Effect of an Intrinsically Disordered Plant Stress Protein on the Properties of Water

Abstract

Dehydrins are plant proteins that are able to protect plants from various forms of dehydrative stress such as drought, cold, and high salinity. Dehydrins can prevent enzymes from losing activity after freeze/thaw treatments. Previous studies had suggested that the dehydrins function by a molecular shield effect, essentially preventing a denatured enzyme from aggregating with another enzyme. Therefore, the larger the dehydrin, the larger the shield and theoretically the more effective the protection. Although this relationship holds for smaller dehydrins, it fails to explain why larger dehydrins are less efficient than would be predicted from their size. Using solvatochromic dyes to probe the solvent features of water, we first confirm that the dehydrins do not bind the dyes, which would interfere with interpretation of the data. We then show that the dehydrins have an effect on three solvent properties of water (dipolarity/polarizability, hydrogen-bond donor acidity and hydrogen-bond acceptor basicity), which can contribute to the protective mechanism of these proteins. Interpretation of these data suggests that although polyethylene glycol and dehydrins have similar protective effects, dehydrins may more efficiently modify the hydrogen-bonding ability of bulk water to prevent enzyme denaturation. This possibly explains why dehydrins recover slightly more enzyme activity than polyethylene glycol.

Figure 1

Dipolarity/polarizability, π^∗, of water in aqueous solution as a function of concentrations of dehydrins K₂, K₄, and K₁₀ and the control protein HSPB6. The parameter π^∗ was calculated using Eq. 1. K₂, dotted circle; K₄, right semifilled circle; K₁₀, top semifilled circle; HSPB6, dotted triangle.

Figure 2

HBD acidity, α, of water in aqueous solution as a function of the concentration of dehydrins K₂, K₄, and K₁₀ and the control protein HSPB6. The parameter α was calculated using Eqs. 3, 4, and 5. K₂, dotted circle; K₄, right semifilled circle; K₁₀, top semifilled circle; HSPB6, dotted triangle.

Figure 3

HBA basicity, β, of water in aqueous solution as a function of the concentration of dehydrins K₂, K₄, and K₁₀ and the control protein HSPB6. The parameter β was calculated using Eq. 2. K₂, dotted circle; K₄, right semifilled circle; K₁₀, top semifilled circle; HSPB6, dotted triangle.

Figure 4

Interrelationship between coefficients z_o, a_o, and b_o characterizing solvent features of water in the presence of dehydrins K₂, K₄, and K₁₀ and the control proteins HSPB6 and ELP (see Eq. 7).

Figure 5

The efficiency of LDH cryoprotection (PD₅₀) as a function of the size of the compound (log(R_h)) and the distance (d_io)(from Eqs. 9 to 10).

Figure 6

Evaluation of intrinsic disorder propensity of (A) K₂, (B) K₄, and (C) K₁₀ dehydrins. Disorder profiles representing per-residue disorder predispositions of these proteins were generated by PONDR VLXT, PONDR VSL2, PONDR VL3, PONDR FIT, IUPred_short, and IUPred_long, with the corresponding results shown by black, red, green, pink, yellow, and blue lines, respectively. Dashed cyan lines show the mean disorder propensity calculated for each protein by averaging disorder profiles of individual predictors. The light pink shadow around the PONDR FIT shows error distribution. In these analyses, the predicted intrinsic disorder scores above 0.5 are considered to correspond to the disordered residues/regions, whereas regions with the disorder scores between 0.2 and 0.5 are considered flexible. To see this figure in color, go online.