NP - Signaling functions

PMID: 35874957 - 2022

N protein exhibits three important roles in coronavirus life cycle

• the primary role is to assemble with genomic RNA into the viral RNA-protein (vRNP) complex ^22. Virion formation occurs via the accumulation of the SARS-CoV-2 structural proteins [S, E, M and N] and gRNA at the ER-Golgi intermediate compartment (ERGIC) membrane. The N protein can interact with a luminal domain of the M protein through C-terminal dimerization domain (CTD) ^(247-365), which may be essential for mediating the recruitment of N-containing RNPs to the ERGIC membrane ^23. Previous study also suggests that the interaction between N protein and the E protein may plays an important role in SARS-CoV-2 assembly ^24. Together with N protein, a single strand of SARS-CoV-2 gRNA forms dense, locally ordered ribonucleoprotein (RNP) regions which may be further organized into more complex arrangements ^25^-27.
• liquid-liquid phase separation (LLPS) with viral genome RNA and potentially facilitates the viral assembly. Many RNA-binding proteins have been found to undergo LLPS with RNA to participate the biological and disease processes ^28^-32. The LLPS is dependent on the length and concentration of ssRNA. What is more, N protein forms typical sphere-like droplets with short ssRNAs, but not solid-like structures with long ssRNAs. The free Zn²+ in cytosol is essential for N protein/RNA LLPS ^21. It is also reported that viral RNA can induce assembly of the N protein into phase-separated condensates in vitro and pinpoints a ~40 residue region in the central intrinsically disordered region (IDR) with a key role in RNA-driven phase separation ^23.
• localize to replication transcription complexes (RTCs) at the early stages of infection, where it enhances replication and the transcription of viral RNA by recruiting host factors ¹⁵,33^-36. Previous studies have demonstrated that N protein and RNA recruited components of the RNA polymerase (RdRp) complex (Nsp7, Nsp8 and Nsp12) are responsible for replicating viral gRNA ³⁷,38. The phosphorylation of the SR domain of the N protein can decrease the degree of recruitment of RdRp components to condensates ^37. The interaction between N and Nsp3 is essential for viral replication. The N-terminal of N has an RNA-binding domain (N-NTD) that can bind the RNA genome, whereas, the C-terminal domain (N-CTD) can interact with the viral Nsp3 ^39.