A parasite-surface trans-sialidase of Trypanosoma cruzi

反式唾液酸酶(TS)是在原生动物锥虫中发现的一种独特的酶。这种酶催化唾液酸从宿主衍生的糖结合物转移到寄生虫表面受体分子上。这种酶是在克氏锥虫的锥虫期首次发现的，克氏锥虫是查加斯病的病原体。转移到寄生虫表面糖蛋白上的唾液酸被认为是细胞入侵和逃避宿主防御系统的重要因素。反式唾液酸酶在类鞭毛虫期高表达，并且这些蛋白的基因以串联阵列的形式排列。另一种类型的反式唾液酸酶在寄生虫的昆虫阶段可检测到。这两种类型的反式唾液酸酶基因定位于不同的染色体上，可能是独立调控的。这两个反式唾液酸酶分子的催化结构域有80%以上的相似性，但这些氨基和羧基末端没有相似之处。这些酶最显著的区别在于它们的C-末端。类鞭毛虫型的C-末端一半是12个氨基酸单位的串联重复序列，其次是GPI锚定结构。表鞭毛体反式唾液酸酶不存在这些重复序列，也不存在GPI。这两种酶都由几个基因家族成员组成，近年来，我们对这些反式唾液酸酶基因家族的异质性进行了分析。对扩增出的DNA片段的序列分析表明，大约一半的基因编码酶失活类型的反式唾液酸酶，在这个200个氨基酸长的区域有30个氨基酸替换。利用细菌表达系统检测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.