Tobamovirus and Dianthovirus Movement Proteins Are Functionally Homologous

AbstractThe movement proteins (MPs) of tobacco mosaic tobamovirus (TMV) and red clover necrotic mosaic dianthovirus (RCNMV) enlarge plasmodesmata size exclusion limits, transport RNA from cell to cell, and bind nucleic acids in vitro. Despite these functional similarities, they have no sequence homology. However, they do appear to have similar secondary structures. We have used transgenic plants expressing either the TMV MP or the RCNMV MP, and a chimeric TMV that encodes the RCNMV MP as its only functional MP gene, to demonstrate that the MPs of TMV and RCNMV are functionally homologous. Further, both TMV and RCNMV can act as helper viruses to allow the cell-to-cell movement of the heterologous movement-defective viruses. These data support the conclusion that, despite other differences, such as particle morphology, host range, and sequence, TMV and RCNMV share a common mechanism for cell-to-cell movement

Conversion in the Requirement of Coat Protein in Cell-to-Cell Movement Mediated by the Cucumber Mosaic Virus Movement Protein

Plant viruses have movement protein (MP) gene(s) essential for cell-to-cell movement in hosts. Cucumber mosaic virus (CMV) requires its own coat protein (CP) in addition to the MP for intercellular movement. Our present results using variants of both CMV and a chimeric Brome mosaic virus with the CMV MP gene revealed that CMV MP truncated in its C-terminal 33 amino acids has the ability to mediate viral movement independently of CP. Coexpression of the intact and truncated CMV MPs extremely reduced movement of the chimeric viruses, suggesting that these heterogeneous CMV MPs function antagonistically. Sequential deletion analyses of the CMV MP revealed that the dispensability of CP occurred when the C-terminal deletion ranged between 31 and 36 amino acids and that shorter deletion impaired the ability of the MP to promote viral movement. This is the first report that a region of MP determines the requirement of CP in cell-to-cell movement of a plant virus

Tobacco mosaic virus movement protein enhances the spread of RNA silencing

Eukaryotic cells restrain the activity of foreign genetic elements, including viruses, through RNA silencing. Although virusesencode suppressors of silencing to support their propagation, viruses may also exploit silencing to regulate host geneexpression or to control the level of their accumulation and thus to reduce damage to the host. RNA silencing in plantspropagates from cell to cell and systemically via a sequence-specific signal. Since the signal spreads between cells throughplasmodesmata like the viruses themselves, virus-encoded plasmodesmata-manipulating movement proteins (MP) mayhave a central role in compatible virus:host interactions by suppressing or enhancing the spread of the signal. Here, we haveaddressed the propagation of GFP silencing in the presence and absence of MP and MP mutants. We show that the proteinenhances the spread of silencing. Small RNA analysis indicates that MP does not enhance the silencing pathway but ratherenhances the transport of the signal through plasmodesmata. The ability to enhance the spread of silencing is maintainedby certain MP mutants that can move between cells but which have defects in subcellular localization and do not supportthe spread of viral RNA. Using MP expressing and non-expressing virus mutants with a disabled silencing suppressingfunction, we provide evidence indicating that viral MP contributes to anti-viral silencing during infection. Our resultssuggest a role of MP in controlling virus propagation in the infected host by supporting the spread of silencing signal. Thisactivity of MP involves only a subset of its properties implicated in the spread of viral RNA