Mapping of antigenic domains in poliovirus VP1 involved in structural rearrangements during virus morphogenesis and antigenic alterations of the virion.
Monoclonal antibodies (mAbs) directed against linear epitopes of the structural polypeptide VP1 of poliovirus type 1, Mahoney (PV1M), were used as sensitive tools to evaluate the accessibility of certain amino acid residues, both during virus morphogenesis and after conformational transitions of the capsid resulting from heat treatment (H- or 80S particles) and cell-receptor interaction (A- or 135S particles). Antibody binding sites were mapped by immunoblotting of VP1 fragments after procaryotic expression and by introduction of nested sets of deletions into recombinant VP1. The binding sites clustered at the amino- and carboxy-termini of the polypeptide, respectively. In 14S particles the amino-terminal sites were accessible for our mAbs, most likely from the inner surface of the particle. The carboxy-terminal sites became inaccessible during formation of pentamers from protomers. As shown by differential reaction of the mAbs, the amino-terminus of VP1 becomes externalized up to residues 41-55, whereas residues 56-67 remain buried during transition to both 80S and 135S particles. Carboxy-terminal residues 280-286 also become accessible to antibody binding on the surface of the altered particles. Since these residues are part of the canyon cleft of VP1, a structural rearrangement indicated by these mAbs is apparently associated with the loss of binding ability of 135S particles to the cellular receptor, which could explain the loss of infectivity of these particles.