Abstract
Anhydrobiosis, one of the most extensively studied forms of cryptobiosis, is induced in certain organisms as a response to desiccation. Anhydrobiotic species has been hypothesized to produce substances that can protect their biological components and/or cell membranes without water. In extremotolerant tardigrades, highly hydrophilic and heat-soluble protein families, cytosolic abundant heat-soluble (CAHS) proteins, have been identified, which are postulated to be integral parts of the tardigrades’ response to desiccation. In this study, to elucidate these protein functions, we performed in vitro and in vivo characterizations of the reversible self-assembling property of CAHS1 protein, a major isoform of CAHS proteins from Ramazzottius varieornatus, using a series of spectroscopic and microscopic techniques. We found that CAHS1 proteins homo-oligomerized via the C-terminal α-helical region and formed a hydrogel as their concentration increased. We also demonstrated that the overexpressed CAHS1 proteins formed condensates under desiccation-mimicking conditions. These data strongly suggested that, upon drying, the CAHS1 proteins form oligomers and eventually underwent sol–gel transition in tardigrade cytosols. Thus, it is proposed that the CAHS1 proteins form the cytosolic fibrous condensates, which presumably have variable mechanisms for the desiccation tolerance of tardigrades. These findings provide insights into molecular strategies of organisms to adapt to extreme environments.
Highlights
Anhydrobiosis, one of the most extensively studied forms of cryptobiosis, is induced in certain organisms as a response to desiccation
The dried CAHS1 proteins on a carbon grid were visualized by transmission electron microscopy (TEM), showing that fibril structures were formed (Fig. 1A)
The infrared (IR) spectroscopic analysis showed that the dehydrated CAHS1 protein adopted an α-helical structure, which was reversibly disrupted upon hydration (Fig. 1B,C)
Summary
Anhydrobiosis, one of the most extensively studied forms of cryptobiosis, is induced in certain organisms as a response to desiccation. It is proposed that the CAHS1 proteins form the cytosolic fibrous condensates, which presumably have variable mechanisms for the desiccation tolerance of tardigrades. These findings provide insights into molecular strategies of organisms to adapt to extreme environments. Trehalose is assumed to exert its protective functions through water replacement and vitrification; these are two distinct but not mutually exclusive mechanisms in protecting the organisms from the harmful effects of d esiccation[12,15] In the former mechanism, accumulating trehalose molecules extensively interact with biomolecular surfaces through hydrogen bonds by replacing the water molecules. These proteins are distinct from LEA proteins based on their conserved sequences, they share some similarities with LEA proteins in terms of the bioinformatic prediction of high propensity for α-helix formation[32,33]
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