O-linked oligosaccharides are acquired by herpes simplex virus glycoproteins in the Golgi apparatus

IgA Nephropathy: Pleiotropic impact of Epstein-Barr virus infection on immunopathogenesis and racial incidence of the disease

IgA nephropathy (IgAN) is an autoimmune disease in which poorly galactosylated IgA1 is the antigen recognized by naturally occurring anti-glycan antibodies, leading to formation of nephritogenic circulating immune complexes. Incidence of IgAN displays geographical and racial disparity: common in Europe, North America, Australia, and east Asia, uncommon in African Americans, many Asian and South American countries, Australian Aborigines, and rare in central Africa. In analyses of sera and cells from White IgAN patients, healthy controls, and African Americans, IgAN patients exhibited substantial enrichment for IgA-expressing B cells infected with Epstein-Barr virus (EBV), leading to enhanced production of poorly galactosylated IgA1. Disparities in incidence of IgAN may reflect a previously disregarded difference in the maturation of the IgA system as related to the timing of EBV infection. Compared with populations with higher incidences of IgAN, African Americans, African Blacks, and Australian Aborigines are more frequently infected with EBV during the first 1-2 years of life at the time of naturally occurring IgA deficiency when IgA cells are less numerous than in late childhood or adolescence. Therefore, in very young children EBV enters "non-IgA" cells. Ensuing immune responses prevent infection of IgA B cells during later exposure to EBV at older ages. Our data implicate EBV-infected cells as the source of poorly galactosylated IgA1 in circulating immune complexes and glomerular deposits in patients with IgAN. Thus, temporal differences in EBV primo-infection as related to naturally delayed maturation of the IgA system may contribute to geographic and racial variations in incidence of IgAN.

Opportunities, Technology, and the Joy of Discovery

My grandparents were immigrants. My paternal grandfather was illiterate. Yet my parents were able to complete college and to become teachers. I had a conventional upbringing in a small town in Florida, graduating from high school in 1960. I was fortunate enough to graduate cum laude from Florida State University and to earn other credentials leading to faculty positions at outstanding institutions of higher education: the University of Chicago and Northwestern University. At a time when women were rarely the leaders of research groups, I was able to establish a well-funded research program and to make contributions to our understanding of viral entry into cells. My best research was done after I became confident enough to seek productive interactions with collaborators. I am grateful for the collaborators and collaborations that moved our field forward and for my trainees who have gone on to successes in many different careers. Expected final online publication date for the Annual Review of Virology, Volume 9 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Pathogenesis of IgA Nephropathy: Current Understanding and Implications for Development of Disease-Specific Treatment

IgA nephropathy, initially described in 1968 as a kidney disease with glomerular “intercapillary deposits of IgA-IgG”, has no disease-specific treatment and is a common cause of kidney failure. Clinical observations and laboratory analyses suggest that IgA nephropathy is an autoimmune disease wherein the kidneys are damaged as innocent bystanders due to deposition of IgA1-IgG immune complexes from the circulation. A multi-hit hypothesis for the pathogenesis of IgA nephropathy describes four sequential steps in disease development. Specifically, patients with IgA nephropathy have elevated circulating levels of IgA1 with some O-glycans deficient in galactose (galactose-deficient IgA1) and these IgA1 glycoforms are recognized as autoantigens by unique IgG autoantibodies, resulting in formation of circulating immune complexes, some of which deposit in glomeruli and activate mesangial cells to induce kidney injury. This proposed mechanism is supported by observations that (i) glomerular immunodeposits in patients with IgA nephropathy are enriched for galactose-deficient IgA1 glycoforms and the corresponding IgG autoantibodies; (ii) circulatory levels of galactose-deficient IgA1 and IgG autoantibodies predict disease progression; and (iii) pathogenic potential of galactose-deficient IgA1 and IgG autoantibodies was demonstrated in vivo. Thus, a better understanding of the structure–function of these immunoglobulins as autoantibodies and autoantigens will enable development of disease-specific treatments.

Virus-Encoded Complement Regulators: Current Status

Viruses require a host for replication and survival and hence are subjected to host immunological pressures. The complement system, a crucial first response of the host immune system, is effective in targeting viruses and virus-infected cells, and boosting the antiviral innate and acquired immune responses. Thus, the system imposes a strong selection pressure on viruses. Consequently, viruses have evolved multiple countermeasures against host complement. A major mechanism employed by viruses to subvert the complement system is encoding proteins that target complement. Since viruses have limited genome size, most of these proteins are multifunctional in nature. In this review, we provide up to date information on the structure and complement regulatory functions of various viral proteins.

The FDA-Approved Drug Nelfinavir Inhibits Lytic Cell-Free but Not Cell-Associated Nonlytic Transmission of Human Adenovirus

Adenoviruses (AdVs) are prevalent and give rise to chronic and recurrent disease. Human AdV (HAdV) species B and C, such as HAdV-C2, -C5, and -B14, cause respiratory disease and constitute a health threat for immunocompromised individuals. HAdV-Cs are well known for lysing cells owing to the E3 CR1-β-encoded adenovirus death protein (ADP). We previously reported a high-throughput image-based screening framework and identified an inhibitor of HAdV-C2 multiround infection, nelfinavir mesylate. Nelfinavir is the active ingredient of Viracept, an FDA-approved inhibitor of human immunodeficiency virus (HIV) aspartyl protease that is used to treat AIDS. It is not effective against single-round HAdV infections. Here, we show that nelfinavir inhibits lytic cell-free transmission of HAdV, indicated by the suppression of comet-shaped infection foci in cell culture. Comet-shaped foci occur upon convection-based transmission of cell-free viral particles from an infected cell to neighboring uninfected cells. HAdV lacking ADP was insensitive to nelfinavir but gave rise to comet-shaped foci, indicating that ADP enhances but is not required for cell lysis. This was supported by the notion that HAdV-B14 and -B14p1 lacking ADP were highly sensitive to nelfinavir, although HAdV-A31, -B3, -B7, -B11, -B16, -B21, -D8, -D30, and -D37 were less sensitive. Conspicuously, nelfinavir uncovered slow-growing round HAdV-C2 foci, independent of neutralizing antibodies in the medium, indicative of nonlytic cell-to-cell transmission. Our study demonstrates the repurposing potential of nelfinavir with postexposure efficacy against different HAdVs and describes an alternative nonlytic cell-to-cell transmission mode of HAdV.

Targeting STT3A-oligosaccharyltransferase with NGI-1 causes herpes simplex virus 1 dysfunction

Herpes simplex virus 1 (HSV-1) is a contagious neurotropic herpesvirus responsible for oral lesions and herpesviral encephalitis. The HSV-1 envelope contains N-glycosylated proteins involved in infection and that are candidate drug targets. NGI-1 is a small-molecule inhibitor of oligosaccharyltransferase (OST) complexes STT3A-OST and STT3B-OST, which catalyze cotranslational and post-translational N-glycosylation, respectively. Because host OSTs attach HSV-1 glycans, NGI-1 might have anti-HSV-1 activity. We evaluated HSV-1 function using NGI-1 and human embryonic kidney 293 knockout lines for OST isoform-specific catalytic and accessory subunits. N-glycosylation of 2 representative envelope proteins (gC and gD) was primarily dependent upon STT3A-OST, but to a large extent replaceable by STT3B-OST. Knockouts impairing STT3A- or STT3B-OST activity, by themselves, did not appreciably affect HSV-1 function (plaque-forming units, normalized to viral particles measured by unglycosylated capsid protein VP5 content). However, with cells lacking STT3B-OST activity (missing the catalytic subunit STT3B or the oxidoreductase subunits magnesium transporter 1/tumor suppressor candidate 3) and thus solely dependent upon STT3A-OST for N-glycosylation, NGI-1 treatment resulted in HSV-1 having cell type-dependent dysfunction (affecting infectivity with Vero cells much more than with the 293 lines). Ablation of post-translational N-glycosylation can therefore make HSV-1 infectivity, and possibly masking of immunogenic peptide epitopes by glycans, highly sensitive to pharmacological inhibition of cotranslational N-glycosylation.-Lu, H., Cherepanova, N. A., Gilmore, R., Contessa, J. N., Lehrman, M. A. Targeting STT3A-oligosaccharyltransferase with NGI-1 causes herpes simplex virus 1 dysfunction.

Experimental Dissection of the Lytic Replication Cycles of Herpes Simplex Viruses in vitro

Herpes simplex viruses type 1 and type 2 (HSV-1 and HSV-2) produce lifelong infections and are highly prevalent in the human population. Both viruses elicit numerous clinical manifestations and produce mild-to-severe diseases that affect the skin, eyes, and brain, among others. Despite the existence of numerous antivirals against HSV, such as acyclovir and acyclovir-related analogs, virus variants that are resistant to these compounds can be isolated from immunosuppressed individuals. For such isolates, second-line drugs can be used, yet they frequently produce adverse side effects. Furthermore, topical antivirals for treating cutaneous HSV infections usually display poor to moderate efficacy. Hence, better or novel anti-HSV antivirals are needed and details on their mechanisms of action would be insightful for improving their efficacy and identifying specific molecular targets. Here, we review and dissect the lytic replication cycles of herpes simplex viruses, discussing key steps involved in cell infection and the processes that yield new virions. Additionally, we review and discuss rapid, easy-to-perform and simple experimental approaches for studying key steps involved in HSV replication to facilitate the identification of the mechanisms of action of anti-HSV compounds.

Global aspects of viral glycosylation

Enveloped viruses encompass some of the most common human pathogens causing infections of different severity, ranging from no or very few symptoms to lethal disease as seen with the viral hemorrhagic fevers. All enveloped viruses possess an envelope membrane derived from the host cell, modified with often heavily glycosylated virally encoded glycoproteins important for infectivity, viral particle formation and immune evasion. While N-linked glycosylation of viral envelope proteins is well characterized with respect to location, structure and site occupancy, information on mucin-type O-glycosylation of these proteins is less comprehensive. Studies on viral glycosylation are often limited to analysis of recombinant proteins that in most cases are produced in cell lines with a glycosylation capacity different from the capacity of the host cells. The glycosylation pattern of the produced recombinant glycoproteins might therefore be different from the pattern on native viral proteins. In this review, we provide a historical perspective on analysis of viral glycosylation, and summarize known roles of glycans in the biology of enveloped human viruses. In addition, we describe how to overcome the analytical limitations by using a global approach based on mass spectrometry to identify viral O-glycosylation in virus-infected cell lysates using the complex enveloped virus herpes simplex virus type 1 as a model. We underscore that glycans often pay important contributions to overall protein structure, function and immune recognition, and that glycans represent a crucial determinant for vaccine design. High throughput analysis of glycosylation on relevant glycoprotein formulations, as well as data compilation and sharing is therefore important to identify consensus glycosylation patterns for translational applications.

The nociceptin receptor inhibits axonal regeneration and recovery from spinal cord injury

Axonal growth after traumatic spinal cord injury is limited by endogenous inhibitors, selective blockade of which promotes partial neurological recovery. The partial repair phenotypes suggest that compensatory pathways limit improvement. Gene expression profiles of mice deficient in Ngr1, which encodes a receptor for myelin-associated inhibitors of axonal regeneration such as Nogo, revealed that trauma increased the mRNA expression of ORL1, which encodes the receptor for the opioid-related peptide nociceptin. Endogenous and overexpressed ORL1 coimmunoprecipitated with immature NgR1 protein, and ORL1 enhanced the O-linked glycosylation and surface expression of NgR1 in HEK293T and Neuro2A cells and primary neurons. ORL1 overexpression inhibited cortical neuron axon regeneration independently of NgR1. Furthermore, regeneration was inhibited by an ORL1 agonist and enhanced by the ORL1 antagonist J113397 through a ROCK-dependent mechanism. Mice treated with J113397 after dorsal hemisection of the mid-thoracic spinal cord recovered greater locomotor function and exhibited lumbar raphespinal axon sprouting. These effects were further enhanced by combined Ngr1 deletion and ORL1 inhibition. Thus, ORL1 limits neural repair directly and indirectly by enhancing NgR1 maturation, and ORL1 antagonists enhance recovery from traumatic CNS injuries in wild-type and Ngr1 null mice.

IgA nephropathy enigma

IgA nephropathy (IgAN) is the leading cause of primary glomerulonephritis in the world. The disease is characterized by the presence of IgA-containing immune complexes in the circulation and in mesangial deposits with ensuing glomerular injury. Although in humans there are two IgA subclasses, only IgA1 molecules are involved. The exclusivity of participation of IgA1 in IgAN prompted extensive structural and immunological studies of the unique hinge region (HR) of IgA1, which is absent in otherwise highly homologous IgA2. HR of IgA1 with altered O-glycans serves as an antigen recognized by autoantibodies specific for aberrant HR glycans leading to the generation of nephritogenic immune complexes. However, there are several unresolved questions concerning the phylogenetic origin of human IgA1 HR, the structural basis of its antigenicity, the origin of antibodies specific for HR with altered glycan moieties, the regulatory defects in IgA1 glycosylation pathways, and the potential approaches applicable to the disease-specific interventions in the formation of nephritogenic immune complexes. This review focuses on the gaps in our knowledge of molecular and cellular events that are involved in the immunopathogenesis of IgAN.

The Human Cytomegalovirus UL116 Gene Encodes an Envelope Glycoprotein Forming a Complex with gH Independently from gL

Human cytomegalovirus (HCMV) is a major cause of morbidity and mortality in transplant patients and is the leading viral cause of birth defects after congenital infection. HCMV infection relies on the recognition of cell-specific receptors by one of the viral envelope glycoprotein complexes. Either the gH/gL/gO or the gH/gL/UL128/UL130/UL131A (Pentamer) complex has been found to fulfill this role, accounting for HCMV entry into almost all cell types. We have studied the UL116 gene product, a putative open reading frame identified by in silico analysis and predicted to code for a secreted protein. Virus infection experiments in mammalian cells demonstrated that UL116 is expressed late in the HCMV replication cycle and is a heavily glycosylated protein that first localizes to the cellular site of virus assembly and then inserts into the virion envelope. Transient-transfection studies revealed that UL116 is efficiently transported to the plasma membrane when coexpressed with gH and that gL competes with UL116 for gH binding. Further evidence for gH/UL116 complex formation was obtained by coimmunoprecipitation experiments on both transfected and infected cells and biochemical characterization of the purified complex. In summary, our results show that the product of the UL116 gene is an HCMV envelope glycoprotein that forms a novel gH-based complex alternative to gH/gL. Remarkably, the gH/UL116 complex is the first herpesvirus gH-based gL-less complex.

IMPORTANCE

HCMV infection can cause severe disease in immunocompromised adults and infants infected in utero The dissection of the HCMV entry machinery is important to understand the mechanism of viral infection and to identify new vaccine antigens. The gH/gL/gO and gH/gL/UL128/UL130/UL131 (Pentamer) complexes play a key role in HCMV cell entry and tropism. Both complexes are formed by an invariant gH/gL scaffold on which the other subunits assemble. Here, we show that the UL116 gene product is expressed in infected cells and forms a heterodimer with gH. The gH/UL116 complex is carried on the infectious virions, although in smaller amounts than gH/gL complexes. No gH/UL116/gL ternary complex formed in transfected cells, suggesting that the gH/UL116 complex is independent from gL. This new gH-based gL-free complex represents a potential target for a protective HCMV vaccine and opens new perspectives on the comprehension of the HCMV cell entry mechanism and tropism.

Inactivation of the Human Cytomegalovirus US20 Gene Hampers Productive Viral Replication in Endothelial Cells

The human cytomegalovirus (HCMV) US12 gene family includes a group of 10 contiguous genes (US12 to US21) encoding predicted seven-transmembrane-domain (7TMD) proteins that are nonessential for replication within cultured fibroblasts. Nevertheless, inactivation of some US12 family members affects virus replication in other cell types; e.g., deletion of US16 or US18 abrogates virus growth in endothelial and epithelial cells or in human gingival tissue, respectively, suggesting a role for some US12 proteins in HCMV cell tropism. Here, we provide evidence that another member, US20, impacts the ability of a clinical strain of HCMV to replicate in endothelial cells. Through the use of recombinant HCMV encoding tagged versions of the US20 protein, we investigated the expression pattern, localization, and topology of the US20-encoded protein (pUS20). We show that pUS20 is expressed as a partially glycosylated 7TMD protein which accumulates late in infection in endoplasmic reticulum-derived peripheral structures localized outside the cytoplasmic virus assembly compartment (cVAC). US20-deficient mutants generated in the TR clinical strain of HCMV exhibited major growth defects in different types of endothelial cells, whereas they replicated normally in fibroblasts and epithelial cells. While the attachment and entry phases in endothelial cells were not significantly affected by the absence of US20 protein, US20-null viruses failed to replicate viral DNA and express representative E and L mRNAs and proteins. Taken together, these results indicate that US20 sustains the HCMV replication cycle at a stage subsequent to entry but prior to E gene expression and viral DNA synthesis in endothelial cells.

IMPORTANCE

Human cytomegalovirus (HCMV) is a major pathogen in newborns and immunocompromised individuals. A hallmark of HCMV pathogenesis is its ability to productively replicate in an exceptionally broad range of target cells, including endothelial cells, which represent a key target for viral dissemination and replication in the host, and to contribute to both viral persistence and associated inflammation and vascular diseases. Replication in endothelial cells depends on the activities of a set of viral proteins that regulate different stages of the HCMV replication cycle in an endothelial cell type-specific manner and thereby act as determinants of viral tropism. Here, we report the requirement of a HCMV protein as a postentry tropism factor in endothelial cells. The identification and characterization of HCMV endotheliotropism-regulating proteins will advance our understanding of the molecular mechanisms of HCMV-related pathogenesis and help lead to the design of new antiviral strategies able to exploit these functions.

Inhibition of O-Linked N-Acetylglucosamine Transferase Reduces Replication of Herpes Simplex Virus and Human Cytomegalovirus

O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) is an essential cellular enzyme that posttranslationally modifies nuclear and cytoplasmic proteins via O-linked addition of a single N-acetylglucosamine (GlcNAc) moiety. Among the many targets of OGT is host cell factor 1 (HCF-1), a transcriptional regulator that is required for transactivation of the immediate-early genes of herpes simplex virus (HSV). HCF-1 is synthesized as a large precursor that is proteolytically cleaved by OGT, which may regulate its biological function. In this study, we tested whether inhibition of the enzymatic activity of OGT with a small molecule inhibitor, OSMI-1, affects initiation of HSV immediate-early gene expression and viral replication. We found that inhibiting OGT's enzymatic activity significantly decreased HSV replication. The major effect of the inhibitor occurred late in the viral replication cycle, when it reduced the levels of late proteins and inhibited capsid formation. However, depleting OGT levels with small interfering RNA (siRNA) reduced the expression of HSV immediate-early genes, in addition to reducing viral yields. In this study, we identified OGT as a novel cellular factor involved in HSV replication. Our results obtained using a small molecule inhibitor and siRNA depletion suggest that OGT's glycosylation and scaffolding functions play distinct roles in the replication cycle of HSV.

IMPORTANCE

Antiviral agents can target viral or host gene products essential for viral replication. O-GlcNAc transferase (OGT) is an important cellular enzyme that catalyzes the posttranslational addition of GlcNAc sugar residues to hundreds of nuclear and cytoplasmic proteins, and this modification regulates their activity and function. Some of the known OGT targets are cellular proteins that are critical for the expression of herpes simplex virus (HSV) genes, suggesting a role for OGT in the replication cycle of HSV. In this study, we found that OGT is required for efficient expression of viral genes and for assembly of new virions. Thus, we identify OGT as a novel host factor involved in the replication of HSV and a potential target for antiviral therapy.

Nelfinavir impairs glycosylation of herpes simplex virus 1 envelope proteins and blocks virus maturation

Nelfinavir (NFV) is an HIV-1 aspartyl protease inhibitor that has numerous effects on human cells, which impart attractive antitumor properties. NFV has also been shown to have in vitro inhibitory activity against human herpesviruses (HHVs). Given the apparent absence of an aspartyl protease encoded by HHVs, we investigated the mechanism of action of NFV herpes simplex virus type 1 (HSV-1) in cultured cells. Selection of HSV-1 resistance to NFV was not achieved despite multiple passages under drug pressure. NFV did not significantly affect the level of expression of late HSV-1 gene products. Normal numbers of viral particles appeared to be produced in NFV-treated cells by electron microscopy but remain within the cytoplasm more often than controls. NFV did not inhibit the activity of the HSV-1 serine protease nor could its antiviral activity be attributed to inhibition of Akt phosphorylation. NFV was found to decrease glycosylation of viral glycoproteins B and C and resulted in aberrant subcellular localization, consistent with induction of endoplasmic reticulum stress and the unfolded protein response by NFV. These results demonstrate that NFV causes alterations in HSV-1 glycoprotein maturation and egress and likely acts on one or more host cell functions that are important for HHV replication.

A vaccine based on the rhesus cytomegalovirus UL128 complex induces broadly neutralizing antibodies in rhesus macaques

Neutralizing antibodies (NAb) are important for interfering with horizontal transmission of human cytomegalovirus (HCMV) leading to primary and congenital HCMV infection. Recent findings have shown that a pentameric virion complex formed by the glycoproteins gH/gL, UL128, UL130, and UL131A (UL128C) is required for HCMV entry into epithelial/endothelial cells (Epi/EC) and is the target of potent NAb in HCMV-seropositive individuals. Using bacterial artificial chromosome technology, we have generated a modified vaccinia Ankara virus (MVA) that stably coexpresses all 5 rhesus CMV (RhCMV) proteins homologous to HCMV UL128C, termed MVA-RhUL128C. Coimmunoprecipitation confirmed the interaction of RhgH with the other 4 RhCMV subunits of the pentameric complex. All 8 RhCMV-naïve rhesus macaques (RM) vaccinated with MVA-RhUL128C developed NAb that blocked infection of monkey kidney epithelial cells (MKE) and rhesus fibroblasts. NAb titers induced by MVA-RhUL128C measured on both cell types at 2 to 6 weeks postvaccination were comparable to levels observed in naturally infected RM. In contrast, MVA expressing a subset of RhUL128C proteins or RhgB glycoprotein only minimally stimulated NAb that inhibited infection of MKE. In addition, following subcutaneous RhCMV challenge at 8 weeks postvaccination, animals vaccinated with MVA-RhUL128C showed reduced plasma viral loads. These results indicate that MVA expressing the RhUL128C induces NAb inhibiting RhCMV entry into both Epi/EC and fibroblasts and limits RhCMV replication in RM. This novel approach is the first step in developing a prophylactic HCMV vaccine designed to interfere with virus entry into major cell types permissive for viral replication, a required property of an effective vaccine.

Herpes simplex virus dances with amyloid precursor protein while exiting the cell

Herpes simplex type 1 (HSV1) replicates in epithelial cells and secondarily enters local sensory neuronal processes, traveling retrograde to the neuronal nucleus to enter latency. Upon reawakening newly synthesized viral particles travel anterograde back to the epithelial cells of the lip, causing the recurrent cold sore. HSV1 co-purifies with amyloid precursor protein (APP), a cellular transmembrane glycoprotein and receptor for anterograde transport machinery that when proteolyzed produces A-beta, the major component of senile plaques. Here we focus on transport inside epithelial cells of newly synthesized virus during its transit to the cell surface. We hypothesize that HSV1 recruits cellular APP during transport. We explore this with quantitative immuno-fluorescence, immuno-gold electron-microscopy and live cell confocal imaging. After synchronous infection most nascent VP26-GFP-labeled viral particles in the cytoplasm co-localize with APP (72.8+/−6.7%) and travel together with APP inside living cells (81.1+/−28.9%). This interaction has functional consequences: HSV1 infection decreases the average velocity of APP particles (from 1.1+/−0.2 to 0.3+/−0.1 µm/s) and results in APP mal-distribution in infected cells, while interplay with APP-particles increases the frequency (from 10% to 81% motile) and velocity (from 0.3+/−0.1 to 0.4+/−0.1 µm/s) of VP26-GFP transport. In cells infected with HSV1 lacking the viral Fc receptor, gE, an envelope glycoprotein also involved in viral axonal transport, APP-capsid interactions are preserved while the distribution and dynamics of dual-label particles differ from wild-type by both immuno-fluorescence and live imaging. Knock-down of APP with siRNA eliminates APP staining, confirming specificity. Our results indicate that most intracellular HSV1 particles undergo frequent dynamic interplay with APP in a manner that facilitates viral transport and interferes with normal APP transport and distribution. Such dynamic interactions between APP and HSV1 suggest a mechanistic basis for the observed clinical relationship between HSV1 seropositivity and risk of Alzheimer's disease.

The Tn antigen-structural simplicity and biological complexity

Glycoproteins in animal cells contain a variety of glycan structures that are added co- and/or posttranslationally to proteins. Of over 20 different types of sugar-amino acid linkages known, the two major types are N-glycans (Asn-linked) and O-glycans (Ser/Thr-linked). An abnormal mucin-type O-glycan whose expression is associated with cancer and several human disorders is the Tn antigen. It has a relatively simple structure composed of N-acetyl-D-galactosamine with a glycosidic α linkage to serine/threonine residues in glycoproteins (GalNAcα1-O-Ser/Thr), and was one of the first glycoconjugates to be chemically synthesized. The Tn antigen is normally modified by a specific galactosyltransferase (T-synthase) in the Golgi apparatus of cells. Expression of active T-synthase is uniquely dependent on the molecular chaperone Cosmc, which is encoded by a gene on the X chromosome. Expression of the Tn antigen can arise as a consequence of mutations in the genes for T-synthase or Cosmc, or genes affecting other steps of O-glycosylation pathways. Because of the association of the Tn antigen with disease, there is much interest in the development of Tn-based vaccines and other therapeutic approaches based on Tn expression.

Virus–complement interactions: an assiduous struggle for dominance

The complement system is a major component of the innate immune system that recognizes invading pathogens and eliminates them by means of an array of effector mechanisms, in addition to using direct lytic destruction. Viruses, in spite of their small size and simple composition, are also deftly recognized and neutralized by the complement system. In turn, as a result of years of coevolution with the host, viruses have developed multiple mechanisms to evade the host complement. These complex interactions between the complement system and viruses have been an area of focus for over three decades. In this article, we provide a broad overview of the field using key examples and up-to-date information on the complement-evasion strategies of viruses.

Characterization of the human cytomegalovirus gH/gL/UL128-131 complex that mediates entry into epithelial and endothelial cells

The entry of human cytomegalovirus (HCMV) into biologically relevant epithelial and endothelial cells involves endocytosis followed by low-pH-dependent fusion. This entry pathway is facilitated by the HCMV UL128, UL130, and UL131 proteins, which form one or more complexes with the virion envelope glycoprotein gH/gL. gH/gL/UL128-131 complexes appear to be distinct from the gH/gL/gO complex, which likely facilitates entry into fibroblasts. In order to better understand the assembly and protein-protein interactions of gH/gL/UL128-131 complexes, we generated HCMV mutants lacking UL128-131 proteins and nonreplicating adenovirus vectors expressing gH, gL, UL128, UL130, and UL131. Our results demonstrate that UL128, UL130, and UL131 can each independently assemble onto gH/gL scaffolds. However, the binding of individual UL128-131 proteins onto gH/gL can significantly affect the binding of other proteins; for example, UL128 increased the binding of both UL130 and UL131 to gH/gL. Direct interactions between gH/UL130, UL130/UL131, gL/UL128, and UL128/UL130 were also observed. The export of gH/gL complexes from the endoplasmic reticulum (ER) to the Golgi apparatus and cell surface was dramatically increased when all of UL128, UL130, and UL131 were coexpressed with gH/gL (with or without gO expression). Incorporation of gH/gL complexes into the virion envelope requires transport beyond the ER. Thus, we concluded that UL128, UL130, and UL131 must all bind simultaneously onto gH/gL for the production of complexes that can function in entry into epithelial and endothelial cells.

Random linker-insertion mutagenesis to identify functional domains of herpes simplex virus type 1 glycoprotein B

Herpes simplex virus glycoprotein B (gB) is one of four glycoproteins essential for viral entry and cell fusion. Recently, an x-ray structure of the nearly full-length trimeric gB ectodomain was determined. Five structural domains and two linker regions were identified in what is probably a postfusion conformation. To identify functional domains of gB, we performed random linker-insertion mutagenesis. Analyses of 81 mutants revealed that only 27 could fold to permit processing and transport of gB to the cell surface. These 27 mutants fell into three categories. Insertions into two regions excluded from the solved structure (the N terminus and the C-terminal cytoplasmic tail) had no negative effect on cell fusion and viral entry activity, identifying regions that can tolerate altered structure without loss of function. Insertions into a disordered region in domain II and the adjacent linker region also permitted partial cell fusion and viral entry activity. Insertions at 16 other positions resulted in loss of cell fusion and viral entry activity, despite detectable levels of cell surface expression. Four of these insertion sites were not included in the solved structure. Two were between residues exposed to a cavity that is too small to accommodate the 5-amino acid insertions, consistent with the solved structure being different from the native prefusion structure. Ten were between residues exposed to the surface of the trimer, identifying regions that may be critical for interactions with other viral proteins or cellular components or for transitions from the prefusion to postfusion state.

Sialic acid on herpes simplex virus type 1 envelope glycoproteins is required for efficient infection of cells

Herpes simplex virus type 1 (HSV-1) envelope proteins are posttranslationally modified by the addition of sialic acids to the termini of the glycan side chains. Although gC, gD, and gH are sialylated, it is not known whether sialic acids on these envelope proteins are functionally important. Digestion of sucrose gradient purified virions for 4 h with neuraminidases that remove both alpha2,3 and alpha2,6 linked sialic acids reduced titers by 1,000-fold. Digestion with a alpha2,3-specific neuraminidase had no effect, suggesting that alpha2,6-linked sialic acids are required for infection. Lectins specific for either alpha2,3 or alpha2,6 linkages blocked attachment and infection to the same extent. In addition, the mobility of gH, gB, and gD in sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels was altered by digestion with either alpha2,3 specific neuraminidase or nonspecific neuraminidases, indicating the presence of both linkages on these proteins. The infectivity of a gC-1-null virus, DeltagC2-3, was reduced to the same extent as wild-type virus after neuraminidase digestion, and attachment was not altered. Neuraminidase digestion of virions resulted in reduced VP16 translocation to the nucleus, suggesting that the block occurred between attachment and entry. These results show for the first time that sialic acids on HSV-1 virions play an important role in infection and suggest that targeting virion sialic acids may be a valid antiviral drug development strategy.

The extracellular domain of herpes simplex virus gE is indispensable for efficient cell-to-cell spread: evidence for gE/gI receptors

Herpes simplex virus (HSV) spreads rapidly and efficiently within epithelial and neuronal tissues. The HSV glycoprotein heterodimer gE/gI plays a critical role in promoting cell-to-cell spread but does not obviously function during entry of extracellular virus into cells. Thus, gE/gI is an important molecular handle on the poorly understood process of cell-to-cell spread. There was previous evidence that the large extracellular (ET) domains of gE/gI might be important in cell-to-cell spread. First, gE/gI extensively accumulates at cell junctions, consistent with being tethered there. Second, expression of gE/gI in trans interfered with HSV spread between epithelial cells. To directly test whether the gE ET domain was necessary for gE/gI to promote virus spread, a panel of gE mutants with small insertions in the ET domain was constructed. Cell-to-cell spread was reduced when insertions were made within either of two regions, residues 256 to 291 or 348 to 380. There was a strong correlation between loss of cell-to-cell spread function and binding of immunoglobulin. gE ET domain mutants 277, 291, and 348 bound gI, produced mature forms of gE that reached the cell surface, and were incorporated into virions yet produced plaques similar to gE null mutants. Moreover, all three mutants were highly restricted in spread within the corneal epithelium, in the case of mutant 277 to only 4 to 6% of the number of cells compared with wild-type HSV. Therefore, the ET domain of gE is indispensable for efficient cell-to-cell spread. These observations are consistent with our working hypothesis that gE/gI can bind extracellular ligands, so-called gE/gI receptors that are concentrated at epithelial cell junctions. This fits with similarities in structure and function of gE/gI and gD, which is a receptor binding protein.

Complement: structure, functions, evolution, and viral molecular mimicry

The complement (C') system has long been recognized as an important mediator of innate immune defense and inflammation. In recent years there is increasing evidence suggesting that complement components may also participate in non-inflammatory and developmental processes. Here we review our current work on the structural-functional aspects of C3-ligand interactions and the rational design of small-sized complement inhibitors. We present a novel, proteomics-based, approach to studying protein-protein interactions within the C' system and discuss our progress in the study of viral immune evasion strategies. Furthermore we discuss the involvement of complement proteins in organ regeneration and hematopoietic development.

Envelopment of human cytomegalovirus occurs by budding into Golgi-derived vacuole compartments positive for gB, Rab 3, trans-golgi network 46, and mannosidase II

Although considerable progress has been made towards characterizing virus assembly processes, assignment of the site of tegumentation and envelopment for human cytomegalovirus (HCMV) is still not clear. In this study, we examined the envelopment of HCMV particles in human lung fibroblasts (HF) HL 411 and HL 19, human umbilical vein endothelial cells, human pulmonary arterial endothelial cells, and arterial smooth muscle cells at different time points after infection by electron microscopy (EM), immunohistochemistry, and confocal microscopy analysis. Double-immunofluorescence labeling experiments demonstrated colocalization of the HCMV glycoprotein B (gB) with the Golgi resident enzyme mannosidase II, the Golgi marker TGN (trans-Golgi network) 46, and the secretory vacuole marker Rab 3 in all cell types investigated. Final envelopment of tegumented capsids was observed at 5 days postinfection by EM, when tegumented capsids budded into subcellular compartments located in the cytoplasm, in close proximity to the Golgi apparatus. Immunogold labeling and EM analysis confirmed staining of the budding compartment with HCMV gB, Rab 3, and mannosidase II in HL 411 cells. However, the markers Rab 1, Rab 2, Rab 7, Lamp 1 (late endosomes and lysosomes), and Lamp 2 (lysosomes) neither showed specific staining of the budding compartment in the immunogold labeling experiments nor colocalized with gB in the immunofluorescent colocalization experiments in any cell type studied. Together, these results suggest that the final envelopment of HCMV particles takes place mainly into a Golgi-derived secretory vacuole destined for the plasma membrane, which may release new infectious virus particles by fusion with the plasma membrane.

Herpes simplex virus gE/gI expressed in epithelial cells interferes with cell-to-cell spread

The herpes simplex virus (HSV) glycoprotein heterodimer gE/gI plays an important role in virus cell-to-cell spread in epithelial and neuronal tissues. In an analogous fashion, gE/gI promotes virus spread between certain cell types in culture, e.g., keratinocytes and epithelial cells, cells that are polarized or that form extensive cell junctions. One mechanism by which gE/gI facilitates cell-to-cell spread involves selective sorting of nascent virions to cell junctions, a process that requires the cytoplasmic domain of gE. However, the large extracellular domains of gE/gI also appear to be involved in cell-to-cell spread. Here, we show that coexpression of a truncated form of gE and gI in a human keratinocyte line, HaCaT cells, decreased the spread of HSV between cells. This truncated gE/gI was found extensively at cell junctions. Expression of wild-type gE/gI that accumulates at intracellular sites, in the trans-Golgi network, did not reduce cell-to-cell spread. There was no obvious reduction in production of infectious HSV in cells expressing gE/gI, and virus particles accumulated at cell junctions, not at intracellular sites. Expression of HSV gD, which is known to bind virus receptors, also blocked cell-to-cell spread. Therefore, like gD, gE/gI appears to be able to interact with cellular components of cell junctions, gE/gI receptors which can promote HSV cell-to-cell spread.

Viral mimicry of the complement system

The complement system is a potent innate immune mechanism consisting of cascades of proteins which are designed to fight against and annul intrusion of all the foreign pathogens. Although viruses are smaller in size and have relatively simple structure, they are not immune to complement attack. Thus, activation of the complement system can lead to neutralization of cell-free viruses, phagocytosis of C3b-coated viral particles, lysis of virus-infected cells, and generation of inflammatory and specific immune responses. However, to combat host responses and succeed as pathogens, viruses not only have developed/adopted mechanisms to control complement, but also have turned these interactions to their own advantage. Important examples include poxviruses, herpesviruses, retroviruses, paramyxoviruses and picornaviruses. In this review, we provide information on the various complement evasion strategies that viruses have developed to thwart the complement attack of the host. A special emphasis is given on the interactions between the viral proteins that are involved in molecular mimicry and the complement system.

Progress in molecular and genetic studies of IgA nephropathy

Several new findings emerged recently from biochemical, genetic, and molecular studies of patients with IgA nephropathy. It appears that immunoglobulin A1-secreting cells of IgA nephropathy patients produce increased amounts of aberrantly glycosylated IgA1 in which the O-linked glycans in the hinge region are deficient in the content of galactose. The galactose-deficient IgA1 in the circulation is recognized by naturally occurring antibodies with anti-glycan specificity, and immune complexes are formed. These circulating immune complexes escape hepatic degradation and eventually are deposited in the kidney mesangium. Resident mesangial cells bind the IgA-containing immune complexes with the involvement of a novel IgA receptor and become activated. A familial form of IgA nephropathy has been linked to chromosome 6q22-23. Recent progress in molecular analyses of IgA nephropathy thus defines this disease as an autoimmune process with a novel IgA mesangial receptor and certain genetically determined traits.

The transmembrane domain of the hepatitis C virus E2 glycoprotein is required for correct folding of the E1 glycoprotein and native complex formation

Hepatitis C virus (HCV) encodes two glycoproteins, E1 and E2, that interact to form both native and aggregated complexes in tissue culture cells. In native complexes, E1 and E2 are associated by noncovalent interactions and such complexes are considered to constitute the authentic interactions between the proteins. By contrast, the proteins are linked by covalent, disulfide bonds in aggregated complexes. From studies with a mutant in which cysteine residues in E1 have been substituted with other amino acids, we show that E1 continues to associate with E2, although the migratory patterns of the proteins on gels are consistent with the formation of aggregated complexes. Therefore, such complexes can be stabilized by noncovalent as well as covalent interactions. To further examine the requirements for native complex formation, segments of foreign glycoproteins were linked to regions of E2. Our data provide direct evidence for the requirement of C-terminal sequences in E2 that contain the transmembrane domain to permit oxidation of E1 and assembly of a native complex. By contrast, native complexes and oxidized E1 are not found in the presence of chimeric proteins containing the E2 ectodomain. These data suggest that interaction of E1 with the E2 transmembrane domain is critical for native complex formation.

Human cytomegalovirus pp28 (UL99) localizes to a cytoplasmic compartment which overlaps the endoplasmic reticulum-golgi-intermediate compartment

Although the assembly of herpesviruses has remained an active area of investigation, considerable controversy continues to surround the cellular location of tegument and envelope acquisition. This controversy is particularly evident when the proposed pathways for alpha- and beta-herpesvirus assembly are compared. We have approached this aspect of human cytomegalovirus (HCMV) assembly, specifically, envelopment, by investigating the intracellular trafficking of viral tegument proteins which localize in the cytoplasms of infected cells. In this study we have demonstrated that the virion tegument protein pp28 (UL99), a true late protein, was membrane associated as a result of myristoylation. A mutation in this protein which prevented incorporation of [(3)H]myristic acid also altered the detergent solubility and intracellular distribution of the protein when it was expressed in transfected cells. Using a panel of markers for intracellular compartments, we could localize the expression of wild-type pp28 to an intracellular compartment which colocalized with the endoplasmic reticulum-Golgi-intermediate compartment (ERGIC), a dynamic compartment of the secretory pathway which interfaces with both the ER and Golgi apparatus. The localization of this viral tegument protein within an early secretory compartment of the cell provided further evidence that the assembly of the HCMV tegument likely includes a cytoplasmic phase. Because pp28 has been shown to be localized to a cytoplasmic assembly compartment in HCMV-infected cells, our findings also suggested that viral tegument protein interactions within the secretory pathway may have an important role in the assembly of the virion.

The C-terminal cytoplasmic tail of herpes simplex virus type 1 gE protein is phosphorylated in vivo and in vitro by cellular enzymes in the absence of other viral proteins

Herpes simplex virus 1 glycoprotein E (gE-1) is highly phosphorylated in culture cells during infection. In this report, it is shown that phosphorylation is mediated by host enzymes in human cells stably transfected with gE, in the absence of other herpesvirus products. In contrast, a tailless gE product (C terminus deletion mutant) is not phosphorylated. By using an in vitro kinase assay combined with linker-insertion mutagenesis, it is shown that casein kinase II catalyses the phosphorylation of the C-terminal domain of the protein. Also, it is demonstrated that the serine residues at positions 476 and/or 477 in the cytoplasmic portion of the protein are the major acceptors for the phosphate groups.

Cellular expression of alphaherpesvirus gD interferes with entry of homologous and heterologous alphaherpesviruses by blocking access to a shared gD receptor

Several human and animal alphaherpesviruses can enter cells via human herpesvirus entry mediator C (HveC), a receptor for viral glycoprotein D (gD). In previous studies with cells expressing unknown entry mediators, cellular expression of alphaherpesvirus gD was shown to inhibit entry of the homologous virus and sometimes also of heterologous alphaherpesviruses. To investigate the mechanism of gD-mediated interference and the basis for cross-interference among alphaherpesviruses, HveC was expressed in cells as the sole entry mediator, in the presence or absence of one of the gDs encoded by herpes simplex virus type 1, pseudorabies virus, or bovine herpesvirus type 1. Cells expressing HveC alone were highly susceptible to entry of all three viruses, whereas cells coexpressing HveC and any one of the gDs were at least partially resistant to infection by each virus. Coexpression of gD with HveC did not cause reduced levels of cell-surface HveC but the HveC had reduced ability to bind to exogenous gD. Coimmunoprecipitation experiments revealed that HveC was complexed with gD in lysates of cells expressing both. Thus, cellular expression of gD can interfere with alphaherpesvirus entry by blocking ligand-binding sites of the gD receptor(s) used for entry and cross-interference can occur because different forms of alphaherpesvirus gD can compete for shared entry receptors.

Accumulation of virion tegument and envelope proteins in a stable cytoplasmic compartment during human cytomegalovirus replication: characterization of a potential site of virus assembly

The assembly of human cytomegalovirus (HCMV) is thought to be similar to that which has been proposed for alphaherpesviruses and involve envelopment of tegumented subviral particles at the nuclear membrane followed by export from the cell by a poorly defined pathway. However, several studies have shown that at least two tegument virion proteins remain in the cytoplasm during the HCMV replicative cycle, thereby suggesting that HCMV cannot acquire its final envelope at the nuclear envelope. We investigated the assembly of HCMV by determining the intracellular trafficking of the abundant tegument protein pp150 (UL32) in productively infected human fibroblasts. Our results indicated that pp150 remained within the cytoplasm throughout the replicative cycle of HCMV and accumulated in a stable, juxtanuclear structure late in infection. Image analysis using a variety of cell protein-specific antibodies indicated that the pp150-containing structure was not a component of the endoplasmic reticulum, (ER), ER-Golgi intermediate compartment, cis or medial Golgi, or lysosomes. Partial colocalization of the structure was noted with the trans-Golgi network, and it appeared to lie in close proximity to the microtubule organizing center. Two additional tegument proteins (pp28 and pp65) and three envelope glycoproteins (gB, gH, and gp65) localized in this same structure late infection. This compartment appeared to be relatively stable since pp150, pp65, and the processed form of gB could be coisolated following cell fractionation. Our findings indicated that pp150 was expressed exclusively within the cytoplasm throughout the infectious cycle of HCMV and that the accumulation of the pp150 in this cytoplasmic structure was accompanied by at least five other virion proteins. These results suggested the possibility that this virus-induced structure represented a cytoplasmic site of virus assembly.

Circulating immune complexes in IgA nephropathy consist of IgA1 with galactose-deficient hinge region and antiglycan antibodies

Circulating immune complexes (CICs) isolated from sera of patients with IgA nephropathy (IgAN) consist of undergalactosylated, mostly polymeric, and J chain-containing IgA1 and IgG antibodies specific for N-acetylgalactosamine (GalNAc) residues in O-linked glycans of the hinge region of IgA1 heavy chains. Antibodies with such specificity occur in sera of IgAN patients, and in smaller quantities in patients with non-IgA proliferative glomerulonephritis and in healthy controls; they are present mainly in the IgG (predominantly IgG2 subclass), and less frequently in the IgA1 isotype. Their specificity for GalNAc was determined by reactivity with IgA1 myeloma proteins with enzymatically removed N-acetylneuraminic acid (NeuNAc) and galactose (Gal); removal of the O-linked glycans of IgA1 resulted in significantly decreased reactivity. Furthermore, IgA2 proteins that lack the hinge region with O-linked glycans but are otherwise structurally similar to IgA1 did not react with IgG or IgA1 antibodies. The re-formation of isolated and acid-dissociated CICs was inhibited more effectively by IgA1 lacking NeuNAc and Gal than by intact IgA1. Immobilized GalNAc and asialo-ovine submaxillary mucin (rich in O-linked glycans) were also effective inhibitors. Our results suggest that the deficiency of Gal in the hinge region of IgA1 molecules results in the generation of antigenic determinants containing GalNAc residues that are recognized by naturally occurring IgG and IgA1 antibodies.

Channel catfish virus gene 50 encodes a secreted, mucin-like glycoprotein

Cells infected with the wild-type (WT) strain of channel catfish virus (CCV) secreted a glycoprotein with an apparent molecular mass (MM) superior to 200 kDa into the culture medium. This protein, designated gp250, was the sole viral glycoprotein detected in the culture medium after [3H]mannose labeling of the infected cells. When cells were infected with the attenuated V60 strain, a glycoprotein of 135 kDa (designated gp135) was detected instead of gp250. Because WT gene 50 is predicted to encode a secreted, mucin-type glycoprotein, we expressed this gene transiently and detected a glycoprotein of the same apparent MM as gp250 in the culture medium of transfected catfish cells. The increased mobility in SDS-PAGE of the secreted V60 glycoprotein correlated with the presence of a major deletion in V60 gene 50. Therefore, we concluded that gp250 in the WT and gp135 in the V60 strains are both likely encoded by gene 50. An important shift in the relative mobility of gp250 in SDS-PAGE was observed after tunicamycin treatment of infected cells labeled with [3H]glucosamine, confirming the presence of N-linked sugars on gp250. We observed variations in the size of PCR products derived from gene 50 amplification in three different field isolates. Such genetic variations are a characteristic feature of mucin genes and are linked to crossing-over events between internal repeated sequences, such as those present in gene 50.

The acceptor specificity of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases

The in vitro and in vivo specificity of the family of peptide:N-acetylgalactosaminyltransferases (GalNAcT) is analyzed on the basis of the reactivity and/or inhibitory activity of peptides and protein segments. The transferases appear to be multi-substrate enzymes with extended active sites containing a least nine subsites that interact cooperatively with a linear segment of at least nine amino acid residues on the acceptor polypeptide. Functional acceptor sites are located on the surface of the protein and extended conformations (beta-strand conformation) are preferred. The acceptor specificity of GalNAc-T can be predicted from the primary structure of the acceptor peptide with an accuracy of 70 to 80%. The same GalNAc-T enzymes catalyze the glycosylation of both serine and threonine residues. The higher in vitro catalytic efficiency toward threonine versus serine is the result of enhanced binding as well as increased reaction velocity, both effects being the result of steric interactions between the active site of the enzyme and the methyl group of threonine. Results from substrate binding studies suggest that GalNAc-T catalyzed transfer proceeds via an ordered sequential mechanism.

How do animal DNA viruses get to the nucleus?

Genome and pre-genome replication in all animal DNA viruses except poxviruses occurs in the cell nucleus (Table 1). In order to reproduce, an infecting virion enters the cell and traverses through the cytoplasm toward the nucleus. Using the cell's own nuclear import machinery, the viral genome then enters the nucleus through the nuclear pore complex. Targeting of the infecting virion or viral genome to the multiplication site is therefore an essential process in productive viral infection as well as in latent infection and transformation. Yet little is known about how infecting genomes of animal DNA viruses reach the nucleus in order to reproduce. Moreover, this nuclear locus for viral multiplication is remarkable in that the sizes and composition of the infectious particles vary enormously. In this article, we discuss virion structure, life cycle to reproduce infectious particles, viral protein's nuclear import signal, and viral genome nuclear targeting.

Structure, functions, and evolution of the third complement component and viral molecular mimicry

The third component of the complement system, C3, is a common denominator in the activation of the classical, alternative, and lectin pathways. The ability of C3 molecule to interact with at least 20 different proteins makes it the most versatile component of this system. Since these interactions are important for phagocytic, immunoregulatory, and immune evasion mechanisms, the analysis of its structure and functions has been a subject of intense research. Here we review our current work on the C3-ligand interactions, C3-related viral molecular mimicry, evolution of the complement system, and identification of C3-based complement inhibitors.

Failure to complement infectivity of EBV and HSV-1 glycoprotein B (gB) deletion mutants with gBs from different human herpesvirus subfamilies

Glycoprotein B (gB) is conserved among the herpesvirus family which infects a broad range of species. To investigate the functional homology of human alpha-herpesviruses, beta-herpesviruses, and gamma-herpesviruses gB proteins, complementation studies were performed with gB genes from each subfamily member using EBV gp110 (EBV gB homologue) and HSV-1 gB null mutants. Neither the alpha-herpesvirus HSV-1 gB gene nor the beta-herpesvirus HCMV gB gene were able to complement the gp110 null mutant. Conversely, neither the beta-herpesvirus HCMV gB or the gamma-herpesvirus EBV gp110 gene were able to complement HSV-1 gB null mutants. To further investigate functional domains of EBV gp110 and HSV-1 gB, gB-gp110 chimeric proteins were constructed. Surprisingly, none of the chimeric proteins were able to complement either HSV-1 gB null mutants or EBV gp110 null mutants. These results demonstrate that there is not sufficient functional homology between the different gBs to allow complementation in other subfamily members of the herpesvirus family.

Role of the extracellular domain of human herpesvirus 7 glycoprotein B in virus binding to cell surface heparan sulfate proteoglycans

In an attempt to identify the human herpesvirus 7 (HHV-7) envelope protein(s) involved in cell surface binding, the extracellular domain of the HHV-7 glycoprotein B (gB) homolog protein was cloned and expressed as a fusion product with the Fc domain of human immunoglobulin G heavy chain gamma1 (gB-Fc) in an eukaryotic cell system. Indirect immunofluorescence followed by flow cytometric analysis revealed specific binding of gB-Fc to the membrane of SupT1 cells but not to other CD4+ T-lymphoblastoid cell lines, such as Jurkat or PM1, clearly indicating that gB-Fc did not bind to the CD4 molecule. This was also suggested by the ability of gB-Fc to bind to CD4-negative fibroblastoid Chinese hamster ovary (CHO) cells. The binding was abrogated by enzymatic removal of cell surface heparan sulfate proteoglycans by heparinase and heparitinase but not by treatment with condroitinase ABC. In addition, binding of the gB-Fc fusion protein to CHO cells was severely impaired in the presence of soluble heparin, as well as when heparan sulfate-deficient mutant CHO cells were used. Consistent with these findings, soluble heparin was found to block HHV-7 infection and syncytium formation in the SupT1 cell line. Although the CD4 antigen is a critical component of the receptor for the T-lymphotropic HHV-7, these findings suggest that heparin-like molecules also play an important role in HHV-7-cell surface interactions required for infection and that gB represents one of the HHV-7 envelope proteins involved in the adsorption of virus-to-cell surface proteoglycans.

Biochemical and functional analysis of the Borna disease virus G protein

The Borna disease virus (BDV) antigenome is comprised of five major open reading frames (ORFs). Products have been reported only for ORFs I, II, and III, encoding N (p40), P (p24/p23), and M (gp18), respectively. ORF IV predicts a 57-kDa protein with several potential glycosylation sites. Analysis of radiolabeled extracts from BDV-infected C6 cells and BHK-21 cells transfected with a Semliki Forest virus vector that contains ORF IV demonstrated the presence of a 94-kDa protein (G protein) which was sensitive to tunicamycin, endoglycosidase F/N-glycosidase, and endoglycosidase H but not to O-glycosidase. Sera from BDV-infected rats detected the G protein and had neutralization activity that was reduced following immunoadsorption with the G protein. Preincubation of cells with the G protein interfered with BDV infectivity. This effect was enhanced by treatment of the G protein with the exoglycosidase alpha-mannosidase and reduced after subsequent treatment with N-acetyl-beta-D-glucosaminidase. In concert these findings indicate that ORF IV encodes a 94-kDa N-linked glycoprotein with extensive high mannose- and/or hybrid-type oligosaccharide modifications. The presence of neutralization epitopes on the G protein and its capacity to interfere with infectivity suggest that the G protein is important for viral entry.

Human interferon reduces surface expression but not total production of herpes simplex virus type 1 glycoproteins gC and gE in heterologous hamster cells

The effect of interferon (IFN) on herpes simplex virus type 1 (HSV-1)-induced glycoproteins gC and gE was investigated in a heterologous IFN/cell model. In this model, the effect on surface expression of the glycoproteins could be studied separately from the effect on virus multiplication. Pretreatment of baby hamster kidney cells (BHK) with heterologous human leukocyte IFN suppressed surface expression of HSV-1-encoded gC and gE but had no influence on total production of the glycoproteins. This was in contrast to the effect on human embryonic fibroblast cells (HE) (homologous IFN and cells), where surface expression as well as total production of glycoproteins were reduced. The surface expression was demonstrated by antibody-sensitized monodisperse polystyrene beads, and immunoblotting and two-dimensional electrophoretic analysis of radioisotope-labeled proteins were used to study the total production.

Disulfide bond structure determination and biochemical analysis of glycoprotein C from herpes simplex virus

A biochemical analysis of glycoprotein C (gC of herpes simplex virus was undertaken to further characterize the structure of the glycoprotein and to determine its disulfide bond arrangement. We used three recombinant forms of gC, gC1(457t), gC1(delta33-123t), and gC2(426t), each truncated prior to the transmembrane region. The proteins were expressed and secreted by using a baculovirus expression system and have been shown to bind to monoclonal antibodies which recognize discontinuous epitopes and to complement component C3b in a dose-dependent manner. We confirmed the N-terminal residues of each mature protein by Edman degradation and confirmed the internal deletion in gC1(delta33-123t). The molecular weight and extent of glycosylation of gC1 (457t), gC1(delta33-123t), and gC2(426t) were determined by treating each protein with endoglycosidases and then subjecting it to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and mass spectrometric analysis. The data indicate that eight to nine of the predicted N-linked oligosaccharide sites on gC1(457t) are occupied by glycans of approximately 1,000 Da. In addition, O-linked oligosaccharides are present on gC1(457t), primarily localized to the N-terminal region (amino acids [aa] 33 to 123) of the protein. gC2(426t) contains N-linked oligosaccharides, but no O-linked oligosaccharides were detected. To determine the disulfide bond arrangement of the eight cysteines of gC1(457t),the protein was cleaved with cyanogen bromide. SDS-PAGE analysis followed by Edman degradation identified three cysteine-containing fragments which are not connected by disulfide linkages. Chemical modification of cysteines combined with matrix-assisted laser desorption ionization mass spectrometry identified disulfide bonds between cysteine 1 (aa 127) and cysteine 2 (aa 144) and between cysteine 3 (aa 286) and cysteine 4 (aa 347). Further proteolysis of the cyanogen bromide-generated fragment containing cysteine 5 through cysteine 8, combined with mass spectrometry and Edman degradation, showed that disulfide bonds link cysteine 5 (aa 386) to cysteine 8 (aa 442) and cysteine 6 (aa 390) to cysteine 7 (aa 419). A similar disulfide bond arrangement is postulated to exist in gC homologs from other herpesviruses.

Protein glycosylation in the endoplasmic reticulum and the Golgi apparatus and cell type-specificity of cell surface glycoconjugate expression: analysis by the protein A-gold and lectin-gold techniques

High resolution immunolabeling applying the protein A-gold technique and carbohydrate cytochemistry using lectin-gold labeling on Lowicryl K4M and thawed-frozen thin sections are most useful approaches for the detection of protein antigens and lectin binding sites in intracellular organelles and the plasma membrane. They provided the basis for modern electron microscopic studies on protein glycosylation reactions and the identification of their subcellular localization as reviewed here. These studies have demonstrated organelle subcompartments and the cell type-specific compartmentation of endoplasmic reticulum and Golgi apparatus-associated glycosylation reactions. The other subject reviewed in this paper is cell surface glycoconjugates, as they are expressed in relation to specific cell types present in various organs and during cellular differentiation processes.

Glycosylation of rat NGF receptor ectodomain in the yeast Saccharomyces cerevisiae

Here we studied the glycosylation of a mammalian protein, the ectodomain of rat nerve growth factor receptor (NGFRe), in Saccharomyces cerevisiae. NGFRe is secreted to the culture medium of S. cerevisiae if it is fused to a polypeptide (hsp 150 delta) carrier. The hsp 150 delta-carrier has 95 serine and threonine residues, which were extensively O-glycosylated. In spite of 41 potential sites, NGFRe lacked O-glycans, whether fused to the carrier or not. Distortion of the conformation of NGFRe by inhibition of disulfide formation did not promote O-glycosylation, whereas N-glycosylation was enhanced. Thus, the serine and threonine residues of the hsp 150 delta-NGFRe fusion protein were highly selectively O-glycosylated.

Glycoprotein 110, the Epstein-Barr virus homolog of herpes simplex virus glycoprotein B, is essential for Epstein-Barr virus replication in vivo

The Epstein-Barr virus (EBV) glycoprotein gp110 has substantial amino acid homology to gB of herpes simplex virus but localizes differently within infected cells and is essentially undetectable in virions. To investigate whether gp110, like gB, is essential for EBV infection, a selectable marker was inserted within the gp110 reading frame, BALF4, and the resulting null mutant EBV stain, B95-110HYG, was recovered in lymphoblastoid cell lines (LCLs). While LCLs infected with the parental virus B95-8 expressed the gp110 protein product following productive cycle induction, neither full-length gp110 nor the predicted gp110 truncation product was detectable in B95-110HYG LCLs. Infectious virus could not be recovered from B95-110HYG LCLs unless gp110 was provided in trans. Rescued B95-110HYG virus latently infected and growth transformed primary B lymphocytes. Thus, gp110 is required for the production of transforming virus but not for the maintenance of transformation of primary B lymphocytes by EBV.

Characterization of the US8.5 protein of herpes simplex virus

In a previous study a novel gene designated as US8.5 was identified in the unique short component of the herpes simplex virus type 1 (HSV1) genome. Epitope tagging experiments suggested the existence of a protein encoded by this gene in HSV1 infected cells. To further analyze the US8.5 gene product and function, a rabbit polyclonal antiserum was raised against a recombinant beta-galactosidase-US8.5 fusion protein expressed in E. coli. This antiserum reacted specifically with a 19 kDa protein in HSV1(F) infected cells as shown by immunoblotting and immunoprecipitation experiments. In addition, using the same antiserum a 21 kDa protein was detected in lysates from cells infected with HSV2(G), which was most likely the HSV2 homolog of US8.5. Kinetic studies indicated that US8.5 is expressed as gamma 1 gene. In addition, the US8.5 protein was immunoprecipitated with the anti-US8.5 serum from 32P-labeled lysates of Vero cells infected with HSV1, demonstrating that the protein is phosphorylated. Immunofluorescence studies localized the US8.5 protein to the nucleoli of HSV1 infected cells.

Herpes simplex virus glycoprotein K is known to influence fusion of infected cells, yet is not on the cell surface

Syncytial mutants of herpes simplex virus (HSV) cause extensive fusion of cultured cells, whereas wild-type HSV primarily causes cell rounding and aggregation. A large fraction of syncytial viruses contain mutations in the UL53 gene, which encodes glycoprotein K (gK). Previously, we demonstrated that wild-type and syncytial forms of gK are expressed at similar levels and possess identical electrophoretic mobilities. Using immunofluorescence, we show that gK is not transported to the surfaces of cells infected with either wild-type or syncytial HSV. Instead, gK accumulates in the perinuclear and nuclear membranes of cells. This finding is in contrast to the behavior of all other HSV glycoproteins described to date, which reach the cell surface. When gK was expressed in the absence of other HSV proteins, using a recombinant adenovirus vector, a similar perinuclear and nuclear pattern was observed. In addition, gK remained sensitive to endoglycosidase H, consistent with the hypothesis that gK does not reach the Golgi apparatus and is retained in the endoplasmic reticulum and nuclear envelope. Therefore, although gK mutations promote fusion between the surface membranes of HSV-infected cells, the glycoprotein does not reach the plasma membrane and, thus, must influence fusion indirectly.