Characterization of FasG segments required for 987P fimbria-mediated binding to piglet glycoprotein receptors

The 987P fimbriae of enterotoxigenic strains of Escherichia coli bind to bo th glycoprotein and glycolipid receptors on the brush borders of piglet ent erocytes. A mutation in lysine residue 117 of the adhesive subunit FasG [fa sG(K117A)] previously shown to abrogate 987P binding to the lipid receptor sulfatide did not affect the interaction with the glycoprotein receptors. B oth the fimbriae and the FasG subunits of the wild type and the fasG (K117A ) mutant bound to the glycoprotein receptors, confirming that lysine 117 wa s not required for binding to the glycoprotein receptors. Truncated FasG mo lecules were used to identify domains required for glycoprotein receptor re cognition. At least two segments which did not include lysine117, namely, r esidues 211 (glutamine) to 220 (serine) and 20 (aspartic acid) to 41 (serin e), were shown to be involved in the FasG-glycoprotein receptor interaction s by ligand-blotting assays. Changing isoleucine 217 or leucine 215 of FasG to alanine abolished the property of a truncated FasG fusion protein to in hibit 987P recognition of its glycoprotein receptors. Thus, the K117 residu e of FasG is required only for binding to the glycolipid receptor, whereas the newly identified hydrophobic residues of the FasG subunit are required specifically for the recognition of the glycoprotein receptor. Taken togeth er, our data indicate that different residues of the FasG adhesin are impor tant in 987P fimbrial binding to sulfatide and glycoprotein receptors, sugg esting different mechanisms of interaction.