Functional conservations of the alkaline nuclease of herpes simplex type 1 and human cytomegalovirus.

The herpes simplex virus type 1 UL12 gene product, alkaline nuclease (AN), appears to be involved in viral DNA processing and capsid egress from the nucleus (Shao, L., Rapp, L. M., and Weller, S. K., Virology 196, 146-162, 1993). Although the HSV-1 AN is not absolutely essential for viral replication in tissue culture, conservation of the AN gene in all herpesviruses suggests an important role in the life cycle of herpesviruses. The counterpart of HSV-1 AN for human cytomegalovirus (HCMV) is the UL98 gene product. To examine whether the HCMV AN could substitute for HSV-1 AN, we performed trans-complementation experiments using a HSV-1 amplicon plasmid carrying the HCMV UL98 gene. Our results indicate (i) HCMV AN can complement the growth of the HSV-1 AN deletion mutant UL12lacZ virus in trans; (ii) a new recombinant virus, UL12laZcUL98/99, appears to be generated by the integration of the HCMV UL98 gene into the HSV-1 UL12lacZ viral genome; (iii) in contrast to its parental HSV-1 UL12lacZ virus, capsids formed in UL12lacZUL98/99-infected Vero cells were able to transport from the nucleus to the cytoplasm and mature into infectious viruses. Our results demonstrate a functional conservation of AN between HSV-1 and HCMV.

Identification of a novel peptide substrate of HSV-1 protease using substrate phage display.

The method of substrate phage display was used to select a preferred substrate from three monovalent display libraries using the HSV-1 protease. The display libraries consisted of four random amino acids, six random amino acids, and a biased library containing four amino acids from the P side of the HSV-1 maturation site followed by four random amino acids. A series of consensus peptides was synthesized based upon the results from these screens and tested in peptide cleavage assays. An eight amino acids consensus peptide (LVLASSSF) derived from the phage results was cleaved as efficiently as a 20-mer maturation site peptide. The selected amino acid sequences also allowed the design of a four amino acid paranitroanilide substrate for continuous assay of HSV-1 protease. Similar to HCMV protease, these results define P4 to P1 as a minimal substrate recognition domain for the HSV-1 protease and suggest that P4 to P1 is the minimal substrate domain which all herpesvirus proteases recognize.

Stimulation of the herpes simplex virus type I protease by antichaeotrophic salts.

The herpes simplex virus type 1 protease is expressed as an 80,000-dalton polypeptide, encoded within the 635-amino acid open reading frame of the UL26 gene. The two known protein substrates for this enzyme are the protease itself and the capsid assembly protein ICP35 (Liu, F., and Roizman, B. (1991) J. Virol. 65, 5149-5156). In this report we describe the use of a rapid and quantitative assay for characterizing the protease. The assay uses a glutathione S-transferase fusion protein containing the COOH-terminal cleavage site of ICP35 as the substrate (GST-56). The protease consists of N0, the NH2-terminal 247 amino acid catalytic domain of the UL26 gene product, also expressed as a GST fusion protein. Upon cleavage with N0, a single 25-mer peptide is released from GST-56, which is soluble in trichloroacetic acid. Using this assay, the protease displayed a pH optimum between 7 and 9 but most importantly had an absolute requirement for high concentrations of an antichaeotrophic agent. Strong salting out salts such as Na2SO4 and KPO4 (> or = 1 M) stimulated activity, whereas NaCl and KCl had no effect. The degree of stimulation by 1.25 M Na2SO4 and KPO4 were 100-150- and 200-300-fold, respectively. Using the fluorescent probe 1-anilino-8-naphthalene sulfonate, the protease was shown to bind the dye in the presence of 1.25 M Na2SO4 or KPO4, but not at low ionic strength or in the presence of 1.25 or 2.2 M NaCl. This binding was most likely at the protease active site because a high affinity cleavage site peptide, but not a control peptide, could displace the dye. In addition to cleaving GST-56, the herpes simplex virus type I protease also cleaved the purified 56-mer peptide. Circular dichroism and NMR spectroscopy showed the peptide to be primarily random coil under physiological conditions, suggesting that antichaeotrophic agents affect the conformation of the substrate as well as the protease.

The effect of internal autocleavage on kinetic properties of the human cytomegalovirus protease catalytic domain.

The 28-kilodalton (kDa) catalytic domain of the human cytomegalovirus (HCMV) protease undergoes autoproteolytic cleavage at an internal site (I site), yielding amino-terminal 15-kDa (N15) and carboxyl-terminal 13-kDa (C13) fragments. I site autocleavage has been postulated to inactivate the protease and provide a mechanism for the negative regulation of enzyme activity during viral infection. We purified recombinant enzymes to demonstrate I site autocleavage in vitro and used site-directed mutagenesis of the I site to stabilize the protease. No difference in the kinetic properties of wild type and stabilized mutant proteases were observed in an in vitro peptide cleavage assay. The consequences of I site cleavage on enzyme activity were investigated two ways. First, autodigestion of the wild type enzyme converted the intact protease to N15 and C13 autocleavage products without a corresponding loss in enzyme activity. Second, genetic constructs encoding the N15 and C13 autocleavage products were generated and expressed separately in Escherichia coli, and each fragment was purified. An active enzyme was reconstituted by refolding a mixture of the purified fragments in vitro to form a noncovalent complex. The kinetic properties of this complex were very similar to the wild type and stabilized enzymes under optimal reaction conditions. We concluded from these studies that I site cleavage does not inactivate the HCMV protease, in the absence of other virally induced factors, and that limited potential exists for the regulation of catalytic activity by I site cleavage.

Identification of the serine residue at the active site of the herpes simplex virus type 1 protease.

Herpes simplex virus type 1 (HSV-1) encodes a protease that is essential for proteolytic processing of itself and of the nucleocapsid-associated protein, ICP35 (infected cell protein 35) (Liu, F., and Roizman, B. (1991) J. Virol. 65, 5149-5156). Inhibitor studies indicated that the HSV-1 protease is sensitive to the serine protease inactivator diisopropyl fluorophosphate (DFP). Inactivation is irreversible and dependent on time and concentration of DFP. Loss of activity correlates linearly with the incorporation of [3H]DFP. Analysis of completely inactivated protease by mass spectrometry indicated a stoichiometry of 1 DFP/protease. In order to identify the specific residue modified by DFP, the protease was labeled with [3H]DFP and subsequently digested with trypsin or chymotrypsin. The peptides resulting from each digestion were separated by reverse phase HPLC, and the radioactivity was recovered in a single peak. Mass spectrometric studies and sequencing analysis by Edman degradation identified Ser-129 as the residue modified by DFP. This residue and the region in which it is found is highly conserved among the herpes viral proteases. These data demonstrate that HSV-1 protease is a serine protease and that Ser-129 is the active site nucleophile.

Investigation of the specificity of the herpes simplex virus type 1 protease by point mutagenesis of the autoproteolysis sites.

The herpes simplex virus type 1 (HSV-1) protease is cleaved at two autoprocessing sites during viral maturation, one of which shares amino acid identity with its substrate, ICP35. Similar autoprocessing sites have been observed within other members of the Herpesviridae. Introduction of point mutations within the autoprocessing sites of the HSV-1 protease indicated that specificity resides within the P4-P1' region of the cleavage sites.

Autoproteolysis of herpes simplex virus type 1 protease releases an active catalytic domain found in intermediate capsid particles.

The UL26 gene of herpes simplex virus type 1 (HSV-1) encodes a 635-amino-acid protease that cleaves itself and the HSV-1 assembly protein ICP35cd (F. Liu and B. Roizman, J. Virol. 65:5149-5156, 1991). We previously examined the HSV protease by using an Escherichia coli expression system (I. C. Deckman, M. Hagen, and P. J. McCann III, J. Virol. 66:7362-7367, 1992) and identified two autoproteolytic cleavage sites between residues 247 and 248 and residues 610 and 611 of UL26 (C. L. DiIanni, D. A. Drier, I. C. Deckman, P. J. McCann III, F. Liu, B. Roizman, R. J. Colonno, and M. G. Cordingley, J. Biol. Chem. 268:2048-2051, 1993). In this study, a series of C-terminal truncations of the UL26 open reading frame was tested for cleavage activity in E. coli. Our results delimit the catalytic domain of the protease to the N-terminal 247 amino acids of UL26 corresponding to No, the amino-terminal product of protease autoprocessing. Autoprocessing of the full-length protease was found to be unnecessary for catalysis, since elimination of either or both cleavage sites by site-directed mutagenesis fails to prevent cleavage of ICP35cd or an unaltered protease autoprocessing site. Catalytic activity of the 247-amino-acid protease domain was confirmed in vitro by using a glutathione-S-transferase fusion protein. The fusion protease was induced to high levels of expression, affinity purified, and used to cleave purified ICP35cd in vitro, indicating that no other proteins are required. By using a set of domain-specific antisera, all of the HSV-1 protease cleavage products predicted from studies in E. coli were identified in HSV-1-infected cells. At least two protease autoprocessing products, in addition to fully processed ICP35cd (ICP35ef), were associated with intermediate B capsids in the nucleus of infected cells, suggesting a key role for proteolytic maturation of the protease and ICP35cd in HSV-1 capsid assembly.

Deletion of the VP16 open reading frame of herpes simplex virus type 1.

VP16 (also called Vmw65 and alpha TIF) is a structural protein of herpes simplex virus type 1 (HSV-1) that trans-induces HSV-1 immediate-early gene transcription. This report describes an HSV-1 VP16 deletion mutant that was constructed and propagated in a cell line transformed with a VP16 expression vector. The VP16 deletion mutant replicated like wild-type HSV-1 during infection of the VP16-expressing cell line. Deletion mutant virions propagated in this cell line contained wild-type, cell-derived VP16 protein that was recruited during virion assembly and was functional for immediate-early gene trans-induction. The mutant failed to replicate during subsequent infection of cells that do not express VP16, as determined in plaque assays and single-step replication assays. The deletion mutant induced nearly normal levels of viral DNA synthesis and capsid production during these infections, but it induced slightly lower levels of viral DNA encapsidation and appeared by transmission electron microscopy to be defective in further steps of virion maturation. A genetic revertant of the deletion mutant that was restored for VP16-coding sequences exhibited fully wild-type replication properties in both VP16-expressing and nonexpressing cells. The absence of VP16 protein synthesis at late times of HSV-1 infection prevents the production of infectious progeny virus and correlates with a profound defect in HSV-1 particle assembly.

Latent herpes simplex virus type 1 transcription in human trigeminal ganglia.

We studied latent herpes simplex virus type 1 gene expression in human trigeminal ganglia. Two transcripts were mapped to a 3.0-kilobase region within the long repeat region and appeared to be located in neuronal nuclei. These viral RNAs were not abundant during lytic replication and may represent an alternative pattern of herpes simplex virus type 1 gene expression involved in the pathogenesis of latent infection.

Latent herpes simplex virus type 1 transcripts in peripheral and central nervous system tissues of mice map to similar regions of the viral genome.

Herpes simplex virus type 1 (HSV-1) DNA and RNA have been detected in peripheral nervous system (PNS) and central nervous system (CNS) tissues of latently infected mice. However, explant methods are successful in reactivating HSV-1 only from latently infected PNS tissues. In this report, latent herpesvirus infections in mouse PNS and CNS tissues were compared by in situ hybridization to determine whether the difference in reactivation was at the level of the virus or the host tissue. It was demonstrated that the HSV-1 transcripts present during latency in the mouse PNS and CNS originated from the same region of the genome, the repeats which bracket the long unique sequence. Therefore, the difference in reactivation with PNS and CNS tissues cannot be accounted for by differences in the extent of the HSV-1 genome transcribed during herpesvirus latency. Latent HSV-1 RNA was detected in the trigeminal ganglia (PNS) and the trigeminal system in the CNS from the mesencephalon to the spinal cord as well as other regions of the CNS not noted previously. Latent HSV-1 RNA was found predominantly in neurons but also in a small number of cells which could not be identified as neuronal cells. It is suggested that host differences in CNS and PNS tissues, rather than differences in latent virus transcription, may be important determinants in the HSV-1 reactivation process in explanted tissues.

Novobiocin and coumermycin A1 inhibit viral replication and the reactivation of herpes simplex virus type 1 from the trigeminal ganglia of latently infected mice.

Herpes simplex virus type 1 was reactivated from the trigeminal ganglia of latently infected mice in a quantitative and time-dependent manner. Novobiocin and coumermycin A1 reversibly inhibited the reactivation of herpes simplex virus type 1. They did not inhibit viral replication in permissive cells (CV-1) but did inhibit replication in cells of neuronal origin (C1300) and acutely infected trigeminal ganglia.