BIOSYNTHESIS AND FUNCTION OF GLYCOSPHINGOLIPIDS AND OTHER GLYCOCONJUGATES

点击翻译按钮获取中文摘要

Cholera toxin (CT) produced by Vibrio cholerae and the heat-labile enterotoxin (LT) produced by certain strains of Escherichia coli are the causative agents of cholera and traveler's diarrhea, respectively. The toxins are structurally, immunologically and functionally similar: each has a pentameric B subunit which binds to specific receptors on the intestinal mucosal cell and an A subunit which is involved in activation of adenylylcyclase. Whereas CT uses the ganglioside GM1 as its only receptor, LT appears to recognize G/M1 as well as other receptors on intestinal cells. In order to identify these alternate LT receptors, we used human intestinal CaCo-2 cells, which behave in culture as differentiated enterocytes, the natural target for the two toxins. CaCo-2 cells bound 8-fold more LT than CT, and LT binding was only partially inhibited by CT-B whereas CT binding was completely inhibited by LT-B. Although G/M1 was the only CaCo-2 glycolipid recognized by both CT and LT, a series of membrane galactoproteins (apparent molecular weights between 92 and 120 kDa) were recognized by LT but not by CT. By using specific glycosidases and transferrin and fetuin as model galactoproteins, we established that the binding determinant for LT on these galactoproteins was galactose (beta1-4)N-acetylglucosamine. Furthermore, we were able to show that these galactoproteins recognized by LT were immunoprecipitated by specific antisera against polyactosylated glycoproteins of the lacto-N-neotetraosyl type. Finally, some of these alternate galactoprotein receptors for LT appear to be functional as activation of adenylylcyclase by LT in CaCo-2 cells was only partially inhibited by CT-B. Thus, despite the high degree of homology between LT and CT, LT can recognize glycoconjugates with carbohydrate structures distinct from the oligosaccharide of GM/1 which is recognized by both toxins. Furthermore, the apparent ability of LT to utilize both glycolipids and glycoproteins as receptors raises the possibility that LT may use more than one pathway for cellular activation. In this regard, we had shown previously that CT bound to neoganglioproteins enters the cell through a different pathway than when bound to G/M1.