The Staphylococcus aureus siderophore receptor HtsA undergoes localized conformational changes to enclose staphyloferrin A in an arginine-rich binding pocket.
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 alpha-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 alpha-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.
Department of Microbiology and Immunology, Life Sciences Institute, The University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada.
Jason C
JC
Grigg
John D
JD
Cooper
Johnson
J
Cheung
David E
DE
Heinrichs
Michael E P
ME
Murphy
D018528:ATP-Binding Cassette Transporters / Q000737:chemistry* / Q000378:metabolism; D000595:Amino Acid Sequence; D001120:Arginine / Q000737:chemistry*; D001425:Bacterial Outer Membrane Proteins / Q000378:metabolism*; D002951:Citrates / Q000737:chemistry* / Q000378:metabolism; D018360:Crystallography, X-Ray / Q000379:methods; D008024:Ligands; D008954:Models, Biological; D009154:Mutation; D009952:Ornithine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D011956:Receptors, Cell Surface / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry; D017386:Sequence Homology, Amino Acid; D013211:Staphylococcus aureus / Q000378:metabolism*
D018528:ATP-Binding Cassette Transporters; D001425:Bacterial Outer Membrane Proteins; D002951:Citrates; D008024:Ligands; D011956:Receptors, Cell Surface; D011994:Recombinant Proteins; C035902:siderophore receptors; C063851:staphyloferrin A; D001120:Arginine; D009952:Ornithine