The Escherichia coli K88 periplasmic chaperone FaeE forms a heterotrimeric complex with the minor fimbrial component FaeH and with the minor fimbrial component FaeI.

K88ab fimbriae are long polymeric protein structures mainly composed of FaeG proteins. The Escherichia coli K88 periplasmic chaperone FaeE is a homodimer and forms a heterotrimeric complex with the K88 major fimbrial component FaeG in the periplasm. In this study the direct interaction of FaeE and the minor K88 fimbrial subunits FaeH and FaeI were investigated. The faeH gene and the faeI gene were subcloned in a pINIIIA1-derivative vector containing the faeE gene. SDS-PAGE using normal and gradient gels and immunoblotting revealed that the subcloned genes were expressed in the periplasm. Analyses of periplasmic fractions by native gel electrophoresis and isoelectric focusing (IEF) showed that FaeE and FaeH, as well as FaeE and FaeI formed protein complexes. These complexes were isolated and purified by FPLC or IEF and native gel electrophoresis. The stoichiometry of the proteins in these complexes was studied by automated Edman degradation and gel image analysis. The results showed that FaeE and FaeH, and FaeE and FaeI formed heterotrimeric E2H and E2I complexes, respectively. In addition to the E2H complex, cells expressing FaeE and FaeH accumulated unbound FaeH in their periplasm. In contrast to the E2G complex, the purified E2H complex was not stable and was partly dissociated in the experimental conditions used, suggesting that the interaction between FaeE and FaeH is not as strong as the interaction of FaeE and FaeG.