A Stretch of 11 Amino Acids in the βB-βC Loop of the Coat Protein of Grapevine Fanleaf Virus Is Essential for Transmission by the Nematode Xiphinema index

Grapevine fanleaf virus (GFLV) andArabis mosaic virus (ArMV) from the genusNepovirus , familySecoviridae , cause a severe degeneration of grapevines. GFLV and ArMV have a bipartite RNA genome and are transmitted specifically by the ectoparasitic nematodesXiphinema index andXiphinema diversicaudatum , respectively. The transmission specificity of both viruses maps to their respective RNA2-encoded coat protein (CP). To further delineate the GFLV CP determinants of transmission specificity, three-dimensional (3D) homology structure models of virions and CP subunits were constructed based on the crystal structure ofTobacco ringspot virus , the type member of the genusNepovirus . The 3D models were examined to predict amino acids that are exposed at the external virion surface, highly conserved among GFLV isolates but divergent between GFLV and ArMV. Five short amino acid stretches that matched these topographical and sequence conservation criteria were selected and substituted in single and multiple combinations by their ArMV counterparts in a GFLV RNA2 cDNA clone. Among the 21 chimeric RNA2 molecules engineered, transcripts of only three of them induced systemic plant infection in the presence of GFLV RNA1. Nematode transmission assays of the three viable recombinant viruses showed that swapping a stretch of (i) 11 residues in the βB-βC loop near the icosahedral 3-fold axis abolished transmission byX. index but was insufficient to restore transmission byX. diversicaudatum and (ii) 7 residues in the βE-αB loop did not interfere with transmission by the twoXiphinema species. This study provides new insights into GFLV CP determinants of nematode transmission.