A non‐canonical Lon proteinase lacking the ATPase domain employs the Ser–Lys catalytic dyad to exercise broad control over the life cycle of a double‐stranded RNA virus

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Abstract
We have identified a region related to the protease domain of bacterial and organelle ATP‐dependent Lon proteases in virus protein 4 (VP4) of infectious bursal disease virus strain P2 (IBDVP2), a two‐segmented double‐stranded RNA virus. Unlike canonical Lons, IBDVP2 VP4 possesses a proteinase activity though it lacks an ATPase domain. Ser652 and Lys692 of IBDVP2 VP4 are conserved across the Lon/VP4 family and are essential for catalysis. Lys692 has the properties of a general base, increasing the nucleophilicity of Ser652; a similar catalytic dyad may function in the other Lons. VP4 can cleave in trans and is responsible for the interdomain proteolytic autoprocessing of the pVP2– VP4–VP3 polyprotein encoded by RNA segment A. VP2, which is later derived from pVP2, and VP3 are major capsid proteins of birnaviruses. Results of the characterization of a range of the IBDVP2 VP4 mutants in cell cultures implicate VP4 in trans‐activation of the synthesis of VP1, putative RNA‐dependent RNA polymerase encoded by RNA segment B, and in cleavage rate‐dependent control of process(es) crucial for the generation of the infectious virus progeny.

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