Structural Studies of Afa/Dr Family of Fimbrial Proteins

Abstract

Pathogenic Escherichia coli and Salmonella spp. are Gram negative bacteria that inhabit the lower intestines of mammals and express virulence factors that are responsible for infecting millions of people with urinary tract infections, diarrhoea and food poisoning. These virulence factors, which target the host through many varied mechanisms, assist these organisms to overcome the host defences and colonize or invade the urinary or gastrointestinal tract. This project focuses on Afa/Dr related fimbrial proteins from different strains of pathogenic E. coli and Salmonella enteritidis involved in the initial contact with host cells. A polymeric fibre of major subunits capped by a minor subunit make up the architecture of the fimbria and both are transported to the cell surface by the chaperone-usher system. The structure of a minor subunit from Enteroaggregative E. coli (EAEC), AafB, was solved by X-ray crystallography at 3.0Å resolution. Potential ligands have been identified using glycoarrays and these interactions have been studied in more detail using nuclear magnetic resonance. Residues were identified that are important for interaction with heparin I-S. The structure of HdaB, another minor subunit from EAEC, was also solved by X-ray crystallography to 2.0 Å, although no ligand was identified. As expected these two homologous proteins show a very similar immunoglobulin-like fold consisting of 9 β-strands. Structural work was also performed on SefD, a minor subunit from S. enteritidis. SefD crystals were not reproducible and molecular replacement failed to obtain phase information. It was possible to reproduce crystals by replacing 100% D2O instead of H2O into the crystallisation and sample buffer, although data collection and processing showed a change in space group. Selenomethionine derivatives did not yield an anomalous signal and soaks using sodium iodide and 5-amino-2, 4, 6-triiodoisophthalic acid failed due to lack of incorporation.