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J. Biol. Chem., Vol. 283, Issue 31, 21325-21333, August 1, 2008

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The Viral Protein Egf1.0 Is a Dual Activity Inhibitor of Prophenoloxidase-activating Proteinases 1 and 3 from Manduca sexta^*

Zhiqiang Lu, Markus H. Beck, Yang Wang, Haobo Jiang, and Michael R. Strand¹

From the Department of Entomology, University of Georgia, Athens, Georgia 30602 and Department of Entomology and Plant Pathology, Oklahoma State University, Stillwater, Oklahoma 74078

Some pathogens are capable of suppressing the melanization response of host insects, but the virulence factors responsible are largely unknown. The insect pathogen Microplitis demolitor bracovirus encodes the Egf family of small serine proteinase inhibitors. One family member, Egf1.0, was recently shown to suppress melanization of hemolymph in Manduca sexta in part by inhibiting the enzymatic activity of prophenoloxidase activating proteinase 3 (PAP3). However, other experiments suggested this viral protein suppresses melanization by more than one mechanism. Here we report that Egf1.0 inhibited the amidolytic activity of PAP1 and dose-dependently blocked processing of pro-PAP1 and pro-PAP3. Consistent with its PAP inhibitory activity, Egf1.0 also prevented processing of pro-phenoloxidase, serine proteinase homolog (SPH) 1, and SPH2. Isolation of Egf1.0-protein complexes from plasma indicated that Egf1.0 binds PAPs through its C-terminal repeat domain. Egf1.0 also potentially interacts with SPH2 and two other proteins, ferritin and gloverin, not previously associated with the phenoloxidase cascade. Overall, our results indicate that Egf1.0 is a dual activity PAP inhibitor that strongly suppresses the insect melanization response.

Received for publication, February 27, 2008 , and in revised form, May 5, 2008.

^* This work was supported, in whole or in part, by National Institutes of Health Grant and GM58634 (to H. J.). Support for the work was also derived from United States Department of Agriculture NRI Grants 2005-05382 and 2007-04944 and the Georgia Agricultural Experiment Station (to M. R. 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.

¹ To whom correspondence should be addressed: Dept. of Entomology, 413 Biological Sciences Bldg., University of Georgia, Athens, GA 30602. Tel.: 706-583-8237; Fax: 706-542-2279; E-mail: mrstrand{at}uga.edu.

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