Studies on molecular interactions in two closely related plant viruses putatively using different mechanisms for cell-to-cell movement

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Brome mosaic virus (BMV) and cowpea chlorotic mottle virus (CCMV) are closely related plant viruses but suggested to use different mechanisms for cell-to-cell movement in view of its coat protein (CP) requirement. CP-defective variants of CCMV move from cell to cell but those of BMV do not. To determine the viralfactor (s) involved in the CP requirement in viral cell-to-cell movement, we made several chimeric viruses, e.g. BMV (ΔCP-GFP) and CCMV (ΔCP GFP), where green fluorescent protein (GFP) gene was substituted for the CP. The 3a movement protein (MP) genes were exchanged between the chimeras to construct BMV (C3a/ΔCP-GFP) and CCMV (B3a/ΔCP-GFP). Although BMV (ΔCP-GFP) did not move from cell to cell and CCMV (ΔCP-GFP) moved, BMV (C3a/ΔCP-GFP) moved and CCMV (B3a/ΔCP-GFP) did not move, demonstrating that bromovirus 3a genes determine the requirement of the CP gene for viral spread. We have showed that BMV 3a binds single-stranded (ss) RNA in vitro. Here, I demonstrated that the CCMV 3a also binds ss-RNA cooperatively without sequence specificity. To examine tubule-forming capacity of the 3a proteins of BMV and CCMV, these proteins were monitored in protoplasts as fusion proteins with GFP. In protoplasts, tubular structures were induced from their surface, when they were inoculated with BMV or CCMV derivatives encoding 3a-GFP fusions. Together, one possible explanation is that BMV and CCMV could take at least two mechanisms for movement. Either virus may move through tubular structures as well as could traffic plasmodesmata as MP-RNA complex. GST pull-down assay showed that BMV MP binds its CP in vitro. Further studies are required to understand correlation between CP requirement of each virus and binding ability of each MP with CP.