Figure 7.

Model of GRV infection and the role of ORF3 protein, fibrillarin, CBs and the nucleolus. Upon GRV infection, the ORF3 protein is produced in the cytoplasm and is targeted to CBs reorganizing them into multiple CBLs. The CBLs then move to and fuse with the nucleolus by an unknown mechanism. Host proteins are likely to be involved in targeting of the ORF3 protein to the CBs, reorganizing CBs and causing their fusion with the nucleolus. One such host protein is fibrillarin and the ORF3 protein causes the relocalization of some of the nuclear/nucleolar fibrillarin pool to the cytoplasm where viral RNPs containing the ORF3 protein, fibrillarin and viral RNA accumulate. When produced in companion cells, the viral RNPs are able to migrate into the phloem sieve elements where they are transported to the rest of the plant to generate a systemic infection. Orange circles—the ORF3 protein; dark blue circles—fibrillarin, light blue circles—other host proteins including coilin and U2B″.