title: Transient weak protein-protein complexes transfer heme across the cell wall of Staphylococcus aureus.
abstract: Iron is an essential nutrient for the bacterial pathogen Staphylococcus aureus . Heme in hemoglobin (Hb) is the most abundant source of iron in the human body and during infections is captured by S. aureus using iron-regulated surface determinant (Isd) proteins. A central step in this process is the transfer of heme between the cell wall associated IsdA and IsdC hemoproteins. Biochemical evidence indicates that heme is transferred via an activated IsdA:heme:IsdC heme complex. Transfer is rapid and occurs up to 70,000 times faster than indirect mechanisms in which heme is released into the solvent. To gain insight into the mechanism of transfer, we modeled the structure of the complex using NMR paramagnetic relaxation enhancement (PRE) methods. Our results indicate that IsdA and IsdC transfer heme via an ultraweak affinity "handclasp" complex that juxtaposes their respective 3(10) helices and β7/β8 loops. Interestingly, PRE also identified a set of transient complexes that could represent high-energy pre-equilibrium encounter species that form prior to the stereospecific handclasp complex. Targeted amino acid mutagenesis and stopped-flow measurements substantiate the functional relevance of a PRE-derived model, as mutation of interfacial side chains significantly slows the rate of transfer. IsdA and IsdC bind heme using NEAr Transporter (NEAT) domains that are conserved in many species of pathogenic Gram-positive bacteria. Heme transfer in these microbes may also occur through structurally similar transient stereospecific complexes.
authors.affiliation: Department of Chemistry and Biochemistry and the UCLA-DOE Institute for Genomics and Proteomics, University of California, Los Angeles, California 90095, USA.
authors.forename: Valerie A
authors.initials: VA
authors.lastname: Villareal
authors.forename: Thomas
authors.initials: T
authors.lastname: Spirig
authors.forename: Scott A
authors.initials: SA
authors.lastname: Robson
authors.forename: Mengyao
authors.initials: M
authors.lastname: Liu
authors.forename: Benfang
authors.initials: B
authors.lastname: Lei
authors.forename: Robert T
authors.initials: RT
authors.lastname: Clubb
mesh_terms: D000942:Antigens, Bacterial / Q000737:chemistry / Q000378:metabolism*; D001692:Biological Transport; D002352:Carrier Proteins / Q000737:chemistry / Q000378:metabolism*; D002473:Cell Wall / Q000737:chemistry / Q000378:metabolism*; D006418:Heme / Q000737:chemistry / Q000378:metabolism*; D019906:Nuclear Magnetic Resonance, Biomolecular; D017433:Protein Structure, Secondary; D013211:Staphylococcus aureus / Q000737:chemistry / Q000378:metabolism*
chemical_list: D000942:Antigens, Bacterial; D002352:Carrier Proteins; C512791:IsdA protein, Staphylococcus aureus; C488727:IsdC protein, Staphylococcus aureus; D006418:Heme