A minor 987P protein different from the structural fimbrial subunit is the adhesin

The 987P fimbriae produced by enterotoxigenic strains of Escherichia coli isolated from piglets mediate bacterial attachment to intestinal epithelial cells. These fimbriae consist essentially of a tight helical arrangement of one structural protein subunit encoded by fasA. Fimbriation and specific adhesion requires the expression of seven additional genes (fasB to fasH). In this study, we investigated whether FasA or another Fas protein, e.g., a potential minor fimbrial component, harbors the binding moiety for the pig 987P receptor glycoproteins. Fas proteins, specifically radiolabeled with an in vivo T7 expression system, were isolated from the periplasm and incubated with receptor-containing brush borders isolated from piglet intestinal epithelial cells. FasG bound best to brush borders, whereas no FasA adhered to them. Additional evidence that FasG, and not FasA, is the 987P adhesin was provided by ligand blotting inhibition assays indicating that FasG alone inhibited fimbrial binding to 987P receptors and that in the absence of FasG, other Fas proteins were not inhibitory. FasG was identified in purified fimbrial preparations with a specific anti-FasG antibody probe. Moreover, FasG was shown to be tightly associated with the fimbrial structure, since it was released only after disassembling fimbriae by heat and sodium dodecyl sulfate treatments. The primary structure of FasG, deduced from the DNA sequence, exhibited 19.1 to 24.4% similarity to FasA and large minor components and/or adhesins of other fimbriae. FasG is the first-described minor fimbrial subunit shown to be essential for both fimbrial biogenesis and specific adhesion.