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首页> 外文期刊>Journal of Molecular Biology >Crystal Structures of Limulus SAP-Like Pentraxin Reveal Two Molecular Aggregations
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Crystal Structures of Limulus SAP-Like Pentraxin Reveal Two Molecular Aggregations

机译：LiSAP戊型毒素的晶体结构揭示了两种分子聚集。

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The serum-amyloid-P-component-like pentraxin from Limulus polyphemus, a recently discovered pentraxin species and important effector protein of the hemolymph immune system, displays two distinct doubly stacked cyclic molecular aggregations, heptameric and octameric. The refined three-dimensional structures determined by X-ray crystallography, both based on the same cDNA sequence, show that each aggregate is constructed from a similar dimer of protomers, which is repeated to make up the ring structure. The native octameric form has been refined at a resolution of 3 A, the native heptameric form at 2.3 A, and the phosphoethanolamine (PE)-bound octameric form at 2.7 A. The existence of the hitherto undescribed heptameric form was confirmed by single-particle analysis using cryo-electron microscopy. In the native structures, the calcium-binding site is similar to that in human pentraxins, with two calcium ions bound in each subunit. Upon binding PE, however, each subunit binds a third calcium ion, with allthree calcium ions contributing to the binding and orientation of the bound phosphate group within the ligand-binding pocket. While the phosphate is well-defined in the electron density, the ethanolamine group is poorly defined, suggesting structural and binding variabilities of this group. Although sequence homology with human serum amyloid P component is relatively low, structural homology is high, with very similar overall folds and a common affinity for PE. This is due, in part, to a "topological" equivalence of side-chain position. Identical side chains that are important in both function and fold, from different regions of the sequence in human and Limulus structures, occupy similar space within the overall subunit fold. Sequence and structure alignment, based on the refined three-dimensional structures presented here and the known horseshoe crab pentraxin sequences, suggest that adaptation and refinement of C-reactive-protein-mediated immune responses in these ancient creatures lacking antibody-based immunity are based on adaptation by gene duplication#
机译：来自poly鱼的血清淀粉样蛋白-P-组分样五环素是最近发现的五环素物种和血淋巴免疫系统的重要效应蛋白，它显示出两种不同的双重堆积的环状分子聚集体，七聚体和八聚体。通过X射线晶体学确定的精制三维结构（均基于相同的cDNA序列）表明，每个聚集体均由相似的二聚体前驱体构成，并重复构成环结构。天然八聚体形式已以3 A的分辨率精制，天然七聚体形式为2.3 A的精制，磷酸乙醇胺（PE）结合的八聚体形式为2.7A。迄今未描述的七聚体形式的存在已通过单颗粒证实低温电子显微镜分析。在天然结构中，钙结合位点与人五味毒素相似，每个亚基中都结合有两个钙离子。然而，在结合PE时，每个亚基结合第三个钙离子，所有三个钙离子有助于配体结合袋中结合的磷酸基团的结合和取向。尽管磷酸盐在电子密度中定义明确，但乙醇胺基团定义不明确，表明该基团的结构和结合变异性。尽管与人血清淀粉样蛋白P组分的序列同源性相对较低，但结构同源性较高，具有非常相似的总体折叠倍数和对PE的共同亲和力。这部分归因于侧链位置的“拓扑”等价。来自人和Li结构中序列不同区域的功能和折叠均重要的相同侧链在整个亚基折叠中占据相似的空间。基于此处介绍的精确三维结构和已知的horse螃蟹五味素序列的序列和结构比对表明，在这些缺乏基于抗体的免疫力的古老生物中，C反应蛋白介导的免疫反应的适应和改进是基于基因复制适应＃

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