Protective Functions of Dehydrins on Proteins and Cell Structures
Dehydrins are distributed over a wide range of organisms
including the higher plants, algae, yeast and cyanobacteria. They accumulate
late in embryogenesis and in nearly all the vegetative tissues during normal
growth conditions and in response to stress leading to cell dehydration (e.g.
drought, low temperature and salinity). It is suggested that the dehydrins
interact with membranes in the interior of cells and reduce dehydration induced
damage (Danyluk et al., 1998). The mechanism of these interactions could be
explained by their ability to replace water and, through their hydroxyl groups,
to solvate cytosolic structures (Baker et al., 1995). Other possible
explanations are that dehydrins prevent interactions between membrane bilayers
or that they are able to chelate ions, alleviating the damaging effect of
increased ion concentrations (Danyluk et al., 1998).
Structural and biochemical studies indicate
that dehydrins are intrinsically disordered proteins (IDPs), i.e. in their
functional state they are devoid of a single and stable tertiary structure
(Tompa, 2009). The molecular function of dehydrins is still poorly
characterized, although several mechanisms have been proposed by which the
consequences of environmental stresses could be mitigated, such as membrane
stabilization, resistance to osmotic pressure and protection of proteins ‒ the
so-called chaperone function (Agoston et al., 2011). It was suggested that this
latter effect is based on a ‘‘molecular shield’’ mechanism, rather than typical
chaperone activity. According to this concept dehydrins are able to inhibit the
interaction between denatured protein molecules, preventing the formation of
aggregates. The structure/function relationship of dehydrins, as IDPs, is much
less well established than that of globular proteins. They are proposed to
function either as entropic chains or by molecular recognition (Tompa, 2005).
It is very probable that dehydrins, being typical IDPs, are able to bind their
partner molecules via short recognition elements. During this
interaction, dehydrin molecules could participate in a structurally adaptive
process termed disorder-to-order transition or induced folding (Fuxreiter et
al., 2004; Agoston et al., 2011).
The number and order of the Y-, S- and K-segments define
different dehydrin sub-classes: YnSKn, YnKn, SKn, Kn and KnS. Each dehydrin
structural type may possess a specific function and tissue distribution. Their
precise function has not been established, but in vitro experiments
indicate that some dehydrins (YSKn-type) bind to lipid vesicles that contain
acidic phospholipids, and others (KnS) bind metals and are able to scavenge
hydroxyl radicals (Asghar et al., 1994, Alsheikh et al., 2003), protect lipid
membranes against peroxidation and are cryoprotective towards enzymes sensitive
to freezing. Dehydrins of the SKn and K sub-classes appear to be directly
involved in cold acclimation processes (Houde
et al., 1995; Danyluk et al.,1998; Zhu et al., 2000; Allagulova et al., 2007).
Those of the YnSKn type are usually low molecular weight, alkaline proteins
that are induced by drought (Xiao and
Nassuth, 2006; Vaseva et al., 2010).
Biochemical analyses of dehydrins have shown that spinach
COR85, maize G50, wheat WSC120 and peach PCA60 have cryoprotective activity
(Houde et al., 1995; Wisniewski et al., 1999). PCA60 also exhibits antifreeze
activity by modifying the normal growth of ice and exhibiting thermal
hysteresis (Wisniewski et al., 1999).
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