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


            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
            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

            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
            faces. The budworm's AFP, however, has two such surfaces, whose
            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
            advantage of a rare but highly regular protein fold known as the
            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,
            molitor's a right-handed. Such a fundamental difference excludes
            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

            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
            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

            Our present understanding of how AFPs work cannot reconcile the
            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
            processes, but rather the physical damage that large crystals of
ice could do to

            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

              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
              3.Sidebottom, C. et al. Heat-stable antifreeze protein from
grass. Nature 406, 256

              Macmillan Magazines Ltd 2000 - NATURE NEWS SERVICE 

                Nature   Macmillan Publishers Ltd 2000 Reg. No. 785998

Brent Thomas

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