SignificanceThe plant photoreceptor UVR8 absorbs UV-B light to regulate UV protection and photomorphogenic responses in plants. Here we show that UVR8 adopts multiple conformations to generate the signaling active state. The conformational diversity of UVR8 was revealed using a native mass spectrometry approach, where the photoreceptor was photoactivated in the ion source. Our analyses show that not only disordered but also ostensibly well-folded regions of UVR8 can adopt highly extended conformations that are likely to enhance interactions with partner proteins to facilitate signal propagation. The methodology employed could be used to investigate the structural dynamics of other proteins that are activated by light.

It is now over 30 years since Demchenko and Ladokhin first posited the potential of the tryptophan red edge excitation shift (REES) effect to capture information on protein molecular dynamics. While there have been many key efforts in the intervening years, a biophysical thermodynamic model to quantify the relationship between the REES effect and protein flexibility has been lacking. Without such a model the full potential of the REES effect cannot be realized. Here, we present a thermodynamic model of the tryptophan REES effect that captures information on protein conformational flexibility, even with proteins containing multiple tryptophan residues. Our study incorporates exemplars at every scale, from tryptophan in solution, single tryptophan peptides, to multitryptophan proteins, with examples including a structurally disordered peptide, de novo designed enzyme, human regulatory protein, therapeutic monoclonal antibodies in active commercial development, and a mesophilic and hyperthermophilic enzyme. Combined, our model and data suggest a route forward for the experimental measurement of the protein REES effect and point to the potential for integrating biomolecular simulation with experimental data to yield novel insights.