The mechanism underlying the organization of Borna disease virus inclusion bodies is unique among mononegaviruses

Inclusion bodies (IBs) are characteristic biomolecular condensates organized by mononegaviruses. Here, we characterize the IBs of Borna disease virus 1 (BoDV-1), a unique mononegavirus that forms IBs in the nucleus, in terms of liquid-liquid phase separation (LLPS). The BoDV-1 phosphoprotein (P) alone induces LLPS and the nucleoprotein (N) is incorporated into the P droplet in vitro. In contrast, co-expression of N and P is required for the formation of IB-like structure in cells. Furthermore, while BoDV-1 P binds to RNA, an excess amount of RNA dissolves the liquid droplets formed by N and P. Notably, the N-terminal intrinsically disordered region of BoDV-1 P is essential to drive LLPS and bind to RNA, suggesting that both abilities could compete with one another. These features are unique among mononegaviruses, and thus this study will contribute to a deeper understanding of LLPS-driven organization and RNA-mediated regulation of biomolecular condensates.