Every year, the influenza virus infects between 2 and 6 million people in France. Other influenza viruses, that are very similar to the human influenza virus, cause epizootics that also threaten the human health, as they can cross the species barrier with new viral forms emerging. In particular, the avian influenza epizootic that has been raging in Europe since October 2021 is affecting domestic and wild birds with unprecedented virulence and contagiousness. Given this situation, the WHO has placed the various influenza virus strains under very close surveillance.
In this context, scientists at CEA-IRIG are seeking to elucidate the molecular mechanisms that enable the influenza virus to mutate or adapt to other species.
The genome of this family of viruses is composed of eight single-stranded RNA molecules. Each RNA fragment is covered with multiple copies of viral nucleoproteins (NP) and the 3’ and 5’ RNA ends interact with an RNA polymerase to form the ribonucleoprotein complex (RNP), (involved in the virus proliferation). Until now, studies of RNPs using cryo-electron microscopy (cryo-EM) had only provided a molecular envelope in which the NP molecules had been positioned imprecisely, and without providing any details of their interaction with the viral RNA.
Thanks to their expertise in the expression and purification of recombinant NP protein, scientists were able to produce significantly more biological material than by starting with viruses, an essential condition for conducting a high-resolution cryo-ME study. Using the Titan Krios electron microscope on ESRF’s CM01 line, they succeeded to generate a final model at 5 Å resolution.
Figure: helically symmetrized cryo-electron microscopy 3D reconstruction of the ribonucleoprotein-like particle at 8.7 Å. The reconstruction displays about two intertwined helical turns. Credit CEA
This enables them to understand how the nucleoprotein molecules interact with each other within this complex and flexible architecture. This cryo-EM 3D reconstruction at sub-nanometric resolution enables for the first time the visualisation of the RNA placement within RNPs.
RNA appears to be involved in structuring the ribonucleoprotein complexes, it can slide freely on the surface of the proteins, most likely to facilitate access to the RNA polymerase and thus enables replication of the virus.