The C-terminal region of the Barley stripe mosaic virus γb protein participates in homologous interactions and is required for suppression of RNA silencing

The 17-kDa, cysteine-rich gammab protein of Barley stripe mosaic virus (BSMV) is a major contributor to viral pathogenesis, although it is dispensable for replication and movement in the ND 18 strain of the virus. Within the C-terminal region of gammab, six coiled-coil heptad repeats, structures known to mediate protein-protein interactions, are predicted between amino acids 95 and 140. In this study, we have demonstrated that gammab engages in homologous interactions and that the C-terminal 67 amino acids of the protein are required for these interactions. The gammab homologous interactions were abrogated by mutations designed to disrupt the coiled-coil motifs with substitutions of glycine residues for hydrophobic residues in the a and d positions of the heptads (gammabNC). Mutations within the gammabNC derivative were also found to destroy the silencing suppression activity of gammab in an Agrobacterium-mediated transient assay. Infectivity experiments to evaluate the gammabNC derivative revealed that this mutant developed symptoms 2 days earlier than the wild-type strain in Chenopodium amaranticolor. In barley, gammabNC elicited more severe bleaching and striping symptoms, similar to those of the previously described 'bleached' phenotype that is observed when mutations are introduced into the C1 and BM motifs. These findings collectively show that gammab interactions mediated by the coiled-coil motif are critical for the virulence and counter defence activities of BSMV in both monocot and dicot hosts.