The nine C-terminal residues of the grapevine fanleaf nepovirus movement protein are critical for systemic virus spread

The grapevine fanleaf virus (GFLV) RNA2-encoded polyprotein P2 is proteolytically cleaved by the RNA1-encoded proteinase to yield protein 2A, 2B MP movement protein and 2C CP coat protein. To further investigate the role of the 2B MP and 2C CP proteins in virus movement, RNA2 was engineered by alternatively replacing the GFLV 2B MP and 2C CP genes with their counterparts from the closely related Arabis mosaic virus (ArMV). Transcripts of all chimeric RNA2s were able to replicate in Chenopodium quinoa protoplasts and form tubules in tobacco BY-2 protoplasts in the presence of the infectious transcript of GFLV RNA1. Virus particles were produced when the GFLV 2C CP gene was replaced with its ArMV counterpart, but systemic virus spread did not occur in C. quinoa plants. In addition, chimeric RNA2 containing the complete ArMV 2B MP gene was neither encapsidated nor infectious on plants, probably because polyprotein P2 was incompletely processed. However, chimeric RNA2 encoding ArMV 2B MP , in which the nine C-terminal residues were those of GFLV 2B MP , formed virus particles and were infectious in the presence of GFLV but not ArMV 2C CP . These results suggest that the nine C-terminal residues of 2B MP must be of the same virus origin as the proteinase for efficient proteolytic processing of polyprotein P2 and from the same virus origin as the 2C CP for systemic virus spread.