A symbiosis-related lectin, SLL-2, from the octocoral Sinularia lochmodes, distributes densely on the cell surface of microalgae, Symbiodinium sp., an endosymbiotic dinoflagellate of the coral, and is also shown to be a chemical cue that transforms dinoflagellates into a nonmotile (coccoid) symbiotic state. SLL-2 binds to the sugar chain of the molecule similar to Forssman antigen pentasaccharide (GalNAcα1-3GalNAcβ1-3 Galα1-4 Galβ1-4Glc) on the surface of microalgae with high affinity. Here we report the crystal structure of the complex between SLL-2 and Forssman antigen tetrasaccharide (GalNAcα1-3GalNAcβ1-3 Galα1-4 Galβ) at 3.4 Å resolution. In an asymmetric unit of the crystal, there are two hexameric molecules with totally 12 sugar recognition sites. At 9 in 12 sites, the first and second saccharides of the Forssman antigen tetrasaccharide bind directly to galactopyranoside binding site of SLL-2, whereas the third and fourth saccharides have no interaction with the SLL-2 hexameric molecule that binds the first saccharide. The sugar chain bends at α-1,4-glycosidic linkage between the third and fourth saccharides toward the position that we defined as a pyranoside binding site in the crystal structure of the complex between SLL-2 and GalNAc. The structure allowed us to suggest a possible binding mode of the Forssman antigen pentasaccharide to SLL-2. These observations support our hypothesis that the binding of SLL-2 to the cell surface sugars of zooxanthella in a unique manner might trigger some physiological changes of the cell to adapt symbiosis with the host coral.

中文翻译：

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八叶凝集素SLL-2与Forssman抗原四糖复合的晶体结构。

一种与共生相关的凝集素SLL-2，来自八叶形的Sinularia lochmodes，密集地分布在微藻Symbiodinium sp。的细胞表面上，Symbiodinium sp。是珊瑚的一种共生鞭毛鞭毛的鞭毛，它也被证明是一种化学提示，可将鞭毛鞭毛藻转变为鞭毛藻。非运动性（球状）共生状态。SLL-2以高亲和力与微藻表面上的类似于Forssman抗原五糖（GalNAcα1-3GalNAcβ1-3Galα1-4Galβ1-4Glc）的分子的糖链结合。在这里，我们报告了SLL-2和Forssman抗原四糖（GalNAcα1-3GalNAcβ1-3Galα1-4Galβ）之间的复合物的晶体结构，其分辨率为3.4。在晶体的不对称单元中，有两个带有总共12个糖识别位点的六聚体分子。在12个站点中的9个站点中，Forssman抗原四糖的第一和第二糖直接与SLL-2的吡喃半乳糖苷结合位点结合，而第三和第四糖与与第一糖结合的SLL-2六聚体分子没有相互作用。糖链在第三和第四种糖之间的α-1,4-糖苷键处向我们定义为SLL-2和GalNAc之间复合物晶体结构中吡喃糖苷结合位点的位置弯曲。该结构使我们能够提出Forssman抗原五糖与SLL-2的可能结合方式。这些观察结果支持了我们的假设，即SLL-2以独特的方式与虫黄藻的细胞表面糖结合可能会触发细胞的某些生理变化，以适应与宿主珊瑚的共生。