Department of Molecular Biology, University of Salzburg, Salzburg 5020, Austria.
Department of Pathophysiology, Medical University of Vienna, Vienna 1090, Austria.
Int J Mol Sci. 2017 Oct 16;18(10):2156. doi: 10.3390/ijms18102156.
Birch pollen allergy affects more than 20% of the European allergic population. On a molecular level, birch pollen allergy can be linked to the two dominant allergens Bet v 1 and Bet v 2. Bet v 2 belongs to the profilin family, which is abundant in the plant kingdom. Importantly, the homologous plant profilins have a conserved cysteine motif with a currently unknown functional relevance. In particular, it is unknown whether the motif is relevant for disulfide formation and to what extent it would affect the profilins' structural, functional and immunological properties. Here we present crystal structures of Bet v 2 in the reduced and the oxidized state, i.e., without and with a disulfide bridge. Despite overall structural similarity, the two structures distinctly differ at their termini which are stabilized to each other in the oxidized, i.e., disulfide-linked state. These structural differences translate into differences in their proteolytic resistance. Whereas the oxidized Bet v 2 is rather resistant towards the endolysosomal protease cathepsin S, it is rapidly degraded in the reduced form. By contrast, both Bet v 2 forms exhibit similar immunological properties as evidenced by their binding to IgE antibodies from birch pollen allergic patients and by their ability to trigger histamine release in a humanized rat basophilic leukemia cells (RBL) assay, independent of the presence or absence of the disulfide bridge. Taken together our findings suggest that the oxidized Bet v 2 conformation should be the relevant species, with a much longer retention time to trigger immune responses.
桦树花粉过敏影响了超过 20%的欧洲过敏人群。从分子水平上看，桦树花粉过敏可以与两种主要过敏原 Bet v 1 和 Bet v 2 联系起来。Bet v 2 属于丝氨酸蛋白酶抑制剂家族，在植物界中含量丰富。重要的是，同源植物丝氨酸蛋白酶抑制剂具有保守的半胱氨酸模体，目前其功能相关性尚不清楚。特别是，目前尚不清楚该模体是否与二硫键形成有关，以及它在多大程度上会影响丝氨酸蛋白酶抑制剂的结构、功能和免疫学特性。在这里，我们展示了还原态和氧化态（即无和有二硫键）下的 Bet v 2 晶体结构。尽管整体结构相似，但这两种结构在其末端明显不同，在氧化态（即二硫键连接）下，它们彼此稳定。这些结构差异转化为它们对蛋白水解的抗性差异。尽管氧化的 Bet v 2 对溶酶体蛋白酶组织蛋白酶 S 具有较强的抗性，但它在还原形式下会迅速降解。相比之下，氧化和还原形式的 Bet v 2 均表现出相似的免疫学特性，这表现在它们与桦树花粉过敏患者的 IgE 抗体结合，以及在人源化大鼠嗜碱性白血病细胞 (RBL) 测定中触发组胺释放的能力，而与二硫键的存在与否无关。总的来说，我们的研究结果表明，氧化的 Bet v 2 构象应该是相关的物种，其引发免疫反应的保留时间要长得多。
