Post-translational processing of the major structural subunit of a type IV fimbria from enteropathogenic Escherichia coli

Abstract

Localized adherence (LA) to epithelial cells is a virulence-associated phenotype of enteropathogenic Escherichia coli (EPEC), a leading cause of infantile diarrhea around the world. An inducible bundle forming pilus (BFP) was proposed to be the adhesin mediating LA. The major structural subunit of BFP (bundlin) is encoded on a large EPEC plasmid by the bfpA gene, a member of the type IV fimbria gene family. Like all type IV pilins, bundlin is synthesized as a precursor which is processed at its N-terminus into the mature form after an atypical signal peptide is cleaved, and, like most fimbrial subunits of any type, bundlin has at its C-terminus two Cysteine residues which could form a disulfide bond. The gene encoding the prepilin peptidase responsible for pre-bundlin N-terminal proteolytic processing, bfpP, was cloned from the EPEC plasmid by functional complementation of a P. aeruginosa prepilin peptidase (pilD) mutant. The predicted product of bfpP is homologous to other prepilin peptidases, including TcpJ of Vibrio cholerae (30% identical amino acids), PulO of Klebsiella oxytoca (29%), and PilD of P. aeruginosa (28%). BfpP and PilD are also functionally interchangeable. Disulfide bond formation at the C-terminal domain of bundlin is catalyzed by the product of a ubiquitous E. coli gene, dsbA. Formation of this disulfide bond is required for the stability of bundlin. Mutants with either cysteine of bundlin replaced by serine fail to express detectable levels of the bundlin polypeptide. The effect of dsbA on bundlin oxidization is independent of signal peptide processing. These results clarify the early steps in biogenesis of a type IV pilus and add to our understanding of the post-translational processing of exported proteins.