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
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