Component of the heterodimer clotting factor G which may play a role in defense mechanisms against fungi (Probable). Initiates a (1->3)-beta-glucan-sensing clotting pathway whereby the alpha subunit binds to glucans containing (1->3)-beta linkages, which are components of the fungal cell wall, and the beta subunit catalyzes the activation of proclotting enzyme (PubMed:7822328). Selective cleavage of 98-Arg-|-Ile-99 bond in Limulus proclotting enzyme to form active clotting enzyme. Binding to (1->3)-beta-D-glucan to alpha subunit, induces autocatalysis and activation of beta subunit (PubMed:7822328). Inhibited by intracellular coagulation inhibitor 3/LICI-3 and to a lesser extend by intracellular coagulation inhibitor 2/LICI-2 (PubMed:7822328, PubMed:8798603). Clotting factor G is a heterodimer composed of two non-covalently associated subunits, alpha and beta (PubMed:7822328). Upon activation, converted to a two-chain active form linked by a disulfide bond (PubMed:7822328). Forms a covalent heterodimer with intracellular coagulation inhibitor 3/LICI-3 (PubMed:8798603). Expressed in the hemocytes (at protein level). Clotting factor G is a component of the Limulus test used to detect bacterial endotoxins in fluids. Belongs to the peptidase S1 family.