Selective polyprotein processing determines norovirus sensitivity to Trim7

Noroviruses are a leading cause of gastroenteritis worldwide, yet the molecular mechanisms of how host antiviral factors restrict norovirus infection are poorly understood. Here we present a CRISPR activation screen that identifies mouse genes that inhibit murine norovirus (MNV) replication. Detailed analysis of the major hit Trim7, demonstrates a potent inhibition of the early stages of MNV replication. Leveraging in vitro evolution, we identify MNV mutants that escape Trim7 restriction by altering the cleavage of the viral NS6-7 polyprotein precursor. NS6, but not the NS6-7 precursor, directly binds the substrate binding domain of Trim7. Surprisingly, the selective polyprotein processing that enables Trim7 evasion inflicts a significant evolutionary burden as viruses with decreased NS6-7 cleavage are strongly attenuated in viral replication and pathogenesis. Our data provides an unappreciated mechanism of viral evasion of cellular antiviral factors through selective polyprotein processing and highlights the evolutionary tradeoffs in acquiring resistance to host restriction factors.