title: alpha 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 alpha domain and converts SKDelta59 into a fibrin-dependent Pg activator through unknown mechanisms. We show that the SK alpha 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, SKDelta59 loses the ability to unfold Glu-Pg during complex formation and becomes incapable of nonproteolytic active site formation. In this manner, SKDelta59 behaves more like staphylokinase than like SK; it requires plasmin to form a functional activator complex, and in this complex SKDelta59 does not protect plasmin from inhibition by alpha(2)-antiplasmin. At the same time, SKDelta59 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. SKDelta59 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 alpha 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 alpha domain makes SKDelta59 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 alpha(2)-antiplasmin.
authors.affiliation: Cardiovascular Biology Laboratory, Harvard School of Public Health, Boston, Massachusetts 02115, USA.
authors.forename: Irina Y
authors.initials: IY
authors.lastname: Sazonova
authors.forename: Brian R
authors.initials: BR
authors.lastname: Robinson
authors.forename: Inna P
authors.initials: IP
authors.lastname: Gladysheva
authors.forename: Francis J
authors.initials: FJ
authors.lastname: Castellino
authors.forename: Guy L
authors.initials: GL
authors.lastname: Reed
mesh_terms: D001665:Binding Sites; D005337:Fibrin / Q000494:pharmacology*; D005341:Fibrinolysin / Q000378:metabolism; D006801:Humans; D008666:Metalloendopeptidases / Q000494:pharmacology*; D010958:Plasminogen / Q000737:chemistry / Q000378:metabolism; D011487:Protein Conformation; D013300:Streptokinase / Q000737:chemistry / Q000494:pharmacology*; D010959:Tissue Plasminogen Activator / Q000494:pharmacology*; D000979:alpha-2-Antiplasmin / Q000378:metabolism
chemical_list: D000979:alpha-2-Antiplasmin; D005337:Fibrin; D010958:Plasminogen; D013300:Streptokinase; D010959:Tissue Plasminogen Activator; D005341:Fibrinolysin; D008666:Metalloendopeptidases; C022653:auR protein, Staphylococcus aureus