In innate immunity, antimicrobial peptides (AMPs) play a vital role in combating microbial pathogens. Among the AMPs identified in Penaeus monodon, only anti-lipopolysaccharide factor isoform 3 (ALFPm3) has been reported to exhibit activity against white spot syndrome virus (WSSV). However, the mechanism(s) involved are still not clear. In the present study, ALFPm3-interacting proteins were screened for from a WSSV library using the yeast two-hybrid screening system, revealing the five potential ALFPm3-interacting proteins of WSSV186, WSSV189, WSSV395, WSSV458 and WSSV471. Temporal transcriptional analysis in WSSV-infected P. monodon revealed that all five of these WSSV gene transcripts were expressed in the late phase of infection (24 h and 48 h post-infection). Of these, WSSV189 that was previously identified as a structural protein, was selected for further analysis and was shown to be an enveloped protein by Western blot and immunoelectron microscopy analyses. The in vitro pull-down assay using recombinant WSSV189 (rWSSV189) protein as bait confirmed the interaction between ALFPm3 and WSSV189 proteins. Moreover, pre-incubation of rWSSV189 protein with rALFPm3 protein interfered with the latter's neutralization effect on WSSV in vivo, as shown by the increased cumulative mortality of shrimp injected with WSSV following prior treatment with pre-incubated rWSSV189 and rALFPm3 proteins compared to that in shrimp pre-treated with rALFPm3 protein. Thus, ALFPm3 likely performs its anti-WSSV action by binding to the envelope protein WSSV189 and possibly other WSSV structural proteins.
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