Mouse Norovirus
Noroviruses (family Caliciviridae) are the major cause of epidemic nonbacterial gastroenteritis in humans but the mechanism of antibody neutralization is unknown, and no structure of an infectious virion has been reported. Murine norovirus (MNV) is the only norovirus that can be grown in tissue culture, studied in an animal model, reverse engineered via an infectious clone, and to which neutralizing antibodies have been isolated. We have determined the cryo-electron microscopy structures of an MNV virion and the virion in complex with neutralizing Fab fragments and found some surprising differences to the other known calcivirus structures.
Norwalk virus structure
Shown here is the atomic structure of Norwalk virus (a norovirus in the Calicivirus family). This is a human virus for which there is currently no cell culture system available. Therefore, this virus was produced as a recombinant virus like particle that forms in baculovirus infected insect cells. The basic architecture is similar to the plant Tombusviruses; it has a shell (S) domain encasing the RNA genome, followed by a protruding (P) domain that forms dimers on the outer virus surface. In rNV, the P domains can be divided into two domains, P1 and P2. The P1 domain rests firmly on the S domain surface and the P2 domain has been implicated in receptor and antibody interactions.
Image reconstruction of mouse norovirus (MNV)
We recently determined the cryo-EM structure of mouse norovirus (MNV). MNV has the advantage over NV in that there is a cell culture and animal system for its study. While there is reasonable sequence homology to NV, the capsid of MNV is clearly different than rNV in the position and orientation of the protruding domains. Below (left) is the image reconstruction of MNV colored according to the radial position. Essentially, the shell domain is yellow/range, the P1 domains is green and the P2 domain is blue. The middle figure is a cutaway of that reconstruction showing that the, very much unlike rNV, the P domains are 'floating' up above the surface of the shell domains. On the far right is a red/blue stereo image of the reconstruction.
Comparison of rNV and MNV models
Shown here in grey is the calculated surface of rNV (top) and MNV (bottom). The blue, green, and red ribbon figures are the atomic models subunits A, B, and C, respectively. Note how in the rNV model, the P domains rest atop the S domains and the P1 domains do not interact. In contrast, the P domains of MNV rise up off the S domain surface by at least 16Å and rotate such that the P1 domains now interact. This effectively forms a 'shell within a shell'.
Antibody mediated neutralization of MNV
In our recent publication, we have shown that a neutralizing mouse monoclonal antibody also neutralizes int the Fab fragment form. Therefore, just we did with human rhinovirus, we examined the structure of MNV complexed with these Fab fragments. Firstly, the 'floating P domains' were also observed here even though this data was collected at a different time and using a different preparation. This strongly suggests that the 'floating' nature of the P domains is not an artifact. Further, this structure also demonstrates that neutralization is independent of large conformational changes in the virion. Future studies are planned to tease out the biological significance of such a large structural difference between rNV and MNV.
Our relevant publications: