title: Haem release from haemopexin by HxuA allows Haemophilus influenzae to escape host nutritional immunity.
abstract: Haemophilus influenzae is an obligate human commensal/pathogen. This haem auxotroph must acquire haem from its host to sustain aerobic growth. Haem-haemopexin complexes are one of the potential sources of haem for this microorganism. Haemopexin is a glycoprotein that binds haem with high affinity (subpicomolar Kd) and involved in haem recycling. HxuA, a cell surface protein, is the key to haem acquisition from haemopexin. In this study, we reconstituted a functional Hxu system from H. influenzae in Escherichia coli K-12 that mediated active haem transport across the outer membrane from haem-haemopexin, in the presence of the inner membrane energy-transducing TonB-ExbB-ExbD complex from H. influenzae. A secreted variant of HxuA, HxuA(dm), was produced in E. coli. HxuA(dm) functionally complemented an hxuA mutant of H. influenzae for haem-haemopexin acquisition. HxuA(dm) interacted with haemopexin and haem-haemopexin, with which it formed high-affinity, stoichiometric complexes. Following the interaction between haem-haemopexin and HxuA(dm), haem was no longer bound to its initial high-affinity site and became accessible to its cognate haem receptor, HxuC. HxuA(dm) and the HxuA(dm)-haemopexin complex do not appear to bind haem at detectable levels (affinities below 10(6) M(-1)). HxuA thus appears to 'release' haem from haem-haemopexin complexes and to prevent haem sequestering by haemopexin.
authors.affiliation: Unité des Membranes Bactériennes, CNRS URA 2172, Département de Microbiologie, Institut Pasteur, 25 rue du Dr Roux, 75724 Paris Cedex 15, France.
authors.forename: Clémence
authors.initials: C
authors.lastname: Fournier
authors.forename: Ann
authors.initials: A
authors.lastname: Smith
authors.forename: Philippe
authors.initials: P
authors.lastname: Delepelaire
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D002151:Calorimetry; D004586:Electrophoresis; D048168:Escherichia coli K12; D006193:Haemophilus influenzae / Q000378:metabolism*; D006418:Heme / Q000378:metabolism*; D006466:Hemopexin / Q000378:metabolism*; D015151:Immunoblotting
chemical_list: D001426:Bacterial Proteins; D006418:Heme; D006466:Hemopexin