Department of Agricultural Sciences, PO Box 27, FI-00014 University of Helsinki, Finland
Potato virus Y (PVY) (genus Potyvirus) is the most economically damaging and widely distributed virus in potato. Spread of PVY in the field is controlled by growing resistant cultivars. The dominant potato gene Nytbr for hypersensitive resistance (HR) controls ordinary PVY strains (PVYO) but is overcome by PVYN strains. Studies with infectious PVY chimeras and mutants indicated that the viral determinants necessary and sufficient to overcome Nytbr reside within the helper component proteinase (HC-Pro) (residues 227 to 327). Specifically, eight residues and the modeled three-dimensional conformation of this HC-Pro region distinguish PVYN from PVYO strains. According to the model, the conserved IGN and CCCT motifs implicated in potyvirus replication and movement, respectively, are situated in a coiled structure and an α-helix, respectively, within this region in PVYO; however, their locations are reversed in PVYN. Two residues (R269 and K270) are crucial for the predicted PVYO-specific HC-Pro conformation. Two viral chimeras triggered Nytbr and induced veinal necrosis in tobacco, which is novel for PVY. One chimera belonged to strain group PVYE. Our results suggest a structure–function relationship in recognition of PVYO HC-Pro by Nytbr, reveal HC-Pro amino acid signatures specific to PVYO and PVYN, and facilitate identification of PVY strains overcoming Nytbr.