Function of VP2 Protein in the Stability of the Secondary Structure of Virus-like Particles of Genogroup II Norovirus at Different pH Levels: Function of VP2 Protein in the Stability of NoV VLPs

VP2 is the minor structural protein of noroviruses (NoV) and may function in NoV particle stability. To determine the function of VP2 in the stability of the NoV particle, we constructed and purified two kinds of virus-like particles (VLPs), namely, VLPs (VP1) and VLPs (VP1+VP2), from Sf9 cells infected with recombinant baculoviruses by using a Bac-to-Bac (R) baculovirus expression system. The two kinds of VLPs were treated with different phosphate buffers (pH 2 to pH 8); the secondary structure was then analyzed by far UV circular dichroism (CD) spectroscopy. Results showed that significant disruptions of the secondary structure of proteins were not observed at pH 2 to pH 7. At pH 8, the percentages of alpha-helix, beta-sheet, and beta-turn in VLPs (VP1) were decreased from 11% to 8%, from 37% to 32%, and from 20% to 16%, respectively. The percentage of coil was increased from 32% to 44%. By contrast, the percentages of alpha-helix, beta-sheet, and beta-turn in VLPs (VP1+VP2) were decreased from 11% to 10%, from 37% to 35%, and from 20% to 19%, respectively. The percentage of coil was increased from 32% to 36%. VLPs (VP1+VP2) was likely more stable than VLPs (VP1), as indicated by the percentage of the secondary structures analyzed by CD. These results suggested that VP2 could stabilize the secondary structure of VLPs under alkaline pH conditions. This study provided novel insights into the molecular mechanism of the function of VP2 in the stability of NoV particles.