Structure, function, and inhibition of the SARS-CoV-2 replication-transcription complex

Project Summary COVID-19, caused by the coronavirus SARS-CoV-2, continues to devastate the world. In less than a year, there have been more than 20 million cases with over 700,000 deaths. The viral RNA-dependent RNA polymerase (RdRp) is the central enzyme responsible for transcription and replication of the viral RNA genome. This enzyme is also a target for the current antiviral, remdesivir, used to ameliorate the severity and duration of this disease. The virus also encodes several nucleic acid processing enzymes, in addition to the RdRp, including a helicase, an endonuclease, an exonuclease, and methyltransferases. However, it is unknown how these enzymes coordinate to transcribe and replicate the viral genome. This proposal builds upon preliminary data of the structure of the helicase, nsp13, in complex with the RdRp and a primed substrate RNA (nsp13-replication/transcription complex or nsp13-RTC). The aims here include completing the structural analysis of this complex by utilizing additional data collected. The result of this aim will provide higher resolution (better than 2.7 Å in some parts the RdRp), providing a rich basis for the development of antiviral inhibitors. Also, having this structure in hand allows for the collaboration with expert developers of antimicrobials, also part of the aims, including the investigation of the structural details of the pre-incorporation state of remdesivir and antivirals produced by human microbiome. The models resulting from the structure of nsp13-RTC serve as foundations to test how the helicase and exonuclease function together with the RdRp. Specifically, real-time fluorescence assays, single-molecule fluorescence resonance energy transfer (FRET), and multi-color fluorescence microscopy will be used to probe the role of the helicase and the exonuclease in unwinding substrate RNA, backtracking, and proofreading. Another aim applies the pipeline used to characterize the nsp13-RTC assembly, which yielded a high- resolution structure of the complex, to other RTC assemblies. Specifically, native electrophoretic mobility assays will be used as a starting point to probe larger assemblies of the RTC. Native mass-spectrometry will then be used to determine the composition and stoichiometry of the complexes. Finally, cryo-EM will be applied to solve the structures of these macromolecular machines. The resulting structures will provide a starting point to elucidate the coordinated functions of these enzymes, provide insight into their mechanisms, and establish novel targets for therapeutics. In summary, this proposal aims to understand at the molecular and structural level how the SARS-CoV-2 nucleic acid processing enzymes coordinate to replicate and transcribe the viral genome, and to provide structure-guided targets for drug discovery, with the ultimate goal of providing relief for the COVID-19 pandemic.