A bacterial virulence factor with a dual role as an adhesin and a solute-binding protein: The crystal structure at 1.5 angstrom resolution of the PEBIa protein from the food-borne human pathogen Campylobacter jejuni

摘要

The PEB1 alpha protein is an antigenic factor exposed on the surface of the food-borne human pathogen Campylobacter jejuni, which has a major role in adherence and host colonisation. PEBla is also the periplasmic binding protein component of an aspartate/glutamate ABC transporter essential for optimal microaerobic growth on these dicarboxylic amino acids. Here, we report the crystal structure of PEBla at 1.5 angstrom resolution. The protein has a typical two-domain alpha/beta structure, characteristic of periplasmic extracytoplasmic solute receptors and a chain topology related to the type II subfamily. An aspartate ligand, clearly defined by electron density in the, interdomain cleft, forms extensive polar interactions with the protein, the majority of which are made with the larger domain. Arg89 and Asp174 form ion-pairing interactions with the rnain chain alpha-carboxyl and alpha-aminogroups, respectively, of the ligand, while Arg67, Thr82, Lys19 and Tyr156 co-ordinate the ligand side-chain carboxyl group. Lys19 and Arg67 line a positively charged groove, which favours binding of Asp over the neutral Asn. The ligand-binding cleft is of sufficient depth to accommodate a glutamate. This is the first structure of an ABC-type aspartate-binding protein, and explains the high affinity of the protein for aspartate and glutamate, and its much weaker binding of asparagine and glutamine. Stopped-flow fluorescence spectroscopy indicates a simple bimolecular mechanism of ligand binding, with high association rate constants. Sequence alignments and phylogenetic analyses revealed PEBla homologues in some Gram-positive bacteria. The alignments suggest a more distant homology with GltI from Escherichia coli, a known glutamate and aspartate-binding protein, but Lys19 and Tyr156 are not conserved in GltI. Our results provide a structural basis for understanding both the solute transport and adhesin/virulence functions of PEBla. (c) 2007 Elsevier Ltd. All rights reserved.