Role of PACS-1 in trafficking of human cytomegalovirus glycoprotein B and virus production

Functions of the UL51 protein during the herpesvirus life cycle

The herpesvirus UL51 protein is a multifunctional tegument protein involved in the regulation of multiple aspects of the viral life cycle. This article reviews the biological characteristics of the UL51 protein and its functions in herpesviruses, including participating in the maintenance of the viral assembly complex (cVAC) during viral assembly, affecting the production of mature viral particles and promoting primary and secondary envelopment, as well as its positive impact on viral cell-to-cell spread (CCS) through interactions with multiple viral proteins and its key role in the proliferation and pathogenicity of the virus in the later stage of infection. This paper discusses how the UL51 protein participates in the life cycle of herpesviruses and provides new ideas for further research on UL51 protein function.

The role of microRNA-485 in neurodegenerative diseases

Neurodegenerative diseases (NDDs) are age-related disorders characterized by progressive neurodegeneration and neuronal cell loss in the central nervous system. Neuropathological conditions such as the accumulation of misfolded proteins can cause neuroinflammation, apoptosis, and synaptic dysfunction in the brain, leading to the development of NDDs including Alzheimer's disease (AD) and Parkinson's disease (PD). MicroRNAs (miRNAs) are small noncoding RNA molecules that regulate gene expression post-transcriptionally via RNA interference. Recently, some studies have reported that some miRNAs play an important role in the development of NDDs by regulating target gene expression. MiRNA-485 (miR-485) is a highly conserved brain-enriched miRNA. Accumulating clinical reports suggest that dysregulated miR-485 may be involved in the pathogenesis of AD and PD. Emerging studies have also shown that miR-485 plays a novel role in the regulation of neuroinflammation, apoptosis, and synaptic function in the pathogenesis of NDDs. In this review, we introduce the biological characteristics of miR-485, provide clinical evidence of the dysregulated miR-485 in NDDs, novel roles of miR-485 in neuropathological events, and discuss the potential of targeting miR-485 as a diagnostic and therapeutic marker for NDDs.

Virus-host protein interactions as footprints of human cytomegalovirus replication

Human cytomegalovirus (HCMV) is a pervasive β-herpesvirus that causes lifelong infection. The lytic replication cycle of HCMV is characterized by global organelle remodeling and dynamic virus-host interactions, both of which are necessary for productive HCMV replication. With the advent of new technologies for investigating protein-protein and protein-nucleic acid interactions, numerous critical interfaces between HCMV and host cells have been identified. Here, we review temporal and spatial virus-host interactions that support different stages of the HCMV replication cycle. Understanding how HCMV interacts with host cells during entry, replication, and assembly, as well as how it interfaces with host cell metabolism and immune responses promises to illuminate processes that underlie the biology of infection and the resulting pathologies.

PACS-1 contains distinct motifs for nuclear-cytoplasmic transport and interacts with the RNA-binding protein PTBP1 in the nucleus and cytosol

Phosphofurin acidic cluster sorting protein 1 (PACS-1) is canonically a cytosolic trafficking protein, yet recent reports have described nuclear roles for PACS-1. Herein, we sought to define the nuclear transport mechanism of PACS-1. We demonstrate that PACS-1 nucleocytoplasmic trafficking is dependent on its interaction with the nuclear transport receptors importin alpha 5 and exportin 1. PACS-1 nuclear entry and exit are defined by a nuclear localization signal (NLS, residues 311-318) and nuclear export signal (NES3, residues 366-375). Mutation of the PACS-1 NLS and NES3 altered the localization of a complex formed between PACS-1 and an RNA-binding protein, polypyrimidine tract-binding protein 1. Overall, we identify the nuclear localization mechanism of PACS-1 and highlight a potential role for PACS-1 in RNA-binding protein trafficking.

Human cytomegalovirus UL42 protein inhibits the degradation of glycoprotein B through inhibition of Nedd4 family ubiquitin E3 ligases

Human cytomegalovirus (HCMV) is a globally ubiquitous pathogen and causes congenital infection as well as opportunistic infection in immunocompromised patients. The HCMV UL42 gene encodes a membrane protein that regulates the function of Nedd4 family ubiquitin E3 ligases through its PPxY motif. As HCMV envelope glycoprotein B (gB) also has a PPxY motif at its C-terminal cytoplasmic domain, we examined whether there was any relationship between UL42 protein with gB. Among the Nedd4 family proteins, Nedd4, Nedd4L, and Itch induced the degradation of gB in transiently expressing cells. The degradation of gB by Nedd4 was inhibited by proteasome inhibitor MG132, lysosome inhibitor chloroquine, and the co-expression of UL42 proteins. Among those Nedd4 family proteins, Itch was re-localized by the co-expression of gB to the perinuclear region of the cytoplasm. A co-immunoprecipitation assay demonstrated an interaction between gB and Itch through its PPxY motif. The 150 kDa gB precursor was aberrantly ubiquitinated, and the total amount of gB was quickly decreased in the absence of UL42. Our results indicate that UL42 prevent the degradation of gB by the inhibition of Nedd4 family proteins. This article is protected by copyright. All rights reserved.

Structure of Epstein-Barr virus tegument protein complex BBRF2-BSRF1 reveals its potential role in viral envelopment

Epstein-Barr virus (EBV) is a γ-herpesvirus associated with the occurrence of several human malignancies. BBRF2 and BSRF1 are two EBV tegument proteins that have been suggested to form a hetero-complex and mediate viral envelopment, but the molecular basis of their interaction and the functional mechanism of this complex remains unknown. Here, we present crystal structures of BBRF2 alone and in complex with BSRF1. BBRF2 has a compact globular architecture featuring a central β-sheet that is surrounded by 10 helices, it represents a novel fold distinct from other known protein structures. The central portion of BSRF1 folds into two tightly associated antiparallel α-helices, forming a composite four-helix bundle with two α-helices from BBRF2 via a massive hydrophobic network. In vitro, a BSRF1-derived peptide binds to BBRF2 and reduces the number of viral genome copies in EBV-positive cells. Exogenous BBRF2 and BSRF1 co-localize at the Golgi apparatus. Furthermore, BBRF2 binds capsid and capsid-associated proteins, whereas BSRF1 associates with glycoproteins. These findings indicate that the BBRF2-BSRF1 complex tethers EBV nucleocapsids to the glycoprotein-enriched Golgi membrane, facilitating secondary envelopment.

The multifunctional protein PACS-1 is required for HDAC2- and HDAC3-dependent chromatin maturation and genomic stability

Phosphofurin acidic cluster sorting protein-1 (PACS-1) is a multifunctional membrane traffic regulator that plays important roles in organ homeostasis and disease. In this study, we elucidate a novel nuclear function for PACS-1 in maintaining chromosomal integrity. PACS-1 progressively accumulates in the nucleus during cell cycle progression, where it interacts with class I histone deacetylases 2 and 3 (HDAC2 and HDAC3) to regulate chromatin dynamics by maintaining the acetylation status of histones. PACS-1 knockdown results in the proteasome-mediated degradation of HDAC2 and HDAC3, compromised chromatin maturation, as indicated by elevated levels of histones H3K9 and H4K16 acetylation, and, consequently, increased replication stress-induced DNA damage and genomic instability.

Interaction of Human Cytomegalovirus Tegument Proteins ppUL35 and ppUL35A with Sorting Nexin 5 Regulates Glycoprotein B (gpUL55) Localization

Human cytomegalovirus (HCMV) is a widespread human pathogen that causes asymptomatic infection in healthy individuals but poses a serious threat to immunocompromised patients. During the late phase of HCMV infection, the viral capsid is transported to the cytoplasmic viral assembly center (cVAC), where it is enclosed by the tegument protein layer and the viral envelope. The cVAC consists of circularly arranged vesicles from the trans-Golgi and endosomal networks. The HCMV gene UL35 encodes ppUL35 and its shorter form, ppUL35A. We have previously shown that the UL35 gene is involved in HCMV assembly, but it is unknown how UL35 proteins regulate viral assembly. Here we show that sorting nexin 5 (SNX5), a component of the retromer and part of the retrograde transport pathway, interacts with UL35 proteins. Expression of wild-type proteins but not mutants defective in SNX5 binding resulted in the cellular redistribution of the cation-independent mannose-6-phosphate receptor (CI-M6PR), indicating that UL35 proteins bind and negatively regulate SNX5 to modulate cellular transport pathways. Furthermore, binding of UL35 proteins to SNX5 was required for efficient viral replication and for transport of the most abundant HCMV glycoprotein B (gB; gpUL55) to the cVAC. These results indicate that ppUL35 and ppUL35A control the localization of the essential gB through the regulation of a retrograde transport pathway. Thus, this work is the first to define a molecular interaction between a tegument protein and a vesicular transport factor to regulate glycoprotein localization.IMPORTANCE Human cytomegalovirus is ubiquitously present in the healthy population, but reactivation or reinfection can cause serious, life-threatening infections in immunocompromised patients. For completion of its lytic cycle, human cytomegalovirus induces formation of an assembly center where mature virus particles are formed from multiple viral proteins. Viral glycoproteins use separate vesicular pathways for transport to the assembly center, which are incompletely understood. Our research identified a viral structural protein which affects the localization of one of the major glycoproteins. We could link this change in glycoprotein localization to an interaction of the structural protein with a cellular protein involved in regulation of vesicle transport. This increases our understanding of how the virus intersects into cellular regulatory pathways to enhance its own replication.

A Tyrosine-Based Trafficking Motif of the Tegument Protein pUL71 Is Crucial for Human Cytomegalovirus Secondary Envelopment

The human cytomegalovirus (HCMV) tegument protein pUL71 is required for efficient secondary envelopment and accumulates at the Golgi compartment-derived viral assembly complex (vAC) during infection. Analysis of various C-terminally truncated pUL71 proteins fused to enhanced green fluorescent protein (eGFP) identified amino acids 23 to 34 as important determinants for its Golgi complex localization. Sequence analysis and mutational verification revealed the presence of an N-terminal tyrosine-based trafficking motif (YXXΦ) in pUL71. This led us to hypothesize a requirement of the YXXΦ motif for the function of pUL71 in infection. Mutation of both the tyrosine residue and the entire YXXΦ motif resulted in an altered distribution of mutant pUL71 at the plasma membrane and in the cytoplasm during infection. Both YXXΦ mutant viruses exhibited similarly decreased focal growth and reduced virus yields in supernatants. Ultrastructurally, mutant-virus-infected cells exhibited impaired secondary envelopment manifested by accumulations of capsids undergoing an envelopment process. Additionally, clusters of capsid accumulations surrounding the vAC were observed, similar to the ultrastructural phenotype of a UL71-deficient mutant. The importance of endocytosis and thus the YXXΦ motif for targeting pUL71 to the Golgi complex was further demonstrated when clathrin-mediated endocytosis was inhibited either by coexpression of the C-terminal part of cellular AP180 (AP180-C) or by treatment with methyl-β-cyclodextrin. Both conditions resulted in a plasma membrane accumulation of pUL71. Altogether, these data reveal the presence of a functional N-terminal endocytosis motif that is an important determinant for intracellular localization of pUL71 and that is furthermore required for the function of pUL71 during secondary envelopment of HCMV capsids at the vAC.IMPORTANCE Human cytomegalovirus (HCMV) is the leading cause of birth defects among congenital virus infections and can lead to life-threatening infections in immunocompromised hosts. Current antiviral treatments target viral genome replication and are increasingly overcome by viral mutations. Therefore, identifying new targets for antiviral therapy is important for future development of novel treatment options. A detailed molecular understanding of the complex virus morphogenesis will identify potential viral as well as cellular targets for antiviral intervention. Secondary envelopment is an important viral process through which infectious virus particles are generated and which involves the action of several viral proteins, such as tegument protein pUL71. Targeting of pUL71 to the site of secondary envelopment appears to be crucial for its function during this process and is regulated by utilizing host trafficking mechanisms that are commonly exploited by viral glycoproteins. Thus, intracellular trafficking, if targeted, might present a novel target for antiviral therapy.

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.

Caught in the act - protein adaptation and the expanding roles of the PACS proteins in tissue homeostasis and disease

Vertebrate proteins that fulfill multiple and seemingly disparate functions are increasingly recognized as vital solutions to maintaining homeostasis in the face of the complex cell and tissue physiology of higher metazoans. However, the molecular adaptations that underpin this increased functionality remain elusive. In this Commentary, we review the PACS proteins - which first appeared in lower metazoans as protein traffic modulators and evolved in vertebrates to integrate cytoplasmic protein traffic and interorganellar communication with nuclear gene expression - as examples of protein adaptation 'caught in the act'. Vertebrate PACS-1 and PACS-2 increased their functional density and roles as metabolic switches by acquiring phosphorylation sites and nuclear trafficking signals within disordered regions of the proteins. These findings illustrate one mechanism by which vertebrates accommodate their complex cell physiology with a limited set of proteins. We will also highlight how pathogenic viruses exploit the PACS sorting pathways as well as recent studies on PACS genes with mutations or altered expression that result in diverse diseases. These discoveries suggest that investigation of the evolving PACS protein family provides a rich opportunity for insight into vertebrate cell and organ homeostasis.

The C-Terminus of Epstein-Barr Virus BRRF2 Is Required for its Proper Localization and Efficient Virus Production

Epstein-Barr virus (EBV) is a human gammaherpesvirus associated with several malignancies. We reported previously that an EBV lytic gene product BRRF2 is involved in the maturation of progeny virus. To analyze the domain(s) needed for efficient production of progeny, we prepared a series of deletion mutants and found two functional domains in the N- and C-terminal regions by complementation assays. Immunofluorescence analyses revealed that BRRF2 lacking the C-terminal region demonstrated aberrant localization in both the nucleus and cytoplasm, whereas wild-type BRRF2 was localized predominantly in the cytoplasm. We also confirmed that wild-type BRRF2 co-localized with Rab5, an endosomal marker, at least partly. Additionally, serine 511 of BRRF2 was phosphorylated during lytic infection; however, a mutant in which the serine was substituted with alanine still augmented the yield as efficiently as did wild-type BRRF2. These results showed that the C-terminal region of BRRF2 is involved in the predominant localization of BRRF2 to the cytoplasm and in the efficient production of infectious virus.

Epigenetic control of mitochondrial cell death through PACS1-mediated regulation of BAX/BAK oligomerization

PCAF and ADA3 associate within the same macromolecular complexes to control the transcription of many genes, including some that regulate apoptosis. Here we show that PCAF and ADA3 regulate the expression of PACS1, whose protein product is a key component of the machinery that sorts proteins among the trans-Golgi network and the endosomal compartment. We describe a novel role for PACS1 as a regulator of the intrinsic pathway of apoptosis and mitochondrial outer membrane permeabilization. Cells with decreased PACS1 expression were refractory to cell death mediated by a variety of stimuli that operate through the mitochondrial pathway, including human granzyme B, staurosporine, ultraviolet radiation and etoposide, but remained sensitive to TRAIL receptor ligation. The mitochondria of protected cells failed to release cytochrome c as a result of perturbed oligomerization of BAX and BAK. We conclude that PCAF and ADA3 transcriptionally regulate PACS1 and that PACS1 is a key regulator of BAX/BAK oligomerization and the intrinsic (mitochondrial) pathway to apoptosis.

Synthetic DNA approach to cytomegalovirus vaccine/immune therapy

There is no licensed vaccine or cure for human cytomegalovirus (CMV), a ubiquitous β-herpes virus that infects 60-95 % of adults worldwide. Infection is a major cause of congenital abnormalities in newborns, contributes to development of childhood cerebral palsy and medulloblastoma, can result in severe disease in immunocompromised patients, and is a major impediment during successful organ transplantation. While CMV has been increasingly associated with numerous inflammatory diseases and cancers, only recently has it been correlated with increased risk of heart disease in adults, the number-one killer in the USA. These data, among others, suggest that subclinical CMV infection, or microinfection, in healthy individuals may play more of a causative role than an epiphenomenon in development of CMV-associated pathologies. Due to the myriad of diseases and complications associated with CMV, an efficacious vaccine would be highly valuable in reducing human morbidity and mortality as well as saving billions of dollars in annual health-care costs and disability adjusted life years (DALY) in the developing world. Therefore, the development of a safe efficacious CMV vaccine or immune therapy is paramount to the public health. This review aims to provide a brief overview on aspects of CMV infection and disease and focuses on current vaccine strategies. The use of new synthetic DNA vaccines might offer one such approach to this difficult problem.

Superresolution imaging of human cytomegalovirus vMIA localization in sub-mitochondrial compartments

The human cytomegalovirus (HCMV) viral mitochondria-localized inhibitor of apoptosis (vMIA) protein, traffics to mitochondria-associated membranes (MAM), where the endoplasmic reticulum (ER) contacts the outer mitochondrial membrane (OMM). vMIA association with the MAM has not been visualized by imaging. Here, we have visualized this by using a combination of confocal and superresolution imaging. Deconvolution of confocal microscopy images shows vMIA localizes away from mitochondrial matrix at the Mitochondria-ER interface. By gated stimulated emission depletion (GSTED) imaging, we show that along this interface vMIA is distributed in clusters. Through multicolor, multifocal structured illumination microscopy (MSIM), we find vMIA clusters localize away from MitoTracker Red, indicating its OMM localization. GSTED and MSIM imaging show vMIA exists in clusters of ~100–150 nm, which is consistent with the cluster size determined by Photoactivated Localization Microscopy (PALM). With these diverse superresolution approaches, we have imaged the clustered distribution of vMIA at the OMM adjacent to the ER. Our findings directly compare the relative advantages of each of these superresolution imaging modalities for imaging components of the MAM and sub-mitochondrial compartments. These studies establish the ability of superresolution imaging to provide valuable insight into viral protein location, particularly in the sub-mitochondrial compartments, and into their clustered organization.

Herpesviruses exploit several host compartments for envelopment

Enveloped viruses acquire their host-derived membrane at a variety of intracellular locations. Herpesviruses are complex entities that undergo several budding and fusion events during an infection. All members of this large family are believed to share a similar life cycle. However, they seemingly differ in terms of acquisition of their mature envelope. Herpes simplex virus is often believed to bud into an existing intracellular compartment, while the related cytomegalovirus may acquire its final envelope from a novel virus-induced assembly compartment. This review focuses on recent advances in the characterization of cellular compartment(s) potentially contributing to herpes virion final envelopment. It also examines the common points between seemingly distinct envelopment pathways and highlights the dynamic nature of intracellular compartments in the context of herpesvirus infections.

Viral and host control of cytomegalovirus maturation

Maturation in herpesviruses initiates in the nucleus of the infected cell, with encapsidation of viral DNA to form nucleocapsids, and concludes with envelopment in the cytoplasm to form infectious virions that egress the cell. The entire process of virus maturation is orchestrated by protein-protein interactions and enzymatic activities of viral and host origin. Viral tegument proteins play important roles in maintaining the structural stability of capsids and directing the acquisition of virus envelope. Envelopment occurs at modified host membranes and exploits host vesicular trafficking. In this review, we summarize current knowledge of and concepts in human cytomegalovirus (HCMV) maturation and their parallels in other herpesviruses, with an emphasis on viral and host factors that regulate this process.

Characterization and intracellular trafficking of Epstein-Barr virus BBLF1, a protein involved in virion maturation

Epstein-Barr virus (EBV) BBLF1 shares 13 to 15% amino acid sequence identities with the herpes simplex virus 1 UL11 and cytomegalovirus UL99 tegument proteins, which are involved in the final envelopment during viral maturation. This study demonstrates that BBLF1 is a myristoylated and palmitoylated protein, as are UL11 and UL99. Myristoylation of BBLF1 both facilitates its membrane anchoring and stabilizes it. BBLF1 is shown to localize to the trans-Golgi network (TGN) along with gp350/220, a site where final envelopment of EBV particles takes place. The localization of BBLF1 at the TGN requires myristoylation and two acidic clusters, which interact with PACS-1, a cytosolic protein, to mediate retrograde transport from the endosomes to the TGN. Knockdown of the expression of BBLF1 during EBV lytic replication reduces the production of virus particles, demonstrating the requirement of BBLF1 to achieve optimal production of virus particles. BBLF1 is hypothesized to facilitate the budding of tegumented capsid into glycoprotein-embedded membrane during viral maturation.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) protein-induced lysosomal translocation of proapoptotic effectors is mediated by phosphofurin acidic cluster sorting protein-2 (PACS-2)

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis of liver cancer cell lines requires death receptor-5 (DR5)-dependent permeabilization of lysosomal membranes. Ligated DR5 triggers recruitment of the proapoptotic proteins Bim and Bax to lysosomes, releasing cathepsin B into the cytosol where it mediates mitochondria membrane permeabilization and activation of executioner caspases. Despite the requirement for lysosome membrane permeabilization during TRAIL-induced apoptosis, little is known about the mechanism that controls recruitment of Bim and Bax to lysosomal membranes. Here we report that TRAIL induces recruitment of the multifunctional sorting protein phosphofurin acidic cluster sorting protein-2 (PACS-2) to DR5-positive endosomes in Huh-7 cells where it forms an immunoprecipitatable complex with Bim and Bax on lysosomal membranes. shRNA-targeted knockdown of PACS-2 prevents recruitment of Bim or Bax to lysosomes, blunting the TRAIL-induced lysosome membrane permeabilization. Consistent with the reduced lysosome membrane permeabilization, shRNA knockdown of PACS-2 in Huh-7 cells reduced TRAIL-induced apoptosis and increased clonogenic cell survival. The determination that recombinant PACS-2 bound Bim but not Bax in vitro and that shRNA knockdown of Bim blocked Bax recruitment to lysosomes suggests that TRAIL/DR5 triggers endosomal PACS-2 to recruit Bim and Bax to lysosomes to release cathepsin B and induce apoptosis. Together, these findings provide insight into the lysosomal pathway of apoptosis.

An interdomain binding site on HIV-1 Nef interacts with PACS-1 and PACS-2 on endosomes to down-regulate MHC-I

HIV-1 Nef pirates PACS-1 and PACS-2 to downregulate MHC-I, but little is known about the Nef–PACS interaction. The sites on Nef and the PACS proteins required for their interaction are identified, and their importance for Nef trafficking and Nef-induced MHC-I downregulation is discussed. The results provide insight into the molecular basis of Nef action.

The human immunodeficiency virus type 1 (HIV-1) accessory protein Nef directs virus escape from immune surveillance by subverting host cell intracellular signaling and membrane traffic to down-regulate cell-surface major histocompatibility complex class I (MHC-I). The interaction of Nef with the sorting proteins PACS-1 and PACS-2 mediates key signaling and trafficking steps required for Nef-mediated MHC-I down-regulation. Little is known, however, about the molecular basis underlying the Nef–PACS interaction. Here we identify the sites on Nef and the PACS proteins required for their interaction and describe the consequences of disrupting this interaction for Nef action. A previously unidentified cargo subsite on PACS-1 and PACS-2 interacted with a bipartite site on Nef formed by the EEEE₆₅ acidic cluster on the N-terminal domain and W₁₁₃ in the core domain. Mutation of these sites prevented the interaction between Nef and the PACS proteins on Rab5 (PACS-2 and PACS-1)- or Rab7 (PACS-1)-positive endosomes as determined by bimolecular fluorescence complementation and caused a Nef mutant defective in PACS binding to localize to distorted endosomal compartments. Consequently, disruption of the Nef–PACS interaction repressed Nef-induced MHC-I down-regulation in peripheral blood mononuclear cells. Our results provide insight into the molecular basis of Nef action and suggest new strategies to combat HIV-1.

Herpesviruses remodel host membranes for virus egress

Herpesviruses replicate their DNA and package this DNA into capsids in the nucleus. These capsids then face substantial obstacles to their release from cells. Unlike other DNA viruses, herpesviruses do not depend on disruption of nuclear and cytoplasmic membranes for their release. Enveloped particles are formed by budding through inner nuclear membranes, and then these perinuclear enveloped particles fuse with outer nuclear membranes. Unenveloped capsids in the cytoplasm are decorated with tegument proteins and then undergo secondary envelopment by budding into trans-Golgi network membranes, producing infectious particles that are released. In this Review, we describe the remodelling of host membranes that facilitates herpesvirus egress.

A role for the SNARE protein syntaxin 3 in human cytomegalovirus morphogenesis

As an enveloped virus, replication of human cytomegalovirus (HCMV) is dependent on interaction with cellular membrane systems. Its final envelopment occurs into intracellular membranes prior to its secretion. However the mechanisms underlying these processes are poorly understood. Here, we show that HCMV infection induces expression of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) syntaxin 3 (STX3), a component of the cellular machinery for membrane fusion. STX3 was located at the plasma membrane and at the assembly site where it was found associated with virus wrapping membranes by immunogold labelling. Depletion of STX3 using RNA interference reduced HCMV production, while expression of a STX3 construct resistant to RNAi inhibition enhanced virus production. Ultrastructural examination of the assembly site in HCMV-infected STX3-depleted cells showed fewer mature virions and more viruses undergoing final envelopment. In contrast, silencing of STX3 did not affect herpes simplex virus type 1 production. The mechanism through which STX3 affected HCMV morphogenesis likely involved late endosomes/lysosomes since STX3 depletion reduced the expression of lysosomal membrane glycoproteins. Our results demonstrate a function for STX3 in HCMV morphogenesis, and unravel a new role for this SNARE protein in late endosomes/lysosomes compartments.

The tegument protein UL71 of human cytomegalovirus is involved in late envelopment and affects multivesicular bodies

Morphogenesis of human cytomegalovirus (HCMV) is still only partially understood. We have characterized the role of HCMV tegument protein pUL71 in viral replication and morphogenesis. By using a rabbit antibody raised against the C terminus of pUL71, we could detect the protein in infected cells, as well as in virions showing a molecular mass of approximately 48 kDa. The expression of pUL71, detected as early as 48 h postinfection, was not blocked by the antiviral drug foscarnet, indicating an early expression. The role of pUL71 during virus replication was investigated by construction and analysis of a UL71 stop mutant (TBstop71). The mutant could be reconstituted on noncomplementing cells proving that pUL71 is nonessential for virus replication in human fibroblasts. However, the inhibition of pUL71 expression resulted in a severe growth defect, as reflected by an up to 16-fold reduced extracellular virus yield after a high-multiplicity infection and a small-plaque phenotype. Ultrastructural analysis of cells infected with TBstop71 virus revealed an increased number of nonenveloped nucleocapsids in the cytoplasm, many of them at different stages of envelopment, indicating that final envelopment of nucleocapsids in the cytoplasm was affected. In addition, enlarged multivesicular bodies (MVBs) were found in close proximity to the viral assembly compartment, suggesting that pUL71 affects MVBs during virus infection. The observation of numerous TBstop71 virus particles attached to MVB membranes and budding processes into MVBs indicated that these membranes can be used for final envelopment of HCMV.

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.

Estrogen induced breast cancer is the result in the disruption of the asymmetric cell division of the stem cell

There is evidence that in the human breast there is a stem cell population that can give rise to many different cell types and have the unique potential to divide asymmetrically. In this way stem cells maintain the stem cell pool and simultaneously generate committed cells that reconstitute the organ for example for preparing the breast for a new pregnancy after the involution from a previous pregnancy and lactation process. In addition to the in vivo models of mammary morphogenesis there are in vitro systems that are more amenable to study in critically determined conditions the ductulogenic pattern of growth of the breast epithelia. Primary mammary epithelial cells grown in collagen matrix are able to form tree-like structures resembling in vivo ductulogenesis. The human breast epithelial cells MCF-10F formed tubules when grown in type I collagen and we demonstrated that treatment of these cells with 17β-estradiol (E(2)) induces phonotypical changes indicative of neoplastic transformation. The transformation of MCF-10F by E(2) is associated with impaired ductal morphogenesis by altering the stem cells unique potential to divide asymmetrically inducing formation of solid masses mimicking intraductal carcinoma that progress to invasive and tumorigenic phenotype. In the present work we present evidence for the mechanism of cell asymmetry leading to normal ductulogenesis and how the normal stem cell is transformed to cancer stem cell by altering this process. Furthermore, we demonstrate that the carcinogenic agent, in this case E(2), induces a defect in the asymmetric cell division program of the normal mammary stem cell.

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.

Early, active, and specific localization of herpes simplex virus type 1 gM to nuclear membranes

Thirteen different glycoproteins are incorporated into mature herpes simplex virus type 1 (HSV-1) virions. Five of them play important roles during entry, while others intervene during egress of the virus. Although HSV-1 gM is not essential in cell culture, its deletion reduces viral yields and promotes syncytium formation. Furthermore, gM is conserved among herpesviruses, is essential for several of them, and can redirect the gD and gH/gL viral glycoproteins from the cell surface to the trans-Golgi network, where gM presumably modulates final capsid envelopment. Late in infection, gM reaches the nuclear envelope and decorates perinuclear virions. This process seemingly requires U(L)31 and U(L)34 and occurs when several markers of the trans-Golgi network have relocalized to the nucleus. However, the precise mechanism of gM nuclear targeting is unclear. We now report that gM is quickly and specifically targeted to nuclear membranes in a virus-dependent manner. This occurs prior to the HSV-1-induced reorganization of the trans-Golgi network and before gM enters the secretory pathway. The presence of a high-mannose glycosylation pattern on gM further corroborated these findings. While gM was targeted to the inner nuclear membrane early in infection, its partners gD, gH, gN, VP22, U(L)31, and U(L)34 did not colocalize with gM. These data suggest that nuclear gM fulfills an early nuclear function that is independent of its known interaction partners and its function in viral egress.

The endoplasmic reticulum chaperone BiP/GRP78 is important in the structure and function of the human cytomegalovirus assembly compartment

We previously demonstrated that the endoplasmic reticulum (ER) chaperone BiP functions in human cytomegalovirus (HCMV) assembly and egress. Here, we show that BiP localizes in two cytoplasmic structures in infected cells. Antibodies to the extreme C terminus, which includes BiP's KDEL ER localization sequence, detect BiP in regions of condensed ER near the periphery of the cell. Antibodies to the full length, N terminus, or larger portion of the C terminus detect BiP in the assembly compartment. This inability of C-terminal antibodies to detect BiP in the assembly compartment suggests that BiP's KDEL sequence is occluded in the assembly compartment. Depletion of BiP causes the condensed ER and assembly compartments to dissociate, indicating that BiP is important for their integrity. BiP and pp28 are in association in the assembly compartment, since antibodies that detect BiP in the assembly compartment coimmunoprecipitate pp28 and vice versa. In addition, BiP and pp28 copurify with other assembly compartment components on sucrose gradients. BiP also coimmunoprecipitates TRS1. Previous data show that cells infected with a TRS1-deficient virus have cytoplasmic and assembly compartment defects like those seen when BiP is depleted. We show that a fraction of TRS1 purifies with the assembly compartment. These findings suggest that BiP and TRS1 share a function in assembly compartment maintenance. In summary, BiP is diverted from the ER to associate with pp28 and TRS1, contributing to the integrity and function of the assembly compartment.

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.

At the crossroads of homoeostasis and disease: roles of the PACS proteins in membrane traffic and apoptosis

The endomembrane system in mammalian cells has evolved over the past two billion years from a simple endocytic pathway in a single-celled primordial ancestor to complex networks supporting multicellular structures that form metazoan tissue and organ systems. The increased organellar complexity of metazoan cells requires additional trafficking machinery absent in yeast or other unicellular organisms to maintain organ homoeostasis and to process the signals that control proliferation, differentiation or the execution of cell death programmes. The PACS (phosphofurin acidic cluster sorting) proteins are one such family of multifunctional membrane traffic regulators that mediate organ homoeostasis and have important roles in diverse pathologies and disease states. This review summarizes our current knowledge of the PACS proteins, including their structure and regulation in cargo binding, their genetics, their roles in secretory and endocytic pathway traffic, interorganellar communication and how cell-death signals reprogramme the PACS proteins to regulate apoptosis. We also summarize our current understanding of how PACS genes are dysregulated in cancer and how viral pathogens ranging from HIV-1 to herpesviruses have evolved to usurp the PACS sorting machinery to promote virus assembly, viral spread and immunoevasion.

Formation and function of Weibel-Palade bodies

Weibel-Palade bodies (WPBs) are secretory organelles used for post-synthesis storage in endothelial cells that can, very rapidly, be triggered to release their contents. They carry a variety of bioactive molecules that are needed to mount a rapid response to the complex environment of cells that line blood vessels. They store factors that are essential to haemostasis and inflammation, as well as factors that modulate vascular tonicity and angiogenesis. The number of WPBs and their precise content vary between endothelial tissues, reflecting their differing physiological circumstances. The particular functional demands of the highly multimerised haemostatic protein von Willebrand Factor (VWF), which is stored in WPBs as tubules until release, are responsible for the cigar shape of these granules. How VWF tubules drive the formation of these uniquely shaped organelles, and how WPB density increases during maturation, has recently been revealed by EM analysis using high-pressure freezing and freeze substitution. In addition, an AP1/clathrin coat has been found to be essential to WPB formation. Following recruitment of cargo at the TGN, there is a second wave of recruitment that delivers integral and peripheral membrane proteins to WPBs, some of which is AP3 dependent.

HIV-1 Nef binds PACS-2 to assemble a multikinase cascade that triggers major histocompatibility complex class I (MHC-I) down-regulation: analysis using short interfering RNA and knock-out mice

Human immunodeficiency virus, type 1, negative factor (Nef) initiates down-regulation of cell-surface major histocompatibility complex-I (MHC-I) by assembling an Src family kinase (SFK)-ZAP70/Syk-phosphoinositide 3-kinase (PI3K) cascade through the sequential actions of two sites, Nef EEEE(65) and PXXP(75). The internalized MHC-I molecules are then sequestered in endosomal compartments by a process requiring Nef Met(20). How Nef assembles the multikinase cascade to trigger the MHC-I down-regulation pathway is unknown. Here we report that EEEE(65)-dependent binding to the sorting protein PACS-2 targets Nef to the paranuclear region, enabling PXXP(75) to bind and activate a trans-Golgi network (TGN)-localized SFK. This SFK then phosphorylates ZAP-70 to recruit class I PI3K by interaction with the p85 C-terminal Src homology 2 domain. Using splenocytes and embryonic fibroblasts from PACS-2(-/-) mice, we confirm genetically that Nef requires PACS-2 to localize to the paranuclear region and assemble the multikinase cascade. Moreover, genetic loss of PACS-2 or inhibition of class I PI3K prevents Nef-mediated MHC-I down-regulation, demonstrating that short interfering RNA knockdown of PACS-2 phenocopies the gene knock-out. This PACS-2-dependent targeting pathway is not restricted to Nef, because PACS-2 is also required for trafficking of an endocytosed cation-independent mannose 6-phosphate receptor reporter from early endosomes to the TGN. Together, these results demonstrate PACS-2 is required for Nef action and sorting of itinerant membrane cargo in the TGN/endosomal system.

HIV-1 Nef assembles a Src family kinase-ZAP-70/Syk-PI3K cascade to downregulate cell-surface MHC-I

HIV-1 Nef, which is required for the efficient onset of AIDS, enhances viral replication and infectivity by exerting multiple effects on infected cells. Nef downregulates cell-surface MHC-I molecules by an uncharacterized PI3K pathway requiring the actions of two Nef motifs-EEEE(65) and PXXP(75). We report that the Nef EEEE(65) targeting motif enables Nef PXXP(75) to bind and activate a trans-Golgi network-localized Src family tyrosine kinase (SFK). The Nef/SFK complex then recruits and phosphorylates the tyrosine kinase ZAP-70, which binds class I PI3K to trigger MHC-I downregulation in primary CD4+ T cells. In promonocytic cells, Nef/SFK recruits the ZAP-70 homolog Syk to downregulate MHC-I, implicating this PI3K pathway in multiple HIV-1 reservoirs. Isoform-specific PI3K inhibitors repress MHC-I downregulation, identifying them as potential therapeutic agents to combat HIV-1. The discovery of this Nef-SFK-ZAP-70/Syk-PI3K signaling pathway explains the hierarchal role of the Nef motifs in effecting immunoevasion.

Contribution of endocytic motifs in the cytoplasmic tail of herpes simplex virus type 1 glycoprotein B to virus replication and cell-cell fusion

The use of endocytic pathways by viral glycoproteins is thought to play various functions during viral infection. We previously showed in transfection assays that herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) is transported from the cell surface back to the trans-Golgi network (TGN) and that two motifs of gB cytoplasmic tail, YTQV and LL, function distinctly in this process. To investigate the role of each of these gB trafficking signals in HSV-1 infection, we constructed recombinant viruses in which each motif was rendered nonfunctional by alanine mutagenesis. In infected cells, wild-type gB was internalized from the cell surface and concentrated in the TGN. Disruption of YTQV abolished internalization of gB during infection, whereas disruption of LL induced accumulation of internalized gB in early recycling endosomes and impaired its return to the TGN. The growth of both recombinants was moderately diminished. Moreover, the fusion phenotype of cells infected with the gB recombinants differed from that of cells infected with the wild-type virus. Cells infected with the YTQV-mutated virus displayed reduced cell-cell fusion, whereas giant syncytia were observed in cells infected with the LL-mutated virus. Furthermore, blocking gB internalization or impairing gB recycling to the cell surface, using drugs or a transdominant negative form of Rab11, significantly reduced cell-cell fusion. These results favor a role for endocytosis in virus replication and suggest that gB intracellular trafficking is involved in the regulation of cell-cell fusion.

Three-dimensional structure of the human cytomegalovirus cytoplasmic virion assembly complex includes a reoriented secretory apparatus

Human cytomegalovirus (HCMV) induces profound changes in infected cell morphology, including a large cytoplasmic inclusion that corresponds to the virion assembly complex (AC). In electron micrographs, the AC is a highly vacuolated part of the cytoplasm. Markers of cellular secretory organelles have been visualized at the outer edge of the AC, and we recently showed that a marker for early endosomes (i.e., early endosome antigen 1) localizes to the center of the AC. Here, we examined the relationship between the AC and components of the secretory apparatus, studied temporal aspects of the dramatic infection-induced cytoplasmic remodeling, examined the three-dimensional structure of the AC, and considered the implications of our observations for models of HCMV virion maturation and egress. We made three major observations. First, in addition to being relocated, the expression levels of some organelle markers change markedly during the period while the AC is developing. Second, based on three-dimensional reconstructions from z-series confocal microscopic images, the observed concentric rings of vesicles derived from the several compartments (Golgi bodies, the trans-Golgi network [TGN], and early endosomes) are arranged as nested cylinders of organelle-specific vesicles. Third, the membrane protein biosynthetic and exocytic pathways from the endoplasmic reticulum to the Golgi bodies, TGN, and early endosomes are in an unusual arrangement that nonetheless allows for a conventional order of biosynthesis and transport. Our model of AC structure suggests a mechanism by which the virus can regulate the order of tegument assembly.

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

The virion envelope of human cytomegalovirus (HCMV) is complex and consists of an incompletely defined number of glycoproteins. The gM/gN protein complex is the most abundant protein component of the envelope. Studies have indicated that deletion of the viral gene encoding either gM or gN is a lethal mutation. Analysis of the amino acid sequence of gM disclosed a C-terminal acidic cluster of amino acids and a tyrosine-containing trafficking motif, both of which are well-described trafficking/sorting signals in the cellular secretory pathway. To investigate the roles of these signals in the trafficking of the gM/gN complex during virus assembly, we made a series of gM (UL100 open reading frame) mutants in the AD169 strain of HCMV. Mutant viruses that lacked the entire C-terminal cytoplasmic tail of gM were not viable, suggesting that the cytoplasmic tail of gM is essential for virus replication. In addition, the gM mutant protein lacking the cytoplasmic domain exhibited decreased protein stability. Mutant viruses with a deletion of the acidic cluster or alanine substitutions in tyrosine-based motifs were viable but exhibited a replication-impaired phenotype suggestive of a defect in virion assembly. Analysis of these mutant gMs using static immunofluorescence and fluorescence recovery after photobleaching demonstrated delayed kinetics of intracellular localization of the gM/gN protein to the virus assembly compartment compared to the wild-type protein. These data suggest an important role of the glycoprotein gM during virus assembly, particularly in the dynamics of gM trafficking during viral-particle assembly.

Sequence requirements for localization of human cytomegalovirus tegument protein pp28 to the virus assembly compartment and for assembly of infectious virus

The human cytomegalovirus UL99 open reading frame encodes a 190-amino-acid (aa) tegument protein, pp28, that is myristoylated and phosphorylated. pp28 is essential for assembly of infectious virus, and nonenveloped virions accumulate in the cytoplasm of cells infected with recombinant viruses with a UL99 deletion. pp28 is localized to the endoplasmic reticulum-Golgi intermediate compartment (ERGIC) in transfected cells, while in infected cells, it is localized together with other virion proteins in a juxtanuclear compartment termed the assembly compartment (AC). We investigated the sequence requirements for pp28 trafficking to the AC and assembly of infectious virus. Our studies indicated that the first 30 to 35 aa were required for localization of pp28 to the ERGIC in transfected cells. Mutant forms of pp28 containing only the first 35 aa localized with other virion structural proteins to cytoplasmic compartments early in infection, but localization to the AC at late times required a minimum of 50 aa. In agreement with previous reports, we demonstrated that the deletion of a cluster of acidic amino acids (aa 44 to 59) prevented wild-type trafficking of pp28 and recovery of infectious virus. A recombinant virus expressing only the first 50 aa was replication competent, and this mutant, pp28, localized to the AC in cells infected with this virus. These findings argued that localization of pp28 to the AC was essential for assembly of infectious virus and raised the possibility that amino acids in the amino terminus of pp28 have additional roles in the envelopment and assembly of the virion other than simply localizing pp28 to the AC.

HIV-1 Nef-induced down-regulation of MHC class I requires AP-1 and clathrin but not PACS-1 and is impeded by AP-2

Major histocompatibility complex class I is down-regulated from the surface of human immunodeficiency virus (HIV)-1-infected cells by Nef, a virally encoded protein that is thought to reroute MHC-I to the trans-Golgi network (TGN) in a phosphofurin acidic cluster sorting protein (PACS) 1, adaptor protein (AP)-1, and clathrin-dependent manner. More recently, an alternative model has been proposed, in which Nef uses AP-1 to direct MHC-I to endosomes and lysosomes. Here, we show that knocking down either AP-1 or clathrin with small interfering RNA inhibits the down-regulation of HLA-A2 (an MHC-I isotype) by Nef in HeLa cells. However, knocking down PACS-1 has no effect, not only on Nef-induced down-regulation of HLA-A2 but also on the localization of other proteins containing acidic cluster motifs. Surprisingly, knocking down AP-2 actually enhances Nef activity. Immuno-electron microscopy labeling of Nef-expressing cells indicates that HLA-A2 is rerouted not to the TGN, but to endosomes. In AP-2-depleted cells, more of the HLA-A2 localizes to the inner vesicles of multivesicular bodies. We propose that depleting AP-2 potentiates Nef activity by altering the membrane composition and dynamics of endosomes and causing increased delivery of HLA-A2 to a prelysosomal compartment.

An acidic cluster of the cytoplasmic tail of the RD114 virus glycoprotein controls assembly of retroviral envelopes

Retroviral core proteins, Gag and envelope (Env) glycoproteins are expressed from distinct cellular areas and therefore need to encounter to assemble infectious particles. The intrinsic cell localisation properties of either viral component or their capacity to mutually interact determines the assembly of infectious particles. Here, we address how Env determinants and cellular sorting proteins allow the Env derived from gamma retroviruses, murine leukemia virus (MLV) and RD114, to travel to or from late endosomes (LE), which may represent the Env assembly site of retroviruses in some cells. The individual expression of MLV Env resulted in its accumulation in LE in contrast to RD114 Env that required the presence of gamma retroviral Gag proteins. To discriminate between intrinsic intracellular Env localisation and gamma retroviral Gag/Env interactions in influencing Env viral incorporation, we studied Env assembly on heterologous lentiviral particles on which they are passively recruited. We found that an acidic cluster present at the C-terminus of the RD114 Env cytoplasmic tail determines its sub-cellular localisation and retrograde transport. Mutation of this motif induced late endosomal concentration of the RD114 Env, correlating with increased viral incorporation and infectivity. Reciprocally, the reinforcement of a poorly functional acidic motif in the MLV Env resulted in a marked decrease of its late endosomal localisation, leading to weakly infectious lentiviral particles with low Env densities. Finally, through upregulation versus downregulation of its cellular expression, we show that phosphofurin acidic-cluster-sorting protein 1 (PACS-1) controls the function of the RD114 Env acidic cluster, assigning to this cellular effector a crucial role in modulation of Env assembly of some retroviruses.

A PACS-1, GGA3 and CK2 complex regulates CI-MPR trafficking

The cation-independent mannose-6-phosphate receptor (CI-MPR) follows a highly regulated sorting itinerary to deliver hydrolases from the trans-Golgi network (TGN) to lysosomes. Cycling of CI-MPR between the TGN and early endosomes is mediated by GGA3, which directs TGN export, and PACS-1, which directs endosome-to-TGN retrieval. Despite executing opposing sorting steps, GGA3 and PACS-1 bind to an overlapping CI-MPR trafficking motif and their sorting activity is controlled by the CK2 phosphorylation of their respective autoregulatory domains. However, how CK2 coordinates these opposing roles is unknown. We report a CK2-activated phosphorylation cascade controlling PACS-1- and GGA3-mediated CI-MPR sorting. PACS-1 links GGA3 to CK2, forming a multimeric complex required for CI-MPR sorting. PACS-1-bound CK2 stimulates GGA3 phosphorylation, releasing GGA3 from CI-MPR and early endosomes. Bound CK2 also phosphorylates PACS-1Ser(278), promoting binding of PACS-1 to CI-MPR to retrieve the receptor to the TGN. Our results identify a CK2-controlled cascade regulating hydrolase trafficking and sorting of itinerant proteins in the TGN/endosomal system.

Processing of human cytomegalovirus UL37 mutant glycoproteins in the endoplasmic reticulum lumen prior to mitochondrial importation

The human cytomegalovirus (HCMV) UL37 glycoprotein (gpUL37) is internally cleaved and its products divergently traffic to mitochondria or are retained in the secretory pathway. To define the requirements for gpUL37 cleavage, residues -1 and -3 of the consensus endoplasmic reticulum (ER) signal peptidase I site within exon 3 (UL37x3) were replaced by bulky tyrosines (gpUL37 cleavage site mutant I). Internal cleavage of this UL37x3 mutant was inhibited, verifying usage of the consensus site at amino acids (aa) 193/194. The full-length mitochondrial species of gpUL37 cleavage site mutant I was N glycosylated and endoglycosidase H sensitive, indicating that ER translocation and processing took place prior to its mitochondrial importation. Moreover, these results suggest that internal cleavage of gpUL37 is not necessary for its N glycosylation. Partial deletion or disruption of the UL37 hydrophobic core immediately upstream of the cleavage site resulted in decreased protein abundance, suggesting that the UL37x3 hydrophobic alpha-helix contributes to either correct folding or stability of gpUL37. Insertion of the UL37x3 hydrophobic core and cleavage site into pUL37(M), a splice variant of gpUL37 which lacks these sequences and is neither proteolytically cleaved nor N glycosylated, resulted in its internal cleavage and N glycosylation. Its NH(2)-terminal fragment, pUL37(M-NH2), was detected more abundantly in mitochondria, while its N-glycosylated C-terminal fragment, gpUL37(M-COOH), was detected predominantly in the ER in a manner analogous to that of gpUL37 cleavage products. These results indicate that UL37x3 aa 178 to 205 are prerequisite for gpUL37 internal cleavage and alter UL37 protein topology allowing N glycosylation of its C-terminal sequences. In contrast, the NH(2)-terminal UL37x1 hydrophobic leader, present in pUL37x1, pUL37(M), and gpUL37, is not cleaved from mature UL37 protein, retaining a membrane anchor for UL37 isoforms during trafficking. Taken together, these results suggest that HCMV gpUL37 undergoes sequential trafficking, during which it is ER translocated, processed, and then mitochondrially imported.

Reconstitution of herpes simplex virus microtubule-dependent trafficking in vitro

Microtubule-mediated anterograde transport of herpes simplex virus (HSV) from the neuronal cell body to the axon terminal is crucial for the spread and transmission of the virus. It is therefore of central importance to identify the cellular and viral factors responsible for this trafficking event. In previous studies, we isolated HSV-containing cytoplasmic organelles from infected cells and showed that they represent the first and only destination for HSV capsids after they emerge from the nucleus. In the present study, we tested whether these cytoplasmic compartments were capable of microtubule-dependent traffic. Organelles containing green fluorescent protein-labeled HSV capsids were isolated and found to be able to bind rhodamine-labeled microtubules polymerized in vitro. Following the addition of ATP, the HSV-associated organelles trafficked along the microtubules, as visualized by time lapse microscopy in an imaging microchamber. The velocity and processivity of trafficking resembled those seen for neurotropic herpesvirus traffic in living axons. The use of motor-specific inhibitors indicated that traffic was predominantly kinesin mediated, consistent with the reconstitution of anterograde traffic. Immunocytochemical studies revealed that the majority of HSV-containing organelles attached to the microtubules contained the trans-Golgi network marker TGN46. This simple, minimal reconstitution of microtubule-mediated anterograde traffic should facilitate and complement molecular analysis of HSV egress in vivo.

Phosphorylation by casein kinase 2 induces PACS-1 binding of nephrocystin and targeting to cilia

Mutations in proteins localized to cilia and basal bodies have been implicated in a growing number of human diseases. Access of these proteins to the ciliary compartment requires targeting to the base of the cilia. However, the mechanisms involved in transport of cilia proteins to this transitional zone are elusive. Here we show that nephrocystin, a ciliary protein mutated in the most prevalent form of cystic kidney disease in childhood, is expressed in respiratory epithelial cells and accumulates at the base of cilia, overlapping with markers of the basal body area and the transition zone. Nephrocystin interacts with the phosphofurin acidic cluster sorting protein (PACS)-1. Casein kinase 2 (CK2)-mediated phosphorylation of three critical serine residues within a cluster of acidic amino acids in nephrocystin mediates PACS-1 binding, and is essential for colocalization of nephrocystin with PACS-1 at the base of cilia. Inhibition of CK2 activity abrogates this interaction and results in the loss of correct nephrocystin targeting. These data suggest that CK2-dependent transport processes represent a novel pathway of targeting proteins to the cilia.

Envelope glycoprotein gB of Kaposi's sarcoma-associated herpesvirus is essential for egress from infected cells

Kaposi's sarcoma-associated herpesvirus (KSHV) envelope glycoprotein gB interacts with cell surface heparan sulfate (HS) and alpha3beta1 integrin and plays roles in the initial binding and entry into the target cells and in the induction of preexisting host cell signal pathways. To define gB function further, using a bacterial artificial chromosome (BAC) system carrying the KSHV genome (BAC36wt-KSHV), we constructed a recombinant virus genome with the gB open reading frame (ORF) deleted by replacing a 2-kb gB ORF with a 1.3-kb Kan(r) gene. Stable 293T cells carrying BAC36wt-KSHV and DeltagBBAC36-KSHV genomes were generated. Transcript analyses and immunoprecipitation reactions confirmed the absence of gB in the 293T-DeltagBBAC36 cells. When monolayers of 293T-BAC36wt and 293T-DeltagBBAC36 cells were induced with tetradecanoylphorbol-13-acetate, infectious virus was detected only from the 293T-BAC36wt cell supernatants. No significant amount of DNase I-resistant viral DNA was detected in the supernatants of 293T-DeltagBBAC36 cells. BAC36wt-KSHV infected the target cells, and in contrast, no viral DNA and transcripts could be detected in cells infected with DeltagBBAC36-KSHV. Electron microscopy of 293T-DeltagBBAC36 cells revealed capsids in the nuclei, cytoplasmic vesicles with core-containing capsids, and occasional enveloped virions in the cytoplasm. However, enveloped virus particles were observed in the extracellular compartments of 293T-BAC36wt cells only and not in 293T-DeltagBBAC36 cells. Transfection of 293T-DeltagBBAC36 cells with plasmid expressing full-length gB restored the recovery of infectious KSHV in the supernatant. These results suggest that, besides its role in virus binding and entry into the target cells, KSHV gB also plays a role in the maturation and egress of virus from the infected cells.

Herpes simplex virus type 1 capsids transit by the trans-Golgi network, where viral glycoproteins accumulate independently of capsid egress

Egress of herpes capsids from the nucleus to the plasma membrane is a complex multistep transport event that is poorly understood. The current model proposes an initial envelopment at the inner nuclear membrane of capsids newly assembled in the nucleus. The capsids are then released in cytosol by fusion with the outer nuclear membrane. They are finally reenveloped at a downstream organelle before traveling to the plasma membrane for their extracellular release. Although the trans-Golgi network (TGN) is often cited as a potential site of reenvelopment, other organelles have also been proposed, including the Golgi, endoplasmic reticulum-Golgi intermediate compartment, aggresomes, tegusomes, and early or late endosomes. To clarify this important issue, we followed herpes simplex virus type 1 egress by immunofluorescence under conditions that slowed intracellular transport and promoted the accumulation of the otherwise transient reenvelopment intermediate. The data show that the capsids transit by the TGN and point to this compartment as the main reenvelopment site, although a contribution by endosomes cannot formally be excluded. Given that viral glycoproteins are expected to accumulate where capsids acquire their envelope, we examined this prediction and found that all tested could indeed be detected at the TGN. Moreover, this accumulation occurred independently of capsid egress. Surprisingly, capsids were often found immediately adjacent to the viral glycoproteins at the TGN.

Antigenic domain 1 is required for oligomerization of human cytomegalovirus glycoprotein B

Human cytomegalovirus (HCMV) glycoprotein B (gB) is an abundant virion envelope protein that has been shown to be essential for the infectivity of HCMV. HCMV gB is also one of the most immunogenic virus-encoded proteins, and a significant fraction of virus neutralizing antibodies are directed at gB. A linear domain of gB designated AD-1 (antigenic domain 1) represents a dominant antibody binding site on this protein. AD-1 from clinical isolates of HCMV exhibits little sequence variation, suggesting that AD-1 plays an essential role in gB structure or function. We investigated this possibility by examining the role of AD-1 in early steps of gB synthesis. Our results from studies using eukaryotic cells indicated that amino acid (aa) 635 of the gB sequence represented the carboxyl-terminal limit of this domain and that deletion of aa 560 to 640 of the gB sequence resulted in loss of AD-1 expression. AD-1 was shown to be required for oligomerization of gB. Mutation of cysteine at either position 573 or 610 in AD-1 resulted in loss of its reactivity with AD-1-specific monoclonal antibodies and gB oligomerization. Infectious virus could not be recovered from HCMV bacterial artificial chromosomes following introduction of these mutations into the HCMV genome, suggesting that AD-1 was an essential structural domain required for gB function in the replicative cycle of HCMV. Sequence alignment of AD-1 with homologous regions of gBs from other herpesviruses demonstrated significant relatedness, raising the possibility that this domain may contribute to multimerization of gBs in other herpesviruses.

Human cytomegalovirus virion proteins

Human cytomegalovirus (HCMV) is the largest member of the family of human herpesviruses. The number of virus encoded proteins and the complexity of their functions in the life cycle of this virus are reflected in the size of its genome. There continues to be some controversy surrounding the exact protein coding capacity of the virus with estimates ranging from 160 open reading frames to more than 200 open reading frames. Very recent studies using mass spectrometry to determine the viral proteome suggests that the number of viral proteins may be even greater than previous estimates. The proteins of the virion capsid have readily identifiable homologous proteins in the capsid of the more extensively studied herpes simplex virus, likely because of similar capsid structure and assembly pathways. In contrast, the tegument and the envelope of HCMV contain a significant number of proteins that lack structural homology to proteins found in either alpha or gamma-herpesviruses. This brief overview discusses some of the general features and possible functions of the HCMV virion structural proteins in the replicative cycle of this virus.

Alphaherpesvirus glycoprotein M causes the relocalization of plasma membrane proteins

Herpesvirus glycoprotein M (gM) is a multiple-spanning integral membrane protein found within the envelope of mature herpesviruses and is conserved throughout the Herpesviridae. gM is defined as a non-essential glycoprotein in alphaherpesviruses and has been proposed as playing a role in controlling final envelopment in a late secretory-pathway compartment such as the trans-Golgi network (TGN). Additionally, gM proteins have been shown to inhibit cell-cell fusion in transfection-based assays by an as yet unclear mechanism. Here, the effect of pseudorabies virus (PRV) gM and the herpes simplex virus type 1 (HSV-1) gM/UL49A complex on the fusion events caused by the HSV-1 glycoproteins gB, gD, gH and gL was investigated. Fusion of cells expressing HSV-1 gB, gD, gH and gL was efficiently inhibited by both PRV gM and HSV-1 gM/UL49A. Furthermore, expression of PRV gM or HSV-1 gM/UL49A, which are themselves localized to the TGN, caused both gD and gH/L to be relocalized from the plasma membrane to a juxtanuclear compartment, suggesting that fusion inhibition is caused by the removal of 'fusion' proteins from the cell surface. The ability of gM to cause the relocalization of plasma membrane proteins was not restricted to HSV-1 glycoproteins, as other viral and non-viral proteins were also affected. These data suggest that herpesvirus gM (gM/N) can alter the membrane trafficking itineraries of a broad range of proteins and this may have multiple functions.