Antifreeze Protein Works From Inside Out

To survive wintry weather, organisms such as fish, insects, and plants have developed antifreeze proteins that inhibit growth of ice crystals in their tissues. Studying one such protein called Maxi from winter flounder, researchers in Canada have discovered that the protein has an unexpected structure. Unlike similar antifreeze proteins—and unlike other proteins, generally—Maxi incorporates sheets of hundreds of water molecules into its core (Science 2014, DOI: 10.1126/science.1247407).

If there is one common theme to the thousands of previously known protein structures, it is that proteins stabilize their structures by packing hydrophobic amino acid side chains into a water-free core, says Kim A. Sharp, a biochemistry professor at the University of Pennsylvania, in a commentary about the work. He notes that Maxi’s structure “flaunts its violation of the anhydrous-core principle.”

The research was done by a group at Queen’s University led by biochemistry graduate student Tianjun Sun and professor Peter L. Davies.

Maxi is related to a smaller protein, called type I antifreeze protein. Both are produced in the liver and circulate in the blood of winter flounder and related species.

The type I protein is composed of three repeating sequences that form an α-helix, with key ice-binding residues arrayed on one side of the helix. Those residues are hydrophobic but nevertheless serve to organize and stabilize a water network around the protein’s surface. That water network then can merge and freeze with ice particles. The protein binding forces further ice growth into a form less stable than normal ice.

Maxi is a homodimer of two helical protein subunits with type I antifreeze-protein-like structures, each having 12 repeating sequences with some joining segments. The two helices fold in half, so Maxi looks like a bundle of four helices 145 Å long and 22 Å wide.

But instead of having the ice-binding residues on the outside of the structure, like type I antifreeze protein, Maxi has residues that are oriented inward, where they help form and anchor two perpendicular sheets of about 200 water molecules each that separate the four helices. The waters are ordered into thin, cagelike structures that roughly repeat with the protein sequence.

The water sheets extend from the protein core outward between the helices, where the sheets can then merge with ice and inhibit its growth, Davies says.

Other researchers have observed similar water molecule arrangements around other hydrophobic protein surfaces—but only at smaller scales. “I haven’t seen such an extensive array as this one” in other proteins, says Jeremy C. Smith, a biophysicist at Oak Ridge National Laboratory and the University of Tennessee. He adds that “the structure is certainly of interest to scientists studying the fascinating properties of confined water.”