Factor G, a new heterodimeric serine protease zymogen sensitive to β-glucan is composed of two subunits α (72 kDa) and β (37 kDa). Subunit α contains a mosaic structure including glucanase like- and xylanase like-domains. On the other hand, subunit β is a serine protease zymogen homologous to the horseshoe crab factor B or proclotting enzyme. It is autocatalytically activated in the presence of long linear (1,3)-β-glucans. Upon activation, both subunits α and β are cleaved at a specific Arg-X bond. The activated factor induces the clot formation through the activation of proclotting enzyme. Since factor G is co-localized with the other coagulation factors in the large granules (unpublished results), it would be released into the hemolymph when the cells are stimulated. The released factor G is activated by β-glucans on fungi or other possible pathogens, resulting in the engulfment of the foreign materials by the coagulin clot. The biochemical evidence described above suggests the following hypothesis on the activation of factor G: In the zymogen form, subunit α sterically hinders the activation site of subunit β. β-Glucan-binding to subunit α then exposes the active site of subunit β, allowing autocatalytic activation through bimolecular interaction between subunit β's. The active subunit β then quickly hydrolyzes the Arg150-Glu151 bond in subunit α. Then, another site in the same subunit is cleaved, which is followed by the inactivation of the protease activity. Since the cDNAs for both subunits α and β are now available, the above hypotheses could be examined in expression experiments of the native and mutated forms of factor G.
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