Diffusion of macromolecules and virus-like particles in human cervical mucus

To determine whether or not large macromolecules and viruses can diffuse through mucus, we observed the motion of proteins, microspheres, and viruses in fresh samples of human cervical mucus using fluorescent recovery after photobleaching and multiple image photography. Two capsid virus-like particles, human papilloma virus (55 nm, approximately 20,000 kDa) and Norwalk virus (38 nm, approximately 10,000 kDa), as well as most of the globular proteins tested (15-650 kDa) diffused as rapidly in mucus as in saline. Electron microscopy of cervical mucus confirmed that the mesh spacing between mucin fibers is large enough (20-200 nm) for small viruses to diffuse essentially unhindered through mucus. In contrast, herpes simplex virus (180 nm) colocalized with strands of thick mucus, suggesting that herpes simplex virus, unlike the capsid virus particles, makes low-affinity bonds with mucins. Polystyrene microspheres (59-1000 nm) bound more tightly to mucins, bundling them into thick cables. Although immunoglobulins are too small to be slowed by the mesh spacing between mucins, diffusion by IgM was slowed by mucus. Diffusion by IgM-Fc(5 mu), the Fc pentamer core of an IgM with all 10 Fab moieties removed, was comparably slowed by mucus. This suggests that the Fc moieties of antibodies make low-affinity bonds with mucins.

Three-Dimensional Structure of Herpes Simplex Virus from Cryo-Electron Tomography

Herpes simplex virus, a DNA virus of high complexity, consists of a nucleocapsid surrounded by the tegument-a protein compartment-and the envelope. The latter components, essential for infectivity, are pleiomorphic. Visualized in cryo-electron tomograms of isolated virions, the tegument was seen to form an asymmetric cap: On one side, the capsid closely approached the envelope; on the other side, they were separated by approximately 35 nanometers of tegument. The tegument substructure was particulate, with some short actin-like filaments. The envelope contained 600 to 750 glycoprotein spikes that varied in length, spacing, and in the angles at which they emerge from the membrane. Their distribution was nonrandom, suggesting functional clustering.

Seeing the herpesvirus capsid at 8.5 A

Human herpesviruses are large and structurally complex viruses that cause a variety of diseases. The three-dimensional structure of the herpesvirus capsid has been determined at 8.5 angstrom resolution by electron cryomicroscopy. More than 30 putative alpha helices were identified in the four proteins that make up the 0.2 billion-dalton shell. Some of these helices are located at domains that undergo conformational changes during capsid assembly and DNA packaging. The unique spatial arrangement of the heterotrimer at the local threefold positions accounts for the asymmetric interactions with adjacent capsid components and the unusual co-dependent folding of its subunits.

Egress of alphaherpesviruses: comparative ultrastructural study

Egress of four important alphaherpesviruses, equine herpesvirus 1 (EHV-1), herpes simplex virus type 1 (HSV-1), infectious laryngotracheitis virus (ILTV), and pseudorabies virus (PrV), was investigated by electron microscopy of infected cell lines of different origins. In all virus-cell systems analyzed, similar observations were made concerning the different stages of virion morphogenesis. After intranuclear assembly, nucleocapsids bud at the inner leaflet of the nuclear membrane, resulting in enveloped particles in the perinuclear space that contain a sharply bordered rim of tegument and a smooth envelope surface. Egress from the perinuclear cisterna primarily occurs by fusion of the primary envelope with the outer leaflet of the nuclear membrane, which has been visualized for HSV-1 and EHV-1 for the first time. The resulting intracytoplasmic naked nucleocapsids are enveloped at membranes of the trans-Golgi network (TGN), as shown by immunogold labeling with a TGN-specific antiserum. Virions containing their final envelope differ in morphology from particles within the perinuclear cisterna by visible surface projections and a diffuse tegument. Particularly striking was the addition of a large amount of tegument material to ILTV capsids in the cytoplasm. Extracellular virions were morphologically identical to virions within Golgi-derived vesicles, but distinct from virions in the perinuclear space. Studies with gB- and gH-deleted PrV mutants indicated that these two glycoproteins, which are essential for virus entry and direct cell-to-cell spread, are dispensable for egress. Taken together, our studies indicate that the deenvelopment-reenvelopment process of herpesvirus maturation also occurs in EHV-1, HSV-1, and ILTV and that membrane fusion processes occurring during egress are substantially different from those during entry and direct viral cell-to-cell spread.

A novel role for phagocytosis-like uptake in herpes simplex virus entry

It is becoming increasingly clear that herpesviruses can exploit the endocytic pathway to infect cells, yet several important features of this process remain poorly defined. Using herpes simplex virus-1 (HSV-1) as a model, we demonstrate that endocytosis of the virions mimic many features of phagocytosis. During entry, HSV-1 virions associated with plasma membrane protrusions followed by a phagocytosis-like uptake involving rearrangement of actin cytoskeleton and trafficking of the virions in large phagosome-like vesicles. RhoA GTPase was activated during this process and the mode of entry was cell type-specific. Clathrin-coated vesicles had no detectable role in virion trafficking as Eps15 dominant-negative mutants failed to affect HSV-1 uptake. Binding and fusion of the virion envelope with the phagosomal membrane is likely facilitated by clustering of nectin-1 (or HVEM) in phagosomes, which was observed in infected cells. Collectively, our data suggests a novel mode of uptake by which the virus can infect both professional and nonprofessional phagocytes.

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.

Visualization of tegument-capsid interactions and DNA in intact herpes simplex virus type 1 virions

Herpes simplex virus type 1 virions were examined by electron cryomicroscopy, allowing the three-dimensional structure of the infectious particle to be visualized for the first time. The capsid shell is identical to that of B-capsids purified from the host cell nucleus, with the exception of the penton channel, which is closed. The double-stranded DNA genome is organized as regularly spaced ( approximately 26 A) concentric layers inside the capsid. This pattern suggests a spool model for DNA packaging, similar to that for some bacteriophages. The bulk of the tegument is not icosahedrally ordered. However, a small portion appears as filamentous structures around the pentons, interacting extensively with the capsid. Their locations and interactions suggest possible roles for the tegument proteins in regulating DNA transport through the penton channel and binding to cellular transport proteins during viral infection.

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.

Axonal transport of herpes simplex virions to epidermal cells: evidence for a specialized mode of virus transport and assembly

To examine the transmission of herpes simplex virus (HSV) from axon to epidermal cell, an in vitro model was constructed consisting of human fetal dorsal root ganglia cultured in the central chamber of a dual-chamber tissue culture system separated from autologous skin explants in an exterior chamber by concentric steel cylinders adhering to the substratum through silicon grease and agarose. Axons grew through the agarose viral diffusion barrier and terminated on epidermal cells in the exterior chamber. After inoculation of HSV onto dorsal root ganglia, anterograde axonal transport of glycoprotein and nucleocapsid antigen was observed by confocal microscopy to appear in exterior chamber axons within 12 h and in epidermal cells within 16 h, moving at 2-3 mm/h. Although both enveloped and unenveloped nucleocapsids were observed in the neuronal soma by transmission electron microscopy, only nucleocapsids were observed in the axons, closely associated with microtubules. Nodule formation at the surface of HSV-infected axons, becoming more dense at the axon terminus on epidermal cells, and patches of axolemmal HSV glycoprotein D expression were observed by scanning (immuno)electron microscopy, probably representing virus emerging from the axolemma. These findings strongly suggest a specialized mode of viral transport, assembly, and egress in sensory neurons: microtubule-associated intermediate-fast anterograde axonal transport of unenveloped nucleocapsids with separate transport of glycoproteins to the distal regions of the axon and assembly prior to virus emergence at the axon terminus.

Glycoprotein D receptor-dependent, low-pH-independent endocytic entry of herpes simplex virus type 1

Two herpes simplex virus type 1 (HSV-1) entry pathways have been described: direct fusion between the virion envelope and the plasma membrane, as seen on Vero cells, and low-pH-dependent endocytosis, as seen on CHO nectin-1 and HeLa cells. In this paper, we studied HSV entry into C10 murine melanoma cells and identified a third entry pathway for this virus. During entry into C10 cells, virion envelope glycoproteins rapidly became protected from the membrane-impermeable chemical cross-linker BS3 and from proteinase K. Protection was gD receptor dependent, and the time taken to detect protected protein was proportional to the rate of virus entry. Ultrastructural examination revealed that virions attached to the surface of C10 cells were localized to membrane invaginations, whereas those on the surface of receptor-negative B78 cells were peripherally attached. Virus entry into C10 cells was energy dependent, and intracellular enveloped virions were seen within membrane-bound vesicles consistent with endocytic entry. Entry was not inhibited by bafilomycin A1 or ammonium chloride, showing that passage of the virion through a low-pH environment was not required for infection. Resistance to similar reagents should therefore not be taken as proof of HSV entry by a nonendosomal pathway. These data define a novel gD receptor-dependent acid-independent endocytic entry pathway for HSV.

Herpes simplex virus glycoproteins gB and gH function in fusion between the virion envelope and the outer nuclear membrane

Herpesviruses must traverse the nuclear envelope to gain access to the cytoplasm and, ultimately, to exit cells. It is believed that herpesvirus nucleocapsids enter the perinuclear space by budding through the inner nuclear membrane (NM). To reach the cytoplasm these enveloped particles must fuse with the outer NM and the unenveloped capsids then acquire a second envelope in the trans-Golgi network. Little is known about the process by which herpesviruses virions fuse with the outer NM. Here we show that a herpes simplex virus (HSV) mutant lacking both the two putative fusion glycoproteins gB and gH failed to cross the nuclear envelope. Enveloped virions accumulated in the perinuclear space or in membrane vesicles that bulged into the nucleoplasm (herniations). By contrast, mutants lacking just gB or gH showed only minor or no defects in nuclear egress. We concluded that either HSV gB or gH can promote fusion between the virion envelope and the outer NM. It is noteworthy that fusion associated with HSV entry requires the cooperative action of both gB and gH, suggesting that the two types of fusion (egress versus entry) are dissimilar processes.

Assembly of herpes simplex virus (HSV) intermediate capsids in insect cells infected with recombinant baculoviruses expressing HSV capsid proteins

The capsid of herpes simplex virus type 1 (HSV-1) is composed of seven proteins, VP5, VP19C, VP21, VP22a, VP23, VP24, and VP26, which are the products of six HSV-1 genes. Recombinant baculoviruses were used to express the six capsid genes (UL18, UL19, UL26, UL26.5, UL35, and UL38) in insect cells. All constructs expressed the appropriate-size HSV proteins, and insect cells infected with a mixture of the six recombinant baculoviruses contained large numbers of HSV-like capsids. Capsids were purified by sucrose gradient centrifugation, and electron microscopy showed that the capsids made in Sf9 cells had the same size and appearance as authentic HSV B capsids. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis demonstrated that the protein composition of these capsids was nearly identical to that of B capsids isolated from HSV-infected Vero cells. Electron microscopy of thin sections clearly demonstrated that the capsids made in insect cells contained the inner electron-translucent core associated with HSV B capsids. In infections in which single capsid genes were left out, it was found that the UL18 (VP23), UL19 (VP5), UL38 (VP19C), and either the UL26 (VP21 and VP24) or the UL26.5 (VP22a) genes were required for assembly of 100-nm capsids. VP22a was shown to form the inner core of the B capsid, since in infections in which the UL26.5 gene was omitted the 100-nm capsids that formed lacked the inner core. The UL35 (VP26) gene was not required for assembly of 100-nm capsids, although assembly of B capsids was more efficient when it was present. These and other observations indicate that (i) the products of the UL18, UL19, UL35, and UL38 genes self-assemble into structures that form the outer surface (icosahedral shell) of the capsid, (ii) the products of the UL26 and/or UL26.5 genes are required (as scaffolds) for assembly of 100-nm capsids, and (iii) the interaction of the outer surface of the capsid with the scaffolding proteins requires the product of the UL18 gene (VP23).

Dynamics of herpes simplex virus capsid maturation visualized by time-lapse cryo-electron microscopy

The capsid of the herpes simplex virus initially assembles as a procapsid that matures through a massive conformational change of its 182 MDa surface shell. This transition, which stabilizes the fragile procapsid, is facilitated by the viral protease that releases the interaction between the shell and the underlying scaffold; however, protease-deficient procapsids mature slowly in vitro. To study procapsid maturation as a time-resolved process, we monitored this reaction by cryo-electron microscopy (cryo-EM). The resulting images were sorted into 17 distinct classes, and three-dimensional density maps were calculated for each. When arranged in a chronological series, these maps yielded molecular movies of procapsid maturation. A single major switching event takes place at stages 8-9, preceded by relatively subtle adjustments in the pattern of interactions and followed by similarly small 'aftershocks'. The primary mechanism underlying maturation is relative rotations of domains of VP5, the major capsid protein.

Manipulation of herpes simplex virus type 1 by dielectrophoresis

The frequency-dependent dielectrophoretic behaviour of an enveloped mammalian virus, herpes simplex virus type 1 is described. It is demonstrated that over the range 10 kHz-20 MHz, these viral particles, when suspended in an aqueous medium of conductivity 5 mS m(-1), can be manipulated by both positive and negative dielectrophoresis using microfabricated electrode arrays. The observed transition from positive to negative dielectrophoresis at frequencies around 4.5 MHz is in qualitative agreement with a simple model of the virus as a conducting particle surrounded by an insulating membrane.

Assembly of VP26 in herpes simplex virus-1 inferred from structures of wild-type and recombinant capsids

The 1250 A diameter herpes simplex virus-1 (HSV-1) capsid shell consists of four major structural proteins, of which VP26 (approximately 12,000 M(r)) is the smallest. Using 400 kV electron cryomicroscopy and computer reconstruction, we have determined the three-dimensional structures of the wild-type capsid and a recombinant baculovirus-generated HSV-1 capsid which lacks VP26. Their difference map demonstrates the presence of VP26 hexamers attached to all the hexons in the wild-type capsid, and reveals that the VP26 molecule consists of a large and a small domain. Although both hexons and pentons are predominantly composed of VP5, VP26 is not present on the penton. Based on the interactions involving VP26 and the hexon subunits, we propose a mechanism for VP26 assembly which would account for its distribution. Possible roles of VP26 in capsid stability and DNA packaging are discussed.

Allosteric signaling and a nuclear exit strategy: binding of UL25/UL17 heterodimers to DNA-Filled HSV-1 capsids

UL25 and UL17 are two essential minor capsid proteins of HSV-1, implicated in DNA packaging and capsid maturation. We used cryo-electron microscopy to examine their binding to capsids, whose architecture observes T = 16 icosahedral geometry. C-capsids (mature DNA-filled capsids) have an elongated two-domain molecule present at a unique, vertex-adjacent site that is not seen at other quasiequivalent sites or on unfilled capsids. Using SDS-PAGE and mass spectrometry to analyze wild-type capsids, UL25 null capsids, and denaturant-extracted capsids, we conclude that (1) the C-capsid-specific component is a heterodimer of UL25 and UL17, and (2) capsids have additional populations of UL25 and UL17 that are invisible in reconstructions because of sparsity and/or disorder. We infer that binding of the ordered population reflects structural changes induced on the outer surface as pressure builds up inside the capsid during DNA packaging. Its binding may signal that the C-capsid is ready to exit the nucleus.

Glycoprotein D or J delivered in trans blocks apoptosis in SK-N-SH cells induced by a herpes simplex virus 1 mutant lacking intact genes expressing both glycoproteins

We have made two stocks of a herpes simplex virus 1 mutant lacking intact U(S)5 and U(S)6 open reading frames encoding glycoproteins J (gJ) and D (gD), respectively. The stock designated gD(-/+), made in cells carrying U(S)6 and expressing gD, was capable of productively infecting cells, whereas the stock designated gD(-/-), made in cells lacking viral DNA sequences, was known to attach but not initiate infection. We report the following. (i) Both stocks of virus induced apoptosis in SK-N-SH cells. Thus, annexin V binding to cell surfaces was detected as early as 8 h after infection. (ii) U(S)5 or U(S)6 cloned into the baculovirus under the human cytomegalovirus immediate-early promoter was expressed in SK-N-SH cells and blocked apoptosis in cells infected with either gD(-/+) or gD(-/-) virus, whereas glycoprotein B, infected cell protein 22, or the wild-type baculovirus did not block apoptosis. (iii) In SK-N-SH cells, internalized, partially degraded virus particles were detected at 30 min after exposure to gD(-/-) virus but not at later intervals. (iv) Concurrent infection of cells with baculoviruses did not alter the failure of gD(-/-) virus from expressing its genes or, conversely, the expression of viral genes by gD(-/+) virus. These results underscore the capacity of herpes simplex virus to initiate the apoptotic cascade in the absence of de novo protein synthesis and indicate that both gD and gJ independently, and most likely at different stages in the reproductive cycle, play a key role in blocking the apoptotic cascade leading to cell death.

Evidence that herpes simplex virus VP16 is required for viral egress downstream of the initial envelopment event

During infection with herpes simplex virus type 1 (HSV-1), VP16 serves multiple functions, including transcriptional activation of viral immediate early genes and downregulation of the virion host shutoff protein vhs. Furthermore, VP16 has been shown to be involved in some aspect of virus assembly and/or maturation. Experiments with a VP16 null virus, 8MA, suggested that VP16 plays a direct role in virion assembly, since removal of VP16 from the HSV-1 genome results in reduced levels of encapsidated DNA and a failure to produce extracellular enveloped particles. However, VP16 null mutants display a severe translational arrest due to unrestrained vhs activity, thus complicating interpretation of these data. We examine here the role of VP16 in virion assembly and egress in the context of a vhs null background, using the virus 8MA/DeltaSma (VP16(-) vhs(-)). Comparison of 8MA and 8MA/DeltaSma with respect to viral DNA accumulation and encapsidation and accumulation of the major capsid protein, VP5, revealed that the 8MA lethal phenotype is only partially due to uncontrolled vhs activity, indicating that VP16 is required in HSV-1 virion formation. Electron microscopy confirmed these results and further showed that VP16 is required for HSV-1 egress beyond the perinuclear space. In addition, we describe the isolation and characterization of an 8MA derivative capable of propagation on Vero cells, due to second site mutations in the vhs and UL53 (gK) genes. Taken together, these results show that VP16 is required for viral egress downstream of the initial envelopment step and further underscore the importance of VP16 in controlling vhs activity within an infected cell.

Anterograde transport of herpes simplex virus type 1 in cultured, dissociated human and rat dorsal root ganglion neurons

The mechanism of anterograde transport of herpes simplex virus was studied in cultured dissociated human and rat dorsal root ganglion neurons. The neurons were infected with HSV-1 to examine the distribution of capsid (VP5), tegument (VP16), and glycoproteins (gC and gB) at 2, 6, 10, 13, 17, and 24 h postinfection (p.i.) with or without nocodazole (a microtubule depolymerizer) or brefeldin A (a Golgi inhibitor). Retrogradely transported VP5 was detected in the cytoplasm of the cell body up to the nuclear membrane at 2 h p.i. It was first detected de novo in the nucleus and cytoplasm at 10 h p.i., the axon hillock at 13 h p.i., and the axon at 15 to 17 h p.i. gC and gB were first detected de novo in the cytoplasm and the axon hillock at 10 h p.i. and then in the axon at 13 h p.i., which was always earlier than the detection of VP5. De novo-synthesized VP16 was first detected in the cytoplasm at 10 to 13 h p.i. and in the axon at 16 to 17 h p.i. Nocodazole inhibited the transport of all antigens, VP5, VP16, and gC or gB. The kinetics of inhibition of VP5 and gC could be dissociated. Brefeldin A inhibited the transport of gC or gB and VP16 but not VP5 into axons. Transmission immunoelectron microscopy confirmed that there were unenveloped nucleocapsids in the axon with or without brefeldin A. These findings demonstrate that glycoproteins and capsids, associated with tegument proteins, are transported by different pathways with slightly differing kinetics from the nucleus to the axon. Furthermore, axonal anterograde transport of the nucleocapsid can proceed despite the loss of most VP16.

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.

Evidence for antiviral activity of glutathione: in vitro inhibition of herpes simplex virus type 1 replication

The role of glutathione (GSH) in the in vitro infection and replication of human herpes simplex virus type 1 (HSV-1) was investigated. Intracellular endogenous GSH levels dramatically decreased in the first 24 h after virus adsorption, starting immediately after virus challenge. The addition of exogenous GSH was not only able to restore its intracellular levels almost up to those found in uninfected cells, but also to inhibit > 99% the replication of HSV-1. This inhibition was concentration-dependent, not related to toxic effects on host cells and also maintained if the exogenous GSH was added as late as 24 h after virus challenge, i.e. when virus infection was fully established. Electron microscopic examination of HSV-1-infected cells showed that GSH dramatically reduced the number of extracellular and intracytoplasmic virus particles, whereas some complete nucleocapsids were still detected within the nuclei of GSH-treated cells. Consistent with this observation, immunoblot analysis showed that the expression of HSV-1-glycoprotein B, crucial for the release and the infectivity of virus particles, was significantly decreased. Data suggest that exogenous GSH inhibits the replication of HSV-1 by interfering with very late stages of the virus life cycle, without affecting cellular metabolism.

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.

The effects of radiation damage on the structure of frozen hydrated HSV-1 capsids

Radiation damage imposes stringent limits on the information content of electron micrographs of biological specimens. In this study, we have investigated its effects on frozen, hydrated specimens and three-dimensional reconstructions calculated from cryomicrographs using capsids of herpes simplex virus as a model system. Multiple-exposure series of micrographs of both B-capsids (which contain no DNA) and C-capsids (which are fully packaged) were recorded and reconstructions were calculated from the first exposures, corresponding to a cumulative electron dose of 6-7 e-/A2, and from later exposures (25-40 e-/A2). Experimental procedures were standardized to ensure that perceived changes in the micrographs and reconstructions would be attributable to radiation damage alone. The effects of the higher doses in both the micrographs and the reconstructions were expressed as a progressive blurring of the finer details, corresponding to a delocalization of structure in the ice-embedded specimens. The resolutions of the reconstructions were quantified according to a form of the Fourier ring correlation coefficient criterion, according to which the first-exposure reconstructions had resolutions of 30-36 A. The fifth-exposure B-capsid reconstruction had comparable nominal resolution, although it exhibited progressively lower correlations at higher spatial frequencies. Qualitatively similar changes in the series of C-capsid reconstructions were observed although they were more pronounced, presumably because these micrographs had lower contrast and signal-to-noise ratios. We infer that the observed changes in the images and reconstructions and the concomitant loss in contrast in the immediate vicinity of the capsid surface may reflect radiation-induced perturbation of molecular structure and/or the release of peptide fragments. Nevertheless, the observed changes are relatively subtle, at least at the operational resolution of this study; overall, our results support earlier indications (M. F. Schmid et al. J. Struct. Biol. 108, 62-68, 1992) that prospects are quite good for tilt-series reconstructions from cryoelectron micrographs, including six to eight views of the same specimen.

Sorting and transport of alpha herpesviruses in axons

The alpha herpesviruses, a subfamily of the herpesviruses, are neurotropic pathogens found associated with most mammalian species. The prototypic member of this subfamily is herpes simplex virus type 1, the causative agent of recurrent cold sores in humans. The mild nature of this disease is a testament to the complex and highly regulated life cycle of the alpha herpesviruses. The cellular mechanisms used by these viruses to disseminate infection in the nervous system are beginning to be understood. Here, we overview the life cycle of alpha herpesviruses with an emphasis on assembly and transport of viral particles in neurons.

Isolation and characterization of herpes simplex virus type 1 mutants defective in the UL6 gene

Previous studies have shown that the protein encoded by herpes simplex virus type 1 (HSV-1) gene UL6 is required for processing and packaging of replicated viral DNA and is a minor component of virions and capsids. In this report, we describe the construction of UL6- HSV-1 mutants with a disrupted UL6 gene using complementing cells and show that they fail to synthesize the UL6 protein or produce infectious virus in noncomplementing cells. The mutants synthesized but failed to process and encapsidate viral DNA and accumulated only immature capsids which lacked the UL6 protein. Immunofluorescence analysis showed that the UL6 protein, when expressed transiently in transfected cells in the absence of other HSV-1 proteins, is localized exclusively to the nucleus. We also investigated an HSV-1 mutant with a defect in gene UL33, the product of which is also thought to be involved in viral DNA processing and packaging. The phenotype of this mutant on noncomplementing cells with regard to failure to process and encapsidate viral DNA, accumulation of immature capsids, and inability to produce infectious virus was the same as that of UL6- viruses. This mutant, however, produced capsids containing the UL6 protein, indicating that association of the UL6 protein with the capsid is independent of the UL33 protein.