Bovine herpesvirus 1 tegument protein VP22 interacts with histones, and the carboxyl terminus of VP22 is required for nuclear localization.

Bovine herpesvirus 1(BHV-1) is an alphaherpesvirus. Structurally, BHV-1 contains an envelope, tegument, and nucleocapsid (28). The tegument structure is a unique feature among herpesviruses but remains poorly defined. A large number of viral proteins participate in the assembly of the tegument (28). Not only are tegument proteins important viral structural proteins, they also play critical roles during infection. The multifaceted roles of tegument proteins give an additional advantage in virus infection, since these tegument proteins can quickly be released into the cell upon viral penetration and exert their functions prior to any viral gene expression. BHV-1 VP22 is a 258-amino-acid (aa) tegument protein, and its homologs are highly conserved among alphaherpesviruses (20). The BHV-1 VP22 deletion mutant virus yields only a slightly lower titer than that of the wild-type virus in tissue-cultured cells, but interestingly, this deletion mutant is asymptomatic and avirulent in infected cattle (19). Thus, VP22 might play an important role during BHV-1 replication in vivo. The nuclear localization of BHV-1 VP22 also suggests that VP22 may have regulatory functions (20). In herpes simplex virus type 1 (HSV-1), VP22 interacts with another tegument protein, VP16, and its gene is classified as essential (8). To date, the exact biological function of VP22 in infection is still unknown. Histones are the most abundant DNA binding proteins in eukaryotic cells, are highly conserved, and are actively transported into the nucleus (2, 13, 18, 21). Two copies each of histone H2A, H2B, H3, and H4 form the octamer core. The octamer core and the DNA wrapped around the core form the basic unit of the chromatin structure called the nucleosome (15). Histone H1 binds the core and linker DNA. Histones are heavily modified proteins in the cell, and such modifications include acetylation, phosphorylation, methylation, and ubiquitination (26). Recent studies have shown that some of these histone modifications play important roles in chromatin remodeling, cell cycle control, and gene regulation (26). Phosphorylation of H3 is linked to chromatin condensation prior to cell division (30). Histone acetyltransferase activity is mapped to a number of transcriptional regulatory proteins such as p300 (also called CBP) (1), PCAF (16, 31), GCN5 (5, 17), and TAF(II)250 (22). Histone acetylation is believed to open up the chromatin structure, keeping DNA accessible to transcriptional factors and facilitating gene activation (6, 26). Conversely, histone deacetylation is linked to gene repression (3, 14, 29). Therefore, histones not only serve as an important structural component of chromatin but also are actively involved in the regulation of key cellular activities. To better understand the biological function of BHV-1 tegument protein VP22, in this report we (i) demonstrate that the nuclear localization of VP22 is independent of other viral factors; (ii) map the functional domains that support the nuclear localization of VP22; (iii) demonstrate the specific association between VP22 and histones; (iv) show that VP22 shares similar antigenic determinants with histones; and (v) demonstrate that, in VP22-expressing cells and BHV-1 infected cells, acetylation of histone H4 is decreased. The attenuation of the VP22 deletion mutant virus in vivo, the ability of VP22 to associate with histones, and the reduced acetylation of histone suggest that VP22 may have regulatory functions during virus replication.