Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, SC 29208, USA; Department of Molecular Physiology and Biological Physics, University of Virginia, 1340 Jefferson Park Avenue, Charlottesville, VA 22908, USA.
Mol Immunol. 2013 Dec;56(4):794-803. doi: 10.1016/j.molimm.2013.07.004. Epub 2013 Aug 23.
The allergen Act d 11, also known as kirola, is a 17 kDa protein expressed in large amounts in ripe green and yellow-fleshed kiwifruit. Ten percent of all kiwifruit-allergic individuals produce IgE specific for the protein. Using X-ray crystallography, we determined the first three-dimensional structures of Act d 11, produced from both recombinant expression in Escherichia coli and from the natural source (kiwifruit). While Act d 11 is immunologically correlated with the birch pollen allergen Bet v 1 and other members of the pathogenesis-related protein family 10 (PR-10), it has low sequence similarity to PR-10 proteins. By sequence Act d 11 appears instead to belong to the major latex/ripening-related (MLP/RRP) family, but analysis of the crystal structures shows that Act d 11 has a fold very similar to that of Bet v 1 and other PR-10 related allergens regardless of the low sequence identity. The structures of both the natural and recombinant protein include an unidentified ligand, which is relatively small (about 250 Da by mass spectrometry experiments) and most likely contains an aromatic ring. The ligand-binding cavity in Act d 11 is also significantly smaller than those in PR-10 proteins. The binding of the ligand, which we were not able to unambiguously identify, results in conformational changes in the protein that may have physiological and immunological implications. Interestingly, the residue corresponding to Glu45 in Bet v 1 (Glu46), which is important for IgE binding to the birch pollen allergen, is conserved in Act d 11, even though it is not in other allergens with significantly higher sequence identity to Bet v 1. We suggest that the so-called Gly-rich loop (or P-loop), which is conserved in all PR-10 allergens, may be responsible for IgE cross-reactivity between Bet v 1 and Act d 11.
过敏原 Act d 11，也称为 kirola，是一种在成熟的绿色和黄色果肉猕猴桃中大量表达的 17 kDa 蛋白。所有猕猴桃过敏个体中有 10%会产生针对该蛋白的特异性 IgE。我们使用 X 射线晶体学确定了 Act d 11 的前三个三维结构，这些结构分别来自大肠杆菌的重组表达和天然来源（猕猴桃）。虽然 Act d 11 在免疫学上与桦树花粉过敏原 Bet v 1 和其他与发病机制相关的蛋白家族 10（PR-10）成员相关，但它与 PR-10 蛋白的序列相似性较低。根据序列，Act d 11 似乎属于主要乳胶/成熟相关（MLP/RRP）家族，但晶体结构分析表明，无论序列同一性较低，Act d 11 的折叠都非常类似于 Bet v 1 和其他 PR-10 相关过敏原。天然和重组蛋白的结构都包括一个未被识别的配体，该配体相对较小（通过质谱实验约为 250 Da），很可能含有一个芳香环。Act d 11 的配体结合腔也明显小于 PR-10 蛋白。我们无法明确识别的配体结合导致蛋白构象发生变化，这可能具有生理和免疫学意义。有趣的是，Bet v 1 中与 IgE 结合重要的Glu45 对应残基（Glu46）在 Act d 11 中是保守的，尽管在与 Bet v 1 具有更高序列同一性的其他过敏原中不是这样。我们认为，所有 PR-10 过敏原中都保守的所谓 Gly-rich 环（或 P 环）可能是导致 Bet v 1 和 Act d 11 之间 IgE 交叉反应的原因。
