Pocket structures of surface protrusions shared among serologically distinct nervous necrosis viruses (NNVs) were predicted in silico to bind to sialylated N-glycans, a host cellular receptor

Abstract

Nervous necrosis virus (NNV), one of the simplest spherical RNA-viruses, causes high mortality in aquaculture facilities worldwide. In this study, we investigated changes in the infectivity and reactivity of serologically distinct NNVs after treatment with 2.0 M urea, which was used as a means to predict a site or sites on surface protrusions responsible for NNV infection. Antigenicity of NNVs declined significantly after urea treatment but their infectivity was maintained, regardless of serological distinctions. The ability of urea-treated NNVs to be neutralized by antisera was also maintained, regardless of serological distinction. Moreover, weak but reliable cross-neutralizing reactions were observed between serologically distinct NNVs even after urea-treatment. These results suggested that common epitopes resistant to urea-treatment were shared among serologically distinct NNVs. Furthermore, two highly conserved amino-acid stretches among NNVs were revealed, and these stretches surround the pocket structures at the apex of surface protrusions. Molecular docking analysis showed that the pocket structure of surface protrusions was a site for maximally stable binding with the terminal moiety of sialylated N-glycans, a common cellular receptor for serologically distinct NNVs. Based on these results, the pocket structures on surface protrusions of NNVs, appear to be sites responsible for binding to sialylated N-glycans. Contrarily, a red-spotted grouper NNV (RGNNV)-specific MAb, neutralized naïve RGNNV but not the urea-treated RGNNV, suggesting that the epitope for this MAb was not present in the pocket structures. This suggests that disappearance of NNV infectivity due to binding with this MAb was the result of indirect inhibition of binding to the cellular receptor.