Protective Functions of Dehydrins on Proteins and Cell Structures

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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).