title: Staphylococcal complement inhibitor: structure and active sites.
abstract: The pathogenic bacterium Staphylococcus aureus counteracts the host immune defense by excretion of the 85 residue staphylococcal complement inhibitor (SCIN). SCIN inhibits the central complement convertases; thereby, it reduces phagocytosis following opsonization and efficiently blocks all downstream effector functions. In this study, we present the crystal structure of SCIN at 1.8 A resolution and the identification of its active site. Functional characterization of structure based chimeric proteins, consisting of SCIN and the structurally but nonfunctional homologue open reading frame-D, indicate an 18-residue segment (Leu-31-Gly-48) crucial for SCIN activity. In all complement activation pathways, chimeras lacking these SCIN residues completely fail to inhibit production of the potent mediator of inflammation C5a. Inhibition of alternative pathway-mediated opsonization (C3b deposition) and formation of the lytic membrane attack complex (C5b-9 deposition) are strongly reduced for these chimeras as well. For inhibition of the classical/lectin pathway-mediated C3b and C5b-9 deposition, the same residues are critical although additional sites are involved. These chimeras also display reduced capacity to stabilize the C3 convertases of both the alternative and the classical/lectin pathway indicating the stabilizing effect is pivotal for the complement inhibitory activity of SCIN. Because SCIN specifically and efficiently inhibits complement, it has a high potential in anti-inflammatory therapy. Our data are a first step toward the development of a second generation molecule suitable for such therapeutic complement intervention.
authors: Medical Microbiology, University Medical Center Utrecht, Heidelberglaan 100, Utrecht, The Netherlands. s.h.m.rooijakkers@umcutrecht.nl
authors: Suzan H M
authors: SH
authors: Rooijakkers
authors: Fin J
authors: FJ
authors: Milder
authors: Bart W
authors: BW
authors: Bardoel
authors: Maartje
authors: M
authors: Ruyken
authors: Jos A G
authors: JA
authors: van Strijp
authors: Piet
authors: P
authors: Gros
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry*; D001665:Binding Sites; D015936:Complement C5a / Q000037:antagonists & inhibitors; D003169:Complement Inactivator Proteins / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006801:Humans; D008969:Molecular Sequence Data; D011487:Protein Conformation; D011993:Recombinant Fusion Proteins / Q000737:chemistry; D013211:Staphylococcus aureus / Q000276:immunology / Q000472:pathogenicity*
chemical_list: D001426:Bacterial Proteins; D003169:Complement Inactivator Proteins; D011993:Recombinant Fusion Proteins; D015936:Complement C5a