A Group 6 Late Embryogenesis Abundant Protein from Common Bean Is a Disordered Protein with Extended Helical Structure and Oligomer-forming Properties

Background: Late embryogenesis-abundant proteins accumulate under water-deficit and are widely distributed in plants and anhydrobiotic organisms. Results: A common bean group-6 LEA protein shows structural disorder, adopts helicity under water deficit or high molecular crowding, and presents oligomeric forms. Conclusion: PvLEA6 protein adopts different conformations and/or a quaternary structure depending on its environment. Significance: Similar characteristics might be present in different LEA proteins, which could be relevant to their function. Late embryogenesis-abundant proteins accumulate to high levels in dry seeds. Some of them also accumulate in response to water deficit in vegetative tissues, which leads to a remarkable association between their presence and low water availability conditions. A major sub-group of these proteins, also known as typical LEA proteins, shows high hydrophilicity and a high percentage of glycine and other small amino acid residues, distinctive physicochemical properties that predict a high content of structural disorder. Although all typical LEA proteins share these characteristics, seven groups can be distinguished by sequence similarity, indicating structural and functional diversity among them. Some of these groups have been extensively studied; however, others require a more detailed analysis to advance in their functional understanding. In this work, we report the structural characterization of a group 6 LEA protein from a common bean (Phaseolus vulgaris L.) (PvLEA6) by circular dichroism and nuclear magnetic resonance showing that it is a disordered protein in aqueous solution. Using the same techniques, we show that despite its unstructured nature, the addition of trifluoroethanol exhibited an intrinsic potential in this protein to gain helicity. This property was also promoted by high osmotic potentials or molecular crowding. Furthermore, we demonstrate that PvLEA6 protein is able to form soluble homo-oligomeric complexes that also show high levels of structural disorder. The association between PvLEA6 monomers to form dimers was shown to occur in plant cells by bimolecular fluorescence complementation, pointing to the in vivo functional relevance of this association.