Diversity in the C3b [corrected] contact residues and tertiary structures of the staphylococcal complement inhibitor (SCIN) protein family

J Biol Chem. 2012 Jan 2;287(1):628-640. doi: 10.1074/jbc.M111.298984. Epub 2011 Nov 15.


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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Animals
  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / metabolism*
  • Bacterial Proteins / pharmacology
  • Complement C3-C5 Convertases / antagonists & inhibitors
  • Complement C3-C5 Convertases / metabolism*
  • Complement C3b / metabolism*
  • Complement C3c / metabolism
  • Crystallography, X-Ray
  • Humans
  • Models, Molecular
  • Protein Structure, Tertiary
  • Staphylococcus aureus / metabolism*


  • Bacterial Proteins
  • Complement C3b
  • Complement C3c
  • Complement C3-C5 Convertases

Associated data

  • PDB/3T46
  • PDB/3T47
  • PDB/3T48
  • PDB/3T49
  • PDB/3T4A