The cytoplasmic tail of glycoprotein M (gpUL100) expresses trafficking signals required for human cytomegalovirus assembly and replication

Human induced pluripotent stem cells are resistant to human cytomegalovirus infection primarily at the attachment level due to the reduced expression of cell-surface heparan sulfate

Cytomegalovirus (CMV), a type of herpes virus, is the predominant cause of congenital anomalies due to intrauterine infections in humans. Adverse outcomes related to intrauterine infections with human cytomegalovirus (HCMV) vary widely, depending on factors such as fetal infection timing, infection route, and viral virulence. The precise mechanism underlying HCMV susceptibility remains unclear. In this study, we compared the susceptibility of neonatal human dermal fibroblast cells (NHDFCs) and human induced pluripotent stem cells (hiPSCs) derived from NHDFCs, which are genetically identical to HCMV, using immunostaining, microarray, in situ hybridization, quantitative PCR, and scanning electron microscopy. These cells were previously used to compare CMV susceptibility, but the underlying mechanisms were not fully elucidated. HCMV susceptibility of hiPSCs was significantly lower in the earliest phase. No shared gene ontologies were observed immediately post-infection between the two cell types using microarray analysis. Early-stage expression of HCMV antigens and the HCMV genome was minimal in immunostaining and in in situ hybridization in hiPSCs. This strongly suggests that HCMV does not readily bind to hiPSC surfaces. Scanning electron microscopy performed using the NanoSuit method confirmed the scarcity of HCMV particles on hiPSC surfaces. The zeta potential and charge mapping of the charged surface in NHDFCs and hiPSCs exhibited minimal differences when assessed using zeta potential analyzer and scanning ion conductance microscopy; however, the expression of heparan sulfate (HS) was significantly lower in hiPSCs compared with that in NHDFCs. Thus, HS expression could be a primary determinant of HCMV resistance in hiPSCs at the attachment level.

IMPORTANCE

Numerous factors such as attachment, virus particle entry, transcription, and virus particle egress can affect viral susceptibility. Since 1984, pluripotent cells are known to be CMV resistant; however, the exact mechanism underlying this resistance remains elusive. Some researchers suggest inhibition in the initial phase of HCMV binding, while others have suggested the possibility of a sufficient amount of HCMV entering the cells to establish latency. This study demonstrates that HCMV particles rarely attach to the surfaces of hiPSCs. This is not due to limitations in the electrostatic interactions between the surface of hiPSCs and HCMV particles, but due to HS expression. Therefore, HS expression should be recognized as a key factor in determining the susceptibility of HCMV in congenital infection in vitro and in vivo. In the future, drugs targeting HS may become crucial for the treatment of congenital CMV infections. Thus, further research in this area is warranted.

Insights into the Transcriptome of Human Cytomegalovirus: A Comprehensive Review

Human cytomegalovirus (HCMV) is a widespread pathogen that poses significant risks to immunocompromised individuals. Its genome spans over 230 kbp and potentially encodes over 200 open-reading frames. The HCMV transcriptome consists of various types of RNAs, including messenger RNAs (mRNAs), long non-coding RNAs (lncRNAs), circular RNAs (circRNAs), and microRNAs (miRNAs), with emerging insights into their biological functions. HCMV mRNAs are involved in crucial viral processes, such as viral replication, transcription, and translation regulation, as well as immune modulation and other effects on host cells. Additionally, four lncRNAs (RNA1.2, RNA2.7, RNA4.9, and RNA5.0) have been identified in HCMV, which play important roles in lytic replication like bypassing acute antiviral responses, promoting cell movement and viral spread, and maintaining HCMV latency. CircRNAs have gained attention for their important and diverse biological functions, including association with different diseases, acting as microRNA sponges, regulating parental gene expression, and serving as translation templates. Remarkably, HCMV encodes miRNAs which play critical roles in silencing human genes and other functions. This review gives an overview of human cytomegalovirus and current research on the HCMV transcriptome during lytic and latent infection.

Deleting UL49.5 in duck plague virus causes attachment, entry and spread defects

Duck plague is a disease with high morbidity and mortality rates, and it causes great losses for the duck breeding industry. Duck plague virus (DPV) is the causative agent of duck plague, and DPV UL49.5 protein (pUL49.5) is homologue of glycoprotein N (gN), which is conserved in herpesviruses. UL49.5 homologues are known to be involved in processes such as immune escape, virus assembly, viral fusion, transporter associated with antigen processing (TAP) inhibition and degradation, and maturation and incorporation of glycoprotein M. However, few studies have focused on the role of gN in the early stage of virus infection cells. In this study, we determined that DPV pUL49.5 was distributed in the cytoplasm and colocalized with the endoplasmic reticulum (ER). Moreover, we found that DPV pUL49.5 was a virion component and nonglycosylated protein. To better explore its function, BAC-DPV-ΔUL49.5 was constructed, and its attachment was only approximately 25 % of the revertant virus. Additionally, the penetration ability of BAC-DPV-ΔUL49.5 has only reached 73 % of the revertant virus. The plaque sizes produced by the UL49.5-deleted virus were approximately 58 % smaller than those produced by the revertant virus. Deleting UL49.5 mainly resulted in attachment and cell-to-cell-spread defects. Taken together, these findings suggest important roles for DPV pUL49.5 in viral attachment, penetration and spread.

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.

Deciphering the Potential Coding of Human Cytomegalovirus: New Predicted Transmembrane Proteome

CMV is a major cause of morbidity and mortality in immunocompromised individuals that will benefit from the availability of a vaccine. Despite the efforts made during the last decade, no CMV vaccine is available. An ideal CMV vaccine should elicit a broad immune response against multiple viral antigens including proteins involved in virus-cell interaction and entry. However, the therapeutic use of neutralizing antibodies targeting glycoproteins involved in viral entry achieved only partial protection against infection. In this scenario, a better understanding of the CMV proteome potentially involved in viral entry may provide novel candidates to include in new potential vaccine design. In this study, we aimed to explore the CMV genome to identify proteins with putative transmembrane domains to identify new potential viral envelope proteins. We have performed in silico analysis using the genome sequences of nine different CMV strains to predict the transmembrane domains of the encoded proteins. We have identified 77 proteins with transmembrane domains, 39 of which were present in all the strains and were highly conserved. Among the core proteins, 17 of them such as UL10, UL139 or US33A have no ascribed function and may be good candidates for further mechanistic studies.

Antibody Titers Against Human Cytomegalovirus gM/gN and gB Among Pregnant Women and Their Infants

Congenital CMV (cCMV) infection can affect infants born to mothers with preconceptional seroimmunity. To prevent cCMV due to nonprimary maternal infection, vaccines eliciting responses exceeding natural immunity may be required. Anti-gM/gN antibodies have neutralizing capacity in-vitro and in animal models, but anti-gM/gN antibodies have not been characterized among seroimmune pregnant women. Paired maternal and infant cord sera from 92 CMV seropositive mothers and their full-term or preterm infants were tested for anti-gM/gN antibody titers in comparison with anti-gB titers and neutralizing activity. Anti-gM/gN titers were significantly lower than anti-gB titers for all groups and did not correlate with serum neutralizing capacity. Further study is needed to determine if higher anti-gM/gN antibody titers might enhance serum neutralizing capacity among seropositive adults.

Recent progress in development of monoclonal antibodies against human cytomegalovirus

Human cytomegalovirus (HCMV) is a ubiquitous pathogen that can cause permanent childhood disabilities following in utero infection and life threatening diseases in immune-compromised individuals such as those post transplantation. Without an effective vaccine, small molecule antiviral drugs are routinely used in high-risk transplant recipients, but the effectiveness of which is limited by side effects and drug resistance. The potentials of antibody-based passive immune therapies alone or in combination with the small molecule antivirals to treat or prevent HCMV infection have been actively studied. In this review, we focus on the recent publications on identification and characterization of monoclonal antibodies that have the potential to be developed as anti-HCMV therapies. We review the progress in clinical evaluation of antibody-based therapies to prevent HCMV-associated diseases.

Characterization of gH/gL/pUL128-131 pentameric complex, gH/gL/gO trimeric complex, gB and gM/gN glycoproteins in a human cytomegalovirus using automated capillary western blots

Human cytomegalovirus (HCMV) is currently a major cause of congenital disease in newborns and organ failure in transplant recipients. Despite decades of efforts, an effective vaccine against HCMV has yet to be developed. However, the discovery of pentameric gH complex on viral surface which contains potent neutralizing epitopes may help enable development of an effective vaccine. In our company ongoing Phase II clinical trial of whole-live virus HCMV vaccine (V160), the pentameric gH complex has been restored on the surface of live attenuated AD169 virus strain. The reconstructed HCMV virus contains a variety of surface glycoproteins including pentameric gH/gL/gUL128-131 complex, trimeric gH/gL/gO complex, gB glycoprotein, and gM/gN heterodimer complex. To further characterize this virus and enable the monitoring of multiple viral antigens during vaccine process development an effective and efficient analytical strategy was required to detect and quantify several viral surface proteins. In this paper, we present an innovative approach based on capillary western blot technology that allows fast and accurate quantitation of pentameric gH/gL/gUL128-131 complex, trimeric gH/gL/gO complex, and gB glycoprotein. This method is suitable for analyzing target proteins in multiple sample types including supernatants from infected cell culture, purification intermediates, concentration bulk, and the final vaccine product. In addition, the capillary western blot-based technology identified a previously unknown biochemical profile present in some HCMV viruses: triplet gH peaks of viral surface proteins in non-reducing environment, which could potentially present a new strategy for specificity and identity testing.

Rat and human cytomegalovirus ORF116 encodes a virion envelope glycoprotein required for infectivity

Herpesviruses encode multiple glycoproteins required for different stages of viral attachment, fusion, and envelopment. The protein encoded by the human cytomegalovirus (HCMV) open reading frame UL116 forms a stable complex with glycoprotein H that is incorporated into virions. However, the function of this complex remains unknown. Herein, we characterize R116, the rat CMV (RCMV) putative homolog of UL116. Two R116 transcripts were identified in fibroblasts with three proteins expressed with molecular weights of 42, 58, and 82 kDa. R116 is N-glycosylated, expressed with late viral gene kinetics, and is incorporated into the virion envelope. RCMV lacking R116 failed to result in productive infection of fibroblasts and siRNA knockdown of R116 substantially reduced RCMV infectivity. Complementation in trans of an R116-deficient virus restored ability of the virus to infect fibroblasts. Finally, UL116 knockdown also decreased HCMV infectivity indicating that R116 and UL116 both contribute to viral infectivity.

Receptors in host pathogen interactions between human cytomegalovirus and the placenta during congenital infection

Cellular receptors in human cytomegalovirus (HCMV) mother to child transmission play an important role in congenital infection. Placental trophoblast cells are a significant cell type in placental development, placental functional processes, and in HCMV transmission. Different cells within the placental floating and chorionic villi present alternate receptors for HCMV cell entry. Syncytiotrophoblasts present neonatal Fc receptors that bind and transport circulating maternal immunoglobulin G across the placental interface which can also be bound to HCMV virions, facilitating viral entry into the placenta and foetal circulation. Cytotrophoblast express HCMV receptors including integrin-α1β1, integrin-αVβ3, epidermal growth factor receptor and platelet-derived growth factor receptor alpha. The latter interacts with HCMV glycoprotein-H, glycoprotein-L and glycoprotein-O (gH/gL/gO) trimers (predominantly in placental fibroblasts) and the gH/gL/pUL128, UL130-UL131A pentameric complex in other placental cell types. The pentameric complex allows viral tropism of placental trophoblasts, endothelial cells, epithelial cells, leukocytes and monocytes. This review outlines HCMV ligands and target receptor proteins in congenital HCMV infection.

Rat Cytomegalovirus Virion-Associated Proteins R131 and R129 Are Necessary for Infection of Macrophages and Dendritic Cells

Cytomegalovirus (CMV) establishes persistent, latent infection in hosts, causing diseases in immunocompromised patients, transplant recipients, and neonates. CMV infection modifies the host chemokine axis by modulating chemokine and chemokine receptor expression and by encoding putative chemokine and chemokine receptor homologues. The viral proteins have roles in cellular signaling, migration, and transformation, as well as viral dissemination, tropism, latency and reactivation. Herein, we review the contribution of CMV-encoded chemokines and chemokine receptors to these processes, and further elucidate the viral tropism role of rat CMV (RCMV) R129 and R131. These homologues of the human CMV (HCMV)-encoded chemokines UL128 and UL130 are of particular interest because of their dual role as chemokines and members of the pentameric entry complex, which is required for entry into cell types that are essential for viral transmission and dissemination. The contributions of UL128 and UL130 to acceleration of solid organ transplant chronic rejection are poorly understood, and are in need of an effective in vivo model system to elucidate the phenomenon. We demonstrated similar molecular entry requirements for R129 and R131 in the rat cells, as observed for HCMV, and provided evidence that R129 and R131 are part of the viral entry complex required for entry into macrophages, dendritic cells, and bone marrow cells.

Guinea pig cytomegalovirus trimer complex gH/gL/gO uses PDGFRA as universal receptor for cell fusion and entry

Species-specific guinea pig cytomegalovirus (GPCMV) causes congenital CMV and the virus encodes homolog glycoprotein complexes to human CMV, including gH-based trimer (gH/gL/gO) and pentamer-complex (PC). Platelet-derived growth factor receptor alpha (gpPDGFRA), only present on fibroblast cells, was identified via CRISPR as the putative receptor for PC-independent GPCMV infection. Immunoprecipitation assays demonstrated direct interaction of gH/gL/gO with gpPDGFRA but not in absence of gO. Expression of viral gB also resulted in precipitation of gB/gH/gL/gO/gpPDGFRA complex. Cell-cell fusion assays determined that expression of gpPDGFRA and gH/gL/gO in adjacent cells enabled cell fusion, which was not enhanced by gB. N-linked gpPDGFRA glycosylation inhibition had limited effect and blocking tyrosine kinase (TK) transduction had no impact on infection. Ectopically expressed gpPDGFRA or TK-domain mutant in trophoblast or epithelial cells previously non-susceptible to GPCMV(PC-) enabled viral infection. In contrast, transient human PDGFRA expression did not complement GPCMV(PC-) infection, a potential basis for viral species specificity.

Pathogen at the Gates: Human Cytomegalovirus Entry and Cell Tropism

The past few years have brought substantial progress toward understanding how human cytomegalovirus (HCMV) enters the remarkably wide spectrum of cell types and tissues that it infects. Neuropilin-2 and platelet-derived growth factor receptor alpha (PDGFRα) were identified as receptors, respectively, for the trimeric and pentameric glycoprotein H/glycoprotein L (gH/gL) complexes that in large part govern HCMV cell tropism, while CD90 and CD147 were also found to play roles during entry. X-ray crystal structures for the proximal viral fusogen, glycoprotein B (gB), and for the pentameric gH/gL complex (pentamer) have been solved. A novel virion gH complex consisting of gH bound to UL116 instead of gL was described, and findings supporting the existence of a stable complex between gH/gL and gB were reported. Additional work indicates that the pentamer promotes a mode of cell-associated spread that resists antibody neutralization, as opposed to the trimeric gH/gL complex (trimer), which appears to be broadly required for the infectivity of cell-free virions. Finally, viral factors such as UL148 and US16 were identified that can influence the incorporation of the alternative gH/gL complexes into virions. We will review these advances and their implications for understanding HCMV entry and cell tropism.

Qualitative Differences in Capsidless L-Particles Released as a By-Product of Bovine Herpesvirus 1 and Herpes Simplex Virus 1 Infections

The alphaherpesvirus family includes viruses that infect humans and animals. Hence, not only do they have a significant impact on human health, but they also have a substantial economic impact on the farming industry. While the pathogenic manifestations of the individual viruses differ from host to host, their relative genetic compositions suggest similarity at the molecular level. This study provides a side-by-side comparison of the particle outputs from the major human pathogen HSV-1 and the veterinary pathogen BoHV-1. Ultrastructural and proteomic analyses have revealed that both viruses have broadly similar morphogenesis profiles and infectious virus compositions. However, the demonstration that BoHV-1 has the capacity to generate vast numbers of capsidless enveloped particles that differ from those produced by HSV-1 in composition implies a divergence in the cell biology of these viruses that impacts our general understanding of alphaherpesvirus morphogenesis.

Despite differences in the pathogenesis and host range of alphaherpesviruses, many stages of their morphogenesis are thought to be conserved. Here, an ultrastructural study of bovine herpesvirus 1 (BoHV-1) envelopment revealed profiles similar to those previously found for herpes simplex virus 1 (HSV-1), with BoHV-1 capsids associating with endocytic tubules. Consistent with the similarity of their genomes and envelopment strategies, the proteomic compositions of BoHV-1 and HSV-1 virions were also comparable. However, BoHV-1 morphogenesis exhibited a diversity in envelopment events. First, heterogeneous primary envelopment profiles were readily detectable at the inner nuclear membrane of BoHV-1-infected cells. Second, the BoHV-1 progeny comprised not just full virions but also an abundance of capsidless, noninfectious light particles (L-particles) that were released from the infected cells in numbers similar to those of virions and in the absence of DNA replication. Proteomic analysis of BoHV-1 L-particles and the much less abundant HSV-1 L-particles revealed that they contained the same complement of envelope proteins as virions but showed variations in tegument content. In the case of HSV-1, the U_L46 tegument protein was reproducibly found to be >6-fold enriched in HSV-1 L-particles. More strikingly, the tegument proteins U_L36, U_L37, U_L21, and U_L16 were depleted in BoHV-1 but not HSV-1 L-particles. We propose that these combined differences reflect the presence of truly segregated “inner” and “outer” teguments in BoHV-1, making it a critical system for studying the structure and process of tegumentation and envelopment.

IMPORTANCE The alphaherpesvirus family includes viruses that infect humans and animals. Hence, not only do they have a significant impact on human health, but they also have a substantial economic impact on the farming industry. While the pathogenic manifestations of the individual viruses differ from host to host, their relative genetic compositions suggest similarity at the molecular level. This study provides a side-by-side comparison of the particle outputs from the major human pathogen HSV-1 and the veterinary pathogen BoHV-1. Ultrastructural and proteomic analyses have revealed that both viruses have broadly similar morphogenesis profiles and infectious virus compositions. However, the demonstration that BoHV-1 has the capacity to generate vast numbers of capsidless enveloped particles that differ from those produced by HSV-1 in composition implies a divergence in the cell biology of these viruses that impacts our general understanding of alphaherpesvirus morphogenesis.

Betaherpesvirus Virion Assembly and Egress

Virions are the vehicle for cell-to-cell and host-to-host transmission of viruses. Virions need to be assembled reliably and efficiently, be released from infected cells, survive in the extracellular environment during transmission, recognize and then trigger entry of appropriate target cells, and disassemble in an orderly manner during initiation of a new infection. The betaherpesvirus subfamily includes four human herpesviruses (human cytomegalovirus and human herpesviruses 6A, 6B, and 7), as well as viruses that are the basis of important animal models of infection and immunity. Similar to other herpesviruses, betaherpesvirus virions consist of four main parts (in order from the inside): the genome, capsid, tegument, and envelope. Betaherpesvirus genomes are dsDNA and range in length from ~145 to 240 kb. Virion capsids (or nucleocapsids) are geometrically well-defined vessels that contain one copy of the dsDNA viral genome. The tegument is a collection of several thousand protein and RNA molecules packed into the space between the envelope and the capsid for delivery and immediate activity upon cellular entry at the initiation of an infection. Betaherpesvirus envelopes consist of lipid bilayers studded with virus-encoded glycoproteins; they protect the virion during transmission and mediate virion entry during initiation of new infections. Here, we summarize the mechanisms of betaherpesvirus virion assembly, including how infection modifies, reprograms, hijacks, and otherwise manipulates cellular processes and pathways to produce virion components, assemble the parts into infectious virions, and then transport the nascent virions to the extracellular environment for transmission.

Phosphorylation of Golgi Peripheral Membrane Protein Grasp65 Is an Integral Step in the Formation of the Human Cytomegalovirus Cytoplasmic Assembly Compartment

Human cytomegalovirus (HCMV) is the largest member of the Herpesviridae and represents a significant cause of disease. During virus replication, HCMV alters cellular functions to facilitate its replication, including significant reorganization of the secretory and endocytic pathways of the infected cell. A defining morphologic change of the infected cell is the formation of a membranous structure in the cytoplasm that is designated the virion assembly compartment (AC), which consists of virion structural proteins surrounded by cellular membranes. The loss of normal Golgi compartment morphology and its relocalization from a juxtanuclear ribbonlike structure to a series of concentric rings on the periphery of the AC represents a readily recognized reorganization of cellular membranes in the HCMV-infected cell. Although trafficking of viral proteins to this compartment is required for the assembly of infectious virions, the functional significance of the reorganization of intracellular membranes like the Golgi membranes into the AC in the assembly of infectious virus remains understudied. In this study, we determined that Golgi membrane ribbon fragmentation increased during the early cytoplasmic phase of virion assembly and that Golgi membrane fragmentation in infected cells was dependent on the phosphorylation of an integral cis-Golgi protein, Grasp65. Inhibition of Golgi membrane fragmentation and of its reorganization into the AC resulted in decreased production of infectious particles and alteration of the incorporation of an essential protein into the envelope of the mature virion. These results demonstrated the complexity of the virus-host cell interactions required for efficient assembly of this large DNA virus.

The human cytomegalovirus (HCMV)-induced reorganization of intracellular membranes that is required for the formation of the viral assembly compartment (AC) has been an area of study over the last 20 years. The significance of this virus-induced structure has been evinced by the results of several studies which showed that relocalization of viral proteins to the AC was required for efficient assembly of infectious virus. In this study, we have identified a mechanism for the fragmentation of the Golgi ribbon in the infected cell en route to AC morphogenesis. Identification of this fundamental process during HCMV replication allowed us to propose that the functional role of Golgi membrane reorganization during HCMV infection was the concentration of viral structural proteins and subviral structures into a single intracellular compartment in order to facilitate efficient protein-protein interactions and the virion protein trafficking required for the assembly of this large and structurally complex virus.

Virion Glycoprotein-Mediated Immune Evasion by Human Cytomegalovirus: a Sticky Virus Makes a Slick Getaway

The prototypic herpesvirus human cytomegalovirus (CMV) exhibits the extraordinary ability to establish latency and maintain a chronic infection throughout the life of its human host. This is even more remarkable considering the robust adaptive immune response elicited by infection and reactivation from latency. In addition to the ability of CMV to exist in a quiescent latent state, its persistence is enabled by a large repertoire of viral proteins that subvert immune defense mechanisms, such as NK cell activation and major histocompatibility complex antigen presentation, within the cell. However, dissemination outside the cell presents a unique existential challenge to the CMV virion, which is studded with antigenic glycoprotein complexes targeted by a potent neutralizing antibody response. The CMV virion envelope proteins, which are critical mediators of cell attachment and entry, possess various characteristics that can mitigate the humoral immune response and prevent viral clearance. Here we review the CMV glycoprotein complexes crucial for cell attachment and entry and propose inherent properties of these proteins involved in evading the CMV humoral immune response. These include viral glycoprotein polymorphism, epitope competition, Fc receptor-mediated endocytosis, glycan shielding, and cell-to-cell spread. The consequences of CMV virion glycoprotein-mediated immune evasion have a major impact on persistence of the virus in the population, and a comprehensive understanding of these evasion strategies will assist in designing effective CMV biologics and vaccines to limit CMV-associated disease.

Epstein-Barr virus glycoprotein gM can interact with the cellular protein p32 and knockdown of p32 impairs virus

The Epstein-Barr virus glycoprotein complex gMgN has been implicated in assembly and release of fully enveloped virus, although the precise role that it plays has not been elucidated. We report here that the long predicted cytoplasmic tail of gM is not required for complex formation and that it interacts with the cellular protein p32, which has been reported to be involved in nuclear egress of human cytomegalovirus and herpes simplex virus. Although redistribution of p32 and colocalization with gM was not observed in virus infected cells, knockdown of p32 expression by siRNA or lentivirus-delivered shRNA recapitulated the phenotype of a virus lacking expression of gNgM. A proportion of virus released from cells sedimented with characteristics of virus lacking an intact envelope and there was an increase in virus trapped in nuclear condensed chromatin. The observations suggest the possibility that p32 may also be involved in nuclear egress of Epstein-Barr virus.

Functional annotation of human cytomegalovirus gene products: an update

Human cytomegalovirus is an opportunistic double-stranded DNA virus with one of the largest viral genomes known. The 235 kB genome is divided in a unique long (UL) and a unique short (US) region which are flanked by terminal and internal repeats. The expression of HCMV genes is highly complex and involves the production of protein coding transcripts, polyadenylated long non-coding RNAs, polyadenylated anti-sense transcripts and a variety of non-polyadenylated RNAs such as microRNAs. Although the function of many of these transcripts is unknown, they are suggested to play a direct or regulatory role in the delicately orchestrated processes that ensure HCMV replication and life-long persistence. This review focuses on annotating the complete viral genome based on three sources of information. First, previous reviews were used as a template for the functional keywords to ensure continuity; second, the Uniprot database was used to further enrich the functional database; and finally, the literature was manually curated for novel functions of HCMV gene products. Novel discoveries were discussed in light of the viral life cycle. This functional annotation highlights still poorly understood regions of the genome but more importantly it can give insight in functional clusters and/or may be helpful in the analysis of future transcriptomics and proteomics studies.

Immunization with cytomegalovirus envelope glycoprotein M and glycoprotein N DNA vaccines can provide mice with complete protection against a lethal murine cytomegalovirus challenge

Human cytomegalovirus virions contain three major glycoprotein complexes (gC I, II, III), all of which are required for CMV infectivity. These complexes also represent major antigenic targets for anti-viral immune responses. The gC II complex consists of two glycoproteins, gM and gN. In the current study, DNA vaccines expressing the murine cytomegalovirus (MCMV) homologs of the gM and gN proteins were evaluated for protection against lethal MCMV infection in a mouse model. Humoral and cellular immune responses, spleen viral titers, and mice survival and body-weight changes were examined. The results showed that immunization with gM or gN DNA vaccine alone was not able to offer good protection, whereas co-immunization with both gM and gN induced an effective neutralizing antibody response and cellular immune response, and provided mice with complete protection against a lethal MCMV challenge. This study provides the first in vivo evidence that the gC II (gM-gN) complex may be able to serve as a protective subunit antigen for future HCMV vaccine development.

Saturated very long chain fatty acids are required for the production of infectious human cytomegalovirus progeny

Human cytomegalovirus hijacks host cell metabolism, increasing the flux of carbon from glucose to malonyl-CoA, the committed precursor to fatty acid synthesis and elongation. Inhibition of acetyl-CoA carboxylase blocks the production of progeny virus. To probe further the role of fatty acid metabolism during infection, we performed an siRNA screen to identify host cell metabolic enzymes needed for the production of infectious cytomegalovirus progeny. The screen predicted that multiple long chain acyl-CoA synthetases and fatty acid elongases are needed during infection, and the levels of RNAs encoding several of these enzymes were upregulated by the virus. Roles for acyl-CoA synthetases and elongases during infection were confirmed by using small molecule antagonists. Consistent with a role for these enzymes, mass spectrometry-based fatty acid analysis with¹³C-labeling revealed that malonyl-CoA is consumed by elongases to produce very long chain fatty acids, generating an approximately 8-fold increase in C26-C34 fatty acid tails in infected cells. The virion envelope was yet further enriched in C26-C34 saturated fatty acids, and elongase inhibitors caused the production of virions with lower levels of these fatty acids and markedly reduced infectivity. These results reveal a dependence of cytomegalovirus on very long chain fatty acid metabolism.

Herpes viruses modulate cellular pathways to generate the building blocks that are necessary for their replication. Human cytomegalovirus alters metabolism of infected cells and causes a dramatic increase in lipid biosynthesis. We have investigated the role of lipid pathways in the viral life cycle and discovered that the virus requires several host enzymes that are responsible for the synthesis of very long chain fatty acids. Interestingly, very long chain fatty acids are substantially increased in the lipids of infected cells and saturated forms of these fatty acids are selectively incorporated into the envelope of the virus. Drugs that inhibit the synthesis of very long chain fatty acids generate virus particles with reduced infectivity. The discovery that human cytomegalovirus depends on the production of particular fatty acids furthers our understanding of virus-host cell interaction and suggests potential novel strategies for antiviral therapies.

Dynein mediates the localization and activation of mTOR in normal and human cytomegalovirus-infected cells

Activation of stress signaling pathways normally leads to inhibition of the mammalian target of rapamycin complex 1 (mTORC1); however, human cytomegalovirus (HCMV) infection maintains mTORC1 activity in the presence of numerous types of stress. We previously demonstrated that HCMV infection maintains mTORC1 activity during amino acid deprivation through a Ras-related GTP-binding (Rag) protein-independent mechanism. This depends on the colocalization of mTOR and its activator, Rheb (Ras homology enriched in brain)-GTP, to a perinuclear position that corresponds to the viral cytoplasmic assembly compartment (AC). The data presented here show that the HCMV-induced, amino acid depletion-resistant perinuclear localization and activation of mTORC1 occurs as early as 8 h post-infection, prior to AC formation. We show that the molecular motor dynein is required for perinuclear localization of mTORC1 in both uninfected and HCMV-infected cells. Association between dynein and mTOR is shown by coimmunoprecipitation, and inhibition of dynein function using RNAi or the small molecule inhibitor ciliobrevin A inhibits mTORC1 activity in both uninfected and HCMV-infected cells. The data suggest that mTORC1 activation requires dynein-dependent transport to a position in the cell where it can be activated. Thus, the HCMV commandeers a cellular dynein-dependent mTORC1 activation mechanism to maintain stress-resistant mTORC1 activity during infection and to form the AC.

Human herpesvirus 6 glycoprotein M is essential for virus growth and requires glycoprotein N for its maturation

Human herpesvirus 6 (HHV-6) is a T-lymphotropic virus belonging to the betaherpesvirus family. Several HHV-6-encoded glycoproteins are required for cell entry and virion maturation. Glycoprotein M (gM) is conserved among all herpesviruses, and therefore thought to have important functions; however, the HHV-6 g has not been characterized. Here, we examined the expression of HHV-6 g, and examined its function in viral replication, using a mutant and revertant gM. HHV-6 g was expressed on virions as a glycoprotein modified with complex N-linked oligosaccharides. As in other herpesviruses, HHV-6 g formed a complex with glycoprotein N (gN), and was transported from the endoplasmic reticulum to the trans-Golgi network only when part of this complex. Finally, a gM mutant virus in which the gM start codon was destroyed was not reconstituted, although its revertant was, indicating that HHV-6 g is essential for virus production, unlike the gM of alphaherpesviruses.

Synaptic vesicle-like lipidome of human cytomegalovirus virions reveals a role for SNARE machinery in virion egress

Human cytomegalovirus induces and requires fatty acid synthesis. This suggests an essential role for lipidome remodeling in viral replication. We used mass spectrometry to quantify glycerophospholipids in mock-infected and virus-infected fibroblasts, as well as in virions. Although the lipid composition of mock-infected and virus-infected fibroblasts was similar, virions were markedly different. The virion envelope contained twofold more phosphatidylethanolamines and threefold less phosphatidylserines than the host cell. This indicates that the virus buds from a membrane with a different lipid composition from the host cell as a whole. Compared with published datasets, the virion envelope showed the greatest similarity to the synaptic vesicle lipidome. Synaptosome-associated protein of 25 kDa (SNAP-25) is a component of the complex that mediates exocytosis of synaptic vesicles in neurons; and its homolog, SNAP-23, functions in exocytosis in many other cell types. Infection induced the relocation of SNAP-23 to the cytoplasmic viral assembly zone, and knockdown of SNAP-23 inhibited the production of virus. We propose that cytomegalovirus capsids acquire their envelope by budding into vesicles with a lipid composition similar to that of synaptic vesicles, which subsequently fuse with the plasma membrane to release virions from the cell.

Human cytomegalovirus UL97 kinase and nonkinase functions mediate viral cytoplasmic secondary envelopment

Previous studies have revealed critical roles for the human cytomegalovirus (HCMV) UL97 kinase in viral nuclear maturation events. We have shown recently that UL97 affects the morphology of the viral cytoplasmic assembly compartment (AC) (M. Azzeh, A. Honigman, A. Taraboulos, A. Rouvinski, and D. G. Wolf, Virology 354:69-79, 2006). Here, we employed a comprehensive ultrastructural analysis to dissect the impact of UL97 on cytoplasmic steps of HCMV assembly. Using UL97 deletion (ΔUL97) and kinase-null (K355M) mutants, as well as the UL97 kinase inhibitor NGIC-I, we demonstrated that the loss of UL97 kinase activity resulted in a unique combination of cytoplasmic features: (i) the formation of pp65-rich aberrant cytoplasmic tegument aggregates, (ii) distorted intracytoplasmic membranes, which replaced the normal architecture of the AC, and (iv) a paucity of cytoplasmic tegumented capsids and dense bodies (DBs). We further showed that these abnormal assembly intermediates did not result from impaired nuclear capsid maturation and egress per se by using 2-bromo-5,6-dichloro-1-(β-d-ribofuranosyl) benzimidizole (BDCRB) to induce the artificial inhibition of nuclear maturation and the nucleocytoplasmic translocation of capsids. The specific abrogation of UL97 kinase activity under low-multiplicity-of-infection conditions resulted in the improved release of extracellular virus compared to that of ΔUL97, despite similar rates of viral DNA accumulation and similar effects on nuclear capsid maturation and egress. The only ultrastructural correlate of the growth difference was a higher number of cytoplasmic DBs, tegumented capsids, and clustered viral particles observed upon the specific abrogation of UL97 kinase activity compared to that of ΔUL97. These combined findings reveal a novel role for UL97 in HCMV cytoplasmic secondary envelopment steps, with a further distinction of kinase-mediated function in the formation of the virus-induced AC and a nonkinase function enhancing the efficacy of viral tegumentation and release.

Varicella-zoster virus glycoprotein M

Glycoprotein M (gM) is conserved among herpesviruses. Important features are its 6-8 transmembrane domains without a large extracellular domain, localization to the virion envelope, complex formation with another envelope glycoprotein, glycoprotein N (gN), and role in virion assembly and egress. In varicella-zoster virus (VZV), the gM homolog is encoded by ORF50. VZV gM is predicted to be an eight-transmembrane envelope glycoprotein with a complex N-linked oligosaccharide. It mainly localizes to the trans-Golgi network, where final virion envelopment occurs. Studies in which VZV gM or its partner gN were disrupted suggest that the gM/gN complex plays an important role in cell-to-cell spread. Here, we summarize the biological features of VZV gM, including our recent findings on its characterization and function.

Human cytomegalovirus tegument protein pUL71 is required for efficient virion egress

The human cytomegalovirus virion is composed of a DNA genome packaged in an icosahedral capsid, surrounded by a tegument of protein and RNA, all enclosed within a glycoprotein-studded envelope. Achieving this intricate virion architecture requires a coordinated process of assembly and egress. We show here that pUL71, a component of the virion tegument with a previously uncharacterized function, is required for the virus-induced reorganization of host cell membranes, which is necessary for efficient viral assembly and egress. A mutant that did not express pUL71 was able to efficiently accumulate viral genomes and proteins that were tested but was defective for the production and release of infectious virions. The protein localized to vesicular structures at the periphery of the viral assembly compartment, and during infection with a pUL71-deficient virus, these structures were grossly enlarged and aberrantly contained a cellular marker of late endosomes/lysosomes. Mutant virus preparations exhibited less infectivity per unit genome than wild-type virus preparations, due to aggregation of virus particles and their association with membrane fragments. Finally, mutant virus particles accumulated within the cytoplasm of infected cells and were localized to the periphery of large structures with properties of lysosomes, whose formation was kinetically favored in mutant-virus-infected cells. Together, these observations point to a role for pUL71 in the establishment and/or maintenance of a functional viral assembly compartment that is required for normal virion trafficking and egress from infected cells.

In addition to causing disease in immunocompromised individuals, human cytomegalovirus is the leading known infectious cause of birth defects. To induce these pathologies, the virus must spread from its site of introduction to various organs and tissues in the body. The processes of viral assembly and egress, which underlie the spread of infection, are incompletely understood. We elucidate a role for a virus-coded protein, pUL71, in these processes and demonstrate the importance of maintaining an intricate, virus-induced reorganization of host cell membranes for efficient virus spread.

Optimal replication of human cytomegalovirus correlates with endocytosis of glycoprotein gpUL132

Envelopment of a herpesvirus particle is a complex process of which much is still to be learned. We previously identified the glycoprotein gpUL132 of human cytomegalovirus (HCMV) as an envelope component of the virion. In its carboxy-terminal portion, gpUL132 contains at least four motifs for sorting of transmembrane proteins to endosomes; among them are one dileucine-based signal and three tyrosine-based signals of the YXXØ and NPXY (where X stands for any amino acid, and Ø stands for any bulky hydrophobic amino acid) types. To investigate the role of each of these trafficking signals in intracellular localization and viral replication, we constructed a panel of expression plasmids and recombinant viruses in which the signals were rendered nonfunctional by mutagenesis. In transfected cells wild-type gpUL132 was mainly associated with the trans-Golgi network. Consecutive mutation of the trafficking signals resulted in increasing fractions of the protein localized at the cell surface, with gpUL132 mutated in all four trafficking motifs predominantly associated with the plasma membrane. Concomitant with increased surface expression, endocytosis of mutant gpUL132 was reduced, with a gpUL132 expressing all four motifs in mutated form being almost completely impaired in endocytosis. The replication of recombinant viruses harboring mutations in single trafficking motifs was comparable to replication of wild-type virus. In contrast, viruses containing mutations in three or four of the trafficking signals showed pronounced deficits in replication with a reduction of approximately 100-fold. Moreover, recombinant viruses expressing gpUL132 with three or four trafficking motifs mutated failed to incorporate the mutant protein into the virus particle. These results demonstrate a role of endocytosis of an HCMV envelope glycoprotein for incorporation into the virion and optimal virus replication.

HCMV-encoded glycoprotein M (UL100) interacts with Rab11 effector protein FIP4

The envelope of human cytomegalovirus (HCMV) consists of a large number of glycoproteins. The most abundant glycoprotein in the HCMV envelope is the glycoprotein M (UL100), which together with glycoprotein N (UL73) form the gM/gN protein complex. Using yeast two-hybrid screening, we found that the gM carboxy-terminal cytoplasmic tail (gM-CT) interacts with FIP4, a Rab11-GTPase effector protein. Depletion of FIP4 expression in HCMV-infected cells resulted in a decrease in infectious virus production that was also associated with an alteration of the HCMV assembly compartment (AC) phenotype. A similar phenotype was also observed in HCMV-infected cells that expressed dominant negative Rab11(S25N). Recently, it has been shown that FIP4 interactions with Rab11 and additionally with Arf6/Arf5 are important for the vesicular transport of proteins in the endosomal recycling compartment (ERC) and during cytokinesis. Surprisingly, FIP4 interaction with gM-CT limited binding of FIP4 with Arf5/Arf6; however, FIP4 interaction with gM-CT did not prevent recruitment of Rab11 into the ternary complex. These data argued for a contribution of the ERC during cytoplasmic envelopment of HCMV and showed a novel FIP4 function independent of Arf5 or Arf6 activity.

Characterization of the varicella-zoster virus ORF50 gene, which encodes glycoprotein M

The ORF50 gene of the varicella-zoster virus (VZV) encodes glycoprotein M (gM), which is conserved among all herpesviruses and is important for the cell-to-cell spread of VZV. However, few analyses of ORF50 gene expression or its posttranscriptional and translational modifications have been published. Here we found that in VZV-infected cells, ORF50 encoded four transcripts: a full-size transcript, which was translated into the gM, and three alternatively spliced transcripts, which were not translated. Using a splicing-negative mutant virus, we showed that the alternative transcripts were nonessential for viral growth in cell culture. In addition, we found that two amino acid mutations of gM, V42P and G301M, blocked gM's maturation and transport to the trans-Golgi network, which is generally recognized as the viral assembly complex. We also found that the mutations disrupted gM's interaction with glycoprotein N (gN), revealing their interaction through a bond that is otherwise unreported for herpesviruses. Using this gM maturation-negative virus, we found that immature gM and gN were incorporated into intracellularly isolated virus particles and that mature gM was required for efficient viral growth via cell-to-cell spread but not for virion morphogenesis. The virus particles were more abundant at the abnormally enlarged perinuclear cisternae than those of the parental virus, but they were also found at the cell surface and in the culture medium. Additionally, in the gM maturation-negative mutant virus-infected melanoma cells, typical syncytium formation was rarely seen, again indicating that mature gM functions in cell-to-cell spread via enhancement of syncytium formation.

Bicaudal D1-dependent trafficking of human cytomegalovirus tegument protein pp150 in virus-infected cells

Human cytomegalovirus (HCMV) virion assembly takes place in the nucleus and cytoplasm of infected cells. The HCMV virion tegument protein pp150 (ppUL32) is an essential protein of HCMV and has been suggested to play a role in the cytoplasmic phase of HCMV assembly. To further define its role in viral assembly and to identify host cell proteins that interact with pp150 during viral assembly, we utilized yeast two-hybrid analyses to detect an interaction between pp150 and Bicaudal D1 (BicD1), a protein thought to play a role in trafficking within the secretory pathway. BicD1 is known to interact with the dynein motor complex and the Rab6 GTPase. The interaction between pp150 and BicD1 was confirmed by coimmunoprecipitation and fluorescence resonance energy transfer. Depletion of BicD1 with short hairpin RNA (shRNA) caused decreased virus yield and a defect in trafficking of pp150 to the cytoplasmic viral assembly compartment (AC), without altering trafficking to the AC of another essential tegument protein, pp28, or the viral glycoprotein complex gM/gN. The C terminus of BicD1 has been previously shown to interact with the GTPase Rab6, suggesting a potential role for Rab6-mediated vesicular trafficking in HCMV assembly. Finally, overexpression of the N terminus of truncated BicD1 acts in a dominant-negative manner and leads to disruption of the AC and a decrease in the assembly of infectious virus. This phenotype was similar to that observed following overexpression of dynamitin (p50) and provided additional evidence that morphogenesis of the AC and virus assembly were dynein dependent.

Human cytomegalovirus final envelopment on membranes containing both trans-Golgi network and endosomal markers

The human cytomegalovirus (HCMV) has been shown to complete its final envelopment on cytoplasmic membranes prior to its secretion to the extracellular medium. However, the nature of these membranes has not been characterized. It is thought that HCMV acquires its final envelope from the trans-Golgi network (TGN), though we and others have previously reported a role for endocytic membranes. Here we studied the localization of cellular markers in HCMV-infected cells and in isolated viruses. Immunofluorescence staining indicated that HCMV induces the recruitment of TGN and endosomal markers to the virus factory. Immuno-gold labelling of isolated viral particles and electron microscopy demonstrated the incorporation of TGN46, endosomal markers early endosomal antigen 1, annexin I, transferrin receptor and CD63, and the cation-independent mannose 6-phosphate receptor, which traffics between the TGN and endosomes into the viral envelope. Virus immunoprecipitation assays demonstrated that virions containing TGN46 and CD63 were infectious. This study reconciles the apparent controversy regarding the nature of the HCMV assembly site and suggests that HCMV has the ability to generate a novel membrane compartment containing markers for both TGN and endosomes, or that the membranes that HCMV uses for its envelope may be vesicles in transit between the TGN and endosomes.

Human cytomegalovirus exploits ESCRT machinery in the process of virion maturation

The endosomal sorting complex required for transport (ESCRT) machinery controls the incorporation of cargo into intraluminal vesicles of multivesicular bodies. This machinery is used during envelopment of many RNA viruses and some DNA viruses, including herpes simplex virus type 1. Other viruses mature independent of ESCRT components, instead relying on the intrinsic behavior of viral matrix and envelope proteins to drive envelopment. Human cytomegalovirus (HCMV) maturation has been reported to proceed independent of ESCRT components (A. Fraile-Ramos et al. Cell. Microbiol. 9:2955-2967, 2007). A virus complementation assay was used to evaluate the role of dominant-negative (DN) form of a key ESCRT ATPase, vacuolar protein sorting-4 (Vps4DN) in HCMV replication. Vps4DN specifically inhibited viral replication, whereas wild-type-Vps4 had no effect. In addition, a DN form of charged multivesicular body protein 1 (CHMP1DN) was found to inhibit HCMV. In contrast, DN tumor susceptibility gene-101 (Tsg101DN) did not impact viral replication despite the presence of a PTAP motif within pp150/ppUL32, an essential tegument protein involved in the last steps of viral maturation and release. Either Vps4DN or CHMP1DN blocked viral replication at a step after the accumulation of late viral proteins, suggesting that both are involved in maturation. Both Vps4A and CHMP1A localized in the vicinity of viral cytoplasmic assembly compartments, sites of viral maturation that develop in CMV-infected cells. Thus, ESCRT machinery is involved in the final steps of HCMV replication.

Control of cytoplasmic maturation events by cytomegalovirus tegument protein pp150

Cytomegalovirus replication depends upon a betaherpesvirus-conserved 150-kDa tegument phosphoprotein (pp150; encoded by UL32) that supports the final steps in virion maturation at cytoplasmic assembly compartments. Amino acid substitutions were introduced into conserved region 1 (CR1) and CR2 of pp150, affecting a region that may interact with nucleocapsids. Two independent CR2 point mutants (N201A and G207A) failed to support viral replication in evaluations by a transient complementation assay or after reconstruction into recombinant viruses. An assembly compartment-like cytoplasmic inclusion developed in UL32 mutant virus-infected cells that was similar to that of wild-type virus-infected cells. The cellular localization of the trans-Golgi marker Golgin-97 suggested differences in the organization of the assembly compartment compared to that of wild-type virus-infected cells. Replication-defective CR2 point mutants exhibited the same phenotype as that of a virus carrying a complete deletion of the UL32 open reading frame in these assays. Electron micrographs of fibroblasts at 3 or 5 days postinfection with a deletion mutant (DeltaUL32) grown on UL32-complementing cells showed a similar number and morphology of capsids in the nucleus, but the cytoplasmic region associated with virion assembly appeared highly vesiculated and contained few recognizable nucleocapsids or complete virus particles. These data demonstrate that the principle role of pp150 is to retain nucleocapsid organization through secondary envelopment at the assembly compartment.

An essential role for the proximal but not the distal cytoplasmic tail of glycoprotein M in murid herpesvirus 4 infection

Murid herpesvirus-4 (MuHV-4) provides a tractable model with which to define common, conserved features of gamma-herpesvirus biology. The multi-membrane spanning glycoprotein M (gM) is one of only 4 glycoproteins that are essential for MuHV-4 lytic replication. gM binds to gN and is thought to function mainly secondary envelopment and virion egress, for which several predicted trafficking motifs in its C-terminal cytoplasmic tail could be important. We tested the contribution of the gM cytoplasmic tail to MuHV-4 lytic replication by making recombinant viruses with varying C-terminal deletions. Removing an acidic cluster and a distal YXXΦ motif altered the capsid distribution somewhat in infected cells but had little effect on virus replication, either in vitro or in vivo. In contrast, removing a proximal YXXΦ motif as well completely prevented productive replication. gM was still expressed, but unlike its longer forms showed only limited colocalization with co-transfected gN, and in the context of whole virus appeared to support gN expression less well. We conclude that some elements of the gM cytoplasmic tail are dispensible for MuHV-4 replication, but the tail as a whole is not.

Nuclear trafficking of the human cytomegalovirus pp71 (ppUL82) tegument protein

The human cytomegalovirus tegument protein pp71 localizes to the nucleus immediately upon infection, and functions to initiate viral gene expression. Analysis of a series of random insertion mutations revealed that sequences within the mid region (MR) of pp71 are important for localization to the nucleus. Fusion of MR sequences with eGFP revealed that amino acids 94 to 300 were sufficient to target proteins to the nucleus. Random substitution mutagenesis within this domain resulted in two double substitution mutants, pp71P203T/T223M and pp71T228M/L275Q, with a predominantly cytoplasmic localization. Disruption of nuclear targeting resulted in relocalization of the fusion proteins to a distinct perinuclear region. Using tandem mass spectrometry, we determined that threonine 223 can be phosphorylated. Mutation of this residue to a phosphomimetic amino acid resulted in abrogation of nuclear targeting. These results strongly suggest that the intracellular trafficking of pp71 is regulated by phosphorylation.

Multimerization of tegument protein pp28 within the assembly compartment is required for cytoplasmic envelopment of human cytomegalovirus

Human cytomegalovirus (HCMV) UL99-encoded pp28 is an essential tegument protein required for envelopment and production of infectious virus. Nonenveloped virions accumulate in the cytoplasm of cells infected with recombinant viruses with the UL99 gene deleted. Previous results have suggested that a key function of pp28 in the envelopment of infectious HCMV is expressed after the protein localizes in the assembly compartment (AC). In this study, we investigated the potential role of pp28 multimerization in the envelopment of the infectious virion. Our results indicated that pp28 multimerized during viral infection and that interacting domains responsible for self-interaction were localized in the amino terminus of the protein (amino acids [aa] 1 to 43). The results from transient-expression and/or infection assays indicated that the self-interaction took place in the AC. A mutant pp28 molecule containing only the first 35 aa failed to accumulate in the AC, did not interact with pp28 in the AC, and could not support virus replication. In contrast, the first 50 aa of pp28 was sufficient for the self-interaction within the AC and the assembly of infectious virus. Recombinant viruses encoding an in-frame deletion of aa 26 to 33 of pp28 were replication competent, whereas infectious virus was not recovered from HCMV BACs lacking aa 26 to 43. These findings suggested that the accumulation of pp28 was a prerequisite for multimerization of pp28 within the AC and that pp28 multimerization in the AC represented an essential step in the envelopment and production of infectious virions.