Factor VII-activating protease (FSAP) is a novel plasma-derived serine protease structurally homologous to tissue-type and urokinase-type plasminogen activators. We demonstrate that plasminogen activator inhibitor-1 (PAI-1), the predominant inhibitor of tissue-type and urokinase-type plasminogen activators in plasma and tissues, is an inhibitor of FSAP as well. We detected PAI-1⅐FSAP complexes in addition to high levels of extracellular RNA, an important FSAP cofactor, in bronchoalveolar lavage fluids from patients with acute respiratory distress syndrome. Hydrolytic activity of FSAP was inhibited by PAI-1 with a second-order inhibition rate constant (K a ) of 3.38 ؎ Factor VII-activating protease (FSAP), 2 also known as plasma hyaluronan-binding protein or plasma hyaluronanbinding serine protease, is a recently described plasma serine protease (1-3), expressed primarily in the liver and found in endothelial and epithelial cells of the lung, kidney, placenta, and pancreas (1, 4). The enzyme circulates as a 64-kDa single-chain zymogen (scFSAP) in plasma, which is autoactivated to an enzymatically active two-chain form (tcFSAP). Two-chain FSAP consists of a 46-kDa heavy chain connected by a disulfide bridge to a 29-kDa light chain containing the catalytic domain (5-7). Autoactivation of scFSAP to tcFSAP is promoted in the presence of negatively charged substances, such as heparin, dextran sulfate, phosphatidylethanolamine, or extracellular RNA (5-8).The role of FSAP in different physiological and pathophysiological conditions is not fully understood. A dual role for FSAP in vitro in hemostasis has been suggested (3, 9). FSAP is a potent activator of coagulation factor VII, thus contributing to the initiation of blood coagulation via the extrinsic pathway (3). Furthermore, FSAP also activates prourokinase-type plasminogen activator (uPA) and thus contributes to plasminogen activation as well (9). Additional links between FSAP and the plasminogen activation system exist; FSAP is highly homologous to uPA and tissue-type plasminogen activator (tPA), and uPA is a potential physiological activator of FSAP (5). Additionally, recent in vitro studies have revealed that hemostatic serine protease inhibitors (serpins), such as C1 inhibitor and ␣ 2 -antiplasmin, are inhibitors of FSAP proteolytic activity (7, 9). These observations prompted us to explore the role of PAI-1, the primary member of this inhibitor family in plasma and tissues (10, 11), as a potential regulator of FSAP. It is known that PAI-1 inhibits uPA and tPA by forming SDS-resistant, catalytically inactive complexes with proteases (12, 13). In this reaction, the reactive center residues P 1 -P 1 Ј (Arg 346 -Met 347 ) in PAI-1 function as an exposed "bait" for the protease by mimicking a putative cleavage site (14 -17). In addition to inhibiting serine proteases, PAI-1 also interacts with different components of the extracellular matrix, including heparin (18, 19) and vitronectin (20 -22). In the presence of heparin, the selectivity of PAI-1 for the inhibition ...