Converts the abundant, but inactive, zymogen plasminogen to plasmin by hydrolyzing a single Arg-Val bond in plasminogen. By controlling plasmin-mediated proteolysis, it plays an important role in tissue remodeling and degradation, in cell migration and many other physiopathological events. During oocyte activation, plays a role in cortical granule reaction in the zona reaction, which contributes to the block to polyspermy (By similarity). Specific cleavage of Arg-|-Val bond in plasminogen to form plasmin. Inhibited by SERPINA5. Heterodimer of chain A and chain B held by a disulfide bond. Binds to fibrin with high affinity. This interaction leads to an increase in the catalytic efficiency of the enzyme due to an increase in affinity for plasminogen. Similarly, binding to heparin increases the activation of plasminogen. Binds to annexin A2, cytokeratin-8, fibronectin and laminin. Binds to mannose receptor and the low-density lipoprotein receptor-related protein (LRP1); these proteins are involved in TPA clearance. Binds LRP1B; binding is followed by internalization and degradation. Forms heterodimer with SERPINA5 (By similarity). In complex with SERPINE1, interacts with SORL1 (By similarity). Both FN1 and one of the kringle domains are required for binding to fibrin. Both FN1 and EGF-like domains are important for binding to LRP1. The FN1 domain mediates binding to annexin A2. The second kringle domain is implicated in binding to cytokeratin-8 and to the endothelial cell surface binding site. The single chain, almost fully active enzyme, can be further processed into a two-chain fully active form by a cleavage after Arg-308 catalyzed by plasmin, tissue kallikrein or factor Xa. Belongs to the peptidase S1 family.