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.

In vitro assembly of the herpes simplex virus procapsid: formation of small procapsids at reduced scaffolding protein concentration.

The herpes simplex virus 1 capsid is formed in the infected cell nucleus by way of a spherical, less robust intermediate called the procapsid. Procapsid assembly requires the capsid shell proteins (VP5, VP19C, and VP23) plus the scaffolding protein, pre-VP22a, a major component of the procapsid that is not present in the mature virion. Pre-VP22a is lost as DNA is packaged and the procapsid is transformed into the mature, icosahedral capsid. We have employed a cell-free assembly system to examine the role of the scaffolding protein in procapsid formation. While other reaction components (VP5, VP19C, and VP23) were held constant, the pre-VP22a concentration was varied, and the resulting procapsids were analyzed by electron microscopy and SDS-polyacrylamide gel electrophoresis. The results demonstrated that while standard-sized (T = 16) procapsids with a measured diameter of approximately 100 nm were formed above a threshold pre-VP22a concentration, at lower concentrations procapsids were smaller. The measured diameter was approximately 78 nm and the predicted triangulation number was 9. No procapsids larger than the standard size or smaller than 78-nm procapsids were observed in appreciable numbers at any pre-VP22a concentration tested. SDS-polyacrylamide gel analyses indicated that small procapsids contained a reduced amount of scaffolding protein compared to the standard 100-nm form. The observations indicate that the scaffolding protein concentration affects the structure of nascent procapsids with a minimum amount required for assembly of procapsids with the standard radius of curvature and scaffolding protein content.

Lytic replication of Kaposi's sarcoma-associated herpesvirus results in the formation of multiple capsid species: isolation and molecular characterization of A, B, and C capsids from a gammaherpesvirus.

Despite the discovery of Epstein-Barr virus more than 35 years ago, a thorough understanding of gammaherpesvirus capsid composition and structure has remained elusive. We approached this problem by purifying capsids from Kaposi's sarcoma-associated herpesvirus (KSHV), the only other known human gammaherpesvirus. The results from our biochemical and imaging analyses demonstrate that KSHV capsids possess a typical herpesvirus icosahedral capsid shell composed of four structural proteins. The hexameric and pentameric capsomers are composed of the major capsid protein (MCP) encoded by open reading frame 25. The heterotrimeric complexes, forming the capsid floor between the hexons and pentons, are each composed of one molecule of ORF62 and two molecules of ORF26. Each of these proteins has significant amino acid sequence homology to capsid proteins in alpha- and betaherpesviruses. In contrast, the fourth protein, ORF65, lacks significant sequence homology to its structural counterparts from the other subfamilies. Nevertheless, this small, basic, and highly antigenic protein decorates the surface of the capsids, as does, for example, the even smaller basic capsid protein VP26 of herpes simplex virus type 1. We have also found that, as with the alpha- and betaherpesviruses, lytic replication of KSHV leads to the formation of at least three capsid species, A, B, and C, with masses of approximately 200, 230, and 300 MDa, respectively. A capsids are empty, B capsids contain an inner array of a fifth structural protein, ORF17.5, and C capsids contain the viral genome.

Capsid structure of Kaposi's sarcoma-associated herpesvirus, a gammaherpesvirus, compared to those of an alphaherpesvirus, herpes simplex virus type 1, and a betaherpesvirus, cytomegalovirus.

The capsid of Kaposi's sarcoma-associated herpesvirus (KSHV) was visualized at 24-A resolution by cryoelectron microscopy. Despite limited sequence similarity between corresponding capsid proteins, KSHV has the same T=16 triangulation number and much the same capsid architecture as herpes simplex virus (HSV) and cytomegalovirus (CMV). Its capsomers are hexamers and pentamers of the major capsid protein, forming a shell with a flat, close-packed, inner surface (the "floor") and chimney-like external protrusions. Overlying the floor at trigonal positions are (alpha beta(2)) heterotrimers called triplexes. The floor structure is well conserved over all three viruses, and the most variable capsid features reside on the outer surface, i.e., in the shapes of the protrusions and triplexes, in which KSHV resembles CMV and differs from HSV. Major capsid protein sequences from the three subfamilies have some similarity, which is closer between KSHV and CMV than between either virus and HSV. The triplex proteins are less highly conserved, but sequence analysis identifies relatively conserved tracts. In alphaherpesviruses, the alpha-subunit (VP19c in HSV) has a 100-residue N-terminal extension and an insertion near the C terminus. The small basic capsid protein sequences are highly divergent: whereas the HSV and CMV proteins bind only to hexons, difference mapping suggests that the KSHV protein, ORF65, binds around the tips of both hexons and pentons.

Herpes simplex virus DNA cleavage and packaging proteins associate with the procapsid prior to its maturation.

Packaging of DNA into preformed capsids is a fundamental early event in the assembly of herpes simplex virus type 1 (HSV-1) virions. Replicated viral DNA genomes, in the form of complex branched concatemers, and unstable spherical precursor capsids termed procapsids are thought to be the substrates for the DNA-packaging reaction. In addition, seven viral proteins are required for packaging, although their individual functions are undefined. By analogy to well-characterized bacteriophage systems, the association of these proteins with various forms of capsids, including procapsids, might be expected to clarify their roles in the packaging process. While the HSV-1 UL6, UL15, UL25, and UL28 packaging proteins are known to associate with different forms of stable capsids, their association with procapsids has not been tested. Therefore, we isolated HSV-1 procapsids from infected cells and used Western blotting to identify the packaging proteins present. Procapsids contained UL15 and UL28 proteins; the levels of both proteins are diminished in more mature DNA-containing C-capsids. In contrast, UL6 protein levels were approximately the same in procapsids, B-capsids, and C-capsids. The amount of UL25 protein was reduced in procapsids relative to that in more mature B-capsids. Moreover, C-capsids contained the highest level of UL25 protein, 15-fold higher than that in procapsids. Our results support current hypotheses on HSV DNA packaging: (i) transient association of UL15 and UL28 proteins with maturing capsids is consistent with their proposed involvement in site-specific cleavage of the viral DNA (terminase activity); (ii) the UL6 protein may be an integral component of the capsid shell; and (iii) the UL25 protein may associate with capsids after scaffold loss and DNA packaging, sealing the DNA within capsids.

Evidence for controlled incorporation of herpes simplex virus type 1 UL26 protease into capsids.

Herpes simplex virus type 1 (HSV-1) capsids are initially assembled with an internal protein scaffold. The scaffold proteins, encoded by overlapping in-frame UL26 and UL26.5 transcripts, are essential for formation and efficient maturation of capsids. UL26 encodes an N-terminal protease domain, and its C-terminal oligomerization and capsid protein-binding domains are identical to those of UL26.5. The UL26 protease cleaves itself, releasing minor scaffold proteins VP24 and VP21, and the more abundant UL26.5 protein, releasing the major scaffold protein VP22a. Unlike VP21 and VP22a, which are removed from capsids upon DNA packaging, we demonstrate that VP24 (containing the protease domain) is quantitatively retained. To investigate factors controlling UL26 capsid incorporation and retention, we used a mutant virus that fails to express UL26.5 (DeltaICP35 virus). Purified DeltaICP35 B capsids showed altered sucrose gradient sedimentation and lacked the dense scaffold core seen in micrographs of wild-type B capsids but contained capsid shell proteins in wild-type amounts. Despite C-terminal sequence identity between UL26 and UL26.5, DeltaICP35 capsids lacking UL26.5 products did not contain compensatory high levels of UL26 proteins. Therefore, HSV capsids can be maintained and/or assembled on a minimal scaffold containing only wild-type levels of UL26 proteins. In contrast to UL26.5, increased expression of UL26 did not compensate for the DeltaICP35 growth defect. While indirect, these findings are consistent with the view that UL26 products are restricted from occupying abundant UL26.5 binding sites within the capsid and that this restriction is not controlled by the level of UL26 protein expression. Additionally, DeltaICP35 capsids contained an altered complement of DNA cleavage and packaging proteins, suggesting a previously unrecognized role for the scaffold in this process.

Isolation of herpes simplex virus procapsids from cells infected with a protease-deficient mutant virus.

Herpes simplex virus type 1 (HSV-1) capsid proteins assemble in vitro into spherical procapsids that differ markedly in structure and stability from mature polyhedral capsids but can be converted to the mature form. Circumstantial evidence suggests that assembly in vivo follows a similar pathway of procapsid assembly and maturation, a pathway that resembles those of double-stranded DNA bacteriophages. We have confirmed the above pathway by isolating procapsids from HSV-1-infected cells and characterizing their morphology, thermal sensitivity, and protein composition. Experiments were carried out with an HSV-1 mutant (m100) deficient in the maturational protease for which it was expected that procapsids-normally, short-lived intermediates-would accumulate in infected cells. Particles isolated from m100-infected cells were found to share the defining properties of procapsids assembled in vitro. For example, by electron microscopy, they were found to be spherical rather than polyhedral in shape, and they disassembled at 0 degrees C, unlike mature capsids, which are stable at this temperature. A three-dimensional reconstruction computed at 18-A resolution from cryoelectron micrographs showed m100 procapsids to be structurally indistinguishable from procapsids assembled in vitro. In both cases, their predominant components are the four essential capsid proteins: the major capsid protein (VP5), the scaffolding protein (pre-VP22a), and the triplex proteins (VP19C and VP23). VP26, a small, abundant but dispensable capsid protein, was not found associated with m100 procapsids, suggesting that it binds to capsids only after they have matured into the polyhedral form. Procapsids were also isolated from cells infected at the nonpermissive temperature with the HSV-1 mutant tsProt.A (a mutant with a thermoreversible lesion in the protease), and their identity as procapsids was confirmed by cryoelectron microscopy. This analysis revealed density on the inner surface of the procapsid scaffolding core that may correspond to the location of the maturational protease. Upon incubation at the permissive temperature, tsProt.A procapsids transformed into polyhedral, mature capsids, providing further confirmation of their status as precursors.

Assembly of the herpes simplex virus procapsid from purified components and identification of small complexes containing the major capsid and scaffolding proteins.

An in vitro system is described for the assembly of herpes simplex virus type 1 (HSV-1) procapsids beginning with three purified components, the major capsid protein (VP5), the triplexes (VP19C plus VP23), and a hybrid scaffolding protein. Each component was purified from insect cells expressing the relevant protein(s) from an appropriate recombinant baculovirus vector. Procapsids formed when the three purified components were mixed and incubated for 1 h at 37 degrees C. Procapsids assembled in this way were found to be similar in morphology and in protein composition to procapsids formed in vitro from cell extracts containing HSV-1 proteins. When scaffolding and triplex proteins were present in excess in the purified system, greater than 80% of the major capsid protein was incorporated into procapsids. Sucrose density gradient ultracentrifugation studies were carried out to examine the oligomeric state of the purified assembly components. These analyses showed that (i) VP5 migrated as a monomer at all of the protein concentrations tested (0.1 to 1 mg/ml), (ii) VP19C and VP23 migrated together as a complex with the same heterotrimeric composition (VP19C1-VP232) as virus triplexes, and (iii) the scaffolding protein migrated as a heterogeneous mixture of oligomers (in the range of monomers to approximately 30-mers) whose composition was strongly influenced by protein concentration. Similar sucrose gradient analyses performed with mixtures of VP5 and the scaffolding protein demonstrated the presence of complexes of the two having molecular weights in the range of 200,000 to 600,000. The complexes were interpreted to contain one or two VP5 molecules and up to six scaffolding protein molecules. The results suggest that procapsid assembly may proceed by addition of the latter complexes to regions of growing procapsid shell. They indicate further that procapsids can be formed in vitro from virus-encoded proteins only without any requirement for cell proteins.

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.

Assembly of the herpes simplex virus capsid: preformed triplexes bind to the nascent capsid.

The herpes simplex virus type 1 (HSV-1) capsid is a T=16 icosahedral shell that forms in the nuclei of infected cells. Capsid assembly also occurs in vitro in reaction mixtures created from insect cell extracts containing recombinant baculovirus-expressed HSV-1 capsid proteins. During capsid formation, the major capsid protein, VP5, and the scaffolding protein, pre-VP22a, condense to form structures that are extended into procapsids by addition of the triplex proteins, VP19C and VP23. We investigated whether triplex proteins bind to the major capsid-scaffold protein complexes as separate polypeptides or as preformed triplexes. Assembly products from reactions lacking one triplex protein were immunoprecipitated and examined for the presence of the other. The results showed that neither triplex protein bound unless both were present, suggesting that interaction between VP19C and VP23 is required before either protein can participate in the assembly process. Sucrose density gradient analysis was employed to determine the sedimentation coefficients of VP19C, VP23, and VP19C-VP23 complexes. The results showed that the two proteins formed a complex with a sedimentation coefficient of 7.2S, a value that is consistent with formation of a VP19C-VP23(2) heterotrimer. Furthermore, VP23 was observed to have a sedimentation coefficient of 4.9S, suggesting that this protein exists as a dimer in solution. Deletion analysis of VP19C revealed two domains that may be required for attachment of the triplex to major capsid-scaffold protein complexes; none of the deletions disrupted interaction of VP19C with VP23. We propose that preformed triplexes (VP19C-VP23(2) heterotrimers) interact with major capsid-scaffold protein complexes during assembly of the HSV-1 capsid.

The product of the herpes simplex virus type 1 UL25 gene is required for encapsidation but not for cleavage of replicated viral DNA.

The herpes simplex virus type 1 (HSV-1) UL25 gene contains a 580-amino-acid open reading frame that codes for an essential protein. Previous studies have shown that the UL25 gene product is a virion component (M. A. Ali et al., Virology 216:278-283, 1996) involved in virus penetration and capsid assembly (C. Addison et al., Virology 138:246-259, 1984). In this study, we describe the isolation of a UL25 mutant (KUL25NS) that was constructed by insertion of an in-frame stop codon in the UL25 open reading frame and propagated on a complementing cell line. Although the mutant was capable of synthesis of viral DNA, it did not form plaques or produce infectious virus in noncomplementing cells. Antibodies specific for the UL25 protein were used to demonstrate that KUL25NS-infected Vero cells did not express the UL25 protein. Western immunoblotting showed that the UL25 protein was associated with purified, wild-type HSV A, B, and C capsids. Transmission electron microscopy indicated that the nucleus of Vero cells infected with KUL25NS contained large numbers of both A and B capsids but no C capsids. Analysis of infected cells by sucrose gradient sedimentation analysis confirmed that the ratio of A to B capsids was elevated in KUL25NS-infected Vero cells. Following restriction enzyme digestion, specific terminal fragments were observed in DNA isolated from KUL25NS-infected Vero cells, indicating that the UL25 gene was not required for cleavage of replicated viral DNA. The latter result was confirmed by pulsed-field gel electrophoresis (PFGE), which showed the presence of genome-size viral DNA in KUL25NS-infected Vero cells. DNase I treatment prior to PFGE demonstrated that monomeric HSV DNA was not packaged in the absence of the UL25 protein. Our results indicate that the product of the UL25 gene is required for packaging but not cleavage of replicated viral DNA.

Structure of the herpes simplex virus capsid: peptide A862-H880 of the major capsid protein is displayed on the rim of the capsomer protrusions.

The herpes simplex virus-1 (HSV-1) capsid shell has 162 capsomers arranged on a T = 16 icosahedral lattice. The major capsid protein, VP5 MW = 149,075) is the structural component of the capsomers. VP5 is an unusually large viral capsid protein and has been shown to consist of multiple domains. To study the conformation of VP5 as it is folded into capsid promoters, we identified the sequence recognized by a VP5-specific monoclonal antibody and localized the epitope on the capsid surface by cryoelectron microscopy and image reconstruction. The epitope of mAb 6F10 was mapped to residues 862-880 by immunoblotting experiments performed with (1) proteolytic fragments of VP5, (2) GST-fusion proteins containing VP5 domains, and (3) synthetic VP5 peptides. As visualized in a three-dimensional density map of 6F10-precipitated capsids, the antibody was found to bind at sites on the outer surface of the capsid just inside the openings of the trans-capsomeric channels. We conclude that these sites are occupied by peptide 862-880 in the mature HSV-1 capsid.

Assembly of herpes simplex virus capsids using the human cytomegalovirus scaffold protein: critical role of the C terminus.

An essential step in assembly of herpes simplex virus (HSV) type 1 capsids involves interaction of the major capsid protein (VP5) with the C terminus of the scaffolding protein (encoded by the UL26.5 gene). The final 12 residues of the HSV scaffolding protein contains an A-X-X-F-V/A-X-Q-M-M-X-X-R motif which is conserved between scaffolding proteins found in other alphaherpesviruses but not in members of the beta- or gamma-herpesviruses. Previous studies have shown that the bovine herpesvirus 1 (alphaherpesvirus) UL26.5 homolog will functionally substitute for the HSV UL26.5 gene (E. J. Haanes et al., J. Virol. 69:7375-7379, 1995). The homolog of the UL26.5 gene in the human cytomegalovirus (HCMV) genome is the UL80.5 gene. In these studies, we tested whether the HCMV UL80.5 gene would substitute for the HSV UL26.5 gene in a baculovirus capsid assembly system that we have previously described (D. R. Thomsen et al., J. Virol. 68:2442-2457, 1994). The results demonstrate that (i) no intact capsids were assembled when the full-length or a truncated (missing the C-terminal 65 amino acids) UL80.5 protein was tested; (ii) when the C-terminal 65 amino acids of the UL80.5 protein were replaced with the C-terminal 25 amino acids of the UL26.5 protein, intact capsids were made and direct interaction of the UL80.5 protein with VP5 was detected; (iii) assembly of intact capsids was demonstrated when the sequence of the last 12 amino acids of the UL80.5 protein was changed from RRIFVA ALNKLE to RRIFVAAMMKLE; (iv) self-interaction of the scaffold proteins is mediated by sequences N terminal to the maturation cleavage site; and (v) the UL26.5 and UL80.5 proteins will not coassemble into scaffold structures. The results suggest that the UL26.5 and UL80.5 proteins form a scaffold by self-interaction via sequences in the N termini of the proteins and emphasize the importance of the C terminus for interaction of scaffold with the proteins that form the capsid shell.

Assembly of the herpes simplex virus capsid: characterization of intermediates observed during cell-free capsid formation.

The herpes simplex virus-1 (HSV-1) capsid is an icosahedral shell approximately 15 nm thick and 125 nm in diameter. Three of its primary structural components are a major capsid protein (VP5; coded by the UL19 gene) and two minor proteins, VP19C (UL38 gene) and VP23 (UL18 gene). Assembly of the capsid involves the participation of two additional proteins, the scaffolding protein (UL26.5 gene) and the maturational protease (UL26 gene). With the goal of identifying morphological intermediates in the assembly process, we have examined capsid formation in a cell-free system containing the five HSV-1 proteins mentioned above. Capsids and capsid-related structures formed during progressively longer periods of incubation were examined by electron microscopy of thin-sectioned specimens. After one minute, 90 minutes and eight hours of incubation the structures observed, respectively, were partial capsids, closed spherical capsids and polyhedral capsids. Partial capsids were two-layered structures consisting of a segment of external shell partially surrounding a region of scaffold. They appeared as wedges or angular segments of closed spherical capsids, the angle ranging from less than 30 degrees to greater than 270 degrees. Partial capsids are suggested to be precursors of closed spherical capsids because, whereas partial capsids were the predominant assembly product observed after one minute of incubation, they were rare in reactions incubated for 45 minutes or longer. Closed spherical capsids were highly uniform in morphology, consisting of a closed external shell surrounding a thick scaffold similar in morphology to the same layers seen in partial capsids. In negatively stained specimens, closed spherical capsids appeared round in profile, suggesting that they are spherical rather than polyhedral in shape. A three-dimensional reconstruction computed from cryoelectron micrographs confirmed that closed spherical capsids are spherical with T = 16 icosahedral symmetry. The reconstruction showed further that, compared to mature HSV-1 capsids, closed spherical capsids are more open structures in which the capsid floor layer is less pronounced. In contrast to closed spherical capsids, polyhedral capsids exhibited distinct facets and vertices, indicating that they are icosahedral like the capsids in mature virions. Upon incubation in vitro, purified closed spherical capsids matured into polyhedral capsids, indicating that the latter arise by angularization of the former. Partial capsids, closed spherical capsids and polyhedral capsids were all found to contain VP5, VP19C, VP23, VP21 and the scaffolding protein; the scaffolding protein being predominantly in the immature, uncleaved form in all cases. Polyhedral capsids and closed spherical capsids were found to differ in their sensitivity to disruption at 2 degrees C. Closed spherical capsids were disassembled while polyhedral capsids were unaffected. Our results suggest that HSV-1 capsid assembly begins with the partial capsid and proceeds through a closed, spherical, unstable capsid intermediate to a closed, icosahedral form similar to that found in the mature virion. Structures resembling HSV-1 partial capsids have been described as capsid assembly intermediates in Salmonella typhimurium bacteriophage P22. HSV-1 capsid maturation from a fragile, spherical state to a robust polyhedral form resembles the prohead maturation events undergone by dsDNA bacteriophages including lambda, T4 and P22. Because of this similarity, we propose the name procapsid for the closed spherical capsid intermediate in HSV-1 capsid assembly.

The herpes simplex virus procapsid: structure, conformational changes upon maturation, and roles of the triplex proteins VP19c and VP23 in assembly.

The proteins coded by the five major capsid genes of herpes simplex virus 1, VP5 (gene UL19), VP19c (UL38), VP23 (UL18), pre-VP22a (UL26.5), and pre-VP21 (UL26), assemble into fragile roundish "procapsids", which mature into robust polyhedral capsids in a transition similar to that undergone by bacteriophage proheads. Here we describe the HSV-1 procapsid structure to a resolution of approximately 2.7 nm from three-dimensional reconstructions of cryo-electron micrographs. Comparison with the mature capsid provides insight into the large-scale conformational changes that take place upon maturation. In the procapsid, the elongated protomers (VP5 subunits) make little contact with each other except around the bases of the hexons and pentons, whereas they are tightly clustered into capsomers in the mature state; the axial channels, which are constricted or blocked in the mature capsid, are fully open; and unlike the well observed 6-fold symmetry of mature hexons, procapsid hexons are distorted into oval and triangular shapes. These deformations reveal a VP5 domain in the inner part of the protrusion wall which participates in inter-protomer bonding in the procapsid and is close to the site where the channel closes upon maturation. Remarkably, there are no direct contacts between neighboring capsomers; instead, interactions between them are mediated by the "triplexes" at the sites of local 3-fold symmetry. This observation discloses the mechanism whereby the triplex proteins, VP19c and VP23, play their essential roles in capsid morphogenesis. In the mature capsid, density extends continuously between neighboring capsomers in the inner "floor" layer. In contrast, there are large gaps in the corresponding region of the procapsid, implying that formation of the floor involves extensive remodeling. Inside the procapsid shell is the hollow spherical scaffold, whose radial density profile indicates that the major scaffold protein, pre-VP22a, is a long molecule (> 24 nm) composed of three domains. Since no evidence of icosahedral symmetry is detected in the scaffold, we infer that (unless higher resolution is required) the scaffold may not be an icosahedral shell but may instead be a protein micelle with a preferred radius of curvature.

Separate functional domains of the herpes simplex virus type 1 protease: evidence for cleavage inside capsids.

The herpes simplex virus type 1 (HSV-1) protease (Pra) and related proteins are involved in the assembly of viral capsids and virion maturation. Pra is a serine protease, and the active-site residue has been mapped to amino acid (aa) 129 (Ser). This 635-aa protease, encoded by the UL26 gene, is autoproteolytically processed at two sites, the release (R) site between amino acid residues 247 and 248 and the maturation (M) site between residues 610 and 611. When the protease cleaves itself at both sites, it releases Nb, the catalytic domain (N0), and the C-terminal 25 aa. ICP35, a substrate of the HSV-1 protease, is the product of the UL26.5 gene. As it is translated from a Met codon within the UL26 gene, ICP35 cd are identical to the C-terminal 329-aa sequence of the protease and are trans cleaved at an identical C-terminal site to generate ICP35 e,f and a 25-aa peptide. Only fully processed Pra (N0 and Nb) and ICP35 (ICP35 e,f) are present in B capsids, which are believed to be precursors of mature virions. Using an R-site mutant A247S virus, we have recently shown that this mutant protease retains enzymatic activity but fails to support viral growth, suggesting that the release of N0 is required for viral replication. Here we report that another mutant protease, with an amino acid substitution (Ser to Cys) at the active site, can complement the A247S mutant but not a protease deletion mutant. Cell lines expressing the active-site mutant protease were isolated and shown to complement the A247S mutant at the levels of capsid assembly, DNA packaging, and viral growth. Therefore, the complementation between the R-site mutant and the active-site mutant reconstituted wild-type Pra function. One feature of this intragenic complementation is that following sedimentation of infected-cell lysates on sucrose gradients, both N-terminally unprocessed and processed proteases were isolated from the fractions where normal B capsids sediment, suggesting that proteolytic processing occurs inside capsids. Our results demonstrate that the HSV-1 protease has distinct functional domains and some of these functions can complement in trans.

Conserved features in papillomavirus and polyomavirus capsids.

Capsids of papilloma and polyoma viruses (papovavirus family) are composed of 72 pentameric capsomeres arranged on a skewed icosahedral lattice (triangulation number of seven, T = 7). Cottontail rabbit papillomavirus (CRPV) was reported previously to be a T = 7laevo (left-handed) structure, whereas human wart virus, simian virus 40, and murine polyomavirus were shown to be T = 7dextro (right-handed). The CRPV structure determined by cryoelectron microscopy and image reconstruction was similar to previously determined structures of bovine papillomavirus type 1 (BPV-1) and human papillomavirus type 1 (HPV-1). CRPV capsids were observed in closed (compact) and open (swollen) forms. Both forms have star-shaped capsomeres, as do BPV-1 and HPV-1, but the open CRPV capsids are approximately 2 nm larger in radius. The lattice hands of all papillomaviruses examined in this study were found to be T = 7dextro. In the region of maximum contact, papillomavirus capsomeres interact in a manner similar to that found in polyomaviruses. Although papilloma and polyoma viruses have differences in capsid size (approximately 60 versus approximately 50 nm), capsomere morphology (11 to 12 nm star-shaped versus 8 nm barrel-shaped), and intercapsomere interactions (slightly different contacts between capsomeres), papovavirus capsids have a conserved, 72-pentamer, T = 7dextro structure. These features are conserved despite significant differences in amino acid sequences of the major capsid proteins. The conserved features may be a consequence of stable contacts that occur within capsomeres and flexible links that form among capsomeres.

Use of weather data and remote sensing to predict the geographic and seasonal distribution of Phlebotomus papatasi in southwest Asia.

Sandfly fever and leishmaniasis were major causes of infectious disease morbidity among military personnel deployed to the Middle East during World War II. Recently, leishmaniasis has been reported in the United Nations Multinational Forces and Observers in the Sinai. Despite these indications of endemicity, no cases of sandfly fever and only 31 cases of leishmaniasis have been identified among U.S. veterans of the Persian Gulf War. The distribution in the Persian Gulf of the vector, Phlebotomus papatasi, is thought to be highly dependent on environmental conditions, especially temperature and relative humidity. A computer model was developed using the occurrence of P. papatasi as the dependent variable and weather data as the independent variables. The results of this model indicated that the greatest sand fly activity and thus the highest risk of sandfly fever and leishmania infections occurred during the spring/summer months before U.S. troops were deployed to the Persian Gulf. Because the weather model produced probability of occurrence information for locations of the weather stations only, normalized difference vegetation index (NDVI) levels from remotely sensed Advanced Very High Resolution Radiometer satellites were determined for each weather station. From the results of the frequency of NDVI levels by probability of occurrence, the range of NDVI levels for presence of the vector was determined. The computer then identified all pixels within the NDVI range indicated and produced a computer-generated map of the probable distribution of P. papatasi. The resulting map expanded the analysis to areas where there were no weather stations and from which no information was reported in the literature, identifying these areas as having either a high or low probability of vector occurrence.

Visualization of three-dimensional density maps reconstructed from cryoelectron micrographs of viral capsids.

Full evaluation of three-dimensional density maps calculated from cryoelectron micrographs of complex supramolecular structures requires that the maps be sifted by a variety of complementary visualization techniques. We present here a primer for a number of such techniques in current widespread use, including surface rendering; serial sections; simulated motion; and real-time manipulation of tiled surfaces displayed on an advanced workstation. The principles on which these techniques operate are briefly reviewed, as are their advantages and limitations, with emphasis on the requirements for visual representation of viral capsid structures. These methods are illustrated in application to a density map of herpes simplex virus type 1 (HSV-1) capsid at 24 A resolution, which reveals more detailed information than heretofore concerning the inner surface of the icosahedral capsid shell and the 150-A-long channels that pass through each of the 162 capsomers.

Herpes simplex virus capsids assembled in insect cells infected with recombinant baculoviruses: structural authenticity and localization of VP26.

Recently, recombinant baculoviruses have been used to show that expression of six herpes simplex virus type 1 genes results in the formation of capsid-like particles. We have applied cryoelectron microscopy and three-dimensional image reconstruction to establish their structural authenticity to a resolution of approximately 2.7 nm. By comparing capsids assembled with and without the expression of gene UL35, we have confirmed the presence of six copies of its product, VP26 (12 kDa), around each hexon tip. However, VP26 is not present on pentons, indicating that the conformational differences between the hexon and penton states of the major capsid protein, VP5, extend to the VP26 binding site.