EPR of site-directed spin-labeled proteins: A powerful tool to study structural flexibility.

Electron Paramagnetic Resonance is a spectroscopic technique which, in combination with site-directed spin-labeling, provides structural and dynamic information about proteins in conditions similar to those of their physiological environment. The information is sequence-resolved, as it is based on probing the local dynamics of a paramagnetic label incorporated as a side chain of a selected amino acid. EPR does not impose a limit on the size of the protein or protein complex, as long as it is amenable to site-directed mutagenesis, and is able to obtain reliable distance distributions between two or more labels (identical or different).. The mean value, width and shape of distance distributions, as well as their dependence upon the state of the protein or interactions with physiological partners, provide insight into order-disorder transitions and the roles of protein flexibility. The main potentialities and limitations of the technique are revised and illustrated with examples of proteins for which order-disorder play an important role. • Electron Paramagnetic Resonance reports on local protein flexibility and disorder. • Site-Directed Spin-Labeling and EPR are used to assess back-bone dynamics. • DEER obtains distance distributions between spin labels in disordered proteins. • Changes in EPR spectra inform about order-disorder transitions in proteins. • Integrating data and modeling tools provide structural and dynamic models for IDPs. [ABSTRACT FROM AUTHOR]