Studies on Sugar Recognition by Animal Lectins

血清甘露聚糖结合蛋白（MBP）是一种c型凝集素，它与以甘露糖或n-乙酰氨基葡萄糖残基终止的糖链结合。一旦MBP与固体载体（如细菌细胞）上的配体糖结合，它就会通过一种称为“凝集素途径”的新途径激活补体。血清中缺乏这种凝集素的患者患有严重的传染病，这一发现清楚地证明了这种凝集素的生理意义。在过去的一年里，我们通过分析人MBP基因5'上游区域启动子活性，研究了MBP mRNA的表达机制。将5'上游区基因片段-845/+69插入到含有荧光素酶基因的报告质粒中，该片段除了包含基本启动子元件CAAT和TATA box外，还包含多个共识序列。将合成的构建体转染到Hep G2细胞中，显示其表达MBP mRNA。用il - 6、TNF-a、INF-g和PMA （phorbol 12-肉豆蔻酸13-乙酸酯）等细胞因子处理转染的细胞对荧光素酶的表达水平没有明显影响，而地塞米松将其表达抑制到对照水平的一半。MBP基因在5'上游区域的逐步缺失导致表达的上下调节，这表明在该启动子区域存在多个负调控元件和正调控元件。

Serum mannan-binding protein (MBP) is a C-type lectin which binds to sugar chains terminated with mannose or N-acetylglucosamine residue. Once MBP binds to ligand sugars on a solid support such as bacterial cells, it activates complement through a new pathway called "lectin pathway". The physiological significance of this lectin was demonstrated clearly by the finding that the patients deficient of the lectin in their serum suffer from serious infectious diseases. In the past one year, we have studied the mechanism of the expression of MBP mRNA by analyzing promoter activity of the 5' upstream region of human MBP gene. 5' upstream region gene fragment -845/+69, which contains several consensus sequences in addition to basic promoter elements CAAT and TATA boxes, was inserted into reporter plasmid containing luciferase gene. The resultant construct was transfected into Hep G2 cells, which was shown to express MBP mRNA.Treatment of the transfected cells with cytokines such as IL6, TNF-a, INF-g and PMA (phorbol 12-myristate 13-acetate) did not show any significant effects on the level of expression of luciferase, while dexamethasone inhibited the expression to one half the control level. Step wise deletions of the MBP gene from 5' upstream region resulted in the up and down regulation of the expression suggesting the presence of multiple negative and positive regulatory elements in this promoter regions.