White spot syndrome virus (WSSV) is a large (Ϸ300 kbp), doublestranded DNA eukaryotic virus that has caused serious disease in crustaceans worldwide. ICP11 is the most highly expressed WSSV nonstructural gene/protein, which strongly suggests its importance in WSSV infection; but until now, its function has remained obscure. We show here that ICP11 acts as a DNA mimic. In crystal, ICP11 formed a polymer of dimers with 2 rows of negatively charged spots that approximated the duplex arrangement of the phosphate groups in DNA. Functionally, ICP11 prevented DNA from binding to histone proteins H2A, H2B, H3, and H2A.x, and in hemocytes from WSSVinfected shrimp, ICP11 colocalized with histone H3 and activated-H2A.x. These observations together suggest that ICP11 might interfere with nucleosome assembly and prevent H2A.x from fulfilling its critical function of repairing DNA double strand breaks. Therefore, ICP11 possesses a functionality that is unique among the handful of presently known DNA mimic proteins.apoptosis ͉ DNase enhancer ͉ crystal structure ͉ shrimp aquaculture T he white spot syndrome virus (WSSV) is an enveloped DNA virus that infects crustaceans and threatens shrimp aquaculture (1-4). The white spot disease caused by WSSV can result in 100% cumulative mortality in farmed shrimps in 2-10 days. Based on studies of individual genes and analysis of the complete genome sequence, the ellipsoid-shaped WSSV has been erected as the type species of a new genus (Whispovirus) of a new virus family Nimaviridae (5). Because of the large size of the viral genome (Ϸ300 kb) and the uniqueness of the encoded proteins, WSSV has not yet been fully characterized.In previous studies, both transcriptomic (WSSV-infected EST database and WSSV DNA microarray) and proteomic (2D electrophoresis) approaches identified ICP11 as a highly expressed WSSV gene/protein (6, 7). The high expression levels of this protein strongly suggest its importance to WSSV infection; but until now, its function has remained unknown. In the present article, we determine the crystal structure of ICP11 and use Far Western assays and indirect immunofluorescence to investigate its function and the factors with which it interacts. We found that ICP11 acts as a DNA mimic that prevents DNA from binding to histone proteins and, thus, disrupts nucleosome assembly. Results ICP11 CrystalStructure. The protein model of ICP11 was built manually into a clear electron density map [supporting information (SI) Fig. S1 A] derived from MAD X-ray diffraction data. The refined structure contains 2 ICP11 molecules as a dimer per asymmetric unit (Fig. 1). The refinement statistics are listed in Table S1. Each monomer consists of a 4-stranded anti-parallel ␤-sheet, a 2-stranded ␤-ribbon, and 2 flanking ␣-helices ( Fig. 1 A and B). In the center of the monomer, 16-aa side chains are associated into a hydrophobic core. Every secondary structural element contributes to this core structure; thus, forming a stable compact globular fold. The nature of the dimer interface is largely...

DNA mimic proteins have DNA-like negative surface charge distributions, and they function by occupying the DNA binding sites of DNA binding proteins to prevent these sites from being accessed by DNA. DNA mimic proteins control the activities of a variety of DNA binding proteins and are involved in a wide range of cellular mechanisms such as chromatin assembly, DNA repair, transcription regulation, and gene recombination. However, the sequences and structures of DNA mimic proteins are diverse, making them difficult to predict by bioinformatic search. To date, only a few DNA mimic proteins have been reported. These DNA mimics were not found by searching for functional motifs in their sequences but were revealed only by structural analysis of their charge distribution. This review highlights the biological roles and structures of 16 reported DNA mimic proteins. We also discuss approaches that might be used to discover new DNA mimic proteins.

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