Some tardigrades can survive extremely desiccated conditions through transition into a state called anhydrobiosis. Anhydrobiotic tardigrades have proteins unique to them and they are thought to be keys to the understanding of unusual desiccation resistance. In fact, previous transcriptome data show that several tardigrade‐specific proteins are significantly upregulated under desiccated conditions. However, their physiological roles and chemical properties have been ambiguous because they show low or no similarity of amino acid sequences to proteins found in other organisms. Here, we report a crystal structure of one of such proteins. This protein shows a β‐sandwich structure composed of 8 β‐strands, three Ca²⁺‐binding sites, and hydrophobic residues on Ca²⁺‐binding (CBD) loops, which resemble characteristics of C2 domain proteins. We therefore conveniently describe this protein as tardigrade C2 domain protein (TC2P). Because the C2 domain functions as a Ca²⁺‐mediated membrane docking module, which is related to signal transduction or membrane trafficking, TC2Ps may play a role in Ca²⁺‐triggered phenomenon under desiccated situations. Our finding provides not only structural insights into a newly discovered desiccation‐related protein family but also insights into the evolution and diversity of C2 domain proteins.

中文翻译：

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对在缓步动物中特别发现的 C2 结构域蛋白的结构洞察

一些缓步动物可以通过过渡到称为脱水生物的状态在极度干燥的条件下生存。无水生物缓步动物具有独特的蛋白质，它们被认为是理解异常抗干燥性的关键。事实上，先前的转录组数据表明，几种缓步动物特异性蛋白质在干燥条件下显着上调。然而，它们的生理作用和化学性质一直是模棱两可的，因为它们与其他生物体中发现的蛋白质的氨基酸序列相似性很低或没有相似性。在这里，我们报告了其中一种蛋白质的晶体结构。该蛋白质显示出由 8 个 β 链、三个 Ca ²⁺结合位点和 Ca ^{2+上的疏水残基组成的 β 夹心结构}结合（CBD）环，类似于 C2 结构域蛋白的特征。因此，我们方便地将这种蛋白质描述为缓步动物 C2 结构域蛋白 (TC2P)。由于C2结构域作为Ca ²⁺介导的膜对接模块，与信号转导或膜运输有关，因此TC2Ps可能在干燥条件下的Ca ²⁺触发现象中发挥作用。我们的发现不仅提供了对新发现的干燥相关蛋白家族的结构见解，还提供了对 C2 域蛋白进化和多样性的见解。