Three epitopes located on the coat protein amino terminus of viruses in the bean common mosaic potyvirus subgroup

Twenty-seven of 29 strains of viruses in the bean common mosaic virus (BCMV) subgroup of legume-infecting potyviruses reacted strongly with one or more of the monoclonal antibodies (MAbs) which are known to be specific for epitopes located along the 50 amino acids which constitute the N-terminal end of the viral coat protein. Approximately one half of the virus strains reacted with the N-terminal epitope specific (NTES) MAb 4G12 which is specific for epitope E/B4, while the other half reacted with NTES MAbs 4 Aff1 or 4F9 which are specific for epitope E/B3. All but two strains contained at least one of these epitopes while no strain contained both. Competitive assays using five sequential, non-overlapping, synthetic, 10mer peptides indicated that the amino acids critical for epitope E/B3 reaction were located at positions 5, 7, and 10 from the N-terminal end of the coat protein. By deduction we postulate that the amino acids critical for epitope E/B4 are located at positions 10, 16, and 17. Because epitope E/B3 requires isoleucine at position 10 for expression whereas epitope E/B4 requires valine to be expressed, no one strain can express both epitopes. Two viruses in our tests (azuki mosaic and Dendrobium mosaic viruses) had deletions in this portion of their sequence explaining their failure to react MAbs specific for either epitope. The critical amino acids for a third epitope, E/B3A, were located at positions 16 and 17. We found no correlation between any of the three N-terminal epitopes defined in this study and the presence or absence of any biological property that we could accurately measure: i.e., symptomatology, host range, or pathotype. However, when coat protein sequences were aligned according to epitope type E/B3 or E/B4, we found that sequences within groups had high levels of identity while between group identities were low. We also found that sequences in the 3′-end non-coding region exhibited similar relationships within and between epitope groups. Two strains of BCMV (NL-4 and RU-1) were found to possess coat protein sequences typical of epitope E/B4 but 3′-NCR sequences typical of epitope E/B3. These data suggest that both strains may be the result of natural recombinants between the two epitope groups.