A parasite-surface trans-sialidase of Trypanosma family

转唾液酸酶（TS）是原生动物锥虫体内发现的一种独特的酶。这种酶催化唾液酸从宿主衍生的糖缀合物转移到寄生虫表面受体分子的反应。该酶首次发现于锥虫（Trypanosomacruzi）的锥虫期（Trypomasitigote），锥虫是南美锥虫病（Chagas' disease）的病原体。转移到寄生虫表面糖蛋白上的唾液酸对细胞入侵和逃避宿主防御系统具有重要作用。基因的活性和非活性形式包含。不同数量的羧基末端十二个氨基酸重复单元，以及氨基末端催化结构域中的一些取代。我们报道了活性的差异不是由于羧基端区域，而是由于氨基端结构域的单个氨基酸的差异，为了检测活性和非活性蛋白基因的比例，我们扩增了对酶活性重要且包含大多数氨基酸取代的区域。我们得到的结果是，一半的基因是活跃的，另一半是不活跃的类型。此外，对这些克隆的测序结果表明，在该区域还存在几种不同类型的替换。然后，我们对TS基因的结构进行了分析，发现不同类型的基因分布在多条染色体上。在Y株中，所有含重复序列的基因都位于一条1.1兆碱基的染色体带中，而缺失重复序列的基因则位于另外两条0.82和0.79兆碱基的染色体上。通过对克氏锥虫TS和兰氏锥虫唾液酸酶的比较分析，有助于确定TS蛋白的特性，从而确定其独特的唾液酸转移反应。

Trans-sialidase (TS) is a unique enzyme found in protozoan Trypanosoma. This enzyme catalyzes sialic acid transfer reaction from host derived glycoconjugates to parasite surface acceptor molecules. This enzyme is first found in trypomasitigote stage of Trypanosoma cruzi, causative agent of Chagas' disease The sialic acids, transferred to the parasite surface glycoprotein are suggested to be important for cell invasion and escape from host defense systems.Sequence analysis of T.cruzi TS genes have shown that trans-sialidase is encoded by several gene family and only some members of this family encode enzymatically active proteins. The active and inactive forms of the genes contain. different numbers of carboxy terminal twelve amino acid repeat units and also some substitutions in the amino terminal catalytic domains. We reported that difference in activity is not due to carboxy-terminal region, but due to single amino acid differences at the animo-terminal domain.To examine the ratio of active and inactive protein genes, we amplified the region where is important for enzyme activity and contains most of amino acid substitutions. We obtained the result taht half of the genes are active and the other half inactive types. In addition, sequencing of these amplifid clones revealed there are several additional types of substitutions in this region.Then, we analyzed TS gene organization and obtained the result that different types of genes are distributed in more than one chromosomal band. In the Y-strain, all repeat-containing genes are localized in one chromosomal band of 1.1 megabases, while the repeat-minus genes are in two other chromosome s of 0.82 and 0.79 megabases.Eichinger's group has obtained the sialidase gene from T.rangeli. A comparative analysis of T.cruzi TS and T.rangeli sialidase is helpful to determine the feature of TS protein which contribute to its unique sialic acid transfer reaction.