A free VP3 C-terminus is essential for the replication of infectious bursal disease virus

Green fluorescent protein (GFP) has been successfully incorporated into the viral-like particles of infectious bursal disease virus (IBDV) with a linker at the C-terminus of VP3 in a baculovirus system. However, when the same locus in segment A was used to express GFP by a reverse genetic (RG) system, no viable GFP-expressing IBDV was recovered. To elucidate the underlying mechanism, cDNA construct of segment A with only the linker sequence (9 amino acids) was applied to generate RG IBDV virus (rIBDV). Similarly, no rIBDV was recovered. Moreover, when the incubation after transfection was extended, wildtype rIBDV without the linker was recovered suggesting a free C-terminus of VP3 might be necessary for IBDV replication. On the other hand, rIBDV could be recovered when additional sequence (up to 40 nucleotides) were inserted at the 3' noncoding region (NCR) adjacent to the stop codon of VP3, suggesting that the burden of the linker sequence was not in the stretched genome size but the disruption of the VP3 function. Finally, when the stop codon of VP3 was deleted in segment A to extend the translation into the 3' NCR without introducing additional genomic sequence, no rIBDV was recovered. Our data suggest that a free VP3 C-terminus is essential for IBDV replication.