This enzyme is closely associated with an endotoxin-sensitive hemolymph coagulation system which may play important roles in both hemostasis and host defense mechanisms. Its active form catalyzes the activation of factor B. Selective cleavage of 103-Arg-|-Ser-104 and 124-Ile-|-Ile-125 bonds in Limulus clotting factor B to form activated factor B. Cleavage of -Pro-Arg-|-Xaa- bonds in synthetic substrates. Activated by Gram-negative bacterial lipopolysaccharides and chymotrypsin. Heterodimer of a light chain and a heavy chain linked by a disulfide bond. Secreted in hemolymph. Belongs to the peptidase S1 family.