A dsRNA Virus Converts Early Endosomes into Stable Replication Organelles Through Maturation Arrest and Retrograde Transport

Abstract

Infectious bursal disease virus (IBDV) and infectious pancreatic necrosis virus (IPNV) are major animal pathogens that impose a heavy burden on the poultry and aquaculture industries. Yet, major aspects of their unique replication program remain unknown. IBDV is unusual among double-stranded RNA viruses in that it replicates in association with host endosomes. We previously demonstrated this association to be orchestrated by the viral protein VP3 and the early endosomal lipid phosphatidylinositol 3-phosphate [PtdIns(3)P], suggesting an essential role for VP3 in forming birnaviruses’ specialized replication compartments. Here, combining high-resolution fluorescence and correlative light-electron microscopy with diverse molecular and biochemical analyses, we define how the IBDV protein VP3 targets and remodels host endosomes, and establish that endosome-associated replication is conserved in members of the Birnaviridae family. We show that IBDV VP3 targets early endosomes via the PtdIns(3)P effector Early Endosomal Antigen 1 (EEA1) to arrest their maturation and prompt their association with the Golgi complex via ESCPE-1-mediated retrograde transport. We further show that IPNV also exploits endosomal membranes for replication in a PtdIns(3)P dependent manner, although for this virus VP3 engages endosomes through an EEA1-independent mechanism yet to be determined. This work uncovers how birnaviruses exploit a single viral protein to orchestrate endosome remodeling and intracellular trafficking to establish specialized replication compartments while revealing both conserved and host-adapted features of birnaviruses replication. Significance Statement Infectious Bursal Disease Virus (IBDV) and Infectious Pancreatic Necrosis Virus (IPNV) are globally important pathogens that threaten poultry and aquaculture production, causing major economic losses and impacting food security. Birnaviruses are unusual among double-stranded RNA viruses and have been proposed as evolutionary intermediates in RNA virus evolution, yet many aspects of their replication strategy remain unclear. Here, we show that two evolutionarily distinct birnaviruses depend on PtdIns(3)P-positive endosomes for replication and identify VP3 as the viral factor that targets and remodels these compartments. We further define how VP3 redirects early endosomes, promoting their retrograde trafficking toward the Golgi complex and revealing how birnaviruses convert transient endosomal membranes into stable replication platforms.