Protein movement during complex-formation between tissue plasminogen activator and plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) rapidly inactivates tissue plasmi nogen activator (tPA). After initial binding and cleavage of the reactive-c entre loop of PAI-1, this complex is believed to undergo a major rearrangem ent. Using surface plasmon resonance and SDS-PAGE, we have studied the infl uence of a panel of monoclonal antibodies on the reaction leading to the fi nal covalent complex. On the basis of these data, we suggest the mechanisms for the action of different classes of inhibitory antibodies. We propose t hat the antibodies which convert PAI-1 into a substrate for tPA do this by means of preventing the conversion of the initial PAI-1/tPA complex into th e final complex by sterical intervention. Moreover, the localisation of the binding epitopes on free PAI-1, as well as on the PAI-1/tPA complex, sugge sts that tPA in the final complex cannot be located near helices E and F, a s has previously been proposed. (C) 1999 Elsevier Science B.V. All rights r eserved.