Purified horseshoe crab factor G: Reconstitution and characterization of the (1→3)-β-D-glucan-sensitive serine protease cascade

Horseshoe crab hemocyte lysate responds to (1→3)-β-D-glucans, initiating an enzymatic cascade, which culminates in clot formation. We have purified to homogeneity the serine protease zymogen factor G, which is directly activated by (1→3)-β-D-glucans and which initiates the hemolymph clotting cascade. Factor G is a heterodimeric protein composed of two noncovalently associated subunits α (72 kDa) and β (37 kDa). In the presence of (1→3)-β-D-glucans such as curdlan and paramylon, factor G is autocatalytically activated to an active serine protease named factor Ḡ. This activation is accompanied by limited proteolysis of both subunits: the 72-kDa subunit α is cleaved to 55-kDa and 17-kDa fragments, and the 37-kDa subunit β is shortened to 34 kDa. Longer incubations with (1→3)-β-D-glucans result in cleavage of the 55-kDa fragment to 46 kDa and the 34-kDa fragment to 32 kDa, with concomitant loss of amidase activity. Reconstitution experiments using purified proteins participating in the hemolymph clotting cascade demonstrate that factor G is capable of activating proclotting enzyme directly, resulting in the conversion of coagulogen to coagulin gel. Thus, purified factor G is shown to be the primary initiator of the (1→3)-β-D-glucan-sensitive coagulation pathway in the horseshoe crab hemocyte lysate.