The bICPO protein encoded by bovine herpesvirus 1 (BHV-1) is believed to activate transcription and consequently productive infection. Expression of full-length bICPO protein is toxic in transiently transfected mouse neuroblastoma cells (neuro-2A) in the absence of other viral genes. However, bICPO does not appear to directly induce apoptosis. Although bICPO is believed to be functionally similar to the herpes simplex virus type 1-encoded ICPO, the only protein domain that is well conserved is a C3HC4 zinc ring finger located near the N terminus of both proteins. Site-specific mutagenesis of the zinc ring finger of bICPO demonstrated that it was important for inducing aggregated chromatin structures in transfected cells and toxicity. The zinc ring finger was also required for stimulating productive infection in bovine cells and for trans-activating the thymidine kinase (TK) promoter of herpes simplex virus type 1. Deletion of amino acids spanning 356-677 of bICPO altered subcellular localization of bICPO and prevented trans-activation of the TK promoter. However, this deletion did not prevent trans-activation of the viral genome. Taken together, these studies indicated that bICPO has several functional domains, including the zinc ring finger, which stimulate productive infection and influence cell survival.
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