Vallbracht, M. | bioRxiv Science

bioRxiv2023

Nucleocapsid condensation drives Ebola viral factory maturation and dispersion

Replication and genome encapsidation of many negative-sense RNA viruses take place in virus-induced membrane-less organelles termed viral factories (VFs). While liquid properties of VFs are believed to control the transition from genome replication to encapsidation, the nucleocapsid assembly, VF maturation and interactions with the cellular environment remain elusive. Here we apply in situ cryo-correlative light and electron tomography to follow nucleocapsid assembly and changes in VF morphology and their liquid properties during Ebola virus infection. We show that Ebola viral nucleocapsids transition from loosely packed helical assemblies in early VFs to condensed cylinders that arrange into highly organized parallel bundles later in infection. Early VFs associate with intermediate filaments and are devoid of other host material, but become progressively accessible to cellular components. Our data suggest that this process is coupled to VF solidification and dispersion, and that changes in liquid properties of VFs promote nucleocapsid transport to budding sites. Highlights- Cryo-ET reveals the molecular architecture of Ebola virus replication compartments - Loosely coiled nucleocapsids transition to condensed cylinders forming bundles - Nucleocapsid condensation drives dispersion of viral factories promoting viral egress - Intermediate filaments associate with and are critical for virus factory formation

microbiology↗

bioRxiv2022

The Ebola virus VP40 matrix undergoes endosomal disassembly essential for membrane fusion

Ebola viruses (EBOVs) are filamentous particles, whose shape and stability are determined by the VP40 matrix. Virus entry into host cells occurs via membrane fusion in late endosomes; however, the mechanism of how the remarkably long virions undergo uncoating including virion disassembly and nucleocapsid release into the cytosol, remains unknown. Here, we investigate the structural architecture of EBOVs entering host cells and discover that the VP40 matrix disassembles prior to membrane fusion. We reveal that VP40 disassembly is caused by the weakening of VP40-lipid interactions driven by low endosomal pH that equilibrates passively across the viral envelope without a dedicated ion channel. We further show that viral membrane fusion depends on VP40 matrix integrity, and its disassembly reduces the energy barrier for fusion stalk formation. Thus, pH-driven structural remodeling of the VP40 matrix acts as a molecular switch coupling viral matrix uncoating to membrane fusion during EBOV entry.

microbiology↗