title: Diversity in the C3b [corrected] contact residues and tertiary structures of the staphylococcal complement inhibitor (SCIN) protein family.
abstract: To survive in immune-competent hosts, the pathogen Staphylococcus aureus expresses and secretes a sophisticated array of proteins that inhibit the complement system. Among these are the staphylococcal complement inhibitors (SCIN), which are composed of three active proteins (SCIN-A, -B, and -C) and one purportedly inactive member (SCIN-D or ORF-D). Because previous work has focused almost exclusively on SCIN-A, we sought to provide initial structure/function information on additional SCIN proteins. To this end we determined crystal structures of an active, N-terminal truncation mutant of SCIN-B (denoted SCIN-B18-85) both free and bound to the C3c fragment of complement component C3 at 1.5 and 3.4 Å resolution, respectively. Comparison of the C3c/SCIN-B18-85 structure with that of C3c/SCIN-A revealed that both proteins target the same functional hotspot on the C3b/C3c surface yet harbor diversity in both the type of residues and interactions formed at their C3b/C3c interfaces. Most importantly, these structures allowed identification of Arg44 and Tyr51 as residues key for SCIN-B binding to C3b and subsequent inhibition of the AP C3 convertase. In addition, we also solved several crystal structures of SCIN-D to 1.3 Å limiting resolution. This revealed an unexpected structural deviation in the N-terminal α helix relative to SCIN-A and SCIN-B. Comparative analysis of both electrostatic potentials and surface complementarity suggest a physical explanation for the inability of SCIN-D to bind C3b/C3c. Together, these studies provide a more thorough understanding of immune evasion by S. aureus and enhance potential use of SCIN proteins as templates for design of complement targeted therapeutics.
authors.affiliation: Division of Cell Biology and Biophysics, School of Biological Sciences, University of Missouri-Kansas City, Kansas City, Missouri 64110.
authors.forename: Brandon L
authors.initials: BL
authors.lastname: Garcia
authors.affiliation: Division of Cell Biology and Biophysics, School of Biological Sciences, University of Missouri-Kansas City, Kansas City, Missouri 64110.
authors.forename: Brady J
authors.initials: BJ
authors.lastname: Summers
authors.affiliation: Department of Pathology and Laboratory Medicine, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104.
authors.forename: Zhuoer
authors.initials: Z
authors.lastname: Lin
authors.affiliation: Division of Cell Biology and Biophysics, School of Biological Sciences, University of Missouri-Kansas City, Kansas City, Missouri 64110.
authors.forename: Kasra X
authors.initials: KX
authors.lastname: Ramyar
authors.affiliation: Department of Pathology and Laboratory Medicine, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104.
authors.forename: Daniel
authors.initials: D
authors.lastname: Ricklin
authors.affiliation: Division of Cell Biology and Biophysics, School of Biological Sciences, University of Missouri-Kansas City, Kansas City, Missouri 64110.
authors.forename: Divya V
authors.initials: DV
authors.lastname: Kamath
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439.
authors.forename: Zheng-Qing
authors.initials: ZQ
authors.lastname: Fu
authors.affiliation: Department of Pathology and Laboratory Medicine, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104.
authors.forename: John D
authors.initials: JD
authors.lastname: Lambris
authors.affiliation: Division of Cell Biology and Biophysics, School of Biological Sciences, University of Missouri-Kansas City, Kansas City, Missouri 64110. Electronic address: GeisbrechtB@umkc.edu.
authors.forename: Brian V
authors.initials: BV
authors.lastname: Geisbrecht
mesh_terms: D000818:Animals; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism* / Q000494:pharmacology; D050577:Complement C3-C5 Convertases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D003179:Complement C3b / Q000378:metabolism*; D015932:Complement C3c / Q000378:metabolism; D018360:Crystallography, X-Ray; D006801:Humans; D008958:Models, Molecular; D017434:Protein Structure, Tertiary; D013211:Staphylococcus aureus / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D003179:Complement C3b; D015932:Complement C3c; D050577:Complement C3-C5 Convertases
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