Host-specific cell-to-cell and long-distance movements of cucumber mosaic virus are facilitated by the movement protein of groundnut rosette virus.
The cucumovirus, cucumber mosaic virus (CMV), requires both the 3a movement protein (MP) and the capsid protein (CP) for cell-to-cell movement. Replacement of the MP of CMV with the MP of the umbravirus, groundnut rosette virus (GRV), which does not encode a CP, resulted in a hybrid virus, CMV(ORF4), which could move cell to cell in Nicotiana tabacum and long distance in N. benthamiana. After replacement of the CMV CP in CMV(ORF4) with the gene encoding the green fluorescent protein (GFP), the hybrid virus, CMV(ORF4.GFP), expressing both the GRV MP and the GFP, could move cell to cell but not systemically in either Nicotiana species. Immunoelectron microscopic analysis of cells infected by the hybrid viruses showed different cellular barriers in the vasculature preventing long-distance movement of CMV(ORF4) in N. tabacum and CMV(ORF4.GFP) in N. benthamiana. Thus the GRV MP, which shows limited sequence similarity to the CMV MP, was able to support CP-independent cell-to-cell movement of the hybrid virus, but CP was still required for long-distance movement and entry of particular vascular cells required functions encoded by different proteins.