Structural and molecular properties of inclusion bodies of mumps virus

Viral RNA synthesis of mononegaviruses occurs in the cytoplasmic membrane-less organelles called as inclusion body (IB). Here, we reported that the IBs of mumps virus (MuV), which is the causative agent of mumps and belongs to the family Paramyxoviridae, had properties of liquid organelles formed by liquid-liquid phase separation. Super-resolution microscopic analysis of the MuV IBs demonstrated that nucleocapsid and phospho-(P) proteins formed the cage-like structure and that viral polymerase was in a reticular pattern and partially co-localized with viral mRNA. In addition, we characterized host RNAs localized in the MuV IBs by a spatial transcriptome analysis and found that G-quadruplex motif sequences containing RNAs (G4-RNAs) were concentrated. In vitro phase separation assay showed that the G4-RNAs interacted with the P protein and enhanced condensation in the P droplets. Taken together, our data show that MuV generates the IBs with a characteristic cage-like structure and host G4-RNAs play an important role in the formation of MuV IBs To characterize host RNAs recruited into the MuV IBs, cDNA libraries were prepared from each sample for IB regions, cytoplasmic regions except for IBs, and cytoplasm of uninfected cells using PIC (Photo Isolation chemistry). The sequences were determined by NGS (Next-Generation Sequencing). Based on the expression profiles obtained from each group, RNAs that were significantly concentrated in the MuV IBs were identified.