α Domain Deletion Converts Streptokinase into a Fibrin-dependent Plasminogen Activator through Mechanisms Akin to Staphylokinase and Tissue Plasminogen Activator*

Abstract

The mechanism of action of plasminogen (Pg) activators may affect their therapeutic properties in humans. Streptokinase (SK) is a robust Pg activator in physiologic fluids in the absence of fibrin. Deletion of a “catalytic switch” (SK residues 1–59), alters the conformation of the SK α domain and converts SKΔ59 into a fibrin-dependent Pg activator through unknown mechanisms. We show that the SK α domain binds avidly to the Pg kringle domains that maintain Glu-Pg in a tightly folded conformation. By virtue of deletion of SK residues 1–59, SKΔ59 loses the ability to unfold Glu-Pg during complex formation and becomes incapable of nonproteolytic active site formation. In this manner, SKΔ59 behaves more like staphylokinase than like SK; it requires plasmin to form a functional activator complex, and in this complex SKΔ59 does not protect plasmin from inhibition by α₂-antiplasmin. At the same time, SKΔ59 is unlike staphylokinase or SK and is more like tissue Pg activator, because it is a poor activator of the tightly folded form of Glu-Pg in physiologic solutions. SKΔ59 can only activate Glu-Pg when it was unfolded by fibrin interactions or by Cl^–-deficient buffers. Taken together, these studies indicate that an intact α domain confers on SK the ability to nonproteolytically activate Glu-Pg, to unfold and process Glu-Pg substrate in physiologic solutions, and to alter the substrate-inhibitor interactions of plasmin in the activator complex. The loss of an intact α domain makes SKΔ59 activate Pg through classical “fibrin-dependent mechanisms” (akin to both staphylokinase and tissue Pg activator) that include: 1) a marked preference for a fibrin-bound or unfolded Glu-Pg substrate, 2) a requirement for plasmin in the activator complex, and 3) the creation of an activator complex with plasmin that is readily inhibited by α₂-antiplasmin.