IDENTIFICATION OF A PLASMINOGEN BINDING REGION IN STREPTOKINASE THAT IS NECESSARY FOR THE CREATION OF A FUNCTIONAL STREPTOKINASE-PLASMINOGEN ACTIVATOR COMPLEX

Streptokinase is a plasminogen activator widely used to treat patients with myocardial infarction. However, streptokinase is not a protease, and must first bind and interact with plasminogen to form an enzymati c complex. By measuring the binding of recombinant streptokinase fragm ents to plasminogen, we have sought, first, to identify a plasminogen binding region in streptokinase and, second, to explore the relation b etween binding (via this region) and the generation of a functional st reptokinase-plasminogen activator complex. Recombinant streptokinase b ound in a saturable and specific manner to human Glu-plasminogen with a dissociation constant of 4.2 x 10(-10) M, Recombinant streptokinase fragments spanning amino acids 1-127 and 1-253 could not be shown to b ind to Glu-plasminogen, whereas fragments spanning amino acids 1-352, 120-352, and 244-414 bound tightly to plasminogen and each fragment co mpletely inhibited the binding of full-length streptokinase to plasmin ogen. Although these latter streptokinase fragments formed a complex w ith plasminogen, enzymatic assays indicated that none of them was capa ble of generating an active site. When the streptokinase region shared by these three fragments, spanning residues 244-352, was expressed, i t also bound plasminogen and competitively inhibited the formation of a functional plasminogen activator complex by full-length streptokinas e, Taken together, these data indicate that streptokinase binds to pla sminogen with high affinity, that a primary binding region for plasmin ogen is located within amino acids 244-352, and that binding via this region is necessary for the generation of a functional plasminogen act ivator complex.