PMID: 22086928

Authors:

Garcia BL, Summers BJ, Lin Z, Ramyar KX, Ricklin D, Kamath DV, Fu ZQ, Lambris JD, Geisbrecht BV

Title:

Diversity in the C3b Convertase Contact Residues and Tertiary Structures of the Staphylococcal Complement Inhibitor (SCIN) Protein Family.

Journal:

J Biol Chem. 2012 Jan 2;287(1):628-40. Epub 2011 Nov 15.

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-B(18-85)) both free and bound to the C3c fragment of complement component C3 at 1.5 and 3.4 A resolution, respectively. Comparison of the C3c/SCIN-B(18-85) 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 Arg(44) and Tyr(51) 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 A limiting resolution. This revealed an unexpected structural deviation in the N-terminal alpha 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.