As for all single-stranded, positive-sense RNA (+RNA) viruses, intracellular RNA synthesis relies on extensive remodeling of host cell membranes that leads to the formation of specialized structures. In the case of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) coronavirus causing COVID-19, endoplasmic reticulum membranes are modified, resulting in the formation of double-membrane vesicles (DMVs), which contain the viral dsRNA intermediate and constitute membrane-bound replication organelles. The non-structural and transmembrane protein nsp3 is a key player in the biogenesis of DMVs and, therefore, represents an interesting antiviral target. However, as an integral transmembrane protein, it is challenging to express for structural biology. The C-terminus of nsp3 encompasses all the membrane-spanning, -interacting, and -remodeling elements. By using a cell-free expression system, we successfully produced the C-terminal region of nsp3 (nsp3C) and reconstituted purified nsp3C into phospholipid nanodiscs, opening the way for structural studies. Negative-stain transmission electron microscopy revealed the presence of nsp3C oligomers very similar to the region abutting and spanning the membrane on the cytosolic side of DMVs in a recent subtomogram average of the SARS-CoV-2 nsp3-4 pore (1). AlphaFold-predicted structural models fit particularly well with our experimental data and support a pore-forming hexameric assembly. Altogether, our data give unprecedented clues to understand the structural organization of nsp3, the principal component that shapes the molecular pore that spans the DMVs and is required for the export of RNA in vivo.
Importance: Membrane remodeling is at the heart of intracellular replication for single-stranded, positive-sense RNA viruses. In the case of coronaviruses, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), this leads to the formation of a network of double-membrane vesicles (DMVs). Targeting DMV biogenesis offers promising prospects for antiviral therapies. This requires a better understanding of the molecular mechanisms and proteins involved. Three non-structural proteins (nsp3, nsp4, and nsp6) direct the intracellular membrane rearrangements upon SARS-CoV-2 infection. All of them contain transmembrane helices. The nsp3 component, the largest and multi-functional protein of the virus, plays an essential role in this process. Aiming to understand its structural organization, we used a cell-free protein synthesis assay to produce and reconstitute the C-terminal part of nsp3 (nsp3C) including transmembrane domains into phospholipid nanodiscs. Our work reveals the oligomeric organization of one key player in the biogenesis of SARS-CoV-2 DMVs, providing basis for the design of future antiviral strategies.
Keywords: SARS-CoV-2; cell-free expression; coronavirus; electron microscopy; membrane remodeling; nanodiscs; nsp3.