Structural basis for suramin binding to the C-terminal domain of the SARS-CoV-2 nucleocapsid protein

The global threat posed by COVID-19 persists, largely due to the high mutability of SARS-CoV-2 and the limited availability of effective antiviral therapeutics. The nucleocapsid (N) protein of SARS-CoV-2 is an attractive drug target because of its high degree of sequence conservation and essential role in viral replication. In this study, we show that suramin, a polysulfonated antiviral compound, binds to the C-terminal domain (N-CTD) of the N protein and interferes with its interaction with RNA. Biolayer interferometry (BLI) shows that suramin has a higher binding affinity for N-CTD (K d, 3.30 μM) than for RNA (Kd, 10.12 μM). Electrophoretic mobility shift assays (EMSAs) further confirms that suramin effectively displaces RNA from N-CTD. NMR titration experiments and site-directed mutagenesis identify the α1-η1 helix (residues 248–262) as the primary suramin binding region, with residues K256, R259 and R262 playing critical roles in ligand recognition. In addition, NMR relaxation and model-free analyses reveal that the α1-η1 helix is highly flexible on the picosecond to nanosecond timescale, a dynamic feature that likely facilitates ligand binding. Furthermore, ITC and EMSA experiments demonstrate that suramin can bind to the full-length N protein at multiple sites and dissociate RNA from the N protein. Taken together, these findings provide structural and biophysical insights into the mechanism of action of suramin and establish a rational basis for the development of targeted antiviral therapies against SARS-CoV-2.