Scanning electron micrograph of six tardigradesTHOMAS BOOTHBYHardy, microscopic animals called tardigrades, also known as water bears, can survive desiccation. Until now, it wasn’t clear exactly how. The results of a study published in Molecular Cell today (March 16) suggest that proteins lacking stable 3-D structures, called tardigrade-specific intrinsically disordered proteins (TDPs), form glass-like solids that protect the animals during drying.

Other organisms achieve desiccation tolerance with a sugar called trehalose, which forms glass-like solids upon drying. For years, researchers assumed that tardigrades used trehalose, too, but many species of water bears only express small amounts of the sugar—likely not enough to confer the substance’s preservative capabilities.

TDPs “seem to work by a mechanism which is similar to this sugar, trehalose,” said coauthor Thomas Boothby, a postdoc at the University of North Carolina, Chapel Hill. “It seems like evolution has basically come up with two different ways to do the same trick.”

Boothby and colleagues identified a group of TDPs during a screen for tardigrade genes upregulated during desiccation. They confirmed that many of the genes’ protein products were disordered, and that these genes were expressed in three different tardigrade species, either constitutively or during drying. When the team used RNA interference (RNAi) to knock down TDP genes, the tardigrades were less able to survive desiccation.

Next, the researchers introduced TDP genes into yeast and bacteria, finding that the proteins increased desiccation tolerance in both organisms. TDPs also preserved the activity of an enzyme, lactate dehydrogenase, during drying in vitro. The authors showed that, upon desiccation, isolated TDPs transform into a glass-like substance, which also is present in—and increases survival of—both yeast that express TDPs and tardigrades during drying.

The study is “very solidly done,” biologist John Crowe of the University of California, Davis, who did not participate in the work, told The Scientist. He added that one direction for future research would be to examine the effects of TDPs on cell membranes.

“It’s been well known for some time that polymers like this can prevent fusion between membranes during drying, but they don’t preserve them completely,” Crowe said. “Small molecules like trehalose or glucose, or some other small sugar, are required in addition. It may well be that the small amount of trehalose that’s found in tardigrades in conjunction with those proteins may do the job. You might need both.”

According to Boothby, another open question is whether tardigrade species other than the three examined in this study use a similar mechanism to protect against desiccation. There are more than 1,200 tardigrade species, divided into two classes.

“It’s going to be really instructive if and when we can start looking at differences between those classes,” said Carl Johansson, an instructor at Fresno City College in California who was not involved in the work.

Probing how TDPs function at a molecular level to protect the animals from desiccation could have applications beyond water bears. “This represents the first step that could be used to improve the capability of other organisms to desiccate in the future,” said coauthor Lorena Rebecchi of the University of Modena and Reggio Emilia in Italy. Rebecchi explained that learning more about tardigrade proteins could eventually enable researchers to safely desiccate plants and other animals.

“People don’t know about tardigrades, but they are very important,” she said. “They have a lot of biological secrets that could be used to improve the quality of human life.”

T.C. Boothby et al., “Tardigrades use intrinsically disordered proteins to survive desiccation,” Molecular Cell, doi:10.1016/j.molcel.2017.02.018, 2017.