The Molecular Basis for Anti-Proteolytic and Non-Proteolytic Functions of Plasminogen Activator Inhibitor Type-1: Roles of the Reactive Centre Loop, the Shutter Region, the Flexible Joint Region and the Small Serpin Fragment

Abstract

The serine proteinase inhibitor plasminogen activator inhibitor type-1 (PAI-1) is the primary physiological inhibitor of the tissuetype and the urokinasetype plasminogen activator (tPA and uPA, respectively) and as such an important regulator of proteolytic events taking place in the circulation and in the extracellular matrix. Moreover, a few nonproteolytic functions have been ascribed to PAI-1, mediated by its interaction with vitronectin or the interaction between the uPAPAI-1 complex bound to the uPA receptor and members of the low density lipoprotein receptor family. PAI-1 belongs to the serpin family, characterised by an unusual conformational flexibility, which governs its molecular interactions. In this review we describe the antiproteolytic and nonproteolytic functions of PAI-1 from both a biological and a biochemical point of view. We will relate the various biological roles of PAI-1 to its biochemistry in general and to the different conformations of PAI-1 in particular. We put emphasis on the intramolecular rearrangements of PAI-1 that are required for its antiproteolytic as well as its nonproteolytic functions.