Are tubules generated by the 3a protein necessary for cucumber mosaic virus movement?

The 3a movement protein of cucumber mosaic virus (CMV), fused to the jellyf ish green fluorescent protein (3a-GFP) generated surface punctate aggregate s as well as tubules protruding from infected tobacco and Nicotiana bentham iana protoplasts, Fluorescent tubules also appeared on the surface of proto plasts prepared from transgenic tobacco plants expressing 3a-GFP, indicatin g that the 3a protein is the only viral component required for the formatio n of the tubules, CMV with a mutation in the gene encoding the 3a protein, M8 CMV, could infect tobacco systemically, but tubules were not detected pr otruding from infected protoplasts when the mutated 3a protein was fused to the GFP [(M8)3a-GFP]. This indicates that the ability of the 3a protein to generate tubules in the surface of protoplasts is not a function required for the spread of CMV in tobacco, On the other hand, the (M8)3a-GFP did not traffic through plasmodesmata interconnecting tobacco epidermal cells, in contrast to the wildtype 3a-GFP, This suggests that there may be a correlat ion between the ability of the 3a protein to assemble tubules in protoplast s and its ability to promote movement within certain tissues.