The central premise that has attracted investment worldwide in structural biology projects is based on the theory that if we can determine the three-dimensional structure of a finite number of proteins, we will find the key to understanding their biological activities. In the last ten years, however, it has become increasingly clear that a large proportion (up to 40%) of proteins encoded by the human genome are intrinsically disordered or contain long disordered domains (> 50 amino acids). Intrinsically disordered proteins (IDPs) are still functional in spite of the lack of a well-defined three-dimensional structure. The classical “structure-function” paradigm does not, therefore, apply to these proteins and new research is needed on the relation between the primary sequence and molecular function, as well as new methods for observing these flexible parts of proteins.

Against this background then, and within the framework of Grenoble’s “Structural Biology Partnership” [6], researchers at CEA, CNRS and the Université Joseph Fourier focused their interest on the nucleoprotein of the measles virus, whose partially disordered nature controls replication of the virus. This protein forms nucleocapsids by binding with the viral RNA (see Figure). The researchers developed a model that reconstructs the structure of the nucleocapsid using experimental data obtained thanks to three complementary techniques: EM, SAXS and NMR. For the first time, this study reveals the disordered domain in situ, i.e. in the context of the entire nucleocapsid.

The results show that this part of the protein, that controls viral transcription and replication, does remain disordered in situ. Further, the results strongly suggest that the protein’s intrinsic flexibility plays a crucial role in its function. The researchers will now be able to use these techniques to examine the structure, the dynamics and the kinetics of the nucleoprotein in situ, in the presence of the RNA polymerase [7] to gain a deeper understanding of the mechanisms involved in the spread of the virus.

Article reference:

Intrinsic disorder in measles virus nucleocapsids. Malene Ringkjøbing Jensen, Guillaume Communie, Euripedes Almeida Ribeiro Jr, Nicolas Martinez, Ambroise Desfosses, Loïc Salmon, Luca Mollica, Frank Gabel, Marc Jamin, Sonia Longhi, Rob W. H. Ruigrok, and Martin Blackledge. PNAS, online, 2011.

[1] Jean-Pierre Ebel Institute of Structural Biology (IBS, CEA-CNRS-Université Joseph Fourier joint institute, Grenoble)

[2]Unit of Virus Host Cell Interactions (UVHCI), UMI 3265 UJF-EMBL-CNRS Grenoble international joint research unit

[3] Biological Macromolecule Architecture and Function, UMR 6098 CNRS and University of Aix-Marseille I and II joint research unit

[4] Electron microscopy gives an overall image of the structured domain of these large objects; the so-called small angle scattering technique measures the diameter of the capsid, thereby enabling us to see the perimeter within which movement of the flexible parts of proteins is produced; NMR (nuclear magnetic resonance) is used to determine the dynamics of the protein’s entire disordered domain with atomic resolution.

[5] Virus nucleocapsid: a complex structure made up of helical RNA associated with 13 nucleoprotein molecules for each turn in the helix.

[6] Structural Biology Partnership: an alliance between key players based in Grenoble and involved in Structural Biology, IBS, EMBL, ESRF and ILL.

[7] RNA polymerase is the enzyme in charge of replicating the virus’ RNA, an essential step in its propagation. It interacts directly with the nucleoprotein.