VP1, the putative RNA-dependent RNA polymerase of infectious bursal disease virus, forms complexes with the capsid protein VP3, leading to efficient encapsidation into virus-like particles

A cDNA corresponding to the coding region of VP1, the putative RNA-dependen t RNA polymerase, of infectious bursal disease virus (IBDV) was cloned and inserted into the genome of a vaccinia virus inducible expression vector. T he molecular mass and antigenic reactivity of VP1 expressed in mammalian ce lls are identical to those of its counterpart expressed in IBDV-infected ce lls. The results presented here demonstrate that VP1 is efficiently incorpo rated into IBDV virus-like particles (VLPs) produced in mammalian cells coe xpressing the IBDV polyprotein and VP1. Incorporation of VP1 into VLPs requ ires neither the presence of IBDV RNAs nor that of the nonstructural polype ptide VP5. Immunofluorescence, confocal laser scanning microscopy, and immu noprecipitation analyses conclusively showed that VP1 forms complexes with the structural polypeptide VP3. Formation of VP1-VP3 complexes is likely to be a key step for the morphogenesis of IBDV particles.