Horseshoe crab (1,3)-beta-D-glucan-sensitive coagulation factor G. A serine protease zymogen heterodimer with similarities to beta-glucan-binding proteins.
Horseshoe crab factor G is an intracellular serine protease zymogen that initiates the (1,3)-beta-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, alpha and beta, which are autocatalytically converted to active factor G in the presence of (1,3)-beta-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 beta 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 alpha, 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 beta-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)-beta-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.
Seki
N
N
Department of Molecular Biology, Graduate School of Medical Science, Kyushu University, Fukuoka, Japan.
Muta
T
T
Oda
T
T
Iwaki
D
D
Kuma
K
K
Miyata
T
T
Iwanaga
S
S
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001779:Blood Coagulation Factors / Q000235:genetics*; D002352:Carrier Proteins / Q000235:genetics; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000235:genetics; D043364:Endo-1,4-beta Xylanases; D015870:Gene Expression; D005796:Genes; D004693:Glucan Endo-1,3-beta-D-Glucosidase / Q000737:chemistry*; D006026:Glycoside Hydrolases / Q000737:chemistry*; D006737:Horseshoe Crabs / Q000737:chemistry*; D008969:Molecular Sequence Data; D009838:Oligodeoxyribonucleotides / Q000737:chemistry; D012333:RNA, Messenger / Q000235:genetics; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D012697:Serine Endopeptidases / Q000235:genetics*
D001779:Blood Coagulation Factors; D002352:Carrier Proteins; D018076:DNA, Complementary; D009838:Oligodeoxyribonucleotides; D012333:RNA, Messenger; D006026:Glycoside Hydrolases; D004693:Glucan Endo-1,3-beta-D-Glucosidase; D043364:Endo-1,4-beta Xylanases; C085383:coagulation factor G, horseshoe crab; D012697:Serine Endopeptidases