Small P particles formed by the Taiwan-native norovirus P domain overexpressed in Komagataella pastoris

The protrusion (P) domain of the major structural protein VP1 of norovirus (NoV) is critical for the host's immune response and receptor binding. Most heterologous P domains expressed in Escherichia coli or Komagataella pastoris (formally known as Pichia pastoris) form P particles consisting of 24 P monomers formed through intermolecular contact in the P regions and an end-linked cysteine tag. The small P particle is only found in P domains with terminal modifications. In this study, the NoV P domain of the most predominant NoV strain GII.4 isolated from Taiwan was expressed in K. pastoris. A high yield of NoV P was obtained using the high-cell density fermentation process in K. pastoris. A large amount of the small P particles and the trimer and dimer complexes formed by 12, 6, and 2 P monomers were observed in both the expression of the NoV P-His and P containing cysteine tag at the N-terminus. Dynamic light scattering and transmission electron microscopy analysis of the purified NoV P-His and P revealed that most of these small P particles are triangle-, square-, and ring-shaped with a diameter of 14-15 nm. The binding ability of purified NoV P-His and P to human histo-blood group antigen was confirmed by a saliva-binding assay. Without terminal modification, small P particles were formed in our study. The amino acid sequence analysis showed only four different amino acids (residue 84, 119, 136, and 313) between the P domain in this study and other investigated GII.4 strains suggesting that these amino acids might play an important role in the P particle formation. The small P particles formed by the Taiwan-native norovirus P domain overexpressed in K. pastoris may provide further information for morphogenesis studies and vaccine development.