Fig. 7.

Two putative models of the effect of Nsp3-S676T on virus replication and compensatory role of N-S194L. (A) Coronaviruses employ Nsp3 SUD to increase the binding between Paip1 and PABP1 to enhance viral mRNA translation. SARS-CoV-2 with the Nsp3-S676T mutation decreases translational stimulation, resulting in diminished virus infectivity. The N-S194L mutation augments N protein expression to compensate for the loss of viral fitness caused by S676T and reverts virus virulence. (B) Nsp3 and nsp4 are the minimal components required for the formation of the DMV spanning pore (30). The N-terminal domains of nsp3, including Ubl-1, HVR, Mac, and SUD, are critical for forming the crown-like shaped gate of the pore at the convex side of DMV. The SUD is a major component of this gate (30). N protein, serving as an RNA chaperone in CoV, is located at the gate and binds to Ubl1 to regulate viral RNA export. T676 in SUD is postulated to increase linker flexibility (Fig. 4F), resulting in increased mobility of the SUD-M relative to the SUD-C and leading to instability of the gate and diminished viral mRNA transport. S194L augments expression of the N protein. Increased levels of N are postulated to stabilize the gate, restoring viral RNA exit from the DMV. Note that the DMV contains two bilayers, but for the sake of simplicity, only one layer is shown.