The noroviruses are now considered the most common cause of outbreaks of nonbacterial gastroenteritis. The norovirus virion comprises 90 protein dimers of the norovirus capsid protein, each of which has two domains: the shell domain and the protruding (P) domain. The shell domain is involved in the formation of the icosahedral shell whereas the P domain forms arch-like protrusions. The P domain has been divided into the P1 subdomain and the P2 subdomain, with the P2 region of the capsid thought to be involved in receptor binding and immunogenicity and includes documented hypervariable sites.

This study examines the changes that occurred in the P2 region of GII.Pe_GII.4 norovirus in the course of its evolution from a precursor phase (2008-2009), to an intermediate phase (2010) and finally to an epidemic phase (2012-2015).The findings are reviewed in relation to the progressive evolution of the virus and to previously documented hypervariable sites in the P2 region.

Twenty-two P2 region amino acid (aa) sequences (166 aa long) from all phases of the evolution of the virus were compared and the changes analysed.  3D computer modelling of the capsid proteins was performed to aid in locating the variable sites on a norovirus capsid protein. 

Twenty sites in the P2 region underwent change and, of these, ten corresponded to previously proposed hypervariable sites and ten to novel hypervariable sites. It was notable that aa changes at two sites, X and Y, only emerged as the epidemic phase progressed.  3D modelling showed their location to not be on the exposed surface of the virion and combined with the nature of the aa changes suggests these sites were important in enhancing the structural integrity of the capsid, which in turn may have facilitated the longer term viability of the virus.

The current study helps establish the validity of previously proposed hypervariable sites in the P2 region as well as indicating new ones. It also provides data on how these sites changed over the evolutionary history of a particular norovirus strain.  Studying how epidemic strains of norovirus evolve may assist in predicting future norovirus epidemics.