Department of Biological Sciences, Graduate School of Science, The University of Tokyo, Tokyo, Japan.
PLoS One. 2012;7(8):e44209. doi: 10.1371/journal.pone.0044209. Epub 2012 Aug 28.
Tardigrades are able to tolerate almost complete dehydration by reversibly switching to an ametabolic state. This ability is called anhydrobiosis. In the anhydrobiotic state, tardigrades can withstand various extreme environments including space, but their molecular basis remains largely unknown. Late embryogenesis abundant (LEA) proteins are heat-soluble proteins and can prevent protein-aggregation in dehydrated conditions in other anhydrobiotic organisms, but their relevance to tardigrade anhydrobiosis is not clarified. In this study, we focused on the heat-soluble property characteristic of LEA proteins and conducted heat-soluble proteomics using an anhydrobiotic tardigrade. Our heat-soluble proteomics identified five abundant heat-soluble proteins. All of them showed no sequence similarity with LEA proteins and formed two novel protein families with distinct subcellular localizations. We named them Cytoplasmic Abundant Heat Soluble (CAHS) and Secretory Abundant Heat Soluble (SAHS) protein families, according to their localization. Both protein families were conserved among tardigrades, but not found in other phyla. Although CAHS protein was intrinsically unstructured and SAHS protein was rich in β-structure in the hydrated condition, proteins in both families changed their conformation to an α-helical structure in water-deficient conditions as LEA proteins do. Two conserved repeats of 19-mer motifs in CAHS proteins were capable to form amphiphilic stripes in α-helices, suggesting their roles as molecular shield in water-deficient condition, though charge distribution pattern in α-helices were different between CAHS and LEA proteins. Tardigrades might have evolved novel protein families with a heat-soluble property and this study revealed a novel repertoire of major heat-soluble proteins in these anhydrobiotic animals.
水熊虫能够通过可逆地切换到非代谢状态来耐受几乎完全的脱水。这种能力被称为脱水休眠。在脱水休眠状态下，水熊虫可以承受各种极端环境，包括太空，但它们的分子基础在很大程度上仍然未知。晚期胚胎丰富蛋白（LEA）是热可溶性蛋白，可以防止其他脱水生物中脱水条件下的蛋白质聚集，但它们与水熊虫脱水休眠的相关性尚不清楚。在这项研究中，我们专注于 LEA 蛋白的热可溶性特性，并使用一种脱水休眠的水熊虫进行了热可溶性蛋白质组学研究。我们的热可溶性蛋白质组学鉴定了五种丰富的热可溶性蛋白。它们都与 LEA 蛋白没有序列相似性，并形成了两个具有不同亚细胞定位的新型蛋白质家族。根据它们的定位，我们将它们命名为细胞质丰富的热可溶性（CAHS）和分泌丰富的热可溶性（SAHS）蛋白家族。这两个蛋白家族在水熊虫中是保守的，但在其他门中没有发现。虽然 CAHS 蛋白在水合状态下是无规则卷曲的，SAHS 蛋白富含β-折叠，但在缺水条件下，这两种蛋白都像 LEA 蛋白一样改变构象为α-螺旋结构。CAHS 蛋白中 19 个氨基酸重复的两个保守重复序列能够在α-螺旋中形成两亲性条纹，表明它们在缺水条件下作为分子盾牌的作用，尽管 CAHS 和 LEA 蛋白的α-螺旋中电荷分布模式不同。水熊虫可能已经进化出具有热可溶性特性的新型蛋白家族，本研究揭示了这些脱水动物中主要热可溶性蛋白的新型谱。
