Temperature-Induced Extended Helix/Random Coil Transitions in a Group 1 Late Embryogenesis-Abundant Protein from Soybean

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Abstract
Group 1 late embryogenesis-abundant (LEA) proteins are a subset of hydrophilins that are postulated to play important roles in protecting plant macromolecules from damage during freezing, desiccation, or osmotic stress. To better understand the putative functional roles of group 1 LEA proteins, we analyzed the structure of a group 1 LEA protein from soybean (Glycine max). Differential scanning calorimetry of the purified, recombinant protein demonstrated that the protein assumed a largely unstructured state in solution. In the presence of trifluoroethanol (50% [w/v]), the protein acquired a 30% α-helical content, indicating that the polypeptide is highly restricted to adopt α-helical structures. In the presence of sodium dodecyl sulfate (1% [w/v]), 8% of the polypeptide chain adopted an α-helical structure. However, incubation with phospholipids showed no effect on the protein structure. Ultraviolet absorption and circular dichroism spectroscopy revealed that the protein existed in equilibrium between two conformational states. Ultraviolet absorption spectroscopy studies also showed that the protein became more hydrated upon heating. Furthermore, circular dichroism spectral measurements indicated that a minimum of 14% of amino acid residues existed in a solvent-exposed, left-handed extended helical or poly (l-proline)-type (PII) conformation at 20°C with the remainder of the protein being unstructured. The content of PII-like structure increased as temperature was lowered. We hypothesize that by favoring the adoption of PII structure, instead of the formation of α-helical or β-sheet structures, group 1 LEA proteins retain a high content of surface area available for interaction with the solvent. This feature could constitute the basis of a potential role of LEA proteins in preventing freezing, desiccation, or osmotic stress damage.