Le virus de la grippe est un virus à ARN négatif appartenant à la famille des Orthomyxoviridae qui se compose de 7 membres dont les virus influenza A, B, C et D. Le génome viral comprend 7 à 8 particules ribonucléoprotéiques (RNP) au sein desquelles l’ARN viral (ARNv) est recouvert de multiples copies de nucléoprotéines (NP) et est associé à l’ARN polymérase virale via ses extrémités 3’ et 5’. Au cours de ce travail de thèse, nous nous sommes tout d’abord focalisés sur l’étude biochimique de NP A et NP B et avons pu mettre en évidence des comportements différents en ce qui concerne leurs propriétés d’oligomérisation en présence ou en absence d’ARN et en fonction de la concentration en sel. Pour la première fois nous avons pu observer une structure similaire aux RNP mais reconstituée uniquement à partir de NP A et d’un ARN de 12 nucléotides. Nous avons pu formuler l’hypothèse que 12 nucléotides de l’ARN serait fixés à la NP avec une forte affinité tandis que le reste de l’ARN fixerait la NP avec une affinité beaucoup plus faible. En parallèle nous avons résolu la structure cristallographique de la nucléoprotéine de la grippe de type D et réaliser la caractérisation de son interaction avec l’importine-α7 humaine. Enfin nous avons étudié la fixation de l’ARN sur NP D et mis en évidence l’importance de l’extrémité C-terminale dans le processus de fixation à l’ARN. Ces informations ont permis de formuler de nouvelles hypothèses quant au fonctionnement du virus de la grippe et permettre d’inscrire ce projet de thèse dans une dynamique globale de lutte contre ce virus. / Influenza virus is a negative RNA virus belongs to the Orthomyxoviridae family which consists of 7 members including influenza viruses A, B, C and D. The viral genome comprises 7 to 8 ribonucleoprotein particles (RNP) in which the viral RNA (vRNA) is coated with multiple copies of nucleoproteins (NP) and is associated with the viral RNA polymerase by its 3 'and 5' ends. In this thesis, we first focused on the biochemical study of NP A and NP B and we demonstrate that there are different behaviors with regard to their oligomerization properties in the presence or absence of RNA and as a function of the salt concentration. For the first time we were able to observe a structure very similar to RNP which was reconstituted only from NP A and a 12 nucleotide RNA. Thus, we formulate the hypothesis that 12 nucleotides of the RNA would bind NP with a very strong affinity while the rest of the RNA would bind NP with a lower affinity. In parallel, we solved the crystallographic structure of the nucleoprotein of influenza D virus and we characterized its interaction with human importin-α7. Finally, we studied the binding of RNA on NP D and we demonstrated the importance of the C-terminal end in the RNA binding process. This thesis project made it possible to formulate new hypotheses concerning the functioning of the influenza virus and to include this thesis project in a global dynamic of combating the influenza virus.

<p>Human noroviruses (HuNoVs) are the leading cause of an acute form of non-bacterial gastroenteritis, where strains belonging to genogroup (G) II are dominant. Upon expression with the baculovirus culture system, virus-like particles (VLPs) of HuNoVs are expected to assemble into T = 3 icosahedral capsid particles resembling the structure of the infectious virion particles. However, some strains were found to assemble into either T = 1 or T = 4 capsids, or a combination of two different capsid forms. In this study, we showed that VLPs of the Virginia 1997 387 (VA387) GII.4 outbreak strain assembled into T = 1, T = 3, and T = 4 capsids upon expression in insect cell culture, the first case for a naturally occurring HuNoV strain to assemble into all three capsid states. TEM analysis revealed that T = 1 icosahedral particles were the most abundant in purified samples, which contrasts previous findings where either T = 3 or T = 4 were the most abundant. We resolved the cryo-EM structures of the T = 1 shell (S) domain, T = 3, and T = 4 particles to 2.24, 2.44, and 3.43 Å, respectively, making them the most resolved norovirus (NoV) structures to date. Single particle cryo-EM 3D analysis showed that the protruding (P) domain of T = 1 and T = 4 VLPs are highly dynamic. Additionally, we showed that T = 3 VLPs are resistant while T = 1 and T = 4 VLPs are sensitive to digestion in the presence of trypsin. This suggested that T = 1 and T = 4 capsids are less stable among the VLPs, which is consistent with the highly dynamic P domain inferred from our cryo-EM 3D analysis. During infection, HuNoVs travel through the gastrointestinal (GI) tract where they encounter a broad range of variable conditions that include pH, ionic strength, and host defenses (e.g., proteases). Our analyses suggest that virions are T = 3 particles as they can survive the GI tract upon exiting the host. We determined the first cryo-EM structure of T = 3 VLPs in complex with the known HuNoV host cell receptor, histo-blood group antigen (HBGA), to a resolution of 2.51 Å, demonstrating that NoV VLPs can serve as a platform in the structural characterization of small ligand molecules. Lastly, we identified a histidine residue retained in the S domain of all identified caliciviruses critical in the assembly of capsids. Our structures and their characterization will contribute to the development of therapeutic agents to combat noroviruses. </p>