PROBING THE RECEPTOR RECOGNITION SITE OF THE FIMH ADHESIN BY FIMBRIAE-DISPLAYED FIMH-FOCH HYBRIDS

Type I fimbriae are surface organelles of Escherichia coil which media te D-mannose-sensitive binding to different host surfaces. This bindin g is conferred by the minor fimbrial component FimH. The binding domai n of the FimH adhesin has been studied by constructing hybrids of FimH and a homologous protein, FocH, originating from F1C fimbriae. F1C fi mbriae do not bind to D-mannosides or confer agglutination of any know n types of erythrocytes or yeast. It was previously shown that the Foc H protein can be readily substituted by the FimH adhesin, resulting in hybrid fimbriae with the same binding characteristics as type 1 fimbr iae. The receptor binding of fimbriae-presented chimeric FimH-FocH hyb rids was studied. FimH-FocH fusions encompassing 72% of the N-terminus of FimH fused to the complementary sector of FocH conferred agglutina tion of erythrocytes and yeast cells at a comparable lever to FimH. Su rprisingly, it was also found that similar fusions containing between 56 and 66% FimH still conferred binding to yeast cells, D-mannose-BSA and D-mannose-beads but did not give rise to agglutination. the recept or binding capacity of fusions containing 50% or less of the FimH N-te rminal region was virtually abolished. The results point to the presen ce of a D-mannose-receptor-binding core domain in FimH, the affinity o f which is modulated by other sectors of the protein to enable binding to extended mannose-containing targets.