CHARACTERIZATION OF THE BINDING OF DIFFERENT CONFORMATIONAL FORMS OF PLASMINOGEN-ACTIVATOR INHIBITOR-1 TO VITRONECTIN - IMPLICATIONS FOR THE REGULATION OF PERICELLULAR PROTEOLYSIS

Plasminogen activator inhibitor type 1 (PAI-1), the primary physiologi c inhibitor of plasminogen activation, is associated with the adhesive glycoprotein vitronectin (Vn) in plasma and the extracellular matrix, In this study we examined the binding of different conformational for ms of PAI-1 to both native and urea-purified vitronectin using a solid -phase binding assay, These results demonstrate that active PAI-1 bind s to urea-purified Vn with approximately 6-fold higher affinity than t o native Vn, In contrast, inactive forms of PAI-1 (latent, elastase-cl eaved, synthetic reactive center loop peptide-annealed, or complexed t o plasminogen activators) display greatly reduced affinities for both forms of adsorbed Vn, with relative affinities reduced by more than 2 orders of magnitude, Structurally, these inactive conformations all di ffer from active PAI-1 by insertion of an additional strand into beta sheet A, suggesting that it is the rearrangement of sheet A that resul ts in reduced Vn affinity, This is supported by the observation that P AI-1 associated with beta-anhydrotrypsin, which does not undergo rearr angement of beta-sheet A, shows no such decrease in affinity, whereas PAI-1 complexed to beta-trypsin, which does undergo sheet A rearrangem ent, displays reduced affinity for Vn similar to PAI-1 plasminogen act ivator complexes, Together these data demonstrate that the interaction between PAI-1 and Vn depends on the conformational state of both prot eins and suggest that the Vn binding site on PAI-1 is sensitive to str uctural changes associated with loss of inhibitory activity.