Arab S, Lingwood C A
Department of Microbiology, Hospital for Sick Children, Toronto, Ontario, Canada.
Glycoconj J. 1996 Apr;13(2):159-66. doi: 10.1007/BF00731490.
The importance of the surrounding lipid environment on the availability of glycolipid carbohydrate for ligand binding was demonstrated by studying the influence of phosphatidylcholine fatty acid chain length on binding of verotoxins (VT1 and VT2c) to their specific cell surface receptor, globotriaosylceramide (Gb3) in the presence of auxiliary lipids both in a microtitre plate surface bilayer film and in a liposome membrane model system. In the microtitre assay, both VT1 and VT2c binding to Gb3 was increased as a function of decreasing PC acyl chain length likely resulting in increased Gb3 exposure. In the liposome assay VT1 binding was similarly modulated, however the effect of VT2c binding was more complex and did not follow a simple function of increased carbohydrate exposure. Earlier work established that C22:1 and C18:1Gb3 fatty acid homologues were the preferred Gb3 receptor containing liposomes, but in C14PC liposomes, binding to C22:1Gb3 (but not C18:1Gb3) was elevated such that this Gb3 species now became the preferred receptor for both toxins. This change in verotoxin/Gb3 homologue binding selectivity in the presence of C14PC did not occur in the microtitre bilayer format. These results are consistent with our proposal that these toxins recognize different epitopes on the Gb3 oligosaccharide. We infer that relative availability of these epitopes for toxin binding in an artificial bilayer is influenced not only by the exposure due to the discrepancy between the fatty acyl chain lengths of Gb3 and PC, but by the physical mode of presentation of the bilayer structure. Such acyl chain length differences have a more marked effect in a supported bilayer film whereas only the largest discrepancies affect Gb3 receptor function in liposomes. The basis of phospholipid modulation of glycolipid carbohydrate accessibility for receptor function is likely complex and will involve phase separation, gel/liquid crystalline transition, packing and lateral mobility within the bilayer, suggesting that such parameters should be considered in the assessment of glycolipid receptor function in cells.
通过在微量滴定板表面双层膜和脂质体膜模型系统中，研究磷脂酰胆碱脂肪酸链长度对维罗毒素（VT1和VT2c）与它们的特异性细胞表面受体——球三糖基神经酰胺（Gb3）结合的影响，证明了周围脂质环境对糖脂碳水化合物用于配体结合的可用性的重要性。在微量滴定分析中，随着磷脂酰胆碱（PC）酰基链长度的缩短，VT1和VT2c与Gb3的结合均增加，这可能导致Gb3暴露增加。在脂质体分析中，VT1的结合受到类似调节，然而VT2c结合的影响更为复杂，并不遵循碳水化合物暴露增加的简单函数关系。早期研究表明C22:1和C18:1 Gb3脂肪酸同系物是含Gb3受体的脂质体的首选，但在C14PC脂质体中，与C22:1 Gb3（而非C18:1 Gb3）的结合增加，使得这种Gb3种类现在成为两种毒素的首选受体。在C14PC存在下，维罗毒素/Gb3同系物结合选择性的这种变化在微量滴定双层形式中并未发生。这些结果与我们的提议一致，即这些毒素识别Gb3寡糖上的不同表位。我们推断，在人工双层中，这些表位用于毒素结合的相对可用性不仅受Gb3和PC脂肪酸酰基链长度差异导致的暴露影响，还受双层结构呈现的物理模式影响。这种酰基链长度差异在支持的双层膜中有更显著的影响，而只有最大的差异影响脂质体中Gb3受体功能。磷脂对糖脂碳水化合物用于受体功能的可及性的调节基础可能很复杂，将涉及相分离、凝胶/液晶转变、双层内的堆积和横向流动性，这表明在评估细胞中糖脂受体功能时应考虑这些参数。
