Structural highlights
Function[SCIN_STAA3] Involved in countering the first line of host defense mechanisms. Efficiently inhibits opsonization, phagocytosis and killing of S.aureus by human neutrophils. Acts by binding and stabilizing human C3 convertases (C4b2a and C3bBb), leading to their inactivation. The convertases are no longer able to cleave complement C3, therefore preventing further C3b deposition on the bacterial surface and phagocytosis of the bacterium. Also prevents C5a-induced neutrophil responses (By similarity). Publication Abstract from PubMedThe 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. Staphylococcal complement inhibitor: structure and active sites.,Rooijakkers SH, Milder FJ, Bardoel BW, Ruyken M, van Strijp JA, Gros P J Immunol. 2007 Sep 1;179(5):2989-98. PMID:17709514[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References | ||||||||||||||||||||
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