Z-RNA and the flipside of the SARS Nsp13 helicase

We present evidence that the severe acute respiratory syndrome coronavirus (SARS) non-structural protein 13 (Nsp13) modulates the Z-RNA dependent regulated cell death pathway of necroptosis ( 1 ). We show that Z-prone sequences (called flipons ( 2 )) exist in coronavirus and provide a signature that enables identification of the animal viruses which have become human pathogens. We also identify a potential RHIM in Nsp13. These two observations allow us to suggest a model in which Nsp13 may regulate Z-RNA-initiated RHIM-dependent cell death outcomes at two steps. The first step involves possible new ATP-independent Z-flipon helicase activity in Nsp13, which is distinct from the activity of the canonical A-RNA helicase. This activity unwinds/quenches nascent Z-RNAs, preventing their sensing by ZBP1. The second step involves RHIM-dependent inhibition of ZBP1, RIPK3 and/or RIPK1, preventing cell death downstream of Z-RNA sensing. Together the RHIM and Z-flipon helicase have the potential to alter the host response to the virus and the effectiveness of drugs targeting the NSP13 helicase.