Structural features of the scaffold interaction domain at the N terminus of the major capsid protein (VP5) of herpes simplex virus type 1

Cryo-electron microscopy structures of capsids and in situ portals of DNA-devoid capsids of human cytomegalovirus

The portal-scaffold complex is believed to nucleate the assembly of herpesvirus procapsids. During capsid maturation, two events occur: scaffold expulsion and DNA incorporation. The portal-scaffold interaction and the conformational changes that occur to the portal during the different stages of capsid formation have yet to be elucidated structurally. Here we present high-resolution structures of the A- and B-capsids and in-situ portals of human cytomegalovirus. We show that scaffolds bind to the hydrophobic cavities formed by the dimerization and Johnson-fold domains of the major capsid proteins. We further show that 12 loop-helix-loop fragments-presumably from the scaffold domain-insert into the hydrophobic pocket of the portal crown domain. The portal also undergoes significant changes both positionally and conformationally as it accompanies DNA packaging. These findings unravel the mechanism by which the portal interacts with the scaffold to nucleate capsid assembly and further our understanding of scaffold expulsion and DNA incorporation.

The Ins and Outs of Herpesviral Capsids: Divergent Structures and Assembly Mechanisms across the Three Subfamilies

Human herpesviruses, classified into three subfamilies, are double-stranded DNA viruses that establish lifelong latent infections within most of the world’s population and can cause severe disease, especially in immunocompromised people. There is no cure, and current preventative and therapeutic options are limited. Therefore, understanding the biology of these viruses is essential for finding new ways to stop them. Capsids play a central role in herpesvirus biology. They are sophisticated vehicles that shelter the pressurized double-stranded-DNA genomes while ensuring their delivery to defined cellular destinations on the way in and out of the host cell. Moreover, the importance of capsids for multiple key steps in the replication cycle makes their assembly an attractive therapeutic target. Recent cryo-electron microscopy reconstructions of capsids from all three subfamilies of human herpesviruses revealed not only conserved features but also remarkable structural differences. Furthermore, capsid assembly studies have suggested subfamily-specific roles of viral capsid protein homologs. In this review, we compare capsid structures, assembly mechanisms, and capsid protein functions across human herpesvirus subfamilies, highlighting the differences.

Hsp90 is involved in pseudorabies virus virion assembly via stabilizing major capsid protein VP5

Many viruses utilize molecular chaperone heat shock protein 90 (Hsp90) for protein folding and stabilization, however, the role of Hsp90 in herpesvirus lifecycle is obscure. Here, we provide evidence that Hsp90 participates in pseudorabies virus (PRV) replication. Viral growth kinetics assays show that Hsp90 inhibitor geldanamycin (GA) abrogates PRV replication at the post-penetration step. Transmission electron microscopy demonstrates that dysfunction of Hsp90 diminishes the quantity of PRV nucleocapsids. Overexpression and knockdown of Hsp90 suggest that de novo Hsp90 is involved in PRV replication. Mechanismly, dysfunction of Hsp90 inhibits PRV major capsid protein VP5 expression. Co-immunoprecipitation and indirect immunofluorescence assays indicate that Hsp90 interacts with VP5. Interestingly, Hsp70, a collaborator of Hsp90, also interacts with VP5, but doesn't affect PRV growth. Finally, inhibition of Hsp90 results in PRV VP5 degradation in a proteasome-dependent manner. Collectively, our data suggest that Hsp90 contributes to PRV virion assembly and replication via stabilization of VP5.

The Apical Region of the Herpes Simplex Virus Major Capsid Protein Promotes Capsid Maturation

The herpesvirus capsid assembles in the nucleus as an immature procapsid precursor built around viral scaffold proteins. The event that initiates procapsid maturation is unknown, but it is dependent upon activation of the VP24 internal protease. Scaffold cleavage triggers angularization of the shell and its decoration with the VP26 and pUL25 capsid-surface proteins. In both the procapsid and mature angularized capsid, the apical region of the major capsid protein (VP5) is surface exposed. We investigated whether the VP5 apical region contributes to intracellular transport dynamics following entry into primary sensory neurons and also tested the hypothesis that conserved negatively charged amino acids in the apical region contribute to VP26 acquisition. To our surprise, neither hypothesis proved true. Instead, mutation of glutamic acid residues in the apical region delayed viral propagation and induced focal capsid accumulations in nuclei. Examination of capsid morphogenesis based on epitope unmasking, capsid composition, and ultrastructural analysis indicated that these clusters consisted of procapsids. The results demonstrate that, in addition to established events that occur inside the capsid, the exterior capsid shell promotes capsid morphogenesis and maturation.IMPORTANCE Herpesviruses assemble capsids and encapsidate their genomes by a process that is unlike those of other mammalian viruses but is similar to those of some bacteriophage. Many important aspects of herpesvirus morphogenesis remain enigmatic, including how the capsid shell matures into a stable angularized configuration. Capsid maturation is triggered by activation of a protease that cleaves an internal protein scaffold. We report on the fortuitous discovery that a region of the major capsid protein that is exposed on the outer surface of the capsid also contributes to capsid maturation, demonstrating that the morphogenesis of the capsid shell from its procapsid precursor to the mature angularized form is dependent upon internal and external components of the megastructure.

Extensive subunit contacts underpin herpesvirus capsid stability and interior-to-exterior allostery

The herpesvirus capsid is a complex protein assembly that includes hundreds of copies of four major subunits and lesser numbers of several minor proteins, all of which are essential for infectivity. Cryo-electron microscopy is uniquely suited for studying interactions that govern the assembly and function of such large functional complexes. Here we report two high-quality capsid structures, from human herpes simplex virus type 1 (HSV-1) and the animal pseudorabies virus (PRV), imaged inside intact virions at ~7-Å resolution. From these, we developed a complete model of subunit and domain organization and identified extensive networks of subunit contacts that underpin capsid stability and form a pathway that may signal the completion of DNA packaging from the capsid interior to outer surface, thereby initiating nuclear egress. Differences in the folding and orientation of subunit domains between herpesvirus capsids suggest that common elements have been modified for specific functions.

A hydrophobic domain within the small capsid protein of Kaposi's sarcoma-associated herpesvirus is required for assembly

Kaposi's sarcoma-associated herpesvirus (KSHV) capsids can be produced in insect cells using recombinant baculoviruses for protein expression. All six capsid proteins are required for this process to occur and, unlike for alphaherpesviruses, the small capsid protein (SCP) ORF65 is essential for this process. This protein decorates the capsid shell by virtue of its interaction with the capsomeres. In this study, we have explored the SCP interaction with the major capsid protein (MCP) using GFP fusions. The assembly site within the nucleus of infected cells was visualized by light microscopy using fluorescence produced by the SCP-GFP polypeptide, and the relocalization of the SCP to these sites was evident only when the MCP and the scaffold protein were also present - indicative of an interaction between these proteins that ensures delivery of the SCP to assembly sites. Biochemical assays demonstrated a physical interaction between the SCP and MCP, and also between this complex and the scaffold protein. Self-assembly of capsids with the SCP-GFP polypeptide was evident. Potentially, this result can be used to engineer fluorescent KSHV particles. A similar SCP-His6 polypeptide was used to purify capsids from infected cell lysates using immobilized affinity chromatography and to directly label this protein in capsids using chemically derivatized gold particles. Additional studies with SCP-GFP polypeptide truncation mutants identified a domain residing between aa 50 and 60 of ORF65 that was required for the relocalization of SCP-GFP to nuclear assembly sites. Substitution of residues in this region and specifically at residue 54 with a polar amino acid (lysine) disrupted or abolished this localization as well as capsid assembly, whereas substitution with non-polar residues did not affect the interaction. Thus, this study identified a small conserved hydrophobic domain that is important for the SCP-MCP interaction.

Protein interactions in the murine cytomegalovirus capsid revealed by cryoEM

Cytomegalovirus (CMV) is distinct among members of the Herpesviridae family for having the largest dsDNA genome (230 kb). Packaging of large dsDNA genome is known to give rise to a highly pressurized viral capsid, but molecular interactions conducive to the formation of CMV capsid resistant to pressurization have not been described. Here, we report a cryo electron microscopy (cryoEM) structure of the murine cytomegalovirus (MCMV) capsid at a 9.1 Å resolution and describe the molecular interactions among the ∼3000 protein molecules in the MCMV capsid at the secondary structure level. Secondary structural elements are resolved to provide landmarks for correlating with results from sequence-based prediction and for structure-based homology modeling. The major capsid protein (MCP) upper domain (MCPud) contains α-helices and β-sheets conserved with those in MCPud of herpes simplex virus type 1 (HSV-1), with the largest differences identified as a "saddle loop" region, located at the tip of MCPud and involved in interaction with the smallest capsid protein (SCP). Interactions among the bacteriophage HK97-like floor domain of MCP, the middle domain of MCP, the hook and clamp domains of the triplex proteins (hoop and clamp domains of TRI-1 and clamp domain of TRI-2) contribute to the formation of a mature capsid. These results offer a framework for understanding how cytomegalovirus uses various secondary structural elements of its capsid proteins to build a robust capsid for packaging its large dsDNA genome inside and for attaching unique functional tegument proteins outside.

Interactions of the Kaposi's Sarcoma-associated herpesvirus nuclear egress complex: ORF69 is a potent factor for remodeling cellular membranes

All herpesviruses encode a complex of two proteins, referred to as the nuclear egress complex (NEC), which together facilitate the exit of assembled capsids from the nucleus. Previously, we showed that the Kaposi's sarcoma-associated herpesvirus (KSHV) NEC specified by the ORF67 and ORF69 genes when expressed in insect cells using baculoviruses for protein expression forms a complex at the nuclear membrane and remodels these membranes to generate nuclear membrane-derived vesicles. In this study, we have analyzed the functional domains of the KSHV NEC proteins and their interactions. Site-directed mutagenesis of gammaherpesvirus conserved residues revealed functional domains of these two proteins, which in many cases abolish the formation of the NEC and remodeling of nuclear membranes. Small in-frame deletions within ORF67 in all cases result in loss of the ability of the mutant protein to induce cellular membrane proliferation as well as to interact with ORF69. Truncation of the C terminus of ORF67 that resides in the perinuclear space does not impair the functions of ORF67; however, deletion of the transmembrane domain of ORF67 produces a protein that cannot induce membrane proliferation but can still interact with ORF69 in the nucleus and can be tethered to the nuclear membrane by virtue of its interaction with the wild-type-membrane-anchored ORF67. In-frame deletions in ORF69 have varied effects on NEC formation, but all abolish remodeling of nuclear membranes into circular structures. One mutant interacts with ORF67 as well as the wild-type protein but cannot function in membrane curvature and fission events that generate circular vesicles. These studies genetically confirm that ORF67 is required for cellular membrane proliferation and that ORF69 is the factor required to remodel these duplicated membranes into circular-virion-size vesicles. Furthermore, we also investigated the NEC encoded by Epstein-Barr virus (EBV). The EBV complex comprised of BFRF1 and BFLF2 was visualized at the nuclear membrane using autofluorescent protein fusions. BFRF1 is a potent inducer of membrane proliferation; however, BFLF2 cannot remodel these membranes into circular structures. What was evident is the superior remodeling activity of ORF69, which could convert the host membrane proliferations induced by BFRF1 into circular structures.

Identification of a varicella-zoster virus replication inhibitor that blocks capsid assembly by interacting with the floor domain of the major capsid protein

A novel anti-varicella-zoster virus compound, a derivative of pyrazolo[1,5-c]1,3,5-triazin-4-one (coded as 35B2), was identified from a library of 9,600 random compounds. This compound inhibited both acyclovir (ACV)-resistant and -sensitive strains. In a plaque reduction assay under conditions in which the 50% effective concentration of ACV against the vaccine Oka strain (V-Oka) in human fibroblasts was 4.25 μM, the 50% effective concentration of 35B2 was 0.75 μM. The selective index of the compound was more than 200. Treatment with 35B2 inhibited neither immediate-early gene expression nor viral DNA synthesis. Twenty-four virus clones resistant to 35B2 were isolated, all of which had a mutation(s) in the amino acid sequence of open reading frame 40 (ORF40), which encodes the major capsid protein (MCP). Most of the mutations were located in the regions corresponding to the "floor" domain of the MCP of herpes simplex virus 1. Treatment with 35B2 changed the localization of MCP in the fibroblasts infected with V-Oka but not in the fibroblasts infected with the resistant clones, although it did not affect steady-state levels of MCP. Overexpression of the scaffold proteins restored the normal MCP localization in the 35B2-treated infected cells. The compound did not inhibit the scaffold protein-mediated translocation of MCP from the cytoplasm to the nucleus. Electron microscopic analysis demonstrated the lack of capsid formation in the 35B2-treated infected cells. These data indicate the feasibility of developing a new class of antivirals that target the herpesvirus MCPs and inhibit normal capsid formation by a mechanism that differs from those of the known protease and encapsidation inhibitors. Further biochemical studies are required to clarify the precise antiviral mechanism.

The ongoing pursuit of a prophylactic HSV vaccine

HSV is among the most common human pathogens in the world. It is known to cause painful, persistent skin lesions, while also being the most common cause of fatal non-epidemic encephalitis as well as the leading cause of corneal blindness. The development of prophylactic vaccines could substantially reduce global health problems associated with HSV. So far, HSV vaccine strategies have shown noticeable efficacy in early development during preclinical phases but remained unsuccessful or unproven in human trials. New understanding of how the immune system mounts a defence against HSV offers practical strategies for vaccine development. A number of promising vaccine candidates are currently awaiting clinical development or already undergoing clinical testing. Therefore, this is a suitable time to assess the progress of HSV vaccine development and consider existing challenges and future improvements needed to achieve an effective prophylactic HSV vaccine.

A domain in the herpes simplex virus 1 triplex protein VP23 is essential for closure of capsid shells into icosahedral structures

VP23 is a key component of the triplex structure. The triplex, which is unique to herpesviruses, is a complex of three proteins, two molecules of VP23 which interact with a single molecule of VP19C. This structure is important for shell accretion and stability of the protein coat. Previous studies utilized a random transposition mutagenesis approach to identify functional domains of the triplex proteins. In this study, we expand on those findings to determine the key amino acids of VP23 that are required for triplex formation. Using alanine-scanning mutagenesis, we have made mutations in 79 of 318 residues of the VP23 polypeptide. These mutations were screened for function both in the yeast two-hybrid assay for interaction with VP19C and in a genetic complementation assay for the ability to support the replication of a VP23 null mutant virus. These assays identified a number of amino acids that, when altered, abolish VP23 function. Abrogation of virus assembly by a single-amino-acid change bodes well for future development of small-molecule inhibitors of this process. In addition, a number of mutations which localized to a C-terminal region of VP23 (amino acids 205 to 241) were still able to interact with VP19C but were lethal for virus replication when introduced into the herpes simplex virus 1 (HSV-1) KOS genome. The phenotype of many of these mutant viruses was the accumulation of large open capsid shells. This is the first demonstration of capsid shell accumulation in the presence of a lethal VP23 mutation. These data thus identify a new domain of VP23 that is required for or regulates capsid shell closure during virus assembly.

Self-assembly of Epstein-Barr virus capsids

Epstein-Barr virus (EBV), a member of the Gammaherpesvirus family, primarily infects B lymphocytes and is responsible for a number of lymphoproliferative diseases. The molecular genetics of the assembly pathway and high-resolution structural analysis of the capsid have not been determined for this lymphocryptovirus. As a first step in studying EBV capsid assembly, the baculovirus expression vector (BEV) system was used to express the capsid shell proteins BcLF1 (major capsid protein), BORF1 (triplex protein), BDLF1 (triplex protein), and BFRF3 (small capsid protein); the internal scaffold protein, BdRF1; and the maturational protease (BVRF2). Coinfection of insect cells with the six viruses expressing these proteins resulted in the production of closed capsid structures as judged by electron microscopy and sedimentation methods. Therefore, as shown for other herpesviruses, only six proteins are required for EBV capsid assembly. Furthermore, the small capsid protein of EBV (BFRF3), like that of Kaposi's sarcoma-associated herpesvirus, was found to be required for assembly of a stable structure. Localization of the small capsid protein to nuclear assembly sites required both the major capsid (BcLF1) and scaffold proteins (BdRF1) but not the triplex proteins. Mutational analysis of BFRF3 showed that the N-terminal half (amino acids 1 to 88) of this polypeptide is required and sufficient for capsid assembly. A region spanning amino acids 65 to 88 is required for the concentration of BFRF3 at a subnuclear site and the N-terminal 65 amino acids contain the sequences required for interaction with major capsid protein. These studies have identified the multifunctional role of the gammaherpesvirus small capsid proteins.

Small capsid protein pORF65 is essential for assembly of Kaposi's sarcoma-associated herpesvirus capsids

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiologic agent for KS tumors, multicentric Castleman's disease, and primary effusion lymphomas. Like other herpesvirus capsids, the KSHV capsid is an icosahedral structure composed of six proteins. The capsid shell is made up of the major capsid protein, two triplex proteins, and the small capsid protein. The scaffold protein and the protease occupy the internal space. The assembly of KSHV capsids is thought to occur in a manner similar to that determined for herpes simplex virus type 1 (HSV-1). Our goal was to assemble KSHV capsids in insect cells using the baculovirus expression vector system. Six KSHV capsid open reading frames were cloned and the proteins expressed in Sf9 cells: pORF25 (major capsid protein), pORF62 (triplex 1), pORF26 (triplex 2), pORF17 (protease), pORF17.5 (scaffold protein), and also pORF65 (small capsid protein). When insect cells were coinfected with these baculoviruses, angular capsids that contained internal core structures were readily observed by conventional electron microscopy of the infected cells. Capsids were also readily isolated from infected cells by using rate velocity sedimentation. With immuno-electron microscopy methods, these capsids were seen to be reactive to antisera to pORF65 as well as to KSHV-positive human sera, indicating the correct conformation of pORF65 in these capsids. When either virus expressing the triplex proteins was omitted from the coinfection, capsids did not assemble; similar to observations made in HSV-1-infected cells. If the virus expressing the scaffold protein was excluded, large open shells that did not attain icosahedral structure were seen in the nuclei of infected cells. The presence of pORF65 was required for capsid assembly, in that capsids did not form if this protein was absent as judged by both by ultrastructural analysis of infected cells and rate velocity sedimentation experiments. Thus, a novel outcome of this study is the finding that the small capsid protein of KSHV, like the major capsid and triplex proteins, is essential for capsid shell assembly.