Proteolytic processing in infectious bursal disease virus: identification of the polyprotein cleavage sites by site-directed mutagenesis.

The infectious bursal disease virus (IBDV), a member of the Birnaviridae family, is the causative agent of an immune depressive disease that affects domesticated and wild avian species. The expression strategy of IBDV includes the synthesis of a 110-kDa polyprotein containing the capsid precursor polypeptides. The polyprotein is autocatalitically processed rendering three polypeptides: NH2-VPX-VP4-VP3-COOH. We have carried out a systematic analysis, using a series of plasmids encoding polyproteins containing either deletions or single amino acid substitutions, to identify the processing sites. The results obtained showed the existence of two sites, 511LAA513 and 754MAA756, that are essential for the processing of the VPX-VP4 and VP4-VP3 precursors, respectively. These sequences are highly conserved among IBDV strains form serotypes 1 and 2. A secondary VPX-VP4 processing site was detected in a 19-amino acid stretch located upstream of the 511LAA513 site. Analyses using versions of the 754MAA756 VP4-VP3 processing site containing conservative and nonconservative amino acid substitutions demonstrated that the specificity of the cleavage is dictated by the conserved AA dipeptide.