Intrinsically disordered proteins help tardigrades survive desiccation

March 16, 2017 § Leave a comment

H. dujardini tardigrade Courtesy of Bob Goldstein and Vicky Madden at UNC Chapel Hill - https://www.flickr.com/photos/waterbears/sets/72157607218607395/

Scanning electrom microscope image of a tardigrade
Courtesy of Bob Goldstein and Vicky Madden at UNC Chapel Hill – https://www.flickr.com/photos/waterbears/sets/72157607218607395/

 

The humble tardigrade, an organism whose name means “slow stepper,” has long been known to survive bursts of ultraviolet radiation, freezing temperatures, the vacuum of space and extreme droughts. But, until now, the mechanisms by which these creatures do so have remained unclear. In a paper published today in the journal Molecular Cell, researchers at the University of North Carolina, Chapel Hill, report that intrinsically disordered proteins unique to tardigrades, who are also known as “water bears,” are responsible for the organisms’ ability to survive extreme desiccation.

As tardigrades dry out, they crank up their production of intrinsically disordered proteins, which lack three-dimensional structures. As the drying progresses, these proteins vitrify around internal cellular components, forming an amorphous glasslike solid.

“It’s a lot more gentle on the cell,” says lead author Thomas Boothby. The solid prevents proteins that are sensitive to desiccation from denaturing and aggregating; otherwise, these proteins would form crystals that would shred DNA and cell components once water is added back to the system.“What we envision is happening is that membranes and proteins are basically being coated in these disordered proteins that form a glassy matrix around them.”

According to Boothby, one of the competing theories has been that tardigrades use the sugar trehalose to form the glassy matrices that protect their cells. In animals that use trehalose to survive desiccation, such as brine shrimp, the sugar makes up around twenty percent of body weight; the concentration in tardigrades has  been observed at about 2 percent. “When you couple that with genetic evidence that tardigrades don’t have the enzyme to make trehalose, it makes us think that they’re probably not producing the sugar themselves. They’re probably getting a little bit of it from their food source,” says Boothby.

When the researchers ran a differential gene analysis on tardigrades that had been subjected to gradual drying, they noticed 11 cytosolic heat-soluble protein transcripts, 19 secreted heat-soluble protein transcripts and two mitochondrial heat-soluble transcripts that were significantly enriched compared with hydrated conditions. All three of these protein families are believed to encode for intrinsically disordered proteins in tardigrades.

This is the first observation that intrinsically disordered proteins confer protection against desiccation in tardigrades, though nearly all organisms contain intrinsically disordered proteins. When the researchers expressed the genes that code for the tardigrade-specific intrinsically disordered proteins in Escherichia coli and Saccharomyces cerevisiae, they found that the organisms exhibited a hundredfold increase in their ability to tolerate desiccation.

“The finding that tardigrade disordered proteins are crucial for the ability of the members of the animal kingdom to survive during extreme desiccation concurs with previous work on the plant desiccation resistance that was shown to be critically dependent on several specific intrinsically disordered proteins,” says Vladimir Uversky at the University of South Florida. “The ability of tardigrade disordered proteins to vitrify represents a novel intrinsic-disorder-based molecular mechanism of protection of biological material from desiccation.”

Boothby and colleagues also noted that when tardigrades were subjected to freezing conditions instead of desiccation, the organisms activated an entirely different set of genes.

Boothby and colleagues are currently exploring the differences between which genes tardigrades activate for different harsh conditions. “Figuring out if they have just general tricks for surviving all these different stresses or if they use specific mechanisms to survive each individual stress is a really interesting question,” he says. “(It) can help us to understand how these different stress tolerances evolved as well as how the animals do them.”

This post was written by John Arnst, ASBMB Today’s science writer. 

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