Role of Limulus Hemocytes in the Biological Defense System.

马蹄蟹血细胞裂解液响应(1*3)- β - d -葡聚糖，启动酶级联，最终形成凝块。我们纯化了丝氨酸蛋白酶酶原，因子G，它直接被(1*3)- d -葡聚糖激活，并启动血淋巴凝血级联反应。因子G是一种异二聚体蛋白，由两个非共价相关亚基α (72 kDa)和β (37 kDa)组成。在(1*3)- d -葡聚糖（如凝乳蛋白聚糖和paramylon）存在的情况下，因子G被自动催化激活为一种活性丝氨酸蛋白酶，称为因子G。这种激活伴随着两个亚基中Arg-X键的有限蛋白水解：72-kDa亚基α被裂解为55-kDa和17-kDa片段，37-kDa亚基β被缩短为34-kDa。利用参与血淋巴凝血级联的纯化蛋白进行的重构实验表明，因子G能够直接激活促凝酶，从而使凝血原转化为凝血凝胶。对这两个亚基的dna分析表明，这两个亚基来自不同的mRNA物种，因此由不同的基因编码。β亚单位是一种由278个残基组成的丝氨酸蛋白酶酶原，与马蹄蟹因子b同源。α亚单位是一种由654个残基组成的新型mozaic蛋白，具有有趣的特征。这个亚基的nh_2末端部分类似于细菌的-1,3-葡聚糖酶。其126个氨基酸的cooh末端具有与木聚糖酶部分同源的重复序列。在这些区域之间有47个氨基酸的三个重复单元，它们与碳水化合物结合蛋白相似。这些可能是因子G的(1*3)- β - d -葡聚糖结合域(s)。因此，因子G是马蹄蟹(1*3)- β - d -葡聚糖敏感凝血途径的主要引发剂，是一种结构独特的异二聚丝氨酸蛋白酶酶原，因此代表了一类新的活性防御蛋白。少

Horseshoe Crab hemocyte lysate responds to (1*3) -beta-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) -D-glucans and which initiates the hemolymph clotting cascade. Factor G is a heterodimeric protein composed of two noncovalently-asociated subnits alpha (72 kDa) and beta (37 kDa). In the prescence of (1*3) -D-glucans such as curdlan and paramylon, factor G is autocatalytically activated to an active serine protease, named factor G.This activation is accompanied by limited proteolyses of Arg-X bonds in both subunits : the 72-kDa subunit alpha is cleaved to 55-kDa and 17-kDa fragments, and the 37-kDa subunit beta is shortened to 34-kDa. 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 coagulo … More gen to coagulin gel. Analyzes of cDNAs for both subunits indicates that the subunits are derived from separate mRNA species and thus encoded by different genes. Subunit beta is a serine protease zymogen consisting of 278 residues, which is homologous to horseshoe crab factor B.Subunit alpha, on the other hand, is a new type of mozaic protein consisting of 654 residues with intriguing features. The NH_2-terminal portion of this subnit 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 there are three repeating units of 47 amino acids, whose similarity to carbohydrate binding proteins. These may be the (1*3) -beta-D-glucan binding domain (s) of factor G.Thus, factor G,the primary initiator of the (1*3) -beta-D-glucan-sensitive coagulation pathway in the horseshoe crab, is a structurally unique heterodimeric serine protease zymogen and as such represents a new class of active defense proteins. Less