A parasite-surface trans-sialidase of Trypanosoma cruzi

反式唾液酸酶（TS）是一种在原生动物锥虫体内发现的独特酶。该酶催化唾液酸从宿主糖缀合物到寄生虫表面受体分子的转移反应。这种酶首先在恰加斯病的病原体克氏锥虫的锥马鞭毛虫阶段发现。唾液酸，转移到寄生虫表面糖蛋白被认为是重要的细胞入侵和逃离宿主防御系统。我们分析了克氏锥虫反式唾液酸酶的基因结构。反式唾液酸酶在锥马线虫时期高度表达，这些蛋白的基因以串联阵列的方式排列。另一种类型的反式唾液酸酶在寄生虫的昆虫阶段被检测到。这两种类型的反式唾液酸酶基因定位在不同的染色体上，可以独立调节。这两种反式唾液酸酶分子的催化结构域有80%以上的相似性，而这些氨基端和羧基端没有相似性。这些酶最显著的区别在于它们的c端。锥马鞭毛虫型的c端一半是12个氨基酸单位的串联重复序列，这些重复序列后面是GPI锚定结构。没有这些重复，也没有GPI存在于附马鞭毛虫反式唾液酸酶。这两种酶都由基因家族的几个成员组成。在过去的几年里，我们分析了这些反式唾液酸酶基因家族的异质性。PCR扩增的DNA片段序列分析表明，大约一半的基因编码酶失活型反式唾液酸酶，在200个氨基酸长的区域内发现30个氨基酸替换。利用细菌表达系统研究了这些取代对酶活性的影响。

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 trypomastigote 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.We have analyzed the gene structure of T.cruzi trans-sialidase. Trans-sialidase is highly expressed in trypomastigote stage and the genes for these proteins are arranged in clusters of tandem array. The other type of trans-sialidase is detectable at the insect stage of parasite, epimastigote. These two types of trans-sialidase genes are localized at the different chromosomes and may be regulated independently. Catalytic domain of these two trans-sialidase molecule shear more than 80% of similarities, however these amino- and carboxyl- terminal regions have no similarities. Most remarkable differences in these enzymes are at their C-terminal. C-terminal half of trypomastigote type is tandem repeats of 12 amino acid unit and these are followed by GPI anchor structure. No these repeat and no GPI exists in epimastigote trans-sialidase. Both of the enzymes consist of several members of gene family.In these last years, We have analyzed heterogeneity of these trans-sialidase gene family. The sequence analysis of PCR amplified DNA fragments suggested that around half of the genes encode enzymatically inactive type of trans-sialidase and 30 amino acid substitutions were found in this region of 200 amino acid long. The effect of these substitutions to the enzyme activity is also examined using bacterial expression system.