Freeze! Insect proteins halt ice growth

In cold climates, cars often need an infusion of antifreeze to survive the winter. Many fish, insects, and plants are no different, but they produce their own protection -- proteins that prevent their insides from turning to ice. 

 Scientists have studied fish antifreeze since the 1960s, but now researchers from Queen's University in Kingston, Ontario, have isolated and analyzed antifreeze proteins from insects. Virginia K. Walker, Peter L. Davies, and their colleagues collected the proteins from the spruce budworm, which is a moth larva, and from the common mealworm, a pest that feeds on grain.

 The budworm protein is up to 30 times more potent than fish proteins, and the mealworm protein is up to 100 times stronger, the group reports in the September Nature Biotechnology and the Aug. 21 Nature, respectively.

"These are some of the most active antifreezes we have encountered so far," says Choy L. Hew, a biochemist at the University of Toronto who studies fish antifreeze proteins. Collecting enough protein from larvae was quite a feat, he adds.

 If put in frozen foods, fish antifreeze proteins could help prevent ice from recrystallizing, as it does, for example, in ice cream left in the freezer too long. Scientists are also exploring the use of antifreeze proteins in preserving organs and tissues for transplants. The greater potency of the insect proteins suggests that they could be used in lower concentrations, says Laurie A. Graham, a coauthor of the Nature study. Unlike conventional antifreezes, such as the ethylene glycol commonly used in cars, the proteins create a phenomenon called thermal hysteresis: They lower the freezing point of water below 0@C without changing the temperature at which ice melts as it is heated. The insect proteins lower the freezing point a maximum of 5.5@C.

 No one knows exactly how the insect antifreeze proteins work, but they seem to bind to the surface of tiny ice crystals and inhibit their growth, Graham says. The proteins contain many repeated sequences of the amino acids serine, threonine, glycine, and cysteine. The arrangement of these hydrophilic, or water-loving, amino acids may match well the arrangement of water molecules in ice.

 Determining the three-dimensional structure of the proteins will provide more clues to how they work, says Hew. "We have solved the structure for fish antifreeze, so we know how it binds to ice, but it doesn't mean that the insect ones will bind in the same manner."

 The Queen's University group does know that the insect proteins affect the growth of ice crystals differently than fish proteins do, says Daniel Doucet, a coauthor of the Nature Biotechnology study. Ice crystals grown in the presence of the fish protein form pointy spicules with sharp edges. Grown with the budworm protein, however, the ice crystals look like smooth, hexagonal disks, which would cause less damage to cells preserved with antifreeze, he suggests.