Viruses interact with receptors on the cell surface to initiate and co-ordinate infection. The distribution of receptors on host cells can be a key determinant of viral tropism and host infection. Unravelling the complex nature of virus-receptor interactions is, therefore, of fundamental importance to understanding viral pathogenesis. Noroviruses are non-enveloped, icosahedral, positive-sense RNA viruses of global importance to human health, with no approved vaccine or antiviral agent available. Here we use murine norovirus as a model for the study of molecular mechanisms of virus-receptor interactions. We show that variation at a single amino acid residue in the major viral capsid protein had a key impact on the interaction between virus and receptor. This variation did not affect virion production or virus growth kinetics, but a specific amino acid was rapidly selected through evolution experiments, and significantly improved cellular attachment when infecting immune cells in suspension. However, reducing plasma membrane mobility counteracted this phenotype, providing insight into for the role of membrane fluidity and receptor recruitment in norovirus cellular attachment. When the infectivity of a panel of recombinant viruses with single amino acid variations was compared in vivo, there were significant differences in the distribution of viruses in a murine model, demonstrating a role in cellular tropism in vivo. Overall, these results highlight the importance of lipid rafts and virus-induced receptor recruitment in viral infection, as well as how capsid evolution can greatly influence cellular tropism, within-host spread and pathogenicity.

Human noroviruses (HNV) are a prevalent cause of gastroenteritis that contribute to >200,000 deaths each year and cost >£40 billion worldwide per annum. There is currently no approved vaccine or therapy, and a greater understanding of the virus life-cycle could help develop new approaches towards disease control. Although HNV infection is usually self-limiting, persistent infections can establish in immunocompromised people - however the underlying mechanisms are poorly understood. Our studies use the murine norovirus (MNV) model system to investigate fundamental virus biology, and several strains of MNV can also persist in the murine host. The primary receptor for MNV, CD300lf, interacts with a network of amino acids (AAs) on the protruding domain of the virus major capsid protein (VP1). We hypothesised that genetic variations leading to changes within this network of AAs could influence the VP1-CD300lf interaction and viral persistence. Bioinformatic analysis of the VP1-receptor interface highlighted variation in just a single AA that correlates with persistent MNV strains. To confirm this AA is important for receptor interactions we conducted in vitro evolution experiments on suspension or adherent grown cells. Passage through suspension cells resulted in the selection of hydrophobic residues at this position co-incidental with a 1.5-fold increase in viral titre. In contrast, small polar residues were maintained at this position during passage on adherent cells. Furthermore, infectivity assays with infectious clones suggest that hydrophobic residues favour infection of suspension cells over adherent cells. Work is ongoing to understand the importance of this AA on viral infectivity and persistence.

The genomes of positive-sense RNA viruses encode polyproteins that are essential for mediating viral replication. These viral polyproteins must undergo proteolysis (also termed polyprotein processing) to generate functional protein units. This proteolysis can be performed by virally-encoded proteases as well as host cellular proteases, and is generally believed to be a key step in regulating viral replication. Hepatitis E virus (HEV) is a leading cause of acute viral hepatitis. The positive-sense RNA genome is translated to generate a polyprotein, termed pORF1, which is necessary and sufficient for viral genome replication. However, the mechanism of polyprotein processing in HEV remains to be determined. In this study, we aimed to understand processing of this polyprotein and its role in viral replication using a combination of in vitro translation experiments and HEV sub-genomic replicons. Our data suggest no evidence for a virally-encoded protease or auto-proteolytic activity, as in vitro translation predominantly generates unprocessed viral polyprotein precursors. However, seven cleavage sites within the polyprotein (suggested by bioinformatic analysis) are susceptible to the host cellular protease, thrombin. Using two sub-genomic replicon systems, we demonstrate that mutagenesis of these sites prevents replication, as does pharmacological inhibition of serine proteases including thrombin. Overall, our data supports a model where HEV uses host proteases to support replication and could have evolved to be independent of a virally-encoded protease for polyprotein processing.