The C-terminus of infectious bursal disease virus VP3 encodes a predicted intrinsically disordered region, which promotes the formation of cytoplasmic puncta and modulates their physical properties

Liquid-liquid phase separation (LLPS) is a phenomenon of growing interest in cell biology. It is a part of the replication cycles of diverse viruses, but our understanding of the molecular basis that underpins the mechanism of phase separation is incomplete. We previously demonstrated that the virus factories of the birnavirus IBDV, a major agricultural pathogen, are biomolecular condensates formed through LLPS. In this study, we discovered that VP3 was necessary but not sufficient for condensates to form, and the minimal components of these structures were VP3, VP1, and likely vRNA. We also discovered that the C-terminal 36 amino acid region of IBDV VP3 encoded a highly dynamic intrinsically disordered region that promoted the formation of the cytoplasmic puncta and modulated their physical properties. This work contributes to a more detailed understanding of birnavirus replication at the molecular level and to the study of LLPS as a phenomenon.