Studies on the interaction between plasminogen activator inhibitor-1 and vitronectin

Functional structure of the somatomedin B domain of vitronectin

The N-terminal somatomedin B domain (SMB) of vitronectin binds PAI-1 and the urokinase receptor with high affinity and regulates tumor cell adhesion and migration. We have shown previously in the crystal structure of the PAI-1/SMB complex that SMB, a peptide of 51 residues, is folded as a compact cysteine knot of four pairs of crossed disulfide bonds. However, the physiological significance of this structure was questioned by other groups, who disputed the disulfide bonding shown in the crystal structure (Cys5-Cys21, Cys9-Cys39, Cys19-Cys32, Cys25-Cys31), notably claiming that the first disulfide is Cys5-Cys9 rather than the Cys5-Cys21 bonding shown in the structure. To test if the claimed Cys5-Cys9 bond does exist in the SMB domain of plasma vitronectin, we purified mouse and rat plasma vitronectin that have a Met (hence cleavable by cyanogen bromide) at residue 14, and also prepared recombinant human SMB variants from insect cells with residues Asn14 or Leu24 mutated to Met. HPLC and mass spectrometry analysis showed that, after cyanogen bromide digestion, all the fragments of the SMB derived from mouse or rat vitronectin or the recombinant SMB mutants are still linked together by disulfides, and the N-terminal peptide (residue 1-14 or 1-24) can only be released when the disulfide bonds are broken. This clearly demonstrates that Cys5 and Cys9 of SMB do not form a disulfide bond in vivo, and together with other structural evidence confirms that the only functional structure of the SMB domain of plasma vitronectin is that seen in its crystallographic complex with PAI-1.

How vitronectin binds PAI-1 to modulate fibrinolysis and cell migration

The interaction of the plasma protein vitronectin with plasminogen activator inhibitor-1 (PAI-1) is central to human health. Vitronectin binding extends the lifetime of active PAI-1, which controls hemostasis by inhibiting fibrinolysis and has also been implicated in angiogenesis. The PAI-1-vitronectin binding interaction also affects cell adhesion and motility. For these reasons, elevated PAI-1 activities are associated both with coronary thrombosis and with a poor prognosis in many cancers. Here we show the crystal structure at a resolution of 2.3 A of the complex of the somatomedin B domain of vitronectin with PAI-1. The structure of the complex explains how vitronectin binds to and stabilizes the active conformation of PAI-1. It also explains the tissue effects of PAI-1, as PAI-1 competes for and sterically blocks the interaction of vitronectin with cell surface receptors and integrins. Structural understanding of the essential biological roles of the interaction between PAI-1 and vitronectin opens the prospect of specifically designed blocking agents for the prevention of thrombosis and treatment of cancer.

Kinetic analysis of the interaction between vitronectin and the urokinase receptor

Although the urokinase receptor (uPAR) binds to vitronectin (VN) and promotes the adhesion of cells to this matrix protein, the biochemical details of this interaction remain unclear. VN variants were employed in BIAcore experiments to examine the uPAR-VN interaction in detail and to compare it to the interaction of VN with other ligands. Heparin and plasminogen bound to VN fragments containing the heparin-binding domain, indicating that this domain was functionally active in the recombinant peptides. However, no significant binding was detected when uPAR was incubated with this domain, and neither heparin nor plasminogen competed with it for binding to VN. In fact, uPAR only bound to fragments containing the somatomedin B (SMB) domain, and monoclonal antibodies (mAbs) that bind to this domain competed with uPAR for binding to VN. Monoclonal antibody 8E6 also inhibited uPAR binding to VN, and this mAb was shown to recognize sulfated tyrosine residues 56 and 59 in the region adjacent to the SMB domain. Destruction of this site by acid treatment eliminated mAb 8E6 binding but had no effect on uPAR binding. Thus, there appears to be a single binding site for uPAR in VN, and it is located in the SMB domain and is distinct from the epitope recognized by mAb 8E6. Inhibition of uPAR binding to VN by mAb 8E6 probably results from steric hindrance.

A method for defining binding sites involved in protein-protein interactions: analysis of the binding of plasminogen activator inhibitor 1 to the somatomedin domain of vitronectin

Plasminogen activator inhibitor type-1 (PAI-1) is bound to vitronectin (VN) in plasma and in the extracellular matrix. We previously employed a domain-swapping approach to show that the high-affinity binding site for PAI-1 in VN is contained within residues 12-30 in the amino-terminal somatomedin B (SMB) domain. In this study, we attempt to further delineate the location of this site by employing a novel approach that is based on the use of monoclonal antibodies (Mabs) together with site-directed mutagenesis. Six separate Mabs were identified that bound to the SMB domain and competed with PAI-1 for binding to VN. The relative affinity of each of the Mabs, and of PAI-1 itself, for binding to individual variants of SMB (prepared by alanine scanning mutagenesis), was then determined and compared in competitive binding experiments. Three separate, partially overlapping Mab epitopes within SMB were defined by these studies, and the PAI-1 binding site was localized to the region between residues 24 and 37. When considered together with the domain swapping data, these studies suggest that the PAI-1 binding site is contained within a common seven-residue region (i.e., residues 24-30) in the SMB domain.

Regulation of cell adhesion by PAI-1

Type I plasminogen activator inhibitor (PAI-1) is the primary inhibitor of tissue- and urokinase-type plasminogen activators. It circulates in plasma complexed with vitronectin (VN), the primary PAI-1 binding protein. The somatomedin B (SMB) domain of VN contains both the high affinity PAI-1 binding site and the specific site for urokinase plasminogen activator receptor (uPAR). PAI-1 is able to regulate uPAR-mediated cell adhesion by competing with uPAR for VN binding. Binding of PAI-1 to SMD may also affect integrin-mediated cell adhesion to VN by hindering integrin binding to the RGD sequence adjacent to the uPAR binding site.

Stability of plasminogen activator inhibitor-1: role of tyrosine221

Using site-directed mutagenesis, changes of Tyr221 in plasminogen activator inhibitor-1 (PAI-1) have provided mutants with normal activity, but with increased stability. At physiological conditions, the transition of the PAI-1 mutants Tyr221His and Tyr221Ser to the latent form was significantly prolonged (half-lives 14.8 and 4.1 h, respectively) as compared to wild-type PAI-1 (2.0 h). Their half-lives, especially for the Tyr221Ser mutant, were even more prolonged in the presence of vitronectin (23.8 and 53.7 h, respectively). While wild-type PAI-1 was more stable at lower pH, the PAI-1 mutants Tyr221His and Tyr221Ser had stability optima at about pH 6.5, but displayed shorter half-lives at pH 5.5.

The use of fluorescent probes to characterize conformational changes in the interaction between vitronectin and plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1), the primary inhibitor of tissue-type plasminogen activator and urokinase, is known to convert readily to a latent form by insertion of the reactive center loop into a central beta-sheet. Interaction with vitronectin stabilizes PAI-1 and decreases the rate of conversion to the latent form, but conformational effects of vitronectin on the reactive center loop of PAI-1 have not been documented. Mutant forms of PAI-1 were designed with a cysteine substitution at either position P1' or P9 of the reactive center loop. Labeling of the unique cysteine with a sulfhydryl-reactive fluorophore provides a probe that is sensitive to vitronectin binding. Results indicate that the scissile P1-P1' bond of PAI-1 is more solvent exposed upon interaction with vitronectin, whereas the N-terminal portion of the reactive loop does not experience a significant change in its environment. These results were complemented by labeling vitronectin with an arginine-specific coumarin probe which compromises heparin binding but does not interfere with PAI-1 binding to the protein. Dissociation constants of approximately 100 nM are calculated for the vitronectin/PAI-1 interaction from titrations using both fluorescent probes. Furthermore, experiments in which PAI-1 failed to compete with heparin for binding to vitronectin argue for separate binding sites for the two ligands on vitronectin.

Identification of a PAI-1 binding site in vitronectin

Active PAI-1 (plasminogen activator inhibitor 1) is bound to vitronectin in plasma and in the extracellular matrix. In this study we aimed at identifying the PAI-1 binding site in vitronectin, which at present is a matter of dispute. Vitronectin was cleaved with trypsin and the fragments were tested for inhibitory effect on the PAI-1/vitronectin interaction using vitronectin-coated microtiter plates. Intact vitronectin and the tryptic digest of vitronectin both caused a 50% reduction in PAI-1 binding at a concentration of about 2 nmol/I. Gel-filtration on Sephadex G-50 superfine of the tryptic peptides resulted in one main peak of inhibitory activity. The elution volume, Kav, was 0.55 indicating (a) medium-size peptide(s). The peptide was further purified by reverse-phase HPLC. Structural analysis revealed that it constituted the 45 NH2-terminal amino-acid residues in vitronectin. The NH2-terminal vitronectin peptide caused a 50% decrease in PAI-1 binding to the vitronectin-coated microtiter plates at a concentration of about 13 nmol/l. Thus, the peptide is a little less effective in this respect than intact vitronectin. Reduced and S-carboxymethylated peptide had no effect on the interaction. The NH2-terminal vitronectin fragment increased the stability of active PAI-1 by about 60%, which is a little less than with intact vitronectin. The peptide also prevented PAI-1 from oxidation with chloramine T. The half-life was prolonged about 4-fold as compared to about 30-fold with intact vitronectin.