Construction and characterization of CV-1P cell lines which constitutively express the simian virus 40 agnoprotein: alteration of plaquing phenotype of viral agnogene mutants

BK Polyomavirus Evades Innate Immune Sensing by Disrupting the Mitochondrial Network and Promotes Mitophagy

Immune escape contributes to viral persistence, yet little is known about human polyomaviruses. BK-polyomavirus (BKPyV) asymptomatically infects 90% of humans but causes premature allograft failure in kidney transplant patients. Despite virus-specific T cells and neutralizing antibodies, BKPyV persists in kidneys and evades immune control as evidenced by urinary shedding in immunocompetent individuals. Here, we report that BKPyV disrupts the mitochondrial network and membrane potential when expressing the 66aa-long agnoprotein during late replication. Agnoprotein is necessary and sufficient, using its amino-terminal and central domain for mitochondrial targeting and network disruption, respectively. Agnoprotein impairs nuclear IRF3-translocation, interferon-beta expression, and promotes p62/SQSTM1-mitophagy. Agnoprotein-mutant viruses unable to disrupt mitochondria show reduced replication and increased interferon-beta expression but can be rescued by type-I interferon blockade, TBK1-inhibition, or CoCl₂-treatment. Mitochondrial fragmentation and p62/SQSTM1-autophagy occur in allograft biopsies of kidney transplant patients with BKPyV nephropathy. JCPyV and SV40 infection similarly disrupt mitochondrial networks, indicating a conserved mechanism facilitating polyomavirus persistence and post-transplant disease.

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BK polyomavirus agnoprotein disrupts mitochondrial membrane potential and network

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Agnoprotein impairs nucleus IRF3 translocation and interferon-β expression

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Agnoprotein facilitates innate immune evasion during the late viral replication phase

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Damaged mitochondria are targeted for p62/SQSTM1 autophagy

Expression of novel proteins by polyomaviruses and recent advances in the structural and functional features of agnoprotein of JC virus, BK virus, and simian virus 40

Polyomavirus family consists of a highly diverse group of small DNA viruses. The founding family member (MPyV) was first discovered in the newborn mouse in the late 1950s, which induces solid tumors in a wide variety of tissue types that are the epithelial and mesenchymal origin. Later, other family members were also isolated from a number of mammalian, avian and fish species. Some of these viruses significantly contributed to our current understanding of the fundamentals of modern biology such as transcription, replication, splicing, RNA editing, and cell transformation. After the discovery of first two human polyomaviruses (JC virus [JCV] and BK virus [BKV]) in the early 1970s, there has been a rapid expansion in the number of human polyomaviruses in recent years due to the availability of the new technologies and brought the present number to 14. Some of the human polyomaviruses cause considerably serious human diseases, including progressive multifocal leukoencephalopathy, polyomavirus-associated nephropathy, Merkel cell carcinoma, and trichodysplasia spinulosa. Emerging evidence suggests that the expression of the polyomavirus genome is more complex than previously thought. In addition to encoding universally expressed regulatory and structural proteins (LT-Ag, Sm t-Ag, VP1, VP2, and VP3), some polyomaviruses express additional virus-specific regulatory proteins and microRNAs. This review summarizes the recent advances in polyomavirus genome expression with respect to the new viral proteins and microRNAs other than the universally expressed ones. In addition, a special emphasis is devoted to the recent structural and functional discoveries in the field of polyomavirus agnoprotein which is expressed only by JCV, BKV, and simian virus 40 genomes.

Agnoprotein Is an Essential Egress Factor during BK Polyomavirus Infection

BK polyomavirus (BKPyV; hereafter referred to as BK) causes a lifelong chronic infection and is associated with debilitating disease in kidney transplant recipients. Despite its importance, aspects of the virus life cycle remain poorly understood. In addition to the structural proteins, the late region of the BK genome encodes for an auxiliary protein called agnoprotein. Studies on other polyomavirus agnoproteins have suggested that the protein may contribute to virion infectivity. Here, we demonstrate an essential role for agnoprotein in BK virus release. Viruses lacking agnoprotein fail to release from host cells and do not propagate to wild-type levels. Despite this, agnoprotein is not essential for virion infectivity or morphogenesis. Instead, agnoprotein expression correlates with nuclear egress of BK virions. We demonstrate that the agnoprotein binding partner α-soluble N-ethylmaleimide sensitive fusion (NSF) attachment protein (α-SNAP) is necessary for BK virion release, and siRNA knockdown of α-SNAP prevents nuclear release of wild-type BK virions. These data highlight a novel role for agnoprotein and begin to reveal the mechanism by which polyomaviruses leave an infected cell.

Structure-based release analysis of the JC virus agnoprotein regions: A role for the hydrophilic surface of the major alpha helix domain in release

Agnoprotein (Agno) is an important regulatory protein of JC virus (JCV), BK virus (BKV) and simian virus 40 (SV40) and these viruses are unable to replicate efficiently in the absence of this protein. Recent 3D-NMR structural data revealed that Agno contains two alpha-helices (a minor and a major) while the rest of the protein adopts an unstructured conformation (Coric et al., 2017, J Cell Biochem). Previously, release of the JCV Agno from the Agno-positive cells was reported. Here, we have further mapped the regions of Agno responsible for its release by a structure-based systematic mutagenesis approach. Results revealed that amino acid residues (Lys22, Lys23, Phe31, Glu34, and Asp38) located either on or adjacent to the hydrophilic surface of the major alpha-helix domain of Agno play critical roles in release. Additionally, Agno was shown to strongly interact with unidentified components of the cell surface when cells are treated with Agno, suggesting additional novel roles for Agno during the viral infection cycle.

[Investigation of Molecular Mechanism of JC virus Viroporin Activity]

Viroporins are small and hydrophobic viral proteins that form pores on host cell membranes, and their expression can increase the permeability of cellular membranes and the production of progeny virus particles. JC virus (JCV) is the causative agent of progressive multifocal leukoenchephalopathy (PML). We demonstrate that JCV Agno, which is the small and hydrophobic protein, andincreases the plasma membrane permeability and virion release, acts as a viroporin. We also demonstrate that an interaction of Agno with a host cellular protein regulates the viroporin activity of Agno. These findings indicate a new paradigm in virus-host interactions regulating viroporin activity and viral replication.

Role of JC virus agnoprotein in virion formation

JC virus (JCV) belongs to the polyomavirus family of double-stranded DNA viruses and causes progressive multifocal leukoencephalopathy in humans. JCV encodes early proteins (large T antigen, small T antigen, and T' antigen) and four late proteins (agnoprotein, and three viral capsid proteins, VP1, VP2, and VP3). In the current study, a novel function for JCV agnoprotein in the morphogenesis of JC virion particles was identified. It was found that mature virions of agnoprotein-negative JCV are irregularly shaped. Sucrose gradient sedimentation and cesium chloride gradient ultracentrifugation analyses revealed that the particles of virus lacking agnoprotein assemble into irregularly sized virions, and that agnoprotein alters the efficiency of formation of VP1 virus-like particles. An in vitro binding assay and immunocytochemistry revealed that agnoprotein binds to glutathione S-transferase fusion proteins of VP1 and that some fractions of agnoprotein colocalize with VP1 in the nucleus. In addition, gel filtration analysis of formation of VP1-pentamers revealed that agnoprotein enhances formation of these pentamers by interacting with VP1. The present findings suggest that JCV agnoprotein plays a role, similar to that of SV40 agnoprotein, in facilitating virion assembly.

Agnoprotein of mammalian polyomaviruses

Polyomaviruses are naked viruses with an icosahedral capsid that surrounds a circular double-stranded DNA molecule of about 5000 base-pairs. Their genome encodes at least five proteins: large and small tumor antigens and the capsid proteins VP1, VP2 and VP3. The tumor antigens are expressed during early stages of the viral life cycle and are implicated in the regulation of viral transcription and DNA replication, while the capsid proteins are produced later during infection. Members of the Polyomaviridae family have been isolated in birds (Avipolyomavirus) and mammals (Orthopolyomavirus and Wukipolyomavirus). Some mammalian polyomaviruses encode an additional protein, referred to as agnoprotein, which is a relatively small polypeptide that exerts multiple functions. This review discusses the structure, post-translational modifications, and functions of agnoprotein, and speculates why not all polyomaviruses express this protein.

The human polyoma JC virus agnoprotein acts as a viroporin

Virus infections can result in a range of cellular injuries and commonly this involves both the plasma and intracellular membranes, resulting in enhanced permeability. Viroporins are a group of proteins that interact with plasma membranes modifying permeability and can promote the release of viral particles. While these proteins are not essential for virus replication, their activity certainly promotes virus growth. Progressive multifocal leukoencephalopathy (PML) is a fatal demyelinating disease resulting from lytic infection of oligodendrocytes by the polyomavirus JC virus (JCV). The genome of JCV encodes six major proteins including a small auxiliary protein known as agnoprotein. Studies on other polyomavirus agnoproteins have suggested that the protein may contribute to viral propagation at various stages in the replication cycle, including transcription, translation, processing of late viral proteins, assembly of virions, and viral propagation. Previous studies from our and other laboratories have indicated that JCV agnoprotein plays an important, although as yet incompletely understood role in the propagation of JCV. Here, we demonstrate that agnoprotein possesses properties commonly associated with viroporins. Our findings demonstrate that: (i) A deletion mutant of agnoprotein is defective in virion release and viral propagation; (ii) Agnoprotein localizes to the ER early in infection, but is also found at the plasma membrane late in infection; (iii) Agnoprotein is an integral membrane protein and forms homo-oligomers; (iv) Agnoprotein enhances permeability of cells to the translation inhibitor hygromycin B; (v) Agnoprotein induces the influx of extracellular Ca(2+); (vi) The basic residues at amino acid positions 8 and 9 of agnoprotein key are determinants of the viroporin activity. The viroporin-like properties of agnoprotein result in increased membrane permeability and alterations in intracellular Ca(2+) homeostasis leading to membrane dysfunction and enhancement of virus release.

Simian virus 40 late proteins possess lytic properties that render them capable of permeabilizing cellular membranes

Many nonenveloped viruses have evolved an infectious cycle that culminates in the lysis or permeabilization of the host to enable viral release. How these viruses initiate the lytic event is largely unknown. Here, we demonstrated that the simian virus 40 progeny accumulated at the nuclear envelope prior to the permeabilization of the nuclear, endoplasmic reticulum, and plasma membranes at a time which corresponded with the release of the progeny. The permeabilization of these cellular membranes temporally correlated with late protein expression and was not observed upon the inhibition of their synthesis. To address whether one or more of the late proteins possessed an inherent capacity to induce membrane permeabilization, we examined the permeability of Escherichia coli that separately expressed the late proteins. VP2 and VP3, but not VP1, caused the permeabilization of bacterial membranes. Additionally, VP3 expression resulted in bacterial cell lysis. These findings demonstrate that VP3 possesses an inherent lytic property that is independent of eukaryotic signaling or cell death pathways.

Cytomegalovirus infection of trophoblast cells elicits an inflammatory response: a possible mechanism of placental dysfunction

OBJECTIVE

We sought to determine whether cytomegalovirus infection of extravillous trophoblast cells induces inflammatory changes that lead to cell death.

STUDY DESIGN

Extravillous trophoblast (HTR-8/SVneo) cells were infected with cytomegalovirus. Cell death assays were performed 8 to 48 hours after infection. The expression and secretion of cytokines (interleukin-6 and -8) preceding cell death was measured by polymerase chain reaction and enzyme-linked immunosorbent assay.

RESULTS

Cell viability (lactate dehydrogenase assay) was reduced approximately 20%, and rates of apoptosis (measured by TdT-mediated dUTP-X nick end labeling [TUNEL] assay) were increased approximately 40% at 16 to 48 hours after infection. Significantly elevated levels of caspase-3 mRNA levels were observed before increased cell death. Interleukin-6 and -8 mRNA expression and protein secretion were up-regulated 8 to 16 hours after cytomegalovirus infection.

CONCLUSION

Placental exposure to cytomegalovirus induces an inflammatory response that precedes invasive trophoblast cell death. Cytomegalovirus may prevent normal placental invasion, which results in adverse reproductive outcomes that are associated with placental dysfunction.

Identification of FEZ1 as a protein that interacts with JC virus agnoprotein and microtubules: role of agnoprotein-induced dissociation of FEZ1 from microtubules in viral propagation

The human polyomavirus JC virus (JCV) is the causative agent of a fatal demyelinating disease, progressive multifocal leukoencephalopathy, and encodes six major proteins, including agnoprotein. Agnoprotein colocalizes with microtubules in JCV-infected cells, but its function is not fully understood. We have now identified fasciculation and elongation protein zeta 1 (FEZ1) as a protein that interacted with JCV agnoprotein in a yeast two-hybrid screen of a human brain cDNA library. An in vitro binding assay showed that agnoprotein interacted directly with FEZ1 and microtubules. A microtubule cosedimentation assay revealed that FEZ1 also associates with microtubules and that agnoprotein induces the dissociation of FEZ1 from microtubules. Agnoprotein inhibited the promotion by FEZ1 of neurite outgrowth in PC12 cells. Conversely, overexpression of FEZ1 suppressed JCV protein expression and intracellular trafficking in JCV-infected cells. These results suggest that FEZ1 promotes neurite extension through its interaction with microtubules, and that agnoprotein facilitates JCV propagation by inducing the dissociation of FEZ1 from microtubules.

The agnoprotein of polyomaviruses: a multifunctional auxiliary protein

The late region of the three primate polyomaviruses (JCV, BKV, and SV40) encodes a small, highly basic protein known as agnoprotein. While much attention during the last two decades has focused on the transforming proteins encoded by the early region (small and large T-antigens), it has become increasingly evident that agnoprotein has a critical role in the regulation of viral gene expression and replication, and in the modulation of certain important host cell functions including cell cycle progression and DNA repair. The importance of agnoprotein is underscored by its expression during lytic infection of glial cells by JCV that occurs in progressive multifocal leukoencephalopathy (PML), and also in some JCV-associated human neural tumors particularly medulloblastoma. In this review, we will discuss the structure and function of agnoprotein in the viral life cycle during the course of lytic infection and the consequences of agnoprotein expression for the host cell.

An overview: Human polyomavirus JC virus and its associated disorders

JC virus (JCV) is a polyomavirus infecting greater than 80% of the human population early in life. Replication of this virus in oligodendrocytes and astrocytes results in the fatal demyelinating disease progressive multifocal leukoencephalopathy (PML) in immunocompromised individuals, most notably acquired immunodeficiency syndrome (AIDS) patients. Moreover, recent studies have pointed to the association of JCV with a variety of brain tumors, including medulloblastoma. The JCV genome encodes for viral early protein, including large and small T antigens and the newly discovered isoform T', at the early phase of infection and the structural proteins VP1, VP2, and VP3 at the late stage of the lytic cycle. In addition, the late gene is responsible for the production of a small nonstructural protein, agnoprotein, whose function is not fully understood. Here, we have summarized some aspects of the JCV genome structure and function, and its associated diseases, including PML and brain tumors.

Functional interaction between JC virus late regulatory agnoprotein and cellular Y-box binding transcription factor, YB-1

Human polyomavirus JC virus (JCV) is a causative agent of progressive multifocal leukoencephalopathy which results from lytic infection of glial cells. Although significant progress has been made in understanding the regulation of JCV gene transcription, the mechanism(s) underlying the viral lytic cycle remains largely unknown. We recently reported that the JCV late auxiliary Agnoprotein may have a regulatory role in JCV gene transcription and replication. Here, we investigated its regulatory function in viral gene transcription through its physical and functional interaction with YB-1, a cellular transcription factor which contributes to JCV gene expression in glial cells. Time course studies revealed that Agnoprotein is first detected at day 3 postinfection and that its level increased during the late stage of the infection cycle. Agnoprotein is mainly localized to the cytoplasmic compartment of the infected cell, with high concentrations found in the perinuclear region. While the position of Agnoprotein throughout the infection cycle remained relatively unaltered, the subcellular distribution of YB-1 between the cytoplasm and nucleus changed. Results from coimmunoprecipitation and glutathione S-transferase pull-down experiments revealed that Agnoprotein physically interacts with YB-1 and that the amino-terminal region of Agnoprotein, between residues 1 and 36, is critical for this association. Further investigation of this interaction by functional assays demonstrated that Agnoprotein negatively regulates YB-1-mediated gene transcription and that the region corresponding to residues 1 to 36 of Agnoprotein is important for the observed regulatory event. Taken together, these data demonstrate that the interaction of the viral late regulatory Agnoprotein and cellular Y-box binding factor YB-1 modulates transcriptional activity of JCV promoters.

The major neutralizing antigenic site on herpes simplex virus glycoprotein D overlaps a receptor-binding domain

Herpes simplex virus (HSV) entry is dependent on the interaction of virion glycoprotein D (gD) with one of several cellular receptors. We previously showed that gD binds specifically to two structurally dissimilar receptors, HveA and HveC. We have continued our studies by using (i) a panel of baculovirus-produced gD molecules with various C-terminal truncations and (ii) a series of gD mutants with nonoverlapping 3-amino-acid deletions between residues 222 and 254. Binding of the potent neutralizing monoclonal antibody (MAb) DL11 (group Ib) was unaffected in forms of gD containing residues 1 to 250 but was greatly diminished in molecules truncated at residue 240 or 234. Both receptor binding and blocking of HSV infection were also affected by these C-terminal truncations. gD-1(234t) bound weakly to both HveA and HveC as determined by enzyme-linked immunosorbent assay (ELISA) and failed to block infection. Interestingly, gD-1(240t) bound well to both receptors but blocked infection poorly, indicating that receptor binding as measured by ELISA is not the only gD function required for blocking. Optical biosensor studies showed that while gD-1(240t) bound HveC with an affinity similar to that of gD-1(306t), the rates of complex formation and dissociation were significantly faster than for gD-1(306t). Complementation analysis showed that any 3-amino-acid deletion between residues 222 and 251 of gD resulted in a nonfunctional protein. Among this set of proteins, three had lost DL11 reactivity (those with deletions between residues 222 and 230). One of these proteins (deletion 222-224) was expressed as a soluble form in the baculovirus system. This protein did not react with DL11, bound to both HveA and HveC poorly as shown by ELISA, and failed to block HSV infection. Since this protein was bound by several other MAbs that recognize discontinuous epitopes, we conclude that residues 222 to 224 are critical for gD function. We propose that the potent virus-neutralizing activity of DL11 (and other group Ib MAbs) likely reflects an overlap between its epitope and a receptor-binding domain of gD.

The agnogene of the human polyomavirus BK is expressed

Primate polyomavirus genomes all contain an open reading frame at the 5' end of the late coding region called the agnogene. A simian virus 40 agnoprotein with unknown functions has previously been demonstrated. We now show that a BK virus agnoprotein appears in the perinuclear area and cytoplasm late in the infectious cycle. It is phosphorylated in vivo and coimmunoprecipitates with a subset of host cell proteins.

Simian virus 40 late mRNA leader sequences involved in augmenting mRNA accumulation via multiple mechanisms, including increased polyadenylation efficiency

We have examined the contribution of 5' leader sequences to expression directed by the simian virus 40 (SV40) late promoter. These studies showed that addition of sequences which contain the late leader 3' splice site to the late promoter led to an increase in the accumulation of mRNA expressed by the promoter. No other sequences within the leader region, between SV40 positions 334 and 560, exhibited a substantial influence on mRNA accumulation. The increase was due, at least in part, to the creation of a spliceable mRNA transcript, since mutation of either the 5' or 3' splice site could attenuate the effect. However, sequences at or near the 3' splice site appeared to play a more important role than did the 5' splice site in bringing about this increase. In many instances, mutation of the 3' splice site also led to the accumulation of extended transcripts, whereas mutation of the 5' splice site did not produce this result in any instance. Analysis of these extended transcripts showed that they retained sequences normally lost upon cleavage and polyadenylation. This finding suggested that mutation of the 3' splice site sequence led to decreases in the efficiency of polyadenylation. We propose that the SV40 late leader sequences positively contribute to expression of the viral late genes by increasing mRNA accumulation via multiple mechanisms, including the enhancement of pre-mRNA polyadenylation efficiency.

Identification of a site on herpes simplex virus type 1 glycoprotein D that is essential for infectivity

Herpes simplex virus glycoprotein D (gD) plays an essential role during penetration of the virus into cells. There is evidence that it recognizes a specific receptor after initial attachment of virions to cell surface heparan sulfate and also that gD-1, gD-2, and gp50 (the pseudorabies virus gD homolog) bind to the same receptor. Although the antigenic structure of gD has been studied intensively, little is known about functional regions of the protein. Antigenic site I is a major target for neutralizing antibodies and has been partially mapped by using deletion mutants and neutralization-resistant viruses. Working on the assumption that such a site may overlap with a functional region of gD, we showed previously that combining two or more amino acid substitutions within site I prevents gD-1 from functioning and is therefore lethal. We have now used a complementation assay to measure the functional activity of a panel of deletion mutants and compared the results with an antigenic analysis. Several mutations cause gross changes in protein folding and destroy functional activity, whereas deletions at the N and C termini have little or no effect on either. In contrast, deletion of residues 234 to 244 has only localized effects on antigenicity but completely abolishes functional activity. This region, which is part of antigenic site Ib, is therefore essential for gD-1 function. The complementation assay was also used to show that a gD-negative type 1 virus can be rescued by gD-2 and by two gD-1-gD-2 hybrids but not by gp50, providing some support for the existence of a common receptor for herpes simplex virus types 1 and 2 but not pseudorabies virus. Alternatively, gp50 may lack a signal for incorporation into herpes simplex virions.

The growth of simian virus 40 (SV40) host range/adenovirus helper function mutants in an African green monkey cell line that constitutively expresses the SV40 agnoprotein

The simian virus 40 T-antigen carboxy-terminal mutants, dlA2459 and dlA2475, are cell line and temperature dependent for growth and plaque formation in monkey kidney cells. Although these mutants did form plaques on BSC-1 cells at 37 degrees C, they were about fivefold less efficient for plaque formation than wild-type simian virus 40. These mutants did not grow in CV-1 cells and did not synthesize agnoprotein in those cells. CV-1 cells which constitutively express the agnoprotein were permissive for mutant plaque formation. However, late mRNAs, virion proteins, and progeny virion yields did not accumulate to wild-type levels during mutant infection of the agnoprotein-producing cells.

Infection of CV1 cells expressing the polyoma virus middle T antigen or the SV40 agnogene product with simian virus 40 host-range mutants

SV40 viruses bearing mutations at the carboxy-terminus of large T antigen exhibit a host-range phenotype: such viruses are able to grow in BSC monkey kidney cells at 37 degrees C, but give at least 10,000-fold lower yields than wild type virus in BSC cells at 32 degrees C or in CV1 monkey kidney cells at either temperature. The block to infection in the nonpermissive cell type occurs after the onset of viral DNA replication. Infectious progeny virions are produced at very low efficiency. Although capsid proteins are synthesized at decreased levels, this does not account for the magnitude of the defect. Presumably some step of virion assembly or maturation is affected in these mutants. We have previously reported that the viral agnogene product, a protein thought to be involved in viral assembly or release, fails to accumulate in CV1 cells infected with host-range mutants. In polyoma virus the middle T antigen plays a role in virion maturation by influencing the phosphorylation of capsid proteins. In this communication we show that host-range mutants fail to undergo productive infection of CV1 cells expressing middle T antigen. These mutants do form plaques on an agnoprotein-expressing cell line. However, the agnoprotein does not seem to act by correcting the mutational block but rather increases the efficiency of plaque formation.

Antigenic and functional analysis of a neutralization site of HSV-1 glycoprotein D

Herpes simplex virus glycoprotein D is a component of the virion envelope and appears to be involved in attachment, penetration, and cell fusion. Monoclonal antibodies (MAbs) against this protein can be arranged in groups, on the basis of a number of biological and biochemical properties. Group I antibodies are type-common, have high complement-independent neutralization titers, recognize discontinuous (conformational) epitopes, and block each other in a binding assay. The sum of their epitopes constitutes antigenic site I of gD. Using a panel of neutralization-resistant mutants, we previously found that group I MAbs can be divided into two subgroups, Ia and Ib, such that mutations selected with Ia antibodies have little or no effect on binding and neutralization by Ib antibodies, and vice versa. Antigenic site I therefore consists of two parts, Ia and Ib. We have now identified the point mutations which prevent neutralization. Two Ib MAbs (DL11 and 4S) selected a Ser to Asn change at residue 140; this alteration creates a new N-linked glycosylation site, which is used. A third Ib MAb (D2) selected a Gln to Leu change at 132. The mutation selected by the Ia MAb HD1 (Ser to Asn at residue 216) is identical to that selected by MAb LP2, another Ia antibody. By using oligonucleotide-directed mutagenesis, we have produced gD genes with combinations of the above mutations. Attempts to recombine these genes into the virus genome were unsuccessful, suggesting that the combinations are lethal. This was confirmed by a complementation assay which measures the ability of gD transiently expressed in transfected Vero cells to rescue the production of infectious virus by the gD-minus mutant F-gD beta.

Efficiency of utilization of the simian virus 40 late polyadenylation site: effects of upstream sequences

The late polyadenylation signal of simian virus 40 functions with greater efficiency than the early polyadenylation signal, in turn affecting steady-state mRNA levels. Two chloramphenicol acetyltransferase (CAT) transient expression vectors, pL-EPA and pL-LPA, that differ only in their polyadenylation signals were constructed by using the early and late polyadenylation signals, respectively. In transfections of Cos, CV-1P, or HeLa cells and subsequent Northern blot analysis of CAT-specific RNA, approximately five times more steady-state CAT mRNA was produced in transfections with pL-LPA than with pL-EPA. The basis for this difference was not related to the specific promoter used or to RNA stability. Overall, the difference in steady-state mRNA levels derived from the two plasmids appeared to be attributable to intrinsic properties of the two polyadenylation signals, resulting in distinctly different cleavage and polyadenylation efficiencies. Additionally, we found that the utilization of the late polyadenylation site was dramatically reduced by deletion of sequences between 48 and 29 nucleotides 5' of the AAUAAA hexanucleotide. This reduction of mRNA levels was shown not to be caused by altered stability of mutant precursor RNAs or mRNAs, suggesting that these upstream sequences constitute an element of the late polyadenylation signal and may cause, at least to some extent, the greater efficiency of utilization of the late polyadenylation site.

Efficiency of utilization of the simian virus 40 late polyadenylation site: effects of upstream sequences

The late polyadenylation signal of simian virus 40 functions with greater efficiency than the early polyadenylation signal, in turn affecting steady-state mRNA levels. Two chloramphenicol acetyltransferase (CAT) transient expression vectors, pL-EPA and pL-LPA, that differ only in their polyadenylation signals were constructed by using the early and late polyadenylation signals, respectively. In transfections of Cos, CV-1P, or HeLa cells and subsequent Northern blot analysis of CAT-specific RNA, approximately five times more steady-state CAT mRNA was produced in transfections with pL-LPA than with pL-EPA. The basis for this difference was not related to the specific promoter used or to RNA stability. Overall, the difference in steady-state mRNA levels derived from the two plasmids appeared to be attributable to intrinsic properties of the two polyadenylation signals, resulting in distinctly different cleavage and polyadenylation efficiencies. Additionally, we found that the utilization of the late polyadenylation site was dramatically reduced by deletion of sequences between 48 and 29 nucleotides 5' of the AAUAAA hexanucleotide. This reduction of mRNA levels was shown not to be caused by altered stability of mutant precursor RNAs or mRNAs, suggesting that these upstream sequences constitute an element of the late polyadenylation signal and may cause, at least to some extent, the greater efficiency of utilization of the late polyadenylation site.

Translational control of synthesis of simian virus 40 late proteins from polycistronic 19S late mRNA

The simian virus 40 (SV40) 19S late mRNA is polycistronic, encoding multiple late proteins: agnoprotein, VP2, and VP3. We constructed a chloramphenicol acetyltransferase (CAT) transient expression vector in which the SV40 sequences between nucleotides 5171 and 1046 (via the SV40 origin of replication and including the late promoter) were inserted 5' to the cat gene; therefore, the AUG for CAT expression occurs after the AUGs for agnoprotein, VP2, and VP3. CAT enzyme activity assayed after transfection of these constructions indicates the level of CAT AUG utilization and, therefore, can be used as a measure of the ability of prior AUGs to intercept scanning ribosomes. Specifically, deletions and point mutations of the viral AUGs resulted in increased CAT enzyme activity owing to increased utilization of the downstream CAT AUG. To compare a variety of mutants, we used the levels of increase to calculate the translational efficiency of the viral AUGs. Some of our data agree with predictions of the modified scanning model (MSM). Little variation in downstream CAT AUG utilization was noted regardless of whether the VP2 AUG (in a weak MSM sequence context) was intact or removed. Hence, a scanning ribosome may easily bypass it. Similar analysis of the VP3 AUG (in a favorable MSM sequence context) demonstrated that it could efficiently intercept ribosomes prior to the downstream AUG. Overall, these data indicate that the structure of the 19S late mRNA and the relative efficiency of translational start codon utilization can account for the VP3/VP2 ratio found in infected cells. The agnoprotein reading frame, depending on how the mRNA precursor is spliced, is either not contained in the mRNA or is terminated near the VP2 AUG. Under these conditions, the ability of the agnoprotein AUG to block downstream CAT AUG utilization was found to be minimal in our assay. However, we directly tested the blocking ability of the agnoprotein AUG under conditions in which the reading frame terminated well after the CAT AUG. Although the agnoprotein AUG lies in a very good sequence context, this direct analysis showed that it interfered minimally with utilization of the CAT AUG when under the control of the SV40 late promoter. However, expected high levels of interference were regained when the late promoter was replaced with the Rous sarcoma virus long terminal repeat.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of herpes simplex virus type 1 glycoprotein D deletion mutants in mammalian cells

Glycoprotein D (gD) is a viron envelope component of herpes simplex virus types 1 and 2. We have previously defined seven monoclonal antibody (MAb) groups which recognize distinct epitopes on the mature gD-1 protein of 369 amino acids. MAb groups VII, II, and V recognize continuous epitopes at residues 11-19, 272-279, and 340-356, respectively. MAb groups I, III, IV, and VI recognize discontinuous epitopes. Recent studies have focused on epitopes I, III, and VI. Using truncated forms of gD generated by recombinant DNA methods and proteolysis, epitopes III, IV, and VI were located within amino acids 1-233. A portion of discontinuous epitope I was located in a region within residues 233-275. For this study, we used recombinant DNA methods to create mutations in the gD-1 gene and studied the effects of those mutations on gD as expressed in mammalian cells. Plasmid pRE4, containing the coding sequence of gD-1 and the Rous sarcoma virus long terminal repeat promoter, was transfected into mammalian cells. The expressed protein, gD-1-(pRE4), was identical in size and antigenic properties to gD-1 from infected cells. Six in-frame deletion mutations were subsequently constructed by using restriction enzymes to excise portions of the gD-1 gene. Plasmids carrying these mutated forms were transfected into cells, and the corresponding proteins were examined at 48 h posttransfection for antigenicity and glycosylation patterns. Three deletions of varying size were located downstream of residue 233. Analysis of these mutants showed that amino acids within the region 234-244 were critical for binding of DL11 (group I), but not for other MAb groups. Three other deletion mutants lost all ability to bind MAbs which recognize discontinuous epitopes. In addition, much of the gD expressed by these mutants was observed to migrate as high-molecular-weight aggregated forms in nondenaturing gels. Each of these mutations involved the loss of a cysteine residue, suggesting that disulfide linkages play an essential role in the formation of discontinuous epitopes. The extent of glycosylation of the mutant gD molecules accumulated at 48 h posttransfection suggested altered carbohydrate processing. In one case, there was evidence for increased O-linked glycosylation. Those proteins which had lost a cysteine residue as part of the deletion did not accumulate molecules processed beyond the high-mannose stage. The results suggest that carbohydrate processing during synthesis of gD is very sensitive to alterations in structure, particularly changes involving cysteine residues.

The contribution of cysteine residues to antigenicity and extent of processing of herpes simplex virus type 1 glycoprotein D

Glycoprotein D (gD) is an envelope component of herpes simplex virus types 1 (gD-1) and 2 (gD-2). The gD-1 polypeptide contains seven cysteine residues among its 369 amino acids; six are located on the N-terminal or luminal portion of the glycoprotein, and a seventh is located in the transmembrane region. Previous studies used a panel of monoclonal antibodies (MAbs) to define gD epitopes as continuous or discontinuous. Purified gD, denatured by reduction and alkylation, loses discontinuous epitopes, whereas continuous epitopes are retained. The contribution of disulfide bonds to maintenance of discontinuous epitopes is, therefore, significant. In the present study, our objective was to determine the contribution of individual cysteine residues to folding of gD-1 into its native conformation. Site-directed oligonucleotide mutagenesis was used to create seven mutants, each with a serine residue replacing a cysteine. The mutated genes were cloned into a eucaryotic expression vector and transfected into COS-1 cells, and the proteins were separated by nondenaturing polyacrylamide gel electrophoresis, followed by immunoblotting. Replacement of cysteine 7 (residue 333) had only a minimal effect on the antigenic properties of gD-1. In contrast, replacement of any one of the other six cysteine residues resulted in either a major reduction or a complete loss of binding of those MAbs that recognize discontinuous epitopes, with no effect on the binding of MAbs which recognize continuous epitopes. These mutations also had profound effects on the extent of oligosaccharide processing of gD-1. This was determined by digestion of the expressed proteins with various endoglycosidases, followed by electrophoresis and Western blotting (immunoblotting) to observe any mobility changes. Three mutant gD proteins which did not express discontinuous epitopes contained only high-mannose-type oligosaccharides, suggesting that processing had not proceeded beyond the precursor stage. Two mutant forms of gD exhibited reduced binding of MAbs to discontinuous epitopes. A small proportion of the molecules which accumulated at 48 h posttransfection contained complex oligosaccharides. One mutant exhibited reduced binding of MAbs to discontinuous epitopes, but was present at 48 h posttransfection only in the precursor form. The cysteine 7 mutant was processed to the same extent as wild-type gD. We conclude that the first six cysteine residues are critical to the correct folding, antigenic structure, and processing of gD-1, and we speculate that they form three disulfide-bonded pairs.

Missense mutations in the VP1 gene of simian virus 40 that compensate for defects caused by deletions in the viral agnogene

Simian virus 40 mutants lacking sequences in the late leader region are viable but produce smaller plaques than does wild-type virus. Within three passages at low multiplicities of infection, virus stocks of several such mutants accumulated variants that synthesized an altered form of the major virion protein, VP1, having a slightly faster mobility in sodium dodecyl sulfate-polyacrylamide gels than did the wild-type protein. Because these variants overgrew the original virus stocks, we consider them to be second-site revertants. By construction and characterization of a series of recombinants, the second-site mutations were shown to map to at least two different regions of the VP1 gene. Nucleotide sequence analysis indicated that single-amino-acid changes were responsible for the rapid mobility of VP1. When combined in cis with either a wild-type or mutant leader region, these VP1 mutations sped up by 10 to 20 h the time course of accumulation of infectious progeny but not of viral DNA or VP1. LP1, the protein encoded by the agnogene, was shown previously to be necessary for the efficient transport of the virion proteins to the nucleus or for their efficient assembly with viral minichromosomes. The VP1 missense mutations reported here compensate for the lack of LP1 by facilitating this process. On the basis of these findings and findings reported previously by us and others, we hypothesize that LP1 facilitates the formation of infectious particles by inhibiting the polymerization of VP1 molecules until the time they interact with viral minichromosomes; the VP1 mutations reported here compensate for the loss of LP1 by lessening the potential of VP1 for self-polymerization.

Selective translation initiation on bicistronic simian virus 40 late mRNA

We described previously a simian virus 40 (SV40) mutant, pSVAdL, that was defective in synthesis of the late viral protein VP1. This mutant, which contains a 100-base-pair fragment of adenovirus DNA encompassing the major late promoter inserted in the SV40 late promoter region (SV40 nucleotide 294), efficiently synthesizes agnoprotein, a protein encoded by the leader region of the same mRNA that encodes VP1. When the agnoprotein AUG initiation codon in pSVAdL was mutated to UUG, agnoprotein synthesis was abolished, and VP1 synthesis was elevated to wild-type levels. Because levels of late mRNA synthesis were not affected by this mutation, these results support a scanning model of translation initiation and suggest that internal translational reinitiation does not occur efficiently in this situation.

Simian virus 40 agnoprotein facilitates perinuclear-nuclear localization of VP1, the major capsid protein

The agnoprotein of simian virus 40 (SV40) is a 61-amino-acid protein encoded in the leader of some late mRNAs. In indirect immunofluorescence studies with antisera against SV40 capsid proteins, we show that mutants which make no agnoprotein display abnormal perinuclear-nuclear localization of VP1, the major capsid protein, but not VP2 or VP3, the minor capsid proteins. In wild-type (WT) SV40-infected CV-1P cells, VP1 was found predominantly in the cytoplasm until 36 h postinfection (p.i.), approximately the time that high levels of agnoprotein became detectable under our infection conditions. Thereafter, VP1 localized rapidly to the perinuclear region and to the nucleus. In contrast, in agnoprotein-minus mutant-infected CV-1P cells, perinuclear-nuclear accumulation of VP1 occurred much less efficiently; a significantly greater fraction of cells with predominantly cytoplasmic fluorescence was observed up to 48 h p.i. At 48 and 60 h p.i., more cells with largely perinuclear and little nuclear staining were seen than in WT-infected controls. In similar analyses with stably transfected cell lines constitutively expressing the agnoprotein, VP1 localized to the nucleus before 30 h p.i., regardless of the infecting virus. Delayed nuclear entry of VP1 in a mutant which makes no agnoprotein was also overcome in a revertant which has a second site point mutation in VP1. This suggests that an alteration of VP1 can partially overcome the defect of the agnogene mutation by enhancement of the rate of its own nuclear localization. Taken together, these results indicate that at least one function of the agnoprotein is to enhance the efficiency of perinuclear-nuclear localization of VP1.