Affinity chromatography on immobilized fibrin monomer, IV. Two fibrin-binding peptides of a chymotryptic digest of human plasma fibronectin

Interaction of the fibronectin COOH-terminal Fib-2 regions with fibrin: further characterization and localization of the Fib-2-binding sites

Incorporation of fibronectin into fibrin clots is important for the formation of a provisional matrix that promotes cell adhesion and migration during wound healing. Previous studies revealed that this incorporation occurs through noncovalent interaction between two NH2-terminal Fib-1 regions of fibronectin (one on each chain) and the alphaC-regions of fibrin, and is further reinforced by factor XIIIa-mediated covalent cross-linking of fibronectin to the fibrin matrix. To clarify the role of another pair of fibrin-binding regions, Fib-2, located at the disulfide-linked COOH-terminal ends of fibronectin, we prepared by limited proteolysis a dimeric 140 kDa (Fib-2)2 fragment containing both Fib-2 regions and tested its interaction with recombinant fragments corresponding to the alphaC regions of fibrin(ogen). In both ELISA and surface plasmon resonance (SPR) experiments 140 kDa (Fib-2)2 bound to the immobilized Aalpha221-610 alphaC-fragment. However, the affinity of binding was substantially lower than that for Fib-1. Ligand blotting and ELISA established that the Fib-2 binding site is located in the connector part of the alphaC region including residues Aalpha221-391. Analysis of the SPR-detected binding of fibronectin to the immobilized Aalpha221-610 alphaC-fragment revealed two types of fibronectin-binding sites, one with high affinity and another one with much lower affinity. Competition experiments revealed about 30% inhibition of the Fib-2 mediated binding by increasing concentrations of Fib-1 fragment suggesting partial overlap of the two sets of binding sites. Based on these results and our previous studies we propose a mechanism of interaction of fibronectin with fibrin in which both Fib-1 and Fib-2 play a role.

Interaction of fibrin(ogen) with fibronectin: further characterization and localization of the fibronectin-binding site

The interaction of fibronectin with fibrin and its incorporation into fibrin clots are thought to be important for the formation of a provisional matrix that promotes cell adhesion and migration during wound healing. However, it is still unclear whether fibronectin interacts with both fibrin and fibrinogen or fibrin only and whether fibronectin binds exclusively to the fibrin(ogen) alphaC domains. To address these questions, we studied the interaction of fibronectin with fibrinogen, fibrin, and their proteolytic and recombinant fragments. In both ELISA and surface plasmon resonance (SPR) experiments, immobilized fibrinogen did not bind fibronectin at all, but after conversion to fibrin, it bound fibronectin with high affinity. To test which regions of fibrin are involved in this binding, we studied the interaction of fibronectin with the fibrin-derived D-D:E(1) complex and a recombinant alphaC fragment (residues Aalpha221-610) corresponding to the alphaC domain that together encompass the whole fibrin(ogen) molecule. In ELISA, when fibronectin was added to the immobilized D-D:E(1) complex or the immobilized alphaC fragment, only the latter exhibited binding. Likewise, when fibronectin was immobilized and the complex or the alphaC fragment was added, only the latter was observed to bind. The selective interaction between fibronectin and the alphaC fragment was confirmed by SPR. The fibronectin-binding site was further localized to the NH(2) terminal connector region of the alphaC domain since in ELISA, the immobilized recombinant Aalpha221-391 sub-fragment bound fibronectin well while the immobilized recombinant Aalpha392-610 sub-fragment exhibited no binding. This finding was confirmed by ligand blotting analysis. Thus, the results provide direct evidence for the existence of a cryptic high-affinity fibronectin-binding site in the Aalpha221-391 region of the fibrinogen alphaC domain that is not accessible in fibrinogen but becomes exposed in fibrin.

Entry of OpaA+ gonococci into HEp-2 cells requires concerted action of glycosaminoglycans, fibronectin and integrin receptors

Heparan sulphate proteoglycans are increasingly implicated as eukaryotic cell surface receptors for bacterial pathogens. Here, we report that Neisseria gonorrhoeae adheres to proteoglycan receptors on HEp-2 epithelial cells but that internalization of the bacterium by this cell type requires the serum glycoprotein fibronectin. Fibronectin was shown to bind specifically to gonococci producing the OpaA adhesin. Binding assays with fibronectin fragments located the bacterial binding site near the N-terminal end of the molecule. However, none of the tested fibronectin fragments supported gonococcal entry into the eukaryotic cells; a 120 kDa fragment carrying the cell adhesion domain with the amino acid sequence RGD even inhibited the fibronectin-mediated uptake of MS11-OpaA. This inhibition could be mimicked by an RGD-containing hexapeptide and by alpha 5 beta 1 integrin-specific antibodies, suggesting that interaction of the central region of fibronectin with integrin receptors facilitated bacterial uptake. Fibronectin was unable to promote gonococcal entry into HEp-2 cells that had been treated with the enzyme heparinase III, which degrades the glycosaminoglycan side-chains of proteoglycan receptors. On the basis of these results, we propose a novel cellular uptake pathway for bacteria, which involves the binding of the pathogen to glycosaminoglycans that, in turn, act as co-receptors facilitating fibronectin-mediated bacterial uptake through integrin receptors. In this scenario, fibronectin would act as a molecular bridge linking to Opa-proteoglycan complex with host cell integrin receptors.

Cross-linking of fibronectin to C-terminal fragments of the fibrinogen alpha-chain by factor XIIIa

Fibronectin binds specifically to fibrin and is covalently cross-linked to the fibrin alpha chain by activated factor XIII (XIIIa). This reaction is important for wound healing. Here we investigate XIIIa-catalyzed cross-linking of fibronectin and some of its fragments to a recombinant fragment representing the COOH-terminal 30 kDa of the fibrin alpha chain (alpha C30K:His 368-Val 610). Only fibronectin and those fragments containing an intact NH2-terminus were able to form cross-linked complexes. As many as 10 of the 17 lysines in alpha C30K can serve as amine donors in this reaction. Analysis of the rate of XIIIa-catalyzed cross-linking of fibronectin NH2-terminal peptides and fragments with alpha C30K revealed that the presence of the first type I "finger" module accelerates the cross-linking reaction; addition of fingers 2-5 had no further effect.

The alternatively spliced V region contributes to the differential incorporation of plasma and cellular fibronectins into fibrin clots

During blood clot formation in vivo, plasma fibronectin (pFN) is cross-linked to fibrin by coagulation factor XIIIa. Cellular FN (cFN), which localizes to connective tissue, is distinguished from pFN by the inclusion of alternatively spliced segments. To determine if these two FNs are functionally equivalent in blood clotting, the cross-linking of rat pFN and cFN to fibrin was compared in an in vitro clotting assay. Fibrinogen and FN were incubated at physiological ratios in the presence of thrombin and factor XIIIa. Cross-linking of FN to fibrin was monitored by SDS-PAGE and immunoblotting. Over 24 h, cFN was incorporated at a significantly slower rate than pFN and was not completely cross-linked to fibrin at a temperature that favors this interaction (0 degrees C). This difference was observed with purified fibrinogens from human, rat, and bovine and with rat plasma and was maintained even after incubation of pFN with rat fibroblasts for several days. Using the same assay, purified recombinant V(+)-V0 and V(+)-V+ FN dimers resembling pFN and cFN, respectively, showed a similar difference in cross-linking kinetics. These results suggest that the asymmetric distribution of the V region among pFN dimers plays a role in regulating its incorporation into blood clots. In fibrin clots, cFN was converted into a set of cross-linked intermediates distinct from those of pFN. For example, while pFN was initially cross-linked into a pFN-fibrin alpha heterodimer, this product was not a major intermediate in clots formed with cFN. This finding, in conjunction with evidence for the formation of factor XIIIa-catalyzed cFN-cFN cross-links, indicated that cFN molecules interact with each other, and with fibrin, differently from pFN. Together, these results show an important functional distinction between pFN and cFN.

Evidence for the presence of a free N-terminal fibronectin 30-kDa domain in human plasma by quantitative determination with an indirect immunosorbent assay

The free N-terminal 30-kDa domain of the fibronectin subunit chains had previously been shown to mediate binding of soluble fibrin to phagocytic cells. In order to demonstrate whether the fragment is available in plasma in a suitable concentration, an indirect immunoassay procedure for its quantitative evaluation was developed. The free form of the 30-kDa domain was separated from fibronectin and the bulk of the plasma proteins by two-step affinity chromatography on gelatin- and heparin-Sepharose. In the eluate of the heparin-Sepharose the 30-kDa fragment was determined by its capacity to inhibit the immune reaction between a specific antiserum and the 30-kDa fragment immobilized on microtiter wells. The procedure offered reproductibility comparable with other immunoassays (coefficient of variation 4.0 to 8.0%); the lowest amount of detectable 30-kDa fragment was 0.1 microgram/ml. In human plasma this method detected for the first time ca. 5 micrograms/ml 30-kDa fragment. This concentration is in the range required for binding of fibrin to cells.

Free N-terminal fibronectin 30-kD-domain mediates binding of soluble fibrin to gelfiltered unstimulated thrombocytes

Gelfiltered unstimulated human platelets neither bound 125-I-fibrinogen nor 125-I-fibrin. Fibrin-binding was, however, stimulated by N-terminal fibronectin 30 kD-and 70 kD-fragments while fibronectin was ineffective. The 30 kD-fragment also stimulated some platelet preparations to bind fibrinogen which, however, was suppressed by minute amounts of the thrombin inhibitor PPACK. PPACK hardly influenced fibrin-binding. Fragment-promoted fibrinogen-binding was also inhibited by a monoclonal antibody recognizing the membrane glycoprotein IIb/IIIa complex known to act as fibrinogen receptor. This antibody failed to influence fragment-stimulated fibrin-binding giving evidence that fibrinogen and fibrin were retained by different receptors. In contrast to 125-I-fibrin its plasmin-derived and 125-I-labelled fragment X was not recognized by the platelets in presence of the fibronectin 30 kD-fragment. Fragment-stimulated binding of 125-I-fibrin showed a lag phase and was completely inhibited by 0.25 mM putrescine as well as by 1 mM EDTA or 0.1 mM N-ethylmaleinimide. Evidently, a cell-attached transamidase was involved in fibrin-binding possibly by forming a ternary complex with fibrin and the fibronectin fragment.

Interaction of small dermatan sulfate proteoglycan from fibroblasts with fibronectin

Immunogold labeling was used to localize the core protein of small dermatan sulfate proteoglycan (DS-PG) on the surface of cultured human fibroblasts. At 4 degrees C, DS-PG core protein was uniformly distributed over the cell surface. At 37 degrees C, gold particles either became rearranged in form of clusters or remained associated with fibrils. Double-label immunocytochemistry indicated the co-distribution of DS-PG core protein and fibronectin in the fibrils. In an enzyme-linked immunosorbent assay, binding of DS-PG from fibroblast secretions and of its core protein to fibronectin occurred at pH 7.4 and at physiological ionic strength. Larger amounts of core protein than of intact proteoglycan could be bound. Fibronectin peptides containing either the heparin-binding domain near the COOH-terminal end or the heparin-binding NH2 terminus were the only fragments interacting with DS-PG and core protein. Competition and replacement experiments with heparin and dermatan sulfate suggested the existence of adjacent binding sites for heparin and DS-PG core protein. It is hypothesized that heparan sulfate proteoglycans and DS-PG may competitively interact with fibronectin.

N-terminal fibronectin 30-kDa fragment mediates the immobilization of soluble fibrin by factor XIIIa-coated polystyrene beads

Polystyrene beads coated with thrombin-activated Factor XIII (plasma transglutaminase, plasma transamidase) retained soluble 125I-fibrin, if a 30-kDa fragment of fibronectin was present. The fragment was obtained by proteolytic cleavage of plasma fibronectin with trypsin, but was also available with plasmin or thrombin. It represented a fibrin-binding domain at the N-terminus of each of the two subunit chains and contained close to its amino end a transamidase-reactive site. Fibronectin was unable to mediate the binding of 125I-fibrin to Factor XIIIa-coated beads. 125I-fibrinogen was hardly recognized by the beads even in presence of the fibronectin fragment. The relatively slow binding of 125I-fibrin was inhibited by 0.15 mM putrescine or by a pretreatment of the coated beads with EDTA or N-ethylmaleinimide indicating the involvement of a transamidation in the binding reaction. Immobilization of 125I-fibrin in presence of the fibronectin fragment is assumed to require a covalent cross-linking of the two ligands at the immobilized transamidase giving rise to a product which is retained strongly. The possibility is discussed that a surface-attached transamidase might act as a fibrin receptor which requires the fibronectin fragment as a cofactor.

The role of fibronectin fragments and cell-attached transamidase on the binding of soluble fibrin to macrophages

Proteolytic fragments from the N-terminus of the fibronectin subunit chains were shown to mediate the binding of 125-I-fibrin to macrophages. With increasing molecular weight of the fragments, binding activity decreased and intact plasma fibronectin was inactive. Fibrin binding to macrophages was a time dependent reaction and proceeded considerably faster than binding of fibrinogen. The binding reaction was inhibited by putrescine suggesting the involvement of a transamidase. Pericellular transamidase was demonstrated on macrophages by incorporation of 14-C-putrescine into fibronectin 30 kD-fragment. Expression of this enzyme appeared to be rate-limiting for the binding reaction which was accelerated after loading the cells with placental transamidase.

A model for the role of hyaluronic acid and fibrin in the early events during the inflammatory response and wound healing

A model is presented outlining the molecular and cellular events that occur during the early stages of the wound healing process. The underlying theme is that there is a specific binding interaction between fibrin, the major clot protein, and hyaluronic acid (HA), a constituent of the wound extracellular matrix. This binding interaction, which could also be stabilized by other cross-linking components, provides the driving force to organize a three-dimensional HA matrix attached to and interdigitated with the initial fibrin matrix. The HA-fibrin matrix plays a major role in the subsequent tissue reconstruction processes. We suggest that HA and fibrin have both structural and regulatory functions at different times during the wound healing process. The concentration of HA in blood and in the initial clot is very low. This is consistent with the proposed interaction between HA and fibrin(ogen), which could interfere with either fibrinogen activation or fibrin assembly and cross-linking. We propose that an activator (e.g. derived from a plasma precursor, platelets or surrounding cells) is produced during the clotting reaction and then stimulates one or more blood cell types to synthesize and secrete HA into the fibrin matrix of the clot. We predict that HA controls the stability of the matrix by regulating the degradation of fibrin. The new HA-fibrin matrix increases or stabilizes the volume and porosity of the clot and then serves as a physical support, a scaffold through which cells trapped in the clot or cells infiltrating from the peripheral edge of the wound can migrate. The HA-fibrin matrix also actively stimulates or induces cell motility and activates and regulates many functions of blood cells, which are involved in the inflammatory response, including phagocytosis and chemotaxis. The secondary HA-fibrin matrix itself is then modified as cells continue to migrate into the wound, secreting hyaluronidase and plasminogen activator to degrade the HA and fibrin. At the same time these cells secrete collagen and glycosaminoglycans to make a more differentiated matrix. The degradation products derived from both fibrin and HA are, in turn, important regulatory molecules which control cellular functions involved in the inflammatory response and new blood vessel formation in the healing wound. The proposed model generates a number of testable experimental predictions.

SV40 transformed fibroblasts recognize the same 140 kD fibronectin chemotactic fragment as non-transformed cells

SV40-virus-transformed human embryonal fibroblasts show an enhanced chemotactic response to the glycoprotein fibronectin. However, they recognize the same chemotactic active region as non-transformed fibroblasts. The result suggests that an enhancement of chemotaxis by fibroblasts which have been transformed with Simian Virus 40 is due not to the utilization of further chemotactic domains in the molecule, but to an increased sensitivity of the cells to the chemoattractant.

Fibrinmonomer binding to macrophages mediated by fibrin-binding fibronectin fragments

Binding of 125-I-fibrinmonomer to peritoneal macrophages was investigated in dependence of plasma fibronectin and of its thrombin- or plasmin-derived fragments. Plasma fibronectin failed to enhance cell binding of 125-I-fibrinmonomer. In contrast, 30kD-fragments derived from the N-termini of the fibronectin subunits improved binding considerably. The association with the cell surface was completely inhibited by EDTA, 2-5 mM putrescine and to about 40 per cent by 0.1 mM dansylcadaverine suggesting that a transamidase-catalyzed cross-linking reaction was involved. Thrombin-derived 200kD-remnants of the fibronectin subunit chains failed to mediate cell binding of 125-I-fibrinmonomer provided they had been deprived of residual thrombin activity. Otherwise they were active and their activity was inhibited by the thrombin inhibitor hirudin. Plasmin-derived 200 kD-fragments were inactive as well.

The immunoreactivity, ligand, and cell binding characteristics of rheumatoid synovial fluid fibronectin

Fibronectin promotes macrophage adherence and expression of Fc receptors, is chemotactic for fibroblasts, and is an opsonin for fibrin and denatured collagen. These properties suggest a role for fibronectin in the modulation of joint inflammation. Since structural modification of the fibronectin molecule has been shown to result in loss or de novo acquisition of opsonic and chemotactic activity, we determined the functional and immunochemical properties of fibronectin isolated from the inflamed joint. Eighty-six percent of synovial fluids obtained from patients with active rheumatoid arthritis (RA) contained fibronectin fragments, and 39% of the fluids no longer displayed the dimeric form. Compared with native fibronectin, RA peptides were as active in promoting synoviocyte chemotaxis and in glycosaminoglycan binding, but displayed lower affinity for fibrin and gelatin. Although comparable with intact protein in augmenting monocyte attachment to gelatin, the RA synovial fluid peptides did not augment monocyte attachment to fibrin. Analysis of whole synovial fluid and isolated fibronectins by enzyme immunoassay showed that the increased fibronectin immunoreactivity, previously reported in RA synovial fluid, measures intact and nearly intact protein and does not measure extensively degraded fragments.

Aggregation and fibril formation of plasma fibronectin by heparin

The precipitation of plasma fibronectin by heparin in dependence on various parameters was investigated. Rising heparin concentration augmented the precipitates up to a maximum beyond which precipitation decreased. Yields close to 80% were obtained at low temperatures, but some precipitation was observed at 37 degrees C as well. Insolubilization was considerably dependent on the ionic strength, indicating that electrostatic forces play a major role in the aggregation of fibronectin. Calcium already prevented precipitation by heparin at low concentrations. If precipitation was performed on hydrophobized glass cover slides, the formation of fibrils visible by phase-contrast microscopy was observed. On hydrophilic surfaces amorphous precipitates were generally obtained, most likely due to trapping of aggregates by adsorption prior to their arrangement to fibrils. The results are discussed on the basis of a model assuming that heparin induces a conformational rearrangement of plasma fibronectin so that masked binding sites responsible for self-association become exposed.

Evidence for a cryptic lectin site in the cell-binding domain of plasma fibronectin

Various proteolytic fragments from the central region of the fibronectin subunit chains containing the main cell-affinity site were applied in cell binding studies using peritoneal macrophages of guinea pigs. A 125I-labelled 23-kDa peptide was relatively well bound by the cells. Attachment to cells was partially inhibited by wheat germ lectin, suggesting a lectin-like site in the cell-binding domain which recognizes oligosaccharide groups with terminal N-acetylglucosamine or N-acetylneuraminic acid. Binding was inhibited by N-acetylneuraminic acid with half-maximal effect at 2 X 10(-3) M. Other inhibitors were a sialic acid rich ganglioside preparation and fetuin, a sialic acid-containing glycoprotein. In contrast to the 23-kDa peptide a 125I-labelled 125-kDa fragment was only weakly bound, although it included the sequence of the 23-kDa peptide on its C-terminus. The residual binding was weakly inhibited by low concentrations of wheat germ lectin and was remarkably improved by higher concentrations. The behavior of the peptide was explained by the presence of a sialic acid-containing oligosaccharide side chain localized outside of the 23-kDa region and interacting with the lectin-like site in the cell-binding sequence. In accord with this suggestion a 95-kDa fragment representing the oligosaccharide-containing part of the 125-kDa peptide was capable of inhibiting at least partially the cell attachment of the 23-kDa piece. The results indicate a lectin-like affinity site in the cell-binding region of fibronectin which is accessible in the 23-kDa peptide, but is masked in the 125-kDa fragment and in fibronectin by a sialic acid-containing oligosaccharide moiety.

Effects of neutral proteases from human leukocytes on plasma fibronectin

Human plasma fibronectin (FN) is not susceptible to collagenase and gelatinase from human blood leukocytes. Leukocytic elastolytic protease (ELP) and cathepsin G (chymotrypsin-like protease, CLP) degrade FN to similar fragments. Among products of proteolysis by ELP and CLP fragments have been identified which bind to gelatin-fragment 40 kd, to fibrin-fragments 55 kd and 30 kd, and to heparin-fragment 30 kd.

Shape and stability of fibronectin in solutions of different pH and ionic strength

At low ionic strength (0.05 M) the sedimentation coefficient of monomeric plasma fibronectin was found to vary from 8S, at pH 3 and 11, to 13.5S at neutral pH. The lower s20,w value indicates a stretched arrangement of the value indicates a stretched arrangement of the two arms of the molecule which was observed in most electron microscopic studies. The higher value is consistent with a still very asymmetric but more condensed shape, which is probably brought about by back-folding and interactions between chain segments of different net charge. A model of this internal association is based on the finding that segments of rather different isoelectric points alternate along the fibronectin chains. A similar pH dependence was observed for a 140-kDa fragment from the middle region of fibronectin which carries segments of low and high isoelectric points at its ends. At high ionic strength (0.35 M) the pH dependence of the sedimentation coefficients was less pronounced and intermediate s20,w values were found. This is expected when both repulsive and attractive interactions are weakened by the electrolyte. It was verified by circular dichroism spectra that the protein was not denatured at pH 3 or 11. Thermal transition curves revealed a destabilization at pH 3 but the thermal denaturation occurred well above 20 degrees C at which the pH dependence of the solution shape was studied.

Common evolutionary origin of the fibrin-binding structures of fibronectin and tissue-type plasminogen activator

Comparison of the primary structures of high-Mr urokinase and tissue-type plasminogen activator reveals a high degree of structural homology between the two proteins, except that tissue activator contains a 43 residue long amino-terminal region, which has no counterpart in urokinase. We show that this segment is homologous with the finger-domains responsible for the fibrin-affinity of fibronectin. Limited proteolysis of the amino-terminal region of plasminogen activator was found to lead to a loss of the fibrin-affinity of the enzyme. It is suggested that the finger-domains of fibronectin and tissue-types plasminogen activator have similar functions and that the finger-domains of the two proteins evolved from a common ancestral fibrin-binding domain.

Generation of fibronectin receptors on macrophages by wheat germ lectin

A chymotrypsin-derived and 125I-labelled 125-kDa fragment of human plasma fibronectin which contained the cell binding site, was only weakly bound by peritoneal macrophages of guinea pigs and binding was not saturable. In presence of wheat germ lectin binding increased proportionally to the logarithm of the lectin concentration. Association of 125I-fragment with cells was partially prevented by non-labelled fragment indicating a saturable receptor-ligand interaction. An apparent affinity constant of about 2--4 x 10(-5) M was evaluated. A considerable fraction of the cell-bound 125I-fragment resisted removal by proteases suggesting that it was internalized. In order to investigate an influence of wheat germ lectin on the binding of 125I-fibronectin by the cells the macrophages were preincubated with the lectin followed by washing and evaluation of 125I-fibronectin binding. A simultaneous incubation of the cells with 125I-fibronectin and lectin was impractical due to partial interaction of the two proteins giving rise to some unspecific precipitates. Preincubation with wheat germ lectin considerably improved the capacity of the macrophages for binding of 125I-fibronectin. Again the binding of 125I-labelled protein could be restricted by unlabelled one. N-acetyl-glucosamine inhibited the binding of 125I-fibronectin by wheat germ lectin-treated cells if applied in the preincubation phase and more effectively, if applied in the final 125I-fibronectin binding assay. N-Acetylneuraminic acid also inhibited this step. In addition to wheat germ lectin concanavalin A was capable of generating fibronectin receptors on the cell surface. Soy bean lectin, however, was ineffective.

Localization of the chemotactic domain in fibronectin

Fragments derived from human plasma fibronectin by enzymatic degradation were tested in the Boyden chamber for chemotactic activity towards various fibroblast strains. The results provide clear evidence that the chemotactic activity is restricted to a defined region of the fibronectin molecule which is the same for various fibroblast strains. The active domain is localized between the collagen binding site and the major heparin binding site, about 170 kDa apart from the N-terminal and about 70 kDa from the C-terminal ends of the two subunit peptide chains.

A large cathepsin D-derived fragment from the central part of the fibronectin subunit chains

A mild cathepsin D digest of fibronectin only contained single-chain peptides of 200, 140 and 70 kDa and double-chain fragments of about 300 and 140 kDa containing the C-terminal disulfide link. Among the single-chain fragments the 200 kDa peptide was a precursor of the 140 kDa and 70 kDa peptides. The latter was correlated to the N-terminal and the former to the central region of the fibronectin subunit chains.

Monoclonal antibodies used as probes for the structural organization of the central region of fibronectin

Two monoclonal mouse antibodies against human plasma fibronectin were compared in their reactivity for proteolytic fragments of the antigen by enzyme immunoassay and immunoblotting. These antibodies were shown to react with two different structures within a short segment (about 30 kDa) located about one-third away from the C-terminus of the fibronectin chains.

Discrimination of different domains in fibronectin on the basis of their stability against urea denaturation

Circular dichroism spectra, thermal transition profiles and proteolytic susceptibility showed that different regions in the multidomain protein fibronectin exhibit different sensitivity against urea denaturation. A 70-kDa fragment obtained by cathepsin D treatment which comprises the N-terminal part of the fibronectin chains, exhibited in 8M urea a spectrum, at 20 degrees C, identical to that of the native fragment and its thermal unfolding was shifted to lower temperatures by only 10 degrees C. The central portions of the fibronectin chains were remarkably unfolded under the same conditions as clearly demonstrated by the spectra and transition profiles of a cathepsin D-raised 125/140-kDa fragment which originates from this region. When fibronectin or its fragments were exposed to 4 or 8M urea at 4 degrees C and the urea subsequently dialysed off, the spectra and transition curves recorded were very similar to those of the native proteins. Nevertheless, this treatment introduced local conformational changes which resulted in the creation of three new cleavage sites for chymotrypsin. The most prominent one was found to be located in the central part of the middle region and no sites were created in the N-terminal 70-kDa region. In the conjunction with sequence information [Petersen et al. (1983) Proc. Natl. Acad. Sci. USA 80, 137-141] it may be concluded that the disulfide rich domains, made up by regions of internal homology of types I and II in the N-terminal portion of fibronectin, exhibit a remarkable conformational stability, whereas the disulfide free middle region which contains type III domains, is much less stable. Some domains in this region are particularly sensitive to urea denaturation and are irreversibly affected already by 4M urea at 4 degrees C. Therefore, the use of high urea concentrations for the elution of fibronectin from affinity columns may lead to an at least partially irreversible unfolding of some domains and a loss of functions associated with these structural elements.