Expression and assembly of Norwalk virus-like particles in plants using a viral RNA silencing suppressor gene

Binary vector-based transient expression of heterologous proteins in plants is a very attractive strategy due to the short time required for proceeding from planning to expression. However, this expression system is limited by comparatively lower yields due to strong post-transcriptional gene silencing (PTGS) in the host plants. The aim of this study was to optimize a procedure for expression of norovirus virus-like particles (VLPs) in plants using a binary vector with co-expression of a PTGS suppressor to increase the yield of the target protein. The effects of four plant viral PTGS suppressors on protein expression were evaluated using green fluorescent protein (GFP) as a reporter. Constructs for both GFP and PTGS suppressor genes were co-infiltrated in Nicotiana benthamiana plants, and the accumulation of GFP was evaluated. The most effective PTGS suppressor was the 126K protein of Pepper mild mottle virus. Therefore, this suppressor was selected as the norovirus capsid gene co-expression partner for subsequent studies. The construct containing the major (vp1) and minor capsid (vp2) genes with a 3'UTR produced a greater amount of protein than the construct with the major capsid gene alone. Thus, the vp1-vp2-3'UTR and 126K PTGS suppressor constructs were co-infiltrated at middle scale and VLPs were purified by sucrose gradient centrifugation. Proteins of the expected size, specific to the norovirus capsid antibody, were observed by Western blot. VLPs were observed by transmission electron microscopy. It was concluded that protein expression in a binary vector co-expressed with the 126K PTGS suppressor protein enabled superior expression and assembly of norovirus VLPs.