Tardigrades' cytoplasmic abundant heat soluble proteins serve as membrane protectors during dehydration

Tardigrades possess extraordinary tolerance to environmental stresses. Recent studies revealed that cytoplasmic and secreted abundant heat soluble proteins (CAHSs and SAHSs) contributed to such extremotolerance. We examined 39 CAHSs and 28 SAHSs from three representative tardigrade species and identified a conserved central region and highly variable terminal regions in both groups. Phylogenetic analysis suggested that the two groups had distinct sequences despite functional similarity. AlphaFold predicted that CAHSs' central region formed a long and amphiphilic α-helix whereas SAHSs' folds into β-barrel. As dehydration caused protein concentration increase, we simulated CAHS oligomerization and found that they preferably dimerized via their central helix motifs. Examination of CAHS dimers revealed a strong inter-helix interaction. The anti-parallel helical dimers resemble lipid-interacting proteins such as ApoE. Empirical tests using mammalian cells expressing the representative RvCAHS3 showed that CAHSs concentrated on intracellular membranes upon dehydration and significantly improved cell survival measured by the stimulation-evoked Ca²⁺ release from internal stores like the endoplasmic reticulum. Hence, CAHSs incline to dimerize and consequently form meshes on intracellular membranes, which protects the integrity and the functionality of membrane-enclosed organelles. Our finding implicates membrane-based strategies to preserve biomolecules, cells, and tissues under challenging conditions or for energy efficient transportation.