Lysine residue 117 of the FasG adhesin of enterotoxigenic Escherichia coliis essential for binding of 987P fimbriae to sulfatide

The FasG subunit of the 987P fimbriae of enterotoxigenic strains of Escheri chia coli was previously shown to mediate fimbrial binding to a glycoprotei n and a sulfatide receptor on intestinal brush borders of piglets. Moreover , the 987P adhesin FasG is required for fimbrial expression, since fasG nul l mutants are nonfimbriated. In this study,fasG was modified by site-direct ed mutagenesis to study its sulfatide binding properties. Twenty single mut ants were generated by replacing positively charged lysine (K) or arginine (R) residues with small, nonpolar alanine (A) residues. Reduced levels of b inding to sulfatide-containing liposomes correlated with reduced fimbriatio n and FasG surface display in four fasG mutants (R27A, R286A, R226A, and R3 68), Among the 16 remaining normally fimbriated mutants with wild-type leve ls of surface-exposed FasG, only one mutant (K117A) did not interact at all with sulfatide-containing liposomes, Four mutants (K117A, R116A, K118A, an d R200A) demonstrated reduced binding to such liposomes, Since complete phe notypic dissociation between the structure and specific function of 987P wa s observed only with mutant K117A, this residue is proposed to play an esse ntial role in the FasG-sulfatide interaction, possibly communicating with t he sulfate group of sulfatide by hydrogen bonding and/or salt bridge format ion. Residues K17, R116, K118, and R200 may stabilize this interaction.