Abiotic Stress Adaptation: Protein Folding Stability and Dynamics - Abiotic Stress: Plant Responses and Applications in Agriculture - page 7

changes and transduction of stress signals triggers activation of molecular response mecha‐
nisms [22]. A general response to environmental stress conditions is the onset of stress pro‐
teins that facilitate protein folding and protect proteins from misfolding and aggregation.
The targets for these so-called chaperons (heat shock proteins HSP, late embryogenesis
abundant LEA proteins) are partially unfolded or misfolded proteins with stretches of hy‐
drophobic residues that are normally buried in the interior of the protein fold now exposed
to the surface. Since aggregates of misfolded proteins can be very stable and the energy bar‐
riers towards the folded state can be of higher energy, chaperons assist the folding process
by helping to overcome the energy barriers and to refold proteins from aggregates [23].
Transcription of many genes is up regulated under stress conditions. Among these genes,
several code for stress-induced proteins that act to improve water movement through mem‐
branes (water channel proteins), detoxification enzymes or enzymes required for osmolyte
biosynthesis [24]. Studies on plants reported that one of the initial responses to water deficit
is the induction of osmolyte (Figure 3) production. Changes in protein expression levels are
required to regulate osmolyte transport and distribution throughout the plant. The accumu‐
lation of low-molecular weight osmolytes (compatible solutes) is well known to protect mac‐
romolecular structure from stress-induced damage. Increased intracellular osmolyte
concentrations on the other hand may affect protein structure and dynamics. Solvent and
(co-)solvent conditions and protein solvent accessibility is of particular importance during
stress periods because it influences ionic strength, pH values and affinity to certain molecu‐
lar groups on the protein surface.
Figure 3. Examples of organic co-solvents (osmolytes): uncharged sugars, polyols and betaines
Accumulated osmolytes within the cells change the interaction of proteins with the solvent
[25] by increasing (kosmotropic) or decreasing (chaotropic) the order of water. Kosmotropic,
or so-called compensatory co-solvents are well-hydrated molecules with little tendency to
aggregate, have no net charge, and strongly hydrogen-bond with water. They are preferen‐
tially solubilized within the bulk of water and preferentially excluded from the protein sur‐
face, which leads to a decrease in the water diffusion around the protein [26, 27]. Although
molecules do not seem to directly interact with the protein surface, they modify protein sta‐
bility by altering solvent properties. According to the "water structure hypothesis" chaotrop‐