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How do icosahedral viruses package their RNA genome?

In collaboration with the Institute of Integrative Biology of the Cell, the Laboratoire Léon Brillouin and the European Synchrotron Radiation Facility, we have elucidated the self-assembly dynamics of an icosahedral plant virus. Starting from purified proteins and genomic RNA, we probed the spontaneous reconstitution of the virus by using time-resolved small-angle X-ray scattering with a high spatiotemporal resolution.

The simplest viruses are made up of a shell of proteins arrayed into an icosahedral structure. The genome, in the form of RNA, is packaged inside the protective shell by the interplay of electrostatic interactions. The survival of viruses partly relies on their ability to self-assemble inside host cells. Although coarse-grained simulations have identified some assembly pathways, few experimental measurements are available to date due to the difficulty of detecting biological molecules in water over a wide range of time scales.

Cowpea chlorotic mottle virus (CCMV) is an icosahedral RNA virus infecting plants. The virus was spontaneously reconstituted from purified proteins and genomic RNA, and its assembly was probed by using time-resolved small-angle X-ray scattering with a synchrotron source. Measurements revealed that the adsorption of proteins on RNA occurs in a few tens of milliseconds, while the structural reorganization of the formed species can take several hours. Before the completion of virus, the proteins are loosely bound on the RNA, which may ensure a good selectivity for the viral genome during assembly in host cell. The structural reorganization is limited by an energy barrier, thus minimizing the misassembly of the protein shell. Quite unexpectedly, viruses packaging synthetic polyelectrolytes are reconstituted more easily than viruses packaging genomic RNA, which should promote further studies on the role of genome in the self-assembly dynamics.

(Top, from left to right) Cryo-electron microscopy images of partially reconstituted viruses, completely reconstituted viruses and reconstitued viruses packaging polyelectrolytes. Scale bar is 30 nm. (Bottom) Schematical representation of a self-assembling virus superimposed on experimental X-ray scattering intensities. [Nat. Commun. 9 (2018) 3071]

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