Functional structure of the somatomedin B domain of vitronectin

Inhibition of autotaxin by bile salts and bile salt-like molecules increases its expression by feedback regulation

BACKGROUND

Autotaxin is an enzyme that converts lysophospholipid into lysophosphatidic acid (LPA), a highly potent signaling molecule through a range of LPA receptors. It is therefore important to investigate which factors play a role in regulating ATX expression. Since we have reported that ATX levels increase dramatically in patients with various forms of cholestasis, we embarked on a study to reveal factors that influence the enzyme activity ATX as well as its expression level in vitro and in vivo.

METHODS

Bile from cholestatic patients was fractionated by HPLC and analyzed for modulation of ATX activity. ATX expression was measured in fibroblasts upon stimulation or inhibition of LPA signaling.

RESULTS

Surprisingly, ATX activity was stimulated by most forms of its product LPA, but it was inhibited by bile salts and bile salt-like molecules, particularly by 3-OH sulfated bile salts and sulfated progesterone metabolites that are known to accumulate during chronic cholestasis and cholestasis of pregnancy, respectively. Activation of fibroblasts by LPA decreased ATX expression by 72%. Conversely, inhibition of LPA signaling increased ATX expression 3-fold, indicating strong feedback regulation by LPA signaling. In fibroblasts, we could verify that inhibition of ATX activity by bile salts induces its expression. Furthermore, induction of cholestasis in mice causes increased plasma ATX activity.

CONCLUSIONS

Multiple biliary compounds that accumulate in the systemic circulation during cholestasis inhibit ATX activity and thereby increase ATX expression through feedback regulation. This mechanism may contribute to increased serum ATX activity in patients with cholestasis.

Insight into vitronectin structural evolution on material surface chemistries: The mediation for cell adhesion

Biomaterial surface chemistry engenders profound consequences on cell adhesion and the ultimate tissue response by adsorbing proteins from extracellular matrix, where vitronectin (Vn) is involved as one of the crucial mediator proteins. Deciphering the adsorption behaviors of Vn in molecular scale provides a useful account of how to design biomaterial surfaces. But the details of structural dynamics and consequential biological effect remain elusive. Herein, both experimental and computational approaches were applied to delineate the conformational and orientational evolution of Vn during adsorption onto self-assembled monolayers (SAMs) terminating with -COOH, -NH₂, -CH₃ and -OH. To unravel the interplay between cell binding and the charge and wettability of material surface, somatomedin-B (SMB) domain of Vn holding the RGD cell-binding motif was employed in molecular dynamics (MD) simulations, with orientation initialized by Monte Carlo (MC) method. Experimental evidences including protein adsorption, cell adhesion and integrin gene expressions were thoroughly investigated. The adsorption of Vn on different surface chemistries showed very complex profiles. Cell adhesion was enabled on all Vn-adsorbed surfaces but with distinct mechanisms mostly determined by conformational change induced reorientation. Higher amount of Vn was observed on negatively charged surface (COOH) and hydrophobic surface (CH₃). However, advantageous orientations defined by RGD loop conditions were only obtained on the charged surfaces (COOH and NH₂). Specifically, COOH surface straightened up the Vn molecules and accumulated them into a higher density, whereas CH₃ surface squashed Vn and stacked them into higher density multilayer by tracking adsorption but with the RGD loops restrained. These findings may have a broad implication on the understanding of Vn functionality and would help develop new strategies for designing advanced biomaterials.

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Unfolded straighten structure of Vn unbinds RGD loop on negatively charged surface.

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Unfolded Vn forms compact multilayers by tracking adsorption on hydrophobic surface.

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Accessible RGD loop of adsorbed Vn endows better cell affinity for charged surface.

Complement C9 binding site and the anti-microbial activity of caprine vitronectin are localized in close proximity in the N-terminal region of the protein

Vitronectin (Vn) is a ligand for complement C9 and modulates its activity that favors bacterial growth and survival. At the same time, the anti-microbial activity of the heparin-binding region of human Vn has been documented. To understand these diverse and opposite functions of the protein, we have analyzed the interaction of caprine Vn with C9 in the homologous system. In a previous study, the C9 binding activity was mapped to the N-fragment of the caprine Vn (N-Vn), representing the first 200 amino acids. Interestingly, this fragment also inhibited bacterial growth. In this study, we have generated four sub-fragments of N-Vn and analyzed C9 binding by ELISA, blot overlay, surface plasmon resonance and circular dichroism spectroscopy. These sub-fragments were also tested for antimicrobial activity against E. coli and S. aureus by drop plate method and analyzing cell death by flow cytometry. Results of these analyses together with previous data suggest that in addition to the second RGD motif (106-108 amino acids), the first 47 residues are also required for C9 binding. The anti-microbial tests employed indicate that the growth inhibitory property is contributed by 101-150 residues of Vn. These results provide an initial insight into two diverse Vn functions.

Characterization of an Extensive Interface on Vitronectin for Binding to Plasminogen Activator Inhibitor-1: Adoption of Structure in an Intrinsically Disordered Region

Small-angle neutron scattering (SANS) measurements were pursued to study human vitronectin, a protein found in tissues and the circulation that regulates cell adhesion/migration and proteolytic cascades that govern hemostasis and pericellular proteolysis. Many of these functions occur via interactions with its binding partner, plasminogen activator inhibitor-1 (PAI-1), the chief inhibitor of proteases that lyse and activate plasminogen. We focused on a region of vitronectin that remains uncharacterized from previous X-ray scattering, nuclear magnetic resonance, and computational modeling approaches and which we propose is involved in binding to PAI-1. This region, which bridges the N-terminal somatomedin B (SMB) domain with a large central β-propeller domain of vitronectin, appears unstructured and has characteristics of an intrinsically disordered domain (IDD). The effect of osmolytes was evaluated using circular dichroism and SANS to explore the potential of the IDD to undergo a disorder-to-order transition. The results suggest that the IDD favors a more ordered structure under osmotic pressure; SANS shows a smaller radius of gyration (R_g) and a more compact fold of the IDD upon addition of osmolytes. To test whether PAI-1 binding is also coupled to folding within the IDD structure, a set of SANS experiments with contrast variation were performed on the complex of PAI-1 with a vitronectin fragment corresponding to the N-terminal 130 amino acids (denoted the SMB-IDD because it contains the SMB domain and IDD in linear sequence). Analysis of the SANS data using the Ensemble Optimization Method confirms that the SMB-IDD adopts a more compact configuration when bound to PAI-1. Calculated structures for the PAI-1:SMB-IDD complex suggest that the IDD provides an interaction surface outside of the primary PAI-1-binding site located within the SMB domain; this binding is proposed to lead to the assembly of higher-order structures of vitronectin and PAI-1 commonly found in tissues.

Identification of a PAI-1-binding site within an intrinsically disordered region of vitronectin

The serine protease inhibitor, plasminogen activator inhibitor Type-1 (PAI-1) is a metastable protein that undergoes an unusual transition to an inactive conformation with a short half-life of only 1-2 hr. Circulating PAI-1 is bound to a cofactor vitronectin, which stabilizes PAI-1 by slowing this latency conversion. A well-characterized PAI-1-binding site on vitronectin is located within the somatomedin B (SMB) domain, corresponding to the first 44 residues of the protein. Another PAI-1 recognition site has been identified with an engineered form of vitronectin lacking the SMB domain, yet retaining PAI-1 binding capacity (Schar, Blouse, Minor, Peterson. J Biol Chem. 2008;283:28487-28496). This additional binding site is hypothesized to lie within an intrinsically disordered domain (IDD) of vitronectin. To localize the putative binding site, we constructed a truncated form of vitronectin containing 71 amino acids from the N-terminus, including the SMB domain and an additional 24 amino acids from the IDD region. This portion of the IDD is rich in acidic amino acids, which are hypothesized to be complementary to several basic residues identified within an extensive vitronectin-binding site mapped on PAI-1 (Schar, Jensen, Christensen, Blouse, Andreasen, Peterson. J Biol Chem. 2008;283:10297-10309). Steady-state and stopped-flow fluorescence measurements demonstrate that the truncated form of vitronectin exhibits the same rapid biphasic association as full-length vitronectin and that the IDD hosts the elusive second PAI-1 binding site that lies external to the SMB domain of vitronectin.

The involvement of autotaxin in renal interstitial fibrosis through regulation of fibroblast functions and induction of vascular leakage

The accumulation of fibroblasts is a critical step in the development of fibrosis, and lysophosphatidic acid (LPA) promotes fibrosis by regulating multiple fibroblast functions. Autotaxin (ATX) is a key LPA-producing enzyme, and we hypothesized that ATX contributes to the development of renal interstitial fibrosis through LPA-mediated effects on fibroblast functions. In a mouse model of renal interstitial fibrosis induced by unilateral ureteral obstruction (UUO), the levels of renal ATX protein and activity increased with the progression of fibrosis in ligated kidneys, despite concurrent reductions in renal ATX mRNA. UUO enhanced vascular permeability in the renal interstitium, and ATX protein localized to areas of vascular leak, suggesting that vascular leak allowed ATX to enter the renal interstitium. In vitro studies showed that ATX induces the migration and proliferation of renal fibroblasts and enhances the vascular permeability of endothelial monolayers. Finally, pharmacological inhibition of ATX partially attenuated renal interstitial fibrosis. These results suggest that during the development of renal fibrosis, ATX accumulates in the renal interstitium and drives fibroblast accumulation and promotes renal interstitial vascular leak, thereby partially contributing to the pathogenesis of renal interstitial fibrosis. Taken together, ATX inhibition may have the potential to be a novel therapeutic strategy to combat renal interstitial fibrosis.

Mass Spectrometry- and Computational Structural Biology-Based Investigation of Proteins and Peptides

Recent developments of mass spectrometry (MS) allow us to identify, estimate, and characterize proteins and protein complexes. At the same time, structural biology helps to determine the protein structure and its structure-function relationship. Together, they aid to understand the protein structure, property, function, protein-complex assembly, protein-protein interaction, and dynamics. The present chapter is organized with illustrative results to demonstrate how experimental mass spectrometry can be combined with computational structural biology for detailed studies of protein's structures. We have used tumor differentiation factor protein/peptide as ligand and Hsp70/Hsp90 as receptor protein as examples to study ligand-protein interaction. To investigate possible protein conformation, we will describe two proteins-lysozyme and myoglobin. As an application of MS-based assignment of disulfide bridges, the case of the spider venom polypeptide Phα1β will also be discussed.

Structural Characterization and Disulfide Assignment of Spider Peptide Phα1β by Mass Spectrometry

Native Phα1β is a peptide purified from the venom of the armed spider Phoneutria nigriventer that has been shown to have an extensive analgesic effect with fewer side effects than ω-conotoxin MVIIA. Recombinant Phα1β mimics the effects of the native Phα1β. Because of this, it has been suggested that Phα1β may have potential to be used as a therapeutic for controlling persistent pathological pain. The amino acid sequence of Phα1β is known; however, the exact structure and disulfide arrangement has yet to be determined. Determination of the disulfide linkages and exact structure could greatly assist in pharmacological analysis and determination of why this peptide is such an effective analgesic. Here, we used biochemical and mass spectrometry approaches to determine the disulfide linkages present in the recombinant Phα1β peptide. Using a combination of MALDI-MS, direct infusion ESI-MS, and nanoLC-MS/MS analysis of the undigested recombinant Phα1β peptide and digested with AspN, trypsin, or AspN/trypsin, we were able to identify and confirm all six disulfide linkages present in the peptide as Cys1-2, Cys3-4, Cys5-6, Cys7-8, Cys9-10, and Cys11-12. These results were also partially confirmed in the native Phα1β peptide. These experiments provide essential structural information about Phα1β and may assist in providing insight into the peptide's analgesic effect with very low side effects. Graphical Abstract ᅟ.

Evidence that vitronectin is a potent migration-enhancing factor for cancer cells chaperoned by fibrinogen: a novel view of the metastasis of cancer cells to low-fibrinogen lymphatics and body cavities

Diluted (1%) plasma induces migration of malignant cell lines much more strongly than potent pro-metastatic factors. To characterize the factor(s) present in diluted plasma responsible for this phenomenon we performed i) heat inactivation, ii) dialysis, iii) proteinase K treatment, and iv) molecular size filtration studies. We found that this remarkable pro-migratory activity of diluted normal plasma is associated with a ~50–100-kD protein that interacts with G_αI protein-coupled receptors and activates p42/44 MAPK and AKT signaling in target cells. Since this pro-migratory activity of 1% plasma decreases at higher plasma concentrations (> 20%), but is retained in serum, we hypothesized that fibrinogen may be involved as a chaperone of the protein(s). To identify the pro-migratory protein(s) present in diluted plasma and fibrinogen-depleted serum, we performed gel filtration and hydrophobic interaction chromatography followed by mass spectrometry analysis. We identified several putative protein candidates that were further tested in in vitro experiments. We found that this pro-migratory factor chaperoned by fibrinogen is vitronectin, which activates uPAR, and that this effect can be inhibited by fibrinogen. These results provide a novel mechanism for the metastasis of cancer cells to lymphatics and body cavities, in which the concentration of fibrinogen is low, and thus suggests that free vitronectin stimulates migration of tumor cells.

Alpha-enolase (ENO1) controls alpha v/beta 3 integrin expression and regulates pancreatic cancer adhesion, invasion, and metastasis

Background

We have previously shown that in pancreatic ductal adenocarcinoma (PDA) cells, the glycolytic enzyme alpha-enolase (ENO1) also acts as a plasminogen receptor and promotes invasion and metastasis formation. Moreover, ENO1 silencing in PDA cells induces oxidative stress, senescence and profoundly modifies PDA cell metabolism. Although anti-ENO1 antibody inhibits PDA cell migration and invasion, little is known about the role of ENO1 in regulating cell-cell and cell-matrix contacts. We therefore investigated the effect of ENO1 silencing on the modulation of cell morphology, adhesion to matrix substrates, cell invasiveness, and metastatic ability.

Methods

The membrane and cytoskeleton modifications that occurred in ENO1-silenced (shENO1) PDA cells were investigated by a combination of confocal microscopy and atomic force microscopy (AFM). The effect of ENO1 silencing was then evaluated by phenotypic and functional experiments to identify the role of ENO1 in adhesion, migration, and invasion, as well as in senescence and apoptosis. The experimental results were then validated in a mouse model.

Results

We observed a significant increase in the roughness of the cell membrane due to ENO1 silencing, a feature associated with an impaired ability to migrate and invade, along with a significant downregulation of proteins involved in cell-cell and cell-matrix adhesion, including alpha v/beta 3 integrin in shENO1 PDA cells. These changes impaired the ability of shENO1 cells to adhere to Collagen I and IV and Fibronectin and caused an increase in RGD-independent adhesion to vitronectin (VN) via urokinase plasminogen activator receptor (uPAR). Binding of uPAR to VN triggers integrin-mediated signals, which result in ERK1-2 and RAC activation, accumulation of ROS, and senescence. In shENO1 cancer cells, the use of an anti-uPAR antibody caused significant reduction of ROS production and senescence. Overall, a decrease of in vitro and in vivo cell migration and invasion of shENO1 PDA cells was observed.

Conclusion

These data demonstrate that ENO1 promotes PDA survival, migration, and metastasis through cooperation with integrins and uPAR.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-016-0385-8) contains supplementary material, which is available to authorized users.

Vitronectin in the ascites of human ovarian carcinoma acts as a potent chemoattractant for ovarian carcinoma: Implication for metastasis by cancer stem cells

Vitronectin has been identified mainly as an adhesion protein that signals through uPAR and selected integrin receptors. In addition to its pro-adhesive properties, we identified recently vitronectin as a main chemoattractant present in diluted plasma/serum that directly stimulates migration of cancer cells. We also found that this pro-migratory activity of vitronectin can be quenched by fibrinogen. Based on this we hypothesized that this may explain preference of cancer cell to metastasize to fibrinogen-low microenvironments such as lymphatics or peritoneal cavity. Based on this, we decided to investigate a role of vitronectin in metastasis of ovarian cancer cells to peritoneal cavity. We tested migratory responsiveness of three human ovarian cancer cell lines to ascites isolated from ovarian cancer patients and characterize possible molecules involved in migration of ovarian cancer cells. The ascites samples were exposed to heat inactivation, proteinase K digested, dialyzed and charcoal stripped. We also performed cut-off filtration analysis and by employing ELISA assays to measure concentration of vitronectin in ascites fluid samples. Finally, we employed shRNA against uPAR and small molecular inhibitors of integrin receptors to assess their involvement in biological effects of vitronectin. From our studies, we found that the similarly to diluted plasma, vitronectin in absence of fibrinogen is a main chemotactic/chemokinetic protein present in ascites fluid. We also found that these pro-migratory properties of vitronectin can be quenched by addition of fibrinogen. Our studies also indicate that both uPAR and integrin receptors on ovarian cancer cells regulate migration of these cells to vitronectin gradient. In summary, we identified free soluble vitronectin as a potent direct chemoattractant for ovarian cancer cells and that its activity is suppressed after binding to fibrinogen. Since in ascites fluids vitronectin is present in free form because of a lack or low level of fibrinogen, this could explain preferences of ovarian cancer stem cells to metastasize within peritoneum. We propose that inhibitors which could sequester soluble vitronectin in similar fashion as fibrinogen, could be employed as a novel anti-metastatic drugs.

Early effects of paroxysmal atrial fibrillation on plasma markers of fibrinolysis

There are no studies to date on the early changes in the hemostasis profile of patients with paroxysmal atrial fibrillation (PAF).Given the key role of the fibrinolytic system in maintaining blood fluidity, our aim was to examine its activity in patients with clinical manifestation of the disease <24 hours.We studied 51 nonanticoagulated patients with a first episode of the disease (26 men, 25 women; mean age 59.84 ± 1.60 years) and 52 controls (26 men, 26 women; mean age 59.50 ± 1.46 years) who matched the patients in terms of gender, age, comorbidities, and conducted treatment. Using enzyme-linked immunoassays and colorimetric assays we assessed the plasminogen activity, tissue plasminogen activator level (t-PA), plasminogen activator inhibitor 1 activity (PAI-1), α2-antiplasmin activity (α2-AP), D-dimer level, and vitronectin level. Blood samples were collected immediately after hospitalization.Patients were hospitalized between the second and twenty fourth hours (mean 8.14 ± 0.76 hours) after the onset of PAF. Compared to controls, plasminogen (159.40 ± 4.81 vs 100.2 ± 2.88%, P < 0.001) and t-PA levels (11.25 ± 0.35 vs 6.05 ± 0.31 ng/mL, P < 0.001) were significantly higher in the patient group. PAI-1 activity (7.33 ± 0.37 vs 15.15 ± 0.52 AU/mL, P < 0.001) and α2-AP (112.9 ± 2.80 vs 125.60 ± 3.74%, P < 0.05) as well as vitronectin plasma levels (134.7 ± 5.83 vs 287.3 ± 10.44 mcg/mL, P < 0.001) were lower in the PAF group. Conversely, the levels of D-dimer in patients were significantly higher (0.53 ± 0.07 vs 0.33 ± 0.02 ng/mL, P < 0.05).Early changes in the fibrinolytic system occur in PAF, suggesting their close relationship with the manifestation of the disease. There is high plasma fibrinolytic activity, during the very first 24 hours of the disease, which is most likely a pathophysiological response to the intensified procoagulation process.

Autotaxin, a lysophospholipase D with pleomorphic effects in oncogenesis and cancer progression

The ectonucleotide pyrophosphatase/phosphodiesterase type 2, more commonly known as autotaxin (ATX), is an ecto-lysophospholipase D encoded by the human ENNP2 gene. ATX is expressed in multiple tissues and participates in numerous key physiologic and pathologic processes, including neural development, obesity, inflammation, and oncogenesis, through the generation of the bioactive lipid, lysophosphatidic acid. Overwhelming evidence indicates that altered ATX activity leads to oncogenesis and cancer progression through the modulation of multiple hallmarks of cancer pathobiology. Here, we review the structural and catalytic characteristics of the ectoenzyme, how its expression and maturation processes are regulated, and how the systemic integration of its pleomorphic effects on cells and tissues may contribute to cancer initiation, progression, and therapy. Additionally, the up-to-date spectrum of the most frequent ATX genomic alterations from The Cancer Genome Atlas project is reported for a subset of cancers.

Structural basis of substrate discrimination and integrin binding by autotaxin

Autotaxin (ATX) or ecto-nucleotide pyrophosphatase/phosphodiesterase-2 (ENPP2) is a secreted lysophospholipase D that generates the lipid mediator lysophosphatidic acid (LPA), a mitogen and chemo-attractant for many cell types. ATX-LPA signaling has roles in various pathologies including tumour progression and inflammation. However, the molecular basis of substrate recognition and catalysis, and the mechanism of interaction with target cells, has been elusive. Here we present the crystal structure of ATX, alone and in complex with a small-molecule inhibitor. We identify a hydrophobic lipid-binding pocket and map key residues required for catalysis and selection between nucleotide and phospholipid substrates. We show that ATX interacts with cell-surface integrins via its N-terminal somatomedin-B-like domains, using an atypical mechanism. Our results define determinants of substrate discrimination by the ENPP family, suggest how ATX promotes localized LPA signaling, and enable new approaches to target ATX with small-molecule therapeutics.

PAI-1 inhibits neutrophil efferocytosis

Phagocytosis of apoptotic cells, also called efferocytosis, is an essential feature of immune responses and critical for the resolution of inflammation. Plasma and tissue levels of plasminogen activator inhibitor-1 (PAI-1), an inhibitor of fibrinolysis, are elevated in inflammatory conditions, including sepsis and acute lung injury, in which activated neutrophils accumulate in tissues and contribute to organ dysfunction. In this study, we explored the potential involvement of PAI-1 in modulating neutrophil efferocytosis. We found enhanced phagocytosis of viable PAI-1 deficient (PAI-1(-/-)) and of wild-type neutrophils treated with anti-PAI-1 antibodies. PAI-1 levels were decreased on the surface of apoptotic neutrophils and the enhanced phagocytosis of apoptotic wild-type neutrophils or of viable PAI-1(-/-) neutrophils was diminished by preincubation with PAI-1. The increased phagocytosis associated with PAI-1 deficiency or blockade depended on both the lipoprotein receptor-related protein (LRP) and its ligand, calreticulin (CRT), because the LRP-mediated increase in phagocytosis of viable neutrophils induced by blockade of CD 47 was abrogated by PAI-1. CRT levels are increased on viable PAI-1(-/-) neutrophils. While CRT colocalizes with PAI-1 on viable neutrophils, markedly diminished colocalization of PAI-1 and CRT was present on apoptotic neutrophils. Our data therefore indicate that PAI-1 serves as a novel "don't eat me" signal for viable and apoptotic neutrophils.

Characterization of a site on PAI-1 that binds to vitronectin outside of the somatomedin B domain

Vitronectin and plasminogen activator inhibitor-1 (PAI-1) are proteins that interact in the circulatory system and pericellular region to regulate fibrinolysis, cell adhesion, and migration. The interactions between the two proteins have been attributed primarily to binding of the somatomedin B (SMB) domain, which comprises the N-terminal 44 residues of vitronectin, to the flexible joint region of PAI-1, including residues Arg-103, Met-112, and Gln-125 of PAI-1. A strategy for deletion mutagenesis that removes the SMB domain demonstrates that this mutant form of vitronectin retains PAI-1 binding (Schar, C. R., Blouse, G. E., Minor, K. M., and Peterson, C. B. (2008) J. Biol. Chem. 283, 10297-10309). In the current study, the complementary binding site on PAI-1 was mapped by testing for the ability of a battery of PAI-1 mutants to bind to the engineered vitronectin lacking the SMB domain. This approach identified a second, separate site for interaction between vitronectin and PAI-1. The binding of PAI-1 to this site was defined by a set of mutations in PAI-1 distinct from the mutations that disrupt binding to the SMB domain. Using the mutations in PAI-1 to map the second site suggested interactions between alpha-helices D and E in PAI-1 and a site in vitronectin outside of the SMB domain. The affinity of this second interaction exhibited a K(D) value approximately 100-fold higher than that of the PAI-1-somatomedin B interaction. In contrast to the PAI-1-somatomedin B binding, the second interaction had almost the same affinity for active and latent PAI-1. We hypothesize that, together, the two sites form an extended binding area that may promote assembly of higher order vitronectin-PAI-1 complexes.

A deletion mutant of vitronectin lacking the somatomedin B domain exhibits residual plasminogen activator inhibitor-1-binding activity

Vitronectin and plasminogen activator inhibitor-1 (PAI-1) are important physiological binding partners that work in concert to regulate cellular adhesion, migration, and fibrinolysis. The high affinity binding site for PAI-1 is located within the N-terminal somatomedin B domain of vitronectin; however, several studies have suggested a second PAI-1-binding site within vitronectin. To investigate this secondary site, a vitronectin mutant lacking the somatomedin B domain (rDeltasBVN) was engineered. The short deletion had no effect on heparin-binding, integrin-binding, or cellular adhesion. Binding to the urokinase receptor was completely abolished while PAI-1 binding was still observed, albeit with a lower affinity. Analytical ultracentrifugation on the PAI-1-vitronectin complex demonstrated that increasing NaCl concentration favors 1:1 versus 2:1 PAI-1-vitronectin complexes and hampers formation of higher order complexes, pointing to the contribution of charge-charge interactions for PAI-1 binding to the second site. Furthermore, fluorescence resonance energy transfer between differentially labeled PAI-1 molecules confirmed that two independent molecules of PAI-1 are capable of binding to vitronectin. These results support a model for the assembly of higher order PAI-1-vitronectin complexes via two distinct binding sites in both proteins.