NTU and Academia Sinica Team Discovers a Novel DNA Mimic Protein
that Binds with Histone and Disrupts Host Chromosomes

Professor Chu-Fang Lo, Dean of the College of Life Sciences of NTU, and Dr. Andrew H. J. Wang, Vice President of Academia Sinica, led a team of the doctoral students from NTU's Institute of Bio-chemical Sciences and Institute of Zoology in a study of the novel functions of ICP11 protein in shrimp White Spot Syndrome Virus. Using structural biology, the researchers found that this protein, designated ICP11, is a DNA mimic protein. They further showed that ICP11 binds with host histone, and disrupts the host's chromosomes, and that it enhances nuclease activity. The researchers successfully expressed ICP11 in human Hela cells, suggesting that its unique functionalities might also be useful for research in higher organisms and human disease. The results of this study were published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) dated December 30th, 2008.

White Spot Syndrome Virus (WSSV; genus: Whispovirus, family: Nimaviridae) is a new large DNA virus, and it is the causative pathogen of White Spot Disease in shrimps. Aquatic crustaceans infected with this virus often have a very high mortality rate, and this can result in great economic loss. Attention was drawn to the WSSV protein ICP11 when transcriptomics and proteomics research showed that it was the most highly expressed non-structural protein gene in the cells of infected shrimps. From this, researchers reasoned that this newly discovered protein must play an important role in the viral infection process.

When X-ray diffraction was used to analyze the crystal structure of this negatively charged acidic protein, researchers discovered that ICP11 dimers could be assembled into filaments whose surface negative charge distribution is very similar to that of the B-form DNA double-stranded helix. This suggested that ICP11 might be a DNA mimic protein.

Further studies were conducted to explore this possibility. After far Western blotting demonstrated that ICP11 could bind to histone, a series of histone-DNA binding assays showed that ICP11 effectively prevented several histone proteins (H2A, H2A.x, H2B, and H3) from binding with DNA. Immunofluorescence analysis further showed that ICP11-bound histones were located outside of the nuclei of both infected shrimp hemocytes and ICP11 transfected human Hela cells. Histones normally translocate to the cell nucleus either to help form nucleosomes or, in the case of H2Ax, to repair DNA double-strand breaks. Therefore, by preventing histones from entering the cell nucleus, ICP11 prevents nucleosome assembly and DNA repair. Meanwhile ICP11 also showed an unexpected ability to enhance the activity of DNase. Taken together, ICP11's ability to enhance DNase activity and prevent DNA repair results in damage to the host DNA that ultimately leads to the death of the host cells.

WSSV ICP11 is now recognized as one of only a handful of DNA mimic proteins presently known to science. Apart from its intrinsic scientific interest, this work is significant not just because it increases our understanding of the viral diseases of economically important aquatic animals, but also because it may be of great benefit in the study of higher organisms and human medical research. The results of this collaborative research also show that in-depth study of virus-host interactions can be usefully pursued in non-model organisms. Indeed, WSSV has a very large genome (~300 kbp) and the function of many WSSV proteins is still unknown. This is potentially a very fruitful and rewarding area for future research-and it is an area that the NTU/AS team will continue to investigate.