title: The crystal structure of the secreted dimeric form of the hemophore HasA reveals a domain swapping with an exchanged heme ligand.
abstract: To satisfy their iron needs, several Gram-negative bacteria use a heme uptake system involving an extracellular heme-binding protein called hemophore. The function of the hemophore is to acquire free or hemoprotein-bound heme and to transfer it to HasR, its specific outer membrane receptor, by protein-protein interaction. The hemophore HasA secreted by Serratia marcescens, an opportunistic pathogen, was the first to be identified and is now very well characterized. HasA is a monomer that binds one b heme with strong affinity. The heme in HasA is highly exposed to solvent and coordinated by an unusual pair of ligands, a histidine and a tyrosine. Here, we report the identification, the characterization and the X-ray structure of a dimeric form of HasA from S. marcescens: DHasA. We show that both monomeric and dimeric forms are secreted in iron deficient conditions by S. marcescens. The crystal structure of DHasA reveals that it is a domain swapped dimer. The overall structure of each monomeric subunit of DHasA is very similar to that of HasA but formed by parts coming from the two different polypeptide chains, involving one of the heme ligands. Consequently DHasA binds two heme molecules by residues coming from both polypeptide chains. We show here that, while DHasA can bind two heme molecules, it is not able to deliver them to the receptor HasR. However, DHasA can efficiently transfer its heme to the monomeric form that, in turn, delivers it to HasR. We assume that DHasA can function as a heme reservoir in the hemophore system.
authors.affiliation: CNRS, Université Pierre et Marie Curie-Paris 6, Laboratoire International Associé-Dispersal and Adaptation in Marine Species, Unité Mixte de Recherche 7139, Station Biologique, F-29682 Roscoff Cedex, France.
authors.forename: Mirjam
authors.initials: M
authors.lastname: Czjzek
authors.forename: Sylvie
authors.initials: S
authors.lastname: Létoffé
authors.forename: Cécile
authors.initials: C
authors.lastname: Wandersman
authors.forename: Muriel
authors.initials: M
authors.lastname: Delepierre
authors.forename: Anne
authors.initials: A
authors.lastname: Lecroisey
authors.forename: Nadia
authors.initials: N
authors.lastname: Izadi-Pruneyre
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism*; D018360:Crystallography, X-Ray / Q000379:methods*; D019281:Dimerization; D004926:Escherichia coli / Q000378:metabolism; D006418:Heme / Q000737:chemistry; D006427:Hemin / Q000737:chemistry; D006639:Histidine / Q000737:chemistry; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D008565:Membrane Proteins / Q000378:metabolism*; D008968:Molecular Conformation; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D012706:Serratia marcescens / Q000378:metabolism*; D014443:Tyrosine / Q000737:chemistry
chemical_list: D001426:Bacterial Proteins; D002352:Carrier Proteins; C093956:HasA protein, Serratia marcescens; D008024:Ligands; D008565:Membrane Proteins; D014443:Tyrosine; D006418:Heme; D006639:Histidine; D006427:Hemin