The UL6 gene product forms the portal for entry of DNA into the herpes simplex virus capsid

During replication of herpes simplex virus type 1 (HSV-1), viral DNA is synthesized in the infected cell nucleus, where DNA-free capsids are also assembled. Genome-length DNA molecules are then cut out of a larger, multigenome concatemer and packaged into capsids. Here we report the results of experiments carried out to test the idea that the HSV-1 UL6 gene product (pUL6) forms the portal through which viral DNA passes as it enters the capsid. Since DNA must enter at a unique site, immunoelectron microscopy experiments were undertaken to determine the location of pUL6. After specific immunogold staining of HSV-1 B capsids, pUL6 was found, by its attached gold label, at one of the 12 capsid vertices. Label was not observed at multiple vertices, at nonvertex sites, or in capsids lacking pUL6. In immunoblot experiments, the pUL6 copy number in purified B capsids was found to be 14.8 +/- 2.6. Biochemical experiments to isolate pUL6 were carried out, beginning with insect cells infected with a recombinant baculovirus expressing the UL6 gene. After purification, pUL6 was found in the form of rings, which were observed in electron micrographs to have outside and inside diameters of 16.4 +/- 1.1 and 5.0 +/- 0.7 nm, respectively, and a height of 19.5 +/- 1.9 nm. The particle weights of individual rings as determined by scanning transmission electron microscopy showed a majority population with a mass corresponding to an oligomeric state of 12. The results are interpreted to support the view that pUL6 forms the DNA entry portal, since it exists at a unique site in the capsid and forms a channel through which DNA can pass. The HSV-1 portal is the first identified in a virus infecting a eukaryote. In its dimensions and oligomeric state, the pUL6 portal resembles the connector or portal complexes employed for DNA encapsidation in double-stranded DNA bacteriophages such as phi29, T4, and P22. This similarity supports the proposed evolutionary relationship between herpesviruses and double-stranded DNA phages and suggests the basic mechanism of DNA packaging is conserved.     

Role of the UL25 protein in herpes simplex virus DNA encapsidation

The herpes simplex virus protein UL25 attaches to the external vertices of herpes simplex virus type 1 capsids and is required for the stable packaging of viral DNA. To define regions of the protein important for viral replication and capsid attachment, the 580-amino-acid UL25 open reading frame was disrupted by transposon mutagenesis. The UL25 mutants were assayed for complementation of a UL25 deletion virus, and in vitro-synthesized protein was tested for binding to UL25-deficient capsids. Of the 11 mutants analyzed, 4 did not complement growth of the UL25 deletion mutant, and analysis of these and additional mutants in the capsid-binding assay demonstrated that UL25 amino acids 1 to 50 were sufficient for capsid binding. Several UL25 mutations were transferred into recombinant viruses to analyze the effect of the mutations on UL25 capsid binding and on DNA cleavage and packaging. Studies of these mutants demonstrated that amino acids 1 to 50 of UL25 are essential for its stable interaction with capsids and that the C terminus is essential for DNA packaging and the production of infectious virus through its interactions with other viral packaging or tegument proteins. Analysis of viral DNA cleavage demonstrated that in the absence of a functional UL25 protein, aberrant cleavage takes place at the unique short end of the viral genome, resulting in truncated viral genomes that are not retained in capsids. Based on these observations, we propose a model where UL25 is required for the formation of DNA-containing capsids by acting to stabilize capsids that contain full-length viral genomes.     

Identification and genetic mapping of a herpes simplex virus capsid protein that binds DNA

Herpes simplex virus virion protein 19C (VP19C) is a constituent of both unenveloped (nuclear) and enveloped (cytoplasmic) capsids. In this paper we report that 32P-labeled DNA, either supercoiled or linear double stranded, efficiently bound to VP19C electrically transferred from denaturing polyacrylamide gels containing electrophoretically separated proteins from purified capsids. Analyses of the polypeptides specified by herpes simplex virus type 1 X herpes simplex type 2 recombinants with respect to electrophoretic mobility and binding of 32P-labeled DNA indicate that VP19C maps at the same location as infected cell polypeptide 32 and is derived from it.     

Herpes simplex virus capsid structure: DNA packaging protein UL25 is located on the external surface of the capsid near the vertices

UL25 is one of seven herpes simplex virus-encoded proteins involved specifically in DNA encapsidation. Its role appears to be to stabilize the capsid so that DNA is prevented from escaping once it has entered. To clarify the function of UL25, we have examined capsids with the goal of defining where it is located. Analysis of trypsin-treated capsids showed that UL25 is sensitive to cleavage like other proteins such as the major capsid and portal proteins that are exposed on the capsid surface. Internal proteins such as the scaffolding protein and protease were not affected under the same experimental conditions. Capsids were also examined by electron microscopy after staining with gold-labeled antibody specific for UL25. Images of stained capsids demonstrated that most labeled sites (71% in C capsids) were at capsid vertices, and most stained C capsids had label at more than one vertex. A quantitative immunoblotting method showed that the capsid contents of UL25 were 56, 20, and 75 copies per capsid in A, B, and C capsids, respectively. Finally, soluble UL25 protein was found to bind in vitro to purified capsids lacking it. The amount of bound UL25 corresponded to the amount present in B capsids, and bound UL25 was found by immunoelectron microscopy to be located predominantly at the capsid vertices. The results are interpreted to suggest that five UL25 molecules are found at or near each of the capsid vertices, where they are exposed on the capsid surface. Exposure on the surface is consistent with the view that UL25 is added to the capsid as DNA is packaged or during late stages of the packaging process.     

The pseudorabies virus homology of the herpes simplex virus UL21 gene product is a capsid protein which is involved in capsid maturation.

We mutagenized, mapped, and sequenced the pseudorabies virus (PRV) homology of gene UL21 of herpes simplex virus type 1. A polyclonal mouse antiserum against the protein encoded by the UL21 homolog was generated and used to monitor the expression and subcellular localization of the UL21-encoded protein. We found that the protein is identical to a previously detected PRV capsid protein. We analyzed viable PRV strains encoding mutant UL21 homologys, truncated by insertion of an oligonucleotide that contains stop codons in all reading frames. In two PRV mutants carrying the oligonucleotide at two sites within the gene, processing of newly replicated viral DNA was impaired. In addition, we show that one of the UL21 mutants has strongly reduced virulence for mice.     

The herpes simplex virus type 1 DNA packaging protein UL17 is a virion protein that is present in both the capsid and the tegument compartments

The UL17 protein of herpes simplex virus type 1 is essential for packaging the viral genome into the procapsid, a spherical assembly intermediate, and is present in the mature virus particle. We have examined the distribution of UL17 in various assembly products and virions to determine which component of the virus particle UL17 is associated with and at what stage in capsid assembly UL17 is required. UL17 was present in the procapsid, in the DNA-containing angularized C capsid, and in two other angularized capsid forms, A and B, that lack DNA and are thought to be dead-end products. The results suggest that UL17 is a minor capsid protein which is incorporated into the procapsid during assembly of the particle. UL17 was also found in virions and in noninfectious structures known as light (L) particles, which possess a tegument and envelope but lack a capsid. The level of UL17 in these particles was much greater than the amount that could be attributed to capsid contamination of the purified L-particle preparation, suggesting that UL17 is also a tegument protein. The finding that virions contain approximately twofold more UL17 than do C capsids provided further support for the idea that UL17 is present in two different structural components within the mature virion. The UL25 packaging protein, which is also present in virions, was not found in significant amounts in L particles, indicating that it is associated only with the capsid. UL6, the third virion-associated packaging protein, was present in slightly increased levels in L particles.     

The UL25 gene product of herpes simplex virus type 1 is involved in uncoating of the viral genome

Studies on the herpes simplex virus type 1 UL25-null mutant KUL25NS have shown that the capsid-associated UL25 protein is required at a late stage in the encapsidation of viral DNA. Our previous work on UL25 with the UL25 temperature-sensitive (ts) mutant ts1204 also implicated UL25 in a role at very early times in the virus growth cycle, possibly at the stage of penetration of the host cell. We have reexamined this mutant and discovered that it had an additional ts mutation elsewhere in the genome. The ts1204 UL25 mutation was transferred into wild-type (wt) virus DNA, and the UL25 mutant ts1249 was isolated and characterized to clarify the function of UL25 at the initial stages of virus infection. Indirect immunofluorescence assays and in situ hybridization analysis of virus-infected cells revealed that the mutant ts1249 was not impaired in penetration of the host cell but had an uncoating defect at the nonpermissive temperature. When ts1249-infected cells were incubated initially at the permissive temperature to allow uncoating of the viral genome and subsequently transferred to the restrictive temperature, a DNA-packaging defect was evident. The results suggested that ts1249, like KUL25NS, had a block at a late stage of DNA packaging and that the packaged genome was shorter than the full-length genome. Examination of ts1249 capsids produced at the nonpermissive temperature revealed that, in comparison with wt capsids, they contained reduced amounts of UL25 protein, thereby providing a possible explanation for the failure of ts1249 to package full-length viral DNA.     

The herpes simplex virus type 1 UL42 gene product: a subunit of DNA polymerase that functions to increase processivity.

Genetic experiments have shown that the products of the herpes simplex virus type 1 (HSV-1) DNA polymerase (UL30) and UL42 genes are both required for viral DNA replication, and a number of studies have suggested that these two proteins specifically interact. We have confirmed and extended these findings. The viral DNA polymerase from HSV-1-infected cells has been purified as a complex containing equimolar quantities of the UL30 (Pol, the catalytic subunit) and UL42 polypeptides. Sedimentation and gel filtration analyses of this complex are consistent with the idea that the complex consists of a heterodimer of Pol and UL42. A complex with identical physical and functional properties was also purified from insect cells coinfected with recombinant baculoviruses expressing the two polypeptides. Therefore, the formation of the Pol-UL42 complex does not require the participation of any other HSV-encoded protein. We have compared the catalytic properties of the Pol-UL42 complex with those of the isolated subunits of the enzyme purified from recombinant baculovirus-infected insect cells. The specific activity of the catalytic subunit alone was nearly identical to that of the complex when assayed on activated DNA. When assayed on a defined template such as singly primed M13 DNA, however, the combination of Pol and UL42 utilized fewer primers and formed larger products than Pol alone. Template challenge experiments demonstrated that the Pol-UL42 complex was more highly processive than Pol alone. Our data are consistent with the idea that the UL42 polypeptide is an accessory subunit of the DNA polymerase that acts to increase the processivity of polymerization.     

The herpes simplex virus 1 UL17 protein is the second constituent of the capsid vertex-specific component required for DNA packaging and retention

The herpes simplex virus (HSV) UL17 and UL25 minor capsid proteins are essential for DNA packaging. They are thought to comprise a molecule arrayed in five copies around each of the capsid vertices. This molecule was initially termed the "C-capsid-specific component" (CCSC) (B. L. Trus et al., Mol. Cell 26:479-489, 2007), but as we have subsequently observed this feature on reconstructions of A, B, and C capsids, we now refer to it more generally as the "capsid vertex-specific component" (CVSC) (S. K. Cockrell et al., J. Virol. 85:4875-4887, 2011). We previously confirmed that UL25 occupies the vertex-distal region of the CVSC density by visualizing a large UL25-specific tag in reconstructions calculated from cryo-electron microscopy (cryo-EM) images. We have pursued the same strategy to determine the capsid location of the UL17 protein. Recombinant viruses were generated that contained either a small tandem affinity purification (TAP) tag or the green fluorescent protein (GFP) attached to the C terminus of UL17. Purification of the TAP-tagged UL17 or a similarly TAP-tagged UL25 protein clearly demonstrated that the two proteins interact. A cryo-EM reconstruction of capsids containing the UL17-GFP protein reveals that UL17 is the second component of the CVSC and suggests that UL17 interfaces with the other CVSC component, UL25, through its C terminus. The portion of UL17 nearest the vertex appears to be poorly constrained, which may provide flexibility in interacting with tegument proteins or the DNA-packaging machinery at the portal vertex. The exposed locations of the UL17 and UL25 proteins on the HSV-1 capsid exterior suggest that they may be attractive targets for highly specific antivirals.     

A mutation in UL15 of herpes simplex virus 1 that reduces packaging of cleaved genomes

Herpesvirus genomic DNA is cleaved from concatemers that accumulate in infected cell nuclei. Genomic DNA is inserted into preassembled capsids through a unique portal vertex. Extensive analyses of viral mutants have indicated that intact capsids, the portal vertex, and all components of a tripartite terminase enzyme are required to both cleave and package viral DNA, suggesting that DNA cleavage and packaging are inextricably linked. Because the processes have not been functionally separable, it has been difficult to parse the roles of individual proteins in the DNA cleavage/packaging reaction. In the present study, a virus bearing the deletion of codons 400 to 420 of U(L)15, encoding a terminase component, was analyzed. This virus, designated vJB27, failed to replicate on noncomplementing cells but cleaved concatemeric DNA to ca. 35 to 98% of wild-type levels. No DNA cleavage was detected in cells infected with a U(L)15-null virus or a virus lacking U(L)15 codons 383 to 385, comprising a motif proposed to couple ATP hydrolysis to DNA translocation. The amount of vJB27 DNA protected from DNase I digestion was reduced compared to the wild-type virus by 6.5- to 200-fold, depending on the DNA fragment analyzed, thus indicating a profound defect in DNA packaging. Capsids containing viral DNA were not detected in vJB27-infected cells, as determined by electron microscopy. These data suggest that pU(L)15 plays an essential role in DNA translocation into the capsid and indicate that this function is separable from its role in DNA cleavage.     

Identification and characterization of the UL56 gene product of herpes simplex virus type 2

The UL56 gene product of herpes simplex virus (HSV) has been shown to play an important role in viral pathogenicity. However, the properties and functions of the UL56 protein are little understood. We raised rabbit polyclonal antisera specific for the UL56 protein of HSV type 2 (HSV-2) and examined its expression and properties. The gene product was identified as three polypeptides with apparent molecular masses ranging from 32 to 35 kDa in HSV-2-infected cells, and at least one species was phosphorylated. Studies of their origins showed that the UL56 protein of HSV-2 is also translated from the upstream in-frame methionine codon that is not present in the HSV-1 genome. Synthesis was first detected at 6 h postinfection and was not abolished by the viral DNA synthesis inhibitor phosphonoacetic acid. Indirect immunofluorescence studies revealed that the UL56 protein localized to both the Golgi apparatus and cytoplasmic vesicles in HSV-2-infected and single UL56-expressing cells. Deletion mutant analysis showed that the C-terminal hydrophobic region of the protein was required for association with the cytoplasmic membrane and that the N-terminal proline-rich region was important for its translocation to the Golgi apparatus and cytoplasmic vesicles. Moreover, the results of protease digestion assays and sucrose gradient fractionation strongly suggested that UL56 is a tail-anchored type II membrane protein associated with lipid rafts. We thus hypothesized that the UL56 protein, as a tail-anchored type II membrane protein, may be involved in vesicular trafficking in HSV-2-infected cells.     

The herpes simplex virus processivity factor, UL42, binds DNA as a monomer

The processivity subunit of the herpes simplex virus DNA polymerase, UL42, is a monomer in solution. However, UL42 is structurally similar to sliding clamp processivity factors, such as PCNA, which encircle DNA as a multimeric ring. We used chemical crosslinking and electrophoretic mobility-shift assays to investigate whether UL42 oligomerizes upon DNA binding. UL42 did not form intermolecular crosslinks upon treatment with glutaraldehyde in the presence of DNA, whereas proteins that are known to be multimers in solution were successfully crosslinked by this treatment. This result suggests that UL42 does not form multimers on DNA. We next analyzed the composition of UL42:DNA complexes using electrophoretic mobility-shift assays. UL42 was mixed with a maltose-binding protein-UL42 fusion protein before being added to DNA. The patterns of electrophoretic mobility of the resultant protein:DNA complexes were those predicted if each isoform of UL42 binds to DNA as a monomer. From this result and the failure of UL42 to form crosslinks, we infer that UL42 binds DNA as a monomer.     

Binding of the herpes simplex virus type 1 UL9 gene product to an origin of viral DNA replication.

The binding of a herpes simplex virus type 1 (HSV-1) encoded polypeptide to a viral origin of DNA replication has been studied by using a gel retardation assay. Incubation of nuclear extract from HSV-1 infected cells with a labelled origin-containing fragment resulted in the formation of a specific retarded complex, the migration of which was further reduced in the presence of an antibody reactive with the UL9 gene product. Introduction of an additional copy of the UL9 gene, under the control of an immediate early (IE) promoter, conferred the ability to express origin binding activity at the non-permissive temperature upon an HSV-1 ts mutant blocked at the IE stage of infection. Endogenous or exogenous proteolytic activity revealed the presence of a relatively protease-resistant domain which retained sequence-specific DNA binding activity. The C-terminal 317 amino acids of the UL9 gene expressed as a fusion protein in Escherichia coli also bound to the origin. Our results demonstrate that the UL9 gene product binds to a viral origin and that sequence specific recognition and binding are specified by the C-terminal 37% of the polypeptide.     

Dynamic interactions of the UL16 tegument protein with the capsid of herpes simplex virus

The UL16 tegument protein of herpes simplex virus is conserved throughout the herpesvirus family. It has been reported to be capsid associated and may be involved in budding by providing an interaction with the membrane-bound UL11 protein. UL16 has been shown to be present in all the major locations that capsids are found (i.e., the nucleus, cytoplasm, and virions), but whether it is actually capsid associated in each of these has not been reported. Therefore, capsids were purified from each compartment, and it was found that UL16 was present on cytoplasmic but not nuclear capsids. In extracellular virions, the majority of UL16 (87%) was once again not capsid associated, which suggests that the interaction is transient during egress. Because herpes simplex virus (HSV) buds into the acidic compartment of the trans-Golgi network (TGN), the effect of pH on the interaction was examined. The amount of capsid-associated UL16 dramatically increased when extracellular virions were exposed to mildly acidic medium (pH 5.0 to 5.5), and this association was fully reversible. After budding into the TGN, capsid and tegument proteins also encounter an oxidizing environment, which is conducive to disulfide bond formation. UL16 contains 20 cysteines, including five that are conserved within a putative zinc finger. Any free cysteines that are involved in the capsid interaction or release mechanism of UL16 would be expected to be modified by N-ethylmaleimide, and, consistent with this, the amount of capsid-associated UL16 dramatically increased when virions were incubated with this compound. Taken together, these data suggest a transient interaction between UL16 and capsids, possibly modified in the acidic compartment of secretory vesicles and requiring a release mechanism that involves cysteines.     

Packaging of genomic and amplicon DNA by the herpes simplex virus type 1 UL25-null mutant KUL25NS

The herpes simplex virus type 1 (HSV-1) mutant KUL25NS, containing a null mutation within the UL25 gene, was isolated and characterized by McNab and coworkers (A. R. McNab, P. Desai, S. Person, L. L. Roof, D. R. Thomsen, W. W. Newcomb, J. C. Brown, and F. L. Homa, J. Virol. 72:1060-1070, 1998). This mutant was able to cleave the concatemeric products of viral DNA replication into monomeric units, but in contrast to wild-type (wt) HSV-1, they were degraded by DNase treatment, indicating that they were not stably packaged into virus capsids. I have examined the packaging of the KUL25NS genome and an HSV-1 amplicon in cells infected with the mutant virus. In contrast to the previous results, a low level of KUL25NS DNA was resistant to DNase digestion, indicating that it was retained in capsids. The proportion of this packaged DNA present as full-length genomes was much lower than in cells infected by wt HSV-1, and there was a significant overrepresentation of the long terminus and underrepresentation of the short terminus. KUL25NS was less impaired in stably packaging amplicon DNA than in packaging its own genome, and the packaged molecules contained approximately equimolar amounts of the two terminal fragments. Below about 100 kbp, the packaged amplicon molecules exhibited an abundance and size distribution similar to those generated using wt HSV-1 as a helper, but the mutant was relatively impaired in packaging longer amplicon molecules. Both packaged genomic and amplicon DNAs were retained in the nuclei of KUL25NS-infected cells. These results suggest that the UL25 protein may play an important role during the later stages of the head-filling process, prior to release of capsids into the cytoplasm.     

Mutations that specifically impair the DNA binding activity of the herpes simplex virus protein UL42.

The herpes simplex virus DNA polymerase is a heterodimer consisting of a catalytic subunit and the protein UL42, which functions as a processivity factor. It has been hypothesized that UL42 tethers the catalytic subunit to the DNA template by virtue of DNA binding activity (J. Gottlieb, A. I. Marcy, D. M. Coen, and M. D. Challberg, J. Virol. 64:5976-5987, 1990). Relevant to this hypothesis, we identified two linker insertion mutants of UL42 that were unable to bind to a double-stranded-DNA-cellulose column but retained their ability to bind the catalytic subunit. These mutants were severely impaired in the stimulation of long-chain-DNA synthesis by the catalytic subunit in vitro. In transfected cells, the expressed mutant proteins localized to the nucleus but were nonetheless deficient in complementing the growth of a UL42 null virus. Thus, unlike many other processivity factors, UL42 appears to require an intrinsic DNA binding activity for its function both in vitro and in infected cells. Possible mechanisms for the activity of UL42 and its potential as a drug target are discussed.     

Localization of herpes simplex virus type 1 UL37 in the Golgi complex requires UL36 but not capsid structures

The herpes simplex virus type 1 (HSV-1) UL37 gene encodes a 120-kDa polypeptide which resides in the tegument structure of the virion and is important for morphogenesis. The goal of this study was to use green fluorescent protein (GFP) to follow the fate of UL37 within cells during the normal course of virus replication. GFP was inserted in frame at the C terminus of UL37 to generate a fluorescent-protein-tagged UL37 polypeptide. A virus designated K37eGFP, which replicated normally on Vero cells, was isolated and was shown to express the fusion polypeptide. When cells infected with this virus were examined by confocal microscopy, the fluorescence was observed to be predominantly cytoplasmic. As the infection progressed, fluorescence began to accumulate in a juxtanuclear structure. Mannosidase II and giantin were observed to colocalize with UL37eGFP at these structures, as judged by immunofluorescence assays. Therefore, UL37 traffics to the Golgi complex during infection. A VP26mRFP marker (red fluorescent protein fused to VP26) was recombined into K37eGFP, and when cells infected with this “dual-color” virus were examined, colocalization of the red (capsid) and green (UL37) fluorescence in the Golgi structure was observed. Null mutations in VP5 (ΔVP5), which abolished capsid assembly, and in UL36 (Δ36) were recombined into the K37eGFP virus genome. In cells infected with K37eGFP/ΔVP5, localization of UL37eGFP to the Golgi complex was similar to that for the parental virus (K37eGFP), indicating that trafficking of UL37eGFP to the Golgi complex did not require capsid structures. Confocal analysis of cells infected with K37eGFP/Δ36 showed that, in the absence of UL36, accumulation of UL37eGFP at the Golgi complex was not evident. This indicates an interaction between these two proteins that is important for localization of UL37 in the Golgi complex and thus possibly for cytoplasmic envelopment of the capsid. This is the first demonstration of a functional role for UL36:UL37 interaction in HSV-1-infected cells.     

Structural characterization of the UL25 DNA-packaging protein from herpes simplex virus type 1

Herpesviruses replicate their double stranded DNA genomes as high-molecular-weight concatemers which are subsequently cleaved into unit-length genomes by a complex mechanism that is tightly coupled to DNA insertion into a preformed capsid structure, the procapsid. The herpes simplex virus type 1 UL25 protein is incorporated into the capsid during DNA packaging, and previous studies of a null mutant have demonstrated that its function is essential at the late stages of the head-filling process, either to allow packaging to proceed to completion or for retention of the viral genome within the capsid. We have expressed and purified an N-terminally truncated form of the 580-residue UL25 protein and have determined the crystallographic structure of the region corresponding to amino acids 134 to 580 at 2.1-Angstroms resolution. This structure, the first for any herpesvirus protein involved in processing and packaging of viral DNA, reveals a novel fold, a distinctive electrostatic distribution, and a unique "flexible" architecture in which numerous flexible loops emanate from a stable core. Evolutionary trace analysis of UL25 and its homologues in other herpesviruses was used to locate potentially important amino acids on the surface of the protein, leading to the identification of four putative docking regions for protein partners.     

Herpes simplex virus 1 DNA packaging proteins encoded by UL6, UL15, UL17, UL28, and UL33 are located on the external surface of the viral capsid

Studies to localize the herpes simplex virus 1 portal protein encoded by UL6, the putative terminase components encoded by UL15, UL 28, and UL33, the minor capsid proteins encoded by UL17, and the major scaffold protein ICP35 were conducted. ICP35 in B capsids was more resistant to trypsin digestion of intact capsids than pUL6, pUL15, pUL17, pUL28, or pUL33. ICP35 required sectioning of otherwise intact embedded capsids for immunoreactivity, whereas embedding and/or sectioning decreased the immunoreactivities of pUL6, pUL17, pUL28, and pUL33. Epitopes of pUL15 were recognized roughly equally well in both sectioned and unsectioned capsids. These data indicate that pUL6, pUL17, pUL28, pUL33, and at least some portion of pUL15 are located at the external surface of the capsid.     

Residues of the UL25 protein of herpes simplex virus that are required for its stable interaction with capsids

The herpes simplex virus 1 (HSV-1) UL25 gene product is a minor capsid component that is required for encapsidation, but not cleavage, of replicated viral DNA. UL25 is located on the capsid surface in a proposed heterodimer with UL17, where five copies of the heterodimer are found at each of the capsid vertices. Previously, we demonstrated that amino acids 1 to 50 of UL25 are essential for its stable interaction with capsids. To further define the UL25 capsid binding domain, we generated recombinant viruses with either small truncations or amino acid substitutions in the UL25 N terminus. Studies of these mutants demonstrated that there are two important regions within the capsid binding domain. The first 27 amino acids are essential for capsid binding of UL25, while residues 26 to 39, which are highly conserved in the UL25 homologues of other alphaherpesviruses, were found to be critical for stable capsid binding. Cryo-electron microscopy reconstructions of capsids containing either a small tag on the N terminus of UL25 or the green fluorescent protein (GFP) fused between amino acids 50 and 51 of UL25 demonstrate that residues 1 to 27 of UL25 contact the hexon adjacent to the penton. A second region, most likely centered on amino acids 26 to 39, contacts the triplex that is one removed from the penton. Importantly, both of these UL25 capsid binding regions are essential for the stable packaging of full-length viral genomes.