Expression of chimeric proteins based on a backbone of the GII.4 norovirus VP1 and their application in the study of a GII.6 norovirus-specific blockade epitope.

The surface-exposed loop regions of the protruding domain of the norovirus (NoV) major capsid protein VP1 can tolerate the insertion of foreign antigens without affecting its assembly into subviral particles. In this study, we investigated the tolerance of the surface-exposed loop region of the GII.4 NoV VP1 by replacing it with homologous or heterologous sequences. We designed a panel of constructs in which the amino acid sequence from position 298-305 of the GII.4 NoV VP1 was replaced by sequences derived from the same region of GI.3, GII.3, GII.6, and GII.17 NoVs as well as neutralizing epitopes of enterovirus type 71 and varicella-zoster virus. The constructs were synthesized and expressed using a recombinant baculovirus expression system. The expression of target proteins was measured by indirect enzyme-linked immunosorbent assay (ELISA), and the assembly of virus-like particles (VLPs) was confirmed by electron microscopy. Our results showed that all of the constructs expressed high levels of target chimeric proteins, and all of the chimeric proteins successfully assembled into VLPs or subviral particles. An in vitro VLP-histo-blood group antigen (HBGA) binding assay revealed that chimeric-protein-containing VLPs did not bind or showed reduced binding to salivary HBGAs, a ligand for NoV particles. The results of an in vitro VLP-HBGA binding blockade assay indicated that the predicted surface-exposed loop region of the GII.6 NoV VP1 may comprise a blockade epitope. In summary, the surface-exposed loop region of the GII.4 NoV VP1 can be replaced by foreign sequences of a certain length. Using this strategy, we found that the predicted surface-exposed loop region of GII.6 NoV VP1 might contain a blockade epitope.