The Staphylococcus aureus Siderophore Receptor HtsA Undergoes Localized Conformational Changes to Enclose Staphyloferrin A in an Arginine-rich Binding Pocket

Abstract

Staphylococcus aureus uses several efficient iron acquisition strategies to overcome iron limitation. Recently, the genetic locus encoding biosynthetic enzymes for the iron chelating molecule, staphyloferrin A (SA), was determined. S. aureus synthesizes and secretes SA into its environment to scavenge iron. The membrane-anchored ATP binding cassette-binding protein, HtsA, receives the ferric-chelate for import into the cell. Recently, we determined the apoHtsA crystal structure, the first siderophore receptor from Gram-positive bacteria to be structurally characterized. Herein we present the x-ray crystal structure of the HtsA-ferric-SA complex. HtsA adopts a class III binding protein fold composed of separate N-and C-terminal domains bridged by a single ␣-helix. Recombinant HtsA can efficiently sequester ferric-SA from S. aureus culture supernatants where it is bound within the pocket formed between distinct N-and C-terminal domains. A basic patch composed mainly of six Arg residues contact the negatively charged siderophore, securing it within the pocket. The x-ray crystal structures from two different ligand-bound crystal forms were determined. The structures represent the first structural characterization of an endogenous ␣-hydroxycarboxylate-type siderophore-receptor complex. One structure is in an open form similar to apoHtsA, whereas the other is in a more closed conformation. The conformational change is highlighted by isolated movement of three loops within the C-terminal domain, a domain movement unique to known class III binding protein structures.

The Solution Structure, Binding Properties, and Dynamics of the Bacterial Siderophore-binding Protein FepB

“…Arg-239). The movement and direct involvement of C-lobe loop residues in ligand binding has been observed previously for the type III substrate-binding proteins HtsA (90) and SirA (91) from Staphylococcus aureus. Interestingly, the location and loop movement of FepB upon ligand binding is similar to residues 228 -258 of HtsA, which moves ϳ12 Å upon substrate binding.…”

Staphylococcus aureus Transporters Hts, Sir, and Sst Capture Iron Liberated from Human Transferrin by Staphyloferrin A, Staphyloferrin B, and Catecholamine Stress Hormones, Respectively, and Contribute to Virulence

“…Interestingly, of all ligands examined, SstD had the highest affinity for the enteric siderophores enterobactin and salmochelin S4, suggesting that under certain favorable conditions, these ligands might be viable iron sources for S. aureus in vivo. The K d values of SstD for its ligands, in the micromolar range, contrast with the K d values in the nanomolar range that were determined for HtsA and SirA for their cognate ligands, staphyloferrin A and staphyloferrin B (24,25). In line with the micromolar range of affinities of E. coli FhuD for several hydroxamate ligands, this might reflect a sacrifice in ligand affinity in lieu of greater ligand diversity.…”

“…SstD is a member of the class III substrate binding protein family, a family which includes the S. aureus Fe(III)-staphyloferrin receptors HtsA (25) and SirA (24) and the heme-binding IsdE protein (26). BLAST analyses uncovered shared sequence identity between SstD and many other annotated iron-ligand receptor proteins, especially receptors for catechol siderophores, including enterobactin, petrobactin, anguibactin, and vibriobactin.…”

“…The molecular genetics and biochemistry for the synthesis of two S. aureus siderophores, staphyloferrin A (SA) and staphyloferrin B (SB), were recently characterized (7,9). Following capture of extracellular iron, the staphyloferrins are recognized by the highly specific receptors HtsA (SA) and SirA (SB), and at least iron, if not the iron-siderophore complex, is actively imported into the cytosol through permeases (3,7,24,25). Furthermore, it has been shown that genomic inactivation of the staphyloferrin biosynthesis loci sfa (SA) and sbn (SB) eliminates siderophore output and severely curtails S. aureus growth in animal serum (3), prompting the question of whether the target of staphylococcal siderophores in serum is transferrin.…”

Deconstructing honeybee vitellogenin: novel 40 kDa fragment assigned to its N terminus

“…In lamprey, the positive and negative charges are evenly distributed. A positively charged patch might attract negatively charged ligands (Cherstvy, 2009;Corsico et al, 2005;Grigg et al, 2010;Honig and Nicholls, 1995;Horii et al, 2004). The interior of the N-sheet is lined by a mainly hydrophobic cavity, in which a lipid molecule is present in the lamprey lipovitellin crystal structure (Thompson and Banaszak, 2002).…”