Biophysical Journal
Volume 120, Issue 6, 16 March 2021, Pages 1020-1030
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Article
Force-dependent stimulation of RNA unwinding by SARS-CoV-2 nsp13 helicase

https://doi.org/10.1016/j.bpj.2020.11.2276Get rights and content
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Abstract

The superfamily 1 helicase nonstructural protein 13 (nsp13) is required for SARS-CoV-2 replication. The mechanism and regulation of nsp13 has not been explored at the single-molecule level. Specifically, force-dependent unwinding experiments have yet to be performed for any coronavirus helicase. Here, using optical tweezers, we find that nsp13 unwinding frequency, processivity, and velocity increase substantially when a destabilizing force is applied to the RNA substrate. These results, along with bulk assays, depict nsp13 as an intrinsically weak helicase that can be activated >50-fold by piconewton forces. Such force-dependent behavior contrasts the known behavior of other viral monomeric helicases, such as hepatitis C virus NS3, and instead draws stronger parallels to ring-shaped helicases. Our findings suggest that mechanoregulation, which may be provided by a directly bound RNA-dependent RNA polymerase, enables on-demand helicase activity on the relevant polynucleotide substrate during viral replication.

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Keith J. Mickolajczyk, Patrick M. M. Shelton, Michael Grasso, and Xiaocong Cao contributed equally to this work.