Serine protease inhibitor. Inhibits TMPRSS7. Is a primary inhibitor of tissue-type plasminogen activator (PLAT) and urokinase-type plasminogen activator (PLAU). As PLAT inhibitor, it is required for fibrinolysis down-regulation and is responsible for the controlled degradation of blood clots. As PLAU inhibitor, it is involved in the regulation of cell adhesion and spreading. Acts as a regulator of cell migration, independently of its role as protease inhibitor. It is required for stimulation of keratinocyte migration during cutaneous injury repair. It is involved in cellular and replicative senescence (By similarity). Plays a role in alveolar type 2 cells senescence in the lung (By similarity). Is involved in the regulation of cementogenic differentiation of periodontal ligament stem cells, and regulates odontoblast differentiation and dentin formation during odontogenesis (By similarity). Forms a heterodimer with TMPRSS7. Interacts with VTN. Binds LRP1B; binding is followed by internalization and degradation. Interacts with PPP1CB. In complex with PLAU/uPA, interacts with PLAUR/uPAR (By similarity). Interacts with SORL1 and LRP1, either alone or in complex with PLAU; these interactions are abolished in the presence of LRPAP1/RAP (By similarity). The ternary complex composed of PLAUR-PLAU-PAI1 also interacts with SORL1 (By similarity). Also interacts with SORL1, when complexed to PLAT/tPA (By similarity). Belongs to the serpin family.