Structure and function of proteins to form complex with receptor for heat-stable enterotoxin

已知由产肠毒素大肠杆菌产生的热稳定肠毒素（STs）可引起人类和动物的急性腹泻。ST的生物学作用是通过与肠上皮细胞膜上的受体蛋白结合而启动的。st受体复合物的形成导致鸟苷酸环化酶的刺激，随后是细胞中cGMP浓度的增加。蛋白激酶与cGMP结合激活，激活囊性纤维化跨膜传导调节因子，导致细胞分泌液体。在从大鼠肠中纯化ST受体蛋白的研究过程中，发现受体蛋白形成受体蛋白复合物。为了阐明ST的信号转导机制，了解ST受体复合物是具有不同蛋白的同聚寡聚物还是异聚寡聚物是很重要的。本研究的目的是阐明与ST受体蛋白形成复合物的蛋白质的结构和功能。为此，设计ST类似物，使其具有与受体蛋白交联的荧光基团，并具有与天然ST对应的受体蛋白的特异性结合能力。研究了ST与大鼠肠膜的结合特性。结果表明，该膜含有两种受体蛋白，一种与ST可交换结合（Rex），另一种与ST不可交换结合（Rnex）。Rnex以两种受体复合物形式存在于膜上(（1)和(2)）。发现(1)和(2)是由共同的蛋白质成分和彼此不同的成分构成的。在这些蛋白质中，共同成分的分析正在进行中。

Heat-stable enterotoxin (STs) produced by enterotoxigenic Escherichia coli are known to cause an acute diarrhea in men and animals. The biological action of ST is initiated by the binding to its receptor proteins on the membrane of intestinal epithelial cell. The formation of a ST-receptor complex leads to stimulation of guanylate cyclase and is followed by increase in cGMP concentration in the cells. Protein kinase is activated by the binding with cGMP and activate Cystic fibrosis transmembrane conductance regulator to result in a fluid secretion from the cells. In a course of study to purify receptor proteins for ST from rat intestine, receptor protein was found to form receptor protein complex. To elucidate the signal transduction mechanism by ST,it is important to know whether ST-receptor complex is the homo-oligomer or the hetero-oligomer with different proteins. Purpose of this study is to elucidate the structure and functions of proteins which forms complex with receptor protein for ST.To accomplish this purpose, ST analogs were designed to have the fluorophore, the functional group for the cross-linking to the receptor protein, and the specific binding ability to the receptor protein corresponding to that of the native ST and were synthesized. Binding characteristics of ST to the membrane prepared from rat intestine were investigated. Results revealed that membrane contained two types of receptor proteins, that is, one binds exchange-ably with ST (Rex) and the other non-echange-ably (Rnex). Rnex existed on membrane in two kinds of receptor complexes ((1) and (2)). (1) and (2) were found to be constructed with common protein components and the different component to each other. Among these proteins, analysis of common components are in progress.