Cobalt(III)-Schiff Base Coordination to Nsp1, a SARS-CoV-2 Protein

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) suppresses host immune defenses through the activity of nonstructural protein 1 (Nsp1), which inhibits host mRNA translation by occluding the mRNA entry channel of the 40S ribosomal subunit. Here, we investigate whether Co(III)(acacen)(NH3)2+ (acacen = bis(acetylacetone)ethylenediamine) can disrupt Nsp1 function through coordination to histidine residues. Using 59Co nuclear magnetic resonance (NMR) spectroscopy, we show that Co(III)(acacen)(NH3)2+ undergoes axial ligand substitution upon interaction with an Nsp1-derived peptide encompassing the functionally critical His165 residue. Computational analysis supports a coordination structure in which an axial NH3 ligand is displaced by a histidine imidazole. Although proteolytic fragmentation followed by mass spectrometric analysis revealed multiple histidine binding sites in full-length Nsp1, assigned to His13, the His81/His83 region, and His165, in vitro translation assays confirmed that the addition of Co(III)(acacen)(NH3)2+ did not restore host protein synthesis.