Probing Structural Transitions in the Intrinsically Disordered C-Terminal Domain of the Measles Virus Nucleoprotein by Vibrational Spectroscopy of Cyanylated Cysteines

Four single-cysteine variants of the intrinsically disordered C-terminal domain of the measles virus nucleoprotein (N_TAIL) were cyanylated at cysteine and their infrared spectra in the C≡N stretching region were recorded both in the absence and in the presence of one of the physiological partners of N_TAIL, namely the C-terminal X domain (XD) of the viral phosphoprotein. Consistent with previous studies showing that XD triggers a disorder-to-order transition within N_TAIL, the C≡N stretching bands of the infrared probe were found to be significantly affected by XD, with this effect being position-dependent. When the cyanylated cysteine side chain is solvent-exposed throughout the structural transition, its changing linewidth reflects a local gain of structure. When the probe becomes partially buried due to binding, its frequency reports on the mean hydrophobicity of the microenvironment surrounding the labeled side chain of the bound form. The probe moiety is small compared to other common covalently attached spectroscopic probes, thereby minimizing possible steric hindrance/perturbation at the binding interface. These results show for the first time to our knowledge the suitability of site-specific cysteine mutagenesis followed by cyanylation and infrared spectroscopy to document structural transitions occurring within intrinsically disordered regions, with regions involved in binding and folding being identifiable at the residue level.