HORSESHOE-CRAB (1,3)-BETA-D-GLUCAN-SENSITIVE COAGULATION FACTOR-G - ASERINE-PROTEASE ZYMOGEN HETERODIMER WITH SIMILARITIES TO BETA-GLUCAN-BINDING PROTEINS

Horseshoe crab factor G is an intracellular serine protease zymogen th at initiates the (1,3)-beta-D-glucan-sensitive hemolymph clotting path way. Unlike other known serine protease zymogens, which are composed o f a single subunit, factor G consists of two distinct subunits, alpha and beta, which are autocatalytically converted to active factor GBAR in the presence of (1,3)-beta-D-glucan. We have now cloned and sequenc ed cDNAs encoding both subunits of factor G. The subunits are derived from separate mRNA species and thus encoded by different genes. Subuni t beta is a serine protease zymogen which consists of 278 residues wit h a calculated molecular mass of 30,846 Da; it exhibits homology to th e serine protease domain of horseshoe crab factor B. Subunit alpha, on the other hand, is a new type of mosaic protein with intriguing featu res. The mature protein consists of 654 residues with a calculated mol ecular mass of 73,916 Da. The NH2-terminal portion of this subunit is similar to bacterial beta-1,3-glucanases. Its 126 amino acid COOH term inus exhibits a repetitive sequence having partial homology to xylanas es. Between these regions are three repeating units of 47 amino acids, whose similarity to carbohydrate-binding proteins suggests that these may be the (1,3)-beta-D-glucan-binding domain(s) of factor G. Factor G, thus, is a structurally unique heterodimeric serine protease zymoge n and as such may represent a new class of active defense proteins.