Mutational analysis of human papillomavirus E4 proteins: identification of structural features important in the formation of cytoplasmic E4/cytokeratin networks in epithelial cells

Genetic variability of the HPV16 early genes and LCR. Present and future perspectives

Human papillomavirus 16 (HPV16) infection is the aetiologic factor for the development of cervical dysplasia and is regarded as highly carcinogen, because it is implicated in more than 50% of cervical cancer cases, worldwide. The tumourigenic potential of HPV16 has triggered the extensive sequence analysis of viral genome in order to identify nucleotide variations and amino acid substitutions that influence viral oncogenicity and subsequently the initiation and progression of cervical cancer. Nowadays, specific mutations of HPV16 DNA have been associated with an increased risk of high-grade squamous intraepithelial lesions and invasive cervical cancer (ICC) development, including E6: Q14H, H78Y, L83V, Ε7: N29S, S63F, E2: H35Q, P219S, T310K, E5: I65V, whereas highly conserved regions of viral DNA have been extensively characterised. In addition, numerous novel HPV16 mutations are observed among the studied populations from various geographic regions, hence advocating that different HPV16 strains seem to emerge with different tumourigenic capacities. The present review focuses on the variability of the early genes and the long control region, emphasising on the association of specific mutations with the development of severe dysplasia. Finally, it evaluates whether specific regions of HPV16 DNA are able to serve as valuable biomarkers for cervical cancer risk.

HPV16 and 18 genome amplification show different E4-dependence, with 16E4 enhancing E1 nuclear accumulation and replicative efficiency via its cell cycle arrest and kinase activation functions

To clarify E1^E4’s role during high-risk HPV infection, the E4 proteins of HPV16 and 18 were compared side by side using an isogenic keratinocyte differentiation model. While no effect on cell proliferation or viral genome copy number was observed during the early phase of either virus life cycle, time-course experiments showed that viral genome amplification and L1 expression were differently affected upon differentiation, with HPV16 showing a much clearer E4 dependency. Although E4 loss never completely abolished genome amplification, its more obvious contribution in HPV16 focused our efforts on 16E4. As previously suggested, in the context of the virus life cycle, 16E4s G2-arrest capability was found to contribute to both genome amplification success and L1 accumulation. Loss of 16E4 also lead to a reduced maintenance of ERK, JNK and p38MAPK activity throughout the genome amplifying cell layers, with 16E4 (but not 18E4) co-localizing precisely with activated cytoplasmic JNK in both wild type raft tissue, and HPV16-induced patient biopsy tissue. When 16E1 was co-expressed with E4, as occurs during genome amplification in vivo, the E1 replication helicase accumulated preferentially in the nucleus, and in transient replication assays, E4 stimulated viral genome amplification. Interestingly, a 16E1 mutant deficient in its regulatory phosphorylation sites no longer accumulated in the nucleus following E4 co-expression. E4-mediated stabilisation of 16E2 was also apparent, with E2 levels declining in organotypic raft culture when 16E4 was absent. These results suggest that 16E4-mediated enhancement of genome amplification involves its cell cycle inhibition and cellular kinase activation functions, with E4 modifying the activity and function of viral replication proteins including E1. These activities of 16E4, and the different kinase patterns seen here with HPV18, 31 and 45, may reflect natural differences in the biology and tropisms of these viruses, as well as differences in E4 function.

In HPV induced lesions, the most abundant protein expressed in the productive stage of viral life cycle is E1^E4 (E4), with its expression being coincident with viral genome amplification. To clarify the role of E4 in the high-risk HPV life cycle, we carried out a comparative analysis of E4 function in HPV16 and 18 using an isogenic keratinocyte cell-line background. Our results show that E1^E4 contributes to virus genome replication efficiency and life cycle completion rather than being essential. These effects were seen more dramatically with HPV16. The difference between HPV16 and HPV18 in our system suggests important tropism differences between these viruses. HPV16 E4’s contribution to the virus life cycle is mediated by several activities, including its G2 arrest function, as well as its role in activating members of the MAPK pathway, including ERK, p38, and most notably pJNK. These 16 E4 functions facilitated the nuclear localization of the E1 virus helicase and enhanced E1/E2 dependent viral genome amplification as well as stabilising E2. We suspect that the massive accumulation of E4 in the upper epithelial layers may however underlie a more critical role for E4 post-genome amplification.

A risk for non-sexual transmission of human papillomavirus?

Human papillomaviruses (HPVs) are estimated to be the most common sexually transmitted virus in humans. The virus is of great interest as it is the etiological agent of cervical cancer. Sexual transmission of HPV is generally accepted, however, non-sexual transmission of the virus is often debated. Here, we review the evidence from basic research and clinical studies that show HPV can survive well outside of its host to potentially be transmitted by non-sexual means. In doing so, we hope to discover problems in current prevention practices and show a need for better disinfectants to combat the spread of HPV.

Replication of human papillomavirus in culture

Human papillomaviruses (HPV) are the major factor in causing cervical cancer as well as being implicated in causing oral and anal cancers. The life cycle of HPV is tied to the epithelial differentiation system, as only native virus can be produced in stratified human skin. Initially, HPV research was only possible utilizing recombinant systems in monolayer culture. With new cell culture technology, systems using differentiated skin have allowed HPV to be studied in its native environment. Here, we describe current research studying native virions in differentiated skin including viral assembly, maturation, capsid protein interactions, and L2 cross-neutralizing epitopes. In doing so, we hope to show how differentiating skin systems have increased our knowledge of HPV biology and identify gaps in our knowledge about this important virus.

Human papillomavirus type 1 E1^E4 protein is a potent inhibitor of the serine-arginine (SR) protein kinase SRPK1 and inhibits phosphorylation of host SR proteins and of the viral transcription and replication regulator E2

The serine-arginine-specific protein kinase SRPK1 is a common binding partner of the E1^E4 protein of diverse human papillomavirus types. We show here for the first time that the interaction between HPV1 E1^E4 and SRPK1 leads to potent inhibition of SRPK1 phosphorylation of host serine-arginine (SR) proteins that have critical roles in mRNA metabolism, including pre-mRNA processing, mRNA export, and translation. Furthermore, we show that SRPK1 phosphorylates serine residues of SR/RS dipeptides in the hinge region of the HPV1 E2 protein in in vitro kinase assays and that HPV1 E1^E4 inhibits this phosphorylation. After mutation of the putative phosphoacceptor serine residues, the localization of the E2 protein was altered in primary human keratinocytes; with a significant increase in the cell population showing intense E2 staining of the nucleolus. A similar effect was observed following coexpression of E2 and E1^E4 that is competent for inhibition of SRPK1 activity, suggesting that the nuclear localization of E2 is sensitive to E1^E4-mediated SRPK1 inhibition. Collectively, these data suggest that E1^E4-mediated inhibition of SRPK1 could affect the functions of host SR proteins and those of the virus transcription/replication regulator E2. We speculate that the novel E4 function identified here is involved in the regulation of E2 and SR protein function in posttranscriptional processing of viral transcripts.

IMPORTANCE

The HPV life cycle is tightly linked to the epithelial terminal differentiation program, with the virion-producing phase restricted to differentiating cells. While the most abundant HPV protein expressed in this phase is the E4 protein, we do not fully understand the role of this protein. Few E4 interaction partners have been identified, but we had previously shown that E4 proteins from diverse papillomaviruses interact with the serine-arginine-specific protein kinase SRPK1, a kinase important in the replication cycles of a diverse range of DNA and RNA viruses. We show that HPV1 E4 is a potent inhibitor of this host cell kinase. We show that E4 inhibits SRPK1 phosphorylation, not only of cellular SR proteins involved in regulating alternative splicing of RNA but also the viral transcription/replication regulator E2. Our findings reveal a potential E4 function in regulation of viral late gene expression through the inhibition of a host cell kinase.

The E4 protein; structure, function and patterns of expression

The papillomavirus E4 open reading frame (ORF) is contained within the E2 ORF, with the primary E4 gene-product (E1^E4) being translated from a spliced mRNA that includes the E1 initiation codon and adjacent sequences. E4 is located centrally within the E2 gene, in a region that encodes the E2 protein's flexible hinge domain. Although a number of minor E4 transcripts have been reported, it is the product of the abundant E1^E4 mRNA that has been most extensively analysed. During the papillomavirus life cycle, the E1^E4 gene products generally become detectable at the onset of vegetative viral genome amplification as the late stages of infection begin. E4 contributes to genome amplification success and virus synthesis, with its high level of expression suggesting additional roles in virus release and/or transmission. In general, E4 is easily visualised in biopsy material by immunostaining, and can be detected in lesions caused by diverse papillomavirus types, including those of dogs, rabbits and cattle as well as humans. The E4 protein can serve as a biomarker of active virus infection, and in the case of high-risk human types also disease severity. In some cutaneous lesions, E4 can be expressed at higher levels than the virion coat proteins, and can account for as much as 30% of total lesional protein content. The E4 proteins of the Beta, Gamma and Mu HPV types assemble into distinctive cytoplasmic, and sometimes nuclear, inclusion granules. In general, the E4 proteins are expressed before L2 and L1, with their structure and function being modified, first by kinases as the infected cell progresses through the S and G2 cell cycle phases, but also by proteases as the cell exits the cell cycle and undergoes true terminal differentiation. The kinases that regulate E4 also affect other viral proteins simultaneously, and include protein kinase A, Cyclin-dependent kinase, members of the MAP Kinase family and protein kinase C. For HPV16 E1^E4, these kinases regulate one of the E1^E4 proteins main functions, the association with the cellular keratin network, and eventually also its cleavage by the protease calpain which allows assembly into amyloid-like fibres and reorganisation of the keratin network. Although the E4 proteins of different HPV types appear divergent at the level of their primary amino acid sequence, they share a recognisable modular organisation and pattern of expression, which may underlie conserved functions and regulation. Assembly into higher-order multimers and suppression of cell proliferation are common to all E4 proteins examined. Although not yet formally demonstrated, a role in virus release and transmission remains a likely function for E4.

Targeting human papillomavirus genome replication for antiviral drug discovery

Human papillomavirus (HPV) infections are a major human health problem; they are the cause of recurrent benign warts and of several cancers of the anogenital tract and head and neck region. Although there are two prophylactic HPV vaccines that could, if used universally, prevent as many as two-thirds of HPV-induced cancers, as well as several cytotoxic and immunomodulatory agents for localized treatment of infections, there are currently no HPV antiviral drugs in our arsenal of therapeutic agents. This review examines the status of past and ongoing research into the development of HPV antivirals, focused primarily upon approaches targeting the replication of the viral genome. The only HPV enzyme, E1, is a DNA helicase that interfaces with the cellular DNA replication machinery to replicate the HPV genome. To date, searches for small molecule inhibitors of E1 for use as antivirals have met with limited success. The lack of other viral enzymes has meant that the search for antivirals has shifted to a large degree to the modulation of protein-protein interactions. There has been some success in identifying small molecule inhibitors targeting interactions between HPV proteins but with activity against a small subset of viral types only. As noted in this review, it is thought that targeting E1 interactions with cellular replication proteins may provide inhibitors with broader activity against multiple HPV types. Herein, we outline the steps in HPV DNA replication and discuss those that appear to provide the most advantageous targets for the development of anti-HPV therapeutics.

Human papillomavirus proteins as prospective therapeutic targets

Human papillomaviruses (HPV) are the causative agents of a subset of cervical cancers that are associated with persistent viral infection. The HPV genome is an ∼8 kb circle of double-stranded DNA that encodes eight viral proteins, among which the products of the E6 and E7 open reading frames are recognized as being the primary HPV oncogenes. E6 and E7 are expressed in pre-malignant lesions as well as in cervical cancers; hence these proteins have been extensively studied as potential targets for HPV therapies and novel vaccines. Here we review the expression and functions of E6 and E7 in the viral vegetative cycle and in oncogenesis. We also explore the expression and functions of other HPV proteins, including those with oncogenic properties, and discuss the potential of these molecules as alternative therapeutic targets.

Identification of a novel bat papillomavirus by metagenomics

The discovery of novel viruses in animals expands our knowledge of viral diversity and potentially emerging zoonoses. High-throughput sequencing (HTS) technology gives millions or even billions of sequence reads per run, allowing a comprehensive survey of the genetic content within a sample without prior nucleic acid amplification. In this study, we screened 156 rectal swab samples from apparently healthy bats (n = 96), pigs (n = 9), cattles (n = 9), stray dogs (n = 11), stray cats (n = 11) and monkeys (n = 20) using a HTS metagenomics approach. The complete genome of a novel papillomavirus (PV), Miniopterus schreibersii papillomavirus type 1 (MscPV1), with L1 of 60% nucleotide identity to Canine papillomavirus (CPV6), was identified in a specimen from a Common Bent-wing Bat (M. schreibersii). It is about 7.5kb in length, with a G+C content of 45.8% and a genomic organization similar to that of other PVs. Despite the higher nucleotide identity between the genomes of MscPV1 and CPV6, maximum-likelihood phylogenetic analysis of the L1 gene sequence showed that MscPV1 and Erethizon dorsatum papillomavirus (EdPV1) are most closely related. Estimated divergence time of MscPV1 from the EdPV1/MscPV1 common ancestor was approximately 60.2–91.9 millions of years ago, inferred under strict clocks using the L1 and E1 genes. The estimates were limited by the lack of reliable calibration points from co-divergence because of possible host shifts. As the nucleotide sequence of this virus only showed limited similarity with that of related animal PVs, the conventional approach of PCR using consensus primers would be unlikely to have detected the novel virus in the sample. Unlike the first bat papillomavirus RaPV1, MscPV1 was found in an asymptomatic bat with no apparent mucosal or skin lesions whereas RaPV1 was detected in the basosquamous carcinoma of a fruit bat Rousettus aegyptiacus. We propose MscPV1 as the first member of the novel Dyolambda-papillomavirus genus.

HPV-18 E2^E4 chimera: 2 new spliced transcripts and proteins induced by keratinocyte differentiation

The Human Papillomavirus (HPV) E4 is known to be synthesized as an E1^E4 fusion resulting from splice donor and acceptor sites conserved across HPV types. Here we demonstrate the existence of 2 HPV-18 E2^E4 transcripts resulting from 2 splice donor sites in the 5' part of E2, while the splice acceptor site is the one used for E1^E4. Both E2^E4 transcripts are up-regulated by keratinocyte differentiation in vitro and can be detected in clinical samples containing low-grade HPV-18-positive cells from Pap smears. They give rise to two fusion proteins in vitro, E2^E4-S and E2^E4-L. Whereas we could not differentiate E2^E4-S from E1^E4 in vivo, E2^E4-L could be formally identified as a 23 kDa protein in raft cultures in which the corresponding transcript was also found, and in a biopsy from a patient with cervical intraepithelial neoplasia stage I-II (CINI-II) associated with HPV-18, demonstrating the physiological relevance of E2^E4 products.

Characterization of beta papillomavirus E4 expression in tumours from Epidermodysplasia Verruciformis patients and in experimental models

This study provides a first characterisation of β-HPV life-cycle events in tumours abscised from EV patients (the human model of β-HPV-induced skin cancer), and shows how changes in E4 expression patterns relate to disease severity. β-HPV life-cycle has also been reconstructed in organotypic raft cultures created using EV-derived keratinocytes. In EV lesions and raft cultures, abundant cytoplasmic E4 expression was detectable in differentiating cells along with viral genome amplification as reported for other HPV types. E4 expression was also seen in PCNA-positive basal cells in some EV skin cancers as well as in tumours from HPV8CER (Complete Early Region) transgenic mice. In these lesions, E4 staining extended throughout the full thickness of the epithelium and was apparent in the markedly atypical cells. The loss of such staining at the tumour border suggests a distinct type of E4 dysregulation that may be exploited as a marker of viral expression during β-HPV-associated skin cancer progression.

Role of calpain in the formation of human papillomavirus type 16 E1^E4 amyloid fibers and reorganization of the keratin network

The human papillomavirus (HPV) type 16 E1^E4 (16E1^E4) protein is expressed in the middle to upper layers of infected epithelium and has several roles within the virus life cycle. It is apparent that within the epithelium there are multiple species of 16E1^E4 that differ in length and/or degree of phosphorylation and that some or all of these can associate with the cellular keratin networks, leading to network disruption. We show here that the cellular cysteine protease calpain cleaves the 16E1^E4 protein after amino acid 17 to generate species that lack the N terminus. These C-terminal fragments are able to multimerize and form amyloid-like fibers. This can lead to accumulation of 16E1^E4 and disruption of the normal dynamics of the keratin networks. The cleavage of E1^E4 proteins by calpain may be a common strategy used by α-group viruses, since we show that cleavage of type 18 E1^E4 in raft culture is also dependent on calpain. Interestingly, the cleavage of 16E1^E4 by calpain appears to be highly regulated as differentiation of HPV genome-containing cells by methylcellulose is insufficient to induce cleavage. We hypothesize that this is important since it ensures that the formation of the amyloid fibers is not prematurely triggered in the lower layers and is restricted to the upper layers, where calpain is active and where disruption of the keratin networks may aid virus release.

E1--E4-mediated keratin phosphorylation and ubiquitylation: a mechanism for keratin depletion in HPV16-infected epithelium

The keratin IF network of epidermal keratinocytes provides a protective barrier against mechanical insult, it is also a major player in absorbing stress in these cells. The human papilloma virus (HPV) type 16 E1--E4 protein accumulates in the upper layers of HPV16-infected epithelium and is known to associate with and reorganise the keratin IF network in cells in culture. Here, we show that this function is conserved amongst a number of HPV alpha-group E1--E4 proteins and that the differentiation-dependent keratins are also targeted. Using time-lapse microscopy, HPV16 E1--E4 was found to effect a dramatic cessation of keratin IF network dynamics by associating with both soluble and insoluble keratin. Network disruption was accompanied by keratin hyperphosphorylation at several sites, including K8 S73, which is typically phosphorylated in response to stress stimuli. Keratin immunoprecipitated from E1--E4-expressing cells was also found to be ubiquitylated, indicating that it is targeted for proteasomal degradation. Interestingly, the accumulation of hyperphosphorylated, ubiquitylated E1--E4-keratin structures was found to result in an impairment of proteasomal function. These observations shed new light on the mechanism of keratin IF network reorganisation mediated by HPV16 E1--E4 and provide an insight into the depletion of keratin co-incident with E1--E4 accumulation observed in HPV-infected epithelium.

Phosphorylation of the human papillomavirus type 16 E1--E4 protein at T57 by ERK triggers a structural change that enhances keratin binding and protein stability

The E1--E4 protein of human papillomavirus type 16 (HPV16) causes cytokeratin reorganization in the middle and upper epithelial layers and is thought to contribute to multiple facets of the virus life cycle. Although little is known as to how HPV16 E1--E4 (16E1--E4) functions are controlled following the first expression of this protein, the finding that low-risk E1--E4 proteins can be phosphorylated in vivo suggests an important role for kinases. Here, we show that 16E1--E4 is phosphorylated by cyclin-dependent kinase 1 (CDK1) and CDK2, extracellular signal-regulated kinase (ERK), protein kinase A (PKA), and PKC alpha, with CDK1/2 serine 32 and ERK threonine 57 phosphorylations representing the two primary events seen in cells in cycle. Interestingly, T57 phosphorylation was found to trigger a structural change in the 16E1--E4 protein that compacts the central fold region, leading to an increase in 16E1--E4 stability and overall abundance in the cell. When compared to wild-type 16E1--E4, a T57D phosphomimic was found to have greatly enhanced keratin-binding ability and an ability to modulate the binding of the unphosphorylated form, with keratin binding protecting the T57-phosphorylated form of 16E1--E4 from proteasomal degradation. In HPV16 genome-containing organotypic rafts, the T57-phosphorylated form was specifically detected in the intermediate cell layers, where productive infection occurs, suggesting that T57 phosphorylation may have a functional role at this stage of the viral life cycle. Interestingly, coexpression with 16E5 and ERK activation enhanced T57 phosphorylation, suggesting that E1--E4 and E5 may work together in vivo. Our data suggest a model in which the expression of 16E5 from the major E1--E4-E5 mRNA promotes T57 phosphorylation of E1--E4 and keratin binding, with dephosphorylation occurring following the switch to late poly(A) usage. Other forms of E1--E4, with alternative functional roles, may then increase in prevalence in the upper layers of the epithelium.

Identification of an arginine-rich motif in human papillomavirus type 1 E1;E4 protein necessary for E4-mediated inhibition of cellular DNA synthesis in vitro and in cells

Productive infections by human papillomaviruses (HPVs) are restricted to nondividing, differentiated keratinocytes. HPV early proteins E6 and E7 deregulate cell cycle progression and activate the host cell DNA replication machinery in these cells, changes essential for virus synthesis. Productive virus replication is accompanied by abundant expression of the HPV E4 protein. Expression of HPV1 E4 in cells is known to activate cell cycle checkpoints, inhibiting G(2)-to-M transition of the cell cycle and also suppressing entry of cells into S phase. We report here that the HPV1 E4 protein, in the presence of a soluble form of the replication-licensing factor (RLF) Cdc6, inhibits initiation of cellular DNA replication in a mammalian cell-free DNA replication system. Chromatin-binding studies show that E4 blocks replication initiation in vitro by preventing loading of the RLFs Mcm2 and Mcm7 onto chromatin. HPV1 E4-mediated replication inhibition in vitro and suppression of entry of HPV1 E4-expressing cells into S phase are both abrogated upon alanine replacement of arginine 45 in the full-length E4 protein (E1;E4), implying that these two HPV1 E4 functions are linked. We hypothesize that HPV1 E4 inhibits competing host cell DNA synthesis in replication-activated suprabasal keratinocytes by suppressing licensing of cellular replication origins, thus modifying the phenotype of the infected cell in favor of viral genome amplification.

Structural analysis reveals an amyloid form of the human papillomavirus type 16 E1--E4 protein and provides a molecular basis for its accumulation

The abundant human papillomavirus (HPV) type 16 E4 protein exists as two distinct structural forms in differentiating epithelial cells. Monomeric full-length 16E1--E4 contains a limited tertiary fold constrained by the N and C termini. N-terminal deletions facilitate the assembly of E1--E4 into amyloid-like fibrils, which bind to thioflavin T. The C-terminal region is highly amyloidogenic, and its deletion abolishes amyloid staining and prevents E1--E4 accumulation. Amyloid-imaging probes can detect 16E1--E4 in biopsy material, as well as 18E1--E4 and 33E1--E4 in monolayer cells, indicating structural conservation. Our results suggest a role for fibril formation in facilitating the accumulation of E1--E4 during HPV infection.

The E1circumflexE4 protein of human papillomavirus interacts with the serine-arginine-specific protein kinase SRPK1

Human papillomavirus (HPV) infections of the squamous epithelium are associated with high-level expression of the E1circumflexE4 protein during the productive phase of infection. However, the precise mechanisms of how E1circumflexE4 contributes to the replication cycle of the virus are poorly understood. Here, we show that the serine-arginine (SR)-specific protein kinase SRPK1 is a novel binding partner of HPV type 1 (HPV1) E1circumflexE4. We map critical residues within an arginine-rich domain of HPV1 E1circumflexE4, and in a region known to facilitate E1circumflexE4 oligomerization, that are requisite for SRPK1 binding. In vitro kinase assays show that SRPK1 binding is associated with phosphorylation of an HPV1 E1circumflexE4 polypeptide and modulates autophosphorylation of the kinase. We show that SRPK1 is sequestered into E4 inclusion bodies in terminally differentiated cells within HPV1 warts and that colocalization between E1circumflexE4 and SRPK1 is not dependent on additional HPV1 factors. Moreover, we also identify SRPK1 binding of E1circumflexE4 proteins of HPV16 and HPV18. Our findings indicate that SRPK1 binding is a conserved function of E1circumflexE4 proteins of diverse virus types. SRPK1 influences important biochemical processes within the cell, including nuclear organization and RNA metabolism. While phosphorylation of HPV1 E4 by SRPK1 may directly influence HPV1 E4 function during the infectious cycle, the modulation and sequestration of SRPK1 by E1circumflexE4 may affect the ability of SRPK1 to phosphorylate its cellular targets, thereby facilitating the productive phase of the HPV replication cycle.

The full-length E1E4 protein of human papillomavirus type 18 modulates differentiation-dependent viral DNA amplification and late gene expression

Activation of the productive phase of the human papillomavirus (HPV) life cycle in differentiated keratinocytes is coincident with high-level expression of E1E4 protein. To determine the role of E1E4 in the HPV replication cycle, we constructed HPV18 mutant genomes in which expression of the full-length E1E4 protein was abrogated. Undifferentiated keratinocytes containing mutant genomes showed enhanced proliferation when compared to cells containing wildtype genomes, but there were no differences in maintenance of viral episomes. Following differentiation, cells with mutant genomes exhibited reduced levels of viral DNA amplification and late gene expression, compared to wildtype genome-containing cells. This indicates that HPV18 E1E4 plays an important role in regulating HPV late functions, and it may also function in the early phase of the replication cycle. Our finding that full-length HPV18 E1E4 protein plays a significant role in promoting viral genome amplification concurs with a similar report with HPV31, but is in contrast to an HPV11 study where viral DNA amplification was not dependent on full-length E1E4 expression, and to HPV16 where only C-terminal truncations in E1E4 abrogated vegetative genome replication. This suggests that type-specific differences exist between various E1E4 proteins.

Role for Wee1 in inhibition of G2-to-M transition through the cooperation of distinct human papillomavirus type 1 E4 proteins

The infectious cycle of human papillomavirus type 1 (HPV1) is accompanied by abundant expression of the full-length E1;E4 protein (17-kDa) and smaller E4 polypeptides (16-, 11-, and 10-kDa) that arise by sequential loss of N-terminal E1;E4 sequences. HPV1 E4 inhibits G(2)-to-M transition of the cell cycle. Here, we show that HPV1 E4 proteins mediate inhibition of cell division by more than one mechanism. Cells arrested by coexpression of E1;E4 (E4-17K) and a truncated protein equivalent to the 16-kDa species (E4-16K) contain inactive cyclin B1-cdk1 complexes. Inactivation of cdk1 is through inhibitory Tyr(15) phosphorylation, with cells containing elevated levels of Wee1, the kinase responsible for inhibitory cdk1 phosphorylation. Consistent with these findings, overexpression of Wee1 enhanced the extent to which E4-17K/16K-expressing cells arrest in G(2), indicating that maintenance of Wee1 activity is necessary for inhibition of cell division induced by coexpression of the two E4 proteins. Moreover, we have determined that depletion of Wee1 by small interfering RNA (siRNA) alleviates the G(2) block imposed by E4-17K/16K. In contrast however, maintenance of Wee1 activity is not necessary for G(2)-to-M inhibition mediated by E4-16K alone, as overexpression or depletion of Wee1 does not influence the G(2) arrest function of E4-16K. Cells arrested by E4-16K expression contain low levels of active cyclin B1-cdk1 complexes. We hypothesize that differential expression of HPV1 E4 proteins during the viral life cycle determines the host cell cycle status. Different mechanisms of inhibition of G(2)-to-M transition reinforce the supposition that distinct E4 functions are important for HPV replication.

The human papillomavirus type 11 E1/\E4 protein is not essential for viral genome amplification

An abundant human papillomavirus (HPV) protein E1/\E4 is expressed late in the virus life cycle in the terminally differentiated layers of epithelia. The expression of E1/\E4 usually coincides with the onset of viral DNA amplification. However, the function of E1/\E4 in viral life cycle is not completely understood. To examine the role of E1/\E4 in the virus life cycle, we introduced a single nucleotide change in the HPV-11 genome to result in a truncation of E1/\E4 protein without affecting the E2 amino acid sequence. This mutated HPV-11 genome was introduced into a human foreskin keratinocyte cell line immortalized by the catalytic subunit of human telomerase, deficient in p16(INK4a) expression, and previously shown to support the HPV-11 life cycle when grown in organotypic raft culture. We have demonstrated that E1/\E4 is dispensable for HPV-11 viral DNA amplification in the late stages of the viral life cycle.

Human papillomaviruses: basic mechanisms of pathogenesis and oncogenicity

Human papillomaviruses (HPVs) are small double-stranded DNA viruses that infect the cutaneous and mucosal epithelium. Infection by specific HPV types has been linked to the development of cervical carcinoma. HPV infects epithelial cells that undergo terminal differentiation and so encode multiple mechanisms to override the normal regulation of differentiation to produce progeny virions. Two viral proteins, E6 and E7, alter cell cycle control and are the main arbitrators of HPV-induced oncogenesis. Recent data suggest that E6 and E7 also play a major role in the inhibition of the host cell innate immune response to HPV. The E1 and E2 proteins, in combination with various cellular factors, mediate viral replication. In addition, E2 has been implicated in both viral and cellular transcriptional control. Despite decades of research, the function of other viral proteins still remains unclear. While prophylactic vaccines to block genital HPV infection will soon be available, the widespread nature of HPV infection requires greater understanding of both the HPV life cycle as well as the mechanisms underlying HPV-induced carcinogenesis.

Human papillomavirus type 16 E1circumflexE4 contributes to multiple facets of the papillomavirus life cycle

The life cycle of human papillomaviruses (HPVs) is tightly linked to the differentiation program of the host's stratified epithelia that it infects. E1(circumflex)E4 is a viral protein that has been ascribed multiple biochemical properties of potential biological relevance to the viral life cycle. To identify the role(s) of the viral E1(circumflex)E4 protein in the HPV life cycle, we characterized the properties of HPV type 16 (HPV16) genomes harboring mutations in the E4 gene in NIKS cells, a spontaneously immortalized keratinocyte cell line that when grown in organotypic raft cultures supports the HPV life cycle. We learned that E1(circumflex)E4 contributes to the replication of the viral plasmid genome as a nuclear plasmid in basal cells, in which we also found E1(circumflex)E4 protein to be expressed at low levels. In the suprabasal compartment of organotypic raft cultures harboring E1(circumflex)E4 mutant HPV16 genomes there were alterations in the frequency of suprabasal cells supporting DNA synthesis, the levels of viral DNA amplification, and the degree to which the virus perturbs differentiation. Interestingly, the comparison of the phenotypes of various mutations in E4 indicated that the E1(circumflex)E4 protein-encoding requirements for these various processes differed. These data support the hypothesis that E1(circumflex)E4 is a multifunctional protein and that the different properties of E1(circumflex)E4 contribute to different processes in both the early and late stages of the virus life cycle.

The human papillomavirus type 11 E1--E4 protein is a transglutaminase 3 substrate and induces abnormalities of the cornified cell envelope

The human papillomavirus (HPV) E1--E4 protein is detected in the cytoplasm of differentiated keratinocytes, near the cornified cell envelope. HPV does not induce lysis of the infected keratinocyte, and the normally durable cornified cell envelope that forms during keratinocyte differentiation would seemingly inhibit viral egress. HPV infection induces abnormalities of the cornified cell envelope, but the exact mechanisms involved are not well understood. We tested whether the HPV 11 E1--E4 protein, which co-localizes the cell envelope and co-purifies with cell envelope fragments, could serve as an in vitro substrate for transglutaminases. We found evidence of E1--E4 cross-linking by endogenous transglutaminases in an in situ assay using frozen sections of human foreskin, and in addition, E1--E4 protein was cross-linked by recombinant transglutaminase 3 (but not transglutaminase 1) in an in vitro cross-linking assay. We also tested whether expression of E1--E4 in differentiated keratinocytes would induce morphologic alterations of cornified cell envelopes. Differentiated keratinocytes expressing E1--E4 were disorganized and pleomorphic compared to control cells, and cell envelopes purified from E1--E4-expressing cells were small, fragmented, and rough bordered compared to the round, smooth bordered cell envelopes from control cells. We conclude from these in vitro experiments that the E1--E4 protein is cross-linked by transglutaminase 3, and that E1--E4 expression in differentiated keratinocytes induces morphologic abnormalities of the cornified cell envelope.

Human papillomavirus infection: biology, epidemiology, and prevention

Over the past several decades, knowledge of the biology and epidemiology of human papillomavirus (HPV) infection has increased tremendously. However, there are still many unanswered questions concerning the interaction of the virus with its host. The virus has been identified as a necessary causal agent for cervical squamous neoplasia and has been linked to the development of neoplasia in several other mucosal sites. The viral oncogenes E6 and E7 are the major players in the virus' scheme to evade the immune system and use the host cell replication machinery to survive. Many risk factors for infection with HPV have been identified; however, the focus now centers on identifying risk factors for persistence of the infection as it is likely that transient infections play a very small role in the overall development of clinical disease. Prevention measures to date have centered around screening programs, mostly for cervical cancer, including the perfection of screening techniques and inclusion of molecular testing for HPV into screening regimens. The development of prophylactic and therapeutic vaccines has also increased as primary prevention measures appear to have the best hope for long-term effects on cancer incidence.

Emerging therapies for human papillomavirus infection

Human papillomavirus (HPV) is the most common sexually transmitted infection, with > 50% of sexually active women being affected. The virus causes a wide variety of benign and pre-malignant epithelial tumours and although most infections are transient, it is estimated that 1% of the sexually active population in the US have clinically apparent genital warts. A subset of genital HPVs, termed high-risk HPVs, is highly associated with the development of genital cancers including cervical carcinoma. Therapies for these HPV related cancers are however outside of the scope of this review. The absence of a simple monolayer cell culture system for analysis and propagation of the virus has substantially retarded progress in the development of diagnostic and therapeutic strategies for HPV infection. In spite of these difficulties, great progress has been made in the elucidation of the molecular controls of virus gene expression, replication and pathogenesis, and there has been some progress in the development of prophylactic and therapeutic vaccines and of other therapies.

Mitochondria as Functional Targets of Proteins Coded by Human Tumor Viruses

Molecular analyses of tumor virus-host cell interactions have provided key insights into the genes and pathways involved in neoplastic transformation. Recent studies have revealed that the human tumor viruses Epstein-Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV), human papillomavirus (HPV), hepatitis B virus (HBV), hepatitis C virus (HCV), and human T-cell leukemia virus type 1 (HTLV-1) express proteins that are targeted to mitochondria. The list of these viral proteins includes BCL-2 homologues (BHRF1 of EBV; KSBCL-2 of KSHV), an inhibitor of apoptosis (IAP) resembling Survivin (KSHV K7), proteins that alter mitochondrial ion permeability and/or membrane potential (HBV HBx, HPV E[wedge]14, HCV p7, and HTLV-1 p13(II)), and K15 of KSHV, a protein with undefined function. Consistent with the central role of mitochondria in energy production, cell death, calcium homeostasis, and redox balance, experimental evidence indicates that these proteins have profound effects on host cell physiology. In particular, the viral BCL-2 homologues BHRF1 and KSBCL-2 inhibit apoptosis triggered by a variety of stimuli. HBx, p7, E1[wedge]4, and p13(II) exert powerful effects on mitochondria either directly due to their channel-forming activity or indirectly through interactions with endogenous channels. Further investigation of these proteins and their interactions with mitochondria will provide important insights into the mechanisms of viral replication and tumorigenesis and could aid in the discovery of new targets for anti-tumor therapy.

Cooperation between different forms of the human papillomavirus type 1 E4 protein to block cell cycle progression and cellular DNA synthesis

Posttranslational modification-oligomerization, phosphorylation, and proteolytic cleavage-of the human papillomavirus (HPV) E4 protein occurs as the infected keratinocytes migrate up through the suprabasal wart layers. It has been postulated that these events modify E4 function during the virus life cycle. In HPV type 1 (HPV1)-induced warts, N-terminal sequences are progressively cleaved from the full-length E4 protein (E1(wedge)E4) of 17 kDa to produce a series of polypeptides of 16, 11 and 10 kDa. Here, we have shown that in human keratinocytes, a truncated protein (E4-16K), equivalent to the 16-kDa species, mediated a G(2) arrest in the cell cycle that was dependent on a threonine amino acid in a proline-rich domain of the protein. Reconstitution of cyclin B1 expression in E4-16K cells reversed the G(2) arrest. Expression of E4-16K also induced chromosomal rereplication, and this was associated with aberrant nuclear morphology. Perturbation of the mitotic cell cycle was a biological activity specific to the truncated protein. However, coexpression of the full-length E1(wedge)E4 protein and the truncated E4-16K protein inhibited normal cellular proliferation and cellular DNA rereplication but did not prevent cells from arresting in G(2). Our findings provide the first evidence to support the hypothesis that proteolytic cleavage of the E1(wedge)E4 protein modifies its function. Also, different forms of the HPV1 E4 protein cooperate to negatively influence keratinocyte proliferation. We predict that these distinct biological activities of E4 act to support efficient amplification of the viral genome in suprabasal keratinocytes.

Functional analysis of the human papillomavirus type 16 E1=E4 protein provides a mechanism for in vivo and in vitro keratin filament reorganization

High-risk human papillomaviruses, such as human papillomavirus type 16 (HPV16), are the primary cause of cervical cancer. The HPV16 E1=E4 protein associates with keratin intermediate filaments and causes network collapse when expressed in epithelial cells in vitro. Here, we show that keratin association and network reorganization also occur in vivo in low-grade cervical neoplasia caused by HPV16. The 16E1=E4 protein binds to keratins directly and interacts strongly with keratin 18, a member of the type I intermediate-filament family. By contrast, 16E1=E4 bound only weakly to keratin 8, a type II intermediate-filament protein, and showed no detectable affinity for the type III protein, vimentin. The N-terminal 16 amino acids of the 16E1=E4 protein, which contains the YPLLXLL motif that is conserved among supergroup A viruses, were sufficient to target green fluorescent protein to the keratin network. When expressed in the SiHa cervical epithelial cell line, the full-length 16E1=E4 protein caused an almost total inhibition of keratin dynamics, despite the phosphorylation of keratin 18 at serine 33, which normally leads to 14-3-3-mediated keratin solubilization. Mutant 16E1=E4 proteins which lack the LLKLL motif, or which have lost amino acids from their C termini, and which were compromised in the ability to associate with keratins did not disturb normal keratin dynamics. 16E1=E4 was found to exist as dimers and hexamers, whereas a C-terminal deletion mutant (16E1=E4Delta87-92) existed as monomers and formed multimeric structures only poorly. Considered together, our results suggest that by associating with keratins through its N terminus, and by associating with itself through its C terminus, 16E1=E4 may act as a keratin cross-linker and prevent the movement of keratins between the soluble and insoluble compartments. The increase in avidity associated with multimeric binding may contribute to the ability of 16E1=E4 to sequester its cellular targets in the cytoplasm.

The ND10 component promyelocytic leukemia protein relocates to human papillomavirus type 1 E4 intranuclear inclusion bodies in cultured keratinocytes and in warts

Human papillomavirus type 1 (HPV1) E4 protein is associated with cytoplasmic and nuclear inclusions in productively infected keratinocytes. Here we have used transient expression of HPV1 E4 (also known as E1E4) protein in keratinocytes to reproduce formation of E4 inclusions. Immunofluorescence analysis showed that progressive formation of inclusions correlated with diminished colocalization between E4 and keratin intermediate filaments (IFs). Our results support a model in which the HPV1 E4-keratin IF association is transient, occurring only at an early stage of inclusion formation. We also demonstrate that E4 induces relocation of the promyelocytic leukemia protein (PML) from multiple intranuclear speckles (ND10 bodies) to the periphery of nuclear E4 inclusions and that this activity is specific to full-length E4 protein. Analysis of HPV1-induced warts demonstrated that nuclear PML-E4 inclusions were present in productively infected keratinocytes, indicating that reorganization of PML occurs during the virus's replication cycle. It has been suggested that ND10 bodies are the sites for papillomavirus genome replication and virion assembly. Our finding that E4 induces reorganization of ND10 bodies in vitro and in vivo is further strong evidence that these domains play an important role in the papillomavirus life cycle. This study indicates that HPV1 is analogous to other DNA viruses that disrupt or reorganize ND10 domains, possibly to increase efficiency of virus infection. We hypothesize that HPV1 E4-induced reorganization of PML is necessary for efficient replication of the virus during the virus-producing phase.

Modulation of the cell division cycle by human papillomavirus type 18 E4

The life cycle of human papillomaviruses (HPVs) is tightly coupled to the differentiation program of their host epithelial cells. HPV E4 gene expression is first observed in the parabasal layers of squamous epithelia, suggesting that the E4 gene product contributes to the mechanism of differentiation-dependent virus replication, although its biological function remains unclear. We analyzed the effect of HPV type 18 E4 on cell proliferation and found that E4 expression induced cell cycle arrest at the G(2)/M boundary. The functional region of E4 necessary for the growth arrest activity was located in the central portion of the molecule, and this activity was independent of the E4-mediated collapse of cytokeratin intermediate filament structures.

Identification of a G(2) arrest domain in the E1 wedge E4 protein of human papillomavirus type 16

Human papillomavirus type 16 (HPV16) is the most common cause of cervical carcinoma. Cervical cancer develops from low-grade lesions that support the productive stages of the virus life cycle. The 16E1 wedge E4 protein is abundantly expressed in such lesions and can be detected in cells supporting vegetative viral genome amplification. Using an inducible mammalian expression system, we have shown that 16E1 wedge E4 arrests HeLa cervical epithelial cells in G(2). 16E1 wedge E4 also caused a G(2) arrest in SiHa, Saos-2 and Saccharomyces pombe cells and, as with HeLa cells, was found in the cytoplasm. However, whereas 16E1 wedge E4 is found on the keratin networks in HeLa and SiHa cells, in Saos-2 and S. pombe cells that lack keratins, 16E1 wedge E4 had a punctate distribution. Mutagenesis studies revealed a proline-rich region between amino acids 17 and 45 of 16E1 wedge E4 to be important for arrest. This region, which we have termed the "arrest domain," contains a putative nuclear localization signal, a cyclin-binding motif, and a single cyclin-dependent kinase (Cdk) phosphorylation site. A single point mutation in the putative Cdk phosphorylation site (T23A) abolished 16E1 wedge E4-mediated G(2) arrest. Arrest did not involve proteins regulating the phosphorylation state of Cdc2 and does not appear to involve the activation of the DNA damage or incomplete replication checkpoint. G(2) arrest was also mediated by the E1 wedge E4 protein of HPV11, a low-risk mucosal HPV type that also causes cervical lesions. The E1 wedge E4 protein of HPV1, which is more distantly related to that of HPV16, did not cause G(2) arrest. We conclude that, like other papillomavirus proteins, 16E1 wedge E4 affects cell cycle progression and that it targets a conserved component of the cell cycle machinery.

Differential immunogenicity of Epstein-Barr virus latent-cycle proteins for human CD4(+) T-helper 1 responses

Human CD4(+) T-helper 1 cell responses to Epstein-Barr virus (EBV) infection are likely to be important in the maintenance of virus-specific CD8(+) memory and/or as antiviral effectors in their own right. The present work has used overlapping peptides as stimulators of gamma interferon release (i) to identify CD4(+) epitopes within four EBV latent-cycle proteins, i.e., the nuclear antigens EBNA1 and EBNA3C and the latent membrane proteins LMP1 and LMP2, and (ii) to determine the frequency and magnitude of memory responses to these proteins in healthy virus carriers. Responses to EBNA1 and EBNA3C epitopes were detected in the majority of donors, and in the case of EBNA1, their antigen specificity was confirmed by in vitro reactivation and cloning of CD4(+) T cells using protein-loaded dendritic cell stimulators. By contrast, responses to LMP1 and LMP2 epitopes were seen much less frequently. EBV latent-cycle proteins therefore display a marked hierarchy of immunodominance for CD4(+) T-helper 1 cells (EBNA1, EBNA3C >> LMP1, LMP2) which is different from that identified for the same proteins with respect to CD8(+)-T-cell responses (EBNA3C > EBNA1 > LMP2 >> LMP1). Furthermore, the range of CD4(+) memory T-cell frequencies in peripheral blood of healthy virus carriers was noticeably lower and narrower than the corresponding range of latent antigen-specific CD8(+)-T-cell frequencies.

Association of the human papillomavirus type 11 E1()E4 protein with cornified cell envelopes derived from infected genital epithelium

The cornified cell envelope (CCE) is an insoluble matrix of covalently linked proteins assembled in differentiating keratinocytes, providing a barrier against external insults. CCEs derived from HPV 11-infected tissue are fragile compared to those derived from healthy epithelium. To study a possible role for the E1()E4 protein, HPV 11-infected epithelium was examined for the distribution of this protein and three CCE proteins. CCEs were then purified from genital epithelium, fragmented, washed to remove nonassociated proteins, and analyzed for E1()E4 protein. In HPV 11-infected tissue, the E1()E4 protein was detected in the region of the CCE in differentiated keratinocytes. Loricrin and cytokeratin 10 (K10) were absent in E1()E4-positive cells, and E1()E4 protein was not detected in cells containing these proteins. E1()E4 protein was detected in immunoblots as a 10- to 11-kDa doublet in extracts of intact CCEs from infected tissue and in extracts of CCE fragments prepared without using reducing agents. Extraction with reducing agents eliminated E1()E4 detection, suggesting that disulfide bonding was involved in the association with CCE fragments. In addition, cyanogen bromide degradation experiments, immunofluorescence, and immunoelectron microscopy provided evidence that E1()E4 protein was associated with CCE fragments by covalent bonds other than disulfide bonds. We conclude that E1()E4 protein expression is associated with profound alterations in detection of loricrin and K10 in HPV 11-infected genital epithelium. The E1()E4 protein copurified with CCEs derived from infected epithelium and could be identified in CCE fragments, suggesting a possible role for E1()E4 in the development of CCE abnormalities.

Asian-american variants of human papillomavirus type 16 have extensive mutations in the E2 gene and are highly amplified in cervical carcinomas

Human-papillomavirus (HPV)-E2 protein is involved in gene-expression regulation and replication of HPV genome. Disruption of the E2 gene during viral integration has been proposed as a mechanism of tumoral progression, since the expression of E6/E7 viral oncogenes is allowed. However, retention of E1/E2 genes and high viral amplification are frequently found in HPV16-positive carcinomas of some populations. In this study, we investigated whether retention of E1/E2 and viral amplification are associated with particular HPV16 E2 variants in cervical carcinomas. HPV16 detection, E1/E2 integrity and viral amplification were explored by Southern blot in 123 cervical carcinomas. HPV16 variants were identified by Southern blot and by sequencing E6, L1/MY and E2 regions. Of 46 HPV16-positive tumors, 34 were positive for E1/E2 and 14 of them showed a variant restriction pattern by mutations in E2. All 14 were Asian-American (AA) variants and, of 11 sub-classified, 6 were AA-a and 5 AA-c. Two E1/E2-negative tumors also contained the AA-c variant, while the remaining HPV16-positive tumors contained only European variants. The E2 gene of AA variants showed 24 mutations, 19 identical in both sub-classes. The 24 mutations were distributed throughout the entire gene and 19 result in 18 amino-acid changes. The AA variants were associated with E1/E2-positive carcinomas with more than 50 viral copies/cell (p = 0.035). The association of Asian-American E2 variants with retention of E1/E2 suggests that E2 variation may be an alternative mechanism de-regulating the expression of viral oncogenes.

Molecular targets for human papillomaviruses: prospects for antiviral therapy

A substantial medical need exists for the development of antiviral medicines for the treatment of diseases associated with infection by human papillomaviruses (HPVs). HPVs are associated with various benign and malignant lesions including benign genital condyloma, common skin warts, laryngeal papillomas and anogenital cancer. Since treatment options are limited and typically not very satisfactory, the development of safe and effective antiviral drugs for HPV could have substantial clinical impact. In the last few years, exciting advances have been made in our understanding of papillomavirus replication and the effects that the virus has on growth of the host cell. Although still somewhat rudimentary, techniques have been developed for limited virion production in vitro offering the promise of more rapid advances in the dissection and understanding of the virus life cycle. Of the 8-10 HPV gene products that are made during infection, only one encodes enzymatic activities, the E1 helicase. Successful antiviral therapies have traditionally targeted viral enzymes such as polymerases, kinases and proteases. In contrast, macromolecular interactions which mediate the functions of E6, E7 and E2 are thought to be more difficult targets for small molecule therapy.

Identification of conserved hydrophobic C-terminal residues of the human papillomavirus type 1 E1E4 protein necessary for E4 oligomerisation in vivo

Previous studies have shown that human papillomavirus (HPV) E4 proteins undergo oligomerisation, although the precise sequences involved have not been identified. Using the yeast two-hybrid system we have identified HPV 1 E4 sequences that are critical to multimerisation. Fusion proteins were created by linking wild-type and mutant E4 proteins to a LexA DNA-binding domain or a B42 transactivation domain. HPV 1 E4:E4 interactions were examined by expression of these fusion proteins in Saccharomyces cerevisiae. This assay showed that (1) amino acid residues 95 to 115 at the carboxy-terminus were critical for oligomerisation and (2) hydrophobic residues (isoleucine 107, phenylalanine 114) in this domain are major determinants in the formation of oligomers. Interestingly, the carboxy-terminal domain shares homology with other E4 proteins of cutaneous HPV types and, furthermore, positions 107 and 114 are conserved residues. Substitution of the conserved aspartate amino acids (residues 110 and 112) did not abrogate E4 oligomerisation. Chemical cross-linking of wart and recombinant (baculovirus-expressed) HPV 1 E4 protein indicated that in solution this viral protein forms complexes consistent in size with either trimers or tetramers. These complexes were resistant to urea denaturation and are not dependent on the formation of disulphide linkages. A mutant protein containing a deletion of residues 110 to 115 was unable to form oligomers following cross-linking supporting a role for this region in mediating E4:E4 interactions. We conclude that oligomerisation of the HPV 1 E4 protein is likely to be mediated by carboxy-terminal residues and that conserved hydrophobic residues of this domain play a major role in E4 oligomerisation.

The genomes of three of four novel HPV types, defined by differences of their L1 genes, show high conservation of the E7 gene and the URR

The DNA genomes of four new human papillomaviruses, HPV 75, HPV 76, HPV 77, and HPV 80, have been cloned, sequenced, and characterized. HPV 75, HPV 76 (both HPV 49-related), and HPV 77 (HPV 29-related) were isolated from benign cutaneous warts and HPV 80 (HPV 15-related) from histologically normal skin. HPV 77 has also been demonstrated in dysplastic warts and squamous cell carcinomas of the skin. The sequence data presented in this study led to a proposed modification of the definition of a new HPV type. The high degree of DNA sequence similarity between the E7 ORF of HPV 77 and HPV 29 (97.7%), as opposed to the E6 (82.8%) and L1 (85.3%) ORFs, might suggest conservation of a specific function or a possible recombinational event. Only the E6 and L1 ORFs of HPV 75 and HPV 76 have a similarity lower than 90%, whereas the DNA sequences of their upstream regulatory regions (URRs) share a similarity of 93%. The E7, E1, and E4 ORFs, as well as the URR of HPV 15 and HPV 80, share sequence similarities higher than 90%. Such a divergence in the similarity between different segments of the virus genomes of closely related HPV types has not been noted to date. A detailed comparative sequence analysis was performed. HPV 75, HPV 76, and HPV 80 revealed features characteristic of truly cutaneous HPV types, whereas HPV 77 shared several characteristics with the mucosal HPV types, some of which may have functional consequences.

Characterization of events during the late stages of HPV16 infection in vivo using high-affinity synthetic Fabs to E4

HPV late gene expression is initiated as an infected basal cell migrates through the differentiating layers of the epidermis, resulting in the onset of vegetative viral DNA replication and the expression of viral late proteins. We have used a large synthetic immunoglobulin library displayed on phage (diversity 6.5 x 10(10) phage) to isolate three Fabs (TVG405, 406, and 407) which recognize distinct epitopes on the E4 late protein of HPV16. A C-terminal monoclonal (TVG404) was generated by hybridoma technology, and N-terminal polyclonal antiserum was prepared by peptide immunization (alpha N-term). The most potent antibody (TVG405) had an affinity for E4 of approximately 1.0 nM. All antibodies recognized the protein in paraffin-embedded archival material, allowing us to map events in the late stages of virus infection. Expression of E4 in vivo does not coincide with synthesis of the major virus coat protein L1, but precedes it by 1 or 2 cell layers in premalignant lesions caused by HPV16 and by up to 20 cell layers in HPV63-induced warts. In higher grade lesions associated with HPV16, E4 is produced in the absence of L1. By contrast, vegetative viral DNA replication and E4 expression correlate exactly and in some lesions begin as the infected epithelial cell leaves the basal layer. Differentiation markers such as filaggrin, loricrin, and certain keratins are not detectable in E4-positive cells, and nuclear degeneration is delayed. HPV16 E4 has a filamentous distribution in the lower epithelial layers, but associates with solitary perinuclear structures in more differentiated cells. Antibodies to the N-terminus of the protein stained these structures poorly. Our findings are compatible with a role for the HPV16 E4 protein in vegetative DNA replication or in modifying the phenotype of the infected cell to favor virus synthesis or virus release. The Fabs will be of value in the evaluation of model systems for mimicking HPV infection in vitro.

Viral proteins of bovine papillomavirus type 4 during the development of alimentary canal tumours

In cattle infection of the upper alimentary canal mucosa by bovine papillomavirus type 4 (BPV-4) results in the development of papillomas which can progress to cancer in animals fed on bracken fern. This paper describes a study of the cellular and subcellular distribution of a number of different BPV-4 products in experimentally-induced BPV-4 tumours. E8 and E4 proteins were detected solely as cytoplasmic antigens in the undifferentiated and differentiated layers of the papilloma, respectively; L2 was detected solely as a nuclear antigen in the differentiated layers, whereas E7 was present in either the nucleus or the cytoplasm depending on the differentiation stage of the keratinocyte. Replicative forms of viral DNA were detected from the spinous to the squamous layers. Viral antigens were not detected during papilloma regression or in carcinomas. E8 was most prominent in early developmental stages, while E4 and L2 were most abundant in mature papillomas. E7 was present in large amounts in both early and mature stages, declining at later stages. These results suggest a temporal and spatial requirement for the expression and function of the viral proteins.

Mutational analysis of the human papillomavirus type 16 E1--E4 protein shows that the C terminus is dispensable for keratin cytoskeleton association but is involved in inducing disruption of the keratin filaments

The function of the human papillomavirus (HPV) E4 proteins is unknown. In cultured epithelial cells the proteins associate with the keratin intermediate filaments (IFs) and, for some E4 types, e.g., HPV type 16 (HPV-16), induce collapse of the keratin networks. An N-terminal leucine-rich motif (LLXLL) is a conserved feature of many E4 proteins. In a previous study we showed that deletion of this region from the HPV-1 and -16 E4 proteins abrogated the localization of the mutant proteins to the keratin cytoskeleton in a simian virus 40-transformed human keratinocyte cell line (S. Roberts, I. Ashmole, L. J. Gibson, S. M. Rookes, G. J. Barton, and P. H. Gallimore, J. Virol. 68:6432-6445, 1994). The E4 proteins of HPV-1 and -16 have little sequence homology except at the N terminus. Therefore, to establish the role of sequences other than those at the N terminus, we have performed a mutational analysis of the HPV-16 E4 protein. The results of the analysis were as follows: (i) similar to findings for the HPV-1 protein, no mutation of HPV-16 E4 sequences (other than the N-terminal leucine motif) results in a mutant protein which fails to colocalize to the keratin IFs; (ii) the C-terminal domain (residues 61 to 92) is not essential for association with the cytoskeleton; and (iii) deletion of C-terminal sequences (residues 84 to 92; LTVIVTLHP) corresponding to part of a domain conserved between mucosal E4 proteins affects the ability of the mutant protein to induce cytoskeletal collapse, despite colocalization with the keratin IFs. Further analysis of this region showed that conserved hydrophobic residues valines 86 and 88 are important. In addition, we show that the HPV-16 E4 protein is detergent insoluble and exists as several disulfide-linked, high-molecular-weight complexes which could represent homo-oligomers. The C-terminal sequences (residues 84 to 92), in particular valines 86 and 88, are important in the formation of these insoluble complexes. The results of this study support our postulate that the E4 proteins include functional domains at the N terminus and the C terminus, with the intervening sequences possibly acting as a flexible hinge.

Transient expression of epidermal filaggrin in cultured cells causes collapse of intermediate filament networks with alteration of cell shape and nuclear integrity

Filaggrin is an intermediate filament-associated protein (IFAP) that aggregates epidermal keratin filaments in vitro and is thought to perform a similar function during terminal differentiation in vivo. To test this function in living cells, we transiently expressed constructs encoding human filaggrin in both simple epithelial cells (COS-7) and rat keratinocytes. Scanning laser confocal microscopy showed that filaggrin-positive cells had collapsed keratin and vimentin intermediate filament (IF) networks, and that filaggrin partially co-localized with the IF networks. Filaggrin was also detected diffusely in the cytoplasm and nucleus. In contrast, when profilaggrin-like constructs, containing five filaggrin domains separated by the linker sequences, were expressed in cultured cells, immunoreactive granules formed. This finding is reminiscent of the insoluble nature of native profilaggrin that accumulates in keratohyalin granules in vivo, suggesting that the linker peptides (present in profilaggrin but not filaggrin) are important for granule formation. Cells expressing filaggrin also displayed disruption of the nucleus and the nuclear envelope; they rounded up and lost attachment to the substratum, in contrast to control cells over-expressing beta-galactosidase. This functional test of filaggrin in living cells supports its role in the reorganization and packing of keratin IF in epidermal differentiation. Moreover, the observed effects on cell morphology and nuclear integrity suggest that filaggrin may contribute to the form of apoptosis associated with terminal differentiation in epidermis.

Identification and characterization of a filament-associated protein encoded by Amsacta moorei entomopoxvirus

A novel protein which is expressed at high levels in insect cells infected with Amsacta moorei entomopoxvirus was identified by our laboratory. This viral gene product migrates as a 25/27-kDa doublet when subjected to electrophoresis on sodium dodecyl sulfate-polyacrylamide gels. It is expressed at late times of infection and is present in infected cells but is absent in purified extracellular virions and occlusion bodies. The gene encoding this polypeptide was mapped on the viral genome, and cDNA clones were generated and sequenced. The predicted protein was shown to be phosphorylated and contained an unusual 10-unit proline-glutamic acid repeat element. A polyclonal antiserum was produced against a recombinant form of the protein expressed in Escherichia coli, and a monoclonal antibody which reacted with the proline-glutamic acid motif was also identified. Immunofluorescence and immunoelectron microscopy techniques revealed that this protein is associated with large cytoplasmic fibrils which accumulate in the cytoplasm between 96 and 120 h postinfection. We subsequently called this viral polypeptide filament-associated late protein of entomopoxvirus. The fibrils containing this polypeptide are closely associated with occlusion bodies and may play a role in their morphogenesis and maturation.