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 mor e of the monoclonal antibodies (MAbs) which are known to be specific for ep itopes located along the 50 amino acids which constitute the N-terminal end of the viral coat protein. Approximately one half of the virus strains rea cted 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 Al 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 u sing five sequential, nonoverlapping, synthetic, 10mer peptides indicated t hat the amino acids critical for epitope E/B3 reaction were located at posi tions 5, 7, and 10 from the N-terminal end of the coat protein. By deductio n we postulate that the amino acids critical for epitope E/B4 are located a t positions 10, 16, and 17. Because epitope E/B3 requires isoleucine at pos ition 10 for expression whereas epitope E/B4 requires valine to be expresse d, no one strain can express both epitopes. Two viruses in our tests (azuki mosaic and Dendrobium mosaic viruses) had deletions in this portion of the ir sequence explaining their failure to react MAbs specific for either epit ope. The critical amino acids for a third epitope, E/B3A, were located at p ositions 16 and 17. We found no correlation between any of the three N-tern inal epitopes defined in this study and the presence or absence of any biol ogical property that we could accurately measure: i.e., symptomatology, hos t range, or pathotype. However, when coat protein sequences were aligned ac cording 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 al so found that sequences in the 3'-end non-coding region exhibited similar r elationships within and between epitope groups. Two strains of BCMV (NL-4 a nd 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 bo th strains may be the result of natural recombinants between the two epitop e groups.