Human Helicase DDX5 is Hijacked by SARS-CoV-2 Nsp13 Helicase to Enhance RNA Unwinding

DEAD-box protein (DDX) 5 plays important roles in multiple aspects of cellular processes that require modulation of the RNA structure. Alongside the canonical role in RNA metabolism, numerous studies have demonstrated that DDX5 influences viral infections by directly interacting with viral proteins. However, the precise functional role of DDX5 during viral infection remains largely unclear. Here, we explore the previously undiscovered ability of DDX5 to interact and synergize with the Nsp13 helicase of SARS-CoV-2. We show that DDX5 exhibits a nanomolar binding affinity to Nsp13. Also, by dissecting DDX5 in its individual domains, we show that the Nsp13-DDX5 interaction is mediated by the RecA1 domain of DDX5. Importantly, we show that DDX5 and Nsp13 synergize in unwinding double-stranded RNA. Consistent with its ability to bind Nsp13, the RecA1 domain of DDX5 acts as a weak inhibitor of the synergic action of the two helicases in the RNA unwinding process. Modeling of the DDX5-Nsp13 complex provides a plausible explanation for the synergic action of the two helicases, in a mechanism that is likely instrumental in the early stage of infection, when the concentration of Nsp13 is still low.