Karlsson K A, Teneberg S, Angström J, Kjellberg A, Hirst T R, Berström J, Miller-Podraza H
Department of Medical Biochemistry, Göteborg University, Sweden.
Bioorg Med Chem. 1996 Nov;4(11):1919-28. doi: 10.1016/s0968-0896(96)00174-5.
The bacterial protein enterotoxins, cholera toxin (CT) of Vibrio cholerae and heat-labile toxin (LT) of Escherichia coli, induce diarrhea by enhancing the secretory activity of the small intestine of man and rabbit (animal model). This physiological effect is mediated by toxin binding to a glycolipid receptor, the ganglioside GM1, Gal beta 3GalNAc beta 4(NeuAc alpha 3)GAl beta 4Glc beta 1Cer. However, LT, but not CT, was recently shown by us to bind also to paragloboside, Gal beta 4GlcNAc beta 3Gal beta 4Glc beta 1Cer, identified in the target cells. By molecular modeling of this tetrasaccharide in the known binding site of LT, the saccharide-peptide interaction was shown to be limited to the terminal disaccharide (N-acetyllactosamine). This sequence is expressed in many glycoconjugates, and we have therefore assayed glycolipids and glycoproteins prepared from the target tissues. In addition to paragloboside, receptor activity for LT was detected in glycoproteins of human origin and in polyglycosylceramides of rabbit. However, CT bound only to GM1. Two variants of LT with slightly different sequences, human (hLT) and porcine (pLT), were identical in their binding to target glycoproteins and polyglycosylceramides, but different regarding paragloboside, which was positive for pLT but negative for hLT. This difference is discussed on basis of modeling, taking in view the difference at position 13, with Arg in pLT and His in hLT. Although N-acetyllactosamine is differently recognized in form of paragloboside by the two toxin variants, we speculate that this sequence in human glycoproteins and rabbit polyglycosylceramides is the basis for the common binding. Much work remains, however, to clear up up this unexpected sophistication in target recognition.
细菌蛋白肠毒素，如霍乱弧菌的霍乱毒素（CT）和大肠杆菌的不耐热毒素（LT），通过增强人和兔子（动物模型）小肠的分泌活性来诱导腹泻。这种生理效应是由毒素与糖脂受体神经节苷脂GM1（Galβ3GalNAcβ4（NeuAcα3）Galβ4Glcβ1Cer）结合介导的。然而，我们最近发现，LT能与在靶细胞中鉴定出的副球蛋白（Galβ4GlcNAcβ3Galβ4Glcβ1Cer）结合，而CT不能。通过对LT已知结合位点中的这种四糖进行分子建模，发现糖肽相互作用仅限于末端二糖（N-乙酰乳糖胺）。该序列在许多糖缀合物中都有表达，因此我们检测了从靶组织制备的糖脂和糖蛋白。除了副球蛋白外，在人源糖蛋白和兔子的多聚糖基神经酰胺中也检测到了LT的受体活性。然而，CT只与GM1结合。两种序列略有不同的LT变体，人源（hLT）和猪源（pLT），它们与靶糖蛋白和多聚糖基神经酰胺的结合情况相同，但对于副球蛋白的结合情况不同，pLT呈阳性而hLT呈阴性。考虑到13位的差异，pLT中的是精氨酸，hLT中的是组氨酸，基于建模对此差异进行了讨论。尽管两种毒素变体以副球蛋白的形式对N-乙酰乳糖胺的识别不同，但我们推测人糖蛋白和兔子多聚糖基神经酰胺中的这个序列是共同结合的基础。然而，要弄清楚这种在靶标识别中意想不到的复杂性，仍有许多工作要做。
