. Adoptive transfer of Thbs1(+/+) monocytic cells or bone marrow reconstitution rescued aneurysm development in Thbs1(-/-) mice. CONCLUSIONS: Thrombospondin-1 expression plays a significant role in regulation of migration and adhesion of mononuclear cells, contributing to vascular inflammation during AAA development.

Ratios of Four STAT3 Splice Variants in Human Eosinophils and Diffuse Large B Cell Lymphoma Cells.

Signal transducer and activator of transcription 3 (STAT3) is a key mediator of leukocyte differentiation and proliferation. The 3' end of STAT3 transcripts is subject to two alternative splicing events. One results in either full-length STAT3α or in STAT3ß, which lacks part of the C-terminal transactivation domain. The other is at a tandem donor (5') splice site and results in the codon for Ser-701 being included (S) or excluded (ΔS). Despite the proximity of Ser-701 to the site of activating phosphorylation at Tyr-705, ΔS/S splicing has barely been studied. Sequencing of cDNA from purified eosinophils revealed the presence of four transcripts (S-α, ΔS-α, S-ß, and ΔS-ß) rather than the three reported in publically available databases from which ΔS-ß is missing. To gain insight into regulation of the two alternative splicing events, we developed a quantitative(q) PCR protocol to compare transcript ratios in eosinophils in which STAT3 is upregulated by cytokines, activated B cell diffuse large B cell Lymphoma (DLBCL) cells in which STAT3 is dysregulated, and in germinal center B cell-like DLBCL cells in which it is not. With the exception of one line of activated B cell DLCBL cells, the four variants were found in roughly the same ratios despite differences in total levels of STAT3 transcripts. S-α was the most abundant, followed by S-ß. ΔS-α and ΔS-ß together comprised 15.6 ± 4.0 % (mean ± SD, n = 21) of the total. The percentage of STAT3ß variants that were ΔS was 1.5-fold greater than of STAT3α variants that were ΔS. Inspection of Illumina's "BodyMap" RNA-Seq database revealed that the ΔS variant accounts for 10-26 % of STAT3 transcripts across 16 human tissues, with less variation than three other genes with the identical tandem donor splice site sequence. Thus, it seems likely that all cells contain the S-α, ΔS-α, S-ß, and ΔS-ß variants of STAT3.

Nonselective assembly of fibrillin 1 and fibrillin 2 in the rodent ocular zonule and in cultured cells: implications for Marfan syndrome.

PURPOSE: Fibrillins are the major constituent of tissue microfibrils, which form the ocular zonule. In Marfan syndrome (MFS), FBN1 mutations lead to ectopia lentis. The goal of this work was to investigate zonule composition and formation in fibrillin-deficient and wild-type mice. METHODS: Immunofluorescence staining of eyes from wild-type, Fbn1-deficient, and Fbn2-deficient mice, as well as other species, was performed using monospecific fibrillin 1 and fibrillin 2 antibodies. The zonule of Fbn1-deficient and Fbn2-deficient mice was studied by electron microscopy. Microfibril formation in vitro was evaluated by immunofluorescence microscopy of cultured nonpigmented ciliary epithelial cells and fibroblasts. RESULTS: A zonule was present in both Fbn1-deficient and Fbn2-deficient mouse eyes. Immunofluorescence demonstrated that the zonule of Fbn1-deficient mice, wild-type mice, rats, and hamsters contained fibrillin 2. The zonule of Fbn2(-/-) mice contained fibrillin 1. Fibrillin 1 and fibrillin 2 colocalized in microfibrils formed in human nonpigmented ciliary epithelium cultures. Like fibrillin 1, fibrillin 2 microfibril assembly was fibronectin dependent and initiated by cell surface punctate deposits that elongated to form microfibrils. CONCLUSIONS: These data suggest that fibrillin 1 assembly and fibrillin 2 assembly share similar mechanisms. Microfibril composition depends substantially on the local levels of fibrillin isoforms and is not highly selective in regard to the isoform. This raises the intriguing possibility that the zonule could be strengthened in MFS by inducing fibrillin 2 expression in ciliary epithelium. The presence of fibrillin 2 in the murine zonule and an intact zonule in Fbn1-knockout mice may limit the utility of rodent models for studying ectopia lentis in MFS.

Complex contributions of fibronectin to initiation and maturation of microfibrils.

Fibrillins constitute the backbone of extracellular multifunctional assemblies present in elastic and non-elastic matrices, termed microfibrils. Assembly of fibrillins into microfibrils and their homoeostasis is poorly understood and is often compromised in connective tissue disorders such as Marfan syndrome and other fibrillinopathies. Using interaction mapping studies, we demonstrate that fibrillins require the complete gelatin-binding region of fibronectin for interaction, which comprises domains FNI6-FNI9. However, the interaction of fibrillin-1 with the gelatin-binding domain of fibronectin is not involved in fibrillin-1 network assembly mediated by human skin fibroblasts. We show further that the fibronectin network is essential for microfibril homoeostasis in early stages. Fibronectin is present in extracted mature microfibrils from tissue and cells as well as in some in situ microfibrils observed at the ultrastructural level, indicating an extended mechanism for the involvement of fibronectin in microfibril assembly and maturation.

Characterization of molecules binding to the 70K N-terminal region of fibronectin by IFAST purification coupled with mass spectrometry.

Fibronectin (Fn) is a large glycoprotein present in plasma and extracellular matrix and is important for many processes. Within Fn the 70 kDa N-terminal region (70k-Fn) is involved in cell-mediated Fn assembly, a process that contributes to embryogenesis, development, and platelet thrombus formation. In addition, major human pathogens including Staphlycoccus aureus and Streptococcus pyogenes bind the 70k-Fn region by a novel form of protein-protein interaction called ß-zipper formation, facilitating bacterial spread and colonization. Knowledge of blood plasma and platelet proteins that interact with 70k-Fn by ß-zipper formation is incomplete. In the current study, we aimed to characterize these proteins through affinity purification. For this affinity purification, we used a novel purification technique termed immiscible filtration assisted by surface tension (IFAST). The foundation of this technology is immiscible phase filtration, using a magnet to draw paramagnetic particle (PMP)-bound analyte through an immiscible barrier (oil or organic solvent) that separates an aqueous sample from an aqueous eluting buffer. The immiscible barrier functions to remove unbound proteins via exclusion rather than dilutive washing used in traditional isolation methods. We identified 31 interactors from plasma, of which only seven were previously known to interact with Fn. Furthermore, five proteins were identified to interact with 70k-Fn from platelet lysate, of which one was previously known. These results demonstrate that IFAST offers advantages for proteomic studies of interacting molecules in that the technique requires small sample volumes, can be done with high enough throughput to sample multiple interaction conditions, and is amenable to exploratory mass spectrometric and confirmatory immuno-blotting read-outs.

α(M)ß(2) integrin-mediated adhesion and motility of IL-5-stimulated eosinophils on periostin.

Periostin is an extracellular matrix protein that is up-regulated by T helper cell type 2 cytokines in the asthmatic airway and implicated in mouse studies as promoting eosinophil recruitment. We asked whether periostin modulates eosinophil adhesion and motility in vitro. Periostin adsorbed to polystyrene supported adhesion of purified human blood eosinophils stimulated by IL-5, IL-3, or granulocyte/macrophage colony-stimulating factor, but did not support adhesion of eosinophils treated with IL-4 or IL-13. The degree of adhesion depended on the concentrations of periostin during coating and activating cytokine during the adhesion assay. Both full-length periostin and alternatively spliced periostin, lacking C-terminal exons 17, 18, 19, and 21, supported adhesion. Adhesion was inhibited by monoclonal antibody to α(M) or ß(2) integrin subunits, but not by antibodies to other eosinophil integrin subunits. Adsorbed periostin also supported α(M)ß(2)-dependent random motility of IL-5-stimulated eosinophils with optimal movement at an intermediate coating concentration. In the presence of IL-5, eosinophils adherent on periostin formed punctate structures positive for filamentous actin, gelsolin, and phosphotyrosine. These structures fit the criteria for podosomes, highly dynamic adhesive contacts that are distinct from classical focal adhesions. The results establish α(M)ß(2) (CD11b/CD18, Mac-1) as an adhesive and promigratory periostin receptor on cytokine-stimulated eosinophils, and suggest that periostin may function as a haptotactic stimulus able to guide eosinophils to areas of high periostin density in the asthmatic airway.

Ail protein binds ninth type III fibronectin repeat (9FNIII) within central 120-kDa region of fibronectin to facilitate cell binding by Yersinia pestis.

The Yersinia pestis adhesin molecule Ail interacts with the extracellular matrix protein fibronectin (Fn) on host cells to facilitate efficient delivery of cytotoxic Yop proteins, a process essential for plague virulence. A number of bacterial pathogens are known to bind to the N-terminal region of Fn, comprising type I Fn (FNI) repeats. Using proteolytically generated Fn fragments and purified recombinant Fn fragments, we demonstrated that Ail binds the centrally located 120-kDa fragment containing type III Fn (FNIII) repeats. A panel of monoclonal antibodies (mAbs) that recognize specific epitopes within the 120-kDa fragment demonstrated that mAb binding to (9)FNIII blocks Ail-mediated bacterial binding to Fn. Epitopes of three mAbs that blocked Ail binding to Fn were mapped to a similar face of (9)FNIII. Antibodies directed against (9)FNIII also inhibited Ail-dependent cell binding activity, thus demonstrating the biological relevance of this Ail binding region on Fn. Bacteria expressing Ail on their surface could also bind a minimal fragment of Fn containing repeats (9-10)FNIII, and this binding was blocked by a mAb specific for (9)FNIII. These data demonstrate that Ail binds to (9)FNIII of Fn and presents Fn to host cells to facilitate cell binding and delivery of Yops (cytotoxins of Y. pestis), a novel interaction, distinct from other bacterial Fn-binding proteins.

Ligation of the fibrin-binding domain by ß-strand addition is sufficient for expansion of soluble fibronectin.

How fibronectin (FN) converts from a compact plasma protein to a fibrillar component of extracellular matrix is not understood. "Functional upstream domain" (FUD), a polypeptide based on F1 adhesin of Streptococcus pyogenes, binds by anti-parallel ß-strand addition to discontinuous sets of N-terminal FN type I modules, (2-5)FNI of the fibrin-binding domain and (8-9)FNI of the gelatin-binding domain. Such binding blocks assembly of FN. To learn whether ligation of (2-5)FNI, (8-9)FNI, or the two sets in combination is important for inhibition, we tested "high affinity downstream domain" (HADD), which binds by ß-strand addition to the continuous set of FNI modules, (1-5)FNI, comprising the fibrin-binding domain. HADD and FUD were similarly active in blocking fibronectin assembly. Binding of HADD or FUD to soluble plasma FN exposed the epitope to monoclonal antibody mAbIII-10 in the tenth FN type III module ((10)FNIII) and caused expansion of FN as assessed by dynamic light scattering. Soluble N-terminal constructs truncated after (9)FNI or (3)FNIII competed better than soluble FN for binding of FUD or HADD to adsorbed FN, indicating that interactions involving type III modules more C-terminal than (3)FNIII limit ß-strand addition to (1-5)FNI within intact soluble FN. Preincubation of FN with mAbIII-10 or heparin modestly increased binding to HADD or FUD. Thus, ligation of FNIII modules involved in binding of integrins and glycosaminoglycans, (10)FNIII and (12-14)FNIII, increases accessibility of (1-5)FNI. Allosteric loss of constraining interactions among (1-5)FNI, (10)FNIII, and (12-14)FNIII likely enables assembly of FN into extracellular fibrils.

IGD motifs, which are required for migration stimulatory activity of fibronectin type I modules, do not mediate binding in matrix assembly.

Picomolar concentrations of proteins comprising only the N-terminal 70-kDa region (70K) of fibronectin (FN) stimulate cell migration into collagen gels. The Ile-Gly-Asp (IGD) motifs in four of the nine FN type 1 (FNI) modules in 70K are important for such migratory stimulating activity. The 70K region mediates binding of nanomolar concentrations of intact FN to cell-surface sites where FN is assembled. Using baculovirus, we expressed wildtype 70K and 70K with Ile-to-Ala mutations in (3)FNI and (5)FNI; (7)FNI and (9)FNI; or (3)FNI, (5)FNI, (7)FNI, and (9)FNI. Wildtype 70K and 70K with Ile-to-Ala mutations were equally active in binding to assembly sites of FN-null fibroblasts. This finding indicates that IGD motifs do not mediate the interaction between 70K and the cell-surface that is important for FN assembly. Further, FN fragment N-(3)FNIII, which does not stimulate migration, binds to assembly sites on FN-null fibroblast. The Ile-to-Ala mutations had effects on the structure of FNI modules as evidenced by decreases in abilities of 70K with Ile-to-Ala mutations to bind to monoclonal antibody 5C3, which recognizes an epitope in (9)FNI, or to bind to FUD, a polypeptide based on the F1 adhesin of Streptococcus pyogenes that interacts with 70K by the ß-zipper mechanism. These results suggest that the picomolar interactions of 70K with cells that stimulate cell migration require different conformations of FNI modules than the nanomolar interactions required for assembly.

Reactivity of the N-terminal region of fibronectin protein to transglutaminase 2 and factor XIIIA.

Transglutaminase 2 (TG2) is secreted by a non-classical pathway into the extracellular space, where it has several activities pertinent to fibronectin (FN), including binding to the gelatin-binding domain of FN and acting as an integrin co-receptor. Glutamines in the N-terminal tail of FN are known to be susceptible to transamidation by both TG2 and activated blood coagulation factor XIII (FXIIIa). We used immunoblotting, limited proteolysis, and mass spectrometry to localize glutamines within FN that are subject to TG2-catalyzed incorporation of dansylcadaverine in comparison to residues modified by FXIIIa. Such analysis of plasma FN indicated that Gln-3, Gln-7, and Gln-9 in the N-terminal tail and Gln-246 of the linker between fifth and sixth type I modules ((5)F1 and (6)F1) are transamidated by both enzymes. Only minor incorporation of dansylcadaverine was detected elsewhere. Labeling of C-terminally truncated FN constructs revealed efficient TG2- or FXIIIa-catalyzed dansylcadaverine incorporation into the N-terminal residues of constructs as small as the 29-kDa fragment that includes (1-5)F1 and lacks modules from the adjacent gelatin-binding domain. However, when only (1-3)F1 were present, dansylcadaverine incorporation into the N-terminal residues of FN was lost and instead was in the enzymes, near the active site of TG2 and terminal domains of FXIIIa. Thus, these results demonstrate that FXIIIa and TG2 act similarly on glutamines at either end of (1-5)F1 and transamidation specificity of both enzymes is achieved through interactions with the intact 29K fragment.