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

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.