Identification of a pseudorabies virus UL12 (deoxyribonuclease) gene

Emodin is a novel alkaline nuclease inhibitor that suppresses herpes simplex virus type 1 yields in cell cultures

BACKGROUND AND PURPOSE

Most antiviral therapies directed against herpes simplex virus (HSV) infections are limited to a small group of nucleoside analogues that target the viral polymerase. Extensive clinical use of these drugs has led to the emergence of resistant viral strains, mainly in immunocompromised patients. This highlights the need for the development of new anti-herpesviral drugs with novel targets. Herein the effects of a plant anthraquinone, emodin, on the HSV-1 alkaline nuclease activity and virus yields were investigated.

EXPERIMENTAL APPROACH

HSV-1 alkaline nuclease activity was examined by nuclease activity assay. Inhibition of virus yields was measured by plaque reduction assay and immunohistochemical staining. Interaction between emodin and alkaline nuclease was analysed by docking technology.

KEY RESULTS

Emodin specifically inhibited the nuclease activity of HSV-1 UL12 alkaline nuclease in a biochemical assay. Plaque reduction assay revealed that emodin reduced the plaque formation with an EC(50) of 21.5+/-4.4 muM. Immunohistochemical staining using the anti-nucleocapsid protein antibody demonstrated that emodin induced the accumulation of viral nucleocapsids in the nucleus in a dose-dependent manner. Docking analysis further suggested that the inhibitory effect of emodin on the UL12 activity may result from the interaction between emodin and critical catalytic amino acid residues of UL12.

CONCLUSIONS AND IMPLICATIONS

Our findings suggest that emodin is a potent anti-HSV agent that inhibits the yields of HSV-1 via the suppression of a novel target, UL12.

Molecular biology of pseudorabies virus: impact on neurovirology and veterinary medicine

Pseudorabies virus (PRV) is a herpesvirus of swine, a member of the Alphaherpesvirinae subfamily, and the etiological agent of Aujeszky's disease. This review describes the contributions of PRV research to herpesvirus biology, neurobiology, and viral pathogenesis by focusing on (i) the molecular biology of PRV, (ii) model systems to study PRV pathogenesis and neurovirulence, (iii) PRV transsynaptic tracing of neuronal circuits, and (iv) veterinary aspects of pseudorabies disease. The structure of the enveloped infectious particle, the content of the viral DNA genome, and a step-by-step overview of the viral replication cycle are presented. PRV infection is initiated by binding to cellular receptors to allow penetration into the cell. After reaching the nucleus, the viral genome directs a regulated gene expression cascade that culminates with viral DNA replication and production of new virion constituents. Finally, progeny virions self-assemble and exit the host cells. Animal models and neuronal culture systems developed for the study of PRV pathogenesis and neurovirulence are discussed. PRV serves asa self-perpetuating transsynaptic tracer of neuronal circuitry, and we detail the original studies of PRV circuitry mapping, the biology underlying this application, and the development of the next generation of tracer viruses. The basic veterinary aspects of pseudorabies management and disease in swine are discussed. PRV infection progresses from acute infection of the respiratory epithelium to latent infection in the peripheral nervous system. Sporadic reactivation from latency can transmit PRV to new hosts. The successful management of PRV disease has relied on vaccination, prevention, and testing.

Complete, annotated sequence of the pseudorabies virus genome

We have obtained the complete DNA sequence of pseudorabies virus (PRV), an alphaherpesvirus also known as Aujeszky's disease virus or suid herpesvirus 1, using sequence fragments derived from six different strains (Kaplan, Becker, Rice, Indiana-Funkhauser, NIA-3, and TNL). The assembled PRV genome sequence comprises 143,461 nucleotides. As expected, it matches the predicted gene arrangement, genome size, and restriction enzyme digest patterns. More than 70 open reading frames were identified with homologs in related alphaherpesviruses; none were unique to PRV. RNA polymerase II transcriptional control elements in the PRV genome, including core promoters, splice sites, and polyadenylation sites, were identified with computer prediction programs. The correlation between predicted and experimentally determined transcription start and stop sites was excellent. The transcriptional control architecture is characterized by three key features: core transcription elements shared between genes, yielding divergent transcripts and a large number of coterminal transcripts; bifunctional transcriptional elements, yielding head-to-tail transcripts; and short repetitive sequences that could function as insulators against improperly terminated transcripts. Many of these features are conserved in the alphaherpesvirus subfamily and have important implications for gene array analyses.

Pseudorabies virus DNA-binding protein stimulates the exonuclease activity and regulates the processivity of pseudorabies virus DNase

The pseudorabies virus (PRV) DNase is an alkaline exonuclease and endonuclease, which exhibits an Escherichia coli RecBCD-like catalytic function. The PRV DNA-binding protein (DBP) promotes the renaturation of complementary single strands of DNA, which is an essential function for recombinase. To investigate the functional and physical interactions between PRV DBP and DNase, these proteins were purified to homogeneity. PRV DBP stimulated the DNase activity, especially the exonuclease activity, in a dose-dependent fashion. Acetylation of DBP by acetic anhydride resulted in a loss of DNA-binding ability and a 60% inhibition of the DNase activity, suggesting that DNA-binding ability of PRV DBP was required for stimulating the DNase activity. PRV DNase behaved in a processive mode; however, it was converted into a distributive mode in the presence of DBP, implying that PRV DBP stimulated the dissociation of DNase from DNA substrates. The physical interaction between DBP and DNase was further analyzed by enzyme-linked immunosorbent assay, and a significant interaction was observed. Thus, these results suggested that PRV DBP interacted with PRV DNase and regulated the DNase activity in vitro.

The exonuclease activity of HSV-1 UL12 is required for in vivo function

The herpes simplex virus type 1 (HSV-1) UL12 gene encodes an alkaline pH-dependent deoxyribonuclease termed alkaline nuclease. A recombinant UL12 knockout mutant, AN-1, is severely compromised for growth, and analysis of this mutant suggests that UL12 plays a role in processing complex DNA replication intermediates (R. Martinez, R. T. Sarisky, P. C. Weber, and S. K. Weller, (1996) J. Virol. 70, 2075-2085). This processing step may be required for the generation of capsids that are competent for egress from the nucleus to the cytoplasm. In this report, we address the question of whether the AN-1 growth phenotype is due to the loss of UL12 catalytic activity. We constructed two point mutations in a highly conserved region (motif II) of UL12 and purified wild-type and mutant enzymes from a baculovirus expression system. Both mutant proteins are stable, soluble, and competent for correct nuclear localization, suggesting that they have retained an intact global conformation. Neither mutant protein, however, exhibits exonuclease activity. In order to examine the in vivo effects of these mutations, we determined whether expression of mutant proteins from amplicon plasmids could complement AN-1. While the wild-type plasmid complements the growth of the null mutant, neither UL12 mutant can do so. Loss of exonuclease activity therefore correlates with loss of in vivo function.

Recombinant pseudorabies virus DNase exhibits a RecBCD-like catalytic function

The pseudorabies virus (PRV) DNase gene has previously been mapped within the PRV genome. To characterize further the enzymic properties of PRV DNase, this enzyme was expressed in Escherichia coli with the use of a pET expression vector. The protein was purified to homogeneity and assayed for nuclease activity in vitro. Recombinant PRV DNase exhibited an alkaline pH preference and an absolute requirement for Mg2+ ions that could not be replaced by Ca2+ and Na+ ions. Further studies showed that PRV DNase exhibited endonuclease, 5'-exonuclease and 3'-exonuclease activities in both single-stranded and double-stranded DNA. This activity occurred randomly and no significant base preference was demonstrated. The multiple biochemical activities of PRV DNase are similar to the activities of Neurospora crassa endo-exonuclease and E. coli RecBCD, two additional enzymes that are involved in recombination. Taken together, the similarity of action between N. crassa endo-exonuclease, E. coli RecBCD, and PRV DNase suggests that PRV DNase might have a role in the process of recombination that occurs during PRV infection.