Intrinsically disordered proteins (IDPs) defy the structure-function paradigm as they fulfill essential biological functions while lacking well-defined secondary and tertiary structures. Conformational and spectroscopic analyses showed that IDPs do not constitute a uniform family, and can be divided into subfamilies as a function of their residual structure content. Residual intramolecular interactions are thought to facilitate binding to a partner and then induced folding. Comprehensive information about experimental approaches to investigate structural disorder and induced folding is still scarce. We herein provide hints to readily recognize features typical of intrinsic disorder and review the principal techniques to assess structural disorder and induced folding. We describe their theoretical principles and discuss their respective advantages and limitations. Finally, we point out the necessity of using different approaches and show how information can be broadened by the use of multiples techniques.

Measles virus is a negative-sense, single-stranded RNA virus within the Mononegavirales order, which includes several human pathogens, including rabies, Ebola, Nipah, and Hendra viruses. The measles virus nucleoprotein consists of a structured N-terminal domain, and of an intrinsically disordered C-terminal domain, N TAIL (aa 401-525), which undergoes induced folding in the presence of the C-terminal domain (XD, aa 459-507) of the viral phosphoprotein. Within N TAIL , an a-helical molecular recognition element (a-MoRE, aa 488-499) involved in binding to P and in induced folding was identified and then observed in the crystal structure of XD. Using small-angle X-ray scattering, we have derived a low-resolution structural model of the complex between XD and N TAIL , which shows that most of N TAIL remains disordered in the complex despite P-induced folding within the a-MoRE. The model consists of an extended shape accommodating the multiple conformations adopted by the disordered N-terminal region of N TAIL , and of a bulky globular region, corresponding to XD and to the C terminus of N TAIL (aa 486-525). Using surface plasmon resonance, circular dichroism, fluorescence spectroscopy, and heteronuclear magnetic resonance, we show that N TAIL has an additional site (aa 517-525) involved in binding to XD but not in the unstructured-to-structured transition. This work provides evidence that intrinsically disordered domains can establish complex interactions with their partners, and can contact them through multiple sites that do not all necessarily gain regular secondary structure.Keywords: measles virus; nucleoprotein; phosphoprotein; intrinsic disorder; induced folding; NMR; CD; SAXS Measles virus (MV) is an enveloped RNA virus within the Morbillivirus genus of the Paramyxoviridae family.Its nonsegmented, negative-sense, single-stranded RNA genome is encapsidated by the viral nucleoprotein (N) within a helical nucleocapsid. This N-RNA complex is used as a template for both transcription and replication. These latter activities are carried out by the viral polymerase complex, which consists of two components, the large protein (L) and the phosphoprotein (P) (for review, see Lamb and Kolakofsky 2001).

Nipah virus (NiV) is a highly pathogenic emergent paramyxovirus causing deadly encephalitis in humans. Its replication requires a constant supply of unassembled nucleoprotein (N(0)) in complex with its viral chaperone, the phosphoprotein (P). To elucidate the chaperone function of P, we reconstituted NiV the N(0)-P core complex and determined its crystal structure. The binding of the N-terminal region of P blocks the polymerization of N by interfering with subdomain exchange between N protomers and keeps N(0) in an open conformation, ready to grasp an RNA molecule. We found that a peptide derived from the N-binding region of P protects cells against viral infection and demonstrated by structure-based mutagenesis that this peptide acts by inhibiting N(0)-P formation. These results provide new insights about the assembly of N along genomic RNA and validate the N(0)-P complex as a target for drug development.

Measles virus (MV) 1 is a member of the Paramyxovirinae sub-family within the Mononegavirales order. Its non-segmented, negative-sense, single-stranded RNA genome is packaged by the viral nucleoprotein (N) within a helical nucleocapsid. Mononegavirales use this N-RNA complex as a template for both transcription and replication. These reactions are carried out by the RNA-dependent RNA polymerase polymerase (L) in conjuction with the phosphoprotein (P) (for a review see Ref.