A Bacterial Cell-Based Assay To Study SARS-CoV-2 Protein-Protein Interactions

Methods for detecting and dissecting the interactions of virally encoded proteins are essential for probing basic viral biology and providing a foundation for therapeutic advances. The dearth of targeted therapeutics for the treatment of coronavirus disease 2019 (COVID-19), an ongoing global health crisis, underscores the importance of gaining a deeper understanding of the interactions of proteins encoded by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Here, we describe the use of a convenient bacterial cell-based two-hybrid (B2H) system to analyze the SARS-CoV-2 proteome. We identified 16 distinct intraviral protein-protein interactions (PPIs), involving 16 proteins. We found that many of the identified proteins interact with more than one partner. Further, our system facilitates the genetic dissection of these interactions, enabling the identification of selectively disruptive mutations. We also describe a modified B2H system that permits the detection of disulfide bond-dependent PPIs in the normally reducing Escherichia coli cytoplasm, and we used this system to detect the interaction of the SARS-CoV-2 spike protein receptor-binding domain (RBD) with its cognate cell surface receptor ACE2. We then examined how the RBD-ACE2 interaction is perturbed by several RBD amino acid substitutions found in currently circulating SARS-CoV-2 variants. Our findings illustrate the utility of a genetically tractable bacterial system for probing the interactions of viral proteins and investigating the effects of emerging mutations. In principle, the system could also facilitate the identification of potential therapeutics that disrupt specific interactions of virally encoded proteins. More generally, our findings establish the feasibility of using a B2H system to detect and dissect disulfide bond-dependent interactions of eukaryotic proteins. IMPORTANCE Understanding how virally encoded proteins interact with one another is essential in elucidating basic viral biology, providing a foundation for therapeutic discovery. Here, we describe the use of a versatile bacterial cell-based system to investigate the interactions of the protein set encoded by SARS-CoV-2, the virus responsible for the current COVID-19 pandemic. We identified 16 distinct intraviral protein-protein interactions, involving 16 proteins, many of which interact with more than one partner. Our system facilitates the genetic dissection of these interactions, enabling the identification of selectively disruptive mutations. We also describe a modified version of our bacterial cell-based system that permits detection of the interaction between the SARS-CoV-2 spike protein (specifically, its receptor-binding domain) and its cognate human cell surface receptor ACE2, and we investigated the effects of spike mutations found in currently circulating SARS-CoV-2 variants. Our findings illustrate the general utility of our system for probing the interactions of virally encoded proteins.