Characterization of cucumber mosaic virus. IV. Movement protein and coat protein are both essential for cell-to-cell movement of cucumber mosaic virus.

cDNA clones of cucumber mosaic virus (CMV) RNA 3 were modified to express the jellyfish green fluorescent protein (GFP) in place of the 3a movement protein (MP) or coat protein (CP), as fusions to the N (GFP-3a) or C (3a-GFP) terminus of the MP or from a separate open reading frame as part of tricistronic RNAs 3. CMV RNA transcripts containing the individual modified RNAs 3 were unable to infect either Nicotiana tabacum or Nicotiana benthamiana systemically. Infection, as measured by confocal microscopy of GFP fluorescence, generally was limited to one to three epidermal cells at each inoculation site. Limited cell-to-cell movement, but not systemic movement, could be detected by complementation involving expression of MP and CP from two different RNA 3 constructs, each also expressing GFP. Infection involving RNA 3 expressing the GFP-3a fusion showed bright granules of variable size distributed predominantly and nonuniformly throughout the cytoplasm and, to a lesser extent, associated with the cell wall in single fluorescent cells, while infections expressing the 3a-GFP fusion showed bright, punctate fluorescence associated only with the cell wall. Infected cells expressing either 3a-GFP or free GFP showed a halo of less bright, fluorescent, neighboring cells, indicating limited movement of GFP. The initially infected cells also allowed movement of 10-kDa fluorescent dextran to the neighboring halo cells, while infection did not spread, suggesting different requirements for movement of either MP or dextran versus RNA.