Inhibition of virulence-promoting disulfide bond formation enzyme DsbB is blocked by mutating residues in two distinct regions

Abstract

Disulfide bonds contribute to protein stability, activity, and folding in a variety of proteins, including many involved in bacterial virulence such as toxins, adhesins, flagella, and pili, among others. Therefore, inhibitors of disulfide bond formation enzymes could have profound effects on pathogen virulence. In the Escherichia coli disulfide bond formation pathway, the periplasmic protein DsbA introduces disulfide bonds into substrates, and then the cytoplasmic membrane protein DsbB reoxidizes DsbA's cysteines regenerating its activity. Thus, DsbB generates a protein disulfide bond de novo by transferring electrons to the quinone pool. We previously identified an effective pyridazinone-related inhibitor of DsbB enzymes from several Gram-negative bacteria. To map the protein residues that are important for the interaction with this inhibitor, we randomly mutagenized by error-prone PCR the E. coli dsbB gene and selected dsbB mutants that confer resistance to this drug using two approaches. We characterized in vivo and in vitro some of these mutants that map to two areas in the structure of DsbB, one located between the two first transmembrane segments where the quinone ring binds and the other located in the second periplasmic loop of DsbB, which interacts with DsbA. In addition, we show that a mutant version of a protein involved in lipopolysaccharide assembly, lptD4213, is synthetically lethal with the deletion of dsbB as well as with DsbB inhibitors. This finding suggests that drugs decreasing LptD assembly may be synthetically lethal with inhibitors of the Dsb pathway, potentiating the antibiotic effects.