A region of mumps virus nucleoprotein affects defective interfering particle production.

Mumps virus (MuV) is a negative-sense, single-stranded RNA virus belonging to the family Paramyxoviridae. MuV causes acute infection of the parotid glands, and the infection can result in severe cases of encephalitis, meningitis and deafness in humans. The non-segmented RNA genome of MuV is encapsidated by the nucleocapsid protein (NP), which forms the ribonucleoprotein (RNP) complex that serves as a template for viral RNA synthesis. To make viral genomic RNA accessible to the viral polymerase, a conformational change within NP occurs. Recently, an atomic model of the NP of MuV was developed with cryogenic-electron microscopy (cryo-EM) using PIV5 NP crystal structure as a homology template. To examine NP's structure and function, we performed mutational analysis of MuV NP at region(s) proposed to play a role in accessing viral RNA. The MuV NP mutants containing G185P, A197Q, Q200R and groups denoted as Top (N63G, P139D, A197Q), Tip (P109E, N121G, A124R) and Bottom (G21S, E29T, P43N, R93Q, R304Q) were first tested in a minigenome system. All mutations resulted in reduced reporter gene activities with Q200R and Bottom having the most severe negative effects. Rescuing of recombinant viruses with these mutations was attempted, and only MuV mutants '185 (G185P)', '197 (A197Q)' and 'Top (N63G, P139D, A197A)' were obtained. The 'Top' MuV mutant exhibited normal growth kinetics at low multiplicities of infection (MOIs); however, at high MOIs, the virus had reduced peak litres than low MOIs. Further analysis indicates that production of defective interfering particles (DI particles or DIPs) was enhanced by the mutant virus, indicating that this region, a known alpha-helix hinge region, is important for full-length genome replication, suggesting that it plays a role in maintaining stability of viral RNA-dependent RNA-polymerase on RNP template during MuV viral RNA synthesis. Understanding the production of DI particles will lead to a better understanding of MuV pathogenesis, as well as its replication/transcription process.