Infectious bursal disease virus polyprotein processing does not involve cellular proteases.

The larger genome segment, segment A, of infectious bursal disease virus (IBDV) encodes VP2, VP3 and VP4 as a precursor polyprotein. The viral protease, VP4, is responsible for self-processing of the polyprotein, however, there are additional secondary precursor products such as VPX whose further processing has not been defined. Expression of IBDV cDNAs in vitro with rabbit reticulocyte lysates in a coupled transcription-translation system and in the Sindbis virus expression system (with BHK-21 and Vero cell cultures) were used to study processing of the polyprotein. In both expression systems, we identified three main gene products with molecular masses of 48, 34, and 30.5 kDa corresponding to VPX, VP3, and VP4, respectively, as found in IBDV-infected Vero cell cultures, although the amount of each product was variable. A translational time course of the polyprotein gene and analyses of products specified by various sub-clones of the full-length cDNA were used to distinguish primary processing products of translation from secondary products generated by proteolytic processing during in vitro coupled transcription-translation expression. The VPX, VP3 and VP4, which are the primary processing products, first appeared after 20 min of incubation and their production was maximum by 75 min of the coupled transcription-translation reaction. Cycloheximide chases demonstrated that there is no secondary processing of VPX (or VP3 and VP4). Thus VP2, the major capsid protein in virions, was not detected either in translation products of rabbit reticulocyte lysates or in lysates of Sindbis virus recombinant-infected cell cultures indicating the absence of secondary processing of VPX to VP2 during foreign expression of the segment A cDNA. We conclude that VPX maturation to VP2 does not involve cellular proteases.