Identification of Major and Minor Chaperone Proteins Involved in the Export of 987P Fimbriae

The 987P fimbriae of Escherichia coli consist mainly of the major subunit, FasA, and two minor subunits, FasF and FasG. In addition to the previously characterized outer membrane or usher protein FasD, the FasB, FasC, and FasE proteins are required for fimbriation. To better understand the roles of these minor proteins, their genes were sequenced and the predicted polypeptides were shown to be most similar to periplasmic chaperone proteins of fimbrial systems. Western blot (immunoblot) analysis and immunoprecipitation of various fas mutants with specific antibody probes identified both the subcellular localizations and associations of these minor components. FasB was shown to be a periplasmic chaperone for the major fimbrial subunit, FasA. A novel periplasmic chaperone, FasC, which stabilizes and specifically interacts with the adhesin, FasG, was identified. FasE, a chaperone-like protein, is also located in the periplasm and is required for optimal export of FasG and possibly other subunits. The use of different chaperone proteins for various 987P subunits is a novel observation for fimbrial biogenesis in bacteria. Whether other fimbrial systems use a similar tactic remains to be discovered.