Serine protease that cleaves the inactive precursor plasminogen at a specific Arg-Val peptide bond, converting it into active plasmin (PubMed:1969415, PubMed:2521625, PubMed:4270330). Secreted as an inactive zymogen, it is recruited and activated at the cell surface through interaction with the urokinase plasminogen activator receptor (uPAR). Cell-surface activation enables localized, plasmin-dependent pericellular proteolysis, regulating extracellular matrix degradation in cell migration and tissue remodeling (PubMed:1829461, PubMed:2521625, PubMed:28849762). Also contributes to fibrinolysis and blood clot clearance by promoting plasmin generation (By similarity). {ECO:0000250|UniProtKB:P06869, ECO:0000269|PubMed:1829461, ECO:0000269|PubMed:1969415, ECO:0000269|PubMed:2521625, ECO:0000269|PubMed:28849762, ECO:0000269|PubMed:4270330}. This is the function of PLAU (plasminogen activator, urokinase, ENSG00000122861).