Sequence addition to the N- or C-terminus of the major capsid protein VP1 of norovirus affects its cleavage and assembly into virus-like particles

Norovirus (NoV) infection is a leading cause of non-bacterial gastroenteritis worldwide and there are currently no effective therapeutics available to target the virus. The norovirus major capsid protein VP1 is a potential candidate for the development of vaccines due to the similar morphology and immunogenicity of its assembled virus-like particles (VLPs) compared to native virions. In this study, we explored the effects of N- and C-terminal sequence additions to the VP1 of a GII.4 NoV during its assembly into VLPs. A series of sequences of different lengths derived from the minor capsid protein VP2 of the GII.4 NoV were added to the N- and C-terminus of VP1. The fusion proteins were expressed using a recombinant baculovirus expression system and the assembly of the expressed fusion proteins was subsequently observed under electron microscopy (EM). Our results indicated that all constructed fusion proteins were successfully expressed with different degrees of enzyme cleavage at the N-terminus. Electron microscopy revealed the successful assembly of VLPs of different sizes for all fusion proteins. An in vitro binding assay for VLP-histo-blood group antigens (HBGAs) indicated that all fusion proteins exhibited similar binding patterns compared with their wild-type VP1. Our results demonstrate that (Xi et al., 1990) [1] NoV VP1 can tolerate foreign sequences at its N- or C-terminus without affecting its ability to assemble into VLPs, and (Jiang et al., 1992) [2] that the cleavage pattern and effects of foreign sequences on the sizes of assembled VLPs observed in this study might represent important experimental data that can be used to elucidate VP1 self-assembly.