Specifically cleaves the zymogen plasminogen to form the active enzyme plasmin. Specific cleavage of Arg-|-Val bond in plasminogen to form plasmin. Inhibited by SERPINA5. Found in high and low molecular mass forms. Each consists of two chains, A and B. The high molecular mass form contains a long chain A which is cleaved to yield a short chain A. Forms heterodimer with SERPINA5. Binds LRP1B; binding is followed by internalization and degradation. Interacts with MRC2. Interacts with PLAUR. In complex with SERPINE1, interacts with PLAUR/uPAR. Interacts with SORL1 and LRP1, either alone or in complex with SERPINE1; these interactions are abolished in the presence of LRPAP1/RAP. The ternary complex composed of PLAUR-PLAU-PAI1 also interacts with SORLA. Phosphorylation of Ser-157 and Ser-325 abolishes proadhesive ability but does not interfere with receptor binding. Produced as an inactive single-chain protein (pro-uPA or sc-uPA), is processed into the active disulfide-linked two-chain form of PLAU/uPA by a proteolytic event mediated, at least, by TMPRSS4. Belongs to the peptidase S1 family.