X-Message-Number: 14135 From: Brent Thomas <> Subject: interesting differences in ice crystal supression proteins Date: Fri, 21 Jul 2000 13:46:06 -0400 from http://helix.nature.com/nsu/000720/000720-11.html lifelines : The budworm in your freezer CHRISTOPHER SURRIDGE Just as car drivers add antifreeze to their cars' radiators, plants and animals have a range of antifreeze proteins (AFPs) to protect them against the cold. Antifreeze proteins were first discovered in Arctic and Antarctic fishes that survive waters below zero degrees Celsius. Now in Nature [20 July 2000], researchers announce the structures of two AFP's from insects and identify a third from rye grass, shedding light on how these proteins work, and how their power might be harnessed for agriculture and medicine. Unlike chemical antifreezes such as ethylene glycol, these proteins do not reduce the conventional 'freezing point' of water. Instead, AFPs coat the surfaces of ice crystals as they form, preventing them from crystallizing further until, with increasing cold, this barrier is overcome and large needle-like crystals appear. By working at the ice/water interfaces they exert their effect at much lower concentrations, while leaving the chemical nature of the bulk water unchanged so that the chemical processes of life can continue unhindered. The temperature on dry land can drop far below the 2 degrees of frost that is the lower limit for sea water. Indeed the most effective AFP so far discovered comes from an insect, the spruce budworm Choristoneura fumiferana. This protein can reduce the point of freezing of water as much as 6 degrees below its melting point. By elucidating the structure of this antifreeze protein, Peter L. Davies and colleagues at the University of Ontario now provide some hints as to how this extreme activity is achieved1. Other AFPs posses a single flat surface in which specific chemical groups are arranged to bind the regimented water molecules in one of an ice crystal's faces. The budworm's AFP, however, has two such surfaces, whose features seem perfect to interact with two different ice crystal faces. Thus this AFP seems to mount a two-pronged defence against the ice. To construct its two ice-binding surfaces, the budworm AFP as taken advantage of a rare but highly regular protein fold known as the beta-helix. This structure looks a little like a spring that has been flattened on three sides to form a triangular cross-section. In this way, a series of nearly identical protein strands stack one above the other in consecutive turns of the spring. Such a repeating structure would be perfectly suited to binding to the crystalline regularity of an ice facet, a prediction borne out by the structure of another insect AFP, this time from the beetle Tenebrio molitor, also solved by Peter Davies's group2. The T. molitor AFP has an incredibly regular structure containing six turns of beta-helix which can be almost exactly superimposed. In a first for AFP structures, water molecules can be seen attached to the ice-binding surface, spaced as they would be in an ice crystal. But there is one major difference between these two AFPs: the budworm's forms a left-handed helix, T. molitor's a right-handed. Such a fundamental difference excludes the possibility that the proteins could have arisen through evolution from a common ancestor. Although intriguing in its own right, the chemistry of AFPs is of more than academic interest. AFPs could improve the storage of blood, tissues and organs, and protect crops from frost and preserve the textures of frozen foods. But the genetic modification of plants (such as tomatoes) to include fish AFPs has proved disappointing. Identifying an antifreeze protein from the perennial rye grass Lolium perenne, a group from Unilever Research UK lead by Peter Lillford suggests that this may be because for plants, prevention of freezing and protection from freezing are not the same thing3. Lolium survives overwintering by accumulating a very stable protein (it survives boiling) - it is this protein that Lillford's team have investigated. Surprisingly it is a very poor antifreeze, only reducing the freezing point of water 0.1 degrees Celsius below its melting point. But it is exceptional at preventing the growth of ice crystals once they are formed, some 200 times better than the archetypal fish antifreeze from the eel pout (Lycodichthys dearborni). Our present understanding of how AFPs work cannot reconcile the difference in these two activities, even though from a biological point of view it makes perfect sense. Plants have a greater ability to lie dormant than animals, so the major danger posed by freezing may not be the stopping of normal cellular processes, but rather the physical damage that large crystals of ice could do to cells. The diversity of antifreeze proteins has always been taken as an indication that these are a disparate group of proteins recruited by evolution to serve a common function. Now even this common function is proving illusory. Diverse members of the group protect organisms from various aspects of the threat of freezing in subtly different fashions. Yet the rewards available once we can understand and harness their capabilities will be immense. Christopher Surridge is a senior biological sciences editor at Nature 1.Graether, S.P. et al. Beta-helix structure and ice-binding properties of a hyperactive antifreeze protein from an insect. Nature 406, 325-328 (2000). 2.Liou, Y-C., Tocilj, A., Davies, P., Jia, Z. Mimicry of ice structure by surface hydroxyls and water of a beta-helix antifreeze protein. Nature 406, 322-524 (2000). 3.Sidebottom, C. et al. Heat-stable antifreeze protein from grass. Nature 406, 256 (2000). Macmillan Magazines Ltd 2000 - NATURE NEWS SERVICE Nature Macmillan Publishers Ltd 2000 Reg. No. 785998 England. Brent Thomas Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=14135