Location of an immunizing determinant within polypeptide VP1 of type O aphthovirus

VP1 is the only structural polypeptide of aphthovirus able to stimulate the production of neutralizing antibody. The region of VP1 responsible for this activity was located by testing various proteolytic fragments of VP1 for their ability to compete for virus-specific antibodies in serum raised against the intact polypeptide. No antigenic activity could be detected in VP1 fragments isolated from trypsin-treated virus. Controlled digestion revealed that trypsin cleaved VP1 in four places in a preferred order, whereas chymotrypsin cut at a maximum of two sites. The initial cuts by the two proteases were made very close to each other, and in each case resulted in a greatly reduced affinity of the VP1 fragments for virus-specific antibodies in anti-VP1 serum. In contrast, aphthovirus was resistant to Staphylococcus aureus V8 protease, and treatment of isolated VP1 with this protease generated a peptide of mol. wt. 8500 which competed efficiently with virus for antiserum to VP1 and for an absorbed antiviral serum specific for trypsin-sensitive sites. The results indicate that these antisera interact specifically with aphthovirus at a single antigenic determinant located on VP1 approximately two-thirds of the way along the polypeptide sequence.