Differences in domain structure between pericellular matrix and plasma fibronectins as revealed by domain-specific antibodies combined with limited proteolysis and S-cyanylation: a preliminary note

The ShdA adhesin binds to the cationic cradle of the fibronectin 13FnIII repeat module: evidence for molecular mimicry of heparin binding

Introduction of Salmonella enterica serotype Typhimurium into food products results from its ability to persist in the intestine of healthy livestock by mechanisms that are poorly understood. The non-fimbrial adhesin ShdA is a fibronectin binding protein required for persistent intestinal carriage of S. Typhimurium. We further investigated the molecular mechanism of ShdA-mediated intestinal persistence by determining the binding-site of this receptor in fibronectin. Analysis of ShdA binding to fibronectin proteolytic fragments and to recombinant fibronectin fusion proteins identified the (13)FnIII repeat module of the Hep-2 domain as the primary binding site for this adhesin. The (13)FnIII repeat module of fibronectin contains a cationic cradle formed by six basic residues (R6, R7, R9, R23, K25 and R54) that is a high affinity heparin-binding site conserved among fibronectin sequences from frogs to man. Binding of ShdA to the (13)FnIII repeat module of fibronectin and to a second extracellular matrix protein, Collagen I, could be inhibited by heparin. Furthermore, binding of ShdA to the Hep-2 domain was sensitive to the ionic buffer strength, suggesting that binding involved ionic interactions. We therefore determined whether amino acid substitutions of basic residues in the cationic cradle of the Hep-2 domain that inhibit heparin binding also abrogate binding of ShdA. Combined substitution of R6S and R7S strongly reduced ShdA binding to (13)FnIII. These data suggest that ShdA binds the Hep-2 domain of fibronectin by a mechanism that may mimic binding of the host polysaccharide heparin.

Human tissue fibronectin: expression of different isotypes in the adult and fetal tissues

The isotype of fibronectin present in human adult and fetal lung tissues was studied by limited proteolysis by cathepsin D followed by immunoblot analysis with domain-specific antibodies. The results indicate that the fibronectin in the adult lung tissue is predominantly the plasma-type whereas the fibronectin in the fetal lung tissue is more related to the cellular-type than to the plasma-type. Thus, it appears that the fibronectin isotype in tissue switches from the cellular-type to the plasma-type during ontogenesis.

Transformed human cells release different fibronectin variants than do normal cells

Fibronectin molecules are dimers composed of subunits whose primary structures may differ. This is due to alternative splicing in at least two regions (ED and IIICS) of the pre-mRNA. Using two monoclonal antibodies specific for two different epitopes of domain 5 (high affinity for heparin), we have quantitatively analyzed the expression of the IIICS sequence in human fibronectins from different sources. The results demonstrated that the percentage of fibronectin subunits containing the IIICS is higher in fibronectins from tumor-derived or simian virus 40-transformed human cells than in fibronectins from human plasma or normal human fibroblasts. Furthermore, we observed that 45-65% of fibronectin subunits from transformed cells or normal embryonic fibroblasts are sialylated on the heparin-binding domain 5, whereas this occurs in only 24-28% of fibronectin subunits from normal adult fibroblasts. On the contrary, no sialylation was observed on domain 5 in fibronectin from human plasma.

Human liver fibronectin complementary DNAs: identification of two different messenger RNAs possibly encoding the alpha and beta subunits of plasma fibronectin

Human fibronectin polymorphism arises from variation in the C-terminal region [e.g., Sekiguchi, K., Siri, A., Zardi, L., & Hakomori, S. (1985) J. Biol. Chem. 260, 5105-5114]. In order to verify the chemical basis of the fibronectin polymorphism, cDNAs encoding the C-terminal region of human liver fibronectin have been isolated, sequenced, and compared with cDNAs encoding so-called "cellular fibronectin" (i.e., fibronectin produced by cultured cells in vitro). Among the five independent cDNAs thus isolated, two cDNAs, named pLF2 and pLF4, differed in the nucleotide sequence at the "type III connecting segment" (IIIcs) region. pLF4 contained 192 bases in this region whereas pLF2 completely lacked these bases. S1 mapping analysis indicated that both cDNAs with and without the 192 bases are faithful copies of two fibronectin mRNA species abundantly present in human liver. Comparison of the liver cDNAs with those coding for cellular fibronectin indicates that the latter cDNAs contain the 75-base and/or 93-base extra segments at the 5' and 3' boundaries of the 192-base IIIcs region. These extra segments have the consensus sequences for the 3' splice sites at their 3' ends, suggesting that fibronectin mRNAs with partial or complete deletion of the IIIcs sequence result from alternative splicing of a primary RNA transcript. Liver fibronectin cDNAs also lacked the 270-based "extra domain" (ED) segment present in some, but not all, cDNAs encoding cellular fibronectin. Thus, cellular fibronectin appears to have three extra peptide segments, encoded by the 75-base and 93-base segments in the IIIcs region and by the 270-base ED region, that are mostly absent in the liver fibronectin.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural differences in the cell binding region of human fibronectin molecules isolated from cultured normal and tumor-derived human cells

Fibronectins isolated from human plasma (pFN) and from the conditioned media of normal (N-cFN) and tumor (T-cFN) human cells were compared by cathepsin D digestion followed by immunostaining of released fragments with the monoclonal antibody 3E3, specific for the cell binding site. Two different staining patterns were obtained, one specific for pFN and N-cFN, the second common to fibronectins from the 3 different kinds of tumors studied. This indicates structural differences between N-cFN and T-cFN in the cell binding region of the fibronectin molecule.

The oncofetal domain of fibronectin defined by monoclonal antibody FDC-6: its presence in fibronectins from fetal and tumor tissues and its absence in those from normal adult tissues and plasma

An IgG1 monoclonal antibody, FDC-6, was established, which defines a unique fibronectin (FN) domain, located between the "Hep-2" and the "Fib-2" domains, in the COOH-terminal region of FNs isolated from hepatoma, sarcoma, and fetal fibroblasts. A systematic study with this antibody indicates the presence of two classes of human FNs. (i) FN from fetal connective tissue, placenta, amniotic fluid, hepatoma, and colon carcinoma as well as cell lines from fetal tissues (WI-38), hepatomas (HuH-6 and HuH-7), and sarcoma (VA13) was characterized by the presence of the FDC-6-defined domain and by a high molecular weight (subunit Mr, 310,000-335,000). (ii) In contrast, FN from normal adult tissues and plasma was characterized by a lower molecular weight (subunit Mr, 285,000-295,000) and lack of reactivity with FDC-6 and is therefore devoid of the FDC-6-defined domain. The FDC-6-defined domain is therefore called the "oncofetal" domain, and FN containing this domain is hereby called "oncofetal FN" (onf FN). The onf FN is similar to the previously known "cellular-form" FN. FN from normal adult tissues and plasma, lacking the oncofetal domain, is hereby called "normal FN" (nor FN). The nor FN is similar to the previously known "plasma-form" FN. Development of FN from fetal to adult form is associated with loss of the oncofetal domain defined by the FDC-6 antibody, and oncogenic transformation is associated with activation in synthesis of the oncofetal domain defined by the FDC-6 antibody.

Elution of fibronectin proteolytic fragments from a hydroxyapatite chromatography column. A simple procedure for the purification of fibronectin domains

Human plasma fibronectin is composed of at least five distinct domains which we refer to as Hep-1/Fib-1, Gel, Cell, Hep-2 and Fib-2 depending on their affinity for heparin (Hep), gelatin (Gel), the cell surface (Cell) or fibrin (Fib). These domains are aligned from the NH2 to the COOH terminus in the above order and can be separated from each other by mild proteolytic digestion. We have studied the elution of fibronectin thermolysin digest from a hydroxyapatite column using a linear gradient (0.5-190 mM) of sodium phosphate buffer. The five major fibronectin domains were eluted from the hydroxyapatite chromatography column in the following order: Gel, Fib-2, Cell, Hep-1/Fib-1 and Hep-2. They were identified on the basis of their molecular mass, affinity to different macromolecules and reaction with domain-specific monoclonal antibodies. All domains except the Cell and Hep-2 domains eluted as single homogeneous peaks. The Cell domain eluted as two different peaks and the Hep-2 domain eluted as four different peaks. This is the first time that heterogeneity of these two domains has been observed. Since chromatography of a fibronectin thermolysin digest on a hydroxyapatite column provides a good separation of the five major fibronectin domains, we have elaborated a procedure in which each fibronectin domain is purified by no more than two steps; hydroxyapatite and molecular exclusion chromatography. Fractionation of fibronectin proteolytic digest on a hydroxyapatite chromatography column should be of great value in the comparative analysis of fibronectin from different sources and in the study of fibronectin heterogeneity. Its use in combination with molecular exclusion chromatography offers a simple and high-yield method for the purification of large amounts of fibronectin domains.

Similarity of the carbohydrate moieties of fibronectins derived from blood plasma and synthesised by cultured endothelial cells

Plasma and cellular fibronectins are reported to be very similar but not identical in chemical structure. We have compared bovine plasma fibronectin with fibronectin secreted by bovine aortal endothelial cells in culture. Techniques were chosen to highlight likely structural differences, particularly in the carbohydrate moieties. Both fibronectins were wholly reactive to monospecific antiserum and behaved identically in two-dimensional gel electrophoresis. The oligosaccharide chains were identical in proportion and degree of sialylation by anion-exchange HPLC. Fractionation of the glycopeptides on immobilised lectins and serotonin showed that both fibronectins contained (i) predominantly biantennary oligosaccharides, (ii) exclusively N-acetylneuraminic acid residues in a non-clustered array and (iii) no L-fucose residues. The overriding structural similarities support the proposal that the endothelium is a site of synthesis of plasma fibronectin in vivo.

Characterization of synovial fluid fibronectin from patients with rheumatic inflammatory diseases and healthy subjects

Synovial fluid fibronectin from normal subjects and from patients who have rheumatic inflammatory diseases has been studied and compared with plasma fibronectin. The average fibronectin concentration in synovial fluids from normal subjects was 172 +/- 69 micrograms/ml; it was 721 +/- 315 and 556 +/- 349 micrograms/ml in synovial fluids from patients with rheumatoid arthritis and osteoarthritis, respectively. This is the first report on fibronectin concentrations in normal synovial fluids. Synovial fluid fibronectin from healthy subjects and patients with rheumatoid arthritis or osteoarthritis showed a molecular weight identical to that of plasma fibronectin. All normal and pathologic synovial fluid fibronectins showed a remarkably lower electrophoretic mobility compared with that of plasma fibronectin, when separated according to net molecular charge on agarose gel. Peptides from thermolysin digests of fibronectin from plasma and synovial fluid, when compared on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, showed distinct differences. These data demonstrate that synovial fluid fibronectin represents a molecular form which is structurally different from that of plasma fibronectin. This suggests that synovial fluid fibronectin is locally synthesized, possibly by a cell type which differs from that responsible for the production of the plasmatic fibronectin pool.