Conserved Glycines Control Disorder and Function in the Cold-Regulated Protein, COR15A

Cold-regulated (COR) 15A is an intrinsically disordered protein (IDP) from Arabidopsis thaliana important for freezing tolerance. During freezing-induced cellular dehydration, COR15A transitions from a disordered to mostly α-helical structure. We tested whether mutations that increase the helicity of COR15A also increase its protective function. Conserved glycine residues were identified and mutated to alanine. Nuclear magnetic resonance (NMR) spectroscopy was used to identify residue-specific changes in helicity for wildtype (WT) COR15A and the mutants. Circular dichroism (CD) spectroscopy was used to monitor the coil⁻helix transition in response to increasing concentrations of trifluoroethanol (TFE) and ethylene glycol. The impact of the COR15A mutants on the stability of model membranes during a freeze⁻thaw cycle was investigated by fluorescence spectroscopy. The results of these experiments showed the mutants had a higher content of α-helical structure and the increased α-helicity improved membrane stabilization during freezing. Comparison of the TFE- and ethylene glycol-induced coil⁻helix transitions support our conclusion that increasing the transient helicity of COR15A in aqueous solution increases its ability to stabilize membranes during freezing. Altogether, our results suggest the conserved glycine residues are important for maintaining the disordered structure of COR15A but are also compatible with the formation of α-helical structure during freezing induced dehydration.