Horseshoe crab (1,3)-β-D-glucan-sensitive coagulation factor G. A serine protease zymogen heterodimer with similarities to β-glucan-binding proteins

Horseshoe crab factor G is an intracellular serine protease zymogen that initiates the (1,3)-β-D-glucan-sensitive hemolymph clotting pathway. Unlike other known serine protease zymogens, which are composed of a single subunit, factor G consists of two distinct subunits, α and β, which are autocatalytically converted to active factor Ḡ in the presence of (1,3)- β-D-glucan. We have now cloned and sequenced cDNAs encoding both subunits of factor G. The subunits are derived from separate mRNA species and thus encoded by different genes. Subunit β is a serine protease zymogen which consists of 278 residues with a calculated molecular mass of 30,846 Da; it exhibits homology to the serine protease domain of horseshoe crab factor B. Subunit α, on the other hand, is a new type of mosaic protein with intriguing features. The mature protein consists of 654 residues with a calculated molecular mass of 73,916 Da. The NH₂-terminal portion of this subunit is similar to bacterial β-1,3-glucanases. Its 126 amino acid COOH terminus exhibits a repetitive sequence having partial homology to xylanases. 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)-β-D-glucan-binding domain(s) of factor G. Factor G, thus, is a structurally unique heterodimeric serine protease zymogen and as such may represent a new class of active defense proteins.