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为末端的糖链结合。一旦MBP与细菌细胞等固体载体上的配基糖结合，它就会通过一种名为凝集素途径的新途径来激活补体。发现血清中凝集素缺乏的患者会患严重的感染性疾病，这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‘上游区域的启动子活性来研究MBP基因表达的机制。将5‘上游区基因片段-845/+69插入含有荧光素酶基因的报告质粒中，该片段除了含有基本启动子元件CAAT和TATA盒外，还含有几个共有的序列。经IL-6、TNF-α、INF-g和佛波酯(PMA)等细胞因子处理后，荧光素酶的表达水平无明显变化，地塞米松可抑制荧光素酶的表达，仅为对照组的一半。5‘上游区MBP基因的逐步缺失导致MBP基因表达的上调和下调，提示该启动子区域存在多种正负调控元件。

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.