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J. Biol. Chem., Vol. 281, Issue 32, 23119-23128, August 11, 2006

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Functional Characterization of the Complement Control Protein Homolog of Herpesvirus Saimiri

ARG-118 IS CRITICAL FOR FACTOR I COFACTOR ACTIVITIES^*

From the National Centre for Cell Science, Pune University Campus, Ganeshkhind, Pune 411007, India

Herpesvirus saimiri (HVS) is a lymphotropic virus that causes T-cell lymphomas in New World primates. It encodes a structural homolog of complement control proteins named complement control protein homolog (CCPH). Previously, CCPH has been shown to inhibit C3d deposition on target cells exposed to complement. Here we have studied the mechanism by which it inactivates complement. We have expressed the soluble form of CCPH in Escherichia coli, purified to homogeneity and compared its activity to vaccinia virus complement control protein (VCP) and human complement regulators factor H and soluble complement receptor 1. The expressed soluble form of CCPH bound to C3b (K_D = 19.2 µM) as well as to C4b (K_D = 0.8 µM) and accelerated the decay of the classical/lectin as well as alternative pathway C3-convertases. In addition, it also served as factor I cofactor and supported factor I-mediated inactivation of both C3b and C4b. Time course analysis indicated that although its rate of inactivation of C4b is comparable with VCP, it is 14-fold more potent than VCP in inactivating C3b. Site-directed mutagenesis revealed that Arg-118, which corresponds to Lys-120 of variola virus complement regulator SPICE (a residue critical for its enhanced C3b cofactor activity), contributes significantly in enhancing this activity. Thus, our data indicate that HVS encodes a potent complement inhibitor that allows HVS to evade the host complement attack.

Received for publication, March 31, 2006 , and in revised form, June 7, 2006.

^* This work was supported by the Wellcome Trust Senior Research Fellowship in Biomedical Science in India (to A. S.). 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.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. 1 and 2.

¹ Supported by a research fellowship from the Council of Scientific and Industrial Research, India. This work was performed in partial fulfillment of a Ph.D. thesis, which will be submitted to the University of Pune, Pune, India.

² To whom correspondence should be addressed. Tel.: 91-20-2569-0922; Fax: 91-20-2569-2259; E-mail: arvindsahu{at}nccs.res.in.

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