Crystal structure of a dimeric mannose-specific agglutinin from garlic: Quaternary association and carbohydrate specificity
Crystal structure of a dimeric mannose-specific agglutinin from garlic: Quaternary association and carbohydrate specificity
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DOI：
10.1006/jmbi.1998.2353
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发表时间：
1999-01-22
影响因子：
5.6
通讯作者：
Vijayan, M
中科院分区：
文献类型：
--
作者：
Chandra, NR;Ramachandraiah, G;Vijayan, M
从大蒜鳞茎中分离到一种甘露糖特异性凝集素，在α- d -甘露糖过量存在下，它具有晶体结构，空间群C2，晶胞参数为a= 203.24，b =43.78，c= 79.27 π，β=112.4°，不对称单元中有两个二聚体。收集X射线衍射数据，标称分辨率为2.4 μ m，并通过分子置换解析结构。该结构的R因子为22.6%，Rfreeof为27.8%，显示出与雪花莲凝集素相似的β-棱柱II折叠，包括三个反平行的四链β-折叠，排列为12链β-桶，内部近似3重对称。然而，这种凝集素是二聚体，不同于雪花莲凝集素，雪花莲凝集素以四聚体存在，尽管它们之间具有高度的序列相似性。两种结构的比较揭示了大蒜凝集素中的一些取代，这些取代使其稳定为二聚体并防止四聚体形成。在每个亚基上已经鉴定出三个甘露糖分子。此外，观察到每个二聚体中另一个可能的甘露糖分子的电子密度，导致每个二聚体中总共有七个甘露糖分子。尽管雪花莲和大蒜凝集素的甘露糖结合位点和整体结构相似，但它们对糖蛋白如GP 120的特异性差异很大。这些差异出现，在某种程度上，是一个直接的后果，寡聚化的差异，这意味着在四元关联的变化可能是一种模式，实现寡糖特异性灯泡凝集素。
A mannose-specific agglutinin, isolated from garlic bulbs, has been crystallized in the presence of a large excess of α- d -mannose, in space group C 2 and cell dimensions, a=203.24,b =43.78, c=79.27Å, β=112.4°, with two dimers in the asymmetric unit. X-ray diffraction data were collected up to a nominal resolution of 2.4Å and the structure was solved by molecular replacement. The structure, refined to an R -factor of 22.6% and an Rfreeof 27.8% reveals a β-prism II fold, similar to that in the snowdrop lectin, comprising three antiparallel four-stranded β-sheets arranged as a 12-stranded β-barrel, with an approximate internal 3-fold symmetry. This agglutinin is, however, a dimer unlike snowdrop lectin which exists as a tetramer, despite a high degree of sequence similarity between them. A comparison of the two structures reveals a few substitutions in the garlic lectin which stabilise it into a dimer and prevent tetramer formation. Three mannose molecules have been identified on each subunit. In addition, electron density is observed for another possible mannose molecule per dimer resulting in a total of seven mannose molecules in each dimer. Although the mannose binding sites and the overall structure are similar in the subunits of snowdrop and garlic lectin, their specificities to glycoproteins such as GP120 vary considerably. These differences appear, in part, to be a direct consequence of the differences in oligomerisation, implying that variation in quaternary association may be a mode of achieving oligosaccharide specificity in bulb lectins.