Abstract
Singapore Grouper Iridovirus (SGIV) is a member of nucleo cytoplasmic large DNA viruses (NCLDV). This paper reports the functional analysis of ORF75R, a major structural protein of SGIV. Immuno fluorescence studies showed that the protein was accumulated in the viral assembly site. Immunogold-labelling indicated that it was localized between the viral capsid shell and DNA core. Knockdown of ORF75R by morpholinos resulted in the reduction of coreshell thickness, the failure of DNA encapsidation, and the low yield of infectious particles. Comparative proteomics further identified the structural proteins affected by ORF75R knockdown. Two-dimensional gel electrophoresis combined with proteomics demonstrated that ORF75R was phosphorylated at multiple sites in SGIV-infected cell lysate and virions, but the vast majority of ORF75R in virions was the dephosphorylated isoform. A kinase assay showed that ORF75R could be phosphorylated in vitro by the SGIV structural protein ORF39L. Addition of ATP and Mg2+ into purified virions prompted extensive phosphorylation of structural proteins and release of ORF75R from virions. These data suggest that ORF75R is a novel scaffold protein important for viral assembly and DNA encapsidation, but its phosphorylation facilitates virion disassembly. Compared to proteins from other viruses, we found that ORF75R shares common features with herpes simplex virus VP22.
Highlights
Example, herpes simplex virus encodes VP22 as a major scaffold protein to help virion stability and the recruitment of several other viral structural proteins[13]
ORF75R accumulates in the viral assembly site (VAS)
ORF75R immunostaining (Fig. 1, green) merged perfectly with the blue dots (Fig. 1, triangles), which indicated that ORF75R was highly accumulated in the VAS
Summary
Example, herpes simplex virus encodes VP22 as a major scaffold protein to help virion stability and the recruitment of several other viral structural proteins[13]. ORF75R is a major structural component in mature SGIV virion[14]. We have previously identified that it was a lipid-binding protein and phosphorylated at multiple sites[15]. Knockdown of ORF18R, another abundant structural protein of SGIV, could affect ORF75R phosphorylation and virion packaging[16]. We demonstrate that ORF75R is a scaffold protein important for viral packaging
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