DsbA is required for stability of the type IV pilin of enteropathogenic Escherichia coli

The periplasmic Escherichia coli enzyme DsbA catalyses the efficient formation of disulphide linkages in numerous extracytoplasmic proteins. Enteropathogenic E. coli, a major cause of infantile diarrhoea worldwide, expresses a type IV fimbria known as the bundle-forming pilus that promotes adherence to tissue-culture cells. In this study, we report that transposon insertions in the dsbA locus abolish adherence and dramatically reduce the level of bundlin, the major structural subunit of the pilus encoded by the bfpA locus. Adherence and bundlin levels are restored by complementation with the cloned dsbA gene. DsbA has no effect on bfpA transcription as measured with bfpA-lacZ fusions. Replacement of either cysteine codon 129 or 179 of bfpA with a serine codon results in reduced levels of bundlin, similar to the effect of the dsbA mutation. As is the case with dsbA mutants, this decreased level of bundlin is not due to decreased transcription. The half-life of bundlin as detected by pulse-chase experiments is dramatically reduced in a dsbA mutant in comparison to the wild type. The effect of DsbA on bundlin oxidation is independent of signal-peptide processing. Thus, we demonstrate that the DsbA enzyme is critical for the biogenesis of a type IV fimbria because of the essential role of a disulphide bond in the stability of the major structural subunit. These data illuminate the early steps in the biogenesis of type IV fimbriae by demonstrating that newly synthesized prepilin is a transmembrane protein accessible to periplasmic and cytoplasmic processing enzymes.