Kita Akiko, Jimbo Mitsuru, Sakai Ryuichi, Morimoto Yukio, Takeuchi Ryota, Tanaka Hiroshi, Takahashi Takashi, Miki Kunio
Research Reactor Institute, Kyoto University, Kumatori, Sennan, Osaka 590-0494, Japan.
School of Marine Biosciences, Kitasato University, 1-15-1, Kitasato, Minami, Sagamihara, Kanagawa 252-0373, Japan.
Glycobiology. 2017 Aug 1;27(8):696-700. doi: 10.1093/glycob/cwx043.
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.
一种来自八放珊瑚柔指软珊瑚的共生相关凝集素SLL-2，密集分布于共生甲藻（Symbiodinium sp.）的细胞表面，共生甲藻是珊瑚的一种内共生双鞭毛藻，并且它还被证明是一种化学信号，可将双鞭毛藻转变为非运动性（球状）共生状态。SLL-2以高亲和力与共生甲藻表面类似于福斯曼抗原五糖（GalNAcα1-3GalNAcβ1-3 Galα1-4 Galβ1-4Glc）的分子糖链结合。在此，我们报道了SLL-2与福斯曼抗原四糖（GalNAcα1-3GalNAcβ1-3 Galα1-4 Galβ）复合物在3.4 Å分辨率下的晶体结构。在晶体的一个不对称单元中，有两个六聚体分子，共有12个糖识别位点。在12个位点中的9个位点，福斯曼抗原四糖的第一和第二个糖直接与SLL-2的吡喃半乳糖苷结合位点结合，而第三和第四个糖与结合第一个糖的SLL-2六聚体分子没有相互作用。糖链在第三和第四个糖之间的α-1,4-糖苷键处向我们在SLL-2与GalNAc复合物晶体结构中定义为吡喃糖苷结合位点的位置弯曲。该结构使我们能够提出福斯曼抗原五糖与SLL-2可能的结合模式。这些观察结果支持了我们的假设，即SLL-2以独特方式与虫黄藻细胞表面糖的结合可能触发细胞的一些生理变化，以适应与宿主珊瑚的共生。
