BIOSYNTHESIS AND FUNCTION OF GLYCOSPHINGOLIPIDS AND OTHER GLYCOCONJUGATES

神经节苷脂G(M1)是霍乱毒素(CT)的细胞表面受体。 寡糖链(G(M1)OS)被B或结合识别 CT的亚单位，而毒素的A亚单位激活腺苷 刺激性G蛋白(Gs)的ADP-核糖化环化酶 循环酶系统。A亚基获得Gs的途径 尚未建立。我们之前已经证明过 G(M1)OS与细胞共价连接产生的新神经节蛋白 表面蛋白是CT的非功能受体。因此，CT与大鼠结合 用G(M1)OS修饰的胶质瘤C6不能刺激腺苷 循环酶。我们最近发现，在氯喹的存在下，这些 新神经节蛋白成为CT的功能性受体。为了进一步 探索新神经节蛋白作为毒素受体的作用，我们共价 人转铁蛋白结合G(M1)OS。以高为界的导数 与HeLa细胞上转铁蛋白受体的亲和力和支持结合 电脑断层扫描。然而，这种毒素不能激活腺苷环化酶 与G(M1)处理的HeLa细胞形成对比。氯喹治疗允许的CTL G(M1)-转铁蛋白对HeLa细胞腺酰环化酶的激活作用 相比之下，无论是对照组还是G(M1)处理组的HeLa或C6的预处理 加入氯喹的细胞对腺苷的毒素刺激没有影响 循环酶。这些结果表明，与G(M1)绑定的CT可以访问 腺酰环化酶通过不同于结合时的途径 新神经节蛋白。大肠埃希菌不耐热肠毒素(LT)结构 与CT类似，它与GM结合，并激活腺苷环化酶。一个人类 肠上皮细胞系与LT有额外的结合部位 笔刷边界膜。这些地点被确定为 半乳蛋白。我们现在正在确定这些结合位点是否可以 作为LT的功能性受体，允许LT进入细胞 通过与CT不同的途径。

Ganglioside G(M1) is the cell surface receptor for cholera toxin (CT). The oligosaccharide chain (G(M1)OS) is recognized by the B or binding subunits of CT whereas the A subunit of the toxin activates adenylyl cyclase by ADP-ribosylation of the stimulatory G protein (Gs) of the cyclase system. The pathway by which the A subunit gains access to Gs has not yet been established. We previously had shown that neoganglioproteins generated by covalently attaching G(M1)OS to cell surface proteins are nonfunction receptors for CT. Thus, CT binds to rat glioma C6 modified with G(M1)OS but is unable to stimulate adenylyl cyclase. We recently found that in the presence of chloroquine, these neoganglioproteins became functional receptors for CT. To further explore the role of neoganglioproteins as toxin receptors,we covalently attached G(M1)OS to human transferrin. The derivative bound with high affinity to transferrin receptors on HeLa cells and supported the binding of CT. The toxin, however, was unable to activate adenylyl cyclase in contrast to G(M1)-treated HeLa cells. Chloroquine treatment allowed CTL activate adenylyl cyclase in HeLa cells treated with G(M1)-transferrin. By contrast, pretreatment of either control or G(M1)-treated HeLa or C6 cells with chloroquine had no effect on toxin stimulation of adenylyl cyclase. These results indicate that CT bound to G(M1) gains access to adenylyl cyclase through a different pathway than when bound to neoganglioproteins. E. coli heat-labile enterotoxin (LT) is structurally similar to CT, binds to GM, and activates adenylyl cyclase. A human intestinal cell line was found to have additional binding sites for LT on the brush border membranes. These sites were identified as galactoproteins. We are now determining whether these binding sites can serve as functional receptors for LT and allow LT to enter the cells through a different pathway than CT.