Recombinant pseudorabies virus DNase exhibits a RecBCD-like catalytic function

Mutagenesis identifies the critical amino acid residues of human endonuclease G involved in catalysis, magnesium coordination, and substrate specificity

Background

Endonuclease G (EndoG), a member of DNA/RNA nonspecific ββα-Me-finger nucleases, is involved in apoptosis and normal cellular proliferation. In this study, we analyzed the critical amino acid residues of EndoG and proposed the catalytic mechanism of EndoG.

Methods

To identify the critical amino acid residues of human EndoG, we replaced the conserved histidine, asparagine, and arginine residues with alanine. The catalytic efficacies of Escherichia coli-expressed EndoG variants were further analyzed by kinetic studies.

Results

Diethyl pyrocarbonate modification assay revealed that histidine residues were involved in EndoG activity. His-141, Asn-163, and Asn-172 in the H-N-H motif of EndoG were critical for catalysis and substrate specificity. H141A mutant required a higher magnesium concentration to achieve its activity, suggesting the unique role of His-141 in both catalysis and magnesium coordination. Furthermore, an additional catalytic residue (Asn-251) and an additional metal ion binding site (Glu-271) of human EndoG were identified.

Conclusion

Based on the mutational analysis and homology modeling, we proposed that human EndoG shared a similar catalytic mechanism with nuclease A from Anabaena.

Mutagenesis identifies the critical regions and amino acid residues of suid herpesvirus 1 DNA-binding protein required for DNA binding and strand invasion

Herpesviral DNA-binding protein (DBP) is a unique protein involved in viral DNA replication and genomic recombination. It binds and stabilizes the single-stranded DNA. It also forms the D-loops and promotes the strand invasion. To identify the functional regions and amino acid residues required for DNA binding and D-loop formation, we characterized several DBP mutants of suid herpesvirus 1 (SuHV-1). Acetic anhydride modification assay showed that lysine residues were critical for DNA binding and D-loop formation. Replacement of highly conserved lysine residues with alanine revealed that Lys-756 and Lys-970 were critical for DNA binding, while Lys-161 participated in DNA binding and D-loop formation. Analysis of nested deleted mutants showed that N-terminal 201 amino acid residues and C-terminal 305 amino acid residues were required for D-loop formation and DNA binding, respectively. In conclusion, these findings suggested that SuHV-1 DBP contained critical regions for DNA binding and D-loop formation, and Lys-161, Lys-756, and Lys-970 were required for DNA binding or D-loop formation.

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.

Antigenicity and receptor-binding ability of recombinant SARS coronavirus spike protein

Severe acute respiratory syndrome (SARS) is an emerging infectious disease associated with a novel coronavirus and causing worldwide outbreaks. SARS coronavirus (SARS-CoV) is an enveloped RNA virus, which contains several structural proteins. Among these proteins, spike (S) protein is responsible for binding to specific cellular receptors and is a major antigenic determinant, which induces neutralizing antibody. In order to analyze the antigenicity and receptor-binding ability of SARS-CoV S protein, we expressed the S protein in Escherichia coli using a pET expression vector. After the isopropyl-beta-D-thiogalactoside induction, S protein was expressed in the soluble form and purified by nickel-affinity chromatography to homogeneity. The amount of S protein recovered was 0.2-0.3mg/100ml bacterial culture. The S protein was recognized by sera from SARS patients by ELISA and Western blot, which indicated that recombinant S protein retained its antigenicity. By biotinylated ELISA and Western blot using biotin-labeled S protein as the probe, we identified 130-kDa and 140-kDa proteins in Vero cells that might be the cellular receptors responsible for SARS-CoV infection. Taken together, these results suggested that recombinant S protein exhibited the antigenicity and receptor-binding ability, and it could be a good candidate for further developing SARS vaccine and anti-SARS therapy.

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.

Identification of a DNA-binding domain and an active-site residue of pseudorabies virus DNase

The pseudorabies virus (PRV) DNase gene has an open reading frame of 1476 nt, capable of coding a 492-residue protein. A previous study showed that PRV DNase is an alkaline exonuclease and endonuclease, exhibiting an Escherichia coli RecBCD-like catalytic function. To analyse its catalytic mechanism further, we constructed a set of clones truncated at the N-terminus or C-terminus of PRV DNase. The deleted mutants were expressed in E. coli with the use of pET expression vectors, then purified to homogeneity. Our results indicate that (1) the region spanning residues 274-492 exhibits a DNA-binding ability 7-fold that of the intact DNase; (2) the N-terminal 62 residues and the C-terminal 39 residues have important roles in 3'-exonuclease activity, and (3) residues 63-453 are responsible for 5'- and 3'-exonuclease activities. Further chemical modification of PRV DNase revealed that the inactivation of DNase by diethyl pyrocarbonate, which was reversible on treatment with hydroxylamine, seemed to be attributable solely to the modification of histidyl residues. Because the herpesviral DNases contained only one well-conserved histidine residue, site-directed mutagenesis was performed to replace His(371) with Ala. The mutant lost most of its nuclease activity; however, it still exhibited a wild-type level of DNA-binding ability. In summary, these results indicate that PRV DNase contains an independent DNA-binding domain and that His(371) is the active-site residue that has an essential role in PRV DNase activity.

Pseudorabies virus early protein 0 trans-activates the TATA-associated promoter by stimulating the transcription initiation

Pseudorabies virus (PRV) early protein 0 (EP0) is a transactivator containing a RING finger domain. To assess the transactivation mechanism of PRV EP0, we performed the in vitro transcription by combining HeLa nuclear extract, purified recombinant EP0 and simple promoter constructs, and evaluated the results by primer extension. The data showed that EP0 could significantly activate the TATA-containing synthetic promoters. Moreover, EP0 activated transcription by stabilizing the formation of transcription initiation complex instead of enhancing the elongation rate. To further understand the role of EP0 on assembling the transcription initiation complex, we performed the pull-down assay using affinity precipitation of proteins from HeLa nuclear extracts and bacterially expressed glutathione-S-transferase EP0 RING finger fusion. The data showed that at least six nuclear proteins physically interacted with the EP0 RING finger. Overall, the transactivation of PRV EP0 is accomplished by enhancing the transcription initiation and is associated with at least six nuclear proteins.

Identification, expression, and characterization of the pseudorabies virus DNA-binding protein gene and gene product

The pseudorabies virus (PRV) gene encoding a DNA-binding protein (DBP) was first identified in this study. The DBP gene has an open reading frame of 3531 nucleotides, capable of coding a 1177-amino-acid polypeptide of 125 kDa. The deduced DBP exhibits a conserved zinc-binding motif and a conserved DNA-binding region, suggesting the similar DNA-binding mechanism occurs among alphaherpesviral DBP homologs. To further identify the biochemical properties of PRV DBP, this protein was expressed in Escherichia coli by using a pET expression vector and purified to homogeneity. The PRV DBP binds cooperatively and preferentially to single-stranded DNA with no significant base preference, judged by agarose gel electrophoresis and competitive nitrocellulose filter binding assays. Taken together, these results suggest that PRV DBP may play an important role in PRV DNA replication by binding cooperatively and nonspecifically to single-stranded DNA that is formed during the replication origin unwinding and replication fork movement.