Epitope mapping of SARS-CoV-2 RBDs by hydroxyl radical protein footprinting reveals the importance of including negative antibody controls.

Understanding protein-protein interaction is essential when designing drugs or investigating biological processes. A variety of techniques can be employed in order to map the regions on proteins that are involved in binding eg., CryoEM, X-ray spectroscopy, linear epitope mapping, or mass spectrometry-based methods. The most commonly utilized mass spectrometry-based techniques are cross-linking and hydrogen-deuterium exchange (HDX). An alternative technique for identifying residues on the three-dimensional structure of proteins, that are involved in binding, can be hydroxyl radical protein footprinting (HRPF). However, this method is currently hampered by high initial cost and complex experimental setup. Here we set out to present a generally applicable method using Fenton chemistry for mapping of epitopes in a standard mass spectrometry laboratory. Furthermore, the described method illustrates the importance of controls on several levels when performing mass spectrometry-based epitope mapping. In particular, the inclusion of a negative antibody control has not previously been widely utilized in epitope mapping by HRPF analysis. In order to limit the number of false positives, we further introduced quantification by TMT labelling, thereby allowing for direct comparison between sample conditions and biological triplicates. Lastly, up to six technical replicates were incorporated in the experimental setup in order to achieve increased depth of the final analysis. Both binding and opening of regions on receptor-binding domain (RBD) from SARS-CoV-2 Spike Protein, Alpha, and Delta variants, were observed. The negative control antibody experiment combined with the high overlap between biological triplicates resulted in the exclusion of 40% of the significantly changed regions, including both binding and opening regions. The final identified binding region was mapped to a three-dimensional structure and agrees with the literature for neutralizing antibodies towards SARS-CoV-2 Spike Protein. The presented method is straightforward to implement for the analysis of HRPF in a generic MS-based laboratory. The high reliability of the data was achieved by increasing the number of technical and biological replicates combined with negative antibody controls.