Error-prone PCR mutagenesis and reverse bacterial two-hybrid screening identify a mutation in asparagine 53 of the Staphylococcus aureus ESAT6-like component EsxB that perturbs interaction with EsxD

The ESAT6-like Secretion System (ESS) of the human pathogen Staphylococcus aureus secretes heterodimeric virulence effectors such as EsxB and EsxD. To gain insights into the nature of EsxB-EsxD interaction, randomly mutated esxB generated by error-prone PCR was co-transformed together with esxD as adenylate cyclase fusion constructs into cyclase-deficient Escherichia coli, followed by reverse bacterial two-hybrid screening. Three color species were observed: dark blue, light blue, and white (no EsxB-EsxD interaction). The esxB from white colonies was subjected to standard PCR to check for gene signal, followed by SDS-PAGE for variant stability assessment. The gene coding for a stable EsxB variant that perturbed interaction with EsxD was further subjected to DNA sequencing. A single point mutation in esxB at position 157 was identified, leading to an amino acid change from asparagine to aspartic acid at position 53 in the resulting protein. Structural modeling of EsxB reveals that N53 is surface exposed. Whereas N53S substitution by site-directed mutagenesis retained heterodimerization with EsxD, N53A substitution abrogated such interaction. In addition, N53D change in EsxB did not alter interaction with EssG, another soluble component of the ESS pathway, suggesting minimal impact of the N53D substitution on EsxB stability and solubility. Taken together, these data provide new insights into the nature of EsxB-EsxD interaction and offer a systematic approach for in vivo analysis of protein-protein interactions of pathogenic bacteria in non-pathogenic hosts.