Viral cell-to-cell movement requires formation of cortical punctate structures containing Red clover necrotic mosaic virus movement protein.

Movement protein (MP) of Red clover necrotic mosaic virus (RCNMV) forms punctate structures on the cortical endoplasmic reticulum (ER) of Nicotiana benthamiana cells, which are associated with viral RNA1 replication (Kaido et al., Virology 395, 232-242. 2009). We investigated the significance of ER-targeting by MP during virus movement from cell to cell, by analyzing the function of a series of MPs with varying length deletions at their C-terminus, either fused or not fused with green fluorescent protein (GFP). The C-terminal 70 amino acids were crucial to ER-localization of MP-GFP and cell-to-cell movement of the recombinant virus encoding it. However, C-terminal deletion did not affect MP functions, such as increasing the size exclusion limit of plasmodesmata, single-stranded RNA binding in vitro, and MP interacting in vivo. We discuss the possible role of this MP region in virus movement from cell to cell.

Endoplasmic reticulum targeting of the Red clover necrotic mosaic virus movement protein is associated with the replication of viral RNA1 but not that of RNA2.

Red clover necrotic mosaic virus (RCNMV) is a positive-strand RNA virus with a bipartite genome. The movement protein (MP) encoded by RNA2 is essential for viral movement. To obtain further insights into the viral movement mechanism, subcellular localizations of RCNMV MP fused with green fluorescent protein (MP:GFP) were examined in Nicotiana benthamiana epidermal cells and protoplasts. The MP:GFP expressed from the recombinant virus first appeared in the cell wall and subsequently was observed on the cortical endoplasmic reticulum (ER) as punctate spots. In contrast, the MP:GFP expressed transiently in the absence of other viral components was localized exclusively in the cell wall. Transient expression of the MP:GFP with a variety of RCNMV components revealed that the ER localization of the MP:GFP was associated with RNA1 replication, or its negative-strand RNA synthesis, but not those of RNA2 or replicase proteins per se. A model of RCNMV cell-to-cell movement is discussed.