Measles virus C protein interferes with Beta interferon transcription in the nucleus

Transcriptional induction of beta interferon (IFN-β) through pattern recognition receptors is a key event in the host defense against invading viruses. Infection of cells by paramyxoviruses, like measles virus (MV) (genus Morbillivirus), is sensed predominantly by the ubiquitous cytoplasmic helicase RIG-I, recognizing viral 5'-triphosphate RNAs, and to some degree by MDA5. While MDA5 activation is effectively prevented by the MV V protein, the viral mechanisms for inhibition of MDA5-independent induction of IFN-β remained obscure. Here, we identify the 186-amino-acid MV C protein, which shuttles between the nucleus and the cytoplasm, as a major viral inhibitor of IFN-β transcription in human cells. Activation of the transcription factor IRF3 by upstream kinases and nuclear import of activated IRF3 were not affected in the presence of C protein, suggesting a nuclear target. Notably, C proteins of wild-type MV isolates, which are poor IFN-β inducers, were found to comprise a canonical nuclear localization signal (NLS), whereas the NLSs of all vaccine strains, irrespective of their origins, were mutated. Site-directed mutagenesis of the C proteins from an MV wild-type isolate and from the vaccine virus strain Schwarz confirmed a correlation of nuclear localization and inhibition of IFN-β transcription. A functional NLS and efficient nuclear accumulation are therefore critical for MV C to retain its potential to downregulate IFN-β induction. We suggest that a defect in efficient nuclear import of C protein contributes to attenuation of MV vaccine strains.