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 Lens, IBDVP2 VP4 possesses a proteinase activity though it lacks an ATPase domain. Ser652 and Lvs692 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 Lens, VP4 can cleave in trails and is responsible for the interdomain proteolytic autoprocessing of the QVP2-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 trails-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