Abstract
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.
Highlights
The adhesive glycoprotein, vitronectin, circulates in human plasma at concentrations of 200 – 400 g1⁄7mlϪ1 and serves as a regulatory protein in humoral defense mechanisms by interacting with macromolecules in the reaction cascades of coagulation and fibrinolysis
More recent work utilizing heterologous expression systems has focused on the somatomedin B domain of vitronectin, which contains eight cysteines thought to form a “disulfide knot” at the N terminus of substituted for serine 338; M347C, recombinant plasminogen activator inhibitor-1 (PAI-1) with cysteine substituted for methionine 347; NBDP1ЈPAI-1, M347C mutant form of PAI-1 labeled with NBD; NBDP9PAI-1, S338C mutant form of PAI-1 labeled with NBD; hydroxycoumarin glyoxal (HOCGO), 7-hydroxycoumarinyl-3-glyoxal; HOCGOVN, vitronectin labeled with HOCGO; PEG, polyethylene glycol; BSA, bovine serum albumin; HRP, horseradish peroxidase
When NBDP1ЈPAI-1 was titrated with vitronectin, a dissociation constant of 100 Ϯ 50 nM was obtained (Fig. 2B). These data indicate that vitronectin induces a conformational change in the reactive center loop of PAI-1 that shifts the probe at the scissile bond into a somewhat more hydrophilic milieu
Summary
The adhesive glycoprotein, vitronectin, circulates in human plasma at concentrations of 200 – 400 g1⁄7mlϪ1 and serves as a regulatory protein in humoral defense mechanisms by interacting with macromolecules in the reaction cascades of coagulation and fibrinolysis (reviewed in Refs. 1–3). Based on the observation that vitronectin alters PAI-1 protease specificity, it can be hypothesized that vitronectin binding causes conformation changes in the reactive center loop as well. In order to localize sequences critical for PAI-1 binding, Deng et al [34] generated chimeras between segments of the vitronectin somatomedin B domain and complementary sequences in other inactive somatomedin B homology domains. These studies indicated that the essential PAI-1 binding determinant was located between residues 12 and 30 of vitronectin, and alanine scanning mutagenesis revealed that all 8 cysteines and Gly-12, Asp-22, Leu-24, Tyr-27, Tyr-28, and Asp-34 are essential to maintain PAI-1-binding activity [34]. A parallel experimental approach was taken in which an arginine-reactive coumarin derivative [38] was used for site-specific labeling of the arginine-rich heparin-binding region of vitronectin
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
