Structural and conformational analysis of SARS CoV 2 N-CTD revealing monomeric and dimeric active sites during the RNA-binding and stabilization: Insights towards potential inhibitors for N-CTD.

The advent of SARS-CoV-2 has become a universal health issue with no appropriate cure available to date. The coronavirus nucleocapsid (N) protein combines viral genomic RNA into a ribonucleoprotein and protects the viral genome from the host's nucleases. Structurally, the N protein comprises two independent domains: the N-terminal domain (NTD) for RNA-binding and C-terminal domain (CTD) involved in RNA-binding, protein dimerization, and nucleocapsid stabilization. The present study explains the structural aspects associated with the involvement of nucleocapsid C-terminal domain in the subunit assembly that helps the RNA binding and further stabilizing the virus assembly by protecting RNA from the hosts exonucleases degradation. The molecular dynamics (MD) simulations of the N-CTD and RNA complex suggests two active sites (site I: a monomer) and (site II: a dimer) with structural stability (RMSD: ~2 Å), Cα fluctuations (RMSF: ~3 Å) and strong protein-ligand interactions were estimated through the SiteMap module of Schrodinger. Virtual screening of 2456 FDA-approved drugs using structure-based docking identified top two leads distinctively against Site-I (monomer): Ceftaroline fosamil (MM-GBSA = -47.12 kcal/mol) and Cefoperazone (-45.84 kcal/mol); and against Site-II (dimer): Boceprevir, (an antiviral protease inhibitor, -106.78 kcal/mol) and Ceftaroline fosamil (-99.55 kcal/mol). The DCCM and PCA of drugs Ceftaroline fosamil (PC1+PC2 = 71.9%) and Boceprevir (PC1 +PC2 = 61.6%) show significant correlated residue motions which suggests highly induced conformational changes in the N-CTD dimer. Therefore, we propose N-CTD as a druggable target with two active binding sites (monomer and dimer) involved in specific RNA binding and stability. The RNA binding site with Ceftaroline fosamil binding can prevent viral assembly and can act as an antiviral for coronavirus.

SARS-CoV-2 nucleocapsid and Nsp3 binding: an in silico study.

Severe acute respiratory syndrome virus 2 (SARS-CoV-2) belongs to the single-stranded positive-sense RNA family. The virus contains a large genome that encodes four structural proteins, small envelope (E), matrix (M), nucleocapsid phosphoprotein (N), spike (S), and 16 nonstructural proteins (nsp1-16) that together, ensure replication of the virus in the host cell. Among these proteins, the interactions of N and Nsp3 are essential that links the viral genome for processing. The N proteins reside at CoV RNA synthesis sites known as the replication-transcription complexes (RTCs). The N-terminal of N has RNA-binding domain (N-NTD), capturing the RNA genome while the C-terminal domain (N-CTD) anchors the viral Nsp3, a component of RTCs. Although the structural information has been recently released, the residues involved in contacts between N-CTD with Nsp3 are still unknown. To find the residues involved in interactions between two proteins, three-dimensional structures of both proteins were retrieved and docked using HADDOCK. Residues at N-CTD were detected in interaction with L499, R500, K501, V502, P503, T504, D505, N506, Y507, I508, T509, K529, K530K532, S533 of Nsp3 and N-NTD to synthesize SARS-CoV-2 RNA. The interaction between Nsp3 and CTD of N protein may be a potential drug target. The current study provides information for better understanding the interaction between N protein and Nsp3 that could be a possible target for future inhibitors.