White spot syndrome viral protein VP9 alters the cellular higher-order chromatin structure

Viral protein 9 (VP9) is a non-structural protein of white spot syndrome virus (WSSV) highly expressed during the early stage of infection. The crystal structure of VP9 suggests that the polymers of VP9 dimers resemble a DNA mimic, but its function remains elusive. In this study, we demonstrated that VP9 impedes histones binding to DNA via single-molecule manipulation. We established VP9 expression in HeLa cells due to the lack of a WSSV-susceptible cell line, and observed abundant VP9 in the nucleus, which mirrors its distribution in the hemocytes of WSSV-infected shrimp. VP9 expression increased the dynamics and rotational mobility of histones in stable H3-GFP HeLa cells as revealed by fluorescent recovery after photobleaching and fluorescence anisotropy imaging, which suggested a loosened compaction of chromatin structure. Successive salt fractionation showed that a prominent population of histones was solubilized in high salt concentrations, which implies alterations of bulk chromatin structure. Southern blotting identified that VP9 alters juxtacentromeric chromatin structures to be more accessible to micrococcal nuclease digestion. RNA microarray revealed that VP9 expression also leads to significant changes of cellular gene expression. Our findings provide evidence that VP9 alters the cellular higher-order chromatin structure, uncovering a potential strategy adopted by WSSV to facilitate its replication.