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

Mus musculus papillomavirus 1 E8^E2 represses expression of late protein E4 in basal-like keratinocytes via NCoR/SMRT-HDAC3 co-repressor complexes to enable wart formation in vivo

High-risk human papillomaviruses (PV) account for approximately 600,000 new cancers per year. The early protein E8^E2 is a conserved repressor of PV replication, whereas E4 is a late protein that arrests cells in G2 and collapses keratin filaments to facilitate virion release. While inactivation of the Mus musculus PV1 (MmuPV1) E8 start codon (E8-) increases viral gene expression, surprisingly, it prevents wart formation in FoxN1nu/nu mice. To understand this surprising phenotype, the impact of additional E8^E2 mutations was characterized in tissue culture and mice. MmuPV1 and HPV E8^E2 similarly interact with cellular NCoR/SMRT-HDAC3 co-repressor complexes. Disruption of the splice donor sequence used to generate the E8^E2 transcript or E8^E2 mutants (mt) with impaired binding to NCoR/SMRT-HDAC3 activates MmuPV1 transcription in murine keratinocytes. These MmuPV1 E8^E2 mt genomes also fail to induce warts in mice. The phenotype of E8^E2 mt genomes in undifferentiated cells resembles productive PV replication in differentiated keratinocytes. Consistent with this, E8^E2 mt genomes induced aberrant E4 expression in undifferentiated keratinocytes. In line with observations for HPV, MmuPV1 E4-positive cells displayed a shift to the G2 phase of the cell cycle. In summary, we propose that in order to enable both expansion of infected cells and wart formation in vivo, MmuPV1 E8^E2 inhibits E4 protein expression in the basal keratinocytes that would otherwise undergo E4-mediated cell cycle arrest. IMPORTANCE Human papillomaviruses (PVs) initiate productive replication, which is characterized by genome amplification and expression of E4 protein strictly within suprabasal, differentiated keratinocytes. Mus musculus PV1 mutants that disrupt splicing of the E8^E2 transcript or abolish the interaction of E8^E2 with cellular NCoR/SMRT-HDAC3 co-repressor complexes display increased gene expression in tissue culture but are unable to form warts in vivo. This confirms that the repressor activity of E8^E2 is required for tumor formation and genetically defines a conserved E8 interaction domain. E8^E2 prevents expression of E4 protein in basal-like, undifferentiated keratinocytes and thereby their arrest in G2 phase. Since binding of E8^E2 to NCoR/SMRT-HDAC3 co-repressor is required to enable expansion of infected cells in the basal layer and wart formation in vivo, this interaction represents a novel, conserved, and potentially druggable target.

EXPRESSION OF E8^E2 IS REQUIRED FOR WART FORMATION BY MOUSE PAPILLOMAVIRUS 1 IN VIVO

Human papillomavirus (HPV) E1 and E2 proteins activate genome replication. E2 also modulates viral gene expression and is involved in the segregation of viral genomes. In addition to full length E2, almost all PV share the ability to encode an E8^E2 protein, that is a fusion of E8 with the C-terminal half of E2 which mediates specific DNA-binding and dimerization. HPV E8^E2 acts as a repressor of viral gene expression and genome replication. To analyze the function of E8^E2 in vivo, we used the Mus musculus PV1 (MmuPV1)-mouse model system. Characterization of the MmuPV1 E8^E2 protein revealed that it inhibits transcription from viral promoters in the absence and presence of E1 and E2 proteins and that this is partially dependent upon the E8 domain. MmuPV1 genomes, in which the E8 ATG start codon was disrupted (E8-), displayed a 10- to 25-fold increase in viral gene expression compared to wt genomes in cultured normal mouse tail keratinocytes in short-term experiments. This suggests that the function and mechanism of E8^E2 is conserved between MmuPV1 and HPVs. Surprisingly, challenge of athymic nude Foxn1^nu/nu mice with MmuPV1 E8- genomes did not induce warts on the tail in contrast to wt MmuPV1. Furthermore, viral gene expression was completely absent at E8- MmuPV1 sites 20 - 22 weeks after DNA challenge on the tail or quasivirus challenge in the vaginal vault. This reveals that expression of E8^E2 is necessary to form tumors in vivo and that this is independent from the presence of T-cells.IMPORTANCE HPV encode an E8^E2 protein which acts as repressors of viral gene expression and genome replication. In cultured normal keratinocytes, E8^E2 is essential for long-term episomal maintenance of HPV31 genomes, but not for HPV16. To understand E8^E2's role in vivo, the Mus musculus PV1 (MmuPV1)-mouse model system was used. This revealed that E8^E2's function as a repressor of viral gene expression is conserved. Surprisingly, MmuPV1 E8^E2 knock out genomes did not induce warts in T-cell deficient mice. This shows for the first time that expression of E8^E2 is necessary for tumor formation in vivo independently of T cell immunity. This indicates that E8^E2 could be an interesting target for anti-viral therapy in vivo.

Mutations in HPV18 E1^E4 Impact Virus Capsid Assembly, Infectivity Competence, and Maturation

The most highly expressed protein during the productive phase of the human papillomavirus (HPV) life cycle is E1^E4. Its full role during infection remains to be established. HPV E1^E4 is expressed during both the early and late stages of the virus life cycle and contributes to viral genome amplification. In an attempt to further outline the functions of E1^E4, and determine whether it plays a role in viral capsid assembly and viral infectivity, we examined wild-type E1^E4 as well as four E1^E4 truncation mutants. Our study revealed that HPV18 genomes containing the shortest truncated form of E1^E4, the 17/18 mutant, produced viral titers that were similar to wild-type virus and significantly higher compared to virions containing the three longer E1^E4 mutants. Additionally, the infectivity of virus containing the shortest E1^E4 mutation was equivalent to wild-type and significantly higher than the other three mutants. In contrast, infectivity was completely abrogated for virus containing the longer E1^E4 mutants, regardless of virion maturity. Taken together, our results indicate for the first time that HPV18 E1^E4 impacts capsid assembly and viral infectivity as well as virus maturation.

The human papillomavirus replication cycle, and its links to cancer progression: a comprehensive review

HPVs (human papillomaviruses) infect epithelial cells and their replication cycle is intimately linked to epithelial differentiation. There are over 200 different HPV genotypes identified to date and each displays a strict tissue specificity for infection. HPV infection can result in a range of benign lesions, for example verrucas on the feet, common warts on the hands, or genital warts. HPV infects dividing basal epithelial cells where its dsDNA episomal genome enters the nuclei. Upon basal cell division, an infected daughter cell begins the process of keratinocyte differentiation that triggers a tightly orchestrated pattern of viral gene expression to accomplish a productive infection. A subset of mucosal-infective HPVs, the so-called ‘high risk’ (HR) HPVs, cause cervical disease, categorized as low or high grade. Most individuals will experience transient HR-HPV infection during their lifetime but these infections will not progress to clinically significant cervical disease or cancer because the immune system eventually recognizes and clears the virus. Cancer progression is due to persistent infection with an HR-HPV. HR-HPV infection is the cause of >99.7% cervical cancers in women, and a subset of oropharyngeal cancers, predominantly in men. HPV16 (HR-HPV genotype 16) is the most prevalent worldwide and the major cause of HPV-associated cancers. At the molecular level, cancer progression is due to increased expression of the viral oncoproteins E6 and E7, which activate the cell cycle, inhibit apoptosis, and allow accumulation of DNA damage. This review aims to describe the productive life cycle of HPV and discuss the roles of the viral proteins in HPV replication. Routes to viral persistence and cancer progression are also discussed.

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.

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.

A cyclin-binding motif in human papillomavirus type 18 (HPV18) E1^E4 is necessary for association with CDK-cyclin complexes and G2/M cell cycle arrest of keratinocytes, but is not required for differentiation-dependent viral genome amplification or L1 capsid protein expression

The G2/M arrest function of human papillomavirus (HPV) E4 proteins is hypothesized to be necessary for viral genome amplification. Full-length HPV18 E1^E4 protein is essential for efficient viral genome amplification. Here we identify key determinants within a CDK-bipartite consensus recognition motif in HPV18 E1^E4 that are critical for association with active CDK-cyclin complexes and in vitro phosphorylation at the predicted CDK phosphorylation site (threonine 23). The optimal cyclin-binding sequence ((43)RRLL(46)) within this E4 motif is required for G2/M arrest of primary keratinocytes and correlates with cytoplasmic retention of cyclin B1, but not cyclin A. Disruption of this motif in the E4 ORF of HPV18 genomes, and the subsequent generation of stable cell lines in primary keratinocytes revealed that this motif was not essential for viral genome amplification or L1 capsid protein induction. We conclude that the HPV18 E4 G2/M arrest function does not play a role in early vegetative events.

Correlation between cyclin B1 immunostaining in cervical biopsies and HPV detection by PCR

In recent years, experimental data on the roles of viruses and cyclin B1 in G2/M arrest has accumulated. The aim of our study was to immunohistochemically examine a series of cervical squamous intraepithelial lesions for the presence of cyclin B1 and test for a possible correlation with morphologic findings or human papillomavirus (HPV) types detected by polymerase chain reaction. One hundred and three biopsies and/or conization specimens were examined, including high-grade lesions (25), low-grade lesions (52), biopsies showing nondiagnostic atypia (15), and biopsies without histopathologic alterations (11). Three patterns of immunopositivity were recognized, according to the level of epithelium exhibiting positively stained cells. Immunoreactivity for cyclin B1 above the basal/parabasal cells correlated with HPV presence. Cyclin B1-stained cells may represent "prekoilocytes," whose eventual progression to koilocytes would depend on several parameters related to the intricacies of HPV infection. In cases of nondiagnostic atypia immunoreactivity, cyclin B1 could be considered as a surrogate test.

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.

G2/M cell cycle arrest in the life cycle of viruses

There is increasing evidence that viral infection, expression of viral protein or the presence of viral DNA causes the host cell cycle to arrest during G2/M. The mechanisms used by viruses to cause arrest vary widely; some involve the activation of the cellular pathways that induce arrest in response to DNA damage, while others use completely novel means. The analysis of virus-mediated arrest has not been proven easy, and in most cases the consequences of arrest for the virus life cycle are not well defined. However, a number of effects of arrest are being investigated and it will be interesting to see to what extent perturbation of the G2/M transition is involved in viral infections.

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.

Molecular biology of human papillomavirus infection and cervical cancer

HPVs (human papillomaviruses) infect epithelial cells and cause a variety of lesions ranging from common warts/verrucas to cervical neoplasia and cancer. Over 100 different HPV types have been identified so far, with a subset of these being classified as high risk. High-risk HPV DNA is found in almost all cervical cancers (>99.7%), with HPV16 being the most prevalent type in both low-grade disease and cervical neoplasia. Productive infection by high-risk HPV types is manifest as cervical flat warts or condyloma that shed infectious virions from their surface. Viral genomes are maintained as episomes in the basal layer, with viral gene expression being tightly controlled as the infected cells move towards the epithelial surface. The pattern of viral gene expression in low-grade cervical lesions resembles that seen in productive warts caused by other HPV types. High-grade neoplasia represents an abortive infection in which viral gene expression becomes deregulated, and the normal life cycle of the virus cannot be completed. Most cervical cancers arise within the cervical transformation zone at the squamous/columnar junction, and it has been suggested that this is a site where productive infection may be inefficiently supported. The high-risk E6 and E7 proteins drive cell proliferation through their association with PDZ domain proteins and Rb (retinoblastoma), and contribute to neoplastic progression, whereas E6-mediated p53 degradation prevents the normal repair of chance mutations in the cellular genome. Cancers usually arise in individuals who fail to resolve their infection and who retain oncogene expression for years or decades. In most individuals, immune regression eventually leads to clearance of the virus, or to its maintenance in a latent or asymptomatic state in the basal cells.

HPV16 E1^E4 protein is phosphorylated by Cdk2/cyclin A and relocalizes this complex to the cytoplasm

The human papillomavirus type 16 E1--E4 protein is expressed abundantly in cells supporting viral DNA amplification, but its expression is lost during malignant progression. In cell culture, 16E1--E4 causes G2 cell cycle arrest by associating with and preventing the nuclear entry of Cdk1/cyclin B1 complexes. Here, we show that 16E1--E4 is also able to associate with cyclin A and Cdk2 during the G2 phase of the cell cycle. Only a weak association was apparent during S-phase, and progression through S-phase appeared unaffected. As with cyclin B1, the interaction of 16E1--E4 with cyclin A is dependent on residues T22/T23 and results in the accumulation of cyclin A in the cytoplasm where it colocalizes with 16E1--E4. 16E1--E4 serine 32 was found to be phosphorylated by Cdk2/cyclin A. We hypothesize that the interaction of 16E1--E4 with cyclin A may serve to increase the efficiency with which 16E1--E4 is able to prevent mitotic entry.

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.

Role of the E1--E4 protein in the differentiation-dependent life cycle of human papillomavirus type 31

The most highly expressed protein in the productive life cycle of human papillomaviruses (HPVs) is E1--E4, but its function is not well understood. To investigate the role of E1--E4, we undertook a genetic analysis in the context of the complete HPV type 31 (HPV31) genome. A mutant HPV31 genome (E4M9) was constructed that contained a stop codon in the E4 open reading frame at amino acid 9 and was silent in the overlapping E2 coding sequence. Wild-type and mutant genomes were transfected into normal human foreskin keratinocytes (HFKs) and selected for drug resistance, and pooled cultures were examined for effects of E1--E4 on viral functions. Southern blot analyses of transfected HFKs demonstrated that cells carrying the E4M9 mutant genomes were maintained as episomes at copy numbers similar to those in keratinocytes transfected with wild-type HPV31. Both sets of cells grew at similar rates, exhibited comparable extensions of life spans, and had equivalent levels of early transcripts. Following suspension of the cells in a semisolid medium, differentiation-dependent genome amplification and late gene expression were significantly decreased in cells maintaining the E4M9 mutant genome compared to those with wild-type HPV31. One explanation for these effects could be a reduction in the number of cells harboring mutant genomes that enter S phase upon differentiation. An analysis of cells containing E4M9 mutant genomes in organotypic raft cultures indicated a reduction in bromodeoxyuridine incorporation in differentiated suprabasal cells compared to that seen in wild-type rafts. Our results indicate that the HPV31 E1--E4 protein plays a significant role in promoting HPV genome amplification and S phase maintenance during differentiation.