Assembly and activation of the plasma KKS on endothelial cells. Plasma PK circulates in complex with HK. The HK•PK complex binds to a multiprotein receptor complex that consists of cytokeratin 1 (CK1), urokinase plasminogen activator receptor (uPAR) and gC1qR. The proteins of the HK•PK receptor complex co-localize on endothelial cell membranes. When HK•PK binds to endothelial cells, PK is rapidly converted to kallikrein (K) by the enzyme prolylcarboxypeptidase (PRCP), which is constitutively active on endothelial cell membranes. The resulting kallikrein autodigests its receptor, HK, to liberate bradykinin (BK), which can liberate tissue plasminogen activator (tPA), nitric oxide (NO), and prostacyclin (PGI₂) from endothelial cells. Kallikrein also activates FXII, which binds to the same multiprotein receptor complex as HK in its absence. In this revised hypothesis for assembly and activation of the proteins of the plasma KKS, FXII is activated by kallikrein after PK activation. ScuPA, single chain urokinase plasminogen activator.