Understanding the phase separation characteristics of nucleocapsid protein provides a new therapeutic opportunity against SARS-CoV-2 Dear EditorTo date, tens of millions of people have been infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), causing the outbreak of the respiratory disease named the coronavirus disease 2019 . As a newly emerged member of the coronavirus family, SARS-CoV-2 is an enveloped positive-strand RNA virus, which has probably the largest genome (approximately 30 kb) among all RNA viruses. The nucleocapsid (N) protein of SARS-CoV-2 is mainly responsible for recognizing and wrapping viral RNA into helically symmetric structures (Malik, 2020). It was also reported that N protein can boost the efficiency of transcription and replication of viral RNA, implying its vital and multifunctional roles in the life cycle of coronavirus (Surjit and Lal, 2008;Chang et al., 2014). Recently, several independent research teams have reported that N protein of SARS-CoV-2 is capable of undergoing liquid-liquid phase separation (LLPS) (Iserman et al., 2020;Perdikari et al., 2020;Savastano et al., 2020).Here, we comprehensively determined the characteristics of the phase separation driven by the N protein of SARS-CoV-2 (termed SARS-CoV-2 N), and found that LLPS is involved in the interplay between the N protein-viral RNA complex of SARS-CoV-2 (termed SARS-CoV-2 N-RNA) and other viral proteins, such as nsp12. Importantly, we identified two small molecules targeting the SARS-CoV-2 N protein, which can intervene the phase separation properties of the N protein-viral RNA-nsp12 (termed SARS-CoV-2 N-RNA-nsp12) complex, thus probably improving the accessibility of other antiviral drugs (e.g., remdesivir) to their viral targets (e. g., nsp12/RdRp).First, IUPred2 (Erdos and Dosztanyi, 2020) and PLAAC (Lancaster et al., 2014) programs showed that SARS-CoV-2 N is highly disordered, and contains three intrinsically disordered regions (IDRs), with one also displaying prion-like activity (Figs. 1A and S1). Then, we expressed and purified the recombinant SARS-CoV-2 N protein with an mEGFP-tag (a monomeric variant of EGFP, A206K) or a His-tag using a prokaryotic expression system to understand the properties of N-driven LLPS in vitro (Fig. S2A and S2B). Confocal fluorescence microscopy showed that SARS-CoV-2 N was readily self-associated to form numerous micron-sized spherical condensates (Fig. 1B and 1C). Further time-lapse observations revealed that the SARS-CoV-2 N condensates fused and coalesced into larger ones upon their intersections (Fig. 1D and Video S1), verifying the liquid-like properties of SARS-CoV-2 N condensates. We also used fluorescence recovery after photobleaching (FRAP) to deeply study the dynamics of internal molecules within the N protein condensates. Recovery of fluorescence within the bleached regions (Fig. 1E) showed that SARS-CoV-2 N can partially freely diffuse within the condensed phase, consistent with their liquid-like behavior. In addition, phase condensation of SARS-CoV-2 ...

No abstract

The ongoing coronavirus disease 2019 (COVID-19) pandemic has raised an urgent need to develop effective therapeutics against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). As a potential antiviral drug target, the nucleocapsid (N) protein of SARS-CoV-2 functions as a viral RNA chaperone and plays vital and multifunctional roles during the life cycle of coronavirus1-3. In this study, we discovered that the N protein of SARS-CoV-2 undergoes liquid-liquid phase separation (LLPS) both in vitro and in vivo, which is further modulated by viral RNA. In addition, we found that, the core component of the RNA-dependent RNA polymerase (RdRp) of SARS-CoV-2, nsp12, preferentially partitions into the N protein condensates. Moreover, we revealed that, two small molecules, i.e., CVL218 and PJ34, can be used to intervene the N protein driven phase separation and loosen the compact structures of the condensates of the N-RNA-nsp12 complex of SARS-CoV-2. The discovery of the LLPS-mediated interplay between N protein and nsp12 and the corresponding modulating compounds illuminates a feasible way to improve the accessibility of antiviral drugs (e.g., remdesivir) to their targets (e.g., nsp12/RdRp), and thus may provide useful hints for further development of effective therapeutic strategies against SARS-CoV-2.

No abstract