The nucleoprotein of measles virus consists of an Nterminal moiety, N CORE , resistant to proteolysis and a C-terminal moiety, N TAIL , hypersensitive to proteolysis and not visible as a distinct domain by electron microscopy. We report the bacterial expression, purification, and characterization of measles virus N TAIL . Using nuclear magnetic resonance, circular dichroism, gel filtration, dynamic light scattering, and small angle x-ray scattering, we show that N TAIL is not structured in solution. Its sequence and spectroscopic and hydrodynamic properties indicate that N TAIL belongs to the premolten globule subfamily within the class of intrinsically disordered proteins. The same epitopes are exposed in N TAIL and within the nucleoprotein, which rules out dramatic conformational changes in the isolated N TAIL domain compared with the full-length nucleoprotein. Most unstructured proteins undergo some degree of folding upon binding to their partners, a process termed "induced folding." We show that N TAIL is able to bind its physiological partner, the phosphoprotein, and that it undergoes such an unstructured-to-structured transition upon binding to the C-terminal moiety of the phosphoprotein. The presence of flexible regions at the surface of the viral nucleocapsid would enable plastic interactions with several partners, whereas the gain of structure arising from induced folding would lead to modulation of these interactions. These results contribute to the study of the emerging field of natively unfolded proteins.

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).

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In colloidal systems, the interplay between the short range attraction and long-range repulsion can lead to a low density associated state consisting of clusters of individual particles. Recently, such an equilibrium cluster phase was also reported for concentrated solutions of lysozyme at low ionic strength and close to the physiological pH. Stradner et al. [(2004) Equilibrium cluster formation in concentrated protein solutions and colloids. Nature 432:492-495] found that the position of the low-angle interference peak in small-angle x-ray and neutron scattering (SAXS and SANS) patterns from lysozyme solutions was essentially independent of the protein concentration and attributed these unexpected results to the presence of equilibrium clusters. This work prompted a series of experimental and theoretical investigations, but also revealed some inconsistencies. We have repeated these experiments following the protein preparation protocols of Stradner et al. using several batches of lysozyme and exploring a broad range of concentrations, temperature and other conditions. Our measurements were done in multiple experimental sessions at three different high-resolution SAXS and SANS instruments. The lowionic-strength lysozyme solutions displayed a clear shift in peak positions with concentration, incompatible with the presence of the cluster phase but consistent with the system of repulsively interacting individual lysozyme molecules. Within the decoupling approximation, the experimental data can be fitted using an effective interparticle interaction potential involving short-range attraction and long-range repulsion. dynamic arrested state ͉ macromolecular solutions ͉ protein interactions ͉ small-angle scattering ͉ structure factor T he arrested dynamics of colloidal systems and protein solutions interacting via short-range interactions have been actively studied both theoretically and experimentally in recent years (1-4). The mode coupling theory and molecular dynamics (MD) simulations have successfully unified seemingly dissimilar dynamical arrest scenarios in colloidal systems (4, 5). In addition to the conventional glassy state induced by the packing constraints, the presence of short-range attraction leads to a different glassy behavior. The apparently diverse type of dynamical arrest, such as gelation, jamming, glassification or non-ergodicity transition, etc., found in attractive systems can be unified in terms of this attractive glass transition (3). As competing shortrange attraction and long-range repulsion are introduced, additional features are observed (5). In particular, at intermediate volume fractions, the colloidal particles can form an equilibrium cluster phase, which in turn stabilizes a low-density arrested state (6). This type of particle clustering process at low volume fractions has been observed for various colloidal systems (7-9).Although the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory successfully describes the microstructure and equilibrium phase behavior of charged colloidal systems ov...

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