Force-dependent stimulation of RNA unwinding by SARS-CoV-2 nsp13 helicase (Mickolajczyk et al., Biophys J 2021)
Force-dependent activation of RNA unwinding by SARS-CoV-2 nsp13. (A) Example traces of nsp13 unwinding an RNA hairpin at varying forces obtained from a single-molecule optical tweezers assay. (B) Schematic illustrating that mechanical forces destabilizing the dsRNA substrate enhance the helicase activity of nsp13.
SARS-CoV-2 possesses an RNA genome and relies on a superfamily 1 helicase non-structural protein 13 (nsp13) to unwind nucleic acid duplexes for viral replication. Currently nsp13 is being intensively studied as a therapeutic target to combat COVID-19. However, the basic enzymatic properties of nsp13 remain poorly understood. In collaboration with the Kapoor Lab, we conduct a detailed biochemical and biophysical characterization of nsp13’s unwinding activities. On our part, we develop an optical tweezers assay to directly observe RNA unwinding by nsp13 in real time. Remarkably, we find that the unwinding frequency, processivity, and velocity of nsp13 all substantially increase when a destabilizing force is applied to the RNA duplex. These results depict nsp13 as an inherently weak helicase that can be potently activated via mechanical stimulation. We speculate that such stimulation could be provided by an RNA-dependent RNA polymerase that associates with nsp13 in the viral replication machinery.