Protein popsicle holds clues to insect antifreeze

Researchers in Canada have discovered a surprising antifreeze protein used by insects living at sub-zero temperatures. Snow fleas, tiny herbivorous insects that are at their most active in the snowy winter months, possess a unique protein that prevents ice crystals from forming.

Fish, plants, insects and bacteria have all evolved antifreeze proteins (AFPs), but Laurie Graham and Peter Davies, at Queen’s University, Kingston, Ontario, have discovered that the snow flea AFP is different.

All AFPs work by binding to ice as it freezes, preventing the growth of ice crystals. The known beetle and moth AFPs use threonine-rich sites to bind to ice. Snow-flea AFPs use glycine-rich sites. This surprised Davies and colleagues because glycine is associated with flexibility in protein structure, and the researchers would have predicted a less structured AFP molecule as a result. Nevertheless, the snow flea can stop ice forming up to 5.8?C degrees below freezing.

Davies’ discovery supports a theory that fish, insects and plants have all evolved independently to cope with colder temperatures in recent geological history. ’We’re seeing quite an amazing diversity of proteins all doing the same job,’ Davies told Chemistry World, ’these are remarkable examples of recent evolution and separate adaptation to a harsh climate.’

Davies now plans to solve the structure of the snow flea AFP in order to find out how the protein works. He has collected snow fleas from snowbanks and puddles and made what he calls ’an AFP popsicle’ which should provide enough AFP to work with.

Christina Cheng, an AFP expert from the University of Illinois, US, notes that the snow flea is a much more primitive insect than beetles and moths. ’It will be interesting when they report what this [snow flea AFP] structure is and how it may bind to ice, as the repetitive glycine units tend to make turns in a polypeptide, and since glycine is in every 3rd residue [in snow flea AFP] there will be lots of turns,’ she said.

Davies said that once properly understood, AFPs could be used to prevent ice crystals forming in frozen foods, or be added to plant genes to protect crops form harsh weather, or even used in medicine to cool organs due to be transplanted without freezing them. Katharine Sanderson