Sequence close to the N-terminus of L2 protein is displayed on the surface of bovine papillomavirus type 1 virions.

The bovine papillomavirus type 1 (BPV1) L2 protein purified from Escherichia coli was used as an antigen to produce monoclonal antibodies (MAbs). A total of 26 individual clones which recognized the BPV1 L2 protein were obtained. Using infectious BPV1 virus particles, 3 of the MAbs were found to interact with BPV1 virus particles. Binding of the MAbs to BPV1 was confirmed by immunoelectron microscopy. A set of 92 13-mer peptides overlapping by 8 amino acids spanning the entire BPV1 L2 protein was synthesized on a membrane and used to map the epitopes recognized by these antibodies. Seventeen linear epitopes were identified. Our results revealed that a sequence toward the N-terminus of the L2 protein (aa 61-123) is displayed on the virus surface, while the remaining L2 sequences are hidden inside the virus capsid. Although the polyclonal antisera raised against BPV1 L2 neutralized the BPV1 virus, we failed to detect any neutralizing activity for the 3 L2-specific monoclonal antibodies which bound to the BPV1 particles. This suggests that extra binding sites may be needed for neutralization. This study prompted us to propose a model about how L1 and L2 proteins may interact during infectious papillomavirus assembly.