Crystallization and X-ray analysis of 23 nm virus-like particles from Norovirus Chiba strain

Norovirus is a major causative pathogen of nonbacterial acute gastroenteritis. Despite the sequence similarity among various strains, noroviruses of different genotypes show different antigenicities and different binding profiles to histo-blood group antigens (HBGAs). To reveal the relationships between the structure of the capsid and the diversity in antigenicity and the HBGA-binding profile, virus-like particles (VLPs) of the Chiba strain that belongs to genogroup I, genotype 4 were crystallized for X-ray structural analysis. Diffraction data were collected and processed at 3.2 A resolution. The crystal belonged to space group I222, with unit-cell parameters a = 290.0, b = 310.4 c = 350.4 A. The possible packing model indicated that the diameter of the particle was 280 A, which was much smaller than the 38 nm VLPs of Norovirus Norwalk strain (NV) with T = 3 icosahedral symmetry and composed of 180 VP1 proteins. The structure was solved by molecular replacement using the structure of the VP1 pentamer of NV 38 nm VLPs as a search model, revealing that the VLPs were smaller particles: 23 nm VLPs with T = 1 icosahedral symmetry, the structure of which has not yet been analyzed at high resolution. The structure of 23 nm VLPs will enable the two different VLPs of Norovirus to be compared, which will provide important information for understanding the structural basis of capsid formation.