title: Interaction of human complement with Sbi, a staphylococcal immunoglobulin-binding protein: indications of a novel mechanism of complement evasion by Staphylococcus aureus.
abstract: Staphylococcal immunoglobulin-binding protein, Sbi, is a 436-residue protein produced by many strains of Staphylococcus aureus. It was previously characterized as being cell surface-associated and having binding capacity for human IgG and beta(2)-glycoprotein I. Here we show using small angle x-ray scattering that the proposed extracellular region of Sbi (Sbi-E) is an elongated molecule consisting of four globular domains, two immunoglobulin-binding domains (I and II) and two novel domains (III and IV). We further show that together domains III and IV (Sbi-III-IV), as well as domain IV on its own (Sbi-IV), bind complement component C3 via contacts involving both the C3dg fragment and the C3a anaphylatoxin domain. Preincubation of human serum with either Sbi-E or Sbi-III-IV is inhibitory to all complement pathways, whereas domain IV specifically inhibits the alternative pathway. Monitoring C3 activation in serum incubated with Sbi fragments reveals that Sbi-E and Sbi-III-IV both activate the alternative pathway, leading to consumption of C3. By contrast, inhibition of this pathway by Sbi-IV does not involve C3 consumption. The observation that Sbi-E activates the alternative pathway is counterintuitive to intact Sbi being cell wall-associated, as recruiting complement to the surface of S. aureus would be deleterious to the bacterium. Upon re-examination of this issue, we found that Sbi was not associated with the cell wall fraction, but rather was found in the growth medium, consistent with it being an excreted protein. As such, our data suggest that Sbi helps mediate bacterial evasion of complement via a novel mechanism, namely futile fluid-phase consumption.
authors.affiliation: Department of Biology and Biochemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom.
authors.forename: Julia D
authors.initials: JD
authors.lastname: Burman
authors.forename: Elisa
authors.initials: E
authors.lastname: Leung
authors.forename: Karen L
authors.initials: KL
authors.lastname: Atkins
authors.forename: Maghnus N
authors.initials: MN
authors.lastname: O'Seaghdha
authors.forename: Lea
authors.initials: L
authors.lastname: Lango
authors.forename: Pau
authors.initials: P
authors.lastname: Bernadó
authors.forename: Stefan
authors.initials: S
authors.lastname: Bagby
authors.forename: Dmitri I
authors.initials: DI
authors.lastname: Svergun
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Foster
authors.forename: David E
authors.initials: DE
authors.lastname: Isenman
authors.forename: Jean M H
authors.initials: JM
authors.lastname: van den Elsen
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism; D002473:Cell Wall / Q000378:metabolism; D003001:Cloning, Molecular; D003176:Complement C3 / Q000737:chemistry*; D015964:Gene Expression Regulation, Bacterial; D006801:Humans; D007074:Immunoglobulin G / Q000737:chemistry; D008954:Models, Biological; D008958:Models, Molecular; D015233:Models, Statistical; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D017464:Receptors, Complement 3d / Q000737:chemistry; D013211:Staphylococcus aureus / Q000378:metabolism*; D020349:Surface Plasmon Resonance
chemical_list: D001426:Bacterial Proteins; D002352:Carrier Proteins; D003176:Complement C3; D007074:Immunoglobulin G; D017464:Receptors, Complement 3d; C112405:Sbi protein, Staphylococcus aureus