Bovine herpesvirus type 1 (BHV-1) is an alphaherpesvirus which is an important pathogen of cattle, causing a variety of clinical manifestations in its natural host (46). BHV-1 virions have a typical herpesvirus structure characterized by the presence of a double-stranded DNA genome enclosed in an icosahedral capsid, the tegument surrounding the capsid, and the outer host-derived lipid envelope bearing virus-encoded glycoproteins. While the major constituents of the viral envelope have been extensively studied (reviewed in reference 17), the proteins present in the tegument and nucleocapsid of BHV-1 have been poorly characterized. Compositionally, the tegument is the most complex compartment of the virion, containing more than 15 viral gene products (32). In addition to their structural role, various regulatory functions, including modulation of transcription (34, 47), kinase activity (39), RNase activity (41), and DNA packaging (43), have been assigned to some tegument proteins, suggesting that these virion constituents function at several stages during virus infection, establishing conditions for efficient viral replication and promoting virus assembly and egress.Although the U L 47 gene product, tegument protein VP8, is the most abundant component of mature BHV-1 virions (5), its function is unknown. Like its herpes simplex virus type 1 (HSV-1) homologue (31), VP8 is posttranslationally modified by phosphorylation (5,23) and by the addition of O-linked carbohydrates (49). Both HSV-1 and BHV-1 U L 47 homologues possess nuclear localization and nuclear export signatures (7,51,53,56), enabling them to shuttle between the nucleus and cytoplasm when expressed in transiently transfected cells (51,56) or during viral infection (52, 53). Furthermore, both proteins exhibit a steady-state nuclear localization at early stages of infection and during transient expression (6,35,49,51,52,56), suggesting a functional role for these homologues in the nucleus. Nucleocytoplasmic shuttling of VP8 is sensitive to treatment with a RNA polymerase II inhibitor, actinomycin D (52). This observation coupled with recently demonstrated RNA binding activity of the HSV-1 and BHV-1