The Water Bear's Alchemy: How Tardigrades Turn Cells into Life-Saving Gel

There is an animal alive today — barely visible without a microscope, lurching around on eight stubby legs — that can do something no complex animal should be able to do. It can dry out completely. Not partially, not dangerously: completely. Its body loses all but a few percent of its water. It stops breathing. Its metabolism drops to 0.01% of normal. And then, sometimes years later, when moisture returns, it wakes up.

This is the tardigrade. And for decades, the central mystery of its remarkable life was: how?

We knew the what. We didn’t know the how. The answer, it turns out, lies in a strange family of proteins found nowhere else in the animal kingdom — proteins that transform the tardigrade’s insides into something like a protective gel or glass, locking cellular machinery in suspended animation until water returns. And thanks to a 2025 crystal structure published in Angewandte Chemie, we finally have atomic-level insight into how they do it.

The Wrong Answer Was Almost Right

If you want to know how an organism survives being dried out, a good place to start is looking for sugars. Many desiccation-tolerant creatures — the brine shrimp, the nematode Caenorhabditis elegans under stress, certain plant seeds — accumulate a disaccharide called trehalose when they dry out. Trehalose forms a glass-like matrix at low humidity, embedding proteins and membranes in an amorphous solid where they can’t unfold, aggregate, or degrade. It’s an elegant solution, and it works.

So when scientists looked at tardigrades — nature’s most extreme desiccation survivors, animals that can take the vacuum of space — the working assumption was: lots and lots of trehalose.

It wasn’t there. Or barely. Tardigrades have very little trehalose compared to organisms like brine shrimp, which lean on it heavily. The sugar can’t be the whole story.

Enter the CAHS Proteins

In 2017, Thomas Boothby and colleagues at the University of North Carolina Chapel Hill published a study in Molecular Cell that changed the field’s understanding entirely. The key wasn’t a small sugar molecule. It was a completely novel family of proteins that had never been seen before — and that exist only in tardigrades.

They called them Cytoplasmic Abundant Heat-Soluble (CAHS) proteins. The name gives you a quick portrait: these proteins are found in the cytoplasm, they’re present in enormous quantities, and they remain soluble even when you heat the cell extract to temperatures that would denature most proteins.

What they discovered was deeply strange. CAHS proteins are intrinsically disordered proteins — a class of proteins that, unlike the precisely folded enzymes we learn about in textbooks, have no fixed three-dimensional structure in solution. They flop around loosely. They’re essentially molecular strings.

But here’s the twist: when Boothby’s team dried these proteins out, something remarkable happened. They formed a glass. A vitrified, amorphous solid that — crucially — embedded and protected other cellular proteins and structures during desiccation. And when water was added back, the glass dissolved. The proteins unfolded back into their disordered state. The process was fully reversible.

The paper showed that tardigrade cells expressing CAHS proteins could survive desiccation where cells without them could not. It wasn’t trehalose doing the heavy lifting. It was these weird, floppy, previously unknown proteins.

The field had found its answer — or so it thought.

A More Complex Picture

As often happens in science, the first answer turned out to be one chapter in a longer story.

In 2021, a team led by Maho Yagi-Utsumi at the RIKEN Center for Biosystems Dynamics Research published something unexpected in Scientific Reports. CAHS proteins weren’t simply forming a featureless glass when dried. A key member of the family, CAHS1, was assembling into fibers — long, structured filaments that formed a fibrous condensate during desiccation and then dissolved back into disordered monomers when water returned. This was a phase transition more elaborate than simple glass formation.

Also in 2021, a parallel Molecular Cell commentary by Kazuharu Arakawa and Keiji Numata challenged the simple glass-transition model, noting that the picture was more nuanced and the exact mechanism of protection was still unresolved.

A more complete picture began to emerge in 2022, when Kenny Nguyen, Hugo Tapia, Thomas Boothby and colleagues published work in Communications Biology that solved another mystery. Yes, tardigrades have little trehalose — but they’re not ignoring it entirely. At the small amounts they do accumulate, trehalose and CAHS proteins act synergistically. The combination, at naturally occurring ratios, protects far better than either alone. The sugar and the protein are partners, not competitors. The absence of massive trehalose doesn’t mean trehalose is irrelevant; it means the tardigrade has evolved a clever two-ingredient system where small amounts of sugar cooperate with large amounts of protein.

Then came a puzzle about mechanism. If CAHS proteins form gels during desiccation, you might assume it’s the gel state that’s protective — that physically entangling everything into a viscous network stops proteins from aggregating or membranes from collapsing. But experiments began to challenge this assumption too. Research published in Scientific Reports in 2023 showed that CAHS D doesn’t work by retaining water in a hydrogel — even in the fully gelled state, it doesn’t measurably reduce water loss. The protection appears to be structural, not hydrological.

This set up a beautiful experiment, published in Protein Science in September 2025 by Nguyen, Biswas, and colleagues. They tested whether the solution state and the gel state each protect different things. The answer was yes: CAHS D’s protective capacity against desiccation-induced damage is not simply a property of being a gel or a liquid. Both phases have distinct protective roles, and the phase transition itself — the transformation between liquid-like disorder and gel-like condensate — may be a feature, not just an intermediate. Each state guards a different aspect of cellular biology during the drying process.

What the Fibers Actually Look Like

By late 2025, the field had strong mechanistic evidence that CAHS proteins go through a phase transition, that fibers form, and that those fibers matter. But nobody had solved the three-dimensional structure of the fibrils.

That changed with a paper published in Angewandte Chemie International Edition in late 2025 by Anas Malki, Jean-Marie Teulon, Emmi Mikkola, Damien Maurin, and Jean-Luc Pellequer. Using X-ray crystallography (with the structure deposited to the Protein Data Bank as 9RKP), atomic force microscopy, and electron microscopy, they solved the crystal structure of the fibril core of CAHS-8 — a member of the CAHS family from the well-studied tardigrade Hypsibius exemplaris.

What they found was architecturally unusual.

CAHS-8 in solution is intrinsically disordered, as expected. But as it assembles into fibrils, a 101-residue stretch forms a single, long alpha-helix. Two of these helices pair up into a coiled-coil dimer — a classic structural motif in biology, where two helices wrap around each other in a gentle supercoil. So far, not too surprising.

Here’s where it gets interesting. The coiled-coil formed by CAHS-8 is atypical. It covers 90 amino acids, and it displays non-canonical periodicities — its helical repeats don’t follow the standard heptad (every seven residues) pattern that most coiled-coils use. This is unusual geometry, and it creates unusual surface chemistry.

These dimers then stack end-to-end via a second coiled-coil interface on the opposing face of the helix, building the fibril like linked train cars. The individual fibrils pair up further, possibly through their unstructured tails — the intrinsically disordered regions that weren’t part of the crystallizable core.

The result is a hierarchical assembly: disordered protein → ordered helix → coiled-coil dimer → stacked fibril → paired fiber. An entire structural architecture that exists only when the tardigrade is drying out, and that vanishes completely when it rehydrates.

Why This Matters Beyond the Tardigrade

The CAHS story is wonderful as pure biology — a unique solution to one of nature’s hardest problems, evolved by a microscopic animal with a name that means “slow walker.” But it carries real-world implications.

Preserving biological molecules is a massive challenge. Vaccines, enzymes, antibodies, gene therapies: all of these require careful cold chains and often lyophilization (freeze-drying) with protective excipients to survive outside a refrigerator. The systems are expensive, fragile, and inaccessible in many parts of the world.

Early work, including a 2017 Biophysical Journal abstract by Piszkiewicz and colleagues, noted that tardigrade CAHS proteins had potential as excipients for biologics — molecules that help preserve pharmaceutical proteins during drying. This is a direct application of what tardigrades have evolved naturally: the ability to keep proteins functional through complete desiccation.

Now that we have the atomic structure of the CAHS-8 fibril, we can move from biomimicry-by-analogy to rational engineering. Understanding exactly how the coiled-coil geometry creates the protective matrix means we can ask: can we design shorter, simpler, cheaper variants that retain the key structural features? Can we tune the phase transition temperature, the gel stiffness, the protection spectrum?

The Animal at the End

I find myself continually amazed that the tardigrade did all of this before we arrived to study it. Evolution, working blindly over hundreds of millions of years, produced an animal that:

• Makes proteins with no fixed structure that spontaneously fold into unusual, non-canonical coiled-coil fibrils when water is removed
• Uses vanishingly small amounts of a sugar molecule to synergize with those proteins for enhanced protection
• Runs this protective phase transition in reverse, completely, when water returns
• Packages all of this into a 0.5-millimeter animal with eight legs that you can find in a roof gutter, a garden, or the windswept ridges of the Himalayas

The 2025 crystal structure of CAHS-8 is a milestone — the first atomic-resolution picture of what the tardigrade’s survival machinery actually looks like when it’s doing its job. But it’s also, in the way that good science always is, a door opening onto the next set of questions.

Why are the periodicities non-canonical? What do the disordered tails do when fibers pair up? How does the cell signal to CAHS proteins that water is leaving — and that it’s time to change?

The tardigrade isn’t telling us all its secrets yet. But it’s telling us more than it ever has.

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Key papers discussed in this post:

• Boothby et al. (2017). “Tardigrades Use Intrinsically Disordered Proteins to Survive Desiccation.” Molecular Cell. DOI: 10.1016/j.molcel.2017.02.018
• Yagi-Utsumi et al. (2021). “Desiccation-induced fibrous condensation of CAHS protein from an anhydrobiotic tardigrade.” Scientific Reports. DOI: 10.1038/s41598-021-00724-6
• Nguyen et al. (2022). “Trehalose and tardigrade CAHS proteins work synergistically to promote desiccation tolerance.” Communications Biology. DOI: 10.1038/s42003-022-04015-2
• Sanchez-Martinez et al. (2023). “The tardigrade protein CAHS D interacts with, but does not retain, water in hydrated and desiccated systems.” Scientific Reports. DOI: 10.1038/s41598-023-37485-3
• Nguyen et al. (2025). “A phase transition modulates the protective function of a tardigrade disordered protein during desiccation.” Protein Science. DOI: 10.1002/pro.70300
• Malki et al. (2025). “Fibril Structure of Desiccation-Protective Tardigrade Protein CAHS-8.” Angewandte Chemie International Edition. DOI: 10.1002/anie.202519912