Cortisol binding sites on SARS-CoV-2 S1 identified by limited proteolysis-coupled mass spectrometry

We used a limited proteolysis-coupled mass spectrometry approach to pinpoint binding sites of cortisol on SARS-CoV-2 S1. Binding target identification is based on the principle that small-ligand binding alters (increases or decreases) the protease accessibility of the target protein37,38. The binding of the ligand (cortisol) to the target protein (SARS-CoV-2 S1) induces perturbations/conformational changes at the binding sites which can lead to either facilitating or preventing proteolysis by non-specific proteases (thermolysin and trypsin)37,38. We subjected SARS-CoV-2 S1 incubated with either cortisol or vehicle to proteolysis by thermolysin followed by trypsin digestion and mass spectrometry-based identification of the resultant peptides. We found that cortisol either facilitated or prevented S1 proteolytic cleavage at discrete sites. Six unique peptides were identified that were only present for SARS-CoV-2 S1 incubated with either cortisol or vehicle.