The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) 1, 2 is the third coronavirus to cross from an animal reservoir to infect humans in the 21st century, after SARS-CoV3, 4 and the Eastern respiratory syndrome Medium (MERS) coronavirus.5 The isolation and sequencing of SARS-CoV-2 was reported in January 2020, and the virus was found to be highly related to SARS and shares a probable origin in bats.2, 6 Since Its appearance in December 2019 in Wuhan, China, the virus has infected more than 10.5 million people and caused more than 500,000 deaths as of July 1, 2020.
The high infectivity of SARS-CoV-2 and the global spread of this ongoing outbreak highlight the urgent need for public health and therapeutic measures to limit new infections. Furthermore, the severity of atypical pneumonia caused by SARS-CoV-2 (COVID-2019), which often requires hospital stays of several weeks and the use of invasive ventilators, 7-9 highlights the need for therapies to decrease the severity of individual infections.
Current therapeutic strategies against SARS-CoV-2 target important points in the life cycle of the virus. The antiviral redeliver, first developed against the Ebola virus, 10, 11 inhibits the viral RNA-dependent RNA polymerases of a variety of coronaviruses, including SARS-CoV-212-14, and has shown promise against SARS- CoV-2 in small-scale trials in both primates and humans.15, 16 Another target is the viral protease (Mopar / 3CLpro), which is required to process viral polyproteins into their active forms.17 Finally, the viral protease transmembrane peak (S) mediates binding to host cells through angiotensin converts enzyme 2 and transmembrane protease, serine 2 proteins, and mediates fusion of viral and host cell membranes.
18-21 As The virus’s most prominent surface component, the spike protein is the primary target for antibodies in patients and is the focus of several current efforts in the development of the SARS-CoV-2 vaccine. Initial trials using plasma-containing antibodies from convalescent COVID-19 patients have also shown promise in reducing the severity of the disease22.
While previous efforts are targeting viral entry, RNA synthesis, and protein processing, less emphasis has been placed on other steps in the viral life cycle so far. A critical step in coronavirus replication is the assembly of viral genomic RNA and nucleocapsid protein (N) into a ribonucleoprotein (RNP) complex, which interacts with membrane protein (M) and is packaged into virions.
Electron microscopy analysis of related beta coronaviruses has suggested that the RNP complex adopts a helical filament structure, 23-28, but recent cryoelectron tomography analysis of intact SARS-CoV-2 virions has revealed a bead-like arrangement in a string of globular RNP complexes that sometimes assemble into stacks that resemble helical filaments.
29 Despite their location within the viral particle rather than on its surface, patients infected with SARS-CoV-2 display antibody responses higher and earlier to the nucleocapsid protein than to the surface tip protein.30, 31 As such, a better understanding of the structure of the SARS-CoV-2 protein N and the structural differences between it and related coronavirus N proteins, including SARS-CoV, can aid in the development of sensitive and specific immunological tests.
Coronavirus N proteins possess a shared domain structure with an N-terminal RNA-binding domain and a C-terminal domain responsible for dimerization. The assembly of protein N into higher-order RNP complexes is not well understood, but probably involves cooperative interactions between the dimerization domain and other regions of the protein, plus bound RNA. 32-40 Here, we present a high-resolution structure of the SARS-CoV-2 N dimerization domain, revealing an entangled dimer similar to that of related beta coronaviruses.
We also analyzed the self-assembly properties of the SARS-CoV-2 protein N and showed that higher-order assembly requires both the dimerization domain and the extended and disordered C-terminus of the protein. Together with other work revealing the structure and RNA-binding properties of the N-terminal domain of the nucleocapsid, these results lay the foundation for a comprehensive understanding of the assembly and architecture of the SARS-CoV-2 nucleocapsid.