Intermolecular Complementation between Two Varicella-Zoster Virus pORF30 Terminase Domains Essential for DNA Encapsidation

Mutagenesis and functional analysis of the varicella-zoster virus portal protein

Herpesviruses replicate by cleaving concatemeric dsDNA into single genomic units that are packaged through an oligomeric portal present in preformed procapsids. In contrast to what is known about phage portal proteins, details concerning herpesvirus portal structure and function are not as well understood. A panel of 65 Varicella-Zoster virus (VZV) recombinant portal proteins with five amino acid in-frame insertions were generated by random transposon mutagenesis of the VZV portal gene, ORF54. Subsequently, 65 VZVLUC recombinant viruses (TNs) were generated via recombineering. Insertions were mapped to predicted portal domains (clip, wing, stem, wall, crown, and β-hairpin tunnel-loop) and recombinant viruses were characterized for plaque morphology, replication kinetics, pORF54 expression, and classified based on replication in non-complementing (ARPE19) or complementing (ARPE54C50) cell lines. The N- and C-termini were tolerant to insertion mutagenesis, as were certain clip sub-domains. The majority of mutants mapping to the wing, wall, β-hairpin tunnel loop, and stem domains were lethal. Elimination of the predicted ORF54 start codon revealed that the first 40 amino acids of the N-terminus were not required for viral replication. Stop codon insertions in the C-terminus showed that the last 100 amino acids were not required for viral replication. Lastly, a putative protease cleavage site was identified in the C-terminus of pORF54. Cleavage was likely orchestrated by a viral protease; however, processing was not required for DNA encapsidation and viral replication. The panel of recombinants should prove valuable in future studies to dissect mammalian portal structure and function.IMPORTANCEThough nucleoside analogs and a live-attenuated vaccine are currently available to treat some human herpesvirus family members, alternate methods of combating herpesvirus infection could include blocking viral replication at the DNA encapsidation stage. The approval of Letermovir provided proof of concept regarding the use of encapsidation inhibitors to treat herpesvirus infections in the clinic. We propose that small-molecule compounds could be employed to interrupt portal oligomerization, assembly into the capsid vertex, or affect portal function/dynamics. Targeting portal at any of these steps would result in disruption of viral DNA packaging and a decrease or absence of mature infectious herpesvirus particles. The oligomeric portals of herpesviruses are structurally conserved, and therefore, it may be possible to find a single compound capable of targeting portals from one or more of the herpesvirus subfamilies. Drug candidates from such a series would be effective against viruses resistant to the currently available antivirals.

The Terminase Complex of Each Human Herpesvirus

The human herpesviruses (HHVs) are classified into the following three subfamilies: Alphaherpesvirinae, Betaherpesvirinae, and Gammaherpesvirinae. These HHVs have distinct pathological features, while containing a highly conserved viral replication pathway. Among HHVs, the basic viral particle structure and the sequential processes of viral replication are nearly identical. In particular, the capsid formation mechanism has been proposed to be highly similar among herpesviruses, because the viral capsid-organizing proteins are highly conserved at the structural and functional levels. Herpesviruses form capsids containing the viral genome in the nucleus of infected cells during the lytic phase, and release infectious virus (i.e., virions) to the cell exterior. In the capsid formation process, a single-unit-length viral genome is encapsidated into a preformed capsid. The single-unit-length viral genome is produced by cleavage from a viral genome precursor in which multiple unit-length viral genomes are tandemly linked. This encapsidation and cleavage is carried out by the terminase complex, which is composed of viral proteins. Since the terminase complex-mediated encapsidation and cleavage is a virus-specific mechanism that does not exist in humans, it may be an excellent inhibitory target for anti-viral drugs with high virus specificity. This review provides an overview of the functions of the terminase complexes of HHVs.

Kaposi's Sarcoma-Associated Herpesvirus ORF67.5 Functions as a Component of the Terminase Complex

Kaposi's sarcoma-associated herpesvirus (KSHV) is a double-stranded DNA (dsDNA) gammaherpesvirus with a poorly characterized lytic replication cycle. However, the lytic replication cycle of the alpha- and betaherpesviruses are well characterized. During lytic infection of alpha- and betaherpesviruses, the viral genome is replicated as a precursor form, which contains tandem genomes linked via terminal repeats (TRs). One genomic unit of the precursor form is packaged into a capsid and is cleaved at the TR by the terminase complex. While the alpha- and betaherpesvirus terminases are well characterized, the KSHV terminase remains poorly understood. KSHV open reading frame 7 (ORF7), ORF29, and ORF67.5 are presumed to be components of the terminase complex based on their homology to other terminase proteins. We previously reported that ORF7-deficient KSHV formed numerous immature soccer ball-like capsids and failed to cleave the TRs. ORF7 interacted with ORF29 and ORF67.5; however, ORF29 and ORF67.5 did not interact with each other. While these results suggested that ORF7 is important for KSHV terminase function and capsid formation, the function of ORF67.5 was completely unknown. Therefore, to analyze the function of ORF67.5, we constructed ORF67.5-deficient BAC16. ORF67.5-deficient KSHV failed to produce infectious virus and cleave the TRs, and numerous soccer ball-like capsids were observed in ORF67.5-deficient KSHV-harboring cells. Furthermore, ORF67.5 promoted the interaction between ORF7 and ORF29, and ORF29 increased the interaction between ORF67.5 and ORF7. Thus, our data indicated that ORF67.5 functions as a component of the KSHV terminase complex by contributing to TR cleavage, terminase complex formation, capsid formation, and virus production. IMPORTANCE Although the formation and function of the alpha- and betaherpesvirus terminase complexes are well understood, the Kaposi's sarcoma-associated herpesvirus (KSHV) terminase complex is still largely uncharacterized. This complex presumably contains KSHV open reading frame 7 (ORF7), ORF29, and ORF67.5. We were the first to report the presence of soccer ball-like capsids in ORF7-deficient KSHV-harboring lytic-induced cells. Here, we demonstrated that ORF67.5-deficient KSHV also formed soccer ball-like capsids in lytic-induced cells. Moreover, ORF67.5 was required for terminal repeat (TR) cleavage, infectious virus production, and enhancement of the interaction between ORF7 and ORF29. ORF67.5 has several highly conserved regions among its human herpesviral homologs. These regions were necessary for virus production and for the interaction of ORF67.5 with ORF7, which was supported by the artificial intelligence (AI)-predicted structure model. Importantly, our results provide the first evidence showing that ORF67.5 is essential for terminase complex formation and TR cleavage.

The human cytomegalovirus decathlon: Ten critical replication events provide opportunities for restriction

Human cytomegalovirus (HCMV) is a ubiquitous human pathogen that can cause severe disease in immunocompromised individuals, transplant recipients, and to the developing foetus during pregnancy. There is no protective vaccine currently available, and with only a limited number of antiviral drug options, resistant strains are constantly emerging. Successful completion of HCMV replication is an elegant feat from a molecular perspective, with both host and viral processes required at various stages. Remarkably, HCMV and other herpesviruses have protracted replication cycles, large genomes, complex virion structure and complicated nuclear and cytoplasmic replication events. In this review, we outline the 10 essential stages the virus must navigate to successfully complete replication. As each individual event along the replication continuum poses as a potential barrier for restriction, these essential checkpoints represent potential targets for antiviral development.

Mutual Interplay between the Human Cytomegalovirus Terminase Subunits pUL51, pUL56, and pUL89 Promotes Terminase Complex Formation

Human cytomegalovirus (HCMV) genome encapsidation requires several essential viral proteins, among them pUL56, pUL89, and the recently described pUL51, which constitute the viral terminase. To gain insight into terminase complex assembly, we investigated interactions between the individual subunits. For analysis in the viral context, HCMV bacterial artificial chromosomes carrying deletions in the open reading frames encoding the terminase proteins were used. These experiments were complemented by transient-transfection assays with plasmids expressing the terminase components. We found that if one terminase protein was missing, the levels of the other terminase proteins were markedly diminished, which could be overcome by proteasome inhibition or providing the missing subunit in trans These data imply that sequestration of the individual subunits within the terminase complex protects them from proteasomal turnover. The finding that efficient interactions among the terminase proteins occurred only when all three were present together is reminiscent of a folding-upon-binding principle leading to cooperative stability. Furthermore, whereas pUL56 was translocated into the nucleus on its own, correct nuclear localization of pUL51 and pUL89 again required all three terminase constituents. Altogether, these features point to a model of the HCMV terminase as a multiprotein complex in which the three players regulate each other concerning stability, subcellular localization, and assembly into the functional tripartite holoenzyme.IMPORTANCE HCMV is a major risk factor in immunocompromised individuals, and congenital CMV infection is the leading viral cause for long-term sequelae, including deafness and mental retardation. The current treatment of CMV disease is based on drugs sharing the same mechanism, namely, inhibiting viral DNA replication, and often results in adverse side effects and the appearance of resistant virus strains. Recently, the HCMV terminase has emerged as an auspicious target for novel antiviral drugs. A new drug candidate inhibiting the HCMV terminase, Letermovir, displayed excellent potency in clinical trials; however, its precise mode of action is not understood yet. Here, we describe the mutual dependence of the HCMV terminase constituents for their assembly into a functional terminase complex. Besides providing new basic insights into terminase formation, these results will be valuable when studying the mechanism of action for drugs targeting the HCMV terminase and developing additional substances interfering with viral genome encapsidation.