A single amino acid in VP2 is critical for the attachment of infectious bursal disease subviral particles to immobilized metal ions and DF-1 cells

Abstract

VP2 protein is the primary host-protective immunogen of infectious bursal disease virus (IBDV). His249 and His253 are two surface histidine residues in IBDV subviral particles (SVP), which is formed by twenty VP2 trimers when the VP2 protein of a local isolate is expressed. Here, a systemic study was performed to investigate His249 or/and His253 on self-assembly, cell attachment and immunogenicity of SVP. Point-mutagenesis of either or both histidine residues to alanine did not affect self-assembly of the SVP, but the SVP lost its Ni-NTA binding affinity when the His253 was mutated. Indirect immunofluorescence assays and inhibitory experiments also showed that His253 is essential for SVP to attach onto the DF-1 cells and to inhibit IBDV infection of DF-1 cells. Finally, enzyme-linked immunosorbent assays and chicken protection assays demonstrated that SVP with a mutation of His253 to alanine induced comparable neutralizing antibody titers in chickens as the wild-type SVP did. It was concluded that VP2's His253, a site not significant for the overall immunogenicity induced by SVP, is crucial for the binding affinity of SVP to Ni-NTA and the attachment of an IBDV host cell line. This is the first paper to decipher the role of His253 played in receptor interaction and immunogenicity.