Comprehensive structural and dynamical view of an unfolded protein from the combination of single-molecule FRET, NMR, and SAXS

Significance Proteins are the most versatile components of the molecular machinery of life. Synthesized as linear polymers of amino acids, proteins start out in their unfolded state and perform their function either in well-defined folded conformations or as intrinsically disordered proteins (IDPs) lacking tertiary structure. Both for the folding process and the properties of IDPs, a quantitative understanding of the conformational distributions and dynamics of unfolded proteins is thus essential. However, reaching this goal has been challenging owing to the large conformational heterogeneity and rapid dynamics of these systems. Here we combine three of the most powerful biophysical methods available to obtain a comprehensive view of an unfolded protein that would not be available from any of the individual methods. The properties of unfolded proteins are essential both for the mechanisms of protein folding and for the function of the large group of intrinsically disordered proteins. However, the detailed structural and dynamical characterization of these highly dynamic and conformationally heterogeneous ensembles has remained challenging. Here we combine and compare three of the leading techniques for the investigation of unfolded proteins, NMR spectroscopy (NMR), small-angle X-ray scattering (SAXS), and single-molecule Förster resonance energy transfer (FRET), with the goal of quantitatively testing their consistency and complementarity and for obtaining a comprehensive view of the unfolded-state ensemble. Using unfolded ubiquitin as a test case, we find that its average dimensions derived from FRET and from structural ensembles calculated using the program X-PLOR-NIH based on NMR and SAXS restraints agree remarkably well; even the shapes of the underlying intramolecular distance distributions are in good agreement, attesting to the reliability of the approaches. The NMR-based results provide a highly sensitive way of quantifying residual structure in the unfolded state. FRET-based nanosecond fluorescence correlation spectroscopy allows long-range distances and chain dynamics to be probed in a time range inaccessible by NMR. The combined techniques thus provide a way of optimally using the complementarity of the available methods for a quantitative structural and dynamical description of unfolded proteins both at the global and the local level.