Structure-function analysis of the C3 binding region of Staphylococcus aureus immune subversion protein Sbi.
Among the recently discovered Staphylococcus aureus immune evasion proteins, Sbi is unique in its ability to interact with components of both the adaptive and innate immune systems of the host. Sbi domains I and II (Sbi-I and Sbi-II) bind IgG. Sbi domain IV (residues 198-266) binds the central complement protein C3. When linked to Sbi-III, Sbi-IV induces a futile consumption of complement via alternative pathway activation, whereas isolated Sbi-IV specifically inhibits the alternative pathway without complement consumption. Here we have determined the three-dimensional structure of Sbi-IV by NMR spectroscopy, showing that Sbi-IV adopts a three-helix bundle fold similar to those of the S. aureus complement inhibitors Efb-C, Ehp, and SCIN. The (1)H-(15)N HSQC spectrum of Sbi-III indicates that this domain, essential for futile complement consumption, is natively unfolded, at least when isolated from the rest of Sbi. Sbi-IV and Sbi-III-IV both bind C3dg with 1:1 stoichiometry and submicromolar affinity. Despite low overall sequence identity, Sbi possesses the same residues as Efb at two positions essential for Efb-C binding to C3d. Mutation to alanine of either of these residues, Arg-231 and Asn-238, abolishes both Sbi-IV binding to C3dg and Sbi-IV alternative pathway inhibition. The almost complete conservation of Sbi-III and Sbi-IV amino acid sequences across more than 30 strains isolated from human and animal hosts indicates that the unique mechanism of Sbi in complement system subversion is a feature of infections of both humans and economically important animals.
Upadhyay
Abhishek
A
Department of Biology and Biochemistry, University of Bath, Bath BA2 7AY, United Kingdom.
Burman
Julia D
JD
Clark
Elizabeth A
EA
Leung
Elisa
E
Isenman
David E
DE
van den Elsen
Jean M H
JM
Bagby
Stefan
S
D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000276:immunology / Q000378:metabolism*; D001665:Binding Sites; D002352:Carrier Proteins / Q000737:chemistry* / Q000276:immunology / Q000378:metabolism*; D003169:Complement Inactivator Proteins / Q000737:chemistry / Q000378:metabolism; D003170:Complement Pathway, Alternative; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D019906:Nuclear Magnetic Resonance, Biomolecular; D011485:Protein Binding; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D016415:Sequence Alignment; D013211:Staphylococcus aureus / Q000737:chemistry* / Q000276:immunology / Q000378:metabolism*; D013329:Structure-Activity Relationship
D001426:Bacterial Proteins; D002352:Carrier Proteins; D003169:Complement Inactivator Proteins; C524396:Ehp protein, Staphylococcus aureus; C112405:Sbi protein, Staphylococcus aureus