Background:LGBP is an important pattern recognition protein (PRP). Results: PmLGBP binds to ␤-1,3-glucan and LPS and could enhance the phenoloxidase (PO) activity. Knockdown shrimp showed decreased PO activity. Conclusion: PmLGBP functions as a PRP for LPS and ␤-1,3-glucan in the proPO system. Significance: PmLGBP is a PRP involved in the proPO system, exhibits LPS and ␤-1,3-glucan binding activity, and can activate the proPO system.

Background: Melanization plays a major role in invertebrate defense. Results: Suppression of shrimp melanization increased the WSSV susceptibility. The viral protein, WSSV453, interferes the proPO system via PmPPAE2 inhibition. Conclusion: Shrimp melanization has an antiviral role. WSSV overcomes this by suppressing the host proPO proteinase cascade. Significance: The regulation of shrimp melanization during WSSV infection was first demonstrated.

Inhibition of the host melanization reaction, activated by the prophenoloxidase activating (proPO) system, is one of the crucial evasion strategies of pathogens. Recently, the shrimp pathogen, white spot syndrome virus (WSSV), was found to inhibit melanization in the shrimp Penaeus monodon. The viral protein WSSV453 was previously shown to interact with PO-activating enzyme 2 (PmPPAE2) and reported to be involved in suppressing the shrimp melanization response after WSSV infection. Here, we characterized how WSSV453 inhibits melanization. WSSV453 is a non-structural viral protein, which was first detected in shrimp haemocytes at 6 hours post-infection (hpi) by WSSV and in shrimp plasma at 24 hpi. We produced recombinant proteins for three components of the P. monodon proPO system: PmproPPAE2, PmproPO1 and PmproPO2. Functional assays showed that active PmPPAE2 processed PmproPO1 and 2 to produce functional PO. Incubation of WSSV453 with PmproPPAE2 dose-dependently reduced PmPPAE2 activity toward PmPO1 or PmPO2. In contrast, WSSV453 had no effect on activated PmPPAE2. The addition of active PmPPAE2 to WSSV-infected shrimp plasma at day 2 post-infection also rescued PO activity. Taken together, these results indicate that the anti-melanization activity of WSSV is due to WSSV453, which interacts with PmproPPAE2 and interferes with its activation to active PmPPAE2.

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