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J. Biol. Chem., Vol. 280, Issue 20, 20120-20125, May 20, 2005

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Attribution of the Various Inhibitory Actions of the Streptococcal Inhibitor of Complement (SIC) to Regions within the Molecule^*

Michael J. Binks, Barbara A. Fernie-King, David J. Seilly, Peter J. Lachmann, and Kadaba S. Sriprakash¶

From the Queensland Institute of Medical Research, Brisbane, Queensland 4006, Australia and the Department of Veterinary Medicine, University of Cambridge, Cambridge CB3 0ES, United Kingdom

Some strains of Streptococcus pyogenes secrete a virulence factor called the streptococcal inhibitor of complement (SIC) function. SIC is a polyfunctional protein that interacts with a number of host proteins and peptides, especially with those that are involved in host defense systems. In addition to inhibiting the complement-mediated lysis of cells, SIC inhibits lysozyme, secretory leukocyte proteinase inhibitor, and -defensins. SIC also binds to proteins associated with the cytoskeleton and thereby may cause cytoskeletal derangement. The SIC molecule has three distinct structural domains constituting the N-proximal short repeat region (SRR), the central long repeat region (LRR), and the C-proximal proline-rich region (PRR). To map various functions to the structural domains, we have analyzed recombinant subclones expressing various parts of SIC and elastase-generated discrete fragments of SIC for binding to various ligands and for determining their biological properties. The results demonstrate the following. (a) SRR alone was sufficient to confer inhibition of complement function. (b) Anti-defensin and anti-lysozyme activities were mapped to the SRR plus LRR. (c) The LRR plus PRR harbored ezrin binding activity.

Received for publication, December 17, 2004 , and in revised form, February 23, 2005.

^* This work was supported by generous grants from the British Heart Foundation (to P. J. L.) and the National Health and Medical Research Council, Australia. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^¶ To whom correspondence should be addressed: Queensland Inst. of Medical Research, 300 Herston Rd., Brisbane, Queensland 4006, Australia. Tel.: 61-7-33620407; Fax: 61-7-38453507; E-mail: sriS{at}qimr.edu.au.

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