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Structural Characterization of Human Norovirus Strain VA387 Virus-like Particles
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.