Characterization of the binding specificity of K88ac and K88ad fimbriae of enterotoxigenic Escherichia coli by constructing K88ac/K88ad chimeric FaeG major subunits.

Enterotoxigenic Escherichia coli (ETEC) strains expressing K88 (F4) fimbriae are the major cause of diarrhea in young pigs. Three antigenic variants of K88 fimbriae (K88ab, K88ac, and K88ad) have been identified among porcine ETEC strains. Each K88 fimbrial variant shows a unique pattern in binding to different receptors on porcine enterocytes. Such variant specificity in fimbrial binding is believed to be controlled by the major subunit (FaeG) of the K88 fimbriae, because the genes coding for the only other fimbrial subunit are identical among the three variants. Uniqueness in binding to host receptors may be responsible for differences in the virulence levels of porcine diarrhea disease caused by K88 ETEC strains. To better understand the relationships between the structure of FaeG proteins and fimbrial binding function, and perhaps virulence in disease, we constructed and expressed various K88ac/K88ad faeG gene chimeras and characterized the binding activity of each K88 chimeric fimbria. After verifying biosynthesis of the chimeric fimbriae, we examined their binding specificities in bacterial adherence assays by using porcine brush border vesicles that are specific to either the K88ac or K88ad fimbria. Results showed that each fimbria switched binding specificity to that of the reciprocal type when a peptide comprising amino acids 125 to 163 was exchanged with that of its counterpart. Substitutions of a single amino acid within this region negatively affected the binding capacity of each fimbria. These data indicate that the peptide including amino acids 125 to 163 of the FaeG subunit is essential for K88 variant-specific binding.