Releasing the genomic RNA sequestered in the mumps virus nucleocapsid.

In a negative strand RNA virus, the genomic RNA is sequestered inside the nucleocapsid when the viral RNA-dependent RNA polymerase uses it as the template for viral RNA synthesis. It must require a conformational change in the nucleocapsid protein (NP) to make the RNA accessible by the viral polymerase during this process. The structure of an empty mumps virus nucleocapsid-like particle is determined to 10.4 Å resolution by cryoEM image reconstruction. By modeling the crystal structure of parainfluenza virus 5 into the density, it is shown that the α-helix close to the RNA became flexible when RNA was removed. Point mutations in this helix resulted in loss of polymerase activities. Since the core of NP is rigid in the nucleocapsid, we suggest that interactions between this region of the mumps virus NP and its polymerase leads to exposure of the sequestered genomic RNA, instead of large NP domain rotations.

IMPORTANCE: Mumps virus (MuV) infection may cause serious diseases including hearing loss, orchitis, oophoritis, mastitis, and pancreatitis. MuV is a negative strand RNA virus, similar to rabies virus or Ebola virus, that has a unique mechanism of viral RNA synthesis. They all make their own RNA-dependent RNA polymerase (RdRp). The viral RdRp uses the genomic RNA inside the viral nucleocapsid as the template to synthesize viral RNAs. Since the template RNA is always sequestered in the nucleocapsid, the viral RdRp must find a way to open it up in order to gain access to the covered template. Our work reported here shows that a helix structural element in the MuV nucleocapsid protein becomes open when the sequestered RNA is released. The amino acids related to this helix are required for RdRp to synthesize viral RNA. We propose that the viral RdRp pulls this helix open to release the genomic RNA.