Ring structure amino acids affect the suppressor activity of melon aphid-borne yellows virus P0 protein

Melon aphid-borne yellows virus (MABYV) is a newly identified polerovirus occurring in China. Here, we demonstrate that the MABYV encoded P0 (P0MA) protein is a strong suppressor of post-transcriptional gene silencing (PTGS) with activity comparable to tobacco etch virus (TEV) HC-Pro. In addition we have shown that the LP F-box motif present at the N-terminus of P0MA is required for suppressor activity. Detailed mutational analyses on P0MA revealed that changing the conserved Trp 212 with non-ring structured amino acids altered silencing suppressor functions. Ala substitutions at positions 12 and 211 for Phe had no effect on P0 suppression-activity, whereas Arg and Glu substitutions had greatly decreased suppressor activity. Furthermore, substitutions targeting Phe at position 30 also resulted in reduced P0 suppression-activity. Altogether, these results suggest that ring structured Trp/Phe residues in P0 have important roles in suppressor activity.