Department of Microbiology and Immunology, Chang-Gung University, Tao-yuan, Taiwan.
Biochimie. 2010 Feb;92(2):147-56. doi: 10.1016/j.biochi.2009.11.001. Epub 2009 Nov 12.
Abrin-a is the most toxic fraction of lectins isolated from Abrus precatorius seeds and belongs to the family of type 2 ribosome inactivating proteins (RIP). This toxin may act as a defense molecule in plants against viruses, fungi and insects, where attachment of abrin-a to the exposed glycans on the surface of target cells is the crucial and initial step of its cytotoxicity. Although it has been studied for over four decades, the recognition factors involved in abrin-a-carbohydrate interaction remains to be clarified. In this study, roles of mammalian glyco-structural units, ligand clusters and polyvalency in abrin-a recognition were comprehensively analyzed by enzyme-linked lectinosorbent binding and inhibition assays. The results indicate that: (i) this toxin prefers oligosaccharides having alpha-anomer of galactose (Gal) at the non-reducing terminal than the corresponding beta-anomer; (ii) Galalpha1-3Galalpha1- (B(alpha)), Galalpha1-4Gal (E), Galbeta1-3GalNAc (T) and Galbeta1-3/4GlcNAc (I/II) related oligosaccharides were the active glyco-structural units; (iii) tri-antennary II(beta), prepared from N-glycan of asialo fetuin, played a dominant role in recognition; (iv) many high-density polyvalent I(beta)/II(beta) and E(beta) glycotopes enhanced the reactivity; (v) the carbohydrate recognition domain of abrin-a is proposed to be a combination of a small cavity type of Gal as major site and a groove type of additional one to tetrasaccharides as subsites with a preference of alpha1-3/4/6Gal, beta1-3GalNAc, beta1-3/4/6GlcNAc, beta1-4/6Glc, beta1-3DAra and beta1-4Man as subterminal sugars; (vi) size of the carbohydrate recognition domain may be as large enough to accommodate a linear pentasaccharide and complementary to Galalpha1-3Galbeta1-4GlcNAc beta1-3Galbeta1-4Glc (gailipenta) sequence. A comparison of the recognition factors and combining sites of abrin-a with ricin, another highly toxic lectin, was also performed to further understand the differences in recognition factors between these two type 2 RIPs.
相思豆毒素 A 是从相思豆种子中分离得到的凝集素中最具毒性的组分，属于 2 型核糖体失活蛋白（RIP）家族。这种毒素可能在植物中作为一种防御分子，抵御病毒、真菌和昆虫的侵害，其中相思豆毒素 A 与靶细胞表面暴露的糖链的附着是其细胞毒性的关键和初始步骤。尽管已经研究了四十多年，但与相思豆毒素 A-碳水化合物相互作用相关的识别因子仍有待阐明。在这项研究中，通过酶联凝集素结合和抑制实验全面分析了哺乳动物糖结构单元、配体簇和多价性在相思豆毒素 A 识别中的作用。结果表明：（i）与相应的β异构体相比，这种毒素更喜欢非还原末端具有α异构体半乳糖（Gal）的寡糖；（ii）Galα1-3Galα1-（B（α））、Galα1-4Gal（E）、Galβ1-3GalNAc（T）和 Galβ1-3/4GlcNAc（I/II）相关寡糖是活性糖结构单元；（iii）由无唾液胎球蛋白 N-糖制备的三触角 II（β）在识别中起主导作用；（iv）许多高密度多价 I（β）/II（β）和 E（β）糖基显著增强了反应性；（v）推测相思豆毒素 A 的碳水化合物识别域是由主要位点的小空腔型 Gal 和额外的槽型组成，对四糖的亚位点具有 α1-3/4/6Gal、β1-3GalNAc、β1-3/4/6GlcNAc、β1-4/6Glc、β1-3DAra 和 β1-4Man 作为末端糖的偏好；（vi）碳水化合物识别域的大小可能足以容纳一个线性五糖，并与 Galα1-3Galβ1-4GlcNAcβ1-3Galβ1-4Glc（gailipenta）序列互补。还比较了相思豆毒素 A 与蓖麻毒素这两种 2 型 RIP 之间的识别因子和结合位点，以进一步了解这两种 RIP 之间识别因子的差异。
